Performance improvement of LiCoO2 by MgF2 surface modification and mechanism exploration

LiCoO2 cathode material synthesized by a sol-gel method is modified by different contents of MgF2 via simple chemical deposition. The structure and morphology of pristine and MgF2-coated LiCoO2 are investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). It is found that the homogeneous MgF2 surface modification does not change the bulk structure of LiCoO2. The effects of MgF2 on the electrochemical behaviors of LiCoO2 are studied. Comparatively, 1 wt.% MgF2-coated LiCoO2 exhibits the best electrochemical performances. Cyclic voltammetry (CV) results confirm that MgF2 surface coating decreases the electrode polarization and improves the structural stability of LiCoO2 during cycling. The results of electrochemical impedance spectroscopy (EIS) and Fourier transform infrared (FTIR) for pristine and 1 wt.% MgF2-coated LiCoO2 further demonstrates that modification layer suppresses the undesirable growth of solid electrolyte interface (SEI) film, which greatly contributes to the improved electrochemical performances after surface modification. Differential scanning calorimetry (DSC) tests show that MgF2 coating also improves the thermal stability of LiCoO2 electrode.