Impact of Solvation Energetics on the Oxygen Reduction Reaction in Alkali-Metal/Oxygen Batteries

Abstract

Alkali-metal/Oxygen batteries are a potential high-energy alternative to Li-ion batteries1. In these systems, the critical performance bottleneck is the oxygen reduction reaction (ORR) which in turn depends on the non-aqueous electrolyte that mediates ion and oxygen transport2. Herein, we show that a multifold increase of the practical discharge power of these batteries can be achieved by tuning the tripartite interactions between the salt cation, anion and the solvent in the electrolyte3. A single descriptor of the solvation energetics, the solvent reorganization energy (λ), is shown to capture the influence of electrolyte composition on both reactant transport (DO2 ) and reaction kinetics (k) at the electrode and found to be broadly applicable to alkali-metal/oxygen (i.e. Li-, Na- and K-O2) batteries. Increasing cation size (from Li+ to K+) doubled k/λ, indicating an increased sensitivity of k to the choice of anion, while variations in DO2 were minimal over this cation size range. These multifarious effects were captured in the Electrochemical Thiele Modulus incorporating Marcus-Hush kinetics and guidelines are presented for the choice of electrolyte solvent and salt when designing systems for high discharge rates. In a model symmetric K-O2 cell4, both the formation of solvent-separated ion-pairs and the existence of unsolvated anions reduced overpotential losses by 200mV compared to electrolytes with partial anion solvation, successfully demonstrating the rational, ab-initio design of electrolytes for high-power alkali-metal/Oxygen batteries.