Effect of Mg2+/F− co-doping on electrochemical performance of LiNi0.5Mn1.5O4 for 5 V lithium-ion batteries

Mg²⁺/F⁻ co-doped LiNi_0.5Mn_1.5O₄ cathode material was synthesized by a facile one-step solid-state process. The effect of Mg²⁺/F⁻ co-doping on grain morphology, phase structure, and electrochemical properties was studied by a series of characterizations. Scanning-electron-microscopy images show that Mg²⁺/F⁻ co-doped LiNi_0.5Mn_1.5O₄ (denoted LNMO-MF) particles grow larger than pure LiNi_0.5Mn_1.5O₄ particles. X-ray diffraction, Raman spectra, Fourier transformation infrared spectroscopy, X-ray photoelectron spectroscopy, and cyclic-voltammetry tests indicate that all samples mainly display a Fd-3m space group and more Mn³⁺ ions in the LNMO-MF sample after Mg²⁺/F⁻ co-doping, which is conducive to increasing the cationic disorder degree and enhancing the electronic conductivity of electrode material. Results show that the LNMO-MF cathode material delivers an excellent rate performance with discharge capacities of 142, 144, 140 136, 132, 124, 115, and 100 mAh g⁻¹ at 0.2, 0.5, 1, 2, 3, 5, 7, and 10C (1C = 140 mAh g⁻¹), respectively. Remarkably, LNMO-MF also shows cycling stability with a capacity retention of 86.2% at 5C after 400 cycles, which is much higher than that of pure LiNi_0.5Mn_1.5O₄ (67.7%). The improvement of LNMO-MF's electrochemical properties could be ascribed to the Mg²⁺/F⁻ co-doping, delivering a more stable structure, better crystallinity, the highest Li⁺ diffusion coefficient, and the lowest charge-transfer resistance.