ABSTRACT
It is well known that many metals and alloys can store a large quantity of lithium through the electrochemical alloying reaction; for example, Li4.4Sn gives a lithium storage capacity of 992 mAh g-1, nearly triple that of conventional graphite (in the form of LiC6, with a theoretical capacity of 372 mAh g-1), and Li4.4Si can even provide a capacity of 4200 mAh g-1. These materials exhibit a great potential in substituting graphite to become the next-generation anode materials used in high-energy lithium-ion batteries. However, the significant volume expansion during lithiation/delithiation cycle, which leads to the pulverization of electrode materials and very rapid capacity decay, brings a huge problem to their practical applications [14, 29, 30]. To conquer this problem, much work has been done, and one of the most promising methods is the introduction of carbon matrix into a nanocomposite electrode material. For example, Derrien et al. [31] have demonstrated the use of Sn-C composite (i.e., Sn nanoparticles dispersed in the carbon matrix, which acts as a buffer) can effectively relieve the strain associated with volume variations of tin while preventing the aggregation of Sn nanoparticles upon cycling and hence leads to a much improved anode material with large capacity of ca. 500 mAh g-1 over several hundred cycles. When referring to carbon matrix, hollow carbon spheres can always be considered a favorable candidate, and recent work [30] has proved that by encapsulating tin nanoparticles into hollow carbon spheres with a uniform size, anode materials with high specific capacity and good cycling performance will be obtained, as explained in detail below. 6.2.4 Transition Metal OxidesAfter 2000, the use of transition metal oxides as anode materials for lithium-ion batteries became more and more popular [32]. However, most of the materials consisting of transition metal oxides suffer from a high overpotential (voltage difference between the
working voltage and thermodynamic equilibrium voltage, and the value is about 1 V) when undergoing conversion reactions for both lithiation and delithiation process. Therefore, the theoretical lithiation capacity can be achieved only when the thermodynamic equilibrium voltage of the material for conversion reaction is higher than 1 V [33]. Cr2O3 and MnO, with their high lithium storage capacity and relatively low thermodynamic equilibrium voltage, are thus more suitable as anode materials for lithium battery with enhanced energy densities [33, 34]. 6.3 CATHODE MATERIALSIn a lithium-ion battery, cathode materials are usually oxides of transition metals that can undergo oxidation to higher valences when lithium ions are removed [6, 35], while maintaining their structural stability over a wide range of composition (for example, from fully charged states to completely discharged states). Among all the cathode materials, LiCoO2 is the first cathode material that has been used in commercialized lithium-ion batteries and is still widely used. With decades of study, the research on the cathode materials for lithium-ion batteries has focused on the following three types of materials: a layered, structured hexagonal oxide (e.g., LiCoO2), a spinel structured oxide (e.g., LiMn2O4), and an olivine structured oxide LiFePO4), as shown in Table 6.1. In addition, transition metal sulfides such as titanium disulfide (TiS2) have also been adopted in the cathode of lithium batteries. 6.3.1 Layered Structured Hexagonal OxideAs one of the most successful cathode materials, LiCoO2 forms a distorted rock-salt structure, the same as α-NaFeO2, in which the cations order in alternating (111) planes [36]. This crystalline structure provides planes of lithium ions through which lithiation and delithiation can occur. Currently, LiCoO2 suffers from two major problems. On the one hand, the availability of cobalt is relatively
lower than that of other transition metals such as manganese, nickel, and iron, and this leads to a much higher cost. On the other hand, the structure of LiCoO2 is not as stable as that of other cathode materials (e.g., LiFePO4), and it undergoes performance degradation or even failure when overcharged [37, 38]. Therefore, it still is highly desired to search for alternative materials to replace LiCoO2. One of the candidates is LiNiO2, which shares the same layered structure as LiCoO2. Compared with LiCoO2, LiNiO2 is lower in cost and higher in energy density; however, it also suffers from serious structural instability [39, 40]. Element doping of cobalt can effectively increase the degree of ordering, thus enhancing its structural stability; the same holds for LiMnO2, which can achieve a higher capacity as well as a better rate capability through the addition of cobalt and nickel to form a composition of Li(Ni 1/3
)O2 [41-43]. 6.3.2 Spinel Structured OxideAnother promising cathode material is LiMn2O4, which forms a spinel structure, and this structure enables manganese to occupy the octahedral sites while lithium predominantly occupies the tetrahedral sites [44]. LiMn2O4 has a lower cost and enhanced safety compared with LiCoO2, but it also has some limitations [45]. One of its limitations comes from the phase change during the charge/discharge cycling, which will then be responsible for its capacity fade [46, 47]. Doping has been widely used to improve the electrochemical performance of LiMn2O4. For example, the addition of iron may lead to an enhancement in the charge plateau of LiMn2O4 at high voltages, the addition of cobalt may help to stabilize the spinel structure of LiMn2O4, thus bettering the capacity retention upon cycling, and the addition of nickel can improve the capacity of LiMn2O4 by decreasing its lattice parameters [48-50]. Other doping atoms, such as aluminum, can also be used to improve the electrochemical performance of LiMn2O4.
