Quantification of Surface Oxygen Depletion and Solid Carbonate Evolution on the First Cycle of LiNi_0.6Mn_0.2Co_0.2O₂ Electrodes

By combining differential electrochemical mass spectrometry (DEMS) with titrations of electrochemically modified LiNi0.6Mn0.2Co0.2O2 (NMC622), we find that coinciding with the onset of CO2 evolution above ∼3.9 V vs Li/Li+ anodic cutoff potentials are several phenomena: (i) degradation of the native surface Li2CO3, (ii) degradation of the electrolyte evolving CO2, (iii) formation of a film of carbonate-like electrolyte degradation products on charge which are (iv) largely reduced and desorb on discharge, (v) near-surface oxygen charge compensation during charge, and (vi) irreversible formation of a transition metal-reduced, oxygen-depleted layer on the surface of NMC622 that persists after discharge. CO2 stemming from electrolyte degradation and Li2CO3 decomposition begins to evolve above ∼3.9 V on charge, discharge, and rest and results from a corrosion-like process involving NMC622, which appears to be distinct from the process that evolves O2. As measured using titrations that quantify surface peroxo-like character, the disordered surface layer that forms during cycling extends deeper into the oxide bulk than would be anticipated simply from the total O2 evolved. The analyses we report here could be used to quantify the role of the electrolyte, surface contaminants, and transition metal oxide composition on outgassing, electrolyte decomposition, and transition metal oxide surface degradation.