Synthesis and electrochemical properties of monoclinic fluorine-doped lithium manganese oxide (LixMnO2−yFy) for lithium secondary batteries

Abstract

A series of monoclinic fluorine-doped lithium manganese oxide (Li_xMnO_2−yF_y) were prepared by the ion exchange of sodium for lithium in Na_xMnO_2−yF_y precursors that were obtained using a high-temperature solid-state reaction. The microstructure and composition of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), ion chromatography (IC), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The different valence states of manganese in material Li_xMnO_2−yF_y were determined by redox titration method. The electrochemical performance of these samples as cathode materials were studied by the galvanostatic and cyclic voltammetry method. These materials had a high crystallite size, which was composed of 5–8 μm grains. The Li_0.86MnO_1.98F_0.02 materials delivered an initial discharge capacity of 129.2 mA h g⁻¹ and gradually increased to a maximum discharge capacity of 210 mA h g⁻¹ at a current density of 50 mA g⁻¹ after 50 cycles. Moreover, the material showed an excellent cycling behavior, even though its original structure transformed into the spinel phase during cycling. The results show that the partial substitution of monoclinic LiMnO₂ with fluorine can improve the cycle stability and high-rate discharge capability of cathode materials.