Formation\nof a Cathode Electrolyte Interphase on High-Voltage Li-ion Cathodes

The spinel oxide LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> (LNMO) currently competes to replace the conventional layered\ntransition\nmetal oxide active material in Li-ion batteries. The high average\noperating potential (4.8 V vs Li<sup>+</sup>/Li) challenges the stability\nof the electrolyte, which, in turn, compromises the lifetime of the\nLi-ion cell. Online electrochemical mass spectrometry (OEMS) is herein\nimplemented to study the degradation processes occurring at the cathode\nsurface. Gases continuously evolve across subsequent cycles as a result\nof electrolyte oxidation, a process that is found to be only potentially\nactivated and independent of electrode surface composition. The subsequent\nformation of protic species autocatalyzes electrolyte salt degradation,\nwhich in turn triggers the corrosion of active material, current collector,\nand conductive carbons. The effectiveness of several well-known electrolyte\nadditives, previously claimed to act as cathode electrolyte interphase\n(CEI) formers, was explored, revealing the efficacy of phosphorus-based\nadditives. Our study provides a rapid and quantifiable approach to\ntackle the major challenge of high-voltage cathode materials, namely,\ntheir stabilization toward the electrolyte and how to identify and\ndevelop an efficient passivating CEI.