XPS Investigation of the Electrolyte Induced Stabilization of LiCoO2 and "AlPO4"-Coated LiCoO2 Composite Electrodes

The "AlPO4" coating has been shown to improve the electrochemical performance of LiCoO2 batteries. We previously showed that the "AlPO4" coating promotes the formation of metal fluorides, which could act as a stable surface film and protect LiCoO2 from continuous degradation upon cycling. In this work, we removed the fluorine source in the LiPF6 salt by using the LiClO4 salt and investigated the effectiveness of the "AlPO4" coating. Interestingly, the "AlPO4" coating was found to improve the voltage efficiency and capacity retention when cycling in the LiPF6 electrolyte, but was detrimental when cycling in the LiClO4 electrolyte. XPS revealed that the "AlPO4" coating promotes the formation of metal fluoride in both electrolytes, with the surface film formed in LiClO4 being more electrically resistive compared to that formed in LiPF6. The source of fluorine in the coated electrode cycled in LiPF6 is largely attributed to the LiPF6 salt whereas the source of fluorine in the coated electrode cycled in LiClO4 is the binder PVDF. We believe that the coating could react with HF impurity in the LiPF6 electrolyte or from the binder PVDF and form stable metal fluoride films on the surface. (C) The Author(s) 2015. Published by ECS.