Promising Routes to a High Li⁺ Transference Number Electrolyte for Lithium Ion Batteries

The continued search for routes to improve the power and energy density of lithium ion batteries for electric vehicles and consumer electronics has resulted in significant innovation in all cell components, particularly in electrode materials design. In this Review, we highlight an often less noted route to improving energy density: increasing the Li + transference number of the electrolyte. Turning to Newman’s original lithium ion battery models, we demonstrate that electrolytes with modestly higher Li + transference numbers compared to traditional carbonate-based liquid electrolytes would allow higher power densities and enable faster charging (e.g., >2C), even if their conductivity was substantially lower than that of conventional electrolytes. Most current research in high transference number electrolytes (HTNEs) focuses on ceramic electrolytes, polymer electrolytes, and ionomer membranes filled with nonaqueous solvents. We highlight a number of the challenges limiting current HTNE systems and suggest additional work on promising new HTNE systems, such as “solvent-in-salt” electrolytes, perfluorinated solvent electrolytes, nonaqueous polyelectrolyte solutions, and solutions containing anion-decorated nanoparticles.