Synthesis and characterization of Ti–Mn and Ti–Fe codoped Li3V2(PO4)3 as cathode material for lithium ion batteries

Abstract

Ti–Mn and Ti–Fe codoped Li3V2(PO4)3 samples, i.e. Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3 (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25), are prepared by a sol–gel method. Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3 are phase-pure when x is not higher than 0.05. LiMnPO4 and LiFePO4 begin to form as impurity phases in Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3, respectively, when x is equal to 0.1. And another impurity of Mn2P2O7 appears in Li3V2−2xTixMnx(PO4)3 when x is equal to 0.2. All these impurities increase with increasing x. XPS analyses indicate that the oxidation states of Ti, Mn and Fe are +4, +2 and +2 respectively. The first charge/discharge capacities of both Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3 at 0.2 C decline with an increase of x. Both the high-rate discharge capability and long term cycling performance of Li3V1.9Ti0.05Fe0.05(PO4)3 are much better than those of Li3V2(PO4)3, which can be attributed to the smaller particle size, larger lattice parameters and better structural stability induced by Ti and Fe codoping. However, the electrochemical performance of Li3V1.9Ti0.05Mn0.05(PO4)3 is worse than that of Li3V2(PO4)3, which is due to the structural instability induced by the incorporation of Mn.