Electronic energy levels at Li-ion cathode-liquid electrolyte interfaces: Concepts, experimental insights, and...

Although electrolyte decomposition is a key issue for the stability of Li-ion batteries and has been intensively investigated in the past, a common understanding of the concepts and involved processes is still missing. In this article, we present an overview on our results obtained with a surface science approach and discuss the implications for the stability window of Li-ion electrolytes under consideration of calculated oxidation potentials from the literature. We find LiCoO₂ valence band-solvent highest occupied molecular orbital offsets that are in agreement with expectations based on ionization potentials, polarization effects, and solvent-salt interactions. In agreement with thermodynamic considerations, our data show that surface layer formation on pristine electrodes occurs inside the electrochemical window as defined by the measured oxidation and reduction potentials, which can be attributed to electrode surface interactions. The results demonstrate that the simple energy level approach commonly used to evaluate the stability window of Li-ion electrolytes has very limited applicability. The perspectives for further investigations of the electronic structure of Li-ion cathode-liquid electrolyte interfaces are discussed.