XPS Investigation of the Electrolyte Induced Stabilization of LiCoO₂and “AlPO₄”-Coated LiCoO₂Composite Electrodes

The "AlPO 4 " coating has been shown to improve the electrochemical performance of LiCoO 2 batteries. We previously showed that the "AlPO 4 " coating promotes the formation of metal fluorides, which could act as a stable surface film and protect LiCoO 2 from continuous degradation upon cycling. In this work, we removed the fluorine source in the LiPF 6 salt by using the LiClO 4 salt and investigated the effectiveness of the "AlPO 4 " coating. Interestingly, the "AlPO 4 " coating was found to improve the voltage efficiency and capacity retention when cycling in the LiPF 6 electrolyte, but was detrimental when cycling in the LiClO 4 electrolyte. XPS revealed that the "AlPO 4 " coating promotes the formation of metal fluoride in both electrolytes, with the surface film formed in LiClO 4 being more electrically resistive compared to that formed in LiPF 6 . The source of fluorine in the coated electrode cycled in LiPF 6 is largely attributed to the LiPF 6 salt whereas the source of fluorine in the coated electrode cycled in LiClO 4 is the binder PVDF. We believe that the coating could react with HF impurity in the LiPF 6 electrolyte or from the binder PVDF and form stable metal fluoride films on the surface.