The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte-cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation

The stability of the valence state of the 3d transition metal ions and the stoichiometry of LiMO2 (M=Co, Ni, Mn) layered oxides at the surface-electrolyte interface plays a crucial role for energy storage applications. The surface oxidation/reduction of the cations caused by the contact of the solids to air or to the electrolyte results in the blocking of the Li-transport through the interface that leads to the fast batteries deterioration. The influence of the end-of-charge voltage on the chemical composition and the oxidation state of 3d transition metal ions, as well as the stability of the solid-electrolyte interface (SEI), which is formed during the electrochemical Li-deintercalation/intercalation of the LiCoO2 (LCO) and Li(Ni,Mn,Co)O2 (LNMCO), have been investigated by X-ray photoelectron spectroscopy (XPS). While the chemical composition of SEI is similar for both layered oxide surfaces, the electrochemical cycling to some critical voltage values lead to the disappearance of the SEI layer. The breaking and the weakening of the chemical bonds in the chemical compounds of the SEI is assigned with the lattice stress induced by the lattice expansion which is stronger at the higher end-of charge voltages. By the analysis of the shape of the 2p and 3s photoelectron emissions we show that the formation of the SEI layer correlates with the partial reduction of the trivalent Co ions at the electrolyte-LCO interface and amount of the Co2+ ions is increased as the SEI vanishes. In opposite, the Mn4+, Co3+ and Ni2+ ions at the SEI-LNMCO interface are stable under the electrochemical cycling to higher end-of-charge voltage. A correlation between deterioration of the LCO and LNMCO batteries and the change of electronic structure at the surface/interface after the electrochemical cycling has been found. The dissolution of the SEI layer might be the reason for the fast deterioration of the Li-ion batteries