Study On Synthesize And Properties Of Non-stoichiometric Lifepo₄ Via Hydrothermal Method

LiFePO₄ cathode material was synthesized by hydrothermal method, which was characterized by XRD, SEM, and galvanostatic charge-discharge cycling test. And the effects of different surfactants agent, raw material ratios, and impurities on the structure and electrochemical performance of LiFePO₄ were systematically investigated.The effect of ionic surfactant CTAB and non-ionic surfactant PEG400 on the structure and properties of LiFePO₄ was studied. The results showed that LiFePO₄ using non-ionic surfactant PEG400 as surfactant exhibited more excellent performance, which showed the round flaky morphology with single-size of about 1μm. The specific discharge capacities were 133.5,122.9,107.4,97.4mAh/g at 0.2C, 1C, 3Cand5C, respectively.The effect of raw materials ratio on structure and properties of LiFePO₄ was studied. The sample with Li: Fe: P = 3.03: 1: 1.04 showed diamond plate morphology with a diameter of about 500nm, and the best electrochemical properties, whose discharge capacities could reach 152.9, 145.3, 136.2, 122.4 mAh/g at 0.2C, 1C, 3C and 5C rates, respectively, with a cycling capacity degradation rate of only 4.1% after 100 cycles at 1C rate. Moreover, the effect of ball-milling treatment on the product performance was studied, and the results showed that ball-milling treatment had little effect on the electrochemical performance was not effected, though the particle size was reduced significantly.LiFePO₄ prepared by hydrothermal method was exposed to saturated air at 120°C for 0, 12 and 24h, respectively, and the effect of saturated air on LiFePO₄ was investigated. The results showed that the Fe3+ content increased from primary 0.5 to 1.0% with the time prolonging. The Fe3+ content in LiFePO₄ was reduced to below 0.1%, which was transformed to Fe3C after calcination at 750℃for 6 h under N2 atmosphere. The initial specific discharge capacities of carbon-coated material exposed to air for 0, 12 and 24h were 148, 149, 146 mAh/g at 1C rate and 131, 130, 120 mAh/g at 3C rate, respectively. It indicated that a shorter exposure time had no obvious effect on the discharge performance of LiFePO₄ at low C-rate, but a longer exposure time led to the deterioration of electrochemical capacity and cycling performance.