Challenges and Opportunities towards Practical Lithium-Sulfur Batteries under Lean Electrolyte Conditions.

The development of energy storage devices has received increasing attentions as a vital transformative technology to realize low-carbon economy and sustainable energy supply. Lithium-sulfur (Li-S) batteries are considered to be one of the most promising next-generation energy storage devices due to their ultrahigh energy density. Despite the extraordinary progress in the last few years, the actual energy density of Li-S batteries is still far from satisfactory to meet the demand for practical applications. Considering the sulfur electrochemistry highly dependent on solid-liquid-solid multi-phase conversion, the electrolyte amount plays a primary role to the practical performances of Li-S cells. Therefore, lean electrolyte volume with low electrolyte/sulfur ratio is essential for practical Li-S batteries to demonstrate their advantages in energy density, yet under which conditions is highly challenging to achieve acceptable electrochemical performances regarding sulfur kinetics, discharge capacity, Coulombic efficiency, and cycling stability especially for high-sulfur-loading cathodes. In this review, the impact of the electrolyte/sulfur ratio on the actual energy density and the economic cost of Li-S batteries is firstly addressed. Challenges and recent progresses of lean-electrolyte Li-S batteries are included according to the sulfur electrochemical processes regarding the dissolution-precipitation conversion and the solid-solid multi-phasic transition. Finally, prospects of future lean-electrolyte Li-S battery design and engineering are discussed.