Effect Of VO₄^3- Doping On Electrochemical Properties Of The Li₃Fe₂（PO₄）₃ Cathode Materials

Nasicon type Li₃Fe₂（PO₄）₃ compound was considered one of the most potential cathodes for lithium ion batteries because of advantages such as stable structure, cheapness, friendliness to environment and high ion conductivity. However, low electronic conductivity greatly limited its application in industry. In this research, we tried to use VO-4^3- partial substitution for the PO43- anion in the Li₃Fe₂（PO₄）₃ compound for the purpose of improving its electronic conductivity. Investigations on microstructures and electrochemical properties of the VO-4^3- doped materials under different conditions revealed that:Heat treatment to the Li₃Fe₂（PO4）2.85（VO4）0.15 xerogel synthesized by sol-gel method indicated that a pure Li₃Fe₂（PO₄）₃ was obtained at the temperature of 600℃, and some secondary phase appeared when the heating temperature higher than 650℃. After mixing with the graphite, the sample treated at 600℃ showed the highest initial discharge capacity of 95.5m Ah/g at a rate of 0.5C. When mixing with several conductors such as graphite, carbon nanotubes, acetylene black and Ketjen black, the Li₃Fe₂（PO4）2.85（VO4）0.15 sample showed the best electrochemical performance when mixing with the Ketjen black, and its initial discharge capacity was 100.24 m Ah/g at a rate of 0.5C, its capacity retention rate was 94.78% after 60 cyclic charging and discharging. Changing the milling process from 220 rpm for 2h to 300 rpm for 4h resulted in an increase in discharging capacity, which was about 103.14 m Ah/g at a rate of 0.5C, and its capacity retention rate is 97.29% after 60 cycles.Increasing the VO-4^3- doping concentration revealed that the sample with VO-4^3- content of 0.45 presented the best electrochemical performance. Its discharge capacity of 113.40 m Ah/g, which was 16.92% larger than the undoped Li₃Fe₂（PO₄）₃, was obtained at 0.5C rate, and a discharge capacity of 96.62 m Ah/g, 20.13% higher than the undoped one, was reached at 2C rate. Cyclic voltammetry showed that VO-4^3- doping not only increased intensities of redox peaks, but also decreased their potential differences in the CV curves. Those results indicated that polarization in the batteries was decreased, and then electrochemical reaction was accelerated by VO-4^3- doping. Investigation on electrochemical impedance spectra（EIS） showed that the charge transfer resistance, Rct, decreased with an increase in the VO-4^3- content and that the sample with the VO-4^3- content of 0.45 showed the minimum Rct of 88.7Ω, which was lower than half of that for the undoped sample. Lithium-ion diffusion coefficient calculated from the EIS curves at low frequencies proved that they also increased with an increase in the VO-4^3- content, the same variation tendency as the Rct, and that the sample with x of 0.45 exhibited the maximum lithium ion diffusion coefficient of 3.84′10-11 cm2/s, which was one order of magnitude larger than the undoped one. In a word, electrochemical performance of the Li₃Fe₂（PO₄）₃ could be greatly improved by VO-4^3- anion doping.