Synthesis and electrochemical performance of Na-modified Li2Fe0.5Mn0.5SiO4 cathode material for Li-ion batteries

Abstract

A series of Li_2−xNa_xFe_0.5Mn_0.5SiO₄/C (x = 0.00, 0.01, 0.03 and 0.05) composites have been synthesized via a refluxing-assisted solid-state reaction, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), galvanostatic charge–discharge measurements, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests. XRD results show that Li_2−xNa_xFe_0.5Mn_0.5SiO₄/C can be well indexed as the structure of two mixed polymorphs with space group P2₁ and Pmn2₁. XPS results confirms that Na not only exists on the surface of Li₂Fe_0.5Mn_0.5SiO₄ particles, but also has been successfully doped into the crystal lattice of Li₂Fe_0.5Mn_0.5SiO₄. Na-doping can significantly improve the discharge capacity and the rate capability of Li₂Fe_0.5Mn_0.5SiO₄/C. The enhanced electrochemical performance can be attributed to the increased electronic conductivity, the decreased charge transfer impedance, and the improved Li-ion diffusion coefficient.