Separator-Free Gel Electrolytes Based on Water-in-Salt Solutions

Abstract

Water-in-salt electrolytes (WiSE) are one of a growing family of water-based lithium ion electrolytes that use high concentrations of non-hydrolyzable lithium salts such as LiTFSI. WiSE are non-flammable and have a much wider electrochemical stability window than typical dilute aqueous electrolytes through two anion-based mechanisms: the formation of a passive anode layer through the decomposition of the TFSI- anion and exclusion of water by the anion from the cathode surface. This work details the formation of a polymer gel network containing WiSE as its liquid phase using commercially available polymer and crosslinker components. WiSE gel electrolytes are thought to be a natural evolution of the water-in-salt concept, as a WiSE gel should allow for WiSE-containing batteries to decouple themselves from typical cylindrical and prismatic battery form factors. Cycling performance and mechanical properties of the gel electrolytes will be discussed, along with fundamental investigations into the electrode-electrolyte interface and the minimization of charge-transfer resistance. Additionally, the kinetics of gel reactions in WiSE as a solvent will be discussed.