High-Capacity, Aliovalently Doped Olivine LiMn_{1–3<i>x</i>/2}V_<i>x</i>□_<i>x</i>/2PO₄ Cathodes without Carbon Coating

Abstract

A substantial amount of Mn²⁺ has been aliovalently substituted by V³⁺ in cation-deficient LiMn_{1–3<i>x</i>/2}V_<i>x</i>□_<i>x</i>/2PO₄ (0 ≤ <i>x</i> ≤ 0.20) by a low-temperature (<300 °C) microwave-assisted solvothermal (MW-ST) process. The necessity of a low-temperature synthesis to achieve higher levels of doping is demonstrated as the solubility of vanadium decreases with the formation of impurity phases on heating the samples to ≥575 °C. Soft X-ray absorption spectroscopy reveals enhanced Mn–O hybridization in the vanadium-doped samples, which is believed to facilitate an increase in capacity with increasing vanadium content in the lattice. For example, a high capacity of 155 mAh/g is achieved above a cutoff voltage of 3 V without any carbon coating for the <i>x</i> = 0.2 sample. The vanadium substitution enhances the overall kinetics of the material by lowering the charge-transfer impedance and increasing the lithium-diffusion coefficient