Non-stoichiometric carbon-coated LiFe: XPO4 as cathode materials for high-performance Li-ion batteries

Abstract

A series of non-stoichiometric carbon-coated lithium iron phosphate (LiFexPO4/C) have been prepared by a solid-state reaction to study the variation of electrochemical performance at different x values. Characterized by XRD in conjunction with Rietveld refinement, Mössbauer, TEM, Raman, etc., it is indicated that the Li-O bond is elongated in the Fe-poor non-stoichiometric lithium iron phosphate with decreasing x value, while the content of Fe2P and graphitization degree of carbon layer in LiFexPO4/C samples is associated with the ratio of x. The powder electronic conductivity increases from 8.33 × 10-2 S cm-1 to 16.67 × 10-2 S cm-1 as the x value decreases from 1.04 to 0.98, which is due to a suitable amount of Fe2P and a superior graphitized carbon layer. Among different Fe/Li ratios, LiFe0.98PO4/C exhibits the highest rate capability of 163.5 mA h g-1 at 0.1C and 93.5 mA h g-1 at 20C, as well as the largest diffusion coefficient of 12.6 × 10-14 cm2 s-1. It is illustrated that the synergy effect of elongated Li-O bonds, moderate Fe2P and graphitized carbon layer results in the high performance of non-stoichiometric LiFexPO4/C.