Lithium-ion Battery Cathode Material Li <sub> 3 </ Sub> V <sub> 2 </ Sub> (po <sub> 4 </ Sub>) <sub> 3 </ Sub> Preparation And Electrochemical Performance

Monoclinic lithium vanadium phosphate Li3V2(PO4)3 cathode material for lithium ion batteries had attracted much attention for stable structure and safety. However, its development was limited due to rigorous synthesis conditions and poor cycle performance. In this paper, Li3V2(PO4)3 cathode material was synthesized respectively using microwave carbothermal reduction method, sol-gel method and microwave sol-gel method, and studied with Al3+ doped. The Li3V2(PO4)3 cathode material was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), charge-discharge tests. We could studied the synthesis process, material modifying, structure characterization and electrochemical performance deeply with these results.First, Li3V2(PO4)3 was synthesized via the microwave carbothermal reduction method, and optimized its synthesis process. As results shown, crystal structure, particle size and surface morphology of Li3V2(PO4)3 were influence by sintering temperatures and sintering time. Li3V2(PO4)3 particles grew perfect gradually with sintering temperature increasing from 800℃to 850℃. When sintering temperature grew up to 900℃, the sample showed the excessive burning phenomenon. After the single factor experiment, the Li3V2(PO4)3 sample synthesized at 850℃for 15min had the best electrochemical performance. The initial charge/discharge capacity were 130 mAh·g-1 and 120 mAh·g-1 at 0.5C. Its discharge capacity remained 101mAh·g-1, the highest capacity retention of 84.2% after 50 cycles.Then, Li3V2(PO4)3 was synthesized by the sol-gel method using the citric acid as chelating agent and carbon source. It was indicated that diffraction peaks of samples were sharp as shown in XRD patterns. The specific surface area of samples were observed large from SEM images, considering good electrochemical performance. The Li3V2(PO4)3 synthesized at 650℃for 20h exerted good electrochemical performance. The initial charge/discharge capacity were 136 mAh·g-1 and 128 mAh·g-1 at 0.5C. The discharge capacity was 117mAh·g-1, the highest capacity retention of 91.4% after 50 cycles.It was revealed excellent electrochemical performance when Li3V2(PO4)3 was synthesized at 650℃for 15min with the microwave sol-gel method. The initial charge/discharge capacity were 135 mAh·g-1 and 130 mAh·g-1 at 0.5C. The discharge capacity was 124mAh·g-1, the highest capacity retention of 95.4% after 50 cycles.It was discussed that the influence of Al3+ doped amount on crystal structure, surface morphology and electrochemical performance in Li3V2(PO4)3 synthesized by the microwave sol-gel method. As known from results, Li3AlxV2-x(PO4)3 (x=0,0.04,0.08) with different Al3+ doped amount appeared monoclinic structure. The defined Al3+ doped amount made particles refining. The electrochemical performance of Li3AlxV2-x(PO4)3(x=0,0.04,0.08)existed much differences with different Al3+ doped amount. The initial charge/discharge capacity of Li3AlxV2-x(PO4)3 (x=0.04) were 130 mAh·g-1 and 126 mAh·g-1 at 0.5C, and discharge capacity remained 122mAh·g-1 with the retention of 96.8% after 50 cycles. The discharge capacity of Li3AlxV2-x(PO4)3 (x=0.04) was 118 mAh·g-1 retaining 93.65% after 100 cycles at 2C.In this paper, the dynamic properties of Li3V2(PO4)3 with different synthesis methods were explored tentatively. The exchange current density and the diffusion coefficient of cathode materials with different synthesis methods were tested by the linear polarization and PITT. It was indicated that the values of exchange current density and the diffusion coefficient which respectively synthesized by microwave carbothermal reduction method, sol-gel method and microwave sol-gel method increased in proper sequence.