Effect of Mn in Li₃V_2-xMn_x(PO₄)₃ as High Capacity Cathodes for Lithium Batteries

Li₃V_2-xMn_x(PO₄)₃ (x = 0, 0.05) cathode materials, which allow extraction of 3 mol of Li from the formula unit, were investigated to achieve a high energy density utilizing multielectron reactions, activated by the V^3+/5+ redox reaction. Structural investigation demonstrates that V³⁺ was replaced by equivalent Mn³⁺, as confirmed by Rietveld refinement of the X-ray diffraction data and X-ray absorption near edge spectroscopy. The substitution simultaneously lowered the band gap energy from 3.4 to 3.2 eV, according to a density functional theory calculation. In addition to the effect of Mn doping, surface carbonization of Li₃V_2-xMn_x(PO₄)₃ (x = 0, 0.05) dramatically increased the electric conductivity up to 10^-3 S cm^-1. As a result, the carbon-coated Li₃V_2-xMn_x(PO₄)₃ (x = 0.05) delivered a high discharge (reduction) capacity of approximately 180 mAh g^-1 at a current of 20 mA g^-1 (0.1 C rate) with excellent retention, delivering approximately 163 mAh g^-1 at the 200th cycle. Even at 50 C (10 A g^-1), the electrode afforded a discharge capacity of 68 mAh g^-1 and delivered approximately 104 mAh g^-1 (1 C) at -10 °C with the help of Mn doping and carbon coating. The synergetic effects such as a lowered band gap energy by Mn doping and high electric conductivity associated with carbon coating are responsible for the superior electrode performances, including thermal properties with extremely low exothermic heat generation (<0.4 J g^-1 for Li_0.02V_1.95Mn_0.05(PO₄)₃), which is compatible with the layered high energy density of LiNi_0.8Co_0.15Al_0.05O₂ and LiNi_0.8Co_0.1Mn_0.1O₂ materials.