Solid-State Batteries Based on Ionogel Electrolytes

Ionogels are pseudo-solid state electrolytes in which an ionic liquid electrolyte is confined in an organic or inorganic matrix. One well established ionogel synthesis route is based on sol-gel chemistry where silica precursors dissolved in an ionic liquid electrolyte undergo hydrolysis and condensation polymerization. By using an ionic liquid as the solvent, no evaporation occurs and the resulting material is a macroscopically rigid, nonporous material in which the ionic liquid is trapped by capillary forces in the nanometer sized pores in the silica network. Because of their unique architecture, ionogels maintain a nanoscale fluidic state and thus mitigate the interfacial resistances which commonly arise at solid-solid interfaces. Using this approach, we have been able to develop pseudo-solid state materials that possess the electrochemical, thermal and chemical properties of the ionic liquid. Moreover, sol-gel synthesis enables the precursor sol to penetrate porous electrodes before solidifying, a feature which is especially attractive for solid-state batteries.<br/> Our recent work has been directed at developing ionogels for anode free solid-state batteries. In this work we take advantage of the ability to solvent exchange the ionic liquid phase with a liquid electrolyte which is more conducive to lithium plating and stripping. We start by synthesizing an ionogel based on dissolving the silane precursors (tetramethyl orthosilicate and methyltrimethoxy silane) in a 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14 FSI) ionic liquid, using acetic acid as a catalyst. After gelation, the ionic liquid is solvent exchanged with an electrolyte consisting of 1 M lithium bis-(tri-fluoromethylsulfonyl)imide (LiTFSI) in 1 : 1 of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with 1 wt% of LiNO₃. We have cycled Li/ionogel/Cu cells for over 500 cycles with minimal interfacial resistance and nearly 100% coulombic efficiency. We have also investiged the use of 3D printed copper electrodes and are able to plate and strip lithium at current densities up to 10 mA cm^-2. Anode-free solid-state batteries based on these solvent exchanged ionogels show that the ability to substitute the desired electrolyte for the ionic liquid offers an important new direction which, when coupled with their thermal and electrochemical stability, makes ionogel electrolyte materials a very promising approach for solid-state batteries.