Preparation and Lithium Storage Properties of Carbon Confined Li3VO4 Nano Materials
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 then compared with the Li3VO4 (N) nano materials without carbon confinement and Li3VO4 (B) materials 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 is the smallest, which is 51 nm, the grain size of Li3VO4 in Li3VO4 (N) is the second (93 nm), and the grain size of Li3VO4 prepared by pure solid-phase method is 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 method. It was found that the samples prepared by hydrothermal and solid-phase method have mesoporous structure, and the Li3VO4 prepared by simple solid-phase method has little porous structure. The BET specific surface area and the pore volume of the carbon confinement sample are 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 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 has 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 are 435, 428, 401, 356, 302 and 280 mAh·g-1, respectively. In particular, after 50 cycles at 5 C, Li3VO4/C can still maintain 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 process 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 and small particle.