关键词: 碳限域, Li₃VO₄, 水热-固相法, 负极材料, 倍率性能

Abstract:

Li₃VO₄, as a promising anode material for lithium ion batteries, has been widely studied because of its low and safe voltage, and large capacity. However, its poor electronic conductivity impedes the practical application of Li₃VO₄ particularly at high rates. In this paper, carbon confined Li₃VO₄ nano materials (Li₃VO₄/C) were synthesized by hydrothermal and solid-phase method, and for comparison, the Li₃VO₄ (N) nano materials without carbon confinement and Li₃VO₄ (B) materials were also synthesized by pure solid-phase method. The composition, structure, morphology and specific surface area of the three samples were studied by XRD, Raman, TEM and N₂ adsorption-desorption tests. It was found that the grain size of Li₃VO₄ in Li₃VO₄/C was the smallest, which is 51 nm, the grain size of Li₃VO₄ in Li₃VO₄ (N) was the second (93 nm), and the grain size of Li₃VO₄ prepared by pure solid-phase method was the largest (113 nm). The thickness of carbon confinement layer in Li₃VO₄/C was 2-4 nm, which was uniformly coated on the surface of Li₃VO₄. And the specific surface area and pore size distribution of the three samples were measured by BET and BJH methods. It was found that the samples prepared by hydrothermal and solid-phase method had mesoporous structure, and the Li₃VO₄ prepared by a simple solid-phase method had the least porous structure. The BET specific surface area and the pore volume of the carbon confinement sample were larger than those of the sample without carbon confinement layer (30.49 m²·g^-1 vs. 26.42 m²·g^-1 and 0.12 cm³·g^-1 vs. 0.05 cm³·g^-1), which is in agreement with the smaller grains of Li₃VO₄/C by XRD analysis, indicating that the carbon layer limits the growth of Li₃VO₄ grains, so as to increase the contact area between active material and electrolyte when the sample is used as the anode material of lithium ion battery. The charge-discharge performances of the synthesized samples as anodes of lithium ion battery were studied. It was found that the Li₃VO₄/C electrode displayed faster lithium ion storage performance than Li₃VO₄ (N) and Li₃VO₄ (B) electrodes. At the rates of 0.1 C, 0.5 C, 1 C, 5 C, 10 C and 20 C, the discharge capacities of Li₃VO₄/C were 435, 428, 401, 356, 302 and 280 mAh·g^-1, respectively. In particular, after 50 cycles at 5 C, Li₃VO₄/C still maintained 92.3% of the initial capacity, which fully reflects the characteristics of larger capacity, higher rate capability and better stability of Li₃VO₄/C electrode. By analyzing the relationship between morphology and electrochemical properties, it is considered that the carbon confinement layer reduces the ohmic polarization of Li₃VO₄ in the processes of charge and discharge, the large specific surface area improves the penetration efficiency of electrolyte, and the small particle size shortens the diffusion path of lithium ions. At the same time, the synthesis method in this work presents a universal strategy for the preparation of other transition metal oxide salts with porous structure and small particle.

Key words: Carbon confinement, Li₃VO₄, Hydrothermal and solid phase method, Anode material, Rate capability