Improved Study On Performance Of Lithium Ion Battery LiFePO₄ Cathode

Olivine-structured LiFePO4 has become one of the most promising cathode material for its wide material source, low row materials cost, and environmental friendliness. Being used as cathode material, it has the characteristic of good thermal stability, excellent circle performance and so on. Its intrinsic crystal structure, however, result in properties of low electronic conductivity and poor discharge capability at high rate, which has been the greatest obstacle for application of LiFePO4 Nowadays, the main methods for improving electronic conductivity of LiFePO4 focus on carbon coating and metal ion doping. In this paper, the main work is to synthesize LiFePO4/C composite with good electrochemical properties.Firstly, LiFePO4/C composite was sintered with different iron phosphate (FePO4·xH2O, FePO4·2H2O and FePO4) as the iron source. The synthetic conditions of LiFePO4/C were optimized by orthonormal experiment using FePO4·2H2O as raw material. The sample prepared in optimized technical conditions has the highest reversible discharge specific capacity of 151.1 mAh·g-1 at 0.1C(1C=170mA·g-1) rate and 128.6 mAh·g-1 at 2C rate. The effects of pretreatment temperature on the crystal structure of FePO4·2H2O, the morphology and electrochemical properties of LiFePO4/C were investigated. The influences of synthesis temperature, synthesis time on the performance of LiFePO4/C were studied. The results showed the optimal conditions for synthesizing LiFePO4/C were as follows:pretreatment temperature of FePO4·2H2O was 500℃, synthesis temperature was 650℃, synthesis time was six hours. The sample prepared in these conditions exhibited excellent cyclability,delivering a discharge specific capacity of 153.0 mAh·g-1 at 0.1C rate after 40 cycles.The doping effects of Fe site on the electrochemical properties of LiFePO4 were investigated. The results showed V5+doping can improve electrochemical performance of LiFePO4, especially the discharge capability at high rate. LiFe0.99V0.01PO4/C has the discharge capacity of 160.6,145.7 and 135 mAh·g-1 at 0.1 C,1C and 2C rate using FePO4 as the iron source; LiFe1-δ-xVxPO4/C(x=0.005) has the discharge capacity of 160.7,147.2 and 139.9 mAh·g-1 at 0.1C,1C and 2C rate using FeC2O4·2H2O as the iron source.The multiple doping Li-ion battery cathode materials Li0.99Nb0.01Fe1-xMgxPO4/C(x=0,0.01,0.02,0.03,0.04) were synthesized by two-step solid state reaction. Special crystal defect existence of the Nb5+, Mg2+ multiple doping samples, can improve electrochemical performance of LiFePO4, which is propitious to the transfer of Li ions. Li0.99Nb0.01Fe0.97Mg0.03PO4/C has the best electrochemical performance. The initial discharge capacity of the sample delivers 153.7,149.7,144.6 and 126.4 mAh·g-1 at 0.2,1,2 and 4C rates, even at 8C rate, the initial discharge capacity of the sample remains 92.2 mAh·g-1. In addition, it shows excellent cycle stability at different rates.