# A Review on Nanocomposite Materials for
## 4 Cathode
### Olivine, LiMPO\({}_{4}\) (M = Fe, Co, Mn and Their Mixtures)  
Olive LiFePO\({}_{4}\) stands out among cathode materials by its superior cycle life and intrinsic safety [97]. Additionally, Fe is one of the most abundant elements on Earth's crust (i.e., even less costly than Mn [63]), making LiFePO\({}_{4}\) highly cost competitive. LiFePO\({}_{4}\) has an average operating voltage of 3.4 V vs. Li/Li\({}^{+}\), and a specific capacity of \(\sim\)170 mAh/g corresponding to one Li per LiFePO\({}_{4}\) formula unit. The enhanced cycle life and safety of LiFePO\({}_{4}\) are associated with its lower operating voltage (i.e., 3.4 V) compared to other cathode materials such as LiCoO\({}_{2}\) (i.e., 4.2 V) and LiMn\({}_{2}\)O\({}_{4}\) (i.e., 4 V). At such low potentials, non-aqueous electrolytes are highly stable. However, LiFePO\({}_{4}\) has one-dimensional Li-ion diffusion channels leading to poor ionic conductivity, and it has an intrinsically low electronic conductivity. Moreover, LiFePO\({}_{4}\)'s density is lower (i.e., 3.6 g/cm\({}^{3}\)) than the layered oxides and spinels (\(\sim\)5 g/cm\({}^{3}\)) leading to poor volumetric energy density. The low electronic conductivity of LiFePO\({}_{4}\) can be enhanced by carbon coating [98] and by decreasing the LiFePO\({}_{4}\) particle size [99], or both. [PERSON] et al. showed that LiFePO\({}_{4}\) nanoplates (\(\sim\)30-40 nm along the b-axis) with a 5 nm amorphous carbon layer coating prepared by the solvothermal method significantly improved the high rate performance by effectively reducing the Li\({}^{+}\) diffusion length as compared to mesoporous spherical LiFePO\({}_{4}\)/C cathodes [100]. However, decreasing the particle size has a negative effect on tap density and accelerates the side reactions due to increased surface area; therefore, carbon coating is the most widely used method to improve the performance of LiFePO\({}_{4}\). It was shown that, the quantity of carbon coating in LiFePO\({}_{4}\) is not always proportional to the performance enhancement because several other parameters also play a key role in the performance of a carbon coated LiFePO\({}_{4}\) such as structure of carbon [101], especially sp\({}^{2}\)/sp\({}^{3}\) and disordered/graphene (D/G) ratios [102], carbon source [103], carbon thickness [104], and processing conditions [105]. The single doping and co-doping of the carbon layer to improve the electronic conductivities are also explored. Phosphorus doped carbon layer on LiFePO\({}_{4}\) has shown to decrease the charge transfer resistance significantly which is considered to be associated with the free carriers donated by the phosphorus. Additionally, phosphorus doped carbon layer improves the capacity and capacity retention at high C rates [106]. In another study, the LiFePO\({}_{4}\) composite with boron and nitrogen co-doped carbon layers were synthesized by the ball milling process followed by calcination [107]. The sequence of doping shown to be critical (i.e., N + B is preferred), and improvements in high-rate capacity are attributed to electron and hole type carriers donated by nitrogen and boron atoms, respectively [107]. Large scale and low-cost material preparation methods are critical for the higher penetration of Li-ion batteries. LiFePO\({}_{4}\) prepared by the carbothermal reduction method, which is suitable for large scale synthesis, showed almost no capacity fade for 400 cycles [108]. Carbon nanotubes (i.e., CNT) are excellent electronic conductors and have high surface area, and their tubular structure can enhance the overall electronic and ionic conductivity of the active cathode/anode materials by connecting the inert \"dead\" zones in these materials [109]. The schematic of the CNT dispersed in a porous LiFePO\({}_{4}\)-CNT composite is shown in Figure 10. The LiFePO\({}_{4}\)-CNT composite performed significantly better than the pristine LiFePO\({}_{4}\) at both low and high C rates [109]. Another superior electronic conductor is the graphene.