  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.
