Electrochemical and Cycle Analysis of Water-in-Salt K-Acetate Electrolyte Zn-Ion Batteries Under Commercially-Relevant Conditions

Abstract

Water-in-salt electrolyte (WiSE) promises high-voltage battery technology with low fire risk. Here we assess potassium acetate (KAc) WiSE for Zn ion batteries under commercially relevant conditions. Rotating disc electrode analysis of WiSE degradation and Zn plating/deplating suggest a solid electrolyte interphase (SEI) layer dominates. Butler-Volmer kinetics and Koutecky-Levich mass-transfer are of secondary importance. Measurements of chemical potential reveal that bulk solvation of H2O (in KAc WiSE or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) WiSE) is an insignificant process compared to SEI blocking. Zinc cycling in KAc WiSE with practical rates (∼0.3 to 8.0 mA cm−2) and areal capacities (>20 mAh cm−2) shows dendrites are less prominent than in KOH, but the SEI layer suppresses the electrochemical reaction too much for commercial feasibility. Dilution or convection of the WiSE alleviates the SEI blocking effects. Cu substrate shows good Zn adhesion, but Ti, Sn, and Ni show poor adhesion. Cathodes made with Chevrel (Mo6S8) reversibly intercalate Zn2+ to form a novel battery technology when paired with Zn foil, but yield <1.0 V cell voltage. Cathodes made with zinc-containing Prussian blue analogues (ZnHCF or ZnMnHCF) yield a voltage near 2.0 V but don't cycle in the present KAc WiSE formulation. Future research directions for KAc WiSE are proposed, focussing on SEI dynamics and Prussian blue compatibility.