High-Capacity, Aliovalently Doped Olivine LiMn_1–3x/2V_x□_x/2PO₄ Cathodes without Carbon Coating

A substantial amount of Mn 2+ has been aliovalently substituted by V 3+ in cation-deficient LiMn 1–3 x /2 V x □ x /2 PO 4 (0 ≤ x ≤ 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 x = 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.