Research On The Synthesis Of High-density LiFePO₄/C And Its Performance

Lithium ion batteries are one of the preferred mobile power-sources. They have the favorable properties of high working voltage, non-memory effect, long cycling life and high energy density. Owing to its low price, good stability, high capacity and long cycling life, LiFePO₄ will become one of the most promising materials for the use of the positive anode materials of Lithium ion batteries. Based on the detailed discussion of the development of lithium ion battery and it's anode materials, LiFePO₄/C was synthesized by precipitation-calcination and solid-state method respectively, and their physical and chemical properties were systemically studied by the methods of XRD, SEM, TEM, IR, laser granularity test, electronic conductivity test and tap-density test etc. And the electrochemical performances were studied by charge/discharge and CV tests.The anode materials of LiFePO₄/C were synthesized by precipitation-calcination. We firstly found the optimized technological conditions through orthogonal tests to synthesize the precursor of NH₄FePO₄·H₂O by precipitation on the base of single factor tests. Under this technological conditions, NH₄FePo₄·H₂O was in the form of round particle, its tap-density was 1.73g/cm3, and Fe²⁺ content 29.83%. Then we found the optimized technological conditions through orthogonal tests to synthesize the anode materials of LiFePO₄/C by calcination. Under this technological conditions, LiFePO₄/C was in the form of a particle resembling balls, its tap-density was 1.68g/cm³, its Fe²⁺ content was 32.29%, carbon content 5.03% and electronic conductivity 5.67×10^-4S/cm. The initial discharge specific capacities were 147.6mAh/g at 0.1C, 136.7mAh/g at 0.5C, and 122.3mAh/g at 1C at room temperature. After 50 cycles, the discharge specific capacities of the sample were 142.8mAh/g, 127.3mAh/g and 106.7mAh/g respectively. And its initial discharge specific capacity was 163.8mAh/g at 0.5C ,at 60℃and good cycling performance was shown. The material had good performance of anti-overcharge and anti-overdischarge.The anode materials of LiFePO₄/C were synthesized by solid-state method. We used orthogonal tests to seek the optimized technological conditions, under which, its tap-density was 1.12g/cm³, its Fe²⁺ content was 30.87%, carbon content 6.22% and its initial discharge specific capacities were 134.2mAh/g at 0.1C, 120.5mAh/g at 0.5C, and 104.7mAh/g at 1C at room temperature. After 100 cycles, the specific capacity was 116.5 mAh/g, at 0.1C, at room temperature, the maintenance rate of the specific capacity was 86.8%.The pure LiFePO₄ was synthesized by precipitation- calcination. And through comparison, we found that carbon wrapped LiFePO₄ can enhance the electronic conductivity, the specific capacity and cycle performance, and inhibit the particles' growth and reunion.