Probing Surface Chemistry Changes Using LiCoO 2 -only Electrodes in Li-Ion Batteries

Abstract

© 2018 The Electrochemical Society. 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 LiCoO 2 powder and thin-film electrodes in LP57 electrolyte as function of potential. Upon charging of LiCoO 2 a marked growth of oxygenated and carbonated species was observed on the surface, consistent with electrolyte oxidation at high potentials. We also demonstrated that LiCoO 2 oxide surface was prone to decompose the salt starting at 4.1 V Li , as evidenced by the increase of LiF and Li x PF y O z 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 Li x CoO 2 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 PF 2 O 2− species at 4.6 V Li observed by solution 19 F-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.