Effect of VO₄³⁻ substitution for PO₄³⁻ on electrochemical properties of the Li₃Fe₂(PO₄)₃ cathode materials

In this research, VO₄³⁻ substitution was used to improve electrochemical properties of the Nasicon Li₃Fe₂(PO₄)₃ cathode material. The VO₄³⁻ substitution resulted in formation of a homogeneous compound Li₃Fe₂(PO₄)_3-x(VO₄)_x in a composition range of x ≤ 0.45; further introduction of VO₄³⁻ led to precipitation of some other phases. It was shown that the VO₄³⁻ substituted samples presented discharging capacity higher than that of bare non-substituted Nasicon and the reported Ti⁴⁺ and Mn²⁺ doped ones. The Li₃Fe₂(PO₄)_2.55(VO₄)_0.45 material exhibited excellent cycling stability and rate capability, and retained a capacity of 91.8 mAh g⁻¹ after 60 cycles at 2C charge-discharge rate. This value is one of the highest reported to date for the Li₃Fe₂(PO₄)₃ compound, and was about 48% higher than that of the latter. The electrochemical performance enhancements for the VO₄³⁻ substituted samples were attributed to the reduction of charge transfer resistance, increase of electrical conductivity, and fast lithium ion diffusion behavior. Hence, the obtained results proved that the VO₄³⁻ anion substitution for PO₄³⁻ is a powerful technique to improve the electrochemical performance of the studied Nasicon compound.