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

Abstract

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 Li3TixV(2-x)(PO4)3/C samples are successfully synthesized by a sol-gel route. The 7Li MAS NMR spectra of the LTxVP/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 (LT0.08VP/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 LT0.08VP/C possesses the minimum charge transfer resistance. The calculation results of cyclic voltammetry illustrate that the Li-ion diffusion coefficient of LT0.08VP/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.