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 1st 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.This work was supported by the German Science Foundation (DFG, CH566/4-1). The authors acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities. This project has received funding from the EU-H2020 research and innovation programme under grant agreement No 654360 having benefitted from the access provided by CNR-IOM in Trieste (Italy) and EPFL in Lausanne (Switzerland) within the framework of the NFFA-Europe Transnational Access Activity (project ID 360). Computational resources to perform ab initio calculations were provided by Swiss Supercomputing center CSCS (Piz Daint) through project No. s836, and by the Italian supercomputing center CINECA through the ISCRA project "Bat-Mat" of the University of Pavia. I.P., E.M., S.N., F.B., and R.E. also acknowledge funding from the EUROFEL project (RoadMap Esfri) and the European Research Council (ERC) "Horizon 2020" Program under Grant No. 805359-FOXON