Adapting sol-gel chemistry for ionogel solid electrolytes

Ionogels have recently attracted considerable interest as pseudo-solid electrolytes based on their 2-phase microstructure in which an ionic liquid (IL) is confined within the mesoporous architecture of a sol-gel derived silica matrix. In this review, we show how sol-gel synthesis has played a central role in the development of ionogel materials. Ionogels are effectively ‘wet gels’. They are formulated to enable hydrolysis and condensation of alkoxysilane precursors but use an IL as the solvent phase. ILs, which are considered to be room temperature molten salts, have minimum vapor pressures and thus do not evaporate. At the nanoscale, the resulting ionogel possesses a nanofluidic state, but macroscopically it is solid. This unique microstructure enables the ionogel to exhibit the excellent electrochemical properties of the IL including high ionic conductivity, a wide electrochemical stability window along with good thermal and mechanical stability. The nanofluidic state ensures that there is excellent electrical contact between the solid electrode and the pseudo-solid electrolyte. This overcomes one of the problems associated with solid-state batteries, namely solid-solid interfaces. Ionogels have already been used in a number of electrochemical applications including lithium-ion and sodium-ion batteries as well as lithium metal and sodium metal batteries. The electrochemical properties of ionogels, their applications in battery systems and future opportunities in consumer electronics, transportation and the grid are highlighted.

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