关键词: 钒改性, 硅酸铁锂, 羧酸, 正极材料, 锂离子电池

Abstract:

The carbon coated 10% vanadium modified lithium iron silicate (Li₂Fe_0.9V_0.1SiO₄/C) composites were prepared by sol-gel method to form precursor and followed by solid state reaction. Effects of different carboxylic acids, namely, citric acid, acetic acid and oxalic acid, on the crystal structures, surface morphologies, interfacial characteristics and electrochemical properties of the composites were systematically investigated. It was found that a mixed P2₁ and Pmn2₁ phase was formed with the major impure phase of iron (Fe) and minor impurity of lithium silicate (Li₂SiO₃). The initial discharge capacities of 144.7, 140.3 and 168.7 mAh•g^-1 were achieved at 0.1C and room temperature, while the maximum capacities of 155.9, 145.3 and 172.0 mAh•g^-1 at the 7^th, 15^th and 2^nd cycles with the capacity retention values of 68.2%, 76.7% and 59.4% were obtained upon 50 cycles for the uses of citric acid, acetic acid and oxalic acid, respectively. Consisting of three carboxyl functional groups, the citric acid based composite contained higher amount of 7.8% residual carbon, the formation of impure Fe phase was promoted, and the larger charge-transfer resistance of 147 Ω was obtained, leading to lower coulombic efficiency and poorer cycle performance. On the contrast, the acetic acid based composite containedone carboxyl functional group only, resulted in the least amount of Fe and the smaller charge-transfer resistance of 73 Ω ,which showed the best cycle performance with the largest capacity retention. However,  carrying two carboxyl functional groups the oxalic acid based composite led to 6.0% residual carbon and larger flower-like morphology, which slightly improved the lithium ion diffusion coefficient, achieving more than one lithium ion (1.05) per formula unit intercalation.

Key words: Vanadium modification, lithium iron silicate, carboxylic acid, cathode material, lithium ion battery