Plating can also be detected using very precise heat measurements of the cell. Using a calorimeter an isothermic heat flow measurement can be done to very precisely measure the heat generation or absorption of the cell. The heat generated in the cell is created by electrical lossses, ionic mixing and chemical reaction. These sources can be divided into irreversible heat losses, caused by ohmic resistance and ion flow resistances in the cell, and reversible heat losses, caused mainly by the reaction entropy of the intercalation and deintercalation during charge and discharge inside the anode active material. The irreversible heat losses always create a heat flow from the cell to the environment and they scale with the current applied to cell following the formula for heat losses in electrical systems:

$$W = I^2 \cdot R$$

Since the relevant part for the cell reactions and processes is the reversible heat, the measurements have to be performed at very low C-rates to reduce the superposition of the irreversible heat losses. At for example 1C the reversible heat only makes up 1.6% of the total heat generation, while it is at 24.5% at C/4 [188]. The irreversible heat losses should be constant up to 80% SOC since the internal resistance is not changing much in this area, but it increases at the end of the charging process because of Li-ion repulsion at high intercalation levels [189]. Calorimetry was successfully used to observe the different intercalation levels and order-disorder transformations in graphite [188]. Downie et al. [92] observed a change on the expected heat flow when a cell approaches plating conditions at 0 V vs Li/Li+. They tested this behaviour on full- and half cells to prove that the thermal response of the anode is much higher than effects on the cathode. They found that the heat flow became significantly lower when plating started since there is a lower entropy connected to metallic deposition compared to intercalation.

This method can be used to detect the plating onset in a specific cell type for very low C-rate which is rarely applicable to a real application, especially applications that favor conditions leading to plating like electric vehicles.

#### **Neutron Diffraction**

Neutron diffraction uses a neutron beam to create a diffraction pattern that changes depending on the material under observation. One big advantage of neutron diffraction is the high penetration depth that allows in-situ measurements in a non-destructive way, and the fact that it is very sensitive to lighter elements like lithium [78, 109]. But while it can not detect metallic metallic lithium directly, it can show the different intercalation states of the graphite and thus be used to indirectly detect the effects of lithium plating and the subsequent diffusion of the lithium depositions into the graphite. Lüders et al. [190] have analyzed the amount of plated lithium by performing neutron diffraction during the relaxation after plating and have obtained the amount of lithium metal that diffused into the anode by measuring the change in intercalation level within the graphite. They performed tests with different charging currents and found a dramatic increase of lithium plating after the current passed C/2 in their experiments, while the diffusion rate of the reversible lithium plated stayed constant.

The biggest disadvantage of neutron diffraction is the need for neutron source which creates the necessity for the nuclear reactor.

#### **Nuclear Magnetic Resonance**

The nuclear magnetic resonance spectroscopy (NMR) is a technique frequently used in chemical analysis. Isotopes that contain an odd number of protons and/or neutrons react to a strong magnetic field by creating an electromagnetic signal that can be used to distinguish between the present elements and molecules. NMR is very precise in identifying monomolecular organic compounds. Since lithium metal contains 3 protons, NMR 7 Li NMR can be used to detect metallic lithium in LIBs caused by lithium plating [7]. The magnetic field however can not penetrate deeply into bulk the material, but Bhattacharyya et al. [191] using the finite penetration depth of the method, showed that is possible to distinguish whether the lithium deposition has a mossy morphology or forms dendrites on a lithium metal anode and also showed a way to quantify the amount of mossy/dendritic lithium deposited. A similar experiment was repeated by Chang et at. [192]. At the same time NMR is capable of detecting the lithium intercalated into the graphite [160] and can show the different intercalation levels. The measurement method for lithium is fast enough to obtain operando snapshots at a reasonable time frame [191]. According to Wandt et al. a disadvantage of NMR is the low spectral resolution.

The usage of NMR is limited by the very small size sample area and the fact that a specific cell enclosure is necessary for the magnetic field to be able to enter the cell.

#### **Electron Paramagnetic Resonance Spectroscopy**

In EPR the same technique as in NMR is used, but instead of exciting the atomic nuclei, in EPR the unpaired free electron spins are excited. This allows EPR the detection of free radicals. EPR just like NMR is also capable of detection metallic lithium [155, 193]. Wandt et al. [194] postulate that EPR should show a higher sensitivity per unit to detect lithium metal compared to NMR. They were able to detect the amount of plated lithium as a function of time and thus could calculate the current that flows into the plating reaction as a function of time by the derivative of the charge amount. Besides that they could also distinguish the portions of the coulombic efficiency that went into the SEI reformation on the surface of the plated lithium and finally calculate the amount of dead lithium remaining after stripping. They did however find a great variance between cells under the same conditions that they attribute to inhomogeneities in cell compression in their test cell. Furthermore the EPR signal was able to show whether the lithium metal was located directly on the anode or was grown into the separator by forming dendrites. They however needed a specialised concentric test cell where the cathode active material was directly coated onto the separator since an aluminum substrate would have shielded the microwave. For EPR as well as NMR the penetration depth, called skin depth, into the material the microwaves are calibrated for, is very small. For lithium metal for example at 9*.*5 GHz the skin depth is only 1*.*1 µm, which means that for bulk materials like lithium metal anodes, only the surface of the sample can be probed [194].

#### **Reference Electrodes**

A reference electrode is an electrode that is introduced into the cell additionally to the anode and cathode to allow the measurement of the potential or the impedance of the anode and cathode independently. This can be done in-situ, and using the right reference electrode, only has a very small effect on the cell performance. This is especially interesting for plating in the case of the anode potential, since a drop of the anode potential to a value lower than 0 V vs Li/Li+ indicates a plating event [111, 134, 195, 196]. It is important to notice however that while a drop to an anode potential lower than 0 V should always indicate plating, it doesn't mean that there could be no plating event in the cell before this potential is reached. This is due to the fact that the potential measured by the reference electrode is only the average potential in the region of the reference electrode. Lithium plating can be a highly localized process, especially if cell defects or regions of different pressure or temperature are involved, which is why the averaged potential might not be an accurate indicator where the conditions for plating are met, since the potential differences on the anode are not taken into account.

For the result of a reference electrode measurement it should also be kept in mind that the measurement includes an ohmic drop within the reference electrode [197].

Nonetheless the reference electrode is a powerful tool for cell analysis. The impedance of the anode measured using EIS can also help to more precisely identify the effects of plating on the SEI since the SEI is represented with a distinguished semi circle in the impedance spectra of the cell, even more pronounced if only the anode impedance is measured.

Such a reference electrode is typically implemented into a test cell produced in a laboratory [134, 195], very often coin cells or small pouch cells, since they can be produced with a small amount of effort and equipment. Another possibility is to implement the reference into a commercial cell [160, 198]. It it can be very challenging however to implement it into a commercial cell and the tampering with the cell can lead to unreliable measurement results. The reference electrode has to meet certain requirements for the measurements to be sound:

#### 4. High reproducibility [199]

In general there are two ways to implement a reference electrode in a cell: interior or exterior implementation [146] visible in figure 2.18. The more common way is to sandwich the reference in between the anode and cathode, surrounded by separator, which is called interior implementation. This way the distance to the anode and cathode are minimized and equidistant, leading to comparable measurement results for both electrodes and there is no extra space needed for the reference in the cell compartment. It is however influencing the electrical field between the electrodes and thus causing a variation in the ion flow, slightly distorting the potentials in the cell [145, 146, 200].

The second method is to keep the reference in a separate compartment connected by excess electrolyte outside of the original cell compartment. This is normally easier to implement and minimizes the influence of the reference on the cell performance but is very susceptible to even a very small misalignment of the electrodes strongly distorting the electrical field in the reference electrode compartment [146]. Because of this, this method is harder to reproduce and can only be used in specialized test cells, like the El-Cell (https://el-cell.com/).

**Figure 2.18:** Schematic of a cell with internal and external reference electrode

If the electrode is placed in an area with a high variance in current density this can also lead to measurement errors [197].

Another important aspect is the material of the reference electrode. In LIBs a lithium-metal reference electrode is useful since it is immediately measuring the desired potential [160, 201, 202, 203]. But the handling of metallic lithium is very difficult and requires a protective atmosphere [204]. The reactivity of lithium also means that an SEI will be forming on the reference electrode, consuming some of the electrolyte and thus influencing the cell performance. Lithium reference electrodes also have the risk of dendrites forming on the reference, causing a short circuit inside the cell [145]. Other materials might not have the correct potential to begin with, but are more stable and don't disturb the cell as much as a lithium metal electrode. This for example includes lithium-titanate, platinum [205] or lithium-tincompound LiySn reference electrodes, but it can be hard to determine the true potential of these electrodes. The lithium-tin version for example has multiple voltage plateaus, making determining the potential especially hard [145]. Another method to circumvent the difficult handling of pure lithium metal is using a copper, gold or tin reference electrode that is plated then inside the cell with lithium metal to function as a lithium metal electrode [145]. This however consumes not only electrolyte but also active lithium inside the cell. Abraham et al. [204] noticed, that the potential of their Li*y*Sn reference electrode was drifting when the cell was stored at higher temperatures but reaching stable voltage plateaus after a while, indicating different lithiation states of the tin alloy. Blyr et al. [199] cold not activate their Li4Ti5O reference electrodes. Their explanation was that due to the initial potential of the electrode of 3 V the impurities in the electrolyte reduce first on the reference electrode, poisoning the electrode and making it unusable. Using energy-dispersive X-ray spectroscopy (EDX) they found large amounts of manganese on the surface of the reference, caused by manganese dissolution from the cathode. They solved this problem by already having the cell cycled between the positive and negative electrode previously, which removed the impurities in the electrolyte. Dolle et al. [144] showed that silver is not a suitable material for a reference electrode in a lithium based cell system since even a small current necessary during the measurement using the reference electrode causes a shift in potential, hinting an unidentified electrochemical reaction on the surface of the electrode.

The dimension and shape of the reference electrode compared to the cell size is the last aspect to consider. A bigger reference electrode can significantly distort the measured potential, especially at high currents [146]. The smaller the electrode, the smaller the influence of the electrode, but the smaller the area inside the cell that can be probed. A very small reference electrode can lead to a very localized measurement, that might not reflect the current cell state. Parts of the reference electrode, that connect to the desired measurement point should also be insulated to prevent mixed potentials [204]. A characteristic length lower than the thickness of the separator is suggested by Dees et al. [146] to have an acceptably low influence.

The reference electrode also has a capacity itself. Not all of the ion flow is flowing around the reference electrode, some of the charge is also passing through it by performing opposite redox-reactions on each side of the electrode. If the current in the cell is high enough, this passing through reaction can polarize the electrode, since its internal capacity is reached [146].

The main challenge for reference electrodes is the Creation of reproducible laboratory cells so that the construction errors in the cell manufacturing do not overshadow the actual effects in the cell that are supposed to be measured during the experiment.

#### **GDOES**

A plasma and argon ion sputtering are used in GDOES to gradually remove material from the surface of the sample. This removed material is added to the plasma and then emits characteristic wavelengths that allow for element identification and quantification. The argon beam ablates the sample layer by layer and this way allows gathering of depth profiling information. The method was used by Ghanbari et al. [124] to identify localized lithium depositions in the anode. The argon beam can sputter through the whole depth of the anode material, from the surface to the substrate and thus give a whole picture of the material composition. They assumed that all measured oxygen percentage is bound in the form of Li2O and thus assume that the rest of the lithium percentage must be in the form of metallic lithium. In this way they categorized 88% of the lithium content in the upper SEI layer as metallic lithium caused by lithium plating beforehand in comparison to a sample with no plating, where no lithium was identified in metallic form. Their measurements also showed that an increased lithium concentration goes to a depth of 3 to 4 µm into the sample. This can either mean that the plating can go deeper into the bulk of the anode material, or that plating creates a thicker surface layer compared to the regular SEI. The second theory is in line with what FIB cross-section pictures revealed in this thesis in chapter 3.1.

While for all other measurement methods so far the cell material stays intact and in some cases the cell is even still operable, GDOES consumes the material so that no further experiments or analysis can be performed on the sample.

#### **Gas Chromatography**

Fang et al. [59] used the fact that metallic lithium reacts with water forming H2-gas to identify the amount of metallic lithium in their samples. They used a titration method by adding pure H2O to their samples and than analyzed the created gas regarding its H2 content to find the amount of metallic lithium that was present in the sample. They call this combination of tritation and gas chromatography titration gas chromatography (TGC). The only other species that could be possibly found in the SEI that also forms hydrogen gas in contact with water is LiH that can be created by impurities during the cell manufacturing. The authors ruled out the existence of LiH in their samples however by testing plating-free samples that have been cycled 10 times to form a substantial SEI using the same technique. In these samples no H2 gas could be found, so no LiH was present in the SEI. The method also allows the quantification of the metallic lithium up to 10 × 10−7 gram. It does not allow any spacial resolution into the sample depth however and has the same disadvantage as GDOES of consuming the whole sample in the process.

# **Experimental and Analysis Chapter** 3

In this chapter first the preceding works are presented and put into context with the present thesis. The preceding works are student theses supervised by the author that laid the foundation for the research done in this thesis. In all tests done in this work cells of the same type or cells that have been utilized before in the student theses are used. This way the results are comparable. In the following chapters the 3 experimental approaches used in this work are introduced and their experimental setup is described. Since the 3 topics require very different analytical methods the evaluation and analysis of the results is directly following each of the experimental setups. The three topics are:

All chapters presented here look at a different approach of detecting or analyzing lithium plating either in operando, as is the case for the voltage and current anomalies, or post-mortem for the XPS measurements and the particle analysis.

#### **3.1 Preceding Works**

Various theses under the supervision of this author have been creating the foundation of this thesis and in this part the most important findings of these theses will be presented. There have been more theses than are presented in this chapter investigating different theories and methods in the field of plating whose results did not yield usable data or that showed a certain method to not work. In this chapter only the theses are presented whose data or results directly influenced the present thesis.

Stief [206] cycled cells at different temperatures and C-Rates to find the conditions that cause plating and observed the change of the anodes for the different conditions using an LSM. In the thesis by Wink [207] the effects of lithium plating have been analyzed with optical methods utilizing an LSM and a FIB. Jonack [208] analyzed 6 different cells from different manufacturers to identify the cell parameters that can favor plating. She used electrical measurements as well as LSM observations in her analysis.

#### **Optical analysis**

Stief cycled ICR18650 HB2 1500mAh LG Chem cells under different conditions. Figure 3.1 shows the conditions under which the cells have been cycled. He used currents from 1C to 3C and a temperature range of 0 ◦C to −20 ◦C. Only for the cell with the lowest current and highest temperature (at 1C and 0 ◦C) no plating could be detected, all other conditions produced plating of differing strength within the cells. The occurrence of plating was detected by evaluating the discharge curve for the characteristic stripping plateau as discussed in Chapter 2.6.1. Figure 3.2 also shows the capacity loss of the cells over the 10 cycles. All cells except for the condition under which not plating was detectable show accelerated degradation caused by plating. All cells at −20 ◦C show almost the same capacity loss which is because all three cells did not have any CC phase and thus the applied current did not affect the measurement. They are all technically charged under the same conditions. The cell charged at −10 ◦C at 3C also shows the same ageing behaviour. This cell also did not have a CC phase which indicates that omitting the CC phase seems to have stronger effect on the occurrence of plating than the temperature. It could be that for all cells showing the highest ageing rate a saturation of metallic lithium deposition was reached and they thus all show similar capacity loss.

**Figure 3.1:** Plating conditions under which the cells in Stiefs work have been cycled denoted by the red crosses.

For all currents at −20 ◦C the overpotential was so high during charge that the CC phase was completely omitted and only the CV phase was present during the charge. During the charging process an anomaly was detectable in the charge voltage during the CC phase as well as in the CV phase of the cells that omitted the CC phase. This effect was not further discussed since it was not the main topic in Stiefs thesis and will be analyzed in more detail later in chapter 3.2. Stief then opened all cells under protective argon atmosphere and took LSM images of the anodes of all cells, visible in figure 3.3.

It is visible in the LSM images that compared to the new cell in the bottom left all cells show changes of the surface morphology. The surfaces get covered with a mossy layer on top of the graphite particles. It can be seen that a

**Figure 3.2:** Capacity loss of the cells tested under different conditions over 10 cycles

higher capacity loss correlates with more surface coverage. But even the cell cycled at 1C and 0 ◦C shows a slight coverage compared to the pristine cell which indicates that even though no plating could be detected in the form of stripping or increased capacity degradation the surface analysis revealed a weak coverage similar to the other cells.

Based on the LSM analysis a FIB located in another institute was used to create a more detailed view of these surface coverage found by Stief in the thesis of Wink. For the optical analysis in the thesis of Wink LG Chem 18650HB2 NMC cells have been cycled under normal and plating conditions at −10 ◦C. One cell was left in its new state, one cell was cycled for 29 cycles at room temperature, three cells have been cycled under plating conditions at −10 ◦C for 1, 29 and 58 cycles respectively. The cells have then been opened

**Figure 3.3:** The anode surfaces of the cells cycled under different conditions from 1C to 3C at 0 to −20 ◦C

under protective argon atmosphere in a glovebox and observed using the LSM and FIB. The first test was to analyze the effect of atmospheric exposure on the samples to see whether transport under protective atmosphere to the FIB device is necessary. Figure 3.5 shows the effects of oxygen and moisture exposure on the samples. It is visible in part A that there is no noticeable effect on the microstructure for a new cell. In B on the other hand drastic changed of the structure are visible, even though the cell only had one cycle under plating conditions. Part C shows a much lower effect than B although the number plating cycles is much higher. That leads to the assumption that after a small amount of cycling the chemical composition of the SEI is different compared to further cycled cells. A possible explanation could be that after one cycle there is still a considerable amount of lithium available for plating and thus there is more reversible plated or passivated lithium in its metallic form in the SEI that is highly reactive. Over time this lithium reacts with parts of the SEI and forms less chemically active compounds and together with the fact that in later cycles the irreversible capacity loss, and thus the inventory of available lithium in the cell, is much lower, the amount of reactive metallic lithium causing degradation at air atmosphere is greatly reduced.

**Figure 3.4:** FIB cross section of A: new cell and B: cell after 29 cycles under normal conditions

Figure 3.4 shows a FIB cross section of the anode of a completely new cell on the left (A) and a cell after 29 cycles under normal conditions (B). Prior to the cross section a thin film of platinum was sputtered on the surface of the anode to improve the FIB cut, visible in the light stripe in the top part of all cross sections. The graphite particles are clearly visible in both images and there is no surface film on the particles detectable. The average SEI should have a thickness of 40 to 80 nm (see chapter 2.2.1) for a new cell, which both of the samples are. So it would be almost invisible at the magnification of the images. There is also no noticeable change between the new cell and the cycled cell, which is to be expected after only 29 cycles.

Figure 3.6 shows the cross sections for one cycle of plating (A), 29 cycles of plating (B) and 58 cycles of plating (C). Even after only one cycle of plating there is a mossy structure visible in the crevasses between the graphite particles. Over the course of additional cycling under plating condition this mossy layer grows out and covers the complete particles. The border between the graphite particle and the surface layer is not clearly visible everywhere, since the lower part of the layer shows a darker color on the images. This means that the lower part of the layer seems to have a different composition. After 29 cycles the layer has a thickness of 7 µm at its thickest part, more

**Figure 3.5:** 96 hours of air exposure of the FIB cross section for A: new cell, B: 1 cycle of plating at −10 ◦C and C: 58 cycles of plating at −10 ◦C (before exposure on the left, after exposure on the right)

than 100 times the thickness of the average SEI of a non-plated cell. After 58 cycles it is visible that the layer has become even thicker, with a thickness of 10 µm at its thickest part. The darker lower part of the layer is even more pronounced. The top of the layer in both image B and C stills shows the mossy structure also visible in image A.

Wink also performed basic image analysis on the FIB images seen in figure 3.6 to separate areas that could be metallic lithium depositions. He used the difference in brightness of the areas that look more solid and sharp compared to the surrounding depositions in the thickened SEI to estimate the volume of those depositions. The brightness of pixel of a FIB image corresponds to the conductivity of the material present, meaning that brighter areas have a higher conductivity. This would be expected by deposited lithium layer that they should have higher conductivity than the surrounding SEI decomposition compounds and thus have a higher brightness in the FIB image. A comparison of the obtain volume estimation to the theoretical volume of the lithium inventory lost during the plating cycles showed that they are in the same order of magnitude. This is a fist indicator that these layers could be the lithium depositions.

**Figure 3.6:** FIB cross section of A: 1 cycle of plating, B: cell after 29 cycles of plating and C: cell after 58 cycles of plating

#### **Comparison of Plating in Cells of Different Manufacturers**

To then seen whether the effects of plating are different in the cells of different manufacturers Jonack [208] analyzed 6 cells of different manufacturers regarding their susceptibility to plating. She used 2 types of 18650 cells made by Sony, Samsung and LG with one cell of each producer being an NCA cell and one being an NMC cell. As discussed in chapter 2.2.1 the SEI layer is less stable in cells with manganese in the cathode active material. This should also lead to expected higher susceptibility to plating in cells with an NMC cathode compared to an NCA cathode. Table 3.1 shows the six different cells used in her work.

**Table 3.1:** Cells used in the work of Jonack [208]

All cells have been analyzed using the LSM and electrically characterized in their new state and after 10 cycles of plating at −20 ◦C. For each experiment 4 cells have been cycled and their average capacity loss was determined. Figure 3.7 shows the irreversible plating induced capacity loss of the cells after the 10 cycles at −20 ◦C.

The most noticeable result is the consistent difference in capacity loss between the NMC and NCA cells, probably caused by the presence of manganese, as already discussed above. All NMC cells show a significantly higher capacity loss than their NCA counterpart from the same manufacturer. The anodes of the cells have then also been optically analyzed using an LSM under protective atmosphere. Figures 3.8 to 3.9 show the LSM images taken from the anode samples. The left pictures always show the new cell, the right picture the aged cell. The top row corresponding to the NMC and the bottom row to the NCA cells of each producer.

**Figure 3.7:** Comparison of the capacity loss of the cells used in Jonacks work

Is is visible in all three images that 10 cycles of plating cause a visible change in the surface structure of the anode. A mossy deposition is covering the anode surface in all samples. But while the surface of the NMC cells seems to be almost fully covered with the deposition there are still graphite particles visible in the images of the NCA samples, especially in the image of the Sony NCA sample, that also showed the lowest capacity loss of all tested cells. Thus there seems to be a correlation between the depositions and the capacity loss caused by plating. In the work of Jonack no further significant correlations could be found between any measured cell parameter, whether electrical or optical, and the irreversible capacity loss. For this reason the cells will be used for further experiments in the XPS and optical section in this thesis in the sections 3.3 and 3.4 to identify other possible differences in the cells that could be important for their plating susceptibility.

**Figure 3.8:** Comparison of the anode surface of the new (left) and plated (right) Sony cells. (top: NMC, bottom: NCA)

**Figure 3.9:** Comparison of the anode surface of the new (left) and plated (right) Samsung cells. (top: NMC, bottom: NCA)

**Figure 3.10:** Comparison of the anode surface of the new (left) and plated (right) LG cells. (top: NMC, bottom: NCA)

#### **3.2 Charge Anomalies**

During the charging process under plating conditions anomalies are visible in the voltage profile during the CC phase as well as in the current profile during the CV phase, as already explained in chapter 2.6.1 and detected in the work of Stief presented in chapter 3.1. These anomalies could potentially not only be used to obtain a deeper understanding of the plating process, but could also help detect plating operando, since the detection only requires a voltage measurement and knowledge of the cell's expected behaviour under non plating conditions.

#### **3.2.1 Overview**

The effect in the CV phase only occurs if the CC phase is very short, i.e. at very low temperatures or very high charge currents if the cell is still at a rather low SOC when the charge end voltage is already reached. Figure 3.11 shows two examples of these anomalies obtained from the cell tests of Stief. The left plot shows the CC phase of a cell charged at 0 ◦C at a current of 2C, the right plot shows the CV phase of a cell of the same type when the CC phase was completely skipped and the charging process solely utilized the CV phase. Both cells show detectable voltage plateaus on discharge, confirming that plating occurred in both cells during charging, as seen in Figure 3.12. The voltage plateau is clearly visible in the discharge voltage curve on the left for the cell charged at −20 ◦C. For the cell charged at 0 ◦C the plateau is less visible, since the plating was not as strong compared to −20 ◦C. There is however an easily visible minimum in the dV/dt curve on the right that denotes the end of the stripping plateau. The ends of the stripping plateaus are marked with the dashed lines for both cells.

In figure 3.11 it is clearly visible that the voltage profile at the beginning of the charge has a change in gradient and is briefly showing a negative voltage gradient. This effect occurs and subsides during the first 20 seconds of the CC phase. This leads to the assumption that it is connected to the lithium metal crystallization process, as discussed in 2.6.1. On the left, the CV phase of a cell charged at −20 ◦C is visible. The cell was also charged with

**Figure 3.11:** Voltage profile (left) during CC phase and current profile (right) during CV phase of two cells under different plating conditions (first cycle)

a current rate of 2C which caused a voltage drop high enough to completely omit the CC phase. The high internal resistance caused by the low cell temperature created an over-potential that caused the cell voltage to rise to the end of charge voltage of 4*.*2 V within the first measurement point. In this case the expected exponentially dropping current during the CV phase shows a bell shape overlayed that subsides only after almost 2000 seconds. The effects for the voltage and current curves should be equivalent since the voltage and current of the cell are connected via ohms law by the internal cell resistance. Both effects are the result of a decease in internal cell resistance, which is comparable to the findings of Koleti et al. [152], who found the impedance of the spectrum to drastically change when plating occurs. They used a single frequency measurement at the point right before the start of the diffusion branch to measure the real-time change of the resistance during the charge process and state that they can detect the onset of plating by the deviation of the resistance from its regular trend. The effects visible in figure 3.11 should also be caused by the same effect. An interesting difference between the results in [152] however is that they detect plating starting at high SOC at around 80 to 90 % while the change of the resistance visible here occurs at the start of the charging process at a very low SOC. Since both effects could also be caused by an increase in cell temperature that lowers the internal resistance the temperatures at the cell surface have also

**Figure 3.12:** Discharge voltage (left) and dV/dt (right) curve for the same measurements as in Figure 3.11 both showing stripping

been recorded during the measurements and are depicted in red. A look at the temperature profiles of both measurements shows a rise in temperature that is to be expected due to ohmic losses. For the CC phase voltage profile on the left the temperature profile is completely de-coupled from the change in voltage and thus is very unlikely to have influenced the voltage anomaly. On the right side the temperature profile is coupled with the current profile during the CV phase but the change in temperature is lagging slightly behind the bell shape and is thus most likely a consequence of the increased current during the bell shape and not the reason for the bell shape to form in the first place. It has to be kept in mind though, that the temperature measured on the surface of the cell is not fully in phase with the cell core temperature which could undermine the previous assumption. Figure 3.13 shows the temperature change of the three cells cycled at −20 ◦C.

As visible in the plot the total change in temperature is not very high, the change is only from −20 ◦C to about −15 ◦C and would likely not cause a drop in the total internal resistance by more than a factor of 3.5 necessary to cause the visible current increase during the bell phase. The factor of 3.5 can be calculated by estimating the current during a normal exponential drop CV phase at the time of the peak of the bell and divide is by the bell peak current. To prove this theory the spectra of a cell of the same type have been measured at −15 ◦C and −20 ◦C. Figure 3.14 shows the

**Figure 3.13:** Temperature change during the CV phase of the cell cycled at −20 ◦C

corresponding spectra on the left and the ratio of the real part of the two different measurements on the right.

**Figure 3.14:** Spectra (left) at -15 and −20 ◦C and ration of the real part of the two spectra over all frequencies (right)

It is visible that the highest change in resistance over the 5 ◦C of difference during the charge can only increase the current by a factor of 1.45. The peak of the bell shape is occurring after about 600 seconds, which corresponds to a frequency of 17 mHz where the factor has an even lower value of around 1.25. This means that while the change in temperature is influencing the peak shape, the main decrease in resistance can not be caused by the temperature change but must rather be linked to the plating process.

Another aspect that strongly links the effect to plating is that the anomaly diminishes over the cycles. Figure 3.15 and 3.16 show 10 cycles of cells under plating conditions at 0 ◦C and −20 ◦C at 2C respectively. For the cell cycled at 0 ◦C is it clearly visible that the voltage anomaly is diminishing over the 10 cycles. For the −20 ◦C cell in figure 3.16 the same can be seen for the bell in the CV phase. Here it is also noticeable that the stripping plateau gets smaller over the cycles as well. In figure 3.15 at 0 ◦C the stripping plateau is very weak and thus it is hard to determine whether the stripping is also decreasing over the cycles but the stripping plateau for the first cycle is visible higher than in the following cycles.

**Figure 3.15:** Change of the voltage curve during the CC phase (left) and the discharge curve (right) over 10 cycles under plating conditions (at 2C and 0 ◦C)

**Figure 3.16:** Change of the voltage curve during the CC phase (left) and the discharge curve (right) over 10 cycles under plating conditions (at 2C and −20 ◦C)

Figure 3.17 shows EIS spectra of the cells cycled at 0 ◦C and −20 ◦C over the 10 cycles. The spectra were recorded after the cell was characterized at room temperature. It is visible that for both cells the overall resistance is decreasing, which correlates with the diminishing anomalies. For the −20 ◦C measurements however the change in resistance is very small and could not solely cause the significant change in the bell shape. This leads to the assumption that there must be a temporary change in resistance during the plating occurrence whose magnitude is decreasing if there is already plating induced capacity loss present. To detect a temporary change in resistance EIS spectra have been measured every 6 minutes for 12 hours after the cell was charged for one time under plating conditions at −20 ◦C.

**Figure 3.17:** EIS spectra of the two cell cycled at 0 ◦C) (left) and −20 ◦C) (right) over the 10 cycles. The spectra have been recorded after characterization at room temperature

The resulting spectra are shown in figure 3.18. Right after the charge was finished the impedance (in blue) was much smaller than the the final impedance after 12 hours, which is accordance with the observations of a decrease of resistance in the voltage and current measurements. This measurement is of course superimposed by a small change in cell temperature due to the ohmic losses heating up the cell in the course of the charge that is also influencing the spectra but a cell of this size (type 18650) is equalizing in temperature within less than 2 hours, so only the first measurements should be affected by the temperature change. The internal resistance is not changing during the whole time and thus seems to be unaffected by the plating. It

**Figure 3.18:** 120 EIS spectra measured every 6 minutes for 12 hours directly after charging under plating conditions at −20 ◦C. (Frequency from 0.1 to 20 kHz)

can be assumed that the change in impedance visible in the spectra is caused by the reversible plating depositions on the surface that are diffusing into the anode over time. The change in impedance could stem from the fact that it is electrochemically favourable for lithium to deposit on existing metallic lithium compared to intercalation into the anode. This should affect to fist semi-circle of the spectrum since that circle depicts the anode double-layer behaviour. The small dent at the beginning of the relaxation is probably caused by the change in temperature of the cell. After that impedance is increasing relatively constantly until a saturation is reached at the end of measurements, which is similar to a diffusion behaviour. This change in impedance is also in good accordance with the findings of Koleti et al. [152].

Further evidence that the anomaly is linked to plating can be found in the correlation of the charge created by the added bell shape and the reversible plating detected during the discharge in the stripping plateau. Figure 3.19 shows the correlation of the charge under the bell shape of the CV phase and the stripping charge during the following discharge.

**Figure 3.19:** Correlation of the charge supplied during the bell anomaly and the corresponding stripping charge detected in the stripping discharge plateau for all cells omitting the CC phase

As discussed in chapter 2.7 during plating a portion of the lithium ions is still intercalating into the anode, while another part is forming the metallic lithium film on the surface. It is assumed that the underlying regular exponential dropping current profile depicts the normal intercalation process, while the superimposed bell shape shows the plating part. To obtain the shape of the current during a regular CV phase a curve composed of two superimposed exponentially dropping functions has been fitted into the measured profile.

$$i\_{exp}(t) = i\_0 \cdot (e^{-\frac{t}{\tau\_1}} + e^{-\frac{t}{\tau\_2}})$$

The fit result was then subtracted from the total measured current curve to obtain just the bell shape. The current profile of the extracted bell shape was then integrated to obtain the current supplied during this time. For the discharge profile the length of the stripping plateau was determined by using the deviation of the voltage curve and then the discharge current accordingly integrated to obtain the reversible amount of plated charge. It has to be kept in mind that there is a small error to the amount of charge, since there is also a small amount of de-intercalation and diffusion of the plating lithium in the anode that superimpose the stripping process. So the stripped amount is always slightly underestimated. The results show a very good correlation of more than 0.9 for all cells tested. This means that a measurement using no CC phase can be used to extract only the portion of the plating current from the total charge current. For the measurements that had a CC phase there is no way to extract the plating current since there is no easily obtainable voltage curve that could be used to isolate just the plating anomaly in the voltage curve. And it can be assumed that for these measurements most or all of the plating has been deposited during the CC phase, since none of the measurements show the bell shape anomaly in the following CV phase.

In this thesis the effect of the current anomaly occuring in the CV phase will be analyzed in detail. The voltage anomaly present in the CC phase will be investigated in future works.

#### **3.2.2 Current Anomaly**

Under normal operating conditions it should not occur that the CC phase is omitted completely, and if this case should occur this could already be seen as an indicator that there is likely going to be plating during the following charge by itself since the over-potentials in the cell are extremely high. These kinds of over-potentials can only be caused by very low temperatures or very high charging currents, both conditions susceptible to plating. Because of this the bell shape can most likely not help in identifying and preventing plating in a real world application. It is nonetheless helpful in getting a better understanding of the plating process since it can be utilized to estimate the portions of the charge current that go into plating and intercalation. This could e.g. be used to characterize cells regarding their plating susceptibility in just one cycle. Additionally compared to just analyzing the stripping plateau during discharge the analysis of the pulse characteristics should give a more accurate estimation of the total amount of plating since it should contain the irreversible as well as the reversible plating fractions. It is also not affected by the intercalation of the plated lithium into the anode over time that reduces the amount of detectable lithium when the discharge plateau is utilized for the analysis.

#### **Bell fitting**

To better understand the bell effect and its change over the cycles, the obtained cycling measurement data is fitted in this chapter and the resulting parameters are analyzed over the cycles.

Kober et al. [209] also examined the CV phase bell anomaly. They identified logistic growth as possible explanation. This kind of growth process can be implemented using the JMAK model [210, 211, 212]. This model describes isothermal phase transformations and crystallization processes of metals that can be applied to the deposition of lithium on the anode surface. The formula gives the amount of charge deposited on the surface by

$$q\left(t\right) = q\_m X\left(t\right) = q\_m \left(1 - e^{-\frac{kt^m}{n}}\right).$$

The derivative of the formula then gives the current flowing into the deposition process.

$$i\left(t\right) = \frac{dQ\left(t\right)}{dt} = q\_m \frac{dX(t)}{dt} = q\_m k t^{n-1} \cdot e^{-k t^n/n}$$

The constant *k* is the rate constant including the nucleation growth rate, the form factor and the rate of the crystal growth. The parameter *n* denotes the Avrami exponent giving information about the underlying transformation mechanisms. For constant growth and nucleation rates, *n* only depends on the dimensions of the growth. A value of 2 denotes one dimensional, e.g. cylindrical growth that could describe dendrites, while a value of three describes a surface coverage expected from mossy plating on a real anode surface [213]. For the deposition of lithium on the anode surface, a dimensionality of 2 to 3 is to be expected [214], since the growth can be either covering the surface in a two dimensional plane or form one dimensional dendrites.

**Figure 3.20:** Effect of the three parameters *qm*, *n* and *k* on the shape of the JMAK function

Figure 3.20 shows influence of the three parameters on the JMAK function. The blue curves show a low value and the most red curve ends with the highest value. The plating amount *qm* visible in the leftmost plot is simply increasing the total amount of charge and thus the area under the function without changing the shape of the overall function. The Avrami exponent *n* in the middle plot is shifting the curve to the left and makes it more narrow for higher values. Higher values for n describe growth in more dimensions, which allows more material to deposit in a shorter amount of time thus narrowing the curve. The last parameter *k* in the right plot shows a similar behaviour with the difference that the effect on the shape of curve seems to be smaller than for *n*. Since *k* describes the growth rate a similar explanation as for *n* can be applied. A fast growth rate will create more deposition in a smaller amount of time.

It has to be kept in mind though that the JMAK equation can only be seen as an approximation since it is based on assumptions that can not be verified for the experiment present or are not very close to real observations in general [215]. These assumptions are:

While the assumption of the nuclei distribution is likely to be true on a macroscopic surface the size of a battery electrode the constant growth rate is limited by the lithium ion diffusion as well as the already present surface coverage in a real cell. It can be assumed however that the growth rate is constant at the beginning of the deposition since the available amount of lithium ions is still sufficient and enough surface area is available for the growth. The equal growth in all directions will be limited by the surface structure but it is not clear how large the influence of this is on the validity of the model function.

To fit the whole curve the JMAK function needs to be superimposed with the underlying intercalation current that can be described with a number of over-layed exponential functions, as described in the chapter before.

$$i\_{exp}(t) = i\_0 \cdot (e^{-\frac{t}{\tau\_1}} + e^{-\frac{t}{\tau\_2}} \dots),$$

For the measurements in this thesis three exponential functions gave good fitting results and since the functions have no physical meaning and are only utilized to best model the shape of the curve, the exact number of used functions is not of importance for the final results. The exponential part can be roughly pre-fitted using a weighting function only weighting the parts of the current measurement not containing the bell shape overlay to estimate good stating parameters for the exponential functions. The current contribution of the JMAK function is

$$i\_{JMAK}\left(t\right) = q\_m k t^{n-1} \cdot \ e^{-\frac{kt^m}{n}}.$$

Where *qm* denotes the maximum amount of charge deposited on the surface. The total fitting function can then be written as

$$i\_{total}\left(t\right) = \ i\_{exp}\left(t\right) + i\_{JMAK}\left(t\right).$$

The fitting was performed using a genetic fitting algorithm in MATLAB. However, only using one JMAK function did not yield satisfactory fitting results. The left part of Figure 3.21 shows the fit for the -20°C measurement at 3C with only one JMAK function.

**Figure 3.21:** Fit with exponential drop and one JMAK functions (left) compared to three JMAK functions (right)

It is visible that the fit has errors at the peak of the bell and in fitting the current decrease when the bell is declining. Fanfoni et al. [216] noticed that for an electrochemical deposition in an electrochemical cell the JMAK equation can only hold true for the initial stage of deposition since new nuclei can be formed on the surface of the already deposited surface layer. For this reason an approach with multiple overlayed JMAK functions is tested. The right part of Figure 3.21 shows the fitting with three JMAK functions overlaying. It can be seen that using multiple JMAK functions in the fit leads to drastically improved fitting results. The exponential functions have only been adjusted slightly by the second fit showing a good consistency of the two fits. The additional JMAK functions needed to improve the fit have a lower amplitude and the maximum is shifted to a later point in time. A theory for this behavior could be that the plating process happens in layers. A large bulk of lithium is forming at the beginning over a wide area of the surface of the anode, represented by the first, larger JMAK function. On top of this a second layer of lithium forms with a small onset, represented by the second, later and lower JMAK function. The lithium ions use the already existing metal lithium as crystal nuclei. Following this theory, the process would continue with more JMAK functions, forming new layers on top of the last one with smaller and smaller JMAK pulses. To test if this behavior proves to be true the fitting has been repeated using a variable number of JMAK functions. Since this also drastically increases the number of parameters to be fitted, it was tested how many superimposed JMAK functions can lead to good results with minimal computation time. One to eight overlayed JMAK functions have been tested. It was assumed that a higher number of JMAK functions would produce such small bell shapes that they can be neglected. An evolutionary algorithm was used since no individual starting parameters are needed, instead the algorithm takes upper and lower boundaries to create the starting population. To improve the fitting the boundaries of the following, smaller JMAK functions are calculated from the boundaries of the preceding functions. Figure 3.22 shows the evolution of the fitting error when using an increasing number of JMAK functions including the confidence band.

The error first drops until around a number of 4 and then stays almost constant. The error increases slightly again for a number greater 4 with a slightly higher standard deviation, since the fitting becomes increasingly unstable for higher numbers due to the increasing number of parameters that have to be fitted (With 8 JMAK functions, the number of parameters is 35). The fitting error thus shows the expected behavior. From the fitting error results, a number of four JMAK functions is chosen as the optimal amount, since beginning with four there is not much of a benefit regarding the resulting error while the computation time is drastically increasing and the standard deviation is lowest for a JMAK number of 4.

During the fitting process two problems with the fitting and the usage of an evolutionary algorithm in particular become apparent. Figure 3.23 shows the fitting results over all 9 cycles for the cell at −20 ◦C that showed no CC phase. The first problem can be seen in the order of the JMAK functions for each cycle. Comparing e.g. the third and forth cycle in the fitting example in figure 3.23 the order of the JMAK functions is changing randomly.

**Figure 3.22:** Evolution of the fitting error for increasing number of JMAK functions

Function 3 in dark red was the JMAK with the highest amplitude in cycle 3, but in cycle 4 the highest amplitude is now function 1 in light blue. This problem is not important as long as only the overall shape of the fitting result is of interest, but since the parameters of the individual JMAK functions and their evolution over the cycles have to be evaluated, the order of the functions is important for a meaningful analysis. To reduce the impact of this problem and to improve the fitting results first a method was implemented into the evolutionary algorithm to adjust the fitting boundaries depending on the fitting result of the preceding cycle. This increases the likelihood that the functions are logically extended into the next cycle. This reduced the fitting error by a factor of up to 3 for some of the measurements and somewhat alleviated the problem but to achieve good fit results the boundaries still had to be kept as large as ± 50% of the fitting result of the previous cycle.

**Figure 3.23:** Fit results example for all cycles of the cell with no CC phase at −20 ◦C and no fitting adjustments

This is why there is still some discontinuity between the order of the fitted JMAK functions between the cycles.

Figure 3.24 shows the fitting results over all 9 cycles for the same cell as figure 3.23 with the boundary updates. It is visible e.g. in cycle 1 and 2 of figure 3.24 that the bell function with the highest amplitude is still changing position. To allow for a consistent analysis of the fitting results the JMAK functions would need to be sorted in logical order. This could have been done by hand, but this way of sorting would most likely lead to strongly biased results. For this reason to reduce human error and biases a method was developed to sort the JMAK functions based of their mathematical likeliness. As a metric of likeliness the location of the peaks as well as their maximum peak height is taken. It is important to take the position as well as the location of the peaks to compare their likeliness since e.g. looking at

**Figure 3.24:** Fit results example 1 with boundary adjustments for all cycles of the cell with no CC phase at −20 ◦C

figure 3.25 and the evolution of the peaks between the first and the second cycle it is visible that all peaks except of the red peak have high likeliness to a corresponding peak. The red peak however is very wide and flat in the first cycle, with its center point been the rightmost of all four peaks but the only possible partner for the red peak in the second cycle, considering the high likeliness of the other three peaks towards their partners, is a very narrow peak that has the leftmost position of all peaks. In this case just considering the peak location would have lead to a sub-optimal result since the the red peak from the first cycle would have been paired with the rightmost peak of the second cycle in consequence causing all other peaks to be paired with much worse partners. For this reason using both location and height is important of the optimal peak allocation. These two parameters are calculated and normalized for each peak. Together with the same parameters of the previous peak they then form two confidence matrices for position as well as peak height. These two matrices are then combined by taking the weighted average of the two values. Using the weighting function the confidence priority can be shifted towards either the peak position or the peak height. Based on the resulting matrix each peak is then assigned to a peak of the previous cycle by simply calculating which permutation of peaks has the highest total confidence.

**Figure 3.25:** Fit results for example 1 with peak sorting for all cycles of the cell with no CC phase at −20 ◦C

Figure 3.25 shows the sorted peaks for the same measurement as example 1 in figure 3.24. The peaks show a more logical order than the original order visible in figure 3.24 with the red peak always being the furthermost left peak, except from the first to second cycle, where the allocation is not clear and the light blue only starting to be consistent from cycle 3 to the last cycle. The algorithm only shows unexpected results when the peak allocation would also not be clear from a manual inspection e.g. visible in cycle 4 and 5 where the order of the peaks with the highest value change position. A closer look however reveals that peak 2 in dark blue is almost identical to its counterpart in the next cycle. This is one example where the algorithm is superior to a manual sorting of the peaks.

**Figure 3.26:** Fit results example 2 with boundary adjustments for all cycles of the cell with no CC phase at −20 ◦C

Figure 3.26 shows the same fit performed a second time on the same measurement data that was already shown in figure 3.24. The fit result is very good in both cases (mean root-mean-square error (RMSE) of 0.0039 for example 1 and 0.0057 for example 2) but it was possible for the fitting algorithm to achieve a good fit with different underlying functions. This is a problem mainly caused by the use of an evolutionary fitting algorithm since for this kind of fitting the fitting parameters are limited less than for other fitting algorithms even using the boundary updates as described before and also by the fact that the fitting of the first cycle has to have a even larger fitting window. Due to the large number of fitting parameters however and the difference of the measured function shapes the creation of stable starting parameters for other fitting algorithms is impractical. It can be seen that for an increasing cycle number the fitting results in figure 3.24 and 3.26 become more similar. This is caused by the fact that the algorithm uses the fitting results of the previous cycles to update the boundary parameters as discussed above, which puts a very high weight on the first fit and then only allows the algorithm to progressively deviate from the initial fit result. To obtain robust data for the analysis of the behaviour over the cycles a Monte Carlo simulation is performed and the fitting repeated multiple times to calculate the statically most likely correct fitting result. To obtain the most likely fitting result from the repeated fittings the fitting errors are clustered using a simple k-means clustering and the center of the largest cluster is chosen as the most plausible result. The final fit result will then be the fit result whose error is closest to the center of this cluster.

#### **Fitting Results**

After fitting the bell curve with 4 JMAK functions for 178 times (this number was the final amount of fits due to available machine time) the most plausible result was chosen and the function order was adjusted as discussed in the previous section. As discussed before 4 JMAK functions have been chosen since for this number of JMAK functions the fitting error was on average the lowest. Figure 3.27 shows the evolution of the 3 parameters of the 4 JMAK functions over the cycles under plating conditions for the 6 cells that showed bell anomalies in the CV phase. Each row represents one JMAK function with the three parameters *qm*, *n* and *k* from left to right plotted over the cycles. The cell cycled at −10 ◦C with a current of 2C had two errors during cycling which could have possibly been caused by a dendrite short circuit. The first of the two cells cycled at −10 ◦C at 3C also had one error occurring during cycling. For these two cells the missing data points have been interpolated. This has to be kept in mind during the interpretation since this cells technically didn't have same number of cycles as the other cells and additional effects caused by the errors could also have an influence of the behaviour of the cells during subsequent cycles after the erroneous cycle. For the cell at 2C the erroneous cycles were cycle 4 and 6, for the cell at 3C the 8th cycle wasn't performed as intended.

**Figure 3.27:** Parameter evolution for 4 JMAK functions over the cycles under plating conditions for all 6 test cells that showed the bell CV phase effect

The red lines in figure 3.27 are showing all cells cycled at −10 ◦C, the blue lines are used for cells cycled at −20 ◦C. It is visible that the parameters are all very unstable. This means that even though 4 JMAK functions showed the lowest fitting error, the fitting parameters had too many degrees of freedom and fitting results can not be interpreted in a meaningful way. Only in the parameter *qm* a trend is visible in that the curves at −20 ◦C show a higher total charge plated and the total amount of charge plated on average decreases for both temperature over the cycles for all JMAK functions. This is to be expected and this parameter could have also been obtained with just fitting the exponential functions and then integrating the remaining current curve without the exponential part. For 4 JMAK functions neither *n* nor *k* show any trends.

For this reason the fitting was repeated using a smaller amount of JMAK functions. This will increase the fitting error by disregarding new nuclei forming on top of the already existing depositions as discussed in the previous section. But it should lead to more stable fitting results and thus allow for a meaningful analysis of the fitting parameters over the cycles.

**Figure 3.28:** Parameter evolution for 2 JMAK functions over the cycles under plating conditions for all 6 test cells that showed the bell CV phase effect

For the fit with two JMAK functions only 10 repeated fits have been performed since the fitting should be much more stable with a significantly reduced amount of parameters to fit. For the parameters shown in figure 3.28 there are already clear trends visible. The total plated charge *qm* is separated in the −20 ◦C measurements that show a decline over the cycles and a higher value in general and the measurements at −10 ◦C which have a lower value and stay constant over the cycles. it also seems like the values at −20 ◦C are approaching the value of the plated charge at −10 ◦C. This means that the plating mechanism must be somehow different at lower temperatures but this difference is fading over the cycles. It could also be that the amount of plating is decreasing in both cases but the effect is not as pronounced at −10 ◦C and not visible after only 9 cycles. The Avrami exponent *n* in the middle plots is still unstable even for only 2 JMAK functions but even though there is not clear distinction between the two test temperatures a general trend of decreasing value over the cycles can be seen for the first JMAK function. This would indicate, that the dimensionality of the growth is shifting from a planar 2D deposition to a more dendritic 1D cylinder shaped deposition over the cycles [213]. If this is true, then this would mean that for this cell the chance of safety hazards caused by dendrite growth would increase with the number of plating cycles. The last parameter *k* also shows a distinction between the two temperatures but in contrast to *qm* here the lower temperature stay rather constant and the higher temperatures at −10 ◦C are increasing over the plating cycles. The parameter *k* is mainly influenced by the growth rate so this would indicate that with progressing cycles the deposition becomes faster. This could theoretically be caused by already existing deposition so that the possible deposition surface is larger at the start and no new nucleation is necessary. This however would be true for both temperatures and does not explain the difference between the measurements at −10 ◦C and −20 ◦C. It is also noticeable that the parameters of the second JMAK function are more unstable than the parameters of the first one and no additional meaning can be gained from the analysis of the set of parameters of the second JMAK function. For this reason the fitting has been repeated with only one JMAK function to see whether the parameters behave differently.

Figure 3.29 shows the parameter evolution with only one JMAK function over the plating cycles. For 1 JMAK function the fitting was very stable and showed the same result in the majority of the monte carlo fit results meaning that in this case most likely a clear global minimum could be detected. Here

**Figure 3.29:** Parameter evolution for 1 JMAK function over the cycles under plating conditions for all 6 test cells that showed the bell CV phase effect

the same trends can be seen that have already been discussed above for the fitting case with 2 JMAK functions but the distinctions between the two test temperatures is much clearer for *qm* and k and the decrease of the Avrami exponent is also much better visible than in the plots for 2 JMAK functions. It can also be seen that there seems to be no difference for different temperatures regarding the dimensionality of the growth since the parameter n is very similar for all cases. It is also better visible that *k* is also increasing at −20 ◦C which is be expected since both temperatures should lead to a higher amount of existing depositions that can increase the deposition speed as discussed before.

Since none of the measurements at −20 ◦C showed any CC phase the current of the measurement is irrelevant and all three can be seen as plating under the same conditions so the results can also be averaged. The same is true for the three measurements at −10 ◦C. Figure 3.30 shows the averaged parameter curves for the two temperatures as well as the standard deviation for each curve.

It is visible that the trends stay the same as discussed above and that the standard deviation is low enough to distinctively separate the curves for the parameters *qm* and *k*. The large increase in the standard deviation for parameter *k* in the later cycles is most likely caused by the erroneous measurements in two of the three measurements that have been averaged.

It is also notable that even though for all samples the amount of measured plating obtained from analyzing the current curves is reduced over the cy-

**Figure 3.30:** Parameter evolution for 1 JMAK function averaged for the different temperatures

cles, the capacity loss is almost constant at the same time, as visible in figure 3.2. If *qm* is really depicting the total amount of plating, i.e. reversible and irreversible plating combined, that would mean that the irreversible plating is not directly linked to the amount of reversible plating since the cells at −20 ◦C show declining amounts of reversible plating but the irreversible capacity loss stays the same over all cycles. Figure 3.31 shows the comparison of the measured stripping charge added to the irreversible capacity loss detected in the characterization after the plating cycle with the parameter *qm* (converted into Ampere hours).

**Figure 3.31:** Comparison between the fitting parameter *qm* and the total plating charge measured

It is visible that for the measurements at −20 ◦C there is very good agreement between the fitted parameter and the measured plating charge. For −10 ◦C however the real measured charge is significantly lower than the fitted parameter. This is probably caused by the fact that the diffusion speed at −10 ◦C is much higher than at −20 ◦C and thus the charge measured in stripping phase during the discharge is underestimating the real amount of irreversible plating since a significant portion of the lithium is already diffused into the anode during the preceding charge. The shapes of the curves follow quite well, except for two outliers for the −10 ◦C measurements that show up after the erroneous measurements. This indicates that *qm* could be a good measure for the real amount of deposited plating and that it should be closer to the real value than the stripping charge.

#### **3.2.3 Summary of the Current Anomaly Results**

In summary the analysis of the current phase anomaly during plating has shown that:

• The obtained parameters could be used to characterize a battery cell regarding its plating susceptibility for battery life prediction.

Future experiments are necessary on cells of different types and manufacturers to investigate whether the effect it generally applicable. To prove that the dimensionalty parameter is connected to the plating deposition morphology further experiments involving more detailed post mortem analysis or in site techniques are necessary.

#### **3.3 XPS Measurements**

In this chapter the results of the XPS measurements performed in this thesis are shown and analyzed. The original goal of these experiments was to identify and locate metallic lithium inside of the SEI layer. This was done to answer the questions whether there is any dead lithium in metallic form present in the SEI after plating or if the lithium reacts with XPS compounds to form new compounds and thus degrades over time and if it is present, where in the SEI this lithium can predominantly be found. XPS was chosen as a method to do this since it is one of the few non-destructive measurements methods that is capable of detecting metallic lithium and at the same time also differentiating between elemental lithium and lithium bound within the various SEI compounds. As discussed before in chapter 3.32 the penetration depth of XPS is very low so that the method can only measure the composition of the top layer of the SEI. To gather information about the whole structure of the SEI layer thus a depth profiling experiment was performed using argon etching to get spectra at different depths into the SEI. It became however clear during the first measurements that the application of the XPS measurement onto the surface of the sample produced a visible change to the material that could not be explained in the beginning. It was also notable that the effect only seemed to appear on samples where a higher content of lithium was to be expected, namely the samples that have been plated and the fully charged anode sample, as explained later in this chapter. Since the effect was however apparently located outside of the measurement area it was initially ignored and the depth profiles have been recorded regardless. It became later clear that even though the effect was visibly not present in the measurement area which is only a few square millimeters large, the effect did influence the measurement results significantly. In the following analysis of the measurements results it became apparent that the results did not fit the expectations and where very difficult to logically analyze. For this reason a series of follow up experiments were conducted to identify the cause of this surface change. The results of these experiments showed that the effect was caused by not insulating the samples during the measurements and utilizing a charge compensation mechanism in the XPS device, which are both normal procedures during XPS measurements. Unfortunately it was not possible to repeat the depth profiling measurements after identifying the problem since the measurement time of the XPS device was limited, it was however possible to utilize the effect and analyze the already measured depth profiling spectra in a meaningful way. The results showed that the effect could be used intentionally in future experiments as a different approach for lithium detection that does not require the destructive method of argon sputtering for depth profiling.

The order in which the experiments are presented in this chapter is not the chronological order in which they have been performed but rather an order that hopefully helps the reader to better understand the measurement results.

#### **3.3.1 Experimental Setup**

For the XPS measurements the same cells of different manufacturers presented in chapter 3.1 (table 3.1) as well as the cell-type used in the FIB measurements in chapter 3.1 have been used. Additionally one sample was taken from pristine anode material that was not used inside a cell (sample A). This sample was used to get a reference of the graphite surface without any electrolyte or SEI but can not be directly compared to the commercial cells used for the rest of the experiments since it was composed of a laboratory created anode material and is thus only used as a reference for the C1s peak and not further used in the interpretation of the results. For all cells except for the pristine material, one measurement was done with a fresh cell that only experienced a characterization cycle and a plated cell that was plated at −20 ◦C. The cells taken from chapter 3.1 have been used in the experiments there [208] where they experienced 10 cycles of plating and afterwards had approximately 6 months of resting time at about 30% SOC before they were characterized again, they were fully discharged after the characterization, then opened and prepared for the XPS measurements, so all residue of reversible plating effects should have diffused into the anode.

To further investigate the influence of the measurement method the samples of the Sony VTC3 also have been electrically insulated, which will be further discussed in chapter 3.3.2.

For these measurements the cells used are new cells of the type LG ICR-18650HB2 NMC, since these were also used in the work of Wink [207] and the FIB measurements shown in chapter 3.1 have been performed on these cells. This cell type will be further abbreviated as LG-FIB. For this cell type two additional cells have been prepared under different conditions. One of the cells was only cycled once to bring it into the desired state, this is technically a new cell. This cell was opened at 100% SOC. The second cell was cycled for only one full cycle at −20 ◦C to produce lithium plating in the cell and then opened. In comparison to the plated cells taken from chapter 3.1, this cell had a much shorter resting time of less than a day before it was opened. The fully charged cell was used to investigate the presence of intercalated lithium on the XPS measurement since even in a fully discharged cell there will still be some amount of intercalated lithium in the anode. For the freshly plated cell this is even more important since reversible plating could still intercalate into the anode graphite even if the cell is dismantled and increase the amount of intercalated lithium. Figure 3.32 shows an overview of the all samples used in the XPS measurements. For the LG-FIB also an etching experiment was performed on the new, freshly plated and fully charge cells to be able to compare the resulting depth profile with the FIB cross sections of the SEI, which will be discussed further in chapter 3.3.4. The pristine graphite was also etched as a reference. All cells will further be referenced using their the abbreviations of figure 3.32 with SAM standing for the cells of the manufacturer Samsung and SO for Sony.

All cells were opened inside a MBRAUN Unilab glove box under argon protective atmosphere with < 10 ppm of moisture and oxygen concentration. All samples from the opened batteries have been taken from the outermost area of the jellyroll. The samples were then transferred to the XPS device inside a gas tight transport box. The XPS device has an attached mini glove box that was used to transfer the samples from the transport box to the XPS test chamber. Within the mini glove box the samples have been exposed to roughly 100 ppm of oxygen atmosphere for a few minutes before

**Figure 3.32:** Overview over all XPS experiments using the different cell type from table 3.1

entering the high vacuum. It was unfortunately not possible to also measure the moisture content in the mini glove box during the transfer. Oswald et al. [175] performed an experiment to determine the influence of environmental conditions on the samples with a one minute exposure time and found that the lithium content on the surface of lithiated graphite drastically increased. They assume that this was caused by diffusion of lithium to the surface of the graphite and subsequent reactions forming Li2CO3. Since transfer conditions prior to the XPS experiments in this study have not been ideal, the effect could also occur to some extend leading to an overestimation of lithium content on the surface of the sample. But since the SEI layer is to be expected much thicker (as shown in chapter 3.1) due to the plating effect on the plated samples this could mitigate the effect for these samples. And it is not clear if the effect can only occur for lithium intercalated into the anode, as seen in their samples, or also for dead lithium within the SEI.

For the XPS measurements a Kratos Axis Supra device located at the Fraunhofer IZM in Berlin was used. It has a base pressure of 2 × 10−9 torr. The photo-electron source is a monochromatic Al K*α* X-ray source (*hν* = 1486*.*7 eV).

In the measurement process for all samples a complete wide spectrum was recorded, followed by higher resolution detail spectra of the elements of interest that can be found within the SEI layer and anode bulk material. These elements are **fluorine** (F1s), found in in the conductive salt LiPF6 and the most common binder within the anode: PVDF, **oxygen** (O1s), found in most of the organic compounds in the electrolyte, **carbon** (C1s), also present in the organic components and main part of the anode active material, **phosphorus** (P2p), part of the conductive salt as well and **lithium** (L1s), as the electrochemical active ion and part of the conductive salt.

This measurement routine was repeated for several etching steps of different length to obtain a depth profile of the material composition in the SEI for the LG-FIB cells, as mentioned before. The exact parameters for the etching experiment will be discussed in chapter 3.3.4.

#### **General Spectrum Analysis Principles**

The first step in the analysis of the measured spectra is the identification of the peaks and their corresponding compounds. The largest problem with the allocation of the peaks are shifts in the spectra. These shifts can be caused by charging of the sample due to the surface charging effect that was already explained in chapter 2.6.2. This is especially difficult for LIB related materials since they can have a varying conductivity with conductive materials in some locations and non conductive materials in other locations when creating a depth profile during the measurement. These shifts due to surface charging always affect the complete spectrum and can be mitigated by using a reference element that is definitely present in the sample. During the measurements the charge neutralization system of the device was always on since the conductivity of the samples is assumed to be very low due to the surface coverage of the SEI. Bear et al. [157] mention that for Kratos Axis systems the charge neutralization often shifts the measured peaks to a lower binding energy. A negative charge on the surface is accelerating the photoelectrons more than the energy they gained from the X-ray exposure while a positive charge on the surface would cause the opposite and slow down the electrons. The electrons are then registered with an erroneous kinetic energy at the analyzer. Before the data is displayed and analyzed the kinetic energy is transferred into the binding energy. This is done by subtracting the kinetic energy from the X-ray energy (in this case 1486*.*7 eV). That means that a high kinetic energy corresponds to low binding energy since a larger part of the X-ray energy was transferred into kinetic energy. Looking at the spectra a negative shift in binding energy means that the kinetic energy of the photo-electrons was larger than it should have been which means that the electrons have been accelerated by a negative charge on the surface. Analog a positive shift means a lower kinetic energy caused by a deceleration of the photo-electrons due to positive charge on the surface.

For the present sample the carbon or AC peak at 284*.*3 eV [170] is chosen as reference since it should be present in all samples and the spectra were shifted accordingly so that the AC peak is also at position of the highest peak in C1s detail spectrum. As mentioned in chapter 2.6.2 the C1s peak is not the ideal reference peak for several reasons but since the samples used in this experiment show very different spectra, the C1s peak was the only peak that could be easily identified in all of the measurements. The value for the AC peak varies slightly in literature between 284*.*3 eV and 285 eV [162, 169, 170, 171, 172] and this specific value was chosen by finding the best fit with the other peaks in the sample. After the shift of the whole spectrum the individual peaks need to be identified. To do this literature values for the expected compounds are taken. These values however also have a great deviation. Wood et al. [176] analyzed the literature values for the most common LIB compounds and found that while the absolute values for the peaks can be very different, the distances between the individual peaks of each compound, e.g. the distance between the BE for Li1s and O1s in Li2O, are very similar for all different sources. They conclude that it is better to use the ∆*BE* as the main indicator for the peak positions. For the compounds present in their study that were of interest for this thesis, namely Li2O, Li2CO3 and LiOH these ∆*BE* values haven been taken. For the other important compounds not present in their study a similar approach was taken. For these compounds multiple literature values have been compared and their ∆*BE* values calculated. Then an average for these values was taken and the resulting ∆*BE* was used in the peak identification. Table 3.2

and 3.3 exemplary show the literature values and the resulting average for LiF and LiPF6 respectively.

**Table 3.2:** XPS peaks for LiF

**Table 3.3:** XPS peaks for LiPF6

The elemental composition of the SEI is approximately 90% C, 4–6% F, 3–4% O and 1% P [29] as well as small amounts of Li in different compounds. All the possible compounds of the SEI layer found in the literature have been added into the analysis in order to identify as many peaks in the measurement as possible:

**Table 3.4:** XPS peaks derived from literature values

Typical uncertainties for the XPS peak positions are around +*/* − 0*.*2 eV. Also since there always is a small inherent measurement error in the peak positions and it is unknown whether the literature values contained some level of shift it is possible to still shift the values by a small amount (<1 eV) to fit them into the right position for this measurement. But this shift has to be consistent over all measurements since it is a shift correction of the original measurement of the literature source. Another thing to keep in mind is the shift of SEI components in the presence of metallic lithium or lithiated graphite [172, 174, 175]. This has to be considered in the fully charged sample and the plated sample. In this case certain peaks could show a shift that is independent of other peaks, since the shift is depending on the conductivity of the compound.

For this analysis all the peak locations have been determined by using the first measurement of the fully discharged new SO-NMC cell, since in this sample the lithium content should be the lowest (apart from the pristine sample that was only used for AC calibration) and thus any lithium induced individual shifts should be neglect-able. The shape of the peaks can also contain valuable information about the represented compounds and elements. Metallic samples often show asymmetric peaks due to coupling of the photoelectrons with the conduction electrons in the metal [185]. Peaks at higher BE are usually wider and insulators also have wider peaks than conductors [185].

Before any spectra can be analyzed properly the background has to be removed. This background occurs since not all electrons excited by the X-ray will reach the detector unhindered, some of the electrons will in-elastically scatter and lose energy before reaching the detector and thus register as electrons with lower kinetic energy (shown as higher binding energy). The background removal of the spectra was performed using a simple iterative Shirley method [229]. The alternative Tougaard's background yields better results for quantitative analysis but requires a wider measurement range around the detail scans of peaks of about 30 eV [177] that was not measured in the experiments present. The Shirley background is also easier to implement and is sufficient for the basic analysis performed on the data in this thesis. The start and endpoints for the Shirley method have been selected by hand for all spectra.

In the following chapter the recorded XPS spectra will be qualitatively analyzed using the compound lines shown in table 3.4. The lines for Li2CO3 and Li*x*PO*y*F*z* as well as LiOH and LiF have been merged at the average value since they are so close together that the compounds can not be separately identified within the precision of the XPS device. All spectra have been shifted to the AC peak, if possible, to compensate for surface charging during the measurement. In some cases the AC peak was not the highest peak in the C1s detail spectrum. In this case the other compound present in all samples, LiF was used to shift the spectra. The amount of shift for each spectrum is also recorded and can be found in the description of each plot. All detail spectra have had their background removed using the Shirley method, as discussed before.

#### **3.3.2 Influence on the Samples**

As discussed in section 2.6.2 the etching process is a destructive process that can change the chemical composition of the surface. But not only the etching can influence the sample, the energy of the X-ray beam itself can also affect the compounds present, as well as the charging of the surface, called electron damage [157]. Figure 3.33 shows the condition of the four samples of the LG-FIB cell before the measurement, after the first spectrum was recorded and after the first and last etching step.

**Figure 3.33:** Overview of the changes in the sample during the XPS measurement and the etching process

All the samples show a blackening in the etching area that gets darker with increasing etching time. Some change of the surface in the etching area is to be expected since the etching is a very destructive process. More interesting is the diagonal stripe-shaped change of the surface in sample C and D that makes the surface become a metallic silverish color. These are both the samples with a high or higher expected lithium content and possible metallic lithium. This effect is not visible in the pristine and the discharged anode so it is likely that it is connected to the lithium content in the sample. Since the plated anode is also discharged before the cell was opened this is a clear indicator that the SEI of the plated sample must have a different composition from the discharged sample without plating. Interestingly this discoloration always seems to appear in the same angle and is not located at the actual measurement area. The measurement area is in the center of the etching area, but smaller than the etching area, so this effect seems to be mostly outside if the measurement area. The effect is also not connected to the etching since it appears after the first measurement before the etching process is started. To further investigate this effect and its potential influence on the measurement results additional tests have been conducted.

#### **Striping Experiments**

The first experiment to investigate the surface stripes that appear on the samples that contain larger amounts of lithium (i.e. the fully charged and 1 cycle plated LG-FIB samples that have been measured beforehand) was to see whether the effect was caused by the X-rays or by other factors. To do this additional fully charged and plated samples have been prepared and subjected to different combinations of measurement-modes in the XPS device. The device has three main parts that work together during the measurement, the X-Ray beam that is causing the sample to emit the photo electrons, the analyser, which is collecting and separating these electrons by energy and the charge neutralizer (CN) that can be optionally enabled to prevent surface charges from building up that may shift the measured peaks. In all previous tests all three parts were on during the measurement, since CN should be used when the surface of the sample is non conductive, which is the case for the SEI covered anode surface. Figure 3.34 shows all possible combinations of the three parts applied on the plated surface of the SO-NMC anode. Figure 3.35 shows the same experiment on the fully charged anode.

The only combination that creates the stripe on the surface in the experiment is the combination of all three parts. Due to the limitation of the device however it is not possible to activate the analyser without activating the X-Ray, but as it is visible in both figures in part (D) both X-Ray and analyser together don't create any surface change without CN. As a conclusion it is most likely that the combination that can not directly be tested, namely analyser and CN, is what is creating the surface changes. It is also visible by looking at both figures 3.34 and 3.35 that the effect is very similar

**Figure 3.34:** Stripe experiment on plated anode with different measurement modes: (A) as received (B) 10 min X-Ray (C) 10 min CN (D) 10 min X-Ray and CN (E) X-Ray and analyser (F) X-Ray, analyser and CN

for the plated and fully charged samples. The stripe seems to be forming at a constant angle to the sample. Figure 3.36 shows the evolution of the stripe on the plated sample over time.

For this experiment all three parts were turned on and the device was doing a regular wide spectrum measurement. It is visible that after around 120 seconds (D) the stripe is fully formed. The effect starts in the top left and propagates following a large circle shape to the lower middle which makes it seem like it is caused by an impact on the sample at a very shallow angle coming from the top left.

Since the effect is connected to the lithium content on the surface of the sample it is most likely caused by electron induced lithium plating, an effect where an electron beam can be used to cause lithium ions from deeper within the sample to be transported to the surface and deposit there as metallic lithium [176, 230]. This effect alone however can not explain the shape of the stripe. The inherent design of the CN system inside of an XPS device should ensure that the coverage of the electrons on the surface is as homogeneous

**Figure 3.35:** Stripe experiment on fully charged anode with different measurement modes: (A) as received (B) 10 min X-Ray (C) 10 min CN (D) 10 min X-Ray and CN (E) X-Ray and analyser (F) X-Ray, analyser and CN

as possible to avoid local charging of the surface. Figure 3.37 shows the schematic of the CN and measurement unit in the Kratos AXIS XPS device that was used in this thesis.

Since the Kratos device does not utilize highly focused X-rays a simple charge compensation system is sufficient. This system consists of a filament that can be heated to emit electrons using the thermoionic effect mounted above the sample and a charge balance plate that is used to give those free electrons some kinetic energy [157]. The magnetic lens underneath the sample that is part of the analyser can then be used to focus these electrons on the analyzed point on the sample. This works since electrons in a magnetic field are subject to a magnetic force perpendicular to their initial movement vector that makes them follow a spiral pathway towards the sample. This motion also causes the electrons to hit the sample from all possible directions avoiding charging due to topological shadowing [157]. This spiral movement may be an explanation why the stripe appears following a curved path on the surface. It is however not clear why the center of the circle that the

**Figure 3.36:** Stripe evolution on the plated anode over time with (A) after 30 s and followed by 30 s steps until (L) after 240 s in full measurement mode with all three parts on

stripe effect would close is far outside of the area of sample since according to the data from Baer et al. [157] the spiral helix should focus the electrons on the center of the sample where measurement takes place. And it is also not possible to explain the strong localization of the effect since all electrons hitting the surface should have similar energies obtained from the acceleration caused by the charge balance plate. Some electrons could have more energy due the thermoionic generation energy distribution but these electrons would also be randomly distributed onto the surface and not clustering in one location. It looks like not all electrons hitting the surface can cause this effect, which means that the effect could on the one hand being coupled to the energy of the electrons. These electrons hitting the affected region would have gained more energy than the electrons hitting the other regions for some reason. On the other hand the effect could also be linked to a non-uniform distribution of the electrons hitting the surface and the affected locations are hit by more electrons than the rest of surface and are subsequently charged up, causing this reaction. The second theory is more likely since there is no obvious way for the electrons to gain more energy and then for only those electrons having more energy to hit one specific spot.

**Figure 3.37:** Schematic of the Kratos AXIS XPS spectrometer charge neutralization (adapted from [157])

This assumed localized surface charging could also not disperse easily since the SEI covered surface has a very low conductivity.

To further investigate the effect in the next experiment multiple stripes were created on the surface of both samples and then the spectra were measured before and after the striping as well as after a 2 day pause to see the change of the stripes over time. Figure 3.38 shows the optical change on the plated sample (top) and fully charged sample (bottom).

The change in hue in the images after striping for both samples (B and E) is due to the camera automatically changing its white balance and not an effect of the experiment. Due to this automatic white balance adjustment it is hard to identify any changes in the color of the sample. It is however easily visible in B and E that there are two distinct areas for each stripe. In the center of each stripe a lighter, less saturated area is visible, while the

**Figure 3.38:** Creation of multiple stripes on the plated sample (top) and fully charged sample (bottom). (A) and (D) initial condition, (B) and (E) after the stripe creation, (C) and (F) after two days

area around this core seems to have a sightly more warm hue. There is also a dark coloration at the edges of the stripes. In C and F it can be seen that the effect somewhat diminishes over time, which could be the metallic lithium reacting with gas traces inside of the vacuum chamber or with some parts of the SEI.

If the effect is in fact electron induced plating, it should not occur on fully discharged samples that have not been plated, and the effect should also not occur if the sample is insulated and thus has a floating potential. To test this hypothesis one sample was taken from discharged, non-cycled cells of each type and one was taken from a plated cell of each type. These cell have been obained and cycled about 6 months prior to opening and have been stored at 30% SOC as described in the beginning of this chapter. This means that the plated cells are not freshly plated like the example visible in figure 3.34 but had ample time for any reversible lithium plating to diffuse into the anode.

So the only additional lithium in plated cells compared to the non-plated cells should be irreversible metallic lithium depositions that completely lost contact and the additional lithium embedded in the different components of the thicker SEI layer.

To capture the striping process the sample was recorded on video during the experiments. During the recording of a spectrum the device repeats the same measurement two times and then averages the result. Interestingly it was also visible in the video that the two consecutive measurements of the same sample showed deviations in some of the detail spectra likely showing the effect of the striping on the measurement results. These spectral results will be discussed later in section 3.3.3. Figure 3.39 shows the changes in the non-plated (fresh) samples before (top) and after one full XPS measurement with CN on. Figure 3.40 shows the same measurement on the 6 plated samples.

**Figure 3.39:** Optical change of all 6 fresh tested cells after a full measurement with CN on (before: top / after: bottom)

In figure 3.39 it is visible that this does not only occur in plated or fully charged cells but can also be faintly visible in fresh discharged samples. Not all samples show the effect, but some changes are visible in the LG-NMC, LG-NCA and SO-NMC samples. It is also noticeable that the shape of the effect is much wider and more spot-like compared to the previous experiments. The spot however is still on the left side of each sample and propagates into a similar direction as before. It is visible by also looking at the plated samples (that all show the effect at different levels) that the shape of the spot is consistent over all samples and thus seems to be rather dependent on the setting during the measurement than any properties of the sample. It could be that the difference in distance of the samples to the filament has an influence on the shape.

**Figure 3.40:** Optical change of all 6 plated tested cells after a full measurement with CN on (before: top / after: bottom)

All plated samples in figure 3.40 show the effect at varying intensities but much weaker than with the freshly plated samples used in the previous experiments. In the case of the LG-NCA cell it is not clear if the fresh sample or the plated sample show a more distinct effect. An explanation for the behaviour could be that in the freshly plated samples lithium metal is still present in the area of the SEI in metallic form or low energy bindings and can be pulled to the surface with the electron inducted plating. In the samples that have been rested for 6 months on the other side all lithium has either diffused into the anode or formed more stable compounds within the SEI over time. It is not clear why the effect is also visible in the fresh samples in figure 3.39 but it could be that different additives or components of the electrolyte facilitate different compounds within the SEI that may be more susceptible to the effect than in the other cells.

**Figure 3.41:** Comparison of the surface changes of a full measurement with CN on insulated samples of the fresh and plated SO-NMC sample (INS denotes the insulated samples) (before: top / after: bottom)

Figure 3.41 shows the comparison of insulated vs. non-insulated samples of the SO-NMC cell. The insulated samples have been attached on the surface of the sample holder using double sided tape and the are thus electrically insulated. This creates a floating potential within the anode and thus should inhibit the electron induced plating effect in the samples. For both insulated samples no change in the surface can be observed. This is another indicator that the effect observed is the electron induced plating effect since it requires the sample to be grounded for the effect to occur. The spectra of all the samples are investigate in the next section.

#### **3.3.3 Influence of Sample Washing and First Spectral Analysis**

Washing the samples taken from the battery with an appropriate solvent is often done prior to sensitive experiments [14, 54, 86, 124, 156, 162, 231, 232]. This is done to remove any battery electrolyte residue or electrolyte salt rests from the surface of the samples [233]. The solvents used for washing are often the same or similar to the solvents used in the electrolyte (e.g. DMC). Mussa et al. however specifically mentions that washing can dissolve parts of the SEI and does not recommend washing the samples after removing them from the battery. In this case washing can falsify the measurement results since the layer is not in the original sample state anymore. Somerville et al. [231] did a study on the effects of washing on the surface film of graphite electrodes. They report that some authors could detect a higher sensitivity to atmosphere after the washing, which could be explained by the removal of a passivating surface film (this effect was also observed in [232]). The paper itself focused on the ratio of VC in the electrolyte, an additive commonly used in LIBs to improve the formation of a stable SEI. They postulate that the washing removes the salt and the low boiling point solvents present in the surface layer (in this case EC). Samples with different VC ratios from 0 to 6 vol.% have been prepared by them and analyzed using XPS, SEM, attenuated total reflectance (ATR) and fourier transform infra-red spectroscopy (FTIR). They found two different films dependent on the VC concentration with the two films showing a different SEM image as well as different behaviour in the material composition. The low VC concentration film in contrary to the higher concentration film is also very sensitive to the washing process and for this film not only the salt deposits are removed but also LiF and Li*x*PF*y* which can only be found in the deeper layers of the SEI. Their results show clearly that whether washing it useful or not strongly depends on the composition of the electrolyte. Andersson et al. [173] report a removal of LiF from the surface by washing in DMC, although LiF is not solvable in DMC and conclude that the LiF depositions are rather loose crystals on the surface than integrated into the SEI.

Additionally there is very little information in literature in what way the washing process is performed. Bach et al. [14] soaked their samples for a total time of 1*.*5 h changing the solvent bath in between, while other authors don't give any specifics.

For the experiment in this work commercials cells have been used, so it is not possible to obtain any data about the composition of the electrolyte since battery producers generally do not make their internal electrolyte recipes public to the competition. For this reason it could not be determined beforehand whether washing the electrodes before testing is beneficial or harmful. So in the first test run measurements of one washed and one none-washed sample from each battery anode where analyzed and the results have been compared. The samples have been washed in two DMC baths for about 30 seconds each and then dried for one hour in the glovebox at less than 5 ppm moisture and oxygen content before they were mounted to the sample holder and transferred to the XPS device. The short washing time was chosen to see whether a short wash was already influencing the measurement results.

**Figure 3.42:** XPS spectra of a pristine graphite anode (Sample A) with no SEI. Comparison between the washed and unwashed sample (Shift: 0*.*0 eV)

Figure 3.42 shows the measurement result for the pristine graphite anode (sample A) that was not inside of a cell prior to the measurement and thus not in contact with any electrolyte prior to the washing. No shift of the spectrum was necessary since the reference peak at 284*.*3 eV for AC was already at the expected location. This was to be expected, since the graphite should have a good conductivity that prevents the surface from charging up and causes a spectral shift. It is visible that the washing is this case is adding material to the surface. The signals for P2p and Li1s become stronger after washing, but they are both still very weak. The signal for F1s gets a small shoulder and the O1s is also increasing. The occurrence of P and Li seems to be the effect of cross pollution between the samples since they have been washed within the same electrolyte bath and likely the salt from the other samples (LiPF6 and/or LiF) was deposited on the surface. Cross contamination in the glovebox in general can also not be ruled out as a source of these signals, since the other cells have also been opened in the glovebox before. The increase in oxygen can not be explained by the salt and may be caused by a reaction of the graphite surface with the electrolyte. The expected carbon peak is clearly visible at around 284*.*3 eV with another carbon peak at 290 eV. The usually used AC peak is at the same position as the graphite peak of the anode active material. Since there should be no SEI on the surface of the sample the second carbon peak is most likely caused by the PVDF binder in the anode since that is located at around 290 eV with another peak at 286 eV which would also explain the shoulder of the carbon peak [161]. The large peak of the fluorine can also be attributed to the binder [163], while the small shoulder at slightly lower energies is probably caused by the above mentioned contamination, also creating small amounts of LiF on the surface, with an F1s peak at around 685 eV. The F1s peak of the LiPF6 is not visible, which may be due to the overlapping stronger PVDF peak that is only 1 to 2 eV higher. It is visible that all peaks are in good accordance with the literature values. The only position slightly off is the P2p peak, which is most likely a measurement error due to the very low signal strength. The test on pristine graphite was performed to analyze the effect of washing on a surface without SEI and the results show that there seems to be at least some interaction of the washing reagent with the graphite surface and that cross-contamination can happen easily even with the very low amounts of original battery electrolyte that stay in the washing solution.

In figure 3.43 the spectra for a fully discharged anode (sample B) are visible. Contrary to sample A the material is taken from a commercial cell and thus has a fully formed SEI. The energy scale was in this case shifted by 1*.*5 eV to calibrate it to the the AC peak at 284*.*3 eV. Compared to the graphite of sample A the surface should have a very low conductivity so

**Figure 3.43:** XPS spectra of a fully discharged graphite anode with no plating (Sample B). Comparison between the washed and unwashed sample (Shift: 1*.*5 eV)

a shift in the spectrum was to be expected. The surface charges up positively because electrons are removed in the form photoelectrons and this slows down further emitted photoelectrons reducing their kinetic energy. A reduction in kinetic energy means a shift to higher BE since the device then assumes that more energy of the X-ray was spent to remove the electron from its binding, leaving less energy to be turned into kinetic energy. The compounds of the SEI should be visible in the spectra and these compounds could potentially damaged or altered by the washing process. The active material and binder visible in sample A should not show up in the spectrum of sample B, since even the SEI of a fresh cell should be much thicker than the penetration depth of the XPS X-ray beam. The cell went through one full cycle before disassembly. In the full spectrum it is already visible that the changes caused by the washing are lager than for sample A. In the spectrum a mixture mainly consisting of ROCO2Li, Li2O, LiF and Li2CO3 is expected on the surface according to the literature possibly with LiOH contamination due to moisture exposure during the transport, and LiPF6 expected to show up in the spectrum before washing. It looks like most of the surface is covered with the salt remains before washing and mainly consists of LiOH and Li2O otherwise. The shoulder of the carbon peak is assigned to carbon in a one oxygen environment that is often found as part of the AC surface contamination since no part of the PVDF should be visible on the surface of the SEI. The P1s peaks are assigned to the LiPF6 salt residuals as well as phosphorous LiPF6 decomposition products. The strong peak for LiOH most likely stems from surface contamination due to moisture exposure during the transfer from the glovebox to the measurement device. It is surprising that the peaks don't shown any ROCO2Li on the surface of the SEI, since in literature the top layer of the SEI is theorized to mainly consist of anorganic compounds as discussed in chapter 2.2.1. It could be that the high amount of Li2O also stems from contamination with atmosphere similar to the LiOH contamination. After washing, the amount of LiPF6 on the surface is reduced, as expected, but at the same time also the amount of LiF seems to be reduced drastically. This is rather unexpected, since LiF should have a very low solubility in the washing reagent [234]. This could mean that the washing process is removing a part of the SEI additionally to removal of the residual salt on the surface or that the LiF is present on the surface as more lose crystals, as discussed before, that do not get dissolved in the washing solution but are simply washed away.

**Figure 3.44:** XPS spectra of a fully charged graphite anode with no plating (Sample C). Comparison between the washed and unwashed sample (Shift: −2*.*9 eV)

The first thing noticeable in figure 3.44 for the fully charged anode (sample C) is a change in the lithium signal compared to sample B, the shapes of the other signals are very similar to the discharged anode in figure 3.43. To compensate for BE shifting however the whole spectrum had to be shifted by −2*.*9 eV to fit the AC reference peak. In light of the electron induced plating effect discussed in chapter 3.3.2 the change of the shift from a positive value in the discharged sample to the charged sample can only be caused by the newly formed lithium plating on the surface. A possible explanation could be that the electrons emitted by the charge compensation device can now easily accumulate at the location of the measurement spot since the surface is covered in a conductive layer of metallic lithium allowing the electrons to flow freely. The underlying material may still be positively charged from the beginning of the measurement before the surface layer was formed and so the electrons from a double layer on surface, charging the surface negatively, causing the negative shift in the spectrum since the leaving photoelectrons are accelerated further towards the detection unit. For sample B and C the SEI should have a similar thickness and composition, so the conductivity of the SEI should not be significantly different. The penetration depth of the XPS methods is very shallow, with only a few atomic layers and thus should not be influenced much by a charged or discharged anode underneath the SEI since only the top surface of the SEI should be measured. But as discussed in chapter 2.6.2 Oswald et al. [172] found that the SEI compound peak positions can be shifted when the underlying graphite is lithiated and they found a dependency of that shift on the level of lithiation. The discharged anode also showed a thinner SEI with a high Li(O)F content. The fully lithiated anode on the other hand showed a thicker SEI. Their greatest shift however was at around 3 eV while for the samples present in this work the shift between no lithiation and the fully lithiated samples is almost 5 eV. Except for the large shift that cannot be fully explained and the lithium signal the rest of the peaks show a similar behaviour to sample B, with the amount of LiPF6 getting slighly reduced by washing while the amount of LiF is getting reduced much more drastically. The surface is again covered in LiOH and Li2O. In this sample the LiO2 could not only be caused by atmospheric exposure but also could be a reaction product of the freshly formed lithium film on the surface since this reaction can happen even in high vacuum due to the extremely high reactivity of metallic lithium with even trace amounts of oxygen [235]. The lithium signal on the other hand shows a very different shape compared to the discharged anode of sample B. The peaks for the LiOH and possible LiPF6 decomposition products stay the same, but a completely new peak appears at around 52 eV. Together with the flat peak at around 60 eV this could be an indicator for metallic lithium, since metallic lithium can mostly be found at 52 eV to 53 eV [165, 176] with plasmon losses between 60 eV and 66 eV. The fact that the plasmon losses are not visible in the discharged sample supports this assumption. This result also strongly supports the theory of the electron induced plating since if this is in fact metallic lithium, this would mean that metallic lithium is present in the top surface of the fully charged anode even if there was no prior detectable plating event.

**Figure 3.45:** XPS spectra of the anode after 1 cycle of plating (Sample D). Comparison between the washed and unwashed sample (Shift: −2*.*9 eV)

The last sample with a plated anode (sample D) visible in 3.45 was also shifted by the same value as sample C (−2*.*9 eV) to the reference AC peak. Here the peaks again show a similar behaviour to sample B and C, except for the Li1s peaks. The presumed lithium metal is here also visible at around 52 eV, but the peak as well as the plasmon loss signal are lower than the signal of the fully charged anode. This points to the fact that the electron induced plating is capable of pulling the lithium out of the LiC6 of the anode and not only dead lithium in the SEI or lithium from other SEI compounds. The question remains however whether the lithium that can be detected on the surface of the plated anode is also pulled from the anode or from dead lithium caused by plating since it can be assumed that after discharging of the plated anode some reversible plating remained that could have caused the anode to not the be fully discharged by diffusing into the anode over time. Even though the cell was opened one day after the plating cycle and cell was kept at −20 ◦C in the temperature chamber until directly prior to the opening the diffusion of reversible plating into the anode can happen even if the cell is already dismantled and does not require the full electrochemical system of an intact cell. But since it was fully discharged the amount of remaining reversible lithium should be very low. Additionally there is also the diffusion inside of the bulk of the graphite anode material that would cause the reversible lithium diffused into the discharged anode to disperse inside the anode and only be present at the surface level with a very low lithium concentration. In conclusion it can not be proven which the source of the surface lithium caused by the electron induced plating truly is, and it likely is a combination of both but the discussion before points at a bigger part being dead lithium from the SEI rather than reversible plating that remained after discharging.

The results for the comparison of the washed versus the unwashed samples shows that while the washing seems to reduce the amount of LiPF6 on the surface it also seems to change the SEI by removing a considerable amount of the LiF that should be part of the SEI and not a surface contamination. It is not really clear why the washing solvent removes a large amount of LiF since its solubility should be very low. It was also shown that cross contamination between the samples is to be expected unless the cleaning solutions are renewed for each sample. These points and the fact that the washing is not really improving the other signals in a significant way lead to the choice to not wash any further samples for the rest of the XPS experiments since the benefits are not really visible and a negative effect on the SEI is very likely.

#### **Spectra Analysis of the Stripe Effect**

In this part the spectra of the samples of the striping experiment are analyzed. Both cells are of the type LG FIB and are also the cells used for the tests in chapter 3.3.3 and that will be analyzed utilizing argon depth profiling in chapter 3.3.4. For the full cell as well as the plated cell a stripe on the surface was created using the combination of options discussed before with activated charge compensation and magnetic lens. For each sample one spectrum was recorded before with no stripe on the sample while the magnetic lens was turned off, so that no stripe was created on the sample. Then the stripe was created by turning the magnetic lens on together with the CN. Then another measurement was done on the location of the stripe with the same settings as the first measurement. Finally the last measurement was performed 2 days later with the sample remaining under high vacuum for the whole time to see how the surface is changing over time. All measurements have much lower electron counts than the spectra recorded in the previous chapter. This is due to the magnetic lens being turned off. The magnetic lens is used to focus the electrons into the analyser. If the magnetic lens is turned off, only a fraction of the photo-electrons reaches the analyser, reducing the signal strength of the resulting spectrum.

Figure 3.46 shows the three measurements for the sample of the fully charged anode. In the original condition (light blue) it can be seen that the surface manly consists of LiF, either LiOH or Li2CO3 as well as some LiPF6 together with its decomposition products. The AC peak is clearly the highest peak in the C1s spectrum. A small amount of LiO2 can also be seen in the O1s detail spectrum. In the Li1s detail spectrum almost no peak at 52 eV is visible, as well as no plasmon loss hinting at lithium on the surface. The visible LiO2 could be due to normal amounts of LiO2 in the SEI. Compared to that the second follow up measurement on the location of the stripe (grey) shows a very different spectrum. All previously found surface compounds are reduced, except for LiO2 and a newly formed lithium peak at around 52 eV that could be metallic lithium. This is to be expected for electron induced lithium plating since the newly formed metallic lithium layer is covering the surface and thus reducing the signal strength of all other surface compounds except for LiO2 that immediately starts forming on the surface of the lithium metal freshly deposited on the surface of the sample. In the check-up measurement after 2 days (red) that was also performed on the location of the stripe more of the compounds found in the first measure-

**Figure 3.46:** XPS spectra of the full anode: original condition (OC), after measurement (AM) and after 2 days under high vacuum. (Shifts: OC −3*.*0 eV, AM −2*.*7 eV, 2 days −3*.*1 eV )

ment can be detected again. The signals for LiF, LiOH or Li2CO3 as well as LiPF6 and its decomposition products are stronger again, only the AC peak does stay at the same level. There is also further strong increase in the amount of LiO2 which is to be expected due to the continuous reaction of trace oxygen with the surface lithium metal.

In the lithium spectrum the 52 eV lithium peak as well as the plasmon peak are diminished compared to the second measurement but still higher than in the measurement of the original condition. This also makes sense since the lithium is transformed into LiO2 and thus the amount of measurable lithium metal on the surface must decrease. The peak corresponding to LiO2 in the lithium spectrum is also higher than in the second measurement.

The fact that the level of AC are not increasing anymore leads to the conclusion that the increase of the LiF and LiPF6 decomposition products visible in the last measurement are not the same that were visible in the first spectrum but rather newly formed products on top or within the metallic lithium layer due to salt rests on the surface that may react with the metallic lithium and any trace gases in the vacuum chamber.

**Figure 3.47:** XPS spectra of the plate anode: original condition (OC), after measurement (AM) and after 2 days under high vacuum. (Shifts: OC −2*.*9 eV, AM −2*.*5 eV, 2 days −3*.*0 eV )

For the plated sample in figure 3.47 the result looks very similar to the full cell. The only noticeable differences are a higher peak for LiPF6 decomposition products in the P2p peak in the follow up measurement on the stripe and a smaller peak for the surface lithium created by the electron induced plating. This difference in the LiPF6 decomposition peak hints at a different composition of the SEI on the plated cell compared to the full cell that experienced no plating. A smaller lithium peak is to be expected since the lithium content close to the surface of the cell should be lower than in the full cell. Other than these two differences the trends of the spectra look identical.

In conclusion all results strongly point at the electron induced plating effect as the result of the creation of the stripes. It not clear however, as discussed before, why the effect only locally appears on the surface when the electron source of the charge compensation should be designed in a way to disperse the electrons on the surface of the sample in a homogeneous matter.

Since the electron induced plating effect is linked to the samples not being insulated during the experiments [230] to further proof that the changes in the surface are in fact caused by lithium on the surface, a plated and a non-plated sample of the SO-NMC cell have been attached to the sample holder using double sided tape to insulate them during the measurement. The optical results of this experiment have already been shown before in Figure 3.41 and no optical change was visible in the insulated samples. Here the spectra are analyzed as well to see if the spectra do not show any change compared to their not insulated counterparts either. Figure 3.48 shows the comparison between the insulated (red) and non-insulated sample of a fresh anode. Figure 3.49 shows the same comparison for the plated samples.

It is visible in figure 3.48 and 3.49 that there are significant differences between the insulated and the non-insulated samples. Both spectra show very similar behaviour to the spectra discussed before in the striping experiment. The non-insulated samples that are influenced by the electron induced plating effect show weaker signals for all spectra except in the area of 52 eV in the Li1s detail spectra and the LiO2 peak in the O1s spectra again most likely caused by the coverage of the surface with lithium metal blocking the X-ray beams from reaching the original SEI compounds underneath. This result was expected for the plated sample but it is unexpected that the spectra also show such a strong difference for the new sample that has a lower lithium content. Since this sample was fully discharged before cell opening that must either mean that even in a fully discharged cell lithium remains in the anode that can be affected by the electron induced plating effect or that there are compounds inside the SEI apart from the irreversible depositions

**Figure 3.48:** XPS spectra of the new anode: not insulated vs. insulated (Shifts: not insulated 1*.*1 eV, insulated 2*.*5 eV)

of electrochemically dead lithium metal left by lithium plating that can be affected the effect even though the lithium is part of a compound in the SEI.

It also has to be kept in mind that the sample of the new cell was subjected to calendar ageing at room temperature for one year before the cell was opened. This could meant that the samples used for the previous stripe experiments had a thinner SEI and also have a different composition of the SEI.

Unfortunately the influence of the striping effect on the measurement results was only understood later into the measurements and in the following analysis of the depth profiling as well as the spectra of the cells if different manufacturers all samples have been grounded and thus subjected to this effect, which was changing the measurements results. Due to limited access to the XPS device the measurements could not be repeated so the implica-

**Figure 3.49:** XPS spectra of the plated anode: not insulated vs. insulated (Shifts: not insulated 1*.*4 eV, insulated 2*.*4 eV)

tions of the effect will be kept in mind during the analysis and all results will be treated accordingly. With the knowledge of the effect occurring during the measurement the results can still be analyzed in meaningful way and the amount of lithium that was deposited on the surface during the measurement can be seen in the lithium peak as well as the LiO2 peak and can be a very helpful indicator of how much available lithium is close to the surface area of the samples. The electron induced plating could be used in the future in a controlled way to pull the lithium that is bound in less strongly from the SEI to surface without the need for depth profiling. For this it is important however to understand which forms of lithium can be pulled to the surface.

#### **3.3.4 XPS Depth Profiling**