# A Review on Nanocomposite Materials for
## 4 Cathode
### Layered Oxides, LiMO\({}_{2}\) (\(M\) = Co, Mn, Ni and Their Mixtures)  
LiCoO\({}_{2}\) is the first layered oxide type cathode material with a practical capacity of 140 mAh/g and average voltage of 3.9 V vs. Li/Li\({}^{+}\). In practice, only half of the lithium (i.e., 1 \(>\) x \(>\) 0.5) stored in Li\({}_{x}\)CoO\({}_{2}\) is extracted because further Li extraction (i.e., x \(<\) 0.5 which corresponds to upper cut-off voltage of 4.2 V) triggers structural transition from a hexagonal to monoclinic phase as well as amplifies the side reactions, and results in quick degradation of the cycle life. Significant number of studies was conducted to understand the capacity fading mechanism over 4.2 V as well as to improve the capacity of LiCoO\({}_{2}\) by pushing the upper cut-off voltage above 4.2 V while keeping the cycle life at a reasonable level (i.e., \(-\)500 cycles) [34; 35; 36; 37]. It was shown by many that the cycle life of LiCoO\({}_{2}\) at higher voltages (i.e., \(>\)4.2 V) can be improved by metal oxide coatings such as Al\({}_{2}\)O\({}_{3}\), SnO\({}_{2}\), ZrO\({}_{2}\), TiO\({}_{2}\), MgO, [38; 39; 40; 41; 42], metal phosphate coatings such as AlPO\({}_{4}\)[43; 44; 45; 46], metal fluoride coatings such as AlF\({}_{3}\) and LaF\({}_{3}\)[47; 48] and multicomponent metal fluoride coatings such as aluminum-tungsten-fluoride (AlW\({}_{x}\)F\({}_{y}\)) [49]. Although the underlying reasons for such an improvement in cycle life of LiCoO\({}_{2}\) at higher cut-off voltages due to coatings are still debatable [35; 50; 51], following mechanisms have been suggested: coating (i) inhibits the structural transformation [34; 52], (ii) acts as a physical barrier, Figure 9, between the electrolyte and the active material, and prevents the trace amounts of hydrogen fluoride (HF) and water present in the electrolyte from reaching the active material thus effectively suppresses cobalt dissolution and the associated oxygen evolution [53; 54], (iii) converts Lewis acids which in return corrode the insulating surface species and improves the electronic conductivity of the solid electrolyte interface (SEI) layer on LiCoO\({}_{2}\)[50]. In addition to the cycle life, rate capability (i.e., electronic conductivity) can also be improved by metal oxide coatings [55]. Additionally, thermal stability of the LiCoO\({}_{2}\) can be improved by a metal oxide coating, which is desirable for safety purposes [37; 43; 55]. The sample preparation technique can also affect the cycle life of LiCoO\({}_{2}\). [PERSON] et al. showed that LiCoO\({}_{2}\) prepared by the eutectic LiNO\({}_{3}\)-LiCl molten salt can attain 80 charge-discharge cycles in the 2.5-4.4 V range without any capacity loss which was associated with the inhibited structural transitions (i.e., hexagonal \(\leftrightarrow\) monoclinic \(\leftrightarrow\) hexagonal) of LiCoO\({}_{2}\)[56]. One of the problems of the LiCoO\({}_{2}\) chemistry for large scale applications is its high cost due to limited Co sources; additionally, Co is a toxic substance, and environmentally friendly commercial scale recycling technologies are yet to be developed [57; 58]. LiNiO\({}_{2}\) is a lower cost cathode compared to LiCoO\({}_{2}\), and has a similar structure to LiCoO\({}_{2}\); however, it is difficult to obtain stoichiometric LiNiO\({}_{2}\), and synthesis conditions significantly affect its compositional and structural properties which impede the widespread utilization of LiNiO\({}_{2}\) cathode [59]. LiNiO\({}_{2}\) exhibits similar structural transformations as in LiCoO\({}_{2}\); thus, the upper cut-off voltage is 4.2 V (i.e., x \(\sim\)0.5), and beyond 4.2 V cycle-life quickly degrades. Several metal oxide coatings such as ZrO\({}_{2}\) have shown to improve the cycling stability of LiNiO\({}_{2}\) by inhibiting the structural transformation as shown by ex-situ X-ray  
Figure 9: Schematic view of the formation of the solid solution at the coating/active material interface.