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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. [ABSTRACT FROM AUTHOR]

To obtain the highest energy density, lithium ion batteries based on layered Li-Ni-Mn-Co oxides need to be charged to above 4.5 V without sacrificing lifetime. Lithium-rich core-shell (CS) structured positive electrodes having a high energy density core material with poor stability against the electrolyte can be protected by a thin layer of a stable shell material. In this work, the effects of the initial shell thickness, sintering temperature and interdiffusion during sintering on the electrochemical performance of CS cathodes were studied for the first time to our knowledge. Full cell coin cells of selected CS samples with electrolyte additives showed good capacity retention. This work will help guide the design of next generation positive electrode materials using CS and coating strategies. [ABSTRACT FROM AUTHOR]