Non-stoichiometric carbon-coated LiFexPO4 as cathode materials for high-performance Li-ion batteries

Abstract

A series of non-stoichiometric carbon-coated lithium iron phosphate (LiFe_xPO₄/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 LiFe_xPO₄/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, LiFe_0.98PO₄/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 LiFe_xPO₄/C.