Towards practical lean-electrolyte Li–S batteries: Highly solvating electrolytes or sparingly solvating electrolytes?

Lithium–sulfur (Li–S) batteries hold great promise to be the next-generation candidate for high-energy-density secondary batteries but in the prerequisite of using low electrolyte-to-sulfur (E/S) ratios. Highly solvating electrolytes (HSEs) and sparingly solvating electrolytes (SSEs), with opposite nature towards the dissolution of polysulfides, have recently emerged as two effective solutions to decrease the E/S ratio and increase the overall practical energy density of Li–S batteries. HSEs featuring with high polysulfide solvation ability have the potential to reduce the E/S ratio by dissolving more polysulfides with less electrolyte, while SSEs alter the sulfur reaction pathway from a dissolution–precipitation mechanism to a quasi-solid mechanism, thereby independent on the use of electrolyte amount. Both HSEs and SSEs show respective effectiveness in lean-electrolyte Li–S batteries, but encounter different challenges to bring Li–S batteries into practical application. This review aims to present a comparative discussion on their unique features and basic electrochemical reaction mechanisms in practical lean-electrolyte Li–S batteries. Emphasis is focused on the current technical challenges and possible solutions for their future development.