Conductivity–Viscosity Decoupling in Concentrated LiTFSI and LiCl Electrolytes

Water-in-salt (WiS) electrolytes are increasingly recognized for their superior electrochemical stability and advantageous transport properties in energy storage applications. These electrolytes, formulated with various anions, such as halogen-containing organic anions and halogen-inorganic anions, exhibit distinct characteristics. While the mechanisms governing conductivity-viscosity decoupling in concentrated electrolytes with halogen-containing organic anion salts have been extensively investigated, the decoupling behavior in concentrated electrolytes with halogen-inorganic anion salts remains inadequately characterized. Herein, clear differences in decoupling behavior between concentrated LiTFSI and LiCl electrolytes, with the decoupling parameter α being less than unity for LiTFSI but greater than unity for LiCl are presented. Structural analysis reveals ordered arrangements at ≈6 Å and 13 Å in the concentrated LiTFSI electrolyte, in contrast to the lack of such ordered structures in concentrated LiCl electrolyte. Additionally, the molecular dynamics simulations indicate that the decoupling in concentrated LiTFSI results from the counteractive movements of TFSI^- nanoclusters, whereas the decoupling observed in concentrated LiCl is attributable to the cooperative motion of undissociated Li⁺-Cl^- pairs. This work elucidates the conduction mechanism in LiCl-based WiS electrolyte, contributing to a deeper understanding of the conductivity-viscosity relationship and guiding the design of electrolytes for enhanced performance in lithium-ion batteries and supercapacitors.