Accelerated lithium ion interchange and enhanced electrochemical performance of lithium vanadium phosphate cathodes by Cr3+ doping

Abstract

Ion doping is often employed as an effective method to enhance the electrochemical performance of monoclinic lithium vanadium phosphate Li₃V₂(PO₄)₃ (LVP) cathode material. However, the functional mechanism of ion doping in LVP cathode has not been fully understood yet. Considering that Cr³⁺ is adjacent to V³⁺ on the periodic table and differs by only one unpaired d-electron, Cr³⁺ was selected as the dopant to investigate its function in doped LVP. A series of Li₃Cr_xV_(2-x)(PO₄)₃/C (LCr_xVP/C) samples were synthesized by a sol-gel route. XRD results indicate that the crystalline structure of LVP remained unchanged, while the reduction in ionic radius led to a decrease in unit cell parameters. The substitution of V³⁺ by Cr³⁺ in LVP resulted in broaden and shifted peaks around 150–180 ppm in ⁷Li NMR spectra, corresponding to Li sites surrounded by 4–5 Cr³⁺ ions with rapid Li-Li interchange. After 500 cycles at 2C rate within the voltage range of 3.0–4.3 V, the optimal sample LCr_0.1VP/C maintained a capacity of 111.9 mAh/g, with a capacity retention rate of 99.8%. After 500 cycles at 1C in the 3.0–4.8 V range, LCr_0.05VP/C had a capacity of 104.1 mAh/g, with a capacity retention rate of 83.8%. Cr³⁺ dopant promotes the rapid interchange among lithium ions at different sites, which accelerates the lithium ion extraction process within the LVP lattice.