Synthesis And Performance Improvement Study Of LiFePO₄ Cathode Material For Lithium Ion Battery

LiFePO₄ cathode material was synthesized by carbothermal reduction method which was constitutive of high temperature solid reaction in two different temperature ranges. The influence of carbon sources, ball-milling method, amounts of carbon and metal ion doping on the structure and electrochemical performance of LiFePO₄ was studied.The influence of seven kinds of different carbon sources, such as citric acid, sucrose and glucose on the performances of the material was investigated. The results show that the morphology and particle size of LiFePO₄/C composite cathode material are varied with different carbon sources, and thus its electrochemical performance is changed. The sample used citric acid as carbon source has the smallest particle size and the best electrochemical performance, which delivered a high capacity of 142.2 mAh/g at C/5 rate. And even after 25 cycles, the discharge capacity was 98% of initial capacity.The precursor was prepared by wet ball-milling and dry ball-milling method. The results indicate that LiFePO₄ prepared by dry ball-milling has a better electrochemical performance than by wet ball-milling. When dry ball-milling for 15 hours, the sample has the smallest particle size and the best electrochemical performance. At the charge and discharge current rate of C/5, the first discharge capacity is about 142.4 mAh/g. And even after 25 cycles, the discharge capacity reduces to 139.5 mAh/g, which is about 98% of initial capacity.Effects of carbon dosage on the crystal structure, morphology and electr- ochemical properties of LiFePO₄ have been discussed in detail. The results show that the excessive carbon can restrain the growing of LiFePO₄ crystal particle effectively, and reduces the volume of unit cell. LiFePO₄/C cathode material which contains 5% carbon has the best electrochemical performance.Li_1-5xNb_xFePO₄/C cathode material was prepared using Nb₂O₅ as doping agent. Because of the doping quantity was low, the crystal parameters has only a little change. But we still find that the volume of unit cell reduces. Through the test of electrochemical performance, we find that sample when x=0.008 has the best electrochemical performance. At the charge and discharge current rate of C/10, C/5, C/2, 1C, the first discharge capacity is about 164.3, 157.2, 121.6 and 106.8 mAh/g respectively。...