Competitive Coordination Decouples Charge Transport from Viscous Relaxation in Water-in-Salt Electrolytes

Abstract Water-in-salt electrolytes provide wide electrochemical stability but suffer from high viscosity and strong ion correlations. Here, we examine how zwitterionic additives alter transport in concentrated LiTFSI electrolytes. Conductivity, viscosity, Raman spectroscopy, and atomistic molecular dynamics simulations show that zwitterion additives lower the absolute conductivity but shift the electrolytes from subionic to superionic Walden behavior. This decoupling arises from competing coordination effects: zwitterionic anionic groups coordinate Li+ and reduce its mobility, while also weakening Li+–TFSI– association, promoting TFSI––ZI+ contacts, and redistributing water away from TFSI–. Consequently, conductivity and viscosity become governed by increasingly distinct molecular responses. Our results demonstrate that competitive coordination provides a route to tune solvation structure, ion correlations, transference behavior, and viscosity-coupled transport in concentrated electrolytes.