Cathode Electrolyte Interphase Formation and Electrolyte Oxidation Mechanism for Ni-Rich Cathode Materials

Herein, we investigate the formation of a cathode electrolyte interphase (CEI) by electrolyte oxidation on a LiNixM1–xO2 (x > 0.5; M, transition metal) layered oxide (Ni-rich) cathode and compare this phenomenon with a Li-rich layered oxide (Li-rich) cathode. Our investigations focused on two electrochemical properties, the potential and kinetics of electrolyte oxidation, studied using hard X-ray photoelectron spectroscopy (HAXPES), soft X-ray absorption spectroscopy, and density functional theory calculations. HAXPES revealed that a thicker CEI formed on the Ni-rich cathode compared to that on the Li-rich cathode, despite the operation potential of the Ni-rich cathode being lower than that of the Li-rich cathode. Thus, the Ni-rich cathode induces the CEI formation through active oxidation of the electrolyte during charge–discharge cycles. The electronic state of the Ni-rich cathode indicates that the antibonding hybrid orbital of the transition metal 3d–O 2p corresponds to the lowest unoccupied molecular orbital energy level, that is, the hole, and lies near the highest occupied molecular orbital energy level of the electrolyte. In addition, the hole concentration in the charged state was found to be significantly increased, in comparison to other active materials, which promotes oxidization of the electrolyte.