Decomposition Reactivities of Carbonate Electrolyte Vs. Localized High Concentration Electrolytes on NaNiO2 Cathode Surface

Abstract

Cathode electrolyte interphase (CEI) layers formed by the electrolyte decomposition on the positive electrode have been thought to be critical for the electrochemical performance of sodium-ion batteries. The conventional carbonate electrolyte decomposition often leads to unstable CEI layers, which are one of the known causes of capacity loss and structural degradation over repeated charging cycles. To solve this obstacle, the conception of localized high concentration electrolyte (LHCE) was proposed to maintain localized solvation structure of active electrolyte salts by using hydrofluoroethers as diluents. However, the cathode decomposition mechanisms of LHCE electrolytes have been rarely studied and the formation of CEI layers at cathode-LHCE interfaces are still unclear. Here, density functional theory is applied to investigate the oxidative decompositions of carbonate electrolyte and LHCE electrolyte on layered NaNiO2 surface. We compared the oxidation potentials of electrolyte molecules, reactions mechanisms of bulk electrolytes and decomposition reactions on (0 0 2) and (1 0 -2) surfaces of NaNiO2. Our results indicated that H-transfer reactions were prone to occur during carbonate electrolyte decomposition, while solvent(diluent) dissociation reactions were the primary pathways for LHCE decomposition. The proton transfer reactions with cathode surface oxygen atoms contribute to the formation of hydroxyl group (-OH). We predicted that the charge reduction of surface nickel atoms bonded with -OH groups is the main reason for the capacity fading and structural transformation of NaNiO2 cathode.