Graphene modified LiMPO$_{4}$ (M$=$Fe, Mn) as a cathode material for lithium ion batteries

We have synthesized LiFePO$_{4}$/graphene nano-composites using sol-gel method by adding water dispersed graphene oxide to the LiFePO$_{4}$ precursors during the synthesis. The graphene oxide was subsequently reduced by annealing the composite at 600$^{\circ}$C for 5h in forming gas (90{\%} Ar and 10{\%} H$_{2})$ which was confirmed by Raman spectroscopy and X-ray Photoelectron spectroscopy. Addition of graphene significantly improved the electronic conductivity of LiFePO$_{4}$. Scanning Electron microscopy and Transmission electron microscopy images show LiFePO$_{4}$ particles being covered uniformly by graphene sheets throughout the material forming a three dimensional conducting network. Cyclic voltammetry results show that composite is a typical two-phase system. Li ion diffusion coefficient calculations show two orders of magnitude enhancement. At low currents, (C/3), the capacity of LiFePO$_{4}$/graphene composite cathode reaches 160 mAh/g, which is very close to the theoretical limit. More significantly, the graphene modified LiFePO$_{4}$ shows a dramatically improved rate capability up to 27C, and excellent charge-discharge cycle stability over 500 stable cycles. In addition to~LiFePO$_{4}$, LiMnPO$_{4}$/graphene composite has also been synthesized and the results will be discussed.