Probing Surface Chemistry Changes Using LiCoO₂-only Electrodes in Li-Ion Batteries

Fundamental understanding of the reactivity between electrode and electrolyte is key to design the safety and life of Li-ion batteries. Herein X-ray photoelectron spectroscopy was used to examine the electrode/electrolyte interface (EEI) on carbon-free, binder-free LiCoO2 powder and thin-film electrodes in LP57 electrolyte as function of potential. Upon charging of LiCoO2 a marked growth of oxygenated and carbonated species was observed on the surface, consistent with electrolyte oxidation at high potentials. We also demonstrated that LiCoO2 oxide surface was prone to decompose the salt starting at 4.1 V-Li, as evidenced by the increase of LiF and LixPFyOz species upon charging. By DFT calculations we proposed a correlation between the interface composition and the thermodynamic tendency of the EC solvent for dissociative adsorption on the LixCoO2 surface, through the generation of reactive acidic OH groups on the oxide surface, which can have a role in the observed salt decomposition. This is consistent with the evidence of HF and PF2O2- species at 4.6 V-Li observed by solution F-19-NMR measurements. Finally we compared EEI composition between composite and model electrodes and discussed the changes and mechanisms induced by the electrode composition or the use of electrolyte additives. We showed that the addition of diphenyl carbonate (DPC) in the electrolyte has a strong impact on the formation of solvent and salt decomposition products at the EEI layer. (C) 2018 The Electrochemical Society.