The electrode-electrolyte interfaces (EEIs) play important roles in lithium ion batteries, often determining the capacity retention, safety, and lifetime. The EEI of the positive electrode is far less understood than its negative counterpart. In addition to possible electrolyte oxidation products, layered cathodes often exhibit pronounced surface phase transformations that can also limit the performance of the battery. For example, recent experiments have correlated charge-transfer impedance growth with surface phase transformations at Li1-x
(NCA) surfaces, which result from surface oxygen loss.  Electrolyte reactions with the active material play a role in surface oxygen loss as coating layers  and electrolyte additives  can mitigate impedance growth and capacity fade. Determining reaction pathways that promote surface oxygen loss is essential to optimizing the selection of coating layers or additives that further improve capacity retention.
Here we report on our surface studies of NCA, to examine the evolution of the cathode-electrolyte interface (CEI) for stressed NCA electrodes held at high voltages (≥4.5V). Using a combination of surface sensitive x-ray absorption spectroscopy and x-ray photoelectron spectroscopy, we correlate transition metal reduction with electrolyte decomposition species at the cathode surface. From comparison of NCA electrodes held under various constant voltage conditions, we examine the influence of voltage and temperature on the formation of electrolyte decomposition species. Our work provides insight into possible reaction mechanisms that influence changes of the CEI layer for electrodes under thermal and electrochemical stress.
This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583
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