Gelified acetate-based water-in-salt electrolyte stabilizing hexacyanoferrate cathode for aqueous potassium-ion batteries

Abstract

Potassium acetate (KAc)-based “Water-in-salt” electrolytes (WiSE) are herein studied by Raman and classical Molecular Dynamics (MD), evidencing the notably suppressed water activity of these WiSEs since water can be effectively coordinated by both the acetate anion and the potassium cation. The overall molecular arrangement is found to approach the “sponge-like” structure observed in certain ionic liquids. With properly tuned composition, such WiSE can also be compatible with Al current collectors, as demonstrated by extensive electrochemical, scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis. The use of low-cost potassium manganese hexacyanoferrate (KMHCF) in conjunction with these electrolytes, however, is affected by poor cycling performance due to the limited stability of the cathode material in the alkaline environment. Promisingly though, the stability of KMHCF is found to improve substantially when the electrolyte is gelified by adding a small amount (2 ​wt%) of carboxymethyl cellulose (CMC), as testified by the enhanced capacity retention as well as the higher Coulombic efficiency (>99.3%). In particular, Mn and Fe dissolution are suppressed and, as suggested by MD simulations, K cations diffusion may be promoted in the gel electrolyte compared to the liquid system.