On energy: Batteries beyond lithium ion

Lithium–sulfur (Li–S) batteries possess overwhelming energy density of 2654 Wh kg−1, and are considered as the next-generation battery technology for energy demanding applications. Flooded electrolytes are ubiquitously employed in cells to ensure sufficient redox kinetics and preclude the interference of the electrolyte depletion due to side reactions with the lithium metal anode. This strategy is capable of enabling long-lasting, high-capacity and excellent-rate battery performances, but it mask the requirements of practical Li–S batteries, where high-sulfur-loading/content and lean electrolyte are prerequisite to realize the energy-dense Li–S batteries. Sparingly and highly solvating electrolytes have emerged as effective yet simple approaches to decrease the electrolyte/sulfur ratio through altering sulfur species and exerting new reaction pathways. Sparingly solvating electrolytes are characterized by few free solvents to solvate lithium polysulfides, rendering a quasi-solid sulfur conversion and decoupling the reaction mechanisms from electrolyte quantity used in cells; while highly solvating electrolytes adopt high-donicity or high-permittivity solvents and take their advantages of strong solvation ability toward polysulfide intermediates, thereby favoring the polysulfide formation and stabilizing unique radicals, which subsequently accelerate redox kinetics. Both solvation chemistry approaches have their respective features to allow the operation of cells under electrolyte-starved conditions. This Review discusses their unique features and basic physicochemical properties in the working Li–S batteries, presents remaining technical and scientific issues and provides future directions for the electrolyte chemistry to attain high-energy Li–S batteries.