Vanadium doping of LiMnPO4 cathode material: Correlation between changes in the material lattice and the enhancement of the electrochemical performance

Different LiMn1-xVxPO4 (x = 0, 0.05, 1, and 0.15) samples of olivine-type structure were successfully synthesized by solvothermal method in ethylene glycol. The effect of vanadium incorporation on the performance of LiMnPO4 was systematically investigated using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, charge/discharge measurements, cyclic voltammetry and electrochemical impedance spectroscopy tests. The control of the vanadium insertion in 4c sites within octahedral sites, particle size, desired morphology and conductivity, was achieved by an adequate thermal treatment. TEM images showed particle sizes of 100 nm and carbon coating thickness of between 3 and 4 nm. The vanadium insertion increases the number of lithium ion diffusion pathways and free charges in the LiMnPO4 structure. For this reason, the doped materials exhibit superior ionic and electronic conductivity to undoped material. Consequently, the LiMn1-xVxPO4 cathode material can retain more than 60% capacity when tested at rates as high as 5C. Vanadium-doped olivine exhibited a capacity of 126 mAhg(-1) at 0.2C and a high cycling stability. (C) 2019 Elsevier Ltd. All rights reserved.