Ion-Pairing Limits Crystal Growth in Metal-Oxygen Batteries

TL;DR

This study investigates how electrolyte salt concentration influences NaO2 crystal growth in Na-O2 batteries and demonstrates that cation-coordinating additives can improve battery capacity by modulating ion pairing.

Contribution

It reveals the impact of salt concentration on crystal growth and introduces crown molecules as additives to enhance discharge capacity without harming rechargeability.

Findings

Higher salt concentrations reduce NaO2 crystal size.

Crown molecules weaken ion pairing and increase capacity.

Additives do not impair battery rechargeability.

Abstract

Aprotic alkali metal-oxygen batteries are widely considered to be promising high specific energy alternatives to Li-ion batteries. The growth and dissolution of alkali metal oxides such as Li2O2 in Li-O2 batteries and NaO2 and KO2 in Na- and K-O2 batteries, respectively, is central to the discharge and charge processes in these batteries. However, crystal growth and dissolution of the discharge products, especially in aprotic electrolytes, is poorly understood. In this work, we chose the growth of NaO2 in Na-O2 batteries as a model system and show that there is a strong correlation between the electrolyte salt concentration and the NaO2 crystal size. With a combination of experiments and theory, we argue that the correlation is a direct manifestation of the strong cation-anion interactions leading to decreased crystal growth rate at high salt concentrations. Further, we propose and experimentally demonstrate that cation-coordinating crown molecules are suitable electrochemically stable electrolyte additives that weaken ion-pairing and enhance discharge capacities in metal-oxygen batteries while not negatively affecting their rechargeability.