Julian Long

## Detecton and characterizaton of lithium platng

**Universitätsverlag der TU Berlin**

Julian Long **Detection and Characterization of Lithium Plating** The scientifc series *Elektrische Energietechnik an der TU Berlin* is edited by:

Elektrische Energietechnik an der TU Berlin | 14

Julian Long

### **Detection and Characterization of Lithium Plating**

Universitätsverlag der TU Berlin

#### **Bibliographic information published by the Deutsche Nationalbibliothek**

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografe; detailed bibliographic data are available on the internet at http://dnb.dnb.de.

#### **Universitätsverlag der TU Berlin, 2023**

https://verlag.tu-berlin.de

Fasanenstr. 88, 10623 Berlin Tel.: +49 (0)30 314 76131 Email<EMAIL_ADDRESS>
Zugl.: Berlin, Techn. Univ., Diss., 2022 Gutachterin: Prof. Dr.-Ing. Julia Kowal Gutachterin: Prof. Dr.-Ing. Ulrike Krewer (Karlsruher Institut für Technologie) Gutachter: Prof. Dr. rer. nat. Dirk Uwe Sauer (RWTH Aachen) Die Arbeit wurde am 13. Juli 2022 an der Fakultät IV unter dem Vorsitz von Prof. Dr.-Ing. Ronald Plath erfolgreich verteidigt.

This work – except where otherwise noted – is licensed under the Creative Commons License CC BY 4.0. https://creativecommons.org/licenses/by/4.0

Cover image: Julian Long | CC BY 4.0

Print: docupoint GmbH Layout/Typesetting: Julian Long

**ISBN 978-3-7983-3278-2 (print) ISBN 978-3-7983-3279-9 (online)**

**ISSN 2367-3761 (print) ISSN 2367-377X (online)**

Published online on the institutional repository of the Technische Universität Berlin: DOI 10.14279/depositonce-16978 https://dx.doi.org/10.14279/depositonce-16978

"Knowledge is knowing a tomato is a fruit; wisdom is not putting it in a fruit salad." (Miles Kington)

## **Acknowledgement**

First of all I want thank Prof. Dr. Julia Kowal for her support and trust. She was always helpful and supportive without putting pressure on me or trying to imprint her style on the thesis. I am very grateful for her supervision. Furthermore I want express my thanks to the Fraunhofer IZM for letting me perform battery material measurements using their XPS device and especially to Steffen Rauschenbach whose patient help and explanations enabled me to do these experiments in the first place. I want to thank all my colleagues that have accompanied me for all these years at the TU for their comradeship, smart inputs and valuable criticisms. Because of all of you I was always looking forward to come to work. I'm also very grateful for my examiners Prof. Dr. Ulrike Krewe and Prof Dr. Dirk Uwe Sauer who came the long way to Berlin to attend my defense. I also want to thank the students mentioned in this thesis whose contributions in their master thesis' made this work possible. Last but not least I want to thank my family for always believing in me and never pushing me to anything I didn't want and my amazing wife for surviving the lock-down year with me without any mayor complaints.

## **Abstract**

Lithium plating is not only the most severe ageing mechanism in lithiumion batteries (LIBs) but also becoming more and more important due the increasing presence of electric vehicles (EVs). In EVs the extreme conditions causing lithium plating, like very high charging currents and low environment temperatures, are much more prevalent than in consumer electronics. Due to the high number of factors that influence the plating process, ranging from the cell geometry to the chemical composition of the electrolyte, a deeper understanding of the plating process is still lacking. Without this knowledge it is hard to design cells in a plating resistant way, or to operate cells under the ideal conditions to minimize plating. This thesis aims at showing different methods to investigate the plating process on three different levels.

The first method is on the cell level, investigating the behaviour of the whole cell during plating. It contains the analysis of the voltage and current profiles that show an atypical behaviour during plating. The focus of the analysis is on the current profile of the constant voltage (CV) phase during charging under low temperature conditions leading to plating. This current profile can be fitted with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) function that describes the electrochemical deposition process of a metallic species on a surface. The resulting fitting parameters can be utilized to characterize the plating behaviour of the cell as well as better estimate the amount of plated lithium than commonly used methods. It can also potentially predict the future safety risk due to dendrite formation.

In the second part the chemical composition of the surface electrolyte interface (SEI) is investigated using X-ray photoelectron spectroscopy (XPS). The composition as well as the mechanical properties of the SEI are strongly influencing the plating process and preliminary work has shown that plating is also changing the morphology of the SEI and increasing its thickness drastically. Cells under different conditions (plated, charged and discharged) as well as cells of different manufacturers have been probed using XPS. During the measurements an unwanted side effect of the experimental setup was discovered that lead to a migration of lithium to the surface of the sample and was distorting the measurement results. Regardless of the effect, it was possible to see that the SEI can have a very different composition in cells of different manufacturers and that plating not only changes the morphology but also the composition of the SEI. The unwanted side effect could furthermore be utilized to identify samples that were plated recently and could be used in further more controlled experiments to localize lithium depositions on plated samples.

In the last part the particle structure of the anode surface of cells of different manufacturers was investigated using a watershed particle detection algorithm on laser scanning microscopy (LSM) images of the anode surfaces. The distributions of the particle sizes have then been compared to the capacity loss in plated cells. It was shown that the capacity loss correlates with parameters extracted from the particle size distributions. It is however necessary to create more data to verify this correlation.

In summary this thesis utilized new methods to detect or characterize plating on different levels of magnification, from the cell level to the chemical composition. New approaches were found to predict a cells future plating behaviour, spatially localize plated areas on the anode and design cells in a plating resistant way.

## **Kurzzusammenfassung**

Lithium Plating ist nicht nur der Alterungsmechanismus in Lithium-Ionen Batterien mit dem größten Kapazitätsverlust, sondern wird auch im Zuge der voranschreitenden Elektrifizierung des Personenverkehrs immer wichtiger. In Elektrofahrzeugen finden sich die extremen Zustände, wie niedrige Ladetemperaturen und hohe Ladeströme, unter denen Plating auftritt, deutlich häufiger als in Unterhaltungstechnik.

Die erste Methode untersucht das Gesamtverhalten der Zelle auf Zellebene. Hier-bei wird das atypische Verhalten der Strom- und Spannunsprofile während des Plating-Vorgangs analysiert. Der Fokus liegt dabei auf der Untersuchung der Konstantstrom-Phase bei niedrigen Temperaturen während der Ladung.   Die Ergebnisse könnten außerdem helfen das Sicherheitsrisiko der Zelle bei Dendritenbildung vorherzusagen.

  Zellen in verschiedenen Zuständen (geplatet, geladen, entladen), sowie Zellen verschiedener Hersteller wurden mit XPS untersucht.

Im letzten Teil wurde die Partikelstruktur der Anoden von Zellen verschiedener Zellhersteller mit Hilfe einer watershed-Partikeldetektion an LSM Bildern untersucht. Die Verteilung der Partikelgrößen wurde mit dem Kapazitätsverlust gleicher Zelle durch Plating verglichen. Es wurde gezeigt, dass der Kapazitätsverlust mit Parametern, die aus den Partikelverteilungen extrahiert wurden, korreliert. Ein größerer Datensatz ist jedoch notwendig, um diese Ergebnisse zu validieren.

Zusammenfassend hat diese Arbeit verschiedene neue Methoden aufgezeigt, um Plating auf verschiedenen Vergrößerungsebenen zu detektieren und zu charakterisieren. Neue Ansätze wurden gefunden, um das Platingverhalten von Zellen vorher-zusagen, lokalisiertes Lithium auf der Oberfläche zu detektieren und Zellen plating-resistenter designen zu können.

## **Contents**

## **List of Figures**

## **List of Tables**

# **Introduction and Motivation Chapter** 1

In the last decade LIBs have consolidated their prime position as the go to battery for all consumer electronics [1]. With the climate protection targets and the demand for a rising amount of electric vehicles for private and public transport, the LIB is also the only technology that has the needed energy density, as well as the development level necessary, to support the transition from fossil fuels to CO2 free transportation. While there are new technologies in development with even higher theoretical energy densities, none of the candidates has reached a technology readiness level yet that could challenge the usability of the LIB [2]. One big concern with batteries in general however is the ongoing degradation through battery ageing during operation that not only increases the running of costs of all EVs but also reduces the customer acceptance of the vehicles, which is vital for a widespread change to electric transportation [3]. With increasing ageing, not only the capacity of a cell fades but also the available power. Presently the main efforts in battery research are targeted towards increasing the energy density to enable higher range on full electric vehicles and to reduce the ageing on the cell to increase the overall lifetime [4, 5]. One of the strongest ageing effects possible in LIBs is the so called lithium plating, which describes the unwanted deposition of metallic lithium on the anode surface during charging. This deposition takes place when the device is charged at very low temperatures, or when very high charging currents are applied. The capacity losses of plating can lead to the end of life of a cell within 10s of cycles, when the nominal cell life can last for hundreds or even thousands of cycles [6]. The plating process can also cause the growth of lithium dendrites, small lithium metal structures that can pierce the separator and cause shortcuts within the cell. These can lead to cell failure or in the worst case to a thermal runaway of the cell, a process where the cell reaction is self-energizing and causing a cell fire [7].

The conditions for lithium plating are rarely met for cells within consumer electronics, since they are normally charged at moderate rates within living areas. This is different for EVs however. Electric cars are supposed to work under all normal outside conditions and thus charging at low temperatures is something that cannot be prevented. Today the vast majority of customers with electric cars also own a house or have access to a garage to charge their cars at higher temperatures under better conditions [8] which is partly because of the still higher price of the cars and the on average better economical situation of house owners, and partly because of the lack of infrastructure in the area of public charging stations. With the target of a much higher percentage of EVs in the future however, this cannot be guaranteed for each owner of an electric car and some of the vehicles will have to be charged at public charging stations outside. To compensate for this effect, it is possible to equip EVs with a heating system that heats up the battery to the optimal charging temperature prior to the charging process. The drawback of this method is for one the energy losses induced by the necessary heating process that increase the running costs of the car and the increased charging time due to the added heating time. In some cases there may not even be enough energy left in the cell to heat up properly to prevent plating during charging. The batteries are heating up due to the ohmic losses during the charging process, but this does not fully prevent the ageing caused by plating. Another aspect that is of vital importance for EVs is the charging speed [9]. To compete with combustion engine cars the charging speed of EVs would have to be in the range of minutes to increase the acceptance of the EVs as a fully capable replacement. This would however require charging rates that greatly exceed the recommended nominal charging speeds for standard commercials LIBs. To fully charge a battery within ten minutes a charging speed of 6 times higher than average recommended data sheet speeds of higher energy cells of 1C is required. It is possible to manufacture cells that can handle higher charging speeds, but this is always at the cost of energy density, which is vital for the range of the EV.

For these very high charging speeds, plating effects are to be expected even at moderate temperatures, leading to very fast cell degradation [7].

Grid stabilization using EVs could be the solution for the strongly fluctuating energy output of renewable energy sources. All EVs that are not currently in use and that are connected to the power grid could be used to stabilize the grid and store excess energy in times of overproduction, while at the same time providing energy in times of high demand [10, 11, 12, 13]. If the vehicles are used in this way, low environment temperatures could be very damaging for the battery cells. In this case, the energy influx is unpredictable and uncontrollable and thus no prior heating is possible. Always keeping the batteries at a safe temperature however would produce very high energy losses.

The amount of available spent batteries is going to increase significantly within the next few years when the first generations of car batteries will reach the end of life. The plating effect is also attributed to sudden cell death at very low state of health [7, 14] when the surface electrolyte interface (SEI) layer is getting very thick. Predicting this mode of cell failure could be very beneficial for second life usage of spent EV batteries.

Understanding and preventing plating could not only increase the general lifetime for LIBs, but also enable the usage of EVs as a grid support for renewable energies, greatly increasing the usability of fully renewable energy sources to reduce CO2 emissions. It can also help making second life applications more reliable and thus extend the commercial value of the batteries. At the same time it increases the safety of LIBs by preventing dendrites that can cause cell failure or dangerous thermal runaways. A deeper understanding of the plating process can also help to increase the usability of metallic lithium in future battery technologies. The usage of metallic lithium as the anode in a lithium battery drastically increases the energy density. The redeposition of the lithium on the anode during the charging process in the battery however is hard to control and leads to very low cycle numbers of those cells [15, 16, 17]. Even for completely new technologies, the usage of lithium is highly likely [2]. Lithium is not only the lightest solid material in existence but it also has the highest negative standard electrode potential.

This enables very lightweight batteries with a high voltage and thus high energy density.

To achieve a better understanding about plating and find new methods to prevent plating or reduce the damaging impact of plating, in this thesis insitu measurements as well as ex-situ post mortem analysis have been applied to cells that were cycled under plating conditions. Electrical measurements for voltage and current as well as electrochemical impedance spectroscopy (EIS) measurements are conducted on cells with prolonged constant-voltage phases to study the lithium metal crystallization process on the anode surface based on the anomaly of the current shape visible under plating conditions in chapter 3.2. EIS measurements are also conducted during the diffusion of the lithium metal into the anode after the charging is completed to see the effect of metallic lithium metal presence on the impedance of the cell. Combining the results of these measurements leads to a comprehensive understanding of plating from the onset during the charge to the consequences of plating afterwards that can be used to help detect plating in operation, but at the same time also help to optimize cell design to reduce the impact of plating. Reducing the lasting effects plating can also help prevent cell degradation caused by the undetectable plating linked to sudden cell deaths.

These in-situ measurements are supported by different post-mortem analysis methods. An LSM has been used for quick surface analysis to check for visible impact of plating on the surface of the anode and also allows for spatially resolved analysis to detect areas of the cell that are affected stronger by plating, visible in chapter 3.1. In chapter 3.4 a study of cells by different manufacturers was also conducted using the LSM to analyze whether certain surface parameters based on the particle size distribution can be linked to a higher plating susceptibility. FIB cross sections in the samples were used in chapter 3.1 to get an understanding of the surface layer changes induced by plating. To analyze the chemical composition of this surface film XPS combined with depth profiling is applied in chapter 3.3.1.

In summary the main questions this thesis aims to answer are the following:

# **State of the Art – LIBs**

LIBs are using lithium ions to store energy. Contrary to the lithium-metal battery (LMB), where lithium exists in its elemental state inside the battery, the lithium in the LIB only exists in ion form, intercalated, as a free ion in the electrolyte or in a chemical compound. Although the LIB has a lower possible energy density compared to the LMB, the LIB became the standard of lithium batteries. The first lithium batteries on the market in the late 1980s used lithium metal as the anode, but following a series of accidents caused by the re-deposition of the lithium during charging and the resulting risk of lithium dendrites growing on the surface, these cells had to be recalled [18]. The subsequent failure to solve the safety problems of the LMB quickly lead to the discovery of alternate anode materials that where able to store the lithium ions in a safer and more reliable way. The LIB is not only safer than the LMB, it also possesses a much higher cyclability due to the more controlled lithium deposition process on the anode. The LIB quickly took over the market for consumer electronics, completely replacing the leading technology at that time, the nickel-based batteries, due to its superior energy density and cycle life. The introduction of this technology enabled a lot of the mobile devices that are a natural part of our lives today: smartphones, tablets, cameras, smartwatches and so on. As of now the LIB is also the only commercially ready technology that can enable full electric transport for private use and is thus an integral part of the worldwide efforts to become free of fossil fuels and reduce the overall CO2 emissions. And even though the demand for high energy density in the sector of stationary energy storage is naturally not as high as for mobile applications, LIBs are starting to see usage in large scale stationary storage for grid stabilization as well [19].

## **2.1 Components and Functionality**

The LIB is, like all electrochemical storage systems, made up primarily of an anode and a cathode, that drive the redox-reaction. Figure 2.1 shows the general schematic of a LIB and all its components. In most commercially available LIBs today, the **anode** is mainly composed of Graphite (C6). This graphite and added binders are pressed onto a copper foil that functions as the current collector, while the **cathode** is a lithium-metal-oxide (LiMeOx) pressed on aluminum foil.

**Figure 2.1:** Schematic of a general LIB

In the LIB the lithium ions are responsible for the storage of energy. During charging and discharging the ions are moved from the anode to the cathode and vice versa. It is not necessarily always the case in all electrochemical systems that the main ion is moved during the reaction. Even though there always has to be an ion moving inside the cell to close the electrical circuit, e.g. in lead-acid batteries the moving ion is a hydrogen ion, not a lead ion.

The **electrolyte** in LIBs is not taking part in the cell reaction as it was the case in its technological predecessors, the nickel-hydroxide and lead-acid batteries, but is solely in the cell for the ion transport. For this reason the amount of electrolyte in the cell can be reduced to a minimum necessary to properly connect all the cell elements. Normally the electrolyte of commercial LIBs is composed of an organic solvent with a lithium salt as the lithium-ion source. The usage of an aqueous electrolyte in cell cells containing lithium is not possible since lithium reacts with water. Typically the electrolyte is composed of a mixture of organic solvents to get the best performance. The most common solvents used are a combination of ethylene carbonate (EC) and dimethyl carbonate (DMC) with lithium hexafluorophosphate (LiPF6) as the lithium containing salt. This solution has a good combination of temperature stability, good ion conductivity, good wetting ability and no co-intercalation. In the first LIBs propylene carbonate (PC) was used as an electrolyte, a remainder of the short-lived LMB period, having an even better lithium-ion conductivity. This however was causing quick anode degradation when the solvent molecules undesirably intercalated into the graphite together with the lithium ions [18]. The electrolyte also plays a major role in the formation of the SEI layer, that impacts the cell performance as well as the cell degradation and will be explained in detail in chapter 2.2.1.

The **separator** is an element in any electrochemical cell that is physically separating the the two electrodes whilst electrically isolating them against each other. At the same time the separator must be permeable for the ion current inside the cell, which in most cases means a porous separator that is permeable for the electrolyte. In LIBs the most common separator is a polymer membrane, e.g. made of polypropylene (PP) or polyethylene (PE). The desired properties of the separator are a low weight, high thermal and mechanical stability and low material costs [20].

#### **2.1.1 Charging and Discharging**

In a fully charged LIB all lithium ions are intercalated inside the anode. Intercalation means that the ions are positioned in between the graphite layers, but no conversion reaction has taken place. It is described by a reversible process that is keeping the structural integrity of the intercalation host material [21]. The fact that there are no chemical reactions that directly transform materials is one factor of the high lifetime of LIBs. Since there is no new compound forming, the stress due to rearrangement of the material during the charging and discharging process is very low. The graphite is increasing in size by less than 10% during the lithium-ion intercalation [22]. When the battery is discharged the lithium ions de-intercalate from the anode by releasing one electron, becoming Li+, and move through the electrolyte to the cathode where they intercalate into the metal-oxide to form a lithiummetal oxide. The LIB has "lithium ion" in its name for the fact that there are only intercalations of lithium ions involved on both sides of the cell reaction, regardless of charge or discharge [23]. The total cell reaction in the discharge direction can then be described as:

$$\text{Li}\_x\text{C}\_6 + \text{Li}\_{1-x}\text{MeO}\_y \rightleftharpoons \text{H}\_6 + \text{LiMeO}\_2\text{O}$$

Where the x describes the ratio of lithium remaining within the anode, this number is equivalent to the state of charge (SOC). The graphite can have different lithiation states that can be seen in plateaus in the open circuit voltage (OCV) [24]. The Me is representative for a group of metals that can be used in the cathode.

During the charging process, this reaction is simply inverted and the lithium ions are moving from the cathode to the anode, where they intercalate into the graphite and take in one electron.

Since LIBs are very sensitive to over- and undercharging it is very important to stay within the voltage limits for the given material combination. For this reason the most common choice of charging method for LIBs is the constant current constant voltage (CCCV) charging method. Using this method the cell is charged with a constant current until the end-of-charge voltage is reached (typically 4*.*2 V) and then that voltage is held during a constant voltage phase while the current drops. When the current reaches a minimum current threshold the charging process is finished. The minimum current is necessary due to self discharge and parasitic currents because of which the charging current can never reach zero in a real application. Figure 2.2 shows the voltage, current and SOC curves for CCCV charging, where *Ich* is the charging current and *Ithres* the threshold current at which the charging process is terminated. *Vmax* and *Vmin* denote the end-of-charge and end-of-discharge voltage window of the battery, respectively.

**Figure 2.2:** Voltage, Current and SOC curves of a typical CCCV charging process

During the CC-phase the majority of charge is put into the battery and the SOC rises linearly. In most cases the battery is charged to around 80% SOC in the CC-phase. The CV-phase then charges the remaining amount to 100% SOC. The CC-phase makes up around half of the total charging time. For this reason a lot of fast-charging methods only use the CC-phase for charging and don't use the full battery capacity.

#### **2.2 Anode**

Graphite is currently the most used anode material for lithium-ion batteries due to its high practical capacity and low potential vs Li, enabling it to store a high amount of energy, and a rather flat discharge curve, giving a constant power output over a big range of the capacity [21]. The anode of a commercial LIB as of today is in most cases made of a slurry of graphite (C6) with added binder and carbon black that is pressed on a copper foil. The binder used is typically polyvinylidenfluorid (PVDF) [25]. The carbon black is added to increase the electric conductivity of the active material. The carbon has the shape of particles with varying sizes depending on the producer. Differently sized particles can have different effects on the cyclelifetime and other parameters of the battery performance, like the available power density.

The anode is typically oversized by around 10% compared to the cathode to prevent lithium plating at the edges [3, 26]. If the electrodes had the same size local overcharging (or overcharging in general, explained in more detail later in chapter 2.5) caused by inhomogeneous capacity losses of the anode could lead to lithium plating in these regions. The increasing field strength at the edges of the anode could also lead to localized lithium plating [27]. This area is sometimes called the anode overhang [28].

**Figure 2.3:** Schematic view of one graphite particle inside the anode active material

A graphite particle consist of basal and edge planes. The edge planes are perpendicular to the layer direction of the graphite crystals. They provide an easy entry point for the lithium ions to intercalate into the graphite. This is why most of the lithium-ion insertion and diffusion into the active material starts from the edge planes. The basal planes are along the layer direction of the graphite crystal and lithium ions cannot intercalate across a basal plane into the graphite, which is why they are the less reactive areas of the graphite particles. The ratio of edge planes to basal planes has a high impact on the electrochemical performance of a given graphite material [29]. The same way the orientation of the particles has a similar influence, with less oriented graphite generally having a worse performace due to a more difficult diffusion for the lithium ions [3].

In the charged state the lithium ions are intercalated into the graphite, located between the graphite planes. Lithium can naturally diffuse into graphite and is evenly distributing in the anode active material over time after the charging is finished. The resulting material of graphite with intercalated lithium ions is called a graphite intercalation compound (GIC) that is characterized by a mixed ionic and electric conduction [21].

The volume expansion of graphite (C6) compared to fully lithiated graphite (LiC6) is approximately at about 10% to 13% when the lithium ions inhabit the free lattice spots within the graphite [21, 30, 31]. This effect is quite low compared to other materials, but still leads to mechanical stress inside the cell during cycling.

#### **2.2.1 The SEI Layer**

The SEI layer does not only have a huge impact on the cell performance, it also is the main reason for cell ageing in LIBs [4]. The SEI is a thin layer on the anode surface that forms due to electrolyte decomposition. There is no electrochemically stable electrolyte for the operating voltage range of a LIB, so a decomposition is unavoidable. Due to the high reactability of water with lithium an aqueous electrolyte is no option in the current state LIBs, only organic electrolytes can be used. The working voltage window in the battery will either cause electrolyte decomposition at the anode or cathode at the respective voltage values of a fully charged of fully discharged cell [30, 32]. The stability window for most of the commonly used electrolyte parts (e.g. EC,DMC) is from around 0*.*8 V to 4*.*7 V vs Li/Li+ [33, 34]. This window is further decreased by the cell temperature leading to electrolyte degradation at the cathode as well for elevated temperatures [35, 36]. As most commercially available LIBs operate from around 0*.*2 V vs Li/Li+ when fully charged to 1*.*7 V vs Li/Li+ when fully discharged, the decomposition takes place on the anode side at high SOC. In the decomposition process not only the components of the electrolyte are consumed but also the lithium ions present in the electrolyte. This reduces the total number of available lithium in the cell and in consequence the capacity of the cell. This addition of lithium ions into the SEI enables lithium ions from the electrolyte to pass through it to reach the anode surface. Without the lithium ions in the SEI the cell could not function since the lithium ions from the electrolyte could not reach the anode surface to intercalate. The decomposition reaction can also take place at voltages inside the stable window, but in this case it was shown that the reaction is reversible since the electrolyte molecules stay intact [37].

Since the SEI is electrically insulating [38, 39], the growth of the layer should in theory stop after its initial formation. A freshly formed SEI has a thickness in range of nano meters [29]. In a real cell however there is always a small current due to electron hopping leading to a slow but constant SEI growth over time. This growth is accelerated with higher temperatures. A thicker SEI is going to have a higher ion flow resistance and will thus increase the total internal cell resistance. The thickness and resistance of the SEI is reported to vary over three orders of magnitude, depending on the system and the test conditions of the experiment [40]. During cell cycling the SEI can also be damaged due to the swelling and contraction of the anode during the lithium intercalation and de-intercalation, facilitating a reformation of the SEI at newly exposed anode surface. There can also be co-intercalation of other electrolyte components and gas evolution during the decomposition process causing extra stress on the SEI, with the highly flammable gas additionally posing a safety risk [37, 41] and causing other ageing effects to occur by facilitating inhomogeneous current distributions and increased pressure in the cell [42, 43]. Due to the fact that the decomposition on the surface of existing SEI is different from the reaction on the anode surface, the mechanisms change with a higher coverage of the electrode surface [32]. The growth of the SEI not only gradually reduces the capacity of the cell but also has a major impact on the self-discharge characteristics, achievable power density and safety of the cell [29].

These effects lead to a number of desired parameters that can create a stable SEI that can lead to a long cell life. The SEI should be as thin as possible while at the same time being highly electrically insulating to prevent further decomposition. Even if the material composition of the layer is technically electrically insulating, it needs to be thick enough to block electron tunneling [30]. But it should also be permeable selectively for lithium ions for a low cell internal resistance [42]. The layer should also be chemically inert towards all other cell components at a wide range of potentials and temperatures to prevent dissolving of parts of the SEI back into the electrolyte. To compensate for the change in anode volume during cycling it should also have a good mechanical stability to prevent breaking and subsequent reformation [29, 32]. The fracturing of the SEI is even reported to have a bigger effect on the capacity fade than the cracking of the underlying graphite particles due to the fact that it has a higher tendency to fracture [44].

To achieve these desired SEI attributes, the cells are normally charged under special conditions by the manufacturer during their first charging cycles. This process is called cell formation. Before the formation process a full coverage of the anode with the electrolyte has to be assured. This is done by the wetting process, which is typically a prolonged period of resting time (sometimes at elevated temperatures) following the electrolyte filling. During the first cycle of the formation 10%–20% of the whole cell capacity are typically consumed for the SEI generation [26, 30, 45]. The factors that the manufacturer can directly control during the formation process are the applied current, the different cut-off voltage levels and the temperature. These all can have a strong influence on the parameters and stability of the formed SEI, creating different microstructural and chemical properties that can lead to a different long-term chemical and mechanical performance [29]. The current during the formation is typically kept at very low C-rates, ranging from C/5 to C/20 [46]. Most of the SEI is theorized to be formed at 0*.*2 V to 1 V vs Li/Li+ [29]. It was e.g. shown by [47] that the SEI forms in two steps at 0.25V and 0.4V vs. Li/Li+. Other parameters that can be adjusted in cell design that also influence the SEI formation are the graphite type, graphite morphology, electrolyte composition and electrolyte additives, but also factors like particle size of the graphite, basal-to-edge-plane ratio, pore size, degree of crystallinity, Li+ diffusion rate and chemical composition of the surface (e.g. impurities) can affect the SEI composition [29, 37, 48]. A commonly used additive is vinylene carbonate (VC), preventing exfoliation during lithium intercalation in the beginning of the SEI formation [49, 50, 51]. Using bigger particles can lead to a smaller capacity loss during SEI formation since the surface to volume ratio is lower in this case, but these particles are also more prone to exfoliation [29]. The whole wetting and formation process can take up to 0.5 to 2 weeks [46] and needs dedicated cycling capacities for each single cell, making it one the most important bottlenecks of cell production.

The resulting layer has a thickness in the range of nanometers, with reported values going from 3 to 120 nm [29, 37, 39, 48, 52, 53], other publications however also report widely varying thicknesses in the micrometer range [41, 48], Jalkanen et al. [54] even observed an increase in thickness in the range of 10 µm for cells cycled at room temperature and 45 ◦C, while the highest temperature of 65 ◦C in their experiment increased the thickness of the anode by 18 µm. Since the intercalation reaction happens on the edge planes of the graphite particles, the potential on these planes is higher than on the basal planes. This leads to a thicker and differently composed SEI on the edge planes [37]. The thickness of the SEI also varies with the position of the electrode, with the SEI generally being thinner towards the center, and dependent on the type of aging, being thicker when it is caused by calendar ageing [55]. All these factors lead to a differing lithium diffusion constant throughout the SEI in different locations [48]. The pressure on the layer also influences the electron tunneling ability, with more tunneling when the layer is under tension and less tunneling if the layer is under pressure [30].

The chemical composition of the SEI layer is also highly dependent on the conditions under which the layer is formed. A loose organic SEI with rather low conductivity for the lithium ions is formed at higher charging anode potentials and more compact inorganic structures with higher ion conductivity at lower potentials [48, 56]. The charging rate on the first formation cycles on the other hand affects the porosity of the layer, with a more porous layer and higher resistance at higher charging rates. Water residue in the graphite can cause HF creation in electrolytes with LiPF6, which is the most commonly used salt in modern lithium ion batteries. The HF can then attack the SEI and cause a drop in performance of the LIB [29].

The overall structure of the SEI can roughly be described as a multistructure bilayer [29, 30, 32, 48]. The separator sites mainly contain a loose layer filled with electrolyte of inorganic lithium carbonates, organic lithium alkali carbonates and polymeric compounds on the side facing the electrolyte, while the graphite site is a more dense layer of LiF, Li2O and Li2CO3 [57, 58]. Figure 2.4 shows the schematic structure of the SEI. Between these two phases a mixed layer can be found. In general it can be said that the SEI compounds closer to electrode are reduced stronger than the compounds in contact with the electrolyte [37]. On average more than half of the SEI is composed of LiF and Li2CO3, the other half is composed of other lithium decomposition products and organic components like ROLi or ROCO2Li. In these compounds the R stands for an organic rest created by the electrolyte decomposition. Compared to the edge sites, on the basal site a higher concentration of lithium carbonates can be found on the electrolyte site, and a lower content of LiF [57, 58]. For the SEI on a lithium metal anode Fang et al. [59] showed that the biggest part of the SEI was composed of Li2CO3 and Li2O using XPS and high resolution transmission electron spectroscopy (HRTEM). The whole composition on the basal sites is also less organic [37]. To prevent the loss of the SEI due to re-dissolution into the electrolyte [32], it is desired to create a layer mainly composed of inorganic components like Li2CO3 rather than ROLi or ROCO2Li. The ROCO2Li can react with traces of CO2 or H2O present to form lithium carbonate [60]. Hao et al. [61] also notes, that the lithium ion concentration in the SEI highly fluctuates due to an heterogeneous SEI structure. The micro structure is also reported to contain nano-crystalline domains of 5-20 nm in size manly consisting of Li2CO3 and Li2CO2 [48].

In some cases elements from the cathode can cross over to the anode and cause SEI growth, as shown for Mn2 + ions [62].

**Figure 2.4:** Schematic of the theorized SEI structure

The analysis of the SEI is very hard due to its very small layer thickness and sensitivity to oxygen and moisture [29]. To get a deeper understanding of the SEI a number of different measurement and modeling approaches have been used, including initio molecular dynamics (AIMD) simulations [63] to get insight into the reaction dynamics, Shinagawa et al. [52] using a monte carlo simulation to determine the relationship between the layer thickness and its lithium ion conductivity, a mathematical model proposed to describe the mechanical stress on the SEI and subsequent fracturing during cycling [44], while atomic force microscopy (AFM) was used in [48] to measure the thickness and coverage of the SEI.

The continuous growth of the SEI layer over the lifetime of the cell depends on a lot of factors. Firstly a higher cell temperature increases the growth rate since it increases the reaction rate of all chemical reactions within the cell. High temperature can also lead to a change in the composition and morphology of existing SEI [34]. Secondly mechanical stress can lead to the breaking and subsequent reformation. This is dependent on the charging speeds and flexibility of the SEI. And lastly the aforementioned crossing over of cathode metals that can influence the chemical reactions within the SEI.

Stiaszny et al. [64] and Fu et al. [65] formed the theory that there is a second layer on top of the SEI that is permeable for all components of the electrolyte but leads to an increased ohmic resistance. It is assumed to have a similar chemical composition to the SEI but only has the ionic resistance of pure electrolyte [65]. This layer can grow into the separator pores and also increase the electrolyte resistance within the separator, further increase the total cell resistance. This would also explain why the separator often sticks to the anode surface when disassembling aged cells.

The SEI layer also has a profound impact of the plating behavior of cells. The thickness and ion conductivity of the layer directly influences the potential drop at the anode surface, which is the main driving force of the plating process. The mechanical properties of the layer can influence the morphology and shape of plating, i.e. whether the plated lithium metal forms as a homogeneous layer or in the form of located dendrites, that can cause potential risk to the cell safety [66].

#### **2.2.2 Lithium Diffusion**

The lithium diffusion inside the graphite is one of the key drivers of lithium plating. Within the cell there are two different diffusion processes for the lithium ions. One is the diffusion through the electrolyte, which is in a magnitude of 0.4 to 1*.*4 × 10−5 cm2 s −1 [67]. The second part of the diffusion is the solid state diffusion in the graphite particles occurring when the concentration of lithium ions is non-homogeneous within the anode. This happens whenever ions are removed from, or added to the anode, i.e. during charging and discharging, since the transfer of the ions from the electrolyte to the anode is faster than the distribution of the ions inside the active material. The ions are capable of moving along the vertical direction between the graphite lattice layer since the bonding between the layers is comparably weak, leaving a gap of 3*.*35 Å in which the ions can move and intercalate [21]. Since the magnitude of the solid state diffusion is much lower than the value for the electrolyte [68] the diffusion inside the graphite is one of the limiting factors to reach high charging speeds and also the key factor for the occurrence of lithium plating. The value of the diffusion for the solid state phase varies strongly from publication to publication, spanning everything from 1×10−15 to 1×10−5 cm2 s −1 [68, 69]. The main cause of this big difference stems from the fact that the diffusion rate is dependent on multiple different parameters.

Firstly, the diffusion is dependent on the temperature, with lower diffusion rate at lower temperatures, that can be described by the Arrhenius equation [68, 70]. The diffusion of lithium ions starts at temperatures well below 0 ◦C, with different literature values of 230 K in [71] while [72] could observe diffusion of a lithium layer into highly oriented graphite at temperatures as low as 100 K. Secondly, it is also dependent on the SOC of the anode, i.e. the amount of lithium that is already intercalated into the [69, 73], although this effect is described to be of lower influence than the effect of the temperature [74, 75]. This make sense as with a higher intercalation depth, the mobility of the lithium ions is hindered by lithium-lithium interactions [69] with the diffusion coefficient having multiple minima over the intercalation levels that can have a difference of up to 2 orders in magnitude [6]. Furthermore the distribution of the lithium ions inside the graphite is not smooth but rather in fixed intercalation levels, called stages. These stages change abruptly when the lithium concentration reaches a certain threshold and are visible in the charge and discharge curves in form of flat plateaus. The intercalation levels are the following [76]:

$$\text{LiC}\_{72} \longleftrightarrow \text{LiC}\_{36} \longleftrightarrow \text{LiC}\_{27} \longleftrightarrow \text{LiC}\_{18} \longleftrightarrow \text{LiC}\_{18} \longleftrightarrow \text{LiC}\_{12} \longleftrightarrow \text{LiC}\_{6} \tag{2.1}$$

The evolution of these stage structures also influences the diffusion coefficient [21]. Another point is that the diffusion along the edge planes is much higher than along the basal planes [7, 69], with the energy barrier for edge plane diffusion being at 0.3 to 0*.*7 eV while the basal plane is one order of magnitude higher with 10 eV [7]. The barrier on the basal plane can be lower however in the presence of defects [77]. Thus the edge to basal rate, particle size and particle orientation of the graphite have a big impact on the diffusion rate as well. This is also true for defects and along the grain boundaries. Here the diffusion is also severely hindered [69]. Lastly, these parameters are also interconnected, with Umegaki et. al [71] showing that the temperature dependency is different for different intercalation stages.

All these influencing factors show that the diffusion coefficient can be vastly different even within the same cell, when particle orientation, size, local temperature or local defects can differ. Thus the lithium plating process, directly affected by the diffusion coefficient, is highly localized on the anode surface and requires a spatially resolved analysis for deeper understanding.

Lithium metal on the surface of the anode, as it is caused by lithium plating, also diffuses into the anode over time. This makes it hard to analyze the plating effect since the depositions are not stable over time. While the diffusion at −20 ◦C can take 13 hours or longer, the diffusion at room temperature can be six times as fast [78].

#### **2.3 Cathode**

The most commonly used metals used in the cathode are nickel, manganese and cobalt. Since all of these metals have different beneficial effects on the cell performance, a mixture of them can create a good compromise that can achieve the best aspects of all the possible metals. Thus most cells today use a Li(Co*x*Ni*y*Mn*z*)O2 mixture, where the fractions of the used materials can vary. These cells are called NMC-cells. Research is aiming at reducing the amount of cobalt in the cells since it is not only the most expensive of the used materials in the cell, it is also mined under questionable conditions for the workers as well as the environment [1]. Apart from these metals there are also lithium-iron-phosphate cells that are cheaper, have a long lifetime and comparably low environmental impact [20] but the downside of a lower energy density, making them less suitable for mobile applications.

Because of the fact that the anode is oversized compared to the cathode, the cathode is never fully charged again after the initial cycle [34].

The influence of the cathode on lithium plating is very small. The only effect that the cathode can have on lithium plating is if the capacity of the cathode is bigger than the anode capacity, e.g. caused by strongly differing ageing rates, and due to manganese cross-over that can influence the parameters of the SEI. This crossing over effect is explained in more detail in the next section.

#### **2.4 Ageing**

In general ageing of all cell components can be divided into calendar ageing and cyclic ageing. Calendar ageing describes the chemical degradation over time due to unwanted mostly unavoidable side reactions. Cyclic ageing on the other hand describes the stress on the cell components during cycling, i.e. in case of a LIB when the lithium ions are shifted from anode to cathode and vice versa. For calendar ageing the main influencing factors are the temperature and the SOC of the cell. The result of battery ageing is typically loss of usable capacity and an increase of the internal resistance, leading to a lower available power density [5]. Depending on the occurring ageing mechanisms, there can be primarily capacity loss, primarily an increase in internal resistance or both. The capacity fade caused by cyclic ageing under normal conditions has shown a linear trend in most cases but in some cases a square root dependency on the charge throughput was observed as well [79, 80, 81, 82].

For the anode the main ageing mechanism is the formation, growth and reformation of the SEI layer that forms on the surface of the graphite particles. This layer consumes lithium ions, reducing the available lithium inventory and at the same time increasing the cell impedance. Lithium loss in the cell is normally mainly attributed to the SEI layer growth [14]. A more detailed description of the ageing mechanism can be found in chapter 2.2.1, since this layer also has a big impact on the lithium plating effect [5].

Another ageing effect is the mechanical stress the anode receives during charging and discharging due to the change in volume of the cell. While the volume change of around 10 to 13% is rather small prolonged cycling can lead to contact loss between the graphite particles, the active material and the current collector and to a change in porosity of the anode material [4]. Other sources of mechanical stress can be the co-intercalation of solvent ions that can cause graphite exfoliation and gas evolution during electrolyte decomposition [5]. These mechanical ageing effects can affect the capacity, due to loss of active material or contact loss with the active material, or internal resistance due to longer diffusion paths [4]. Over time the pores of the anode material can also be clogged with solvent decomposition products, leading to a higher charge transfer resistance [3].

The strongest effect on the capacity loss has the lithium plating, explained in more detail in chapter 2.5. Here an unwanted deposition of metallic lithium on the anode surface can happen under specific conditions, including charging at low temperatures and high charge currents [4].

Overall the capacity fade even under the same conditions is not necessarily linear with time and the shape of the capacity loss function can change depending on the ageing conditions [82]. While the cathode of a LIB also plays a major role for the performance of the cell, the effect of the cathode on cell ageing and especially lithium plating is low compared to the anode. Lithium plating does not happen on the cathode side since the potential of the cathode is very far away from the potential of metallic lithium. One noteworthy ageing mechanism that also affects the SEI growth on the anode side of cell and thus can have an impact on lithium plating is the transition metal dissolution and migration [3, 4, 5, 34]. Here the metal ions of the cathode can be dissolved into the electrolyte and then move over to the anode surface, where they can take part in the chemical processes at the SEI and change the structure, morphology and growth rate of the SEI, often leading to a higher capacity loss and self discharge. This effect is most pronounced with cathodes containing manganese and has a considerably lower impact with cobalt-nickel mixed oxides [83, 84]. This metal dissolution process is intensified by high operation temperatures and HF traces in the cell caused by remaining moisture from the cell productions reacting with the lithium salt [4, 43, 85, 86]. On the cathode surface an SEI similar to the anode SEI can form at the end of charge [34].

The impact on the cell performance of the cathode layer however is much lower on the cell performance. This layers can be isolating and thus lead to an in-homogeneous current distribution in the cathode [3] though.

The cathode is also similarly to the anode subject to mechanical stress during the insertion of lithium ions, which can cause crystal distortion [5].

The overall ageing process in NMC - graphite cells is reported by [87] to take part in two stages. The first stage is loss on lithium due to film growth, mainly SEI on the anode surface. The second state starts at very low SOH of 50% or less, containing active material loss and cathode metal crossing over.

#### **2.5 The Lithium Plating Process**

Lithium plating is a special ageing process exclusively occuring at the anode of graphite anode LIBs [88] that can lead to strong capacity loss [7],increase of the cell impedance and a loss of coulombic efficiency [89], while posing a potential safety risk at the same time [7, 40, 88]. It describes the deposition of metallic lithium on the anode surface instead of the desired intercalation. This can happen since the anode potential of a fully charged cell is very close to the potential of metallic lithium [4], with a fully lithiated cell only having a potential of 85 to 100 mV vs. Li/Li+ [23] with the last phase transition from LiC12 to LiC6 observed to be at an even lower potential of 65 mV vs. Li/Li+ [70]. Thus if the potential at the anode surface drops further even by small amount, it is thermodynamically favoured for the lithium to deposit on the surface instead of intercalating into the anode material [7, 38, 90, 91]. Downie at al. [92] also found a drop at 28 mV in the dQ/dV curve, signalling the complete filling of all energetically favourable next-nearest neighbor sites between the graphene sheets, marking a point where the lithium ions have to find sites between turbostratically misaligned layers, which is causing a sudden potential drop, bringing the potential closer to the plating onset potential. Although in literature the reasons why this potential drop occurs are various, mainly mentioned are high charge currents [6, 23, 70, 88, 91] and low temperatures [6, 7, 70, 88, 91] resulting in slow solid state diffusion speeds, but the list is also containing mismatched electrodes [93], cell pressure [14, 94, 95], local defects [14, 49, 91, 94, 95, 96], a thick SEI layer [66], high charge voltages [23, 91] and overcharging [40] – showing that plating is dependent on the operation conditions and cell design parameters at the same time [23]. All the reasons are however interconnected and lead to the same outcome of a lower anode potential which is the actual reason for the plating process taking place. Whenever the potential anywhere on the anode drops below 0 V vs. Li/Li+ plating can occur. Figure 2.5 shows the interconnection of the parameters that facilitate plating conditions.

**Figure 2.5:** Conditions that can lead to lithium plating

The polarization of the anode causing this low potential is caused by an ohmic drop due to the charge tranfer resistance of the SEI, the charge transfer overpotential of the electrochemical double layer on the anode surface and the diffusion overpotential [97]. The creation of the overpotential occurs instantaneously under plating conditions, only slightly delayed by the double layer capacitance time constant of a few milliseconds [70]. Baker and Verbrugge [98] however assume that small amounts of metallic lithium always coexist with the intercalated species, even at potentials well below the plating onset of 0 V vs. Li/Li+, caused by the so called under potential deposition. At the same time Verbrugge [99] showed that negative potentials for short periods of time do not necessarily have to lead to plating. It has to be kept in mind that plating can occur even when the SOC of the cell is not very high, since the the potential drop just has to be high enough to cause the potential to go lower than 0 V vs. Li/Li+. This can be caused by a very high current or by the fact that the diffusion speed is so low that the surface of the graphite particle behaves like a fully charged cell, while the active mass bulk is not lithiated to the same level. This was shown by Birkenmaier et al. [91] where plating was detected using changes in the cell thickness in a cycling range of 40–60%. In most cases cells are charged using the CCCV charging method. During the charge most of the plating happens in the CV phase since the cell has reached a very high potential and the anode potential is closest to 0 V vs Li/Li+ in this phase [100].

The most reliable way to produce plating in LIBs is charging at low temperatures. Discharging a LIB at low temperatures can slightly reduce the available capacity with no further negative effects on the cell while charging under the same conditions will likely cause lithium plating [6]. Being a distinctiveness of lithium plating compared to regular ageing mechanisms like the SEI growth, plating becomes more severe the lower the cell temperature. For the other ageing mechanisms normally an Arrhenius behaviour regarding the temperature can be seen, with higher temperatures increasing the reaction speed of unwanted chemical side reactions inside the cell and thus accelerating ageing [7]. The reason why a lower cell temperature promotes plating is the lower solid state diffusion speed within the anode graphite at these temperatures compared to normal operating temperatures [6, 7, 101]. If the influx of lithium ions intercalated into the anode is higher than the ability of the ions to distribute within the graphite via diffusion, the surface of the anode reaches high lithiation and thus a potential close to 0 V vs Li/Li+ very fast. A high current has the same effect of increasing the influx of lithium ions, but at the same time the high current also increases the overpotential, pushing the potential into the plating region even faster. There are observations in literature that plating can occur as high as 10 ◦C, but the interconnections between the temperature of the cell and the charging current can make it very hard to identify any strict rule at which temperature plating starts. It is also very hard to reliably identify the occurrence of plating during operation at normal operation temperatures, since the diffusion of the deposited lithium into the anode can occur very fast in this temperature range. The charging current is increasing the overpotential at the anode due to the potential drop at the SEI layer, thus also favouring plating, but at the same time higher currents lead to higher ohmic heat losses in the cell increasing the overall cell temperature and thus hindering plating. For this reason the onset point of plating for current and temperature is heavily dependent on the thermal properties and subsequently on the cell geometry and size. Smaller cells have more surface area compared to their volume and because of this have more favourable thermal properties to expel any heat losses to the surroundings while bigger cells heat up more easily under the same conditions (similar cooling and C-rates). The dependency of plating on the charging current can severely limit the ability of the cells to fast charge and recuperate braking energy [7], which is becoming more and more important with an increasing amount of electrical cars on the streets.

In the same way the external thermal management of the cell influences the effect. A very effective external cooling mechanism can reduce the impact of the heat losses and also increase the likelihood of plating caused by higher currents. To investigate lithium plating it is very important to have a good cooling system for the tested cells during the experiments, because otherwise the measured cell surface temperature can be vastly different from the temperature inside the cell. This can also help to prevent strong temperature gradients inside the cell that can lead to different plating behavior at different locations on the anode. This is especially important when investigation methods based on external electrical measurements are used, since these can always only depict an averaged information about the cell state.

Increased pressure inside the cell can locally increase the electrochemical activity, leading to a higher local current. Fuchs et. al [95] applied pressure to a pouch cell using a steel ball and observed lithium plating especially at the borders of the pressure point. This effect is caused by an increased current at the edges of the defect that was already proved by Oldham in 1981 [102]. Since these kinds of localized effects often only have a marginal influence on the temperature, higher local currents can easily lead to plating without the usual hindering effect of higher temperatures. The same effect can happen when local defects due to manufacturing errors or mechanical stress are present within the cell. Inactive surface regions can still have reachable active material underneath, which causes a higher current at the borders of these inactive regions. These regions can be part of the anode surface, but the same behavior can be observed for defects in the separator, locally blocking ion flow between the electrodes, also causing increased current densities at the borders of the defect [96].

It has been observed early that plating often occurs at the edges of the anode possibly even under conditions that normally do not favour plating [23, 93]. This is due to the fact that the electrical field is stronger at the corners, again also leading to a higher current in these areas. For this reason, and to prevent anode overcharge, the anode in commercial cells is always oversized to disperse the excess ion influx to the overhang anode. This behavior however can still be found if the anode and cathode are not well aligned and the anode edge is within the cathode area, possibly due to a manufacturing error in laboratory cells where the alignment is often done manually. This edge behaviour can also be seen in defects in the cell that normally create inactive areas that are also having an edge around them locally increasing the electrical field. These defects could be a loss of contact between separator and electrode [103], local electrolyte drying, delamination of the current collector [4], clogged separator pores [96], copper plating caused by deep discharge [43], high local tortuosity variation [104] or simple defects caused in production [105] or due to mechanical stress [106, 107]. It was e.g. also shown by Matadi et al. [108] where gas generation was creating electrochemical inactive areas in the cell during high temperature calendar ageing tests, causing lithium plating at the edges of these areas in subsequent checkup tests. Cannarella and Arnold [96] created artificial defects in coin cells observing the same effect and also found that the impact of the effect is decreasing after a defect size of less than 300 µm and completely disappears for defects smaller than 100 µm. The locations of these spots in commercial cells in applications are however hard to predict since they can be dependent on the design parameters of the cell as well as the cell orientation while testing [7].

While not common in commercial cells, overcharging can also be the cause of lithium plating. When the anode is fully saturated with lithium the remaining lithium has to deposit on the surface instead. This can happen when parts of the anode become electrochemically inactive due to ageing while the lithium ions are located in the cathode material or when the cathode is oversized compared to the anode [40]. Under these circumstances there is an overabundance of lithium ions that can not completely intercalate into the anode. This can be expressed in the ratio of anode capacity to cathode capacity [23]. For this reason the electrode balancing is an important factor in increasing the cell life, the anode capacity must under all circumstances be higher than the cathode capacity [40, 109, 110]. It was already shown by Arora et al. in 1999 [40] that plating starts earlier when there is no excess negative electrode.

This size and shape of the cell also influences the plating locations and onset since longer diffusion ranges have a higher probability of showing inhomogeneities along the way leading to an unbalanced current distribution [89]. But even when the cell is not fully charged, a high SOC is promoting plating since the potential is closer to the critical 0 V vs Li/Li+ and a lower overpotential can still be causing plating compared a to lower SOC.

Another influence can be found in the material properties of the anode material. Waldmann et al. [111] found that the temperature dependency of the anode potential is different for graphite compared to amorphous carbon. The deposition is also influenced by the anode kinetics, which depend e.g. on the tortuosity and porosity of the anode material [7]. The values for tortuosity and porosity are found to be linked to each other [112] and can have a dependency on the material orientation as well [113].

Bach et al. [14] also found that in cells containing manganese in the cathode (typically NMC) the areas covered by plating contained a much higher amount of manganese crossed over from the cathode than the non plated areas. They found concentrations of up to twelve times the base line manganese content in areas with mossy lithium structures. This shows that manganese has an influence on the SEI composition as shown in chapter 2.2.1 and thus also indirectly influences the plating behaviour. Arora et al. [40] showed that bigger graphite particles are more susceptible to lithium plating since the diffusion length in those particles is longer and thus can more easily become the limiting factor. Smaller particles exhibit a larger surface area however and lead to more capacity loss during the formation. The same is true for a thicker electrode where the limitations of the solid state diffusion can lead to plating more easily than in thinner electrodes [40]. This means that high energy cells are more prone to plating than high power cells.

During plating often a loss in coulombic efficiency can be observed that is linked to the loss of active lithium and electrolyte due to SEI formation on the new exposed lithium metal surfaces that can break through the existing SEI [114]. For lithium metal batteries some researches suggest that it could also be caused by an increased tutuosity between the electrode and the electrolyte [115, 116].

To initiate the plating process an additional overpotential needs to be overcome at the anode surface which is based on the nucleation barrier of the lithium ions [97]. The effect is theorized to start underneath the SEI layer since the lithium ions need an electron to deposit as metallic lithium and the SEI has very low to no conductivity [38, 39, 117], as discussed in chapter 2.2.1. An additional reason why it is unlikely for the plating to deposit on top of the SEI is the fact that the potential on the SEI surface is higher than on the anode surface, since the SEI itself poses a resistance to ion movement in the cell. This means that the potential is less likely to be at the plating condition of 0 V vs Li/Li+ on the SEI surface compared to the anode material surface. For this reason the SEI plays an important role in the way the lithium is deposited and how severe the capacity losses caused by the plating are. Hein and Latz [88] showed in simulations that the plating process starts at the particles on the surface of the anode and only spreads into the bulk of the material over time. Even when plating is taking place and metallic lithium is deposited on the anode surface, intercalation of a portion of the ions is still occurring at the same time. These two processes are always overlapping [7, 23] as seen in figure 2.6 on the left side. When the deposition begins however the deposition part quickly starts to dominate the reaction and the intercalation current is strongly decreased [40].

**Figure 2.6:** The plating process during charging (left) and the subsequent discharge after plating (right)

The ratio of the intercalation current and the plating current can be described using [70]

$$I\_{total}(\eta) = I\_{intercalation}(\eta) + I\_{planting}(\eta)$$

Where both of the currents are defined by a Butler Volmer equation

$$I(\eta) = i\_0 A \left( \mathbf{e}^{\frac{\alpha nF}{RT}\eta} - \mathbf{e}^{-\frac{(1-\alpha)nF}{RT}\eta} \right).$$

With the electrode surface area A, the transfer coefficient *³*, gas and faraday constants R and F, the temperature T, the overpotential *¸* and the exchange current density *j*0.

For the intercalation current the exchange current density is dependent on the graphite Li concentration *cs* and the electrolyte Li+ concentration *cl* as well as the maximum concentration of lithium in the graphite *cmax* [118].

$$j\_0 = k(cl)^\alpha (c\_{max} - c\_s)^\alpha (c\_s)^{(1-\alpha)s}$$

Where k is a rate constant and (*cmax*−*cs*) describes the number of available intecalation spots.

The current density of the plating process on existing lithium is strongly dependent on the surface structure of the already plated lithium, with freshly deposited lithium facilitating the highest current density. This density can vary by two orders of magnitude [119].

The deposited metallic lithium has several effects on the cell. First of all, since the volume of the metallic lithium is bigger than the volume of the intercalated specimen, the cell swells, increasing the pressure on all cell components. Secondly, since the deposition is underneath the SEI, it becomes deformed or stretched. Depending on how flexible the SEI layer is and how much and how fast the lithium is deposited, it can break or rip open, exposing the surface of the plated lithium to the electrolyte. The potential of the metallic lithium is even lower than that of LiC6, thus the electrolyte decomposition forming the SEI is also occurring on the surface of the metallic lithium when exposed to the electrolyte [120]. This leads to additional SEI formation on the lithium surface causing loss of electrolyte and additional loss of lithium ions [70]. This newly formed SEI layer also has a different chemical composition since it was formed on metallic lithium rather than the anode graphite [66] and can isolate lithium metal grains that become inactive lithium as a consequence [70]. Because of the importance of the SEI the electrolyte can have a strong influence on the plating effect [6, 23], first visible in the change of the coulombic efficiency during the SEI formation [66]. The main potential drop in the cell is attributed to the SEI layer at the anode-electrolyte interface, with the electrolyte itself having a much lower ohmic resistance [66]. But since the composition of the electrolyte has direct influence on the structure of the SEI, the electrolyte has an indirect influence on the SEI resitance as well [23]. This additional consumption of electrolyte can cause a cell to dry out much faster compared to a cell operated under non-plating conditions and the extended SEI growth can clog the pores thereby decreasing accessible surface area or cause contact loss [54]. The electrical conductivity of a pure lithium film lies at around 10 × 105 S cm−1 while the regular SEI composed mostly of Li2CO3 shows a conductivity of 10 × 10−8 S cm−1 [40].

The lithium on the surface of the anode is not stable over time. If the cell is left to recover after plating, the lithium will diffuse into the graphite and eventually reach an intercalated state [88]. The time needed for the lithium to diffuse into the anode strongly depends on the temperature and can range from a few seconds at elevated temperatures to hours in a subzero environment. It depends on the solid state diffusion speed discussed in more detail in chapter 2.2.2. The second way the lithium on the surface can be removed is during subsequent discharges. When the cell is discharged after plating took place it behaves similar to a metallic lithium cell in the first discharge phase since consuming the metallic lithium is energetically favourable compared to de-intercalation. This effect is called lithium stripping and can be used to estimate the amount of plated lithium [88], it will be discussed further in chapter 2.6.1 taking a closer look at the influence of plating on voltage and current profiles during charge and discharge. During stripping however there is also a de-intercalation current, just like there is an intercalaton current during the lithium deposition, this can be seen on the right side of figure 2.6. Because of this superimposed de-intercalation current and the lithium diffusion that also still occurs at the same time, the amount of plated lithium on the surface will always be underestimated using the aforementioned method of using the stripping voltage profile. Gireaud et al. [94] observed, that the stripping preferably starts at the grain boundaries of a lithium metal surface, since these locations contain the highest interfacial energies and that there are different stripping mechanisms dependent on the current applied. It was shown for cells with metallic lithium that the stripping reaction can be stabilized by using different additives and increasing the cell pressure [17]. Fang et al. [59] also showed that in these cells the stripping rate can change the morphology of the plated lithium, but has almost no effect on the chemical composition of the SEI. They also noted a different effect of stripping depending on the present electrolyte. Experiments with metallic lithium deposition on copper also have shown that the corrosion of the lithium on the surface can not be fully stopped by the newly formed SEI on the lithium metal surface [66]. This means that on graphite parallel to the diffusion into the anode the lithium is also partly permanently lost due to corrosion. Since the deposition of lithium stretches the SEI layer, after either diffusion into the anode or stripping voids can remain between SEI and anode surface as visible in figure 2.7, further increasing the internal resistance of the whole cell and creating more current inhomogeneities than can lead to further plating.

Depending on the treatment of the cell after plating occurred, parts of the lithium can lose contact and become so called dead lithium that can not take place in the cell reaction anymore, but new SEI can still form on the surface of the dead lithium, possibly causing further electrolyte loss [7]. An example for dead lithium can be seen in figure 2.7. This reduces the available cell capacity. Dead lithium can be created when parts of the lithium loses contact with the lithium metal bulk and are then covered with newly formed SEI effectively electrically isolating that part of the lithium. There are cases however where lithium temporarily loses contact and can be recovered at the end of discharge due to the structural change within the cell during discharge [6]. The likeliness of this effect also depends on the morphology of the lithium deposition and other factors, like the discharge current. Petzl et al. [38] observed a dependency of the reversibility of deposited lithium on the SOC, showing high levels of reversibility for moderate SOC and low levels for high and low SOC, with the lower SOC also showing a higher temperature dependency. This dead lithium is also theorized to increase the cells vulnerability to a thermal runaway, increasing the safety risks of the cell [23]. Other macroscopic effects caused by the plating are a decrease in energy efficiency and an increasing cell resistance [7].

The morphology of the plated lithium is dependent on the applied current [121, 122, 123] and temperature [124]. The most important observed forms of deposition are shown in figure 2.8: dendritic growth, homogeneous plating and moss-like growth.

**Figure 2.7:** Effects caused by lithium plating

In case of the dendritic growth, the lithium forms very sharp tips that quickly pierce the SEI and can also bridge the separator causing short circuits [66] that can lead to safety hazards like thermal runaways [17] or degenerate cell performance [88] by increasing the self discharge rate and consuming big amounts of electrolyte due to SEI formation of their big surface area [95]. Orsini et al. [122, 123] showed that dendritic growth can be linked to a high current density. Since the tip of such a metallic needle-like structure produces a very high electrical field the further growth and lithium deposition is intensified there, creating a process of self-energizing growth [125]. These dendrites are most likely to grow in areas with defects and surface imperfections since the current distribution inhomogeneities are most focused in these areas [126]

The most common form is the mossy deposition [94] facilitated by low current densities [122, 123], also having a high surface are and thus high electrolyte consumption. Due to this high surface area the plated cell can have lower surface resistance than a newly fabricated cell [66].

**Figure 2.8:** The most common types of lithium plating morphology

Deposition around the edge area of the anode is called marginal deposition [7] and can be seen in pouch cells [127]. Another form of deposition is local lithium deposition, prominent for cell cycling at higher temperatures and high charging currents [54, 108, 124, 128]. Homogeneous lithium plating on the other hand is mostly found when cycling at low temperatures [38, 111, 124, 129, 130, 131, 132]. To distinguish between the part of the lithium that diffuses into the anode or can be stripped again during discharge and the permanently lost lithium, the part that can be reused is called reversible plating while the losses are irreversible plating. Ideally all of the deposited lithium could be reused, so that the goal can either be to completely suppress the plating effect in the first place or try to solely facilitate reversible plating.

The dead lithium and SEI formation induced electrolyte consumption are the two main reasons of the drastic capacity loss seen in most cases of lithium plating. Both processes reduce the lithium inventory in the cell, thus reducing the available capacity. However, the layer of dead lithium and newly formed SEI also influences the cell impedance [6], as will be seen in chapter 2.6.1 looking at the usage of EIS measurements to identify plating in cells.

The main safety risks involved in plating are the piercing of the separator due to dendrite growth, visible in figure 2.7, possibly causing a short circuit facilitating cell heating that can cause a thermal runaway of the cell and exothermic reactions caused by the dead lithium in cell [7]. Since the deposition of metallic lithium also causes increased pressure in the cell caused by the bigger volume of plated lithium compared to intercalated lithium, this increased pressure can also pose a safety risk to the cell, as the cell is not designed for these conditions. Another important safety aspect is the effect of lithium plating on the thermal runaway behavior of the cells. Fleischhammer et al. [133] found that cells cycled at −10 ◦C showed a lower temperature onset for a thermal runaway and self heating rate than cells aged under normal conditions, or at high C-rates. This means that lithium plating has a long term effect on the cell safety additionally to the immediate risk of dendrite short circuits inside the cell. They also found a temperature drop at 180*.*5 ◦C corresponding to the melting temperature of lithium, further pointing at lithium plating as the cause of the different onset behaviour.

Since the process of lithium plating is dependent on so many variables the capacity loss caused can be very unpredictable [6]. In some cases the capacity of the cell somewhat recovered when the cell was characterized at room temperature after plating cycles [7]. During the cycling process the lithium deposition process can be changing since the surface morphology of the anode is changing with each new plating cycle [66]. The capacity loss caused by plating can take different shapes. In some cases a linear capacity loss was observed [111, 133, 134] hinting at constant loss of available lithium in the cell, while Petzl et al. [130] saw a decelerated capacity loss, where less and less irreversible plating was caused each cycle. Koleti et al. [100] showed that cells with prior ageing conditions and thus a ticker SEI are less pron to plating, which is some what contradictory, since the potential drop at the thicker SEI should be higher. Some publications even link lithium plating with accelerated capacity loss and a sub-sequential sudden cell death after a certain density of lithium in the SEI is reached. These cells show a linear capacity loss behavior before suddenly ageing much faster after a certain capacity threshold is reached, normally in the range of 70 to 80% SOH [14, 79, 86, 135, 136, 137, 138, 139]. This effect was not seen in cells aged at elevated temperatures although plating was confirmed [54], leading to the assumption that this phenomenon is limited to certain kinds of deposition morphology. Anseán et al. [120] also showed that the plating in their experiment was not induced by a single event but by gradual cell degradation at normal conditions. They showed that the plating inducing condition was caused by a slow gradual loss of delithiated anode material, eventually leading to cell imbalance that facilitated plating. It is also suggested that the sudden increase in capacity loss is caused by local drying of the electrolyte that is consumed forming new SEI on the plated lithium [140] which then causes a defect area prone to even more plating. Bach et al. [14] supported these results by showing that the sudden increase in capacity loss is linked to heterogeneous ageing within the cell, in their experiments caused by varying pressure inside the cell. Broussely et al. [139] were able to link the sudden degradation to inactive lithium in the cell. While this effect is not really of importance for applications in their first life due to most cells considered at their end of life in this capacity range anyway, it makes re-use and second life behavior of the cells much harder to predict and also poses a challenge in accurate lifetime prediction. Bach et al. [14] saw almost not change in the charge and discharge profiles prior to the onset of the increased capacity fade, making the effect very hard to detect, especially when no information about the cell history is available.

### **2.6 Lithium Plating Detection**

In this chapter the different methods than can be used to detect lithium plating in situ as well as post mortem are discussed. The methods used in this thesis and discussed in more detail are electrical measurements, including EIS measurements and partly with reference electrodes as in situ methods, LSM and FIB imaging for optical post mortem analysis and XPS for post mortem material analysis.

The rest of the methods are shortly explained and discussed regarding their potential and disadvantages to give broad overview but they have not been utilized in this thesis.

#### **2.6.1 Relevant in Situ Measurements**

In situ measurements allow the characterization of the cell without destroying it and can potentially also been applied while the cell is in operation, being called operando, able to show time dependent porcesses within the cell. In situ methods have the potential to be applied in a real world application if the measurement effort is not too high and no specialized equipment is required for the measurement.

#### **Electrochemical Impedance Measurements**

EIS is a technique used very frequently in battery research to characterize battery cells and get a non destructive insight into the processes of the electrochemical reactions. EIS is either applying an alternating voltage or current signal and measuring the response of the system. If the system is time-invariant and linear, this response can be used to create an impedance spectrum over different frequencies. The resulting spectrum can be interpreted to describe different chemical processes in the cell. A battery cell however is an inherently not time-invariant nor linear system so the amplitude of the imprinted signal must be chosen in a way that the system stays in the same state during the measurement and no change in SOC or temperature occurs in that time period. The lowest frequency measured must be chosen accordingly to prevent the cell from changing its SOC too much and thus become not time-invariant anymore [141]. This is necessary since the resulting EIS spectrum is dependent on the SOC and the temperature of the cell. The shape of the spectrum is also dependent on the ageing state of the cell. Figure 2.9 shows an exemplary spectrum with three different frequency areas. The high frequency (HF) area on the left side contains the information about the ohmic resistance in the cell. The point where the spectrum intersects with the real axis describes the true ohmic resistance in the cell caused by the resistance of the active materials, current collectors and the ion flow resistance of the electrolyte combined. The higher frequencies create an inductive part in the cell that is not of further interest in this work. In the middle frequency (MF) area the impedance describes the charge transfer resistance at the surfaces of the anode and the cathode and the resistance of the SEI. This area is of special interest in the context of lithium plating since the plating effect is influenced by and influences the SEI. The low frequency (LF) area contains a linear slope showing the solid state diffusion within the anode and cathode active materials. The actual frequencies for these areas strongly depend on the cells used in the experiment. For smaller cells the overall impedance is bigger and the intersection point with the real axis is shifted drastically to higher frequencies, often by one or more magnitudes. This requires very different test design when using laboratory vs commercial cells since the cells often have very different capacities.

**Figure 2.9:** Idealized EIS spectrum showing the three different frequency ranges: HF – high frequency, MF – middle frequency and LF – low frequency

The most common elements in EIS spectra of batteries are semi-circles with a negative imaginary part. These semi circles generally describe reactions taking place in different places of the cell. For most LIBs two partly overlapping semi circles can be seen, with each describing the reaction kinetics at one electrode. This can be used to distinguish between effects that happen at either electrode to e.g. find electrode specific ageing effects [54, 64]. It was shown for LiFePO4 cells for example that the cathodic influence on the SEI dominated semi circle can be neglected in the frequency range that is interesting for plating [142].

To better describe the spectra and be able to create a model based on them, they are often fitted with equivalent electrical circuits. To test whether a measured spectrum is valid, a relation called the Kramers-Kronig (KK)- Relation can be used. It connects all imaginary parts of the spectrum to the real parts and vice versa and can be used to verify that linearity and time-independence have not been violated during the measurement [86]. To identify and analyze plating using EIS first the other influences of SOC, temperature and SOH and the spectrum must be clear. Stiazny et al. [86] for example showed that the resistance of the semicircle that corresponds to the SEI becomes less resistive with increasing ageing while the total cell impedance is rising. Zhang et al. [143] observed that the charge transfer resistance is higher for low SOC and that it becomes the dominating part of the full impedance for low temperatures. It was also observed by Dolle et al. [144] that in their experiment to semicircles of the two electrodes became more distinct with increasing ageing. Jansen et al. [145] found that negative and positive electrode are equally increasing the impedance at low temperatures. Dees et al. [146] used a reference electrode to separately measure the resistances of both electrodes and observed that the positive electrode has higher impedance compared to the negative. At low temperatures often used for plating experiments the internal resistance increases drastically, with most of the impedance rise happening in the MF area [145], which shows that the most limiting factor for low temperature charging lies not in the lower conductivity of the electrolyte but rather in the charge transfer at the anode electrolyte interface. This effect is relatively independent of the exact active material in use. Because of that the effect also seems to be independent of the SEI since every material has its own type of SEI [145]. Wang et al. [147] on the other hand postulate the main effect for bad low temperature resistance stems from the SEI that, according to their measurements, has an increase of resistance by a factor of 30 from 25 ◦C to −30 ◦C while the electrolyte resistance only increases by a factor of 10 in the same temperature range. Barsoukov et al. [148] also showed that the spectra can change depending on the thickness of the active material, while Hsien et al. [149, 150] made the same observation to electrode alignment, making comparisons between different cells using EIS very difficult.

Using this general information about the different influencing factors on the spectra, Liu et al. [151], using information obtained from the spectra of the negative electrode, calculated an estimation of the lithium plating onset current for their cells. They used the resistance of the negative electrode to calculate the overpotential at the electrode electrolyte interface and postulated the onset of plating at the potential of 0*.*08 V vs Li/Li+ and could determine the onset current this way. This result also shows that cells with a lower negative electrode resistance are less prone to plating. They however did not include the diffusion overpotentials in their equation. Koleti et al. [152] found a difference in the impedance especially pronounced at the turning point in the spectrum between the last semi circle and the diffusion part for cells during the plating process. They specifically measured only this frequency (0*.*05 Hz) during the charging process and the cells that where subject to plating showed a decrease in the impedance while the non plated cells shows an increase instead. They where able to identify the point of plating onset as a function of the SOC using this measurement.

In summary, EIS measurements can be used to analyze plating but since the EIS signal that is imprinted onto the cell is charging and discharging the cell, it can cause plating or stripping itself during the measurement and must thus be controlled very carefully when measuring cells at low temperatures or cells with deposited lithium on the anode surface. It is also hard to measure the full spectrum of a cell a low temperatures since the required minimum frequencies get very low due to the low cell diffusion and very low EIS frequencies can cause the cell SOC to shift during the measurement violation that stationary condition of the cell.

#### **Electrical Measurements**

The easiest way for the in-situ or operando detection of lithium plating is via voltage and current measurements. These measurements are always available in some form in every real world application since every LIB battery pack needs a battery management system (BMS) and thus are a very good basis for plating analysis. The two main ways to identify plating are using the discharge voltage curve [6, 23, 38, 120, 141] or the voltage relaxation curve [141, 153] after a plating event. If the cell is discharged directly after a plating event the discharge curve will show a plateau in the voltage at the beginning of the discharge as visible in the upper plot of figure 2.10. This plateau appears since the stripping reaction that removes the metallic lithium deposition from the surface is energetically more favourable than deintercalation and thus is dominating the total reaction as long as there is sufficient metallic lithium present on the anode surface. The width of this plateau together with the applied discharge current can be used as an estimate for the amount of plated lithium. But it has to be kept in mind that during the stripping here will still be some diffusion into the graphite (as discussed in chapter 2.5) and even though the stripping reaction is dominating there will be a small part of deintercalation at locations where the lithium depositions are not present or are consumed earlier due to a thinner layer. This means that the amount of plated lithium will always be underestimated using this method [88, 153]. Hein et al. [88] also showed in simulations for different micro-structures that the width of the stripping plateau is also dependent on the spatial distribution and micro-structure of the anode, which is likely based on the effects of the micro-structure on the solid-state diffusion within the anode. The diffusion into the graphite is also dependent on the temperature, becoming much faster at higher temperatures, leading to an even higher error in the quantification and making it harder to use this method as a plating validation at room temperature [153]. It should also be kept in mind that the cell is also heating up due to ohmic losses during the discharge, pronouncing that effect even more. This method can also only show the reversible part of the plated lithium since the dead lithium that lost contact to the anode cannot be stripped. To better make the changes in voltage curve visible often the differential voltage curve *dV/dQ* is used for analysis, visible in figure 2.11 [141, 153]. Fan at al. [6] detected two peaks during discharge, with one of the peaks disappearing when the cell is heated prior to the discharge. They assume that the second peak is dead lithium that is isolated by the SEI but becomes reactivated at the end of the discharge due to expansion of the SEI. The second peak is also not visible during subsequent discharges following plating in the same cell and only appears at charge rates higher than 0.2C. Uhlmann et al. [97] detected a bend in the voltage curve of lithium half cells, indicating the start of lithium plating. It was observed that the stripping plateau becomes shorter which increasing cycle numbers under plating conditions [100], which means that the amount of reversible plating is reduced. That however not necessarily means that the amount of irreversible plating is reduced at the same time.

A similar effect can be seen when the cell is not discharged by observing the relaxation behaviour after a plating event. As visible in the lower plot of figure 2.10 there is also a plateau in the relaxation voltage. Under normal conditions during the relaxation the overpotentials of the ionic concentration gradients in the electrolyte and the concentration gradients of the intercalated lithium ions within the anode dissipate according to the time constants of those processes and the voltage approaches the open circuit voltage meaning an equal distribution of the intercalated lithium and the lithium ions in the electrolyte. With lithium metal present the voltage reaches a constant plateau when the intercalation current of the lithium deposition equals to the diffusion speeds within the anode material, keeping the potential constant as long as there is sufficient lithium present on the surface [97]. This method has the advantage that no extra heat is transferred into the system. But the relaxation voltage cannot as easily be used for the quantification of the plated lithium amount compared to the discharge voltage method since the diffusion speed and thus the time of the plating plateau is strongly depending on the cell temperature. At higher temperatures the diffusion speed may even be too fast to detect in the relaxation [7]. That temperature however is normally still changing due to heat losses of the preceding charging event dissipating [141]. The plateau can also only be detected above a certain lithium threshold, since the superimposed potential of the the deintercala-

**Figure 2.10:** Changes in the the discharge curve (top) and relaxation curve (bottom) after a plating event

tion is otherwise dominating [153]. Koleti et al. [100] found that the length of the CV phase during charging has a big influence on the detect-ability of the plating effect in the voltage since a very prolonged CV phase can lead to intercalation of the plated lithium while the charging is still going on. This can happen when the charging current becomes lower than the intercalation current of the deposited lithium.

Another way to detect the effects of plating in the cell is a simple capacity loss estimation. A cell with a plating history will show a much higher capacity loss compared to a cell aged under normal conditions for the same number

**Figure 2.11:** Measured stripping voltage with corresponding dV/dQ curve

of cycles. Figure 2.12 shows an exemplary capacity curve for plating and normal cycling. This capacity loss however can only be used to detect the occurrence of plating and estimate the amount of irreversible lithium losses in cell. Paired with the voltage curve analysis, this can be used to determine the relation of reversible versus irreversible capacity loss.

One big disadvantage of all these methods is that they can only detect and/or quantify plating after it has occurred. This means that they can be used as tools to get a better understanding of when and how plating can occur, but they can not be used during the charge to detect and possibly prevent plating. There are however also effects on the voltage and current during the plating event since the internal resistance of the cell is changing during the lithium deposition. The effects can be seen in the CC-phase on the left side of figure 2.10 as well as in the CV-phase on the right side of figure 2.13 [100, 141]. Both effects stem from the same origin, the change of internal resistance, but the effect is more visible in the current curve, where a pulse overlaying the normal exponential decrease of the current is observed. In the voltage curve the effect is overlayed by the more complex shape of the OCV and thus harder to analyze. A closer analysis of this effect can be found in chapter 3.2. All of the electrical methods can however only show

**Figure 2.12:** Comparison of the capacity loss under plating conditions and under normal conditions

the average state of the cell and have no spatial resolution. Combined with a reference electrode however they can give some degree of spacial resolution, depending on the size and location of the reference electrode. Another effect that is contributing to the change in voltage and current is the nucleation of the lithium crystals at the start of the plating [154], which was made visible in operando using electron paramagnetic resonance spectroscopy (EPR) by Wandt et al. [155]. This effect can be seen in the electrochemical deposition of different metals and describes the extra energy needed to create a crystal seed as a starting point. It should also be kept in mind that while in all real life applications voltage and current data hast to be measured for the BMS, the resolution of this data might not always be sufficient for the plating related analysis [141].

**Figure 2.13:** Anomaly in the current of the CV-Phase during a plating event

#### **2.6.2 Relevant Post Mortem Analysis**

Post mortem analysis requires disassembly or destruction of the cell. It allows for a more thorough analysis of the cell, being able to show spatially resolved results ranging from highly detailed imagery to chemical component analysis. It can be used to gather a deeper understanding of the mechanisms and reaction within the cell, but can not be utilized operando without destroying the cell.

#### **Optical Methods**

Optical methods include scanning electron microscopy (SEM), FIB microscopy, LSM and light microscopy, but also just visual inspection. All these methods can create pictures of very small structures with varying degrees of resolution. SEM can produce the pictures with the most resolution but requires a vacuum and cannot show any color information. FIB pictures are in the same category but the FIB is often used for surface modification and instead the picture is created with an additional SEM. The FIB beam can cut into the material and thus allows observation of cross sections in the material while the other methods normally only allow observation of the surface. LSM does not have the resolution of SEM but can be operated under normal atmosphere and has a very good resolution in the z axis, allowing for surface roughness analysis but also can not really show any color information. Only

light microscopy and a simple visual inspections can show color, but light microscopy has the lowest resolution of the microscopy methods. All methods can only create a visual representation but do not show any information about the materials present. They can be used to perform particle analysis, characterize material dimensions and the material morphology. Optical methods are often used in conjunction with other methods that can identify material composition or to explain changes in electrochemical measurements. Almost all optical methods require opening of the cell. Some methods like X-ray microtomography can be used in-situ, but they require a specialized test cell with a casing that is transparent for the used radiation and normally have a lower resolution than ex-situ methods.

When graphite becomes intercalated by lithium the color of the graphite changes from a dark grey/black over a dark red to a golden color tone. This can be used to identify regions of the cell that e.g. did not fully charge or discharge. In the context of plating optical methods are often used to show changes in the surface structure of the anode caused by the plating process. It is however not possible to prove the existence of metallic lithium on the surface using optical methods, only to show that an irregular change on the surface was occurring during plating. To determine whether this layer is mostly electrolyte decomposition or metallic lithium requires different methods that can distinguish chemical composition like XPS or gas chromatography.

Other applications of optical analysis are e.g. in the work of Fang et al. [59], who used cryogenic FIB-SEM and cryogenic transmission electron microscopy (TEM) to show the influence of the plating-morphology on stripping. It was shown that a low turtuosity bulky deposition leads to less dead lithium since such a structure can easier keep contact to the anode during discharge. Uhlmann et al. [97] used in-situ light microscopy to observe the lithium plating process live. They saw a grey coating appear after the anode reached the maximum intercalation level, visible in a golden coloring of the graphite. They also found net like structures using SEM that only existed in cells that exhibited plating behaviour in the electrical measurements and postulated that these structures are metallic lithium depositions.

In summary optical methods are powerful tools to quickly identify or quantify surface changes cause by plating but they can not gather any information about the chemical composition and can thus not be used to prove the existence of metallic. They can and should be used in conjunction with other methods that can identify chemical compounds.

#### **XPS**

XPS utilizes the photoelectric effect to distinguish between different materials, composites and even the oxidation states of the materials in the sample [156]. For XPS a high energy X-ray with a specific energy is applied to the sample in an area of at least 10 µm diameter [157]. This energy will cause the electrons of different orbitals of the material to become a free photo-electron with an energy level corresponding to the binding energy of that electron. The kinetic energy of these photo-electrons can then be measured and using the excitation energy of the X-ray source transferred into a binding energy. After the excitation is finished the electron will fall back to its original orbital and in the process release the energy difference from the excited to the original state in form of another photon-electron called an Auger-electron, whose energy is roughly described by the difference between the binding states [158]. Figure 2.14 shows the emission process of a photo-electron and 2.17 the auger electron generation.

By using an electric field to filter the photo-electrons by energy a spectrum can be measured with different intensities for the different energy levels corresponding to the binding states of the originally exited electrons. Some of the emitted electrons will have inelastic scatter interaction within the material of the sample and lose energy because of that. These electrons will cause the so called background in the spectrum causing an underlying increase of all following signals after each material peak [158]. XPS is capable of detecting all elements except for hydrogen and helium and is thus suitable for lithium detection and SEI analysis. The method was frequently used for LIB and LMB surface analysis [26, 55, 86, 156, 159, 160, 161, 162, 163, 164, 165, 166] and e.g. for additive optimizations [167] or coatings [168] for both anodes and cathodes. XPS only has a penetration depth of a few atomic

**Figure 2.14:** Depiction of the photo-electron emission process in XPS

layers (around 5 nm) and is thus only applicable for surface analysis [169]. The penetration depth can be calculated using the following formula [170]:

$$d = 3
λ
sin(
α)$$

Here *³* is the emission angle and *¼* is the attenuation length of electrons in a solid. *¼* depends on the element that is exited and the energy of the X-ray beam used for the excitement. The penetration depth of the measurement thus also depends on the element the electron is released from [158] and the intensity of the photoelectron peak will approximately decrease exponentially with penetration depth into the sample. Combined with an etching process and repeated XPS measurements however a depth profile of the material composition for a sample can be achieved [161, 171]. This etching is normally done by argon sputtering. Oswald et al. [172] however noticed an enrichment of lithium in the sputtering area of fully lithiated graphite which can cause an overestimation of the actual lithium content. The sputtering process is also highly destructive to the surface and can lead to the decomposition of certain components or remove some materials faster and thus cause an uneven surface structure [157, 173]. Andersson et al. [173] report that Li2CO3 can be decomposed to LiO2 due to the Ar sputtering process. Shiraishi et al. [166] and Bhattacharya et al. [48] noticed that the high energy of an XPS beam spot could also alter the chemical composition of the SEI. Shiraishi et al. [166] specifically tested the effect of the X-ray beam and Ar sputtering on the surface and also observed this decomposition, but since they only saw a small decomposition effect of LiOCO3 to LiO2 they deemed its effect insignificant for the analysis. Aurbach et al. [156] noticed a change in the surface appearance for a lithium electrode for an emission current of 5 mA that was not occurring at 3 mA and concluded that lithium can be altered by higher energy beams. Oswald et al. [174] however stated that they could never observe a change of spectra caused by the X-rays. They also mentioned in another publication that after the sputtering a rest time is necessary to allow for charge equalisation. They used a rest time of 2 minutes in their experiments [175]. They however also mention that the sputtering can damage the bonds especially for oxides and thus a lower ion energy or bigger ion mass is preferred. It was also shown by Wood et al. [176] that even under ultra high vacuum (UHV) conditions a monolayer of contamination could be found on metallic lithium after only 15 minutes so that the detection of lithium metal is only possible when using sputtering right before the measurement.

Monitoring lithium on a graphite surface is made difficult by the fact that the photoionization cross section of lithium is only 6% of that of C1s [72] and thus the signal strength of lithium is by more than a magnitude lower than that of the carbon. Additionally the amount of carbon in the sample is also much higher worsening the effect even more. Lithium also has the lowest sensitivity factor of the list of elements than can be detected using XPS, so in order to get a signal for Li that is strong enough the integration time must be big enough to achieve a good signal-to-noise ratio (SNR) [176].

Another aspect of XPS is the unintended charging of the surface due to the X-ray beam if the conductivity of the sample material is not high enough. This leads to a shift in the whole spectrum since the energy of all photoelectrons emitted is altered by the electrical field caused by the surface charges [176]. To compensate this effect an electron gun can be used to disperse the surface charge for non or low conductivity materials. The problem with battery materials, and in particular for the SEI, is the fact that the conductivity can vary widely, especially when etching is performed on the surface. In this case in some locations the SEI may already be removed, exposing graphite with a good conductivity, while in other locations the isolating SEI is still present. In this case using charge compensation can easily overcompensate and create a shift in the opposite direction by overcharging the surface. Using no charge compensation on the other hand may cause a shift of the spectra over time and cause the measurement to drift between two peaks [158]. Another complication can come from dipole layer formation in materials with different phases of non- or semiconducting materials, as it also can be the case for the SEI layer [157]. The charging effect can also not be uniformly distributed on the surface, especially when different materials with varying conductivities are present or charges can be trapped at interface films or multiphase structures or when the surface roughness is very high in some areas of the sample and thus parts of the surface are shadowed from the X-ray [157, 177]. In these cases not only a shift in the spectrum can occur but also a broadening of peaks. In the worst case the charge buildup can lead to changes of materials in the sample [157]. To compensate for shifts of the whole spectra it is useful to utilize the position of an element that is present in the material analyzed in high quantities as a reference to shift all spectra to the right position. In the case of XPS on anode LIB battery materials often the C1s peak of an adventitious carbon (AC) contamination that can be found on all materials within the battery is used as a reference, but the exact energy level used varies throughout the related publications from 285 eV to 284*.*3 eV [162, 169, 170, 171, 172]. Some authors also use materials common to LIBs, like LiF or Li2CO3 as a reference [174]. Baer et al. [157] criticize the usage of AC as a reliable reference element for several reasons, including the undefined chemical state, possible differential charging or a lower then necessary intensity of the signal.

Depending on the energy the X-ray can also potentially cause damage to the sample and change the chemical composition. Dedryvère et al. [178] for example did their experiments at −140 ◦C to avoid this degradation.

The recorded spectrum of an XPS measurement shows peaks for all binding states of the present elements. The intensity of these peaks is given by the number of electrons collected at the energy level of this particular binding state. The sensitivity of detection of specific elements can be as low as 0.1%, but can differ by a factor of 100 by elements [157]. The result is also influenced by the location of the material in the surface region [158]. If the material is not present in the top atomic layer, the intensity of the peak can be reduced. The width of the peak is also a measure for the exact bond present. If a peak is broader than expected that normally means that more chemical bonds are contributing to this peak, but it can also be caused by unwanted side effects like an X-ray induced change of the sample or local surface charging [158].

Figure 2.15 shows an example spectrum of a new LIB anode after discharging. The spectrum covers the whole binding energy (BE) range that the device can measure. The count value corresponds to the number of photo electrons created at this specific energy level. The peaks are showing compounds that are present in high concentration in the sample. All higher energy levels create scattered photo electrons that are visible in the lower energy levels which creates this stair like effect in the spectrum that is called the background, as described above. The wide spectrum is generally only used to get a overview of the elements and compounds present in the sample. In the further measurement steps spectra with a higher resolution focused on areas of interest are recorded. Figure 2.16 shows such a detail spectrum of the same sample in the energy range of 275 to 300 eV, where carbon peaks are located. To interpret a spectrum the first step is to identify the peaks visible in the spectrum. The example spectrum clearly shows the carbon peak (AC) and two peaks corresponding to the binder in the anode material PVDF. The PVDF contains carbon as well as fluorine, which means that it creates a peak in the carbon spectrum visible here as well as in the fluorine spectrum in the range of 680 to 700 eV. One of the two binder peaks is overlapping with the carbon peak, which makes it harder to clearly distinguish the two peaks. In the lower part of the plot the fitted background is also visible that has to be removed before further analysis is performed on the spectrum.

**Figure 2.15:** Example wide spectrum of a discharged LIB anode

From the raw measurement these peaks only allow identification of the present compound but no quantification. To get information about the element distribution within the sample it is necessary to calculate the atomic percentages from the measurement. To do this a factor for each element is needed that describes the average amount of electrons that can leave the sample. These factors are called Scofield's relative sensitivity factors (RSFs) [179, 180]. They are necessary since the penetration depth depends on the element and thus the intensities, or counts of electrons received at the detector, will not be comparable without that correction factor. The factor also

**Figure 2.16:** Example detail spectrum of the C1s area of a discharged LIB anode

depends on the angle of the X-ray source hitting the sample and the RSF factor tables are always calculated for a certain impact angle [158].

XPS peaks can have the shape of a Gauss or Lorentz function, or a combination of both. The peaks can also show a certain asymmetry. Asymmetry in peaks can be caused by an overlap of close binding state signals, in which case there are actually multiple signals present, but also by vibrational excitation and in the case of metallic samples by multi electron excitation [181]. The free holes in metallic samples can interact with the electrons freed by the photo-electron reaction and create so called shake up events that can create a tail attached the main peak for that material [182]. Over a wider spectrum all elements can create extra peaks in the higher BE regions caused by inelastic scattering of the photoelectrons. This effect is called plasmon excitation [181]. For lithium metal for example the main peak is at about 55 eV with plasmon losses located between 66 and 60 eV [176]. The knowledge of the connected plasmon peak can greatly help in the identification of present species, especially if the main peaks are overlapping with other materials.

For LIB battery materials Oswald et al. observed [172] that all elements containing in the SEI are shifted by about 3 eV to higher binding energy when measured on lithiated graphite. These shifts are especially strong for insulating species in the SEI [175]. They also noticed that Argon was shifted as well, while gold and copper did not shift in the presence of lithium metal. Because of this implanted Argon from the sputtering process can be used to detect these shifts [176]. For a lithium free sample the implanted Argon can be found at 242 eV and it shifts towards 245 eV for higher lithium concentrations. They assume that this shift is caused by electrostatic polarization of the elements close to the lithium. This behaviour can possibly also be caused by metallic lithium depositions due to plating [174]. This was also confirmed by Maibach et al. [183] and according to them is caused by an electrostatic double layer forming between lithium and the SEI components. Tallmann et al. [184] could see a strong contribution of the PVDF F1s signal on a normal anode, but the signal was dampened by the metallic film on the surface in the case of lithium deposition. Helpful for the identification of lithium in the sample in particular can be the fact that the peak shapes of conductive samples are usually less wide (by 0*.*5 eV) compared to insulators and the majority of the SEI compounds are non-conductive [185]. Metallic samples also show more asymmetry due to interaction of the photo-electrons with the conduction electrons [185]. Additionally the auger electrons (or plasmon losses) can also be used to help identify complex compounds like the SEI. Figure 2.17 shows the auger effect, where the vacancy left by the photoelectric effect is filled up by an electron from a higher orbital. This effect releases the auger electron that also creates peaks in spectrum. It is important to notice that these auger peak positions do not represent a binding energy since they are interactions with electrons of different binding energies and also that this effect is, contrary to the photoelectric peaks, not dependent on the X-ray excitation energy.

**Figure 2.17:** Depiction of the auger electron emission process in XPS

When trying to analyze the peaks and elemental ratios it is important to keep in mind that homogeneous distribution of elements in the detection volume can lead to under- or overestimation of element percentages in different layers and lead to wrong results [176, 177]. Due to the shifts caused by surface charging the absolute BE values can be hard to interpret, this is why in some cases the ∆BE values can be used for material identification [176]. Additionally the so called valence-band spectra can help to identify certain composites [176], a slope close to 0 eV can be used to identify metallic species in the sample. The valence band spectra can show all species present on the sample. If the number of species is low the interpretation can be simple [163]. Naille et al. [163] e.g. found an indication of ROCO2Li species at 17 eV. Using samples from different areas of the battery electrode XPS can also give a somewhat spatially resolved distribution of the elemental composition of the surface. Moertti et al. [161] e.g. found that more electrolyte decomposition occurred at the edges of the electrodes compared to the center as well as at the outer layers of the cell stack under all ageing conditions. In the case of lithium plating only a small number of papers used XPS. Mussa et al. [55] tried to find lithium depositions in samples cycled at high C rates, but could only find LiF and Li2CO3 on the surface of the sample. They however used the amplitude of the 283 eV C1s peak to estimate the thickness of the SEI, with a higher peak contribution hinting at a thinner SEI. Tallmann et al. [184] modified the surface of the anode with nanoscale layers of Cu and Ni to make them more resistant to plating and observed the effect using XPS, but they only looked at the composition of the SEI and didn't do any depth profiling in combination with XPS.

In this thesis it was planned to used XPS to try to identify the location of the dead lithium within the SEI by performing a depth profile. During the measurement however another effect was discovered, most likely caused by the charged compensation of the XPS device that facilitated a lithium migration to the surface of the anode, falsifying the results of the depth profile but opening a different approach of surface analysis.

#### **2.6.3 Other Methods for Lithium Plating Detection**

In this chapter the other most common methods used in literature for plating detection and characterization are briefly described to show an overview of the available tools. This list is not exhaustive as there are more methods and new ways of lithium plating detection and analysis are still developed. This list can also be divided into in-situ and post mortem methods. The coulombic efficiency measurements can be used operando when the measurement equipment is precise enough. The measurement of the cell thickness as well as the calorimetry also can be used operando, but they require a special setup around the cell. Neutron Diffraction, Nuclear Magnetic Resonance, Electron Paramagnetic Resonance Spectroscopy and reference electrodes work operando as well but additionally require a specialized cell design. Gas Chromatography and glow-discharge optical emission spectroscopy (GDOES) can only be used post mortem.

#### **Coulombic Efficiency**

Lithium plating compared to the normal intercalation mechanism in the cell has a lower coulombic efficiency (CE) since the metallic lithium on the anode surface in creating a fresh SEI on its surface. This is an electrochemical reaction consuming electrons that are lost during the charging process. It is also theorized however that unreacted dead lithium on the SEI might cause in increase in tortuosity at the electrode/electrolyte interface and thus cause a decrease in CE [115, 116]. Using very precise cycling devices, this change in CE can be used to find the onset of lithium plating under a given condition. The typical CE of a LIB lies at 99.5% at room temperature [6]. Fang et al. [59] observed that for a CE of less than 95% deposition of unreacted lithium is dominating the side reactions, while for efficiencies of more than 95% the Li reaction with the SEI is higher and that the amount of unreacted lithium has a linear relationship with the CE . The CE should be getting very close to 1 when charging with very high rates since there is only very little time for parasitic side reactions to take place within one cycle but at a certain threshold this trend is changing due to the onset of lithium plating [129]. Burns et al. where able to identify the plating onset current using the CE data at different temperatures.

The high requirement on the measurement equipment and the fact a fully cycle is necessary to calculate the CE means that this method can not really be used for lithium plating prevention and not suitable for a real world even though it is technically an operando method.

#### **Cell Thickness**

The fact that a lithium deposition takes more space than the intercalation of the lithium into the graphite lattice can be used to detect lithium plating in pouch cells. In theory 1 A h of charge plated on the anode surface should increase the cell volume by 0*.*37 cm3 , which can be calculated from the molar volume of litihum metal compared to the volume of the graphite-lithium intercalation material [91, 131]. In reality this value will be higher, since the deposition is not homogeneous but most of the time in mossy or dendritic form. It also has to be kept in mind that this volume change is superimposed by the expansion of the anode during simultaneous intercalation, the subsequent shrinkage of the cathode material due to de-intercalation and possible SEI growth and gassing [7]. The volume change of the anode however is at around 10% when fully lithiated, while the cathode shows even lower variation, with no more than 0.1 to 1% change on volume, depending on the cathode material [145, 186], so that the influences of the electrode volume changes can be neglected when the plating amount is significant. Bitzer et al. [131] also showed that in their experiment gassing was not present due to the lower testing temperatures, but did detect gassing when plating was present. The two main methods used for the thickness measurements are mechanical gauges [24, 131, 187] or laser scanning methods [91, 127]. The laser scanning method has a lower influence on the cell, since no mechanical contact is necessary, but requires more measurement effort. Both methods also allow for a spatially resolved measurement on the cell surface, with the laser scanning method offering a much higher resolution, since the laser can easily periodically scan the whole cell surface. Rieger et al. [127] measured their cell from both sides at the same time to make the measurement more robust and showed that even at 17 ◦C, compared to cells at 25 ◦C and 40 ◦C, an increase in thickness was visible, that could be linked to lithium plating by subsequent post mortem analysis of the cells. They found a film on the anode surface of the 17 ◦C cell with a thickness between 15 and 30 µm. This method is only usable on pouch cell to detect lithium plating though, since the casing of round and prismatic cells is to rigid to measure any change in cell thickness.

#### **Calorimetry**