A High-Voltage Gel Electrolyte with a Low Salt Concentration for Quasi-Solid-State Flexible Supercapacitors

Safe and eco-friendly aqueous electrolytes with wide electrochemical stability windows (ESWs) and wide temperature operating windows play a vital role in the development of high-energy and sustainable energy storage devices. In this study, we demonstrate a gel-type aqueous lithium bis-trifluoromethanesulfonimide (LiTFSI) electrolyte using glycerol as a cosolvent and polyvinyl alcohol (PVA) as a gel-forming agent. Adding water-miscible glycerol and PVA greatly decreases the number of “free” water molecules due to the H-bond interactions between water–glycerol and water–PVA molecules and suppresses the water splitting reaction. Thus, the ESW of aqueous electrolytes with low LiTFSI concentrations is significantly extended. Gel-type electrolytes with a low LiTFSI concentration of only 2m can achieve an ESW of 3.0 V, which is equivalent to an ESW value of 21m for LiTFSI aqueous solution without glycerol and PVA (superconcentrated or “water-in-salt” LiTFSI electrolyte). In addition, the temperature operating window is from −20 to +60 °C, which is much wider compared to that of the liquid 21m LiTFSI electrolyte. We assemble quasi-solid-state flexible supercapacitors using carbon nanotube sponge as the electrode and a gel-type LiTFSI electrolyte. The quasi-solid-state supercapacitors exhibit an actual operational voltage of 2.4 V, excellent flexibility, thermal stability, and good electrochemical performance at a low LiTFSI concentration in electrolytes.