Li3V2-xMnx(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 V3+/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 Li3V2-xMnx(PO4)3 (x = 0, 0.05) dramatically increased the electric conductivity up to 10-3 S cm-1. As a result, the carbon-coated Li3V2-xMnx(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 Li0.02V1.95Mn0.05(PO4)3), which is compatible with the layered high energy density of LiNi0.8Co0.15Al0.05O2 and LiNi0.8Co0.1Mn0.1O2 materials.
Keywords: band gap energy; battery; carbon coating; cathode; lithium; lithium vanadium phosphate.