New tetrabutylphosphonium organic ionic plastic crystals incorporating borate anions

Abstract

Organic plastic crystals have seen rapid growth in their development and potential application as solid-state electrolytes in electrochemical devices. Two tetrabutylphosphonium ([P₄₄₄₄]⁺) salts incorporating non-halogenated bis(butane-1,4-dioxy)borate ([B(bdo)₂]⁻) and tetrakis(pyrazolyl)borate ([B(pyr)₄]⁻) anions have been synthesised and their physicochemical properties characterised. Both [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] are solid under ambient conditions and display solid–solid thermal phase transitions suggesting the formation of plastic crystal phases, which was confirmed by observation of significant ionic conductivity and ion mobility in the solid state. [P₄₄₄₄][B(pyr)₄] showed more extensive polymorphism and greater ionic conductivity, and from the ³¹P linewidth NMR the phosphonium cation exhibited diffusion at lower temperatures. Therefore, these are of interest as potential new halogen-free solid-state electrolyte materials.