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 Li 3 V 2 (PO 4 ) 3 samples, i.e. Li 3 V 2 − 2 x Ti x Mn x (PO 4 ) 3 and Li 3 V 2 − 2 x Ti x Fe x (PO 4 ) 3 ( x  = 0, 0.05, 0.1, 0.15, 0.2 and 0.25), are prepared by a sol–gel method. Li 3 V 2 − 2 x Ti x Mn x (PO 4 ) 3 and Li 3 V 2 − 2 x Ti x Fe x (PO 4 ) 3 are phase-pure when x is not higher than 0.05. LiMnPO 4 and LiFePO 4 begin to form as impurity phases in Li 3 V 2 − 2 x Ti x Mn x (PO 4 ) 3 and Li 3 V 2 − 2 x Ti x Fe x (PO 4 ) 3 , respectively, when x is equal to 0.1. And another impurity of Mn 2 P 2 O 7 appears in Li 3 V 2 − 2 x Ti x Mn x (PO 4 ) 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 Li 3 V 2 − 2 x Ti x Mn x (PO 4 ) 3 and Li 3 V 2 − 2 x Ti x Fe x (PO 4 ) 3 at 0.2 C decline with an increase of x . Both the high-rate discharge capability and long term cycling performance of Li 3 V 1.9 Ti 0.05 Fe 0.05 (PO 4 ) 3 are much better than those of Li 3 V 2 (PO 4 ) 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 Li 3 V 1.9 Ti 0.05 Mn 0.05 (PO 4 ) 3 is worse than that of Li 3 V 2 (PO 4 ) 3 , which is due to the structural instability induced by the incorporation of Mn.