High performance LiV0.96Mn0.04PO4F/C cathodes for lithium-ion batteries

Abstract

The LiV_0.96Mn_0.04PO₄F/C composite was prepared by partial substitution of vanadium with manganese and in situ carbon coating of LiVPO₄F via a modified carbothermal reduction route. Rietveld refinement of X-ray diffraction (XRD) patterns shows that the crystalline lattice is smaller than the value reported in the literature. While electron energy loss spectroscopy (EELS) on a selected area of high resolution transmission electron microscopy (HRTEM) demonstrates that Mn is successfully doped into LiVPO₄F, X-ray photoelectron spectroscopy (XPS) indicates the co-existence of Mn³⁺ and Mn⁴⁺. With ∼6.29 wt% residual carbon (estimated by thermogravimetric analysis, TGA) included, the specific discharge capacity of LiV_0.96Mn_0.04PO₄F/C at 0.1 C and 6 C could reach 138 mA h g⁻¹ and 98 mA h g⁻¹, respectively. The capacity retention after 1000 cycles at 1 C is about 90%, corresponding to only 0.01% loss per cycle. Electrochemical impedance spectroscopy (EIS) shows that the charge transfer resistance (R_ct) is significantly reduced by Mn substitution, and the lithium diffusion coefficient was calculated to be 1.34 × 10⁻¹³ cm² s⁻¹, which is an order of magnitude higher than that of pristine LiVPO₄F/C. Moreover, LiV_0.96Mn_0.04PO₄F/C retains 74% of its initial capacity after 500 cycles at 1 C at 65 °C, indicating its potential application at high temperatures.