Unraveling the Relationship between Ti⁴⁺ Doping and Li⁺ Mobility Enhancement in Ti⁴⁺ Doped Li₃V₂(PO₄)₃

The electrochemical properties of Li₃V₂(PO₄)₃ (LVP) cathode of lithium ion batteries are often improved by ion doping. Nevertheless, the mechanism of ion doping has not been fully understood. Here, Ti⁴⁺ has been chosen as a typical dopant with similar atomic radius to the six-coordinated V³⁺. A series of Li₃Ti_xV_(2-x)(PO₄)₃/C samples are successfully synthesized by a sol-gel route. The ⁷Li MAS NMR spectra of the LT_xVP/C demonstrate that the doping of Ti⁴⁺ can enhance the mobility of Li ions. The results of electrochemical properties tests show that moderate Ti⁴⁺ doping is able to improve the high rate capability of the materials by increasing the electronic conductivity and Li-ion diffusion coefficient. The optimal sample (LT_0.08VP/C) exhibits the best cycling behavior and rate capability, which can deliver 110.85 mAh/g and capacity retention of 99.36% at 10 C after 100 cycles. Electrochemical impedance spectroscopy results indicate that LT_0.08VP/C possesses the minimum charge transfer resistance. The calculation results of cyclic voltammetry illustrate that the Li-ion diffusion coefficient of LT_0.08VP/C has been improved. By combining the information extracted from a series of electrochemical characterizations and NMR tests, a structural model of Li⁺ vacancy is proposed to explain the improving of Li⁺ mobility.