The Impact of Electrolyte Additives and Upper Cut-off Voltage on the Formation of a Rocksalt Surface Layer in LiNi0.8Mn0.1Co0.1O2 Electrodes

Many literature reports show that layered Li-Ni-Mn-Co oxides (NMC) have a surface reconstruction to a rocksalt (Fm3m) structure which is claimed to be responsible for the increase in cell impedance during high voltage cycling. It is important to determine if appropriate electrolyte additives can suppress the surface reconstructions of NMC materials. LiNi0.8Mn0.1Co0.1O2 (NMC811)/Graphite pouch cells with different electrolyte additives and different upper cutoff potentials were charge-discharge cycled and the electrodes were recovered for z-contrast scanning transmission electron microscope (STEM) studies. It was found that there was no significant surface layer growth for cells cycled between 2.8 and 4.1 V. For cells with an upper cutoff voltage of 4.3 V, the electrodes from cells with control electrolyte (no additives) showed the thickest surface layer. The electrolyte additives vinylene carbonate (VC) and prop-1-ene-1,3-sultone (PES) were found to suppress the growth of the surface layer. However, cells with PES showed a more rapid capacity fade than control cells or cells with 2% VC showing that, at least for NMC811/graphite cells with PES or VC additives, failure cannot only be solely ascribed to a growing rocksalt surface layer. Other processes, for example associated with electrolyte oxidation, are believed to be responsible for failure.