High-Capacity, Aliovalently Doped Olivine LiMn1-3x/2Vx□x/2PO4 Cathodes without Carbon Coating

A substantial amount of Mn2+ has been aliovalently substituted by V3+ in cationdeficient LiMn1-3x/2Vx square x/2PO4 (0 <= x <= 0.20) by a low-temperature (<300 degrees 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 degrees 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.