Synthesis And Modification Of LiFePO₄ As A Cathode Material For Lithium-ion Batteries

In this paper, LiFePO4 cathode material was synthesized by ball mill method with Li2CO3, FeC2O4, NH4H2PO4 as the raw material. Carbon coating, vanadium doped and fluorine doped were wsed in the synthetise route. The crystalline structure, morphology, particle size and the surface appearance of coated carbon layer were characterized by XRD, SEM and HRTEM techniques. The electrochemical properties of the samples were tested on the Neware multichannel cycler, the CV curve and impedance performance were carried out with electrochemical workstation.The electrochemical properties of LiFePO4 material were improved greatly by using PVA as carbon source and the electrochemical properties were best when the PVA’s mass fraction was 10%. At 0.5 C, the discharge specific capacity was 134.0 mAh/g for the first cycle and 133.8 mAh/g for the 100th cycle, little or no damping capacity. But the initial discharge specific capacity was much lower than the theoretical capacity and the specific capacity had a quick attenuation under large charge/discharge current. So another carbon source was needed for modification research.After using pitch as carbon source the electrochemical properties of LiFePO4 were improved compared to PVA, and the electrochemical properties were best when the pitch’s mass fraction was 8%. At 0.5 C, the discharge specific capacity was 153.6 mAh/g for the first cycle and 147.2 mAh/g for the 100th cycle. The capacity retention rate was 95.8%. The specific discharge capacities were 164 mAh/g,156 mAh/g,150 mAh/g,131 mAh/g,102 mAh/g,64 mAh/g and 28 mAh/g respectively corresponding to 0.1 C,0.5 C,1 C,2 C,4 C,8 C and 16 C. But the specific capacity of LiFePO4 sample was low under the large current discharge. So doping modification was needed.The experiment results showed that the performances were improved by doping V. The electrochemical properties were best when the proportion of V was 3%. At 0.5 C, the discharge specific capacity was 155.5 mAh/g for the first cycle and 151.3 mAh/g for the 100th cycle. So the capacity retention rate was 97.3%. The specific discharge capacities were 160 mAh/g at 0.1 C,155 mAh/g at 0.5 C,149 mAh/g at 1 C,143 mAh/g at 2C,138 mAh/g at 4C,129mAh/g at 8C,111 mAh/g at 16C.To improve the initial specific capacity, LiF was used to study anionic doping modification. When the doping ratio was 1% of F, at 0.5 C, the discharge specific capacity was 158.7 mAh/g for the first cycle and 158.1 mAh/g for the 100th cycle, the capacity retention rate was 99.6%. The specific discharge capacities were 162 mAh/g at 0.1 C,159 mAh/g at 0.5 C,157 mAh/g at 1 C,151 mAh/g at 2 C,144 mAh/g at 4C and 132 mAh/g at 8 C. The materials had the best discharge specific capacity and cycle performances.