Effect of the Different Carbon Source of Nanocomposite LiFePO4

Lithium iron phosphate (LiFePO4 exhibiting theoretical capacity of ~170 mAh/g and a flat charge/discharge profile at ~ 3.4 V Li/Li. Additionally, the cost effectiveness, environmental benevolently, thermal stability has made LiFePO4 as one of the most attractive cathode materials for rechargeable Li-ion batteries. 1 Li has slow diffusivity in LiFePO4 (DLi ~ 10 – 10 cm/s) compared to the widely used layered LiCoO2 ((DLi ~ 10 – 10 cm/s). , As a result, only 60% of the capacity could be obtained for the original LiFePO4 in the early work, and its capacity decreases remarkably at larger current density or at high C-rate. Hence, for successful commercial acceptance several material related issues, which can be grouped into three categories, should be addressed. Synthesis of phase pure olivine lithium iron phosphate at relatively low processing temperatures. The literature says that it is important to select proper precursor material as well as optimize process parameters to keep iron in its 2 valence state to suppress the formation of impurity phases and use of carbon with the precursor materials and calcinations at inert ambient have been found useful in limiting the formation of impurity phase. To overcome the poor ionic as well as electronic conductivities of LiFePO4, carbon source coating has been described to improve the Li-ion kinetics. The control on the particle size in the nano-regime along with a narrow particle size distribution of the synthesized LiFePO4 cathode materials is found beneficial to increase capacity, and rate capability. In the case of LiFePO4 the highest discharge capacity obtained was around 163mAh/g with C/10 rate. However, in terms of rate capability best results reported so far is 125 mAh/g with 197C . Hence, the optimization of the synthesis process to obtain phase pure nano-crystalline C-LiFePO4 composite cathode materials is of importance for its reliable cathode application. In the present case, a solidstate route under nitrogen ambient has been adopted to synthesize different source of carbon C-LiFePO4 composite cathodes and their structural and electrochemical properties have been studied and compared. XRD patterns of C-LiFePO4 materials (calcined at 700°C in nitrogen ambient) are shown in Fig. 1a. In the figure all the major reflections were indexed based on orthorhombic olivine structure (Pnma). XRD pattern of CLiFePO4 clearly shows the ideal orthorhombic olivine structure (JCPDS card no.40-1499) without any impurity phase and the calculated pattern based on Reitveld, fit with the experimental result. The lattice parameters were a = 10.328 (1) A, b= 6.0084 (6) and c = 4.6991 (6) A with agreement parameters Rp = 2.57 % and Rwp = 3.33 and matches quite well with the existing literature. 6