Study on structure and electrochemical performance of Tm3+-doped monoclinic Li3V2(PO4)3/C cathode material for lithium-ion batteries

Monoclinic Li3V2-xTmx(PO4)3/C with different Tm3+-doping contents (x=0, 0.01, 0.03, 0.05) have been successfully synthesized for the first time via a conventional solid-state reaction. The effect of Tm3+-doping on the crystalline structure, morphology and electrochemical performance of Li3V2(PO4)3/C has been investigated in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical measurements. The same phase-pure monoclinic structure of Tm3+-doped Li3V2(PO4)3/C sample can be obtained from XRD results while the particle size is smaller than that of pristine Li3V2(PO4)3/C. The Li3V1.97Tm0.03(PO4)3/C composite exhibits the most excellent electrochemical performance among all the samples. In the voltage range of 3.0-4.8 V, the Li3V1.97Tm0.03(PO4)3/C sample delivers a very high initial discharge capacity of 181.2 mAhg−1 at 0.1 C, significantly higher than that of pristine Li3V2(PO4)3/C (170.1 mAhg−1). Moreover, it can still deliver a discharge capacity of 141 mAhg−1 and 123.3 mAhg−1 at 5.0 and 10.0 C, respectively, and sustain 95.2% and 93.4% of capacity retention after 20 cycles. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results demonstrate that the reversibility of pristine Li3V2(PO4)3/C is enhanced and the charge-transfer resistance is reduced after Tm3+-doping at V sites. These improved electrochemical performance can be contributed to the appropriate addition of Tm3+-doping in Li3V2(PO4)3/C system by enhancing structural stability and electrical conductivity.