Interface reactivity of in-situ formed LiCoO2 - PEO solid-state interfaces investigated by X-ray photoelectron spectroscopy: Reaction products, energy level offsets and double layer formation

Interfaces involving ceramic solid electrolytes remain the biggest challenge for all solid-state batteries due to their rigid and brittle nature resulting in high interface resistances. A solution for this problem are solid polymer electrolytes (SPE) with better mechanical and wetting-behavior. A commonly used polymer electrolyte is PEO with LiTFSI, which reaches sufficient ionic conductivity but suffers from side reactions on high voltage cathodes. To use PEO, it is necessary to understand the underlying reaction processes that occur e.g., at the LiCoO2 electrode. In this contribution, our surface science approach to investigate battery interfaces is transferred to PEO using the oligomer poly (ethylene glycol) with LiTFSI deposited by thermal evaporation to study the interface formation towards LiCoO2. The stability of PEG and LiTFSI during evaporation is shown with XPS and mass spectroscopy. The interfaces of LiCoO2/PEG and LiCoO2/PEG with LiTFSI are studied under UHV condition on sputtered LiCoO2 thin films. A reaction layer is formed that is limited to the upper surface of LiCoO2. Additionally, electron and Li+-ion transfer from the LiCoO2 into the reaction phase has been observed. The wetting-behavior of the interfaces are investigated with SEM. Energy level diagrams for both interfaces are drawn, explaining the contact formation properties.