## 2 Electrodes and Electrolytes for Lithium-Based Batteries
### Electrodes
#### 2.1.2 Cathodes  
Lithium transition metal oxides have an \(\alpha\)-NaFeO\({}_{2}\)-type structure, which is a distorted rock salt superstructure. The general formula of lithium transition metal oxides can be written as LiMO\({}_{2}\) (M = V, Cr, Fe, Co and Ni). With a theoretical capacity of 274 mAh/g, lithium cobalt oxide (LiCoO\({}_{2}\), LCO) is the first layered transition metal oxide cathode successfully commercialized. The success of this material as a cathode for LIBs is easily evidenced by its utilization in most portable electronic devices. Unfortunately, the theoretical capacity is unrealized due to the presence of phase transitions during the lithiation and delithiation processes [125; 126; 127]. Doping has been widely employed to suppress the order-disorder transitions of LCO, the transition from hexagonal to monoclinic structure [128]. Predominantly, Mg, Zr, Al, La, Ni, Fe, Cr, Mn and Ti have been used to suppress detrimental phase transitions which lead to crack formation due to the associated large mechanical stresses. Liu et al. [129] demonstrated that doping with La and Al enhances the capacity (190 mAh/g at 0.1 C) of LCO and maintains a 96% capacity retention after 50 cycles. According to this study, La doping increases the c-axis distance and the doped Al acts as a positive charge center, which is expected to suppress the order-disorder transition. The Li\({}^{+}\) diffusion coefficients were shown to improve from \(6.5\times 10^{-11}\) cm\({}^{2}\)/s (pristine) to \(1.2\times 10^{-10}\) cm\({}^{2}\)/s (doped LCO) during the initial charge stage. Manganese-based oxide (LiMnO\({}_{2}\) or LiMn\({}_{2}\)O\({}_{4}\)) cathodes are widely seen as an economical replacement of Co. Although these materials have merits in abundance, they suffer from first cycle irreversible capacity, low discharge capacity retention, low rate capability and substantial charge-discharge voltage hysteresis. Recently, Uyama et al. [130] reported a tetragonal phase of LiMnO\({}_{2}\) synthesized at high pressures (5-12 GPa) that shows a dramatic capacity increase during the initial cycles to 185 mAh/g. Tian et al. [131] showed that the Mn-ion migration into the Li layer significantly blocks Li\({}^{+}\) diffusion in monoclinic LiMnO\({}_{2}\) by first-principle calculations. The cubic spinel LiMn\({}_{2}\)O\({}_{4}\) (LMO) received more attention than other phases because the layered LiMnO\({}_{2}\) is shown to be a metastable phase [132]. However, manganese dissolution from the spinel phase is a challenge to achieve long cycle life batteries [133; 134]. Using the LMO planar structure, Hirayama et al. [135] showed that the (111) surface is more stable than (110) in carbonate-based electrolytes. Tang et al. [136] observed the formation of the Mn\({}_{3}\)O\({}_{4}\) phase, a good source of soluble Mn\({}^{2+}\), on the LMO surface after cycling. Doping of the spinel phase with Al [137], Yt [138], Ce [139], Nb or PO\({}_{4}\)[140] is useful for stabilizing capacity and mitigating the discharge voltage decay (Figure 3). Figure 3: Calculated formation energy of Al-doped cubic spinel LiMn\({}_{2}\)O\({}_{4}\) (LMO). Reprinted from ref. [137] with permission of Elsevier. In 1994, a high voltage cathode, LiNiVO\({}_{4}\) (LNVO) with an inverse spinel structure and space group Fd-3 m (O\({}_{\text{h}}\)[7]) (Figure 4), was introduced, exhibiting an operating potential of 4.8 V (vs. Li/Li\({}^{+}\)) with 1 M LiPF\({}_{6}\) PC/EC/DMC (25/25/50) electrolyte [141; 142]. These high voltage materials are good candidates for SSBs cathodes as some of the solid-state electrolytes presented an electrochemical stability limit higher than 5 V [143]. A similar class of high voltage inverse spinel vanadate cathodes includes LiCoVO\({}_{4}\) and LiMnVO\({}_{4}\)[144]. LNVO has been prepared in several different ways [145]. Thongtem et al. [146; 147] developed a synthesis method using malic and tartaric acid complexes to prepare nanocrystallites. The inverse-spinel structure has been prepared by a sol-gel method [148; 149]. Qin et al.