Halide-Free Synthesis of New Difluoro(oxalato)borate [DFOB]^- -Based Ionic Liquids and Organic Ionic Plastic Crystals

The implementation of next-generation batteries requires the development of safe, compatible electrolytes that are stable and do not cause safety problems. The difluoro(oxalato)borate ([DFOB]^- ) anion has been used as an electrolyte additive to aid with stability, but such an approach has most commonly been carried out using flammable solvent electrolytes. As an alternative approach, utilisation of the [DFOB]^- anion to make ionic liquids (ILs) or Organic Ionic Plastic Crystals (OIPCs) allows the advantageous properties of ILs or OIPCs, such as higher thermal stability and non-volatility, combined with the benefits of the [DFOB]^- anion. Here, we report the synthesis of new [DFOB]^- -based ILs paired with triethylmethylphosphonium [P₁₂₂₂ ]⁺ , and diethylisobutylmethylphosphonium [P_122i4 ]⁺ . We also report the first OIPCs containing the [DFOB]^- anion, formed by combination with the 1-ethyl-1-methylpyrrolidinium [C₂ mpyr]⁺ cation, and the triethylmethylammonium [N₁₂₂₂ ]⁺ cation. The traditional synthetic route using halide starting materials has been successfully replaced by a halide-free tosylate-based synthetic route that is advantageous for a purer, halide free product. The synthesised [DFOB]^- -based salts exhibit good thermal stability, while the ILs display relatively high ionic conductivity. Thus, the new [DFOB]^- -based electrolytes show promise for further investigation as battery electrolytes both in liquid and solid-state form.