Thackeray et al. (2007) proposed a strategy to stabilize a layered Li[MnxNiyCoz]O2 electrode by integrating a structurally compatible component that was electrochemically inactive between 4 and 3 V, aiming to increase the stability of layered electrodes over a wider compositional range without compromising the power or cycle life of the cells. Li2MnO3 is the best stabilized component for Li[MnxNiyCoz]O2 electrode because it is electrochemically inactive over a potential window of 2.0~4.4 V in which Li[MnxNiyCoz]O2 component is electrochemically active and operates as a true insertion electrode. In Li2MnO3, all manganese ions are tetravalent and cannot be oxidized further. Lithium insertion into Li2MnO3, with a concomitant reduction of the manganese ions, is also prohibited because there are no energetically favourable interstitial sites for the guest ions. Under such conditions, Li2MnO3 component acts as a stabilizing unit in the electrode structure. (Thackeray et al., 2007) When charged to higher potentials, typically 4.5 V, it is possible to activate the Li2MnO3 component during the initial charge by removing Li+ with a concomitant loss of oxygen. The removal of two Li+ from the Li2MnO3 component and the reinsertion of only one Li+ into the resulting MnO2 component, exceptionally high reversible capacities (230–250 mAh/g) can be obtained. For example, Li et al. (2011) synthesized a high-voltage layered Li[Li0.2Mn0.56Ni0.16Co0.08]O2 cathode material, a solid solution between Li2MnO3 and LiMn0.4Ni0.4Co0.2O2, by co-precipitation method followed by a hightemperature annealing. After the initial decay, no obvious capacity fading was observed when cycling the material at different rates. Steady-state reversible capacities of 220 mAh/g at 0.2 C, 190 mAh/g at 1 C, 155 mAh/g at 5 C, and 110 mAh/g at 20 C were achieved in long-term cycle tests within the voltage cutoff limits of 2.5 and 4.8 V at 20 C.

#### **<IP_ADDRESS> Heterogeneous core-shell structure**

50 Lithium Ion Batteries – New Developments

LiPON film enhances the lithium-ion migration through the interface between the surface of LiCoO2 particles and the electrolyte, suppressing the surface reaction between the electrode

Metals Mn, Ni, and Co can all be accommodated in the layered metal oxide structure, giving a range of compositions Li[MnxNiyCoz]O2 (*x* + *y* + *z* = 1). A solution of Li[MnxNiyCoz]O2 may possess improved performances, such as thermal stability due to the synergetic effect of three ions. Recently, intense efforts have been directed towards the development of Li[MnxNiyCoz]O2 as a possible replacement for LiCoO2. However, layered Li[MnxNiyCoz]O2 structures tend to become unstable at high levels of delithiation, which is attributed largely to the highly oxidizing nature of tetravalent Co and Ni that results in the loss of oxygen and the migration of the transition metal ions to the lithium-depleted layer. In spite of numerous valuable works, the cycle life and thermal stability of Li[MnxNiyCoz]O2 still need further improvement. Several strategies have been used to improve the electrochemical

Coating of the surfaces of Li[MnxNiyCoz]O2 with Al2O3, LiAlO2, AlPO4, and ZrO2 has been widely attempted, leading to significantly improved electrochemical properties. For example, Hu et al. (2009) coated the surface of LiNi1/3Co1/3Mn1/3O2 with a uniform nanosized layer of ZrO2. ZrO2-coated LiNi1/3Co1/3Mn1/3O2 exhibits an improved rate capability and cycling stability under a high cut-off voltage of 4.5 V, owing to the ability of ZrO2 layer in preventing the direct contact of the active material with the electrolyte. In this case, ZrO2

Thackeray et al. (2007) proposed a strategy to stabilize a layered Li[MnxNiyCoz]O2 electrode by integrating a structurally compatible component that was electrochemically inactive between 4 and 3 V, aiming to increase the stability of layered electrodes over a wider compositional range without compromising the power or cycle life of the cells. Li2MnO3 is the best stabilized component for Li[MnxNiyCoz]O2 electrode because it is electrochemically inactive over a potential window of 2.0~4.4 V in which Li[MnxNiyCoz]O2 component is electrochemically active and operates as a true insertion electrode. In Li2MnO3, all manganese ions are tetravalent and cannot be oxidized further. Lithium insertion into Li2MnO3, with a concomitant reduction of the manganese ions, is also prohibited because there are no energetically favourable interstitial sites for the guest ions. Under such conditions, Li2MnO3 component acts as a stabilizing unit in the electrode structure. (Thackeray et al., 2007) When charged to higher potentials, typically 4.5 V, it is possible to activate the Li2MnO3 component during the initial charge by removing Li+ with a concomitant loss of oxygen. The removal of two Li+ from the Li2MnO3 component and the reinsertion of only one Li+ into the resulting MnO2 component, exceptionally high reversible capacities (230–250 mAh/g) can be obtained. For example, Li et al. (2011) synthesized a high-voltage layered Li[Li0.2Mn0.56Ni0.16Co0.08]O2 cathode material, a solid solution between Li2MnO3 and LiMn0.4Ni0.4Co0.2O2, by co-precipitation method followed by a hightemperature annealing. After the initial decay, no obvious capacity fading was observed when cycling the material at different rates. Steady-state reversible capacities of 220 mAh/g

performances of Li[MnxNiyCoz]O2 cathode, which can be summarized as follows:

play a similar role as it does in ZrO2 coated LiCoO2 (Chen et al., 2002).

**<IP_ADDRESS> Mixing with other layered components**

surface and electrolyte during cyclings.

**2.2.2 Li(Mn,Ni,Co)O2**

**<IP_ADDRESS> Surface modifications**

Recently, Sun and co-workers proposed a novel core/shell concept that construct layered structured LiNixCoyMnzO2 cathode with a core enriched in Ni for high capacity and a shell enriched in Mn for high stability and cycling performance. They reported that core–shell cathode material Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 exhibited an excellent cyclability and thermal stability (Sun et al., 2005). As shown in Figure 12, the core-shell structure consists of a Ni-rich LiNi0.8Co0.1Mn0.1O2 core that delivered a high capacity and a Mn-rich Li[Ni0.5Mn0.5]O2 shell that provides a structural and thermal stability in highly delithiated states. After that, Sun et al. (2009) found a structural mismatch between the core and the shell: Voids of tens of nanometres between the core and shell appear in the core-shell powders after cycling. A shell with a gradient in chemical composition was proposed to grow onto the surfaces of a core material, forming a constant chemical composition. The concentration-gradient cathode material (Li(Ni0.64Co0.18Mn0.18)O2) thus achieved shows a high capacity of 209 mAh/g, which retains 96% after 50 charge/discharge cycles under an aggressive test profile (55 oC between 3.0 and 4.4 V), as shown in Figure 13. In a latest work by Koenig and Sun (Koenig et al., 2011), particles with internal gradients in transition metal composition was synthesized using a coprecipitation reaction with a control over the process conditions. By this, compositions and structures can be rationally integrated into the synthesis process, and further gradient coreshell structure LiNixCoyMnzO2 materials will become a future choice for cathode in LIBs.

Fig. 12. a) SEM image and b) Energy dispersive spectroscopic line scan of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 particles; c) initial charge/discharge curves of Li/Li[Ni0.8Co0.1Mn0.1]O2 and Li/Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 cell in a voltage range of 3.0-4.3 V; and d) differential scanning calorimetry traces of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 and Li[Ni0.8Co0.1Mn0.1]O2 at a charged state to 4.3 V. (Sun et al., 2005)

Heterogeneous Nanostructured Electrode Materials

LiO4 tetrahedra in green, and oxygen ions in red.

materials to the electrolyte.

Li/Li+. (Fang et al., 2008)

for Lithium-Ion Batteries – Recent Trends and Developments 53

situated furthest away from the 16d site of all the interstitial tetrahedral (8a, 8b and 48f) and octahedra (16c). Each of the 8a-tetrahedron faces is shared with an adjacent, vacant 16c site. This combination of structural features in the stoichiometric spinel compound constitutes a very stable structure, as shown in Figure 14. LiMn2O4 has a three-dimensional lithium diffusion path, in which every plane is suitable to exchange lithium ion from active

Fig. 14. Three-dimensional crystal structure of LiMn2O4. MnO6 octahedra are shown in blue,

LiMn2O4 has a theoretical capacity of 148 mAh/g and a voltage plateau of about 4 V. A typical cyclic voltammogram of LiMn2O4 is shown in Figure 15. Two couples of reversible redox peaks are observed during the charge/discharge processes. The split of the redox peaks into two couples shows that the electrochemical reaction of the extraction and insertion of lithium ions occurs in two stages. The first oxidation peak is ascribed to the removal of Li+ from half of the tetrahedral sites in which Li–Li interactions exist, whereas the second oxidation peak is attributed to the removal of Li+ from the remaining tetrahedral sites where no Li–Li interactions exist. The whole process can be described as follows:

> 2 0.5 2 4 0.5 0.5 2 *Li e MnO Li Mn O*

Fig. 15. Cyclic voltammograms of LiMn2O4 within the voltage range from 3.0 to 4.5 V versus

(1)

0.5 2 4 2 4 0.5 0.5 *Li e Li Mn O LiMn O* (2)

Fig. 13. a) Schematic diagram of cathode particle with Ni-rich core surrounded by concentration-gradient outer layer; b) a scanning electron micrograph of a typical particle; c) cycling performance of half cells based on Li(Ni0.8Co0.1Mn0.1)O2, Li(Ni0.46Co0.23Mn0.31)O2 and concentration-gradient material cycled between 3.0 and 4.4 V at 55 oC by applying a constant current rate of 0.5 C (95 mA/g); and d) differential scanning calorimetry traces showing a heat flow from the reaction between the electrolyte and different electrode materials when charged to 4.3 V. (Sun et al., 2009)

#### **2.3 Three-dimensional cathode nanocomposites**

Three-dimensional framework structures have many crosslinked channels which must be sufficiently large to accommodate the lithium ion. The advantages of three-dimensional frameworks are represented by i) a possibility of avoiding, for steric reasons, the coinsertion of bulky species such as solvent molecules; and ii) the smaller degree of expansion/contraction of the framework structure upon lithium insertion/extraction (Winter et al., 1998).

#### **2.3.1 LiMn2O4**

LiMn2O4 is regarded as one of the most promising cathodes because of its high output voltage, high natural abundance, low toxicity, and low cost. LiMn2O4 crystallizes in a spinel structure with the space group of Fd3m. In the LiMn2O4 spinel structure, a cubic closepacked (ccp) array of oxygen ions occupy the 32e position, Mn ions are located in the 16d site, and Li in the 8a site. Mn ions have an octahedral coordination to oxygen, and the MnO6 octahedra share edges in a three-dimensional host for Li guest ions. The 8a tetrahedral site is

Fig. 13. a) Schematic diagram of cathode particle with Ni-rich core surrounded by

materials when charged to 4.3 V. (Sun et al., 2009)

**2.3 Three-dimensional cathode nanocomposites**

(Winter et al., 1998).

**2.3.1 LiMn2O4**

concentration-gradient outer layer; b) a scanning electron micrograph of a typical particle; c) cycling performance of half cells based on Li(Ni0.8Co0.1Mn0.1)O2, Li(Ni0.46Co0.23Mn0.31)O2 and concentration-gradient material cycled between 3.0 and 4.4 V at 55 oC by applying a constant current rate of 0.5 C (95 mA/g); and d) differential scanning calorimetry traces showing a heat flow from the reaction between the electrolyte and different electrode

Three-dimensional framework structures have many crosslinked channels which must be sufficiently large to accommodate the lithium ion. The advantages of three-dimensional frameworks are represented by i) a possibility of avoiding, for steric reasons, the coinsertion of bulky species such as solvent molecules; and ii) the smaller degree of expansion/contraction of the framework structure upon lithium insertion/extraction

LiMn2O4 is regarded as one of the most promising cathodes because of its high output voltage, high natural abundance, low toxicity, and low cost. LiMn2O4 crystallizes in a spinel structure with the space group of Fd3m. In the LiMn2O4 spinel structure, a cubic closepacked (ccp) array of oxygen ions occupy the 32e position, Mn ions are located in the 16d site, and Li in the 8a site. Mn ions have an octahedral coordination to oxygen, and the MnO6 octahedra share edges in a three-dimensional host for Li guest ions. The 8a tetrahedral site is situated furthest away from the 16d site of all the interstitial tetrahedral (8a, 8b and 48f) and octahedra (16c). Each of the 8a-tetrahedron faces is shared with an adjacent, vacant 16c site. This combination of structural features in the stoichiometric spinel compound constitutes a very stable structure, as shown in Figure 14. LiMn2O4 has a three-dimensional lithium diffusion path, in which every plane is suitable to exchange lithium ion from active materials to the electrolyte.

Fig. 14. Three-dimensional crystal structure of LiMn2O4. MnO6 octahedra are shown in blue, LiO4 tetrahedra in green, and oxygen ions in red.

LiMn2O4 has a theoretical capacity of 148 mAh/g and a voltage plateau of about 4 V. A typical cyclic voltammogram of LiMn2O4 is shown in Figure 15. Two couples of reversible redox peaks are observed during the charge/discharge processes. The split of the redox peaks into two couples shows that the electrochemical reaction of the extraction and insertion of lithium ions occurs in two stages. The first oxidation peak is ascribed to the removal of Li+ from half of the tetrahedral sites in which Li–Li interactions exist, whereas the second oxidation peak is attributed to the removal of Li+ from the remaining tetrahedral sites where no Li–Li interactions exist. The whole process can be described as follows:

$$0.5Li^{+} + 0.5e + 2\lambda MnO\_{2} \Leftrightarrow Li\_{0.5}Mn\_{2}O\_{4} \tag{1}$$

$$0.5Li^{+} + 0.5e + Li\_{0.5}Mn\_{2}O\_{4} \Leftrightarrow LiMn\_{2}O\_{4} \tag{2}$$

Fig. 15. Cyclic voltammograms of LiMn2O4 within the voltage range from 3.0 to 4.5 V versus Li/Li+. (Fang et al., 2008)

Heterogeneous Nanostructured Electrode Materials

materials that have been employed to date.

(Sahan et al., 2008)

**2.3.2 LiNi0.5Mn1.5O4**

for Lithium-Ion Batteries – Recent Trends and Developments 55

may act as a physical protection barrier to depress Mn2+ dissolution by forming epitaxial layers on the underlying coated spinel, improving the structural stability of the cathode. It has also been suggested that ZnO coatings may act as HF scavenger to be capable of neutralizing the acid species. However, it is not yet clear which of these various processes is most important to the stabilization of a LiMn2O4 electrode surface. Recently, Ouyang et al., (2010) reported the results of ab initio studies of the atomic and electronic structure of clean and Al2O3 covered LixMn2O4 (001) surfaces (x=0 or 1). They gave a clear picture that only Mn3+ ions occur at the clean LiMn2O4 (001) surfaces due to a lower coordination with O atoms, while Mn atoms in bulk LiMn2O4 exhibit a mixed Mn3+/Mn4+ oxidation states. An Al2O3 ad-layer inhibits the formation of Mn3+ at the LixMn2O4 (001) surfaces and thus has a more complex role than just separating the cathode from the electrolyte as is commonly assumed. Key variables that influence the coating efficacy remain to be identified and optimized for most of the coating

Fig. 17. Cycling performance of different materials at 1C discharge rate at room temperature.

Ni-doped spinel oxide, LiNi0.5Mn1.5O4, is another promising three-dimensional cathode material for future high power battery applications due to its large reversible capacity (147 mAh/g) at a high operating voltage around 4.7 V. LiNi0.5Mn1.5O4 spinel is fundamentally different from pure spinels as all redox activity takes place on Ni and Mn remains in +4 state. LiNi0.5Mn1.5O4 has two different crystal structures of the space groups of *Fd-3m* or *P4332*  depending on Ni ordering in the lattice. As shown in Figure 18, *Fd-3m* type LiNi0.5Mn1.5O4 is a disordering distribution, the Ni and Mn, Li and O atoms are occupied in the 16d octahedral sites, 8a tetrahedral sites and 32e sites, respectively. In this case, Ni and Mn atoms are randomly distributed in the 16d sites. The *P4332* type LiNi0.5Mn1.5O4 is has an ordered distribution, in which Ni, Mn, and Li atoms occupythe 4a, 12d, and 4c sites, respectively. O ions occupy the 8c and 24e sites. In this case, Ni and Mn atoms are ordered regularly (Santhanam et al., 2010). It has been reported that LiNi0.5Mn1.5O4 with *Fd-3m* space

Since the application of LiMn2O4 in high power systems requires the development of fast kinetic electrodes which appears nowadays possible thanks to the use of nanostructured morphologies, various synthesis methods have been widely investigated to prepare nanostructured LiMn2O4 and these nanostructured methods have been demonstrated to be highly effective in greatly enhancing the electrochemical performance of LiMn2O4 electrode materials. It is found that the electrochemical performance of LiMn2O4 electrodes strongly depends on the morphology and the crystalline phase. For example, in our previous work (Fang et al., 2008), we verified that the electrochemical performance of LiMn2O4 could be improved by altering the starting materials, as shown in Figure 16. In particular, α-MnO2 nanorods are proved to be a quite promising starting material for the preparation of highly crystallized and high performance LiMn2O4. After that, Ding et al. (2011) and Lee et al. (2010) prepared single-crystalline LiMn2O4 nanotubes and nanowires from β-MnO2 and α-MnO2 precursors, respectively, which also leads to an improved electrochemical performance.

Fig. 16. a) Charge/discharge curves and b) cycling performance of LiMn2O4 prepared from α-MnO2 nanorods (A) and commercial MnO2 (B) in the voltage range from 3.0 to 4.5 V versus Li/Li+ at a current density of 20 mA/g. (Fang et al., 2008)

Unfortunately, LiMn2O4 shows a fast capacity fading during the cyclings, which cannot be ultimately solved by nanostructured approaches only. The capacity fading of LiMn2O4 has been ascribed to the dissolution of Mn, Jahn–Teller distortion of Mn3+, and decomposition of electrolyte solution on electrode. Of the above factors, dissolution of surface Mn3+ with the electrolyte is now regarded as the most important reason for the capacity fading. Surface modification of the cathode electrode has been a successful strategy to solve the capacity fading problem, which could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling. In recent years, various types of coating materials, including oxides, metals, fluorides, phosphate, polymers, carbon materials, and other electrode materials have been explored to improve the electrochemical stability of LiMn2O4 cathodes. For example, Sahan et al. (2008) reported that Li2O–2B2O3(LBO)-coated LiMn2O4 electrode prepared by a solution method shows an excellent cycling behaviour without any capacity loss even after 30 cycles at room temperature and a 1- C rate, as plotted in Figure 17. The improved cycling stability is also achieved by coating with [Li, La]TiO3 (Jung et al., 2011) and Li4Ti5O12 (Liu et al., 2007). Recent developments in the surface modification have been reviewed by Yi et al. (2009). Despite the numerous studies in this area, a functional understanding of coated LiMn2O4 electrodes remains incomplete. In the case of metal oxides, it appears likely that some of the metal oxides such as MgO, Al2O3, ZrO2

Since the application of LiMn2O4 in high power systems requires the development of fast kinetic electrodes which appears nowadays possible thanks to the use of nanostructured morphologies, various synthesis methods have been widely investigated to prepare nanostructured LiMn2O4 and these nanostructured methods have been demonstrated to be highly effective in greatly enhancing the electrochemical performance of LiMn2O4 electrode materials. It is found that the electrochemical performance of LiMn2O4 electrodes strongly depends on the morphology and the crystalline phase. For example, in our previous work (Fang et al., 2008), we verified that the electrochemical performance of LiMn2O4 could be improved by altering the starting materials, as shown in Figure 16. In particular, α-MnO2 nanorods are proved to be a quite promising starting material for the preparation of highly crystallized and high performance LiMn2O4. After that, Ding et al. (2011) and Lee et al. (2010) prepared single-crystalline LiMn2O4 nanotubes and nanowires from β-MnO2 and α-MnO2 precursors, respectively, which also leads to an improved electrochemical performance.

Fig. 16. a) Charge/discharge curves and b) cycling performance of LiMn2O4 prepared from α-MnO2 nanorods (A) and commercial MnO2 (B) in the voltage range from 3.0 to 4.5 V

Unfortunately, LiMn2O4 shows a fast capacity fading during the cyclings, which cannot be ultimately solved by nanostructured approaches only. The capacity fading of LiMn2O4 has been ascribed to the dissolution of Mn, Jahn–Teller distortion of Mn3+, and decomposition of electrolyte solution on electrode. Of the above factors, dissolution of surface Mn3+ with the electrolyte is now regarded as the most important reason for the capacity fading. Surface modification of the cathode electrode has been a successful strategy to solve the capacity fading problem, which could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling. In recent years, various types of coating materials, including oxides, metals, fluorides, phosphate, polymers, carbon materials, and other electrode materials have been explored to improve the electrochemical stability of LiMn2O4 cathodes. For example, Sahan et al. (2008) reported that Li2O–2B2O3(LBO)-coated LiMn2O4 electrode prepared by a solution method shows an excellent cycling behaviour without any capacity loss even after 30 cycles at room temperature and a 1- C rate, as plotted in Figure 17. The improved cycling stability is also achieved by coating with [Li, La]TiO3 (Jung et al., 2011) and Li4Ti5O12 (Liu et al., 2007). Recent developments in the surface modification have been reviewed by Yi et al. (2009). Despite the numerous studies in this area, a functional understanding of coated LiMn2O4 electrodes remains incomplete. In the case of metal oxides, it appears likely that some of the metal oxides such as MgO, Al2O3, ZrO2

versus Li/Li+ at a current density of 20 mA/g. (Fang et al., 2008)

may act as a physical protection barrier to depress Mn2+ dissolution by forming epitaxial layers on the underlying coated spinel, improving the structural stability of the cathode. It has also been suggested that ZnO coatings may act as HF scavenger to be capable of neutralizing the acid species. However, it is not yet clear which of these various processes is most important to the stabilization of a LiMn2O4 electrode surface. Recently, Ouyang et al., (2010) reported the results of ab initio studies of the atomic and electronic structure of clean and Al2O3 covered LixMn2O4 (001) surfaces (x=0 or 1). They gave a clear picture that only Mn3+ ions occur at the clean LiMn2O4 (001) surfaces due to a lower coordination with O atoms, while Mn atoms in bulk LiMn2O4 exhibit a mixed Mn3+/Mn4+ oxidation states. An Al2O3 ad-layer inhibits the formation of Mn3+ at the LixMn2O4 (001) surfaces and thus has a more complex role than just separating the cathode from the electrolyte as is commonly assumed. Key variables that influence the coating efficacy remain to be identified and optimized for most of the coating materials that have been employed to date.

Fig. 17. Cycling performance of different materials at 1C discharge rate at room temperature. (Sahan et al., 2008)

#### **2.3.2 LiNi0.5Mn1.5O4**

Ni-doped spinel oxide, LiNi0.5Mn1.5O4, is another promising three-dimensional cathode material for future high power battery applications due to its large reversible capacity (147 mAh/g) at a high operating voltage around 4.7 V. LiNi0.5Mn1.5O4 spinel is fundamentally different from pure spinels as all redox activity takes place on Ni and Mn remains in +4 state.

LiNi0.5Mn1.5O4 has two different crystal structures of the space groups of *Fd-3m* or *P4332*  depending on Ni ordering in the lattice. As shown in Figure 18, *Fd-3m* type LiNi0.5Mn1.5O4 is a disordering distribution, the Ni and Mn, Li and O atoms are occupied in the 16d octahedral sites, 8a tetrahedral sites and 32e sites, respectively. In this case, Ni and Mn atoms are randomly distributed in the 16d sites. The *P4332* type LiNi0.5Mn1.5O4 is has an ordered distribution, in which Ni, Mn, and Li atoms occupythe 4a, 12d, and 4c sites, respectively. O ions occupy the 8c and 24e sites. In this case, Ni and Mn atoms are ordered regularly (Santhanam et al., 2010). It has been reported that LiNi0.5Mn1.5O4 with *Fd-3m* space

Heterogeneous Nanostructured Electrode Materials

various current densities. (Fang et al., 2007)

**<IP_ADDRESS> Coating by metal oxides**

current density of 900 mA/g.

for Lithium-Ion Batteries – Recent Trends and Developments 57

space group of *Fd-3m*. The discharge capacity of LiNi0.5Mn1.5O4 reaches up to 134 mAh/g at a current density of 20 mA/g, while it retains as high as 110 mAh/g even at a high

Fig. 20. a) Transmission electron micrograph and b) IR spectrum of LiNi0.5Mn1.5O4;

fluoride, and carbon have been used to modify the surfaces of LiNi0.5Mn1.5O4.

c) Charge/discharge curves of LiNi0.5Mn1.5O4 cycled at current densities of 20 mA/g (solid line) and 160 mA/g (dash line); and d) Discharge curves of the as-prepared LiNi0.5Mn1.5O4 at

The another one problem is that LiNi0.5Mn1.5O4 still undergoes decomposition of electrolyte and dissolution of Mn and Ni, particularly under high voltage and elevated temperature conditions. The construction of heterogeneous nanostructure by coating some chemically stable compounds on LiNi0.5Mn1.5O4 is a most widely used strategy to enhance the electrochemical properties of LiNi0.5Mn1.5O4. This strategy has the advantages of the delivery of the original capacity due to no reduction in the amount of electrochemically active elements in the parent oxide and the suppression of the corrosion reactions between the high voltage spinel cathode and electrolyte. In this regard, various materials such as metal oxides, metal,

Surface modifications of LiNi0.5Mn1.5O4 spinel material by metal oxides such as ZnO (Arrebola et al., 2010), ZrO2 (Wu et al., 2010), Bi2O3 (Liu et al., 2009), and SiO2 (Fan et al., 2007) have been investigated. Metal oxides coating act as a protective layer against the attack by HF that generated from the decomposition of LiPF6 salt in the electrolyte, which effectively suppresses the corrosion reaction. For example, Sun et al. (2002) reported that the

group has a better electrochemical performance than that of spinel with *P4332* space group due to a 2.5 orders of magnitude faster electronic conductivity.

Fig. 18. Spinel structure of LiNi0.5Mn1.5O4 (*Fd-3m*) showing the diffusion path of Li+. (Redrawn from Ref. (Xia et al., 2007))

A typical cyclic voltammogram of *Fd-3m* type LiNi0.5Mn1.5O4 is shown in Figure 19. Three well-defined reversible peaks are observed during the charge/discharge processes. The appearance of 4 V peak is due to Mn3+ which is formed by oxygen loss during the hightemperature calcinations. Two major peaks appearing between 4.5 and 5 V during charge/discharge cyclings are due to the redox couples Ni2+/Ni3+ and Ni3+/Ni4+ or ordering of lithium and vacancies at x = 0.5. For the ordered spinel, the 4V peaks are absent because oxidation states of Ni and Mn were +2 and +4, respectively (Santhanam, et al., 2010).

Fig. 19. Cyclic voltammogram of LiNi0.5Mn1.5O4 electrode cycled between 3.5 and 5 V vs. Li/Li+ at 0.2 mVs-1 scan rate (Xia et al., 2007)

There are two main problems with this LiNi0.5Mn1.5O4 spinel material: One is that it is difficult to prepare pure spinel LiNi0.5Mn1.5O4 due to the formation of LixNi1−xO as a second phase at high calcination temperatures. Developing low-temperature synthesis method of nanostructure LiNi0.5Mn1.5O4 is an effective strategy to solve this problem. In our previous work, we discovered a low-temperature solid-state route for single-phase LiNi0.5Mn1.5O4 (Fang et al., 2007). As shown in Figure 20, LiNi0.5Mn1.5O4 shows a dimension of about 200 nm and has a cubic spinel structure, which can be indexed in a

group has a better electrochemical performance than that of spinel with *P4332* space group

Fig. 18. Spinel structure of LiNi0.5Mn1.5O4 (*Fd-3m*) showing the diffusion path of Li+.

oxidation states of Ni and Mn were +2 and +4, respectively (Santhanam, et al., 2010).

Fig. 19. Cyclic voltammogram of LiNi0.5Mn1.5O4 electrode cycled between 3.5 and 5 V vs.

There are two main problems with this LiNi0.5Mn1.5O4 spinel material: One is that it is difficult to prepare pure spinel LiNi0.5Mn1.5O4 due to the formation of LixNi1−xO as a second phase at high calcination temperatures. Developing low-temperature synthesis method of nanostructure LiNi0.5Mn1.5O4 is an effective strategy to solve this problem. In our previous work, we discovered a low-temperature solid-state route for single-phase LiNi0.5Mn1.5O4 (Fang et al., 2007). As shown in Figure 20, LiNi0.5Mn1.5O4 shows a dimension of about 200 nm and has a cubic spinel structure, which can be indexed in a

A typical cyclic voltammogram of *Fd-3m* type LiNi0.5Mn1.5O4 is shown in Figure 19. Three well-defined reversible peaks are observed during the charge/discharge processes. The appearance of 4 V peak is due to Mn3+ which is formed by oxygen loss during the hightemperature calcinations. Two major peaks appearing between 4.5 and 5 V during charge/discharge cyclings are due to the redox couples Ni2+/Ni3+ and Ni3+/Ni4+ or ordering of lithium and vacancies at x = 0.5. For the ordered spinel, the 4V peaks are absent because

due to a 2.5 orders of magnitude faster electronic conductivity.

(Redrawn from Ref. (Xia et al., 2007))

Li/Li+ at 0.2 mVs-1 scan rate (Xia et al., 2007)

space group of *Fd-3m*. The discharge capacity of LiNi0.5Mn1.5O4 reaches up to 134 mAh/g at a current density of 20 mA/g, while it retains as high as 110 mAh/g even at a high current density of 900 mA/g.

Fig. 20. a) Transmission electron micrograph and b) IR spectrum of LiNi0.5Mn1.5O4; c) Charge/discharge curves of LiNi0.5Mn1.5O4 cycled at current densities of 20 mA/g (solid line) and 160 mA/g (dash line); and d) Discharge curves of the as-prepared LiNi0.5Mn1.5O4 at various current densities. (Fang et al., 2007)

The another one problem is that LiNi0.5Mn1.5O4 still undergoes decomposition of electrolyte and dissolution of Mn and Ni, particularly under high voltage and elevated temperature conditions. The construction of heterogeneous nanostructure by coating some chemically stable compounds on LiNi0.5Mn1.5O4 is a most widely used strategy to enhance the electrochemical properties of LiNi0.5Mn1.5O4. This strategy has the advantages of the delivery of the original capacity due to no reduction in the amount of electrochemically active elements in the parent oxide and the suppression of the corrosion reactions between the high voltage spinel cathode and electrolyte. In this regard, various materials such as metal oxides, metal, fluoride, and carbon have been used to modify the surfaces of LiNi0.5Mn1.5O4.

#### **<IP_ADDRESS> Coating by metal oxides**

Surface modifications of LiNi0.5Mn1.5O4 spinel material by metal oxides such as ZnO (Arrebola et al., 2010), ZrO2 (Wu et al., 2010), Bi2O3 (Liu et al., 2009), and SiO2 (Fan et al., 2007) have been investigated. Metal oxides coating act as a protective layer against the attack by HF that generated from the decomposition of LiPF6 salt in the electrolyte, which effectively suppresses the corrosion reaction. For example, Sun et al. (2002) reported that the

Heterogeneous Nanostructured Electrode Materials

O atoms are in red and VO5 units in blue.

along the *a*-*b* plane into the interlayer space follows the equation:

**2.4 Vanadium pentoxide (V2O5)**

for Lithium-Ion Batteries – Recent Trends and Developments 59

V2O5 is a typical intercalation compound with a layered structure, and therefore is a very promising electrode material for LIBs as it offers the essential advantages of low-cost, abundant sources, high energy density, and better safety. As seen from Figure 21, V2O5 has an orthorhombic crystal structure which can be described as layers of VO5 square pyramids that share edges and corners. The sixth V-O bond in the *c*-direction consists of the weak electrostatic interactions, which facilitates the insertion of various ions and molecules between the layers. Li-ion insertion and electronic transport occur more easily along the *a*-*b* plane rather than through the layers of the *c*-axis (Chan et al., 2007). Lithium intercalates

Fig. 21. Layered crystal structure of bulk V2O5 with boxed region indicating the unit cell.

The theoretical capacity of V2O5 with two lithium insertions/extractions is about 294 mAh/g, much higher than those of the commonly used cathode materials like LiFePO4 (170 mAh/g), making it a promising cathode material for high power batteries. Since the nanostructured approaches have been recently demonstrated to be highly effective in enhancing the electrochemical performance of electrode materials, various nanostructured V2O5 has been studied to show the improved electrochemical properties as featured by high capacity and remarkable reversibility. For example, Sides et al. (2005) reported the preparation of monodisperse V2O5 fibers with an improved electrochemical performance because of the reduced lithium ion diffusion path and increased electrolyte-accessible surface area. Cao et al. (2005) have synthesized V2O5 with highly ordered superstructures, in which nanoparticles interconnect to form nanorods, and these rods circle around to form hollow microspheres. The specific capacity of these V2O5 hollow microspheres is 286.4 mAh/g at 0.2 C charge/discharge rate, which is very close to the theoretical capacity of V2O5. However, the Li-ion diffusion coefficient (10−12 cm2/s) and the electronic conductivity (10−2–10−3 S/cm) in crystalline V2O5 are inherently too low to sustain a large specific capacity at high charge/discharge rates, which cannot be simply altered by the nanostructured strategies mentioned above. In order to improve the electrochemical performance, many heterogeneous nanostructured forms of V2O5 composites have been reported in the literature, and the usefulness of these designs has been demonstrated.

+ - V O +xLi +xe Li V O 2 5 22 5 (3)

surface modification of LiNi0.5Mn1.5O4 by ZnO significantly improves the capability at elevated temperature. ZnO-coated electrode delivers a capacity of 137 mAh/g without any capacity loss even after 50 cycles at 55 ◦C. ZnO coating layer scavenged fluorine anions by transforming HF to ZnF2, which keeps the original particle morphology and reduces an increase in interfacial impedance between cathode and electrolyte. Arrebola et al (2010) have also confirmed the role of scavenging HF of ZnO coating layer, in which ZnO coating is beneficial for enhancing the rate capability of LiNi0.5Mn1.5O4. In the case of SiO2 coated LiNi0.5Mn1.5O4 reported by Fan et al. (2007), LiNi0.5Mn1.5O4 spinel is modified by a porous, nanostructured, amorphous SiO2 surface layer, which is a good HF consumer that can neutralize HF and yields Si-F species. Nanocomposites displayed an obviously improved capacity retention rate at 55 oC, which is beneficial from the lower HF content in electrolyte resulting from the reaction between SiO2 and HF and the mechanical separation effect of the SiO2 coating result in relatively lower content of LiF in the surfaces of the coated LiNi0.5Mn1.5O4 cathode materials. Very recently, Liu et al. (2009) modified the surface of the 5 V cathode material by Bi2O3 coating, which rendered both excellent rate capability and improved cycling performance. Further investigation indicates that Bi2O3 is reduced on the cathode surfaces during the electrochemical cycling to metallic Bi, an electronic conductor. Thus, "Bi2O3" modified layer acts as a protection shell and a fast electron transfer channel as well, which renders an excellent rate capability and good cycling performance.

#### **<IP_ADDRESS> Coating by metals**

Metals such as Zn (Alcantara et al., 2004), Au (Arrebola et al., 2007) and Ag (Arrebola et al., 2005) have also been coated onto the surfaces of spinel LiNi0.5Mn1.5O4 which significantly improves the electrochemical performance. For instance, Arrebola et al. (2007) found that Au coating on LiNi0.5Mn1.5O4 has a beneficial effect in increasing the capacity by hindering the unwanted reactions and simultaneously preventing the active material particles from reacting with the decomposition products such as HF. These authors studied the adverse effect of Ag treatment on the electrochemical performance of LiNi0.5Mn1.5O4. It is found that Ag treatment has only limited benefit at low current densities, while at moderate and high current densities, the capacity delivered is relatively low, even lower than that of the untreated spinel.

#### **<IP_ADDRESS> Coating by fluoride**

Coating of spinel LiNi0.5Mn1.5O4 with BiOF can also improve the electrochemical performances (Kang et al., 2010). For instance, BiOF-coated LiNi0.5Mn1.5O4 shows a significantly improved capacity retention of 84.5% for 70 cycles at 55 ◦C, while the uncoated one exhibits a capacity retention of only 31.3%. The rate capability of the BiOF-coated LiNi0.5Mn1.5O4 is also significantly enhanced. Such improvements in electrochemical performance are attributed to the scavenging HF by BiOF layer from the electrolyte.

#### **<IP_ADDRESS> Coating by carbon**

Yang et al. (2011) coated the spinel LiNi0.5Mn1.5O4 with a small amount of conductive carbon by the carbonization of sucrose. It is found that carbon coating greatly enhances the discharge capacity, rate capability, and cycling stability of the LiNi0.5Mn1.5O4 without degrading the spinel structure. LiNi0.5Mn1.5O4 modified with optimal 1 wt% sucrose can deliver a large capacity of 130 mAh/g at 1 C discharge rate with a high retention of 92% after 100 cycles and a high 114 mAh/g at 5 C discharge rate. The improved electrochemical performances upon carbon-coating are the consequence of the suppression of SEI layer as well as the enhancements in the electronic conductivity and Li+ diffusion.

#### **2.4 Vanadium pentoxide (V2O5)**

58 Lithium Ion Batteries – New Developments

surface modification of LiNi0.5Mn1.5O4 by ZnO significantly improves the capability at elevated temperature. ZnO-coated electrode delivers a capacity of 137 mAh/g without any capacity loss even after 50 cycles at 55 ◦C. ZnO coating layer scavenged fluorine anions by transforming HF to ZnF2, which keeps the original particle morphology and reduces an increase in interfacial impedance between cathode and electrolyte. Arrebola et al (2010) have also confirmed the role of scavenging HF of ZnO coating layer, in which ZnO coating is beneficial for enhancing the rate capability of LiNi0.5Mn1.5O4. In the case of SiO2 coated LiNi0.5Mn1.5O4 reported by Fan et al. (2007), LiNi0.5Mn1.5O4 spinel is modified by a porous, nanostructured, amorphous SiO2 surface layer, which is a good HF consumer that can neutralize HF and yields Si-F species. Nanocomposites displayed an obviously improved capacity retention rate at 55 oC, which is beneficial from the lower HF content in electrolyte resulting from the reaction between SiO2 and HF and the mechanical separation effect of the SiO2 coating result in relatively lower content of LiF in the surfaces of the coated LiNi0.5Mn1.5O4 cathode materials. Very recently, Liu et al. (2009) modified the surface of the 5 V cathode material by Bi2O3 coating, which rendered both excellent rate capability and improved cycling performance. Further investigation indicates that Bi2O3 is reduced on the cathode surfaces during the electrochemical cycling to metallic Bi, an electronic conductor. Thus, "Bi2O3" modified layer acts as a protection shell and a fast electron transfer channel as

well, which renders an excellent rate capability and good cycling performance.

the capacity delivered is relatively low, even lower than that of the untreated spinel.

performance are attributed to the scavenging HF by BiOF layer from the electrolyte.

well as the enhancements in the electronic conductivity and Li+ diffusion.

Metals such as Zn (Alcantara et al., 2004), Au (Arrebola et al., 2007) and Ag (Arrebola et al., 2005) have also been coated onto the surfaces of spinel LiNi0.5Mn1.5O4 which significantly improves the electrochemical performance. For instance, Arrebola et al. (2007) found that Au coating on LiNi0.5Mn1.5O4 has a beneficial effect in increasing the capacity by hindering the unwanted reactions and simultaneously preventing the active material particles from reacting with the decomposition products such as HF. These authors studied the adverse effect of Ag treatment on the electrochemical performance of LiNi0.5Mn1.5O4. It is found that Ag treatment has only limited benefit at low current densities, while at moderate and high current densities,

Coating of spinel LiNi0.5Mn1.5O4 with BiOF can also improve the electrochemical performances (Kang et al., 2010). For instance, BiOF-coated LiNi0.5Mn1.5O4 shows a significantly improved capacity retention of 84.5% for 70 cycles at 55 ◦C, while the uncoated one exhibits a capacity retention of only 31.3%. The rate capability of the BiOF-coated LiNi0.5Mn1.5O4 is also significantly enhanced. Such improvements in electrochemical

Yang et al. (2011) coated the spinel LiNi0.5Mn1.5O4 with a small amount of conductive carbon by the carbonization of sucrose. It is found that carbon coating greatly enhances the discharge capacity, rate capability, and cycling stability of the LiNi0.5Mn1.5O4 without degrading the spinel structure. LiNi0.5Mn1.5O4 modified with optimal 1 wt% sucrose can deliver a large capacity of 130 mAh/g at 1 C discharge rate with a high retention of 92% after 100 cycles and a high 114 mAh/g at 5 C discharge rate. The improved electrochemical performances upon carbon-coating are the consequence of the suppression of SEI layer as

**<IP_ADDRESS> Coating by metals**

**<IP_ADDRESS> Coating by fluoride**

**<IP_ADDRESS> Coating by carbon**

V2O5 is a typical intercalation compound with a layered structure, and therefore is a very promising electrode material for LIBs as it offers the essential advantages of low-cost, abundant sources, high energy density, and better safety. As seen from Figure 21, V2O5 has an orthorhombic crystal structure which can be described as layers of VO5 square pyramids that share edges and corners. The sixth V-O bond in the *c*-direction consists of the weak electrostatic interactions, which facilitates the insertion of various ions and molecules between the layers. Li-ion insertion and electronic transport occur more easily along the *a*-*b* plane rather than through the layers of the *c*-axis (Chan et al., 2007). Lithium intercalates along the *a*-*b* plane into the interlayer space follows the equation:

$$\rm V\_2O\_5 + xLi^+ + xe^- \to Li\_2V\_2O\_5 \tag{3}$$

Fig. 21. Layered crystal structure of bulk V2O5 with boxed region indicating the unit cell. O atoms are in red and VO5 units in blue.

The theoretical capacity of V2O5 with two lithium insertions/extractions is about 294 mAh/g, much higher than those of the commonly used cathode materials like LiFePO4 (170 mAh/g), making it a promising cathode material for high power batteries. Since the nanostructured approaches have been recently demonstrated to be highly effective in enhancing the electrochemical performance of electrode materials, various nanostructured V2O5 has been studied to show the improved electrochemical properties as featured by high capacity and remarkable reversibility. For example, Sides et al. (2005) reported the preparation of monodisperse V2O5 fibers with an improved electrochemical performance because of the reduced lithium ion diffusion path and increased electrolyte-accessible surface area. Cao et al. (2005) have synthesized V2O5 with highly ordered superstructures, in which nanoparticles interconnect to form nanorods, and these rods circle around to form hollow microspheres. The specific capacity of these V2O5 hollow microspheres is 286.4 mAh/g at 0.2 C charge/discharge rate, which is very close to the theoretical capacity of V2O5. However, the Li-ion diffusion coefficient (10−12 cm2/s) and the electronic conductivity (10−2–10−3 S/cm) in crystalline V2O5 are inherently too low to sustain a large specific capacity at high charge/discharge rates, which cannot be simply altered by the nanostructured strategies mentioned above. In order to improve the electrochemical performance, many heterogeneous nanostructured forms of V2O5 composites have been reported in the literature, and the usefulness of these designs has been demonstrated.

Heterogeneous Nanostructured Electrode Materials

composites. (Yamada et al., 2007)

al., 2011)

for Lithium-Ion Batteries – Recent Trends and Developments 61

porous carbons in V2O5 sol under a reduced pressure. Nanoporous V2O5/carbon composites thus prepared exhibit a large capacity of more than 100 mAh/g and good rate capability of 80% at 5.0 A/g (Figure 22b). The good performance is explained by electric double layer

capacitance of large surface area and high rate lithium insertion to V2O5 gel.

Fig. 22. a) TEM image of V2O5/C composites; and b) rate capability of V2O5[n]/C

Fig. 23. a) Schematic diagram showing the strategy for coating V2O5 on SnO2 nanowires (NWs) grown on a stainless-steel (SS) substrate; b) FE-SEM image of the typical V2O5 loaded SnO2 nanowires; the inset indicates the diameter is about 100 nm; c) the galvanostatic charge/discharge curves of the SVNs electrode at different current densities; and d) rate performance at different current densities, and the inset showing the Ragone plots. (Yan et

#### **2.4.1 Mixed with conducting secondary phase**

To enhance the high rate capability of V2O5 cathode materials, the creation of heterogeneous nanostructures from nanoscale V2O5 and conducting secondary phase is one of the most widely used strategies. There are three kinds of mainly adopted conducting materials, viz. conducting conjugated polymers (CCPS), carbon materials, and various metal oxides. CCPS possess a high inherent conductivity and is able to undergo practically fully reversible redox processes. The heterogeneous nanostructures from V2O5 and CCPS combine a high capacity and ability to intercalate lithium ions in V2O5, and high cyclability, conductivity, and other properties of CCPS. For example, Posudievsky et al., (2011) prepared two-component guest– host nanocomposites which are composed of conducting polymers (polyaniline, polypyrrole and polythiophene) and V2O5. The nanocomposites are capable of reversible cycling as the positive electrode in a lithium ion cell, and retain their capacity over one hundred full charge/discharge cycles. The higher discharge capacity, stability in prolonged charge/discharge cycling, and improved diffusion of lithium ions in the nanocomposites, are the consequences of pillaring by the conducting polymer macromolecules in the layers of the inorganic matrix.

Carbon coating has been applied to V2O5 cathode to give good electronic conductivity and stability (Odani et al., 2006; koltypin et al., 2007). For example, koltypin et al. (2007) have prepared carbon-coated V2O5 nanoparticles. The carbon coated V2O5 electrodes demonstrate a higher capacity, much better rate capability, and very good stability upon cycling and aging at elevated temperatures when compared to the electrodes that comprise the microparticles of V2O5.

Other than CCPS and carbon, inorganic oxides such as TiO2 (Takahashi et al, 2006) and WO3 (Cai et al, 2011) have been introduced with an aim to improve the electrochemical properties by modifying the crystallinity of V2O5. For example, Cai et al (2011) found that V2O5–WO3 composites films show the enhanced Li-ion intercalation properties compared to pure V2O5 or WO3 films. At a high current density of 1.33 A/g, V2O5–WO3 film with a V2O5/WO3 molar ratio of 10/1 exhibits the highest capacities of 200 mAh/g at the first cycle and 132 mAh/g after 50 cycles, while pure V2O5 film delivers discharge capacities of 108 mAh/g at the first cycle and 122 mAh/g after 50 cycles. The enhanced Li-ion intercalation properties of the composite films are ascribed to the reduced crystallinity, increased porosity, and the enhanced surface area.

#### **2.4.2 Thin layer loaded on a continuous conductive matrix**

To improve the high rate electrochemical property of V2O5, rapid and continuous electronic and ionic transportation path are required. Reducing the dimension of V2O5 is a most effective method to shorten the Li ion diffusion path and fasten the Li ion transportation, since Li-ion diffusion coefficient of V2O5 (10−12 cm2/s) cannot be altered simply by nanostructure strategy. Also, the disadvantage of the low electronic conductivity (10−2–10−3 S/cm) of V2O5 can be effectively overcome by the introduction of conductive secondary phase. So, if uniformly ultra thin layer of V2O5 is formed on a continuous conductive matrix, rapid and continuous electronic and ionic transportation path can be achieved. For example, Yamada et al. (2007) coated thin V2O5 layer on porous carbon, as shown in Figure 22a. In their work, the porous carbons were prepared using SiO2 colloidal crystals as the templates with three dimensionally interconnected pores, and then V2O5/carbon composites were achieved by immersing the

To enhance the high rate capability of V2O5 cathode materials, the creation of heterogeneous nanostructures from nanoscale V2O5 and conducting secondary phase is one of the most widely used strategies. There are three kinds of mainly adopted conducting materials, viz. conducting conjugated polymers (CCPS), carbon materials, and various metal oxides. CCPS possess a high inherent conductivity and is able to undergo practically fully reversible redox processes. The heterogeneous nanostructures from V2O5 and CCPS combine a high capacity and ability to intercalate lithium ions in V2O5, and high cyclability, conductivity, and other properties of CCPS. For example, Posudievsky et al., (2011) prepared two-component guest– host nanocomposites which are composed of conducting polymers (polyaniline, polypyrrole and polythiophene) and V2O5. The nanocomposites are capable of reversible cycling as the positive electrode in a lithium ion cell, and retain their capacity over one hundred full charge/discharge cycles. The higher discharge capacity, stability in prolonged charge/discharge cycling, and improved diffusion of lithium ions in the nanocomposites, are the consequences of pillaring by the conducting polymer macromolecules in the layers

Carbon coating has been applied to V2O5 cathode to give good electronic conductivity and stability (Odani et al., 2006; koltypin et al., 2007). For example, koltypin et al. (2007) have prepared carbon-coated V2O5 nanoparticles. The carbon coated V2O5 electrodes demonstrate a higher capacity, much better rate capability, and very good stability upon cycling and aging at elevated temperatures when compared to the electrodes that comprise the

Other than CCPS and carbon, inorganic oxides such as TiO2 (Takahashi et al, 2006) and WO3 (Cai et al, 2011) have been introduced with an aim to improve the electrochemical properties by modifying the crystallinity of V2O5. For example, Cai et al (2011) found that V2O5–WO3 composites films show the enhanced Li-ion intercalation properties compared to pure V2O5 or WO3 films. At a high current density of 1.33 A/g, V2O5–WO3 film with a V2O5/WO3 molar ratio of 10/1 exhibits the highest capacities of 200 mAh/g at the first cycle and 132 mAh/g after 50 cycles, while pure V2O5 film delivers discharge capacities of 108 mAh/g at the first cycle and 122 mAh/g after 50 cycles. The enhanced Li-ion intercalation properties of the composite films are ascribed to the reduced crystallinity, increased porosity, and the

To improve the high rate electrochemical property of V2O5, rapid and continuous electronic and ionic transportation path are required. Reducing the dimension of V2O5 is a most effective method to shorten the Li ion diffusion path and fasten the Li ion transportation, since Li-ion diffusion coefficient of V2O5 (10−12 cm2/s) cannot be altered simply by nanostructure strategy. Also, the disadvantage of the low electronic conductivity (10−2–10−3 S/cm) of V2O5 can be effectively overcome by the introduction of conductive secondary phase. So, if uniformly ultra thin layer of V2O5 is formed on a continuous conductive matrix, rapid and continuous electronic and ionic transportation path can be achieved. For example, Yamada et al. (2007) coated thin V2O5 layer on porous carbon, as shown in Figure 22a. In their work, the porous carbons were prepared using SiO2 colloidal crystals as the templates with three dimensionally interconnected pores, and then V2O5/carbon composites were achieved by immersing the

**2.4.1 Mixed with conducting secondary phase**

of the inorganic matrix.

microparticles of V2O5.

enhanced surface area.

**2.4.2 Thin layer loaded on a continuous conductive matrix**

porous carbons in V2O5 sol under a reduced pressure. Nanoporous V2O5/carbon composites thus prepared exhibit a large capacity of more than 100 mAh/g and good rate capability of 80% at 5.0 A/g (Figure 22b). The good performance is explained by electric double layer capacitance of large surface area and high rate lithium insertion to V2O5 gel.

Fig. 22. a) TEM image of V2O5/C composites; and b) rate capability of V2O5[n]/C composites. (Yamada et al., 2007)

Fig. 23. a) Schematic diagram showing the strategy for coating V2O5 on SnO2 nanowires (NWs) grown on a stainless-steel (SS) substrate; b) FE-SEM image of the typical V2O5 loaded SnO2 nanowires; the inset indicates the diameter is about 100 nm; c) the galvanostatic charge/discharge curves of the SVNs electrode at different current densities; and d) rate performance at different current densities, and the inset showing the Ragone plots. (Yan et al., 2011)

Heterogeneous Nanostructured Electrode Materials

cycles with little capacity fade.

of LIBs in practical uses.

TNHCs cycled at a rate of C/5. (Zhang et al., 2008)

for Lithium-Ion Batteries – Recent Trends and Developments 63

conductivity, prevents aggregation, improves chemical stability, and buffers the stress of the inner nanoscale active material. Zhang et al. (2008) reported tin nanoparticles encapsulated elastic hollow carbon spheres (TNHCs) with an uniform size. As shown in Figure 24, in the TNHCs composites, multiple tin nanoparticles with a diameter of less than 100 nm are encapsulated in one thin hollow carbon sphere with a thickness of only 20 nm. This special structure results in both the content of Sn up to 70% by weight and the void volume in carbon shell as high as 70–80% by volume. This void volume and elasticity of thin carbon spherical shell efficiently accommodate the volume change of tin nanoparticles due to the Li-Sn alloying-dealloying reactions, and thus prevent the pulverization of electrode. As a result, this composite shows a decent cycle ability with a high initial capacity of more than 800 mAh/g, and after 100 cycles it shows only a moderate decrease in its specific capacity to 550 mAh/g. Coaxial SnO2@carbon hollow nanospheres have also been reported by Lou et al. (2009). In this new type of nanoarchitecture, the carbon shell is tightly attached to the SnO2 shell, beneficial for the mechanical reinforcement and the enhanced electronic conduction. The composites deliver a discharge capacity of around 460 mAh/g after 100

Fig. 24. a) TEM image of TNHCs; b) discharge/charge capacity profiles of TNHCs in 5 mV–3 V (vs.Li+/Li) voltage window and C/5. Inset shows the first discharge/charge profiles of

Recently, Yang et al. (2011) reported TiO2(B) @carbon composite nanowires by a two-step hydrothermal process with subsequent heat treatment in argon. As shown in Figure 25, the nanostructures exhibit a unique feature of having TiO2(B) encapsulated inside and an amorphous carbon layer coating outside. The carbon shell accelerates the diffusion of both electrons and lithium ions, ensuring a high electrode–electrolyte contact area and eventually making a greater contribution to the capacity. TiO2(B) core also acts as an effective mechanical support to alleviate the stress produced during lithium intercalation/ deintercalation and preserve the one dimensional core/shell structure even after 100 charge/discharge cycles. The composite nanowires exhibit a high reversible capacity of 560 mAh/g after 100 cycles at a

Above reports have fully demonstrated the benefits of heterogeneous core-shell nanostructures. In this core-shell strategy, the void volume in the core-shell configuration should be optimized according to the actual volume variation of electrode materials during the lithium insertion and extraction, in order to avoid decreasing the volume energy density

current density of 30 mA/g, and excellent cycling stability and rate capability.

Recently, Yan et al. (2011) have adopted a simple gas-phase-based method to synthesize V2O5 loaded SnO2 nanowires on the stainless-steel substrate (Figures 23a and b). V2O5 loaded SnO2 nanowires exhibit a high rate capability and very good cycling stability (Figure 23c). SnO2/V2O5 core/shell-nanowires deliver a high power density of about 60 kW/kg while the energy density remains at 282 wh/kg (Figure 23d). The improved electrochemical performance can be ascribed to the synergic effect of V2O5 thin layer and SnO2 nanowires. V2O5 layer coated on SnO2 is very thin, which could shorten the ion-diffusion time and enable a high utilization of V2O5. The free-standing SnO2 nanowires create porous channels, providing effective electrolyte transport and a large contact interface between the electrolyte and active materials. These findings open up new opportunities in the development of highperformance energy-storage devices used for micro-systems, thin-film devices, and other functional devices.

#### **3. Heterogeneous nanostructured anode materials**

Due to the low theoretical capacity of 372 mAh/g for graphitic carbon, intensive research has been conducted to search for the alternative anode materials with higher capacities and rate capabilities. Many other anode materials, such as Sn, Sb, Si, Ge, SnO2, and Co3O4 have been reported to have larger lithium storage capacities than graphitic carbon via the formation of alloys with lithium or through the reversible reactions with lithium ions. However, these anode materials suffer from the huge volume variation during the lithium insertion/extraction process which leads to the pulverization problem and moreover a rapid deterioration in capacity. Reducing the size of these anode materials to nanometer scale can partially suppress the volume variation, and designing the structure of these anode materials as hollow spheres or porous particles can improve the lithium storage capacity and their initial Coulombic efficiency to a certain degree. Unfortunately, the capacity decay of the electrode along with the charge/discharge cycling still cannot be completely avoided (Liang et al., 2009). Recently, heterogeneous nanostructured anode materials, which are composed of multi-nanocomponents have been shown to exhibit an improved rate and cyclying performance due to the synergic effect between the functional nanocomponents. In this section, we introduced the recent trends and developments of these heterogeneous nanostructured anode materials, including carbonaceous nanocomposites, polymer/inorganic oxides nanocomposites, and binder-free thin-film nanocomposites.

#### **3.1 Carbonaceous composites anode materials**

Carbonaceous composites materials have been widely used for anodes. Though there are many different types of carbon materials, such as amorphous carbon, carbon nanotubes, and graphene, etc., all of them can not only significantly enhance the electronic conductivity of electrode materials, but also lead to the stabilized SEI films on anodes, which result in the improved rate and cycling performance. However, the synergic effect between carbon materials and other lithium intercalation materials is very complicated. Even when the same carbon material is adopted, the synergic effect could be significantly different due to the varied combination states of nanocomponents.

#### **3.1.1 Core-shell nanocomposites**

Core–shell nanocomposites for carbon-based anodes refer to a core energy dense nanoparticle coated with a thin amorphous carbon shell, which enhances the electrical

Recently, Yan et al. (2011) have adopted a simple gas-phase-based method to synthesize V2O5 loaded SnO2 nanowires on the stainless-steel substrate (Figures 23a and b). V2O5 loaded SnO2 nanowires exhibit a high rate capability and very good cycling stability (Figure 23c). SnO2/V2O5 core/shell-nanowires deliver a high power density of about 60 kW/kg while the energy density remains at 282 wh/kg (Figure 23d). The improved electrochemical performance can be ascribed to the synergic effect of V2O5 thin layer and SnO2 nanowires. V2O5 layer coated on SnO2 is very thin, which could shorten the ion-diffusion time and enable a high utilization of V2O5. The free-standing SnO2 nanowires create porous channels, providing effective electrolyte transport and a large contact interface between the electrolyte and active materials. These findings open up new opportunities in the development of highperformance energy-storage devices used for micro-systems, thin-film devices, and other

Due to the low theoretical capacity of 372 mAh/g for graphitic carbon, intensive research has been conducted to search for the alternative anode materials with higher capacities and rate capabilities. Many other anode materials, such as Sn, Sb, Si, Ge, SnO2, and Co3O4 have been reported to have larger lithium storage capacities than graphitic carbon via the formation of alloys with lithium or through the reversible reactions with lithium ions. However, these anode materials suffer from the huge volume variation during the lithium insertion/extraction process which leads to the pulverization problem and moreover a rapid deterioration in capacity. Reducing the size of these anode materials to nanometer scale can partially suppress the volume variation, and designing the structure of these anode materials as hollow spheres or porous particles can improve the lithium storage capacity and their initial Coulombic efficiency to a certain degree. Unfortunately, the capacity decay of the electrode along with the charge/discharge cycling still cannot be completely avoided (Liang et al., 2009). Recently, heterogeneous nanostructured anode materials, which are composed of multi-nanocomponents have been shown to exhibit an improved rate and cyclying performance due to the synergic effect between the functional nanocomponents. In this section, we introduced the recent trends and developments of these heterogeneous nanostructured anode materials, including carbonaceous nanocomposites, polymer/inorganic

Carbonaceous composites materials have been widely used for anodes. Though there are many different types of carbon materials, such as amorphous carbon, carbon nanotubes, and graphene, etc., all of them can not only significantly enhance the electronic conductivity of electrode materials, but also lead to the stabilized SEI films on anodes, which result in the improved rate and cycling performance. However, the synergic effect between carbon materials and other lithium intercalation materials is very complicated. Even when the same carbon material is adopted, the synergic effect could be significantly different due to the

Core–shell nanocomposites for carbon-based anodes refer to a core energy dense nanoparticle coated with a thin amorphous carbon shell, which enhances the electrical

functional devices.

**3. Heterogeneous nanostructured anode materials**

oxides nanocomposites, and binder-free thin-film nanocomposites.

**3.1 Carbonaceous composites anode materials**

varied combination states of nanocomponents.

**3.1.1 Core-shell nanocomposites**

conductivity, prevents aggregation, improves chemical stability, and buffers the stress of the inner nanoscale active material. Zhang et al. (2008) reported tin nanoparticles encapsulated elastic hollow carbon spheres (TNHCs) with an uniform size. As shown in Figure 24, in the TNHCs composites, multiple tin nanoparticles with a diameter of less than 100 nm are encapsulated in one thin hollow carbon sphere with a thickness of only 20 nm. This special structure results in both the content of Sn up to 70% by weight and the void volume in carbon shell as high as 70–80% by volume. This void volume and elasticity of thin carbon spherical shell efficiently accommodate the volume change of tin nanoparticles due to the Li-Sn alloying-dealloying reactions, and thus prevent the pulverization of electrode. As a result, this composite shows a decent cycle ability with a high initial capacity of more than 800 mAh/g, and after 100 cycles it shows only a moderate decrease in its specific capacity to 550 mAh/g. Coaxial SnO2@carbon hollow nanospheres have also been reported by Lou et al. (2009). In this new type of nanoarchitecture, the carbon shell is tightly attached to the SnO2 shell, beneficial for the mechanical reinforcement and the enhanced electronic conduction. The composites deliver a discharge capacity of around 460 mAh/g after 100 cycles with little capacity fade.

Fig. 24. a) TEM image of TNHCs; b) discharge/charge capacity profiles of TNHCs in 5 mV–3 V (vs.Li+/Li) voltage window and C/5. Inset shows the first discharge/charge profiles of TNHCs cycled at a rate of C/5. (Zhang et al., 2008)

Recently, Yang et al. (2011) reported TiO2(B) @carbon composite nanowires by a two-step hydrothermal process with subsequent heat treatment in argon. As shown in Figure 25, the nanostructures exhibit a unique feature of having TiO2(B) encapsulated inside and an amorphous carbon layer coating outside. The carbon shell accelerates the diffusion of both electrons and lithium ions, ensuring a high electrode–electrolyte contact area and eventually making a greater contribution to the capacity. TiO2(B) core also acts as an effective mechanical support to alleviate the stress produced during lithium intercalation/ deintercalation and preserve the one dimensional core/shell structure even after 100 charge/discharge cycles. The composite nanowires exhibit a high reversible capacity of 560 mAh/g after 100 cycles at a current density of 30 mA/g, and excellent cycling stability and rate capability.

Above reports have fully demonstrated the benefits of heterogeneous core-shell nanostructures. In this core-shell strategy, the void volume in the core-shell configuration should be optimized according to the actual volume variation of electrode materials during the lithium insertion and extraction, in order to avoid decreasing the volume energy density of LIBs in practical uses.

Heterogeneous Nanostructured Electrode Materials

rates. (Wu et al., 2011)

for Lithium-Ion Batteries – Recent Trends and Developments 65

Fig. 27. a) TEM images of the Fe2O3-filled CNTs; and b) cycling performance of the Fe2O3-filled CNT measured at a rate of 35 mA/g between 0.02 and 2.5 V (vs. Li+/Li). (Yu et al., 2010)

Fig. 28. a) TEM images of CoO@CNT; and b) cycling performance of CoO@CNT at high-

The other strategy is encapsulating the electrode materials in the interior space of CNT (Wang et al., 2009; Wu et al., 2010; Yu et al., 2010; Wu et al., 2011). For example, Yu et al. (2010) synthesized Fe2O3/CNT nanocomposites by AAO template method. As shown in Figure 27, in this nanocomposite, Fe2O3 nanoparticles with a mean diameter of ~9 nm are filled homogeneously into the hollow core of high aspect ratio CNT. The Fe2O3-filled CNTs exhibit a high initial discharge capacity of 2081 mAh/g, and a reversible capacity of 768 mAh/g is obtained after 40 cycles, while the Coulombic efficiency steadily keeps at higher than 95%. Fe2O3-filled CNTs also show a superior cycling performance and rate capability, which can be attributed to the small size of Fe2O3 particles and the confinement effect as well as the good electrical conductivity of CNTs. Very recently, Wu et al. (2011) reported a self-assembled echinus-like nanostructure that consists of mesoporous CoO nanorod@carbon nanotube core–shell materials, as shown in Figure 28a. This core–shell nanostructure shows a high capacity (703–746 mAh/g in 200 cycles) and a long cycle life (0.029% capacity loss per cycle) at a high current rate of 3580 mA/g, as shown in Figure 28b. The excellent electrochemical energy storage should be attributed to the increased electrical conductivity, mechanical stability and electrochemical activity of porous CoO materials in the presence of a carbon nanotube overlayer. This core-shell nanostructure not only

Fig. 25. a) HRTEM image of a section of a TiO2(B)@carbon composite nanowire; b) capacity vs. cycle number curves from the first cycle to the 100th cycle for the TiO2(B)@carbon composite nanowires at a current density of 30 mA/g. (Yang et al., 2011)

#### **3.1.2 Carbon-nanotube-based nanocomposites**

Unique properties of carbon nanotube (CNT) such as high conductivity, nanotexture, resiliency, are very helpful for LIBs, where pure CNT and/or their composites play the role of electrode materials. CNT provides an ideal conducting path to ensure a good charge propagation and can serve as a perfect resilient skeleton with stretchable properties that gives a significant improvement in electrochemical cyclability especially when volumetric changes take place during the charge/discharge processes. In this regard, CNTs have been widely used as the conductive additives or conductive matrix for the anode materials. There are two main strategies to synthesize CNT heterogeneous nanostructures: One is interconnecting the electrode materials with the exterior surface of CNT by various mixing procedures. For example, Zhang et al. (2009) prepared CNT@SnO2 hybrid nanostructures with excellent lithium storage performance by uniformly loading SnO2 nanoparticles onto a cross-stacked CNT network. Xia et al. (2010) prepared caterpillar-like nanoflaky MnO2/CNT nanocomposites, in which highly porous interconnected MnO2 nanoflakes are uniformly coated on the surface of CNT. This nanoflaky MnO2/CNT nanocomposite electrode exhibits a large reversible capacity of 801 mAh/g for the first cycle without capacity fading for the first 20 cycles (Figure 26).

Fig. 26. a) TEM images of one single CNT coated with porous MnO2 layer; and b) voltage profiles of a MnO2/CNT nanocomposite electrode cycled between 0.01 and 3 V at a current density of 200 mA/g. (Xia et al., 2010)

 Fig. 25. a) HRTEM image of a section of a TiO2(B)@carbon composite nanowire; b) capacity vs. cycle number curves from the first cycle to the 100th cycle for the TiO2(B)@carbon

Unique properties of carbon nanotube (CNT) such as high conductivity, nanotexture, resiliency, are very helpful for LIBs, where pure CNT and/or their composites play the role of electrode materials. CNT provides an ideal conducting path to ensure a good charge propagation and can serve as a perfect resilient skeleton with stretchable properties that gives a significant improvement in electrochemical cyclability especially when volumetric changes take place during the charge/discharge processes. In this regard, CNTs have been widely used as the conductive additives or conductive matrix for the anode materials. There are two main strategies to synthesize CNT heterogeneous nanostructures: One is interconnecting the electrode materials with the exterior surface of CNT by various mixing procedures. For example, Zhang et al. (2009) prepared CNT@SnO2 hybrid nanostructures with excellent lithium storage performance by uniformly loading SnO2 nanoparticles onto a cross-stacked CNT network. Xia et al. (2010) prepared caterpillar-like nanoflaky MnO2/CNT nanocomposites, in which highly porous interconnected MnO2 nanoflakes are uniformly coated on the surface of CNT. This nanoflaky MnO2/CNT nanocomposite electrode exhibits a large reversible capacity of 801 mAh/g for the first cycle without capacity fading for the

Fig. 26. a) TEM images of one single CNT coated with porous MnO2 layer; and b) voltage profiles of a MnO2/CNT nanocomposite electrode cycled between 0.01 and 3 V at a current

composite nanowires at a current density of 30 mA/g. (Yang et al., 2011)

**3.1.2 Carbon-nanotube-based nanocomposites**

first 20 cycles (Figure 26).

density of 200 mA/g. (Xia et al., 2010)

Fig. 27. a) TEM images of the Fe2O3-filled CNTs; and b) cycling performance of the Fe2O3-filled CNT measured at a rate of 35 mA/g between 0.02 and 2.5 V (vs. Li+/Li). (Yu et al., 2010)

Fig. 28. a) TEM images of CoO@CNT; and b) cycling performance of CoO@CNT at highrates. (Wu et al., 2011)

The other strategy is encapsulating the electrode materials in the interior space of CNT (Wang et al., 2009; Wu et al., 2010; Yu et al., 2010; Wu et al., 2011). For example, Yu et al. (2010) synthesized Fe2O3/CNT nanocomposites by AAO template method. As shown in Figure 27, in this nanocomposite, Fe2O3 nanoparticles with a mean diameter of ~9 nm are filled homogeneously into the hollow core of high aspect ratio CNT. The Fe2O3-filled CNTs exhibit a high initial discharge capacity of 2081 mAh/g, and a reversible capacity of 768 mAh/g is obtained after 40 cycles, while the Coulombic efficiency steadily keeps at higher than 95%. Fe2O3-filled CNTs also show a superior cycling performance and rate capability, which can be attributed to the small size of Fe2O3 particles and the confinement effect as well as the good electrical conductivity of CNTs. Very recently, Wu et al. (2011) reported a self-assembled echinus-like nanostructure that consists of mesoporous CoO nanorod@carbon nanotube core–shell materials, as shown in Figure 28a. This core–shell nanostructure shows a high capacity (703–746 mAh/g in 200 cycles) and a long cycle life (0.029% capacity loss per cycle) at a high current rate of 3580 mA/g, as shown in Figure 28b. The excellent electrochemical energy storage should be attributed to the increased electrical conductivity, mechanical stability and electrochemical activity of porous CoO materials in the presence of a carbon nanotube overlayer. This core-shell nanostructure not only

Heterogeneous Nanostructured Electrode Materials

**3.2 Polymer/inorganic oxides nanocomposites**

improve the structural stability.

length, and easier interaction with lithium.

**3.2.1 Polypyrrole (PPy)**

for Lithium-Ion Batteries – Recent Trends and Developments 67

Fig. 29. a) SEM images of graphene-encapsulated Co3O4 and b) comparison of the cycle performance of graphene-encapsulated Co3O4 (■), mixed Co3O4/graphene composite (●), and bare Co3O4 electrodes (▲) over 30 cycles (current density=74 mA/g). (Yang et al. 2010)

Heterogeneous nanostructures that consist of the composites of inorganic oxides and organic conducting polymers have been proven to be an effective approach to improve the high-rate electrochemical properties of LIBs. Organic conducting polymers provide a conducting backbone for the nanoscale inorganic oxides and accelerate the charge transfer between them, limit formation of a surface electrochemical interface (SEI) layer to improve the chemical stability, and buffer the stress of the inner nanoscale inorganic oxides to

PPy is a chemical compound formed from a number of connected pyrrole ring structures and it is the first polyacetylene-derivative to show a high conductivity. Chew et al. (2007) reported a novel nano-silicon/PPy composite by chemical ploymerization. The cycling stability of Si/PPy electrodes is significantly improved when compared to that of the bare Si anodes. PPy plays a key role in improving the electrochemical performance of the composites: (i) PPy in the composites is a conducting polymer, which increases the conductivity of the samples. (ii) PPy can act as a conductive binder, increasing the contact among particles, therefore the particle-to-particle resistance will be decreased, thus reducing the irreversible reactions with the electrolyte. (iii) PPy is an effective component that buffers the great volume changes during the cycling process, thus improving the cyclability of the Si electrode. Similarly, Cui et al. (2011) have synthesized SnO2@ PPy nanocomposites by an one-pot oxidative chemical polymerization method. The nanocomposites exhibit high discharge/charge capacities and favourable cycling. For SnO2@PPy nanocomposites with 79 wt% SnO2, the electrode reaction kinetics is controlled by the diffusion of Li+ in the nanocomposite. The calculated diffusion coefficiency of lithium ions in SnO2@PPy nanocomposite with 79 wt% SnO2 is 6.7×10−8 cm2 s−1, while the lithium-alloying activation energy at 0.5 V is 47.3 kJ mol−1, which is obviously lower than that for the bare SnO2. The enhanced electrode performance with the SnO2@PPy nanocomposite is originated from the advantageous nanostructures that allow a better structural flexibility, shorter diffusion

increases the electronic and ionic conductivity due to the presence of highly conductive CNT, but also improves the stability during the repeated ionic intercalation due to the flexible CNT, which act as a buffer to alleviate the volume expansion of metal oxides.

#### **3.1.3 Graphene-based nanocomposites**

In the family of carbon nanostructures, graphene is the youngest member but has attracted enormous recent interest. Graphene is a two-dimensional macromolecular sheet of carbon atoms with a honey comb structure. It is an excellent substrate to host active nanomaterials for LIBs applications due to its high conductivity, large surface area, flexibility, and chemical stability. In this regard, chemically modified graphene materials have been used to form the heterogeneous nanostructured materials with Si (Chou et al., 2010), SnO2 (Paek et al., 2009), Co3O4 (Yang et al., 2010), TiO2 (Li et al., 2011), Fe2O3 (Zhu et al., 2011), Mn3O4 (Wang et al., 2010), and SnSe2 (Choi et al., 2011). Improved electrochemical performances are thus achieved, owing to the synergic effect between graphene and active nanoparticles. In these nanocomposites, the ultrathin flexible graphene layers can provide a support for anchoring well-dispersed active nanoparticles and work as a highly conductive matrix for enabling good contact between them, which can also effectively prevent the volume expansion/contraction and aggregation of the active nanoparticles during Li charge/discharge processes. Meanwhile, the anchoring of active nanoparticles on graphene effectively reduces the degree of restacking of graphene sheets and consequently keeps a highly active surface area and to some extent, increases the lithium storage capacity and cyclic performance (Wu et al., 2010). For example, Paek et al. (2009) reported SnO2/graphene nanocomposites, in which graphene nanosheets are homogeneously distributed between the loosely packed SnO2 nanoparticles. This nanocomposite exhibits a charge capacity of 570 mAh/g after 30 cycles with about 70 % retention of the reversible capacity. Recently, Zhang et al. (2010) synthesized Fe3O4/graphene composites by depositing Fe3+ in the interspaces of graphene sheets. In the composites, Fe3O4 nanoparticles are dispersed on graphene sheets, which show a high reversible capacity, as well as a significantly enhanced cycling performance (about 650 mAh/ g after 50 cycles) and high rate capabilities (350 mAh/g at 5 C).

Though graphene sheets can effectively buffer the strain from the volume change of metals or metal oxides during the charge/discharge processes and preserve the high electrical conductivity of the overall electrode, the metal and metal oxide nanoparticles are still prone to strong aggregation during the cycle processes because of non-intimate contact between graphene layers and active nanoparticles, leading to a slow capacity fading. An alternative strategy for solving the aggregation problem of metal and metal oxides is to confine them within individual graphene shells. Yang et al. (2010) reported graphene-encapsulated Co3O4 nanoparticles prepared by co-assembly between negatively charged graphene oxide and positively charged oxide nanoparticles. This assembly enables a good encapsulation of electrochemically active metal oxide nanoparticles by graphene sheets, thus leading to a remarkable lithium-storage performance such as highly reversible capacity and excellent cycle performance. A very high and stable reversible capacity of about 1100 mAh/g in the initial 10 cycles, and 1000 mAh/g after 130 cycles is achieved for the graphene-encapsulated Co3O4 electrode, which is superior over those of Co3O4/graphene composite or bare Co3O4 electrodes (Figure 29).

increases the electronic and ionic conductivity due to the presence of highly conductive CNT, but also improves the stability during the repeated ionic intercalation due to the

In the family of carbon nanostructures, graphene is the youngest member but has attracted enormous recent interest. Graphene is a two-dimensional macromolecular sheet of carbon atoms with a honey comb structure. It is an excellent substrate to host active nanomaterials for LIBs applications due to its high conductivity, large surface area, flexibility, and chemical stability. In this regard, chemically modified graphene materials have been used to form the heterogeneous nanostructured materials with Si (Chou et al., 2010), SnO2 (Paek et al., 2009), Co3O4 (Yang et al., 2010), TiO2 (Li et al., 2011), Fe2O3 (Zhu et al., 2011), Mn3O4 (Wang et al., 2010), and SnSe2 (Choi et al., 2011). Improved electrochemical performances are thus achieved, owing to the synergic effect between graphene and active nanoparticles. In these nanocomposites, the ultrathin flexible graphene layers can provide a support for anchoring well-dispersed active nanoparticles and work as a highly conductive matrix for enabling good contact between them, which can also effectively prevent the volume expansion/contraction and aggregation of the active nanoparticles during Li charge/discharge processes. Meanwhile, the anchoring of active nanoparticles on graphene effectively reduces the degree of restacking of graphene sheets and consequently keeps a highly active surface area and to some extent, increases the lithium storage capacity and cyclic performance (Wu et al., 2010). For example, Paek et al. (2009) reported SnO2/graphene nanocomposites, in which graphene nanosheets are homogeneously distributed between the loosely packed SnO2 nanoparticles. This nanocomposite exhibits a charge capacity of 570 mAh/g after 30 cycles with about 70 % retention of the reversible capacity. Recently, Zhang et al. (2010) synthesized Fe3O4/graphene composites by depositing Fe3+ in the interspaces of graphene sheets. In the composites, Fe3O4 nanoparticles are dispersed on graphene sheets, which show a high reversible capacity, as well as a significantly enhanced cycling performance (about 650 mAh/ g after 50 cycles) and high

Though graphene sheets can effectively buffer the strain from the volume change of metals or metal oxides during the charge/discharge processes and preserve the high electrical conductivity of the overall electrode, the metal and metal oxide nanoparticles are still prone to strong aggregation during the cycle processes because of non-intimate contact between graphene layers and active nanoparticles, leading to a slow capacity fading. An alternative strategy for solving the aggregation problem of metal and metal oxides is to confine them within individual graphene shells. Yang et al. (2010) reported graphene-encapsulated Co3O4 nanoparticles prepared by co-assembly between negatively charged graphene oxide and positively charged oxide nanoparticles. This assembly enables a good encapsulation of electrochemically active metal oxide nanoparticles by graphene sheets, thus leading to a remarkable lithium-storage performance such as highly reversible capacity and excellent cycle performance. A very high and stable reversible capacity of about 1100 mAh/g in the initial 10 cycles, and 1000 mAh/g after 130 cycles is achieved for the graphene-encapsulated Co3O4 electrode, which is superior over those of Co3O4/graphene composite or bare Co3O4

flexible CNT, which act as a buffer to alleviate the volume expansion of metal oxides.

**3.1.3 Graphene-based nanocomposites**

rate capabilities (350 mAh/g at 5 C).

electrodes (Figure 29).

Fig. 29. a) SEM images of graphene-encapsulated Co3O4 and b) comparison of the cycle performance of graphene-encapsulated Co3O4 (■), mixed Co3O4/graphene composite (●), and bare Co3O4 electrodes (▲) over 30 cycles (current density=74 mA/g). (Yang et al. 2010)

#### **3.2 Polymer/inorganic oxides nanocomposites**

Heterogeneous nanostructures that consist of the composites of inorganic oxides and organic conducting polymers have been proven to be an effective approach to improve the high-rate electrochemical properties of LIBs. Organic conducting polymers provide a conducting backbone for the nanoscale inorganic oxides and accelerate the charge transfer between them, limit formation of a surface electrochemical interface (SEI) layer to improve the chemical stability, and buffer the stress of the inner nanoscale inorganic oxides to improve the structural stability.

#### **3.2.1 Polypyrrole (PPy)**

PPy is a chemical compound formed from a number of connected pyrrole ring structures and it is the first polyacetylene-derivative to show a high conductivity. Chew et al. (2007) reported a novel nano-silicon/PPy composite by chemical ploymerization. The cycling stability of Si/PPy electrodes is significantly improved when compared to that of the bare Si anodes. PPy plays a key role in improving the electrochemical performance of the composites: (i) PPy in the composites is a conducting polymer, which increases the conductivity of the samples. (ii) PPy can act as a conductive binder, increasing the contact among particles, therefore the particle-to-particle resistance will be decreased, thus reducing the irreversible reactions with the electrolyte. (iii) PPy is an effective component that buffers the great volume changes during the cycling process, thus improving the cyclability of the Si electrode. Similarly, Cui et al. (2011) have synthesized SnO2@ PPy nanocomposites by an one-pot oxidative chemical polymerization method. The nanocomposites exhibit high discharge/charge capacities and favourable cycling. For SnO2@PPy nanocomposites with 79 wt% SnO2, the electrode reaction kinetics is controlled by the diffusion of Li+ in the nanocomposite. The calculated diffusion coefficiency of lithium ions in SnO2@PPy nanocomposite with 79 wt% SnO2 is 6.7×10−8 cm2 s−1, while the lithium-alloying activation energy at 0.5 V is 47.3 kJ mol−1, which is obviously lower than that for the bare SnO2. The enhanced electrode performance with the SnO2@PPy nanocomposite is originated from the advantageous nanostructures that allow a better structural flexibility, shorter diffusion length, and easier interaction with lithium.

Heterogeneous Nanostructured Electrode Materials

Li+ ions to release the extra capacity.

symbols, Li extraction. (Xiao et al., 2010)

**3.3 Heterogeneous nanoarchitectured thin-film electrodes**

for Lithium-Ion Batteries – Recent Trends and Developments 69

unexpectedly high lithium storage over the theoretical capacity of SnO2. Lithium is stored in the PEG–SnO2 nanocomposites, where the crosslinked SnO2 particles pave the electron paths and the revolved EO segments provide the coordination centers. When the composite electrode is charged, those Li+ ions coordinated with EO segments are reduced and deposited, and more Li+ ions from electrolytes will be reduced at this site to form the lithium clusters. When the composite electrode is discharged, lithium clusters transform into

Fig. 30. a) TEM images for exfoliated PEO/MoS2 = 0.05 composite; and b) comparison of cycling stability for unexfoliated MoS2 and exfoliated MoS2 with different amounts of PEO. All cells were cycled between 0.01 and 3.0 V at 50 mA/g. Solid symbols, Li insertion; open

Researchers have developed a new structural strategy for high performance electrodes, namely, ''nanoarchitectured thin-film electrode,'' which is composed of a direct growth of highly conductive one-dimensional (1D) metal nanorods onto a metallic current collector followed by attachment of active materials on the surface of 1D metal nanorods. Due to the merit of this unique structure, LIBs can achieve high specific capacities and superior rate capabilities because of the formation of 3D conductive networks by direct connections among active materials, highly conductive 1D pathways, and a metallic current collector. Additionally, the large free space secured by 1D nanostructures effectively alleviates the huge volume changes that occur during Li+ insertion/extraction, leading to a suppressed electrode degradation (Park et al., 2011). This strategy obviates the use of binders, electron conducting additives, and solvents traditionally used for electrode fabrication.  For example, carbon-coated SnO2 nanorod array directly grown on Fe–Co–Ni alloy substrate (Ji et al. 2010) shows a good capacity retention at 585 mAh/g after 50 cycles at 500 mA/g and cyclability (stable 320 mAh/g at 3,000 mA/g). Ortiz et al. (2010) fabricated nanoarchitectured crystalline SnO nanowires /TiO2 nanotubes composites electrodes. As shown in Figure 31, the composites consist of tin and SnO nanowires grown onto TiO2

#### **3.2.2 Polyaniline (PANI)**

PANI is a conducting polymer of the semi-flexible rod polymer family. It has the advantages of ease of fabrication, high conductivity, and good flexibility, so it can be used to prepare the composite anode of varied microstructures. Nickel foam-supported NiO/PANI composites with a porous net-like morphology have been prepared by a chemical bath deposition technique (Huang et al., 2008). These composites are constructed by NiO nanoflakes coated by PANI layer. As an anode for lithium ion batteries, NiO/PANI composites exhibit a weaker polarization as compared to NiO film. The specific capacity after 50 cycles for NiO/PANI film is 520 mAh/g at 1 C, higher than that of 440 mAh/g for NiO film. These improvements are attributed to the fact that PANI enhances the electrical conduction of the electrode and keeps the NiO film stable during cyclings. Similarly, He et al. (2008) prepared SnO2-PANI composite by microemulsion polymerization method. The composite is composed of amorphous PANI and SnO2 nanoparticles, in which SnO2 is coated with PANI. SnO2-PANI composite to show a reversible capacity of 657.6 mAh/g with a capacity loss per cycle at only 0.092% after 80 cycles. Recently, Cai et al. (2010) reported Si/PANI nanocomposite synthesized by chemical polymerization of aniline and nano-silicon. Si/PANI composite maintains a capacity of 1870 mAh/g up to 25 cycles with a slight loss per cycle, showing an excellent rate performance. The high stable cyclability and improved rate performance are attributed to the fact that the nano-silicon particles are electrically connected by the nest-like PANI so that dispersity of the nano-silicon will be greatly enhanced and the conductivity could be improved. Moreover, Si/PANI composite accommodates a large volume expansion because of the flexible nest-like PANI matrix.

#### **3.2.3 Poly(ethylene glycol) (PEG) /polyethylene oxide (PEO)**

PEG or PEO refers to an oligomer or polymer of ethylene oxide. Two names are chemically synonymous, but historically PEG has tended to refer to oligomers and polymers with a molecular mass below 20,000 g/mol, while PEO to polymers with a molecular mass above 20,000 g/mol. PEG or PEO has a chain structure of HO-CH2-(CH2-O-CH2-)n-CH2-OH. PEG and PEO have been widely investigated as the electrolytes in lithium batteries. The ionic transport mechanism is that Li+ ions move from one coordination site to another revolved by the polymer chains. On one hand, the reaction between lithium and PEG is not reversible at ambient conditions. On the other, PEO is stable even in contact with lithium at 60 oC. Hence, until very recently, Xiao et al. (2010) and Xiong et al. (2011) reported the improved lithium storage by introduction of PEO and PEG, respectively, there have been no literature reports that ethylene oxide units in polymers are used for reversible lithium storage. In the case of MoS2/PEO nanocomposite, PEO is inserted into the interlay spacing of exfoliated MoS2 to form Li0.12(PEO)yMoS2. A small amount of PEO (Li0.12(PEO)0.05MoS2) not only doubles the initial capacity of the pristine MoS2 but significantly improves the capacity retention. This nanocomposite delivers a charge/discharge capacity of above 900 mAh/g after 50 cycles, as shown in Figure 30, during which PEO stabilizes the disordered structure of the exfoliated MoS2 throughout the cycling regime to accommodate more Li+ ions and accelerates the Li+ transportation between MoS2 sheets and electrolyte. While higher amounts of PEO may decrease the lithium accessibility and electronic conductivity, resulting in a higher internal resistance and charge transfer. Xiong et al. reported a novel nanonet that is composed of SnO2 nanoparticles and PEG chains. The synergic properties and functionalities of these two components enable this composite to exhibit an

PANI is a conducting polymer of the semi-flexible rod polymer family. It has the advantages of ease of fabrication, high conductivity, and good flexibility, so it can be used to prepare the composite anode of varied microstructures. Nickel foam-supported NiO/PANI composites with a porous net-like morphology have been prepared by a chemical bath deposition technique (Huang et al., 2008). These composites are constructed by NiO nanoflakes coated by PANI layer. As an anode for lithium ion batteries, NiO/PANI composites exhibit a weaker polarization as compared to NiO film. The specific capacity after 50 cycles for NiO/PANI film is 520 mAh/g at 1 C, higher than that of 440 mAh/g for NiO film. These improvements are attributed to the fact that PANI enhances the electrical conduction of the electrode and keeps the NiO film stable during cyclings. Similarly, He et al. (2008) prepared SnO2-PANI composite by microemulsion polymerization method. The composite is composed of amorphous PANI and SnO2 nanoparticles, in which SnO2 is coated with PANI. SnO2-PANI composite to show a reversible capacity of 657.6 mAh/g with a capacity loss per cycle at only 0.092% after 80 cycles. Recently, Cai et al. (2010) reported Si/PANI nanocomposite synthesized by chemical polymerization of aniline and nano-silicon. Si/PANI composite maintains a capacity of 1870 mAh/g up to 25 cycles with a slight loss per cycle, showing an excellent rate performance. The high stable cyclability and improved rate performance are attributed to the fact that the nano-silicon particles are electrically connected by the nest-like PANI so that dispersity of the nano-silicon will be greatly enhanced and the conductivity could be improved. Moreover, Si/PANI composite accommodates a large volume expansion because of the flexible nest-like PANI matrix.

PEG or PEO refers to an oligomer or polymer of ethylene oxide. Two names are chemically synonymous, but historically PEG has tended to refer to oligomers and polymers with a molecular mass below 20,000 g/mol, while PEO to polymers with a molecular mass above 20,000 g/mol. PEG or PEO has a chain structure of HO-CH2-(CH2-O-CH2-)n-CH2-OH. PEG and PEO have been widely investigated as the electrolytes in lithium batteries. The ionic transport mechanism is that Li+ ions move from one coordination site to another revolved by the polymer chains. On one hand, the reaction between lithium and PEG is not reversible at ambient conditions. On the other, PEO is stable even in contact with lithium at 60 oC. Hence, until very recently, Xiao et al. (2010) and Xiong et al. (2011) reported the improved lithium storage by introduction of PEO and PEG, respectively, there have been no literature reports that ethylene oxide units in polymers are used for reversible lithium storage. In the case of MoS2/PEO nanocomposite, PEO is inserted into the interlay spacing of exfoliated MoS2 to form Li0.12(PEO)yMoS2. A small amount of PEO (Li0.12(PEO)0.05MoS2) not only doubles the initial capacity of the pristine MoS2 but significantly improves the capacity retention. This nanocomposite delivers a charge/discharge capacity of above 900 mAh/g after 50 cycles, as shown in Figure 30, during which PEO stabilizes the disordered structure of the exfoliated MoS2 throughout the cycling regime to accommodate more Li+ ions and accelerates the Li+ transportation between MoS2 sheets and electrolyte. While higher amounts of PEO may decrease the lithium accessibility and electronic conductivity, resulting in a higher internal resistance and charge transfer. Xiong et al. reported a novel nanonet that is composed of SnO2 nanoparticles and PEG chains. The synergic properties and functionalities of these two components enable this composite to exhibit an

**3.2.3 Poly(ethylene glycol) (PEG) /polyethylene oxide (PEO)**

**3.2.2 Polyaniline (PANI)**

unexpectedly high lithium storage over the theoretical capacity of SnO2. Lithium is stored in the PEG–SnO2 nanocomposites, where the crosslinked SnO2 particles pave the electron paths and the revolved EO segments provide the coordination centers. When the composite electrode is charged, those Li+ ions coordinated with EO segments are reduced and deposited, and more Li+ ions from electrolytes will be reduced at this site to form the lithium clusters. When the composite electrode is discharged, lithium clusters transform into Li+ ions to release the extra capacity.

Fig. 30. a) TEM images for exfoliated PEO/MoS2 = 0.05 composite; and b) comparison of cycling stability for unexfoliated MoS2 and exfoliated MoS2 with different amounts of PEO. All cells were cycled between 0.01 and 3.0 V at 50 mA/g. Solid symbols, Li insertion; open symbols, Li extraction. (Xiao et al., 2010)

#### **3.3 Heterogeneous nanoarchitectured thin-film electrodes**

Researchers have developed a new structural strategy for high performance electrodes, namely, ''nanoarchitectured thin-film electrode,'' which is composed of a direct growth of highly conductive one-dimensional (1D) metal nanorods onto a metallic current collector followed by attachment of active materials on the surface of 1D metal nanorods. Due to the merit of this unique structure, LIBs can achieve high specific capacities and superior rate capabilities because of the formation of 3D conductive networks by direct connections among active materials, highly conductive 1D pathways, and a metallic current collector. Additionally, the large free space secured by 1D nanostructures effectively alleviates the huge volume changes that occur during Li+ insertion/extraction, leading to a suppressed electrode degradation (Park et al., 2011). This strategy obviates the use of binders, electron conducting additives, and solvents traditionally used for electrode fabrication.  For example, carbon-coated SnO2 nanorod array directly grown on Fe–Co–Ni alloy substrate (Ji et al. 2010) shows a good capacity retention at 585 mAh/g after 50 cycles at 500 mA/g and cyclability (stable 320 mAh/g at 3,000 mA/g). Ortiz et al. (2010) fabricated nanoarchitectured crystalline SnO nanowires /TiO2 nanotubes composites electrodes. As shown in Figure 31, the composites consist of tin and SnO nanowires grown onto TiO2

Heterogeneous Nanostructured Electrode Materials

level of 90 mAh/g.

**4. Summary and outlook**

charge/discharge rates.

for Lithium-Ion Batteries – Recent Trends and Developments 71

Recently, Krishnan et al. (2011) synthesized a functionally strain-graded C-Al-Si anode composite using DC magnetron sputtering. As shown in Figure 33, the composites consist of an array of nanostructures each comprising an amorphous carbon nanorod (∼170 nm long) with an intermediate layer of aluminium (∼13 nm thick) that is finally capped by a silicon nanoscoop (∼40 nm thick) on the very top. The gradation in strain arises from the graded levels of volumetric expansion in these three components on alloying with lithium. The introduction of aluminium as an intermediate layer enables the gradual transition of strain from carbon to silicon, thereby minimizing the mismatch at interfaces between differentially strained materials and enabling the stable operation of electrode under high-rate charge/discharge conditions. C-Al-Si composites can provide an average capacity of about 412 mAh/g with a power output of 100 kW/kg continuously over 100 cycles. Even when the power output is as high as 250 kW/kg, the average capacity over 100 cycles is still at the

Fig. 33. a) Cross-sectional SEM view of C-Al-Si nanoscoop structures deposited on a Si wafer. A magnified cross-section image is also shown with the C, Al, and Si regions demarcated; and b) Capacity as a function of cycle index shown for C-Al-Si electrodes at 51.2, 76.8, and 128 A/g over 100 cycles. The empty symbols represent the discharge capacity while the filled symbols represent the charge capacity in each case. (Krishnan et al., 2011)

In this chapter, we have reviewed the recent trends and developments of using heterogeneous nanostructures as the electrode materials for LIBs. It has been illustrated that heterogeneous nanostructured electrodes materials have advantages over the conventional single component nanostructured electrode materials for LIBs, though one has to face some significant challenges before these heterogeneous nanostructured can be large-scale utilized in practical LIBs. There are many novel heterogeneous nanostructured cathode and anode materials that have been reported to show a high rate capability. But the dream is still there as to design and/or develop novel electrodes with much higher performance at high

For example, one has optimized the performance of plenty of LIBs by extensively studying the influences of the morphology, particle size, loading content, and mixing procedure of different components, while the synergic mechanism of heterogeneous nanostructured

nanotubes by anodization of titanium and tin electrodeposition. The 2 μm length tin-based nanowires supported on self-organized TiO2 provide a real capacities of 95 and 140 μA h cm-2 (∼675 mAh/g) under high rates of 4 C and 2 C, keeping 70 and 85% initial capacity over 50 cycles, respectively.

Fig. 31. a) SEM image of tin nanowires supported onto TiO2 nanotube arrays; and b) cycle life performance under galvanostatic regime at a current density of 100 μAcm-2. (Oritz et al., 2010)

These heterogeneous nanoarchitectured thin-film electrodes have demonstrated to display a stable capacity over many cycles, whereas an increase in film thickness is urgently required to provide the sufficiently active material because stress-induced cracking would lead to a poor cycling performance and rate capability owing to an increase in film thickness. An alternative strategy for tackling this problem is to design the functional structures that buffer the stress during the charge/discharge processes. Zhang et al. (2010) synthesized a Ni nanocone-array (NCA) supported Si architecture composed of many cylinders with regular domes on top to serve as the anode material for LIBs. In this configuration, Ni NCAs facilitate the charge collection and transport, supporting the electrode structure and acting as the inactive confining buffers. These nanostructured Si electrodes show an impressive electrochemical performance with a high capacity of around 2400 mAh/g at 0.2 C rate over 100 cycles with superior capacity retention of 99.8% per cycle. These Ni NCAs also exhibit an excellent lithium storage capability at high charging and discharging rates of 1 C or 2 C (Figure 32).

Fig. 32. a) Schematic diagram that illustrates the fabrication of a nickel nanocone-array supported silicon anode architecture; and b) discharge/charge capacity and Coulombic efficiency versus cycle number for a nanostructured Si electrode at 1 C and 2 C rates after 0.2 C for 5 cycles with a cut-off voltage of 10 mV–1.6 V. (Zhang et al., 2010)

nanotubes by anodization of titanium and tin electrodeposition. The 2 μm length tin-based nanowires supported on self-organized TiO2 provide a real capacities of 95 and 140 μA h cm-2 (∼675 mAh/g) under high rates of 4 C and 2 C, keeping 70 and 85% initial capacity over

Fig. 31. a) SEM image of tin nanowires supported onto TiO2 nanotube arrays; and b) cycle life performance under galvanostatic regime at a current density of 100 μAcm-2. (Oritz et al., 2010)

These heterogeneous nanoarchitectured thin-film electrodes have demonstrated to display a stable capacity over many cycles, whereas an increase in film thickness is urgently required to provide the sufficiently active material because stress-induced cracking would lead to a poor cycling performance and rate capability owing to an increase in film thickness. An alternative strategy for tackling this problem is to design the functional structures that buffer the stress during the charge/discharge processes. Zhang et al. (2010) synthesized a Ni nanocone-array (NCA) supported Si architecture composed of many cylinders with regular domes on top to serve as the anode material for LIBs. In this configuration, Ni NCAs facilitate the charge collection and transport, supporting the electrode structure and acting as the inactive confining buffers. These nanostructured Si electrodes show an impressive electrochemical performance with a high capacity of around 2400 mAh/g at 0.2 C rate over 100 cycles with superior capacity retention of 99.8% per cycle. These Ni NCAs also exhibit an excellent lithium storage capability at high charging and discharging rates of 1 C or 2 C

Fig. 32. a) Schematic diagram that illustrates the fabrication of a nickel nanocone-array supported silicon anode architecture; and b) discharge/charge capacity and Coulombic efficiency versus cycle number for a nanostructured Si electrode at 1 C and 2 C rates after 0.2

C for 5 cycles with a cut-off voltage of 10 mV–1.6 V. (Zhang et al., 2010)

50 cycles, respectively.

(Figure 32).

Recently, Krishnan et al. (2011) synthesized a functionally strain-graded C-Al-Si anode composite using DC magnetron sputtering. As shown in Figure 33, the composites consist of an array of nanostructures each comprising an amorphous carbon nanorod (∼170 nm long) with an intermediate layer of aluminium (∼13 nm thick) that is finally capped by a silicon nanoscoop (∼40 nm thick) on the very top. The gradation in strain arises from the graded levels of volumetric expansion in these three components on alloying with lithium. The introduction of aluminium as an intermediate layer enables the gradual transition of strain from carbon to silicon, thereby minimizing the mismatch at interfaces between differentially strained materials and enabling the stable operation of electrode under high-rate charge/discharge conditions. C-Al-Si composites can provide an average capacity of about 412 mAh/g with a power output of 100 kW/kg continuously over 100 cycles. Even when the power output is as high as 250 kW/kg, the average capacity over 100 cycles is still at the level of 90 mAh/g.

Fig. 33. a) Cross-sectional SEM view of C-Al-Si nanoscoop structures deposited on a Si wafer. A magnified cross-section image is also shown with the C, Al, and Si regions demarcated; and b) Capacity as a function of cycle index shown for C-Al-Si electrodes at 51.2, 76.8, and 128 A/g over 100 cycles. The empty symbols represent the discharge capacity while the filled symbols represent the charge capacity in each case. (Krishnan et al., 2011)

### **4. Summary and outlook**

In this chapter, we have reviewed the recent trends and developments of using heterogeneous nanostructures as the electrode materials for LIBs. It has been illustrated that heterogeneous nanostructured electrodes materials have advantages over the conventional single component nanostructured electrode materials for LIBs, though one has to face some significant challenges before these heterogeneous nanostructured can be large-scale utilized in practical LIBs. There are many novel heterogeneous nanostructured cathode and anode materials that have been reported to show a high rate capability. But the dream is still there as to design and/or develop novel electrodes with much higher performance at high charge/discharge rates.

For example, one has optimized the performance of plenty of LIBs by extensively studying the influences of the morphology, particle size, loading content, and mixing procedure of different components, while the synergic mechanism of heterogeneous nanostructured

Heterogeneous Nanostructured Electrode Materials

LIBs.

**5. Acknowledgment**

192, ISSN 0022-0728

pp.A178–A184, ISSN 0013-4651

**6. References**

for Lithium-Ion Batteries – Recent Trends and Developments 73

Fig. 34. Future heterogeneous nanoarchitectured thin–film electrodes. The backbone material is in green, the branch material in red, and conductive polymer in yellow.

The progress described here suggests that future research should explore more structurally and compositionally complex hierarchical composite nanostructures with internal void/pore space. Considering the potential large-scale energy applications of LIBs, such as in hybrid and plug-in electric vehicles or electrical power storage, mass production possibility, manufacturability, safety factors, and cost of the heterogeneous nanostructrued materials will also be important to the eventual success in their practical applications.

It is imperative that we as scientists and engineers put all our efforts into exploiting more technologically advanced heterogeneous nanostructures to fulfil the future requirements of

This work was financially supported by NSFC (No. 91022018, 21025104), National Basic Research Program of China (No. 2011CBA00501, 2011CB935904), Fujian Program (No.

Alcantara, R.; Jaraba, M.; Lavela, P. & Tirado, J.-L. (2004). X-ray diffraction and

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2009HZ0004-1), and FJIRSM fund (No. SZD09003-1, 2010KL002).

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components remains to be further clarified and key variables that determine the interface interaction between different components have to be identified. Over the last five years, tremendous progress has been made in addressing these challenges. Through a survey of those studies from the viewpoints of the synergic mechanism of heterogeneous nanostructured components, one can find that maximizing both ion and electron transportations in the cathodes and anodes while maintaining the good cycling stability can be fundamentally important in designing and developing novel electrodes for HEVs, PHEVs or EVs. Despite the variations of the reported performance, a consistent trend has been emerging that high power density of 60 kW/kg and high energy density of 282 Wh/kg for SnO2/V2O5 core/shell composite thin-film cathodes and average capacities of ∼412 mAh/g and a power output of∼100 kW/kg for C-Al-Si composites thin-film anodes with rather impressive cycling lifetimes up to 100 cycles at practical charge/discharge rate can be reproducibly achieved. High rate performance and long cycling lifetimes have most frequently been observed when four critical factors are properly addressed: i) use of nanoscale-dimension electroactive materials that allow a rapid lithium ion diffusion; ii) efficient electronic and ionic conduction from electro-active materials to the current collectors that can be maintained during the cycling; iii) incorporation of void/free space or softer inactive material matrices or additives to accommodate the volume variation; iv) good adhesion between different heterogeneous components by constructing special gradient structure can be formed, which can effectively tolerate the different expansion and contraction efficiency of heterogeneous components during the continual lithium insertion/extraction.

Certainly, it is realized that all current heterogeneous nanostructures have their own drawbacks. For example, heterogeneous nanoarchitectured thin-film electrodes can become mechanically unstable when grown excessively long in the axial direction. This will limit their uses as on-chip power sources for MEMS devices and other applications where the areal footprint is at a premium and a high areal energy density is needed. Here, we tend to propose a future heterogeneous nanoarchitectured thin-film electrode as shown in Figure 34. This proposed heterogeneous nanoarchitectured thin-film electrode is composed of a well ordered branched array with a conformal polymer coating layer grown on a conductive substrate. The backbone is an active electrode material, while the branched is active electrode materials with the role of optimizing the lithium ion/electron transportation or inactive electrode materials to depress the side interaction between electrode and electrolyte. The conformal polymer coating layer acts as a conductive layer and binders. There are a few advantages of this proposed structure: (i) the interconnectivity of the electrode material provides a 3-D continuous lithium ion and electron pathway and can increase the mechanical strength of branched arrays; (ii) the interconnected pores accommodate the volume variations of the electrode material and allow the electrolyte to rapidly penetrate and uniformly contact the electrode material; (iii) 3-D interconnected coating of polymer facilitates a fast electron transfer into the 3-D continuous framework, allowing the electrons to directly transport to the electrode materials due to its conformality, effectively alleviating the stress due to its resiliency that ensures mechanically stability for longer arrays, and acting as a binder to strengthen the adhesion between the branched and backbone materials; and (iv) merits of the material being directly contacted with a conductive substrate, in which electrons can easily transfer from the conductive substrate to the electroactive backbone and branched material as well as to the coating conductive polymer shell.

Fig. 34. Future heterogeneous nanoarchitectured thin–film electrodes. The backbone material is in green, the branch material in red, and conductive polymer in yellow.

The progress described here suggests that future research should explore more structurally and compositionally complex hierarchical composite nanostructures with internal void/pore space. Considering the potential large-scale energy applications of LIBs, such as in hybrid and plug-in electric vehicles or electrical power storage, mass production possibility, manufacturability, safety factors, and cost of the heterogeneous nanostructrued materials will also be important to the eventual success in their practical applications.

It is imperative that we as scientists and engineers put all our efforts into exploiting more technologically advanced heterogeneous nanostructures to fulfil the future requirements of LIBs.

### **5. Acknowledgment**

This work was financially supported by NSFC (No. 91022018, 21025104), National Basic Research Program of China (No. 2011CBA00501, 2011CB935904), Fujian Program (No. 2009HZ0004-1), and FJIRSM fund (No. SZD09003-1, 2010KL002).

#### **6. References**

72 Lithium Ion Batteries – New Developments

components remains to be further clarified and key variables that determine the interface interaction between different components have to be identified. Over the last five years, tremendous progress has been made in addressing these challenges. Through a survey of those studies from the viewpoints of the synergic mechanism of heterogeneous nanostructured components, one can find that maximizing both ion and electron transportations in the cathodes and anodes while maintaining the good cycling stability can be fundamentally important in designing and developing novel electrodes for HEVs, PHEVs or EVs. Despite the variations of the reported performance, a consistent trend has been emerging that high power density of 60 kW/kg and high energy density of 282 Wh/kg for SnO2/V2O5 core/shell composite thin-film cathodes and average capacities of ∼412 mAh/g and a power output of∼100 kW/kg for C-Al-Si composites thin-film anodes with rather impressive cycling lifetimes up to 100 cycles at practical charge/discharge rate can be reproducibly achieved. High rate performance and long cycling lifetimes have most frequently been observed when four critical factors are properly addressed: i) use of nanoscale-dimension electroactive materials that allow a rapid lithium ion diffusion; ii) efficient electronic and ionic conduction from electro-active materials to the current collectors that can be maintained during the cycling; iii) incorporation of void/free space or softer inactive material matrices or additives to accommodate the volume variation; iv) good adhesion between different heterogeneous components by constructing special gradient structure can be formed, which can effectively tolerate the different expansion and contraction efficiency of heterogeneous components during the continual lithium

Certainly, it is realized that all current heterogeneous nanostructures have their own drawbacks. For example, heterogeneous nanoarchitectured thin-film electrodes can become mechanically unstable when grown excessively long in the axial direction. This will limit their uses as on-chip power sources for MEMS devices and other applications where the areal footprint is at a premium and a high areal energy density is needed. Here, we tend to propose a future heterogeneous nanoarchitectured thin-film electrode as shown in Figure 34. This proposed heterogeneous nanoarchitectured thin-film electrode is composed of a well ordered branched array with a conformal polymer coating layer grown on a conductive substrate. The backbone is an active electrode material, while the branched is active electrode materials with the role of optimizing the lithium ion/electron transportation or inactive electrode materials to depress the side interaction between electrode and electrolyte. The conformal polymer coating layer acts as a conductive layer and binders. There are a few advantages of this proposed structure: (i) the interconnectivity of the electrode material provides a 3-D continuous lithium ion and electron pathway and can increase the mechanical strength of branched arrays; (ii) the interconnected pores accommodate the volume variations of the electrode material and allow the electrolyte to rapidly penetrate and uniformly contact the electrode material; (iii) 3-D interconnected coating of polymer facilitates a fast electron transfer into the 3-D continuous framework, allowing the electrons to directly transport to the electrode materials due to its conformality, effectively alleviating the stress due to its resiliency that ensures mechanically stability for longer arrays, and acting as a binder to strengthen the adhesion between the branched and backbone materials; and (iv) merits of the material being directly contacted with a conductive substrate, in which electrons can easily transfer from the conductive substrate to the electroactive backbone and

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