Energy Level Alignment at Cobalt Phosphate/Electrolyte Interface: Intrinsic Stability vs. Interfacial Chemical Reactions in 5 V Lithium Ion Batteries

The intrinsic stability of the 5V LiCoPO₄ - LiCo₂P₃O₁₀ thin film (carbon free) cathode material coated with MoO₃ thin layer is studied by comprehensive synchrotron electron spectroscopy in-situ approach combined with first principle calculations. The atomicmolecular level study demonstrates fully reversible electronic properties of the cathode after the 1^st electrochemical cycle. The polyanionic oxide is not involved into chemical reactions with the fluoroethylene containing liquid electrolyte even when being charged to 5.1 V vs. Li⁺/Li. The high stability of the cathode is explained on the base of the developed energy level model. In contrast, chemical composition of the cathode-electrolyte interface evolves continuously by involving MoO₃ in the decomposition reaction with consequent leaching of oxide from the surface. The proposed mechanisms of chemical reactions are attributed to an external electrolyte oxidation via charge transfer from the relevant electron level to the MoO₃ valence band state, as well as to an internal electrolyte oxidation via proton transfer to the solvents. This study gives deeper inside into the development of both a doping strategy to enhance electronic conductivity of high voltage cathode materials, and an efficient surface coating against unfavorable interfacial chemical reactions.