Insights into Solid-Electrolyte Interphase Induced Li-Ion Degradation from in Situ Auger Electron Spectroscopy

Surface reactions occurring on LiMn₂O₄, LiCoO₂, LiNiO₂, Li[Ni_1/3Mn_1/3Co_1/3]O₂, and LiFePO₄ during charging and overcharging are studied by in situ and ex situ Auger electron spectroscopy. Carbon surface stability at the cathode solid-electrolyte interphase (SEI), associated with carbonate formation, decomposition, and CO/CO₂ evolution, on different electrodes during cycling correlates with their cycle life. To understand how associated CO and CO₂ evolution affects cycle stability, LiMn₂O₄ is cycled in flowing gas. Flowing Ar enhances cycle life by a factor of 2, while flowing Ar with 1% CO₂ reduces cycle life by a factor of 2. CO₂ is proposed to degrade cycle life by trapping Li and metal ions as carbonate in the anode SEI.