Improved electrochemical performance of spinel LiMn_1.5Ni_0.5O₄ through MgF₂ nano-coating

A spinel LiMn_1.5Ni_0.5O₄ (LMNO) cathode material synthesized by a sol-gel method is modified by MgF₂ nano-coating via a wet coating strategy. The results of X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) showed that the MgF₂ nano-coating layers do not physically change the bulk structure of the pristine material. Compared with the pristine compound, the MgF₂-coated LMNO electrodes display enhanced cycling stabilities. Particularly, the 5 wt% MgF₂-coated LMNO demonstrates the best reversibility, with a capacity retention of 89.9% after 100 cycles, much higher than that of the pristine material, 69.3%. The dQ/dV analysis and apparent Li⁺ diffusion coefficient calculation prove that the kinetic properties are enhanced after MgF₂ surface modification, which partly explains the improved electrochemical performances. Electrochemical impedance spectroscopy (EIS) and Fourier transform infrared spectroscopy (FTIR) data confirm that the MgF₂ coating layer helps in suppressing the fast growth of the solid electrolyte interface (SEI) film in repeated cycling, which effectively stabilizes the spinel structure. Additionally, differential scanning calorimetry (DSC) tests show that the MgF₂ nano-coating layer also helps in enhancing the thermal stability of the LMNO cathode.