Super-Concentrated Alkali Hydroxide Electrolytes for Rechargeable Zn Batteries

Abstract

Increasing the salt concentration of an electrolyte to over 10 M brings new solute-solvent interactions that define an emerging class of super-concentrated electrolytes for rechargeable batteries. To this class we introduce a super-concentrated alkaline electrolyte. Nearly saturated with KOH (at 15 M), the aqueous solution displays a broad electrochemical stability window (>2.5 V on Au) while retaining an exceptionally high ionic conductivity (>0.27 S/cm at 25 °C). Without a solid-electrolyte interphase, we can confirm the role of electron transfer kinetics in determining the stability as opposed to the thermodyanmic effect computed based on activity coefficients. The compositionally simple solution also allows spectroscopies and ab-initio molecular dynamics simulation to identify a novel mechanism of OH− structural diffusion, where unlike the conventional Grotthuss mechanism through hydrogen-bond networks, electrostatic forces sustain proton hopping and break the common stability-conductivity tradeoff. A high ZnO solubility in the electrolyte further mitigate the issue of passivation when a Zn anode is deeply discharged. These unique properties enable a NiOOH||Zn battery to deliver a cumulative capacity >10 Ah/cm2 at 40 mA/cm2 to meet practical needs of safe, inexpensive energy storage.