Amphiphilic salts as single-component, solvent-free, lithium electrolytes

Abstract

Electrolytes with enhanced thermal stability are sought for next-generation lithium batteries. In this work, we discuss the synthesis, thermal properties, morphology, and ionic conductivity of single-component, solvent-free electrolytes composed of lithium salts with amphiphilic anions. These salts exhibit nanoscale phase segregation between the ionic domains and aliphatic tails of the amphiphilic anions. It is found that for a series of lithium salts with a decane tail, the ionic conductivity is correlated with ion pair binding energy. The ionic conductivity is highest for the decane tailed salt with the –sulfonyl(trifluoromethanesulfonyl)imide head group (LiC10TFSI) at 5.6 × 10^-7 S/cm at 70°C, with salts with – sulfonyl(phenylsulfonyl)imide and –sulfonylazanide anions exhibiting lower conductivity. A salt with an octadecane tail and TFSI headgroup (LiC18TFSI) has further improved ionic conductivity, 10 to 1000 times higher depending on the temperature and 3.6 × 10^-5 S/cm at 70°C. LiC18TFSI is a smectic ionic liquid crystal at intermediate temperatures as confirmed through X-ray scattering experiments and molecular dynamics simulations, whereas LiC10TFSI is suspected to be a disordered ionic liquid at the measurement conditions, highlighting the importance of ionic aggregate morphology on bulk ionic conductivity of electrolytes with ion clusters.