LiMn₂O₄ surface chemistry evolution during cycling revealed by in situ auger electron spectroscopy and X-ray photoelectron spectroscopy

This work utilizes in situ electrochemical and analytical characterization during cycling of LiMn₂O₄ (LMO) equilibrated at different potentials in an ultrahigh vacuum (UHV) environment. The LMO reacts with organic molecules in the vacuum to form a high surface concentration of Li₂CO₃ (≈50% C) during initial charging to 4.05 V. Charging to higher potentials reduces the overall Li₂CO₃ concentration (≈15% C). Discharging to 3.0 V increases the Li₂CO₃ concentration (≈30% C) and over discharging to 0.1 V again reduces its concentration (≈15% C). This behavior is reproducible over 5 cycles. The model geometry utilized suggests that oxygen from LMO can participate in redox of carbon, where LMO contributes oxygen to form the carbonate in the solid electrolyte interphase (SEI). Similar results were obtained from samples cycled ex situ, suggesting that the model in situ geometry provides reasonably representative information about surface chemistry evolution. Carbon redox at LMO and the inherent voltage instability of the Li₂CO₃ likely contributes significantly to its capacity fade.