The Role of Electrolyte in the First-Cycle Transformations of LiNi0.6Mn0.2Co0.2O2

Here we present the role of the electrolyte in the outgassing and formation of the peroxo-like surface layer on LiNi0.6Mn0.2Co0.2O2 (NMC622) in the first cycle. Results are given for the cyclic carbonates ethylene carbonate (EC) and propylene carbonate; the linear carbonates dimethyl carbonate and diethyl carbonate (DEC); and the glymes dimethoxyethane and tetraethylene glycol dimethyl ether. The salts lithium bis(trifluoromethanesulfonyl)imide and lithium hexafluorophosphate and the additives vinylene carbonate (VC) and fluoroethylene carbonate are also compared for the mixed carbonate EC/DEC electrolyte system. Although the gas evolution is distinct for different electrolytes, we find that the formation of the peroxo-like surface layer on NMC622 during the first cycle is largely independent of the identity of the electrolyte and tends to increase in magnitude with the attainable charge capacity for each electrolyte. Our results indicate that initial surface transformations are inevitable and are largely a material function of the NMC622. In all electrolytes tested except linear carbonates and the additive VC, Li2CO3 degradation to CO2 accounted for the maximum source of carbon dioxide > 4.6 V vs Li/Li+ in the first cycle.