# A Review on Nanocomposite Materials for
## 4 Cathode
### Spinel, LiM\({}_{2}\)O\({}_{4}\) (M = Mn, and Mixtures of Co and Ni)  
Spinel LiMn\({}_{2}\)O\({}_{4}\) has three-dimensional Li ion conduction channels which translate to excellent rate capability; additionally, it is easy to synthesize LiMn\({}_{2}\)O\({}_{4}\) from various precursors. Mn is a low cost and an earth abundant material. Spinel LiMn\({}_{2}\)O\({}_{4}\) is also inherently safe due to its thermal stability [74]. On the other hand, the major problems associated with LiMn\({}_{2}\)O\({}_{4}\) are Mn dissolution especially at high operating and storage temperatures (i.e., 55 \({}^{\circ}\)C) and the phase transition (i.e., Jahn-Teller distortion) that occurs if LiMn\({}_{2}\)O\({}_{4}\) is over discharged. Li\({}_{\text{x}}\)Mn\({}_{2}\)O\({}_{4}\) has two voltage plateaus at \(\sim\)4 V (x = 0 to 1.0) and \(\sim\)3 V (x = 1.0 to 2.0), and phase transition from cubic to tetragonal structure can be prevented if LiMn\({}_{2}\)O\({}_{4}\) is not discharged below 3.5 V [75]. The over discharge of LiMn\({}_{2}\)O\({}_{4}\) results in significant capacity fade and poor cycling hence should be avoided. The Mn dissolution is more problematic and occurs at both 3 V and 4 V voltage plateaus. It was shown by [PERSON] et al. that Mn dissolution is a function of surface area of LiMn\({}_{2}\)O\({}_{4}\) particles indicating Mn dissolution is originated at the active material-electrolyte interface [76]. The LiMn\({}_{2}\)O\({}_{4}\) surface acts as a catalyst at high open circuit voltages and oxidizes the non-aqueous electrolyte producing harmful by-products, and higher temperatures accelerates these side reactions further. The dissolved Mn ions can also migrate to the anode side and get reduced and precipitate as Mn metal at the anode surface which can damage the structure of the solid electrolyte interface and increase the charge transfer resistance [77]. To circumvent the Mn dissolution problem, passivation layer needs to be formed between the active material and the electrolyte. Various coatings and dopants such as lithium boron oxide (LBO) glass [76], SiO\({}_{2}\)[78], SnO\({}_{2}\)[79], MgO [80], Ag [81; 82], Al [83], carbon nanotube (CNT) [84], LiCoO\({}_{2}\)[85] as well as mixing with other cathode materials such as NMC [86] have shown to suppress Mn dissolution hence improves the storage life, cycle life and/or capacity fading [87]. In particular, LBO glass was the first reported coating for LiMn\({}_{2}\)O\({}_{4}\); it has a favorable ionic conductivity and can withstand oxidation at high potentials (i.e., 4 V) as well as molten LBO glass has good wetting properties. The LBO glass surface treatment improves the storage performance, but has a detrimental effect on cycling life which is associated with the reduction of average Mn valence state on the surface by boron containing solid species [76]. On the other hand, Al doping of LiMn\({}_{2}\)O\({}_{4}\) increased the average valance of Mn, and showed enhanced cycling performance at high temperatures [83]. Li and Al co-doped manganese oxide spinel compounds prepared by polyvinyl pyrrolidone precursor method improved the initial discharge capacity and capacity retention [88]. LiMn\({}_{2}\)O\({}_{4}\)/CNT composites are also reported with improved capacity retention and high rate capability for flexible electronics applications [84]. More stable cathode materials such as LiCoO\({}_{2}\) are also utilized as a barrier coating for LiMn\({}_{2}\)O\({}_{4}\)[85]. A 5 nm LiCoO\({}_{2}\) coating prepared by micro-emulsion technique improved the cycle life and reduced the self-discharge of LiMn\({}_{2}\)O\({}_{4}\); however, resulted in slight decrease in capacity of pristine LiMn\({}_{2}\)O\({}_{4}\) due to Li storage inability of the LiCoO\({}_{2}\) coating [85].