Enhancement in electrochemical performance of LiFePO4 -carbon nano composite materials for lithium ion batteries

LiFePO4 has attracted great interest as a cathode material for lithium ion batteries due to its reasonably high theoretical capacity (170mAh/g), thermal stability, high Li ion reversibility and low cost. However, prohibitively low electronic conductivity (∼10-9 S/cm) of LiFePO4 leads to high impedance, low capacity and low rate capability. To overcome this bottleneck, we have developed multiple approaches to improve the conductivity of LiFePO4. Motivated by the outstanding electronic and mechanical properties as well as high surface area of graphene, we prepared LiFePO4/graphene nano-composites by a sol-gel method. The phase purity of the nano LiFePO4/Graphene composite was confirmed by X-Ray diffraction. Addition of graphene improved the electronic conductivity of LiFePO4 by six orders of magnitude. Scanning electron microscopy and transmission electron microscope images show LiFePO4 particles being covered uniformly by graphene sheets throughout the material forming a three-dimensional conducting network. At low currents and charging rate of C/3, the capacity of the composite cathode reaches 160 mAh/g, which is very close to the theoretical limit. More significantly, the LiFePO4-graphene composite shows a dramatically improved rate capability up to 27C and excellent charge-discharge cycle stability over 500 stable cycles. To further improve the conductivity of LiFePO4 and thus its rate capability, we optimized the concentration of the Fe2P metallic impurity phase by tuning the annealing temperature. X-ray diffraction shows that samples annealed at 600° C are nearly phase pure while those treated at higher temperatures contain Fe2P and Li3PO4 impurity phases, which increase with increasing annealing temperature. Mossbauer spectroscopy and magnetic measurements were used to quantify the amount of Fe2P impurity phase. Scanning electron microscopy measurement reveals a noticeable increase in particle size as the annealing temperature increases from 700 °C to 900 °C. Optimal results are obtained in LiFePO4/C samples annealed at 700 °C, which show the lowest charge transfer resistance, highest Li-ion diffusion coefficient, the highest specific capacity of 166 mAh/g at a rate of 1C and the best rate capability among all samples. In addition, we have also studied the effect of doping In3+ on the Fe site and found that the addition of indium significantly improves the electronic conductivity leading to further improvement in capacity and rate capability.