Tuning anion solvation energetics enhances potassium–oxygen battery performance

Abstract

The oxygen reduction reaction (ORR) is a critical reaction in secondary batteries based on alkali metal chemistries. The nonaqueous electrolyte mediates ion and oxygen transport and determines the heterogeneous charge transfer rates by controlling the nature and degree of solvation. This study shows that the solvent reorganization energy (λ) correlates well with the oxygen diffusion coefficient (DO2) and with the ORR rate constant (k) in nonaqueous Li–, Na–, and K–O2 cells, thereby elucidating the impact of variations in the solvation shell on the ORR. Increasing cation size (from Li+ to K+) doubled k=λ, indicating an increased sensitivity of k to the choice of anion, while variations in DO2were minimal over this cation size range. At the level of a symmetric K–O2 cell, both the formation of solvent-separated ion pairs [K+–(DMSO)n–ClO4− + (DMSO)m–ClO4−] and the anions being unsolvated (in case of PF6−) lowered ORR activation barriers with a 200-mV lower overpotential for the PF6− and ClO4− electrolytes compared with OTf− and TFSI− electrolytes with partial anion solvation [predominantly K+–(DMSO)n–OTf−]. Balancing transport and kinetic requirements, KPF6 in DMSO is identified as a promising electrolyte for K–O2 batteries.