On the Formation of a Cathode Electrolyte Interphase on High Voltage Li-ion Cathodes

The spinel oxide LiNi0.5Mn1.5O4 (LNMO) currently competes to replace the conventional layered transition metal oxide active material in Li-ion batteries. The high average operating potential (4.70 V vs. Li+/Li) challenges the stability of the electrolyte, which in turn compromises the lifetime of the Li-ion cell. Online electrochemical mass spectrometry (OEMS) is herein implemented to study the degradation processes occurring at the cathode surface. Gases continuously evolve across subsequent cycles as a result of electrolyte oxidation, a process that is found to be only potential-activated and independent of electrode surface composition. The subsequent formation of protic species autocatalyzes electrolyte salt degradation, which in turn triggers the corrosion of active material, current collector, and conductive carbons. The effectiveness of several well-known electrolyte additives, previously claimed to act as cathode electrolyte interphase (CEI) formers, were explored revealing the efficacy of phosphorus-based additives. Our study provides a rapid and quantifiable approach to tackle the major challenge of high voltage cathode materials, namely its stabilisation towards the electrolyte, and how to identify and develop an efficient CEI.