Plastic Crystals Utilising Small Ammonium Cations and Sulfonylimide Anions as Electrolytes for Lithium Batteries

Organic ionic plastic crystals (OIPCs) are increasingly promising as a class of solid-state electrolyte for developing safer lithium batteries. However, their advancement relies on expanding the range of well-characterised cation/anion combinations. Here, we report the synthesis and characterization of OIPCs utilising small ammonium cations tetramethylammonium ([N₁₁₁₁]⁺), triethylmethylammonium ([N₁₂₂₂]⁺) and tetraethylammonium ([N₂₂₂₂]⁺), chosen to encourage significant rotational and translational motion, with the charge-diffuse and electrochemically stable bis(fluorosulfonyl)imide ([FSI]ˉ) and bis(trifluoromethanesulfonyl)imide ([NTf₂]ˉ) anions. To investigate the physico-chemical properties of the OIPCs, the free volume was measured by positron annihilation spectroscopy (PALS) and correlated with the ionic conductivity and thermal analysis (DSC). Solid-state NMR analysis of the salts, is also reported. The salts with the less symmetric cation, [N₁₂₂₂][FSI] and [N₁₂₂₂][NTf₂], were identified as the most promising electrolyte materials, and thus the electrochemical properties after mixing with 10 and 90 mol% lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂), respectively, were investigated. This study demonstrates the efficacy of these OIPC materials as new quasi-solid state electrolytes with advantageous properties such as high conductivity, good thermal and electrochemical properties, the ability to incorporate high lithium salt concentrations and support efficient lithium electrochemistry.