Polymer-Supported Aqueous Electrolytes for Lithium Ion Batteries: II. Conductivity Gain Upon Polymerization and Double Glass Transition in LiTFSI + H2O + PDA Gels

Polymerization via ultraviolet irradiation of LiTFSI21mH2O + PDA liquid mixtures turned most of these liquids into gels or solids. Here, LiTFSI21mH2O denotes a 21 m aqueous solution of lithium bis(trifluoromethanesulfonyl)imide, and PDA stands for a monomer of poly(ethylene glycol) diacrylate of Mn 575. Systematic thermoconductometric measurement on these electrolytes, both before and after the polymerization, showed the gel electrolytes to be considerably more conductive than their precursor liquid mixtures, especially at lower temperatures. A parallel measurement of glass transition temperature, theta (g), revealed in these gel electrolytes a unique double glass transition enveloping two sub-transitions each with its own theta (g)'s. These and a number of other related experimental observations can be consistently and clearly explained based on the existence of a solution and a polymer substructure in the polymerized electrolytes, and on these substructures becoming codominant in the gels. The exceptional conductivity in these gel electrolytes points to a promising direction to formulating a polymer-supported aqueous electrolyte with a set of desirable physical traits and an uncompromising conductivity.