Development of new solid-state electrolytes based on a hexamethylguanidinium plastic crystal and lithium salts

Abstract

There is an urgent need to improve the safety of lithium batteries, mainly due to the flammable and volatile organic solvent-based electrolytes currently used. Organic ionic plastic crystals (OIPCs) are showing promise as safe alternative solid-state electrolytes. The new OIPC hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI]) displays excellent properties, such as high conductivity and a wide electrochemical stability window, making it a promising material as a starting point for the development of new solid state electrolytes. In this work, mixtures of [HMG][FSI] with lithium salts of bis(trifluoromethanesulfonyl)imide ([TFSI]) or bis(fluorosulfonyl)imide ([FSI]) were prepared and the effects of lithium salt addition and the influence of anion on the electrolyte properties were investigated. Simultaneously, the effect of the salt concentration was studied by the addition of increasing amounts of lithium salt in [HMG][FSI] (10, 50 and 90 mol% of LiFSI and LiTFSI). The thermal properties, ionic conductivities, self-diffusion properties, electrochemical stabilities and lithium transport properties were evaluated for all electrolytes and the neat OIPC. Overall, all of the prepared quasi-solid state electrolytes showed good transport properties and supported reversible deposition and stripping of lithium, thus highlighting an important new avenue for the development of OIPCs as solid-state electrolytes for lithium batteries.