Hydrothermal Synthesis Of LiFePO₄ And Li₃V₂(PO₄)₃ Cathode Materials For Lithium Ion Batteries

Polyaniline doped lithium iron phosphate (PAn-LiFePO4) cathod materials were synthesized by hydrothermal method, water and ethanol/water mixtures were used as solvent, respectively. At the same time, another cathode material, lithium vanadium phosphate [Li3V2(PO4)], was also pretreated by hydrothermal method, and then carbon coated by carbon thermal reduction. The effects of raw materials, reaction conditions and the solvent on the performance of products were studied. The structure of the products was analyzed by FT-IR, X-ray diffraction and elemental analysis. The rate performance, cycling performance and cyclic voltammetry properties of products were tested, respectively.The olivine structure LiFcPO4 without any impurity was prepared by hydrothermal method at 160℃for 12h when the mole ratio of LiOH, FeSO4 and NH4H2PO4 was 2:1:1, and the water was used as solvent. However, the products present poor electrochemical performance. PAn-LiFePO4 composites were prepared by hydrothermal method in the same condition. When doped with 20wt% polyaniline, the initial discharge capacity of PAn-LiFePO4 was 160mAh/g at 0.1 C rate. After 50th cycle under 0.1C rate, the capacity still kept 95.9% of initial capacity. The SEM images showed that polyaniline particles were distributed on and around the LiFePO4 particles evenly, and the conductivity of the materials was improved accordingly.The water/ethanol mixture was also used as solvent for the hydrothermal synthesis of PAn-LiFePO4 composites. The obtained products presented better electrochemical performance than those obtained using water as solvent. The volume ratio of water to ethanol didn't affect the structure of LiFePO4. The PAn-LiFePO4 composite showed better performance when the volume ratio of water to ethanol was 1:2, which initial discharge capacity was 160mAh/g at 0.1C rate. After 50th cycle under 1C rate, the maintenance of initial discharge capacity was 96.3%. The SEM images showed that regular diamond-shaped sheet structure was obtained by hydrothermal method. The uniform particle size and the large specific surface of products led to the good electrochemical performance.The Li3V2(PO4)3/C cathode materials were synthesized by hydrothermal pretreatment and high temperature sintering. After the raw materials were pretreated by hydrothermal method, the sintering time of Li3V2(PO4)3 reduced to 2 hours. Compared with the traditional high temperature solid-state reaction, the reaction time was shortened by 90%, and the sintering temperature was also reduced significantly. The initial discharge capacity of the sample sintered at 800℃was 177mAh/g at 0.1 C rate (90% of theoretical capacity). After 50th cycle under 1C rate, the maintenance of initial discharge capacity was 92.3%. The discharge capacity of 125mAh/g can still be held at 2C rate, which indicated that the products had good cycle and rate properties.