A symmetrical ionic liquid/Li salt system for rapid ion transport and stable lithium electrochemistry

Contrary to the accepted wisdom that avoids cation symmetry for the sake of optimum electrolyte properties, we reveal outstanding behaviour for the diethylpyrrolidinium cation ([C2epyr]), in combination with the bis(fluorosulfonyl)imide (FSI) anion and Li[FSI]. The equimolar [C2epyr][Li][FSI]2 is a liquid with high conductivity, high Li transference number and >90% lithium metal cycling efficiency. The high level of performance for these electrolytes invites consideration of a new class of electrolytes for lithium batteries.