Lithium–Sulfur Batteries under Lean Electrolyte Conditions: Challenges and Opportunities

Abstract

The development of energy‐storage devices has received increasing attention as a transformative technology to realize a 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 is highly dependent on solid‐liquid‐solid multi‐phase conversion, the electrolyte amount plays a primary role in the practical performances of Li–S cells. Therefore, a lean electrolyte volume with low electrolyte/sulfur ratio is essential for practical Li–S batteries, yet under these conditions it 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 addressed. Challenges and recent progress are presented in terms of the sulfur electrochemical processes: 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.

Optimizing Redox Reactions in Aprotic Lithium–Sulfur Batteries

“…Although Li 2 S is the final product of Li–S reaction, the redox reactions between solid‐state S and Li 2 S include several steps, which undergo compositional and structural conversions. [ 18,19 ] During discharge, S 8 is progressively reduced to LiPSs and eventually to Li 2 S through the steps below (Figure 1b). The initial plateau (≈2.4 V) represents the two‐phase reaction from solid S 8 to liquid long‐chain LiPSs (Li 2 S x , 6 < x ≤ 8) (Step I).…”

“…To some extent, the dissolution of LiPSs is conducive to the S reduction process; however, this dissolution will also lead to the loss of active materials and thereby reduce the Coulombic efficiency and cycle stability of Li–S batteries. [ 19 ] Therefore, a comprehensive understanding of the properties of LiPSs and its dissolution mechanisms are critical to address these issues.…”

“…Redox mediators (RMs) are molecules with reversible redox couples that can be electrochemically oxidized or reduced on the cathode surface and then diffuse to react with the active materials. [ 236 ] They enable the redox of insoluble species to be coupled to the electrode surface without requiring electronic contact (Figure 15b). [ 234 ] An RMs with proper equilibrium potential ( E RMs < E Li2S ) can address the sluggish kinetic in Li–S batteries by redox‐mediated process.…”

Efficient polysulfide blocker from conductive niobium nitride@graphene for Li-S batteries

“…A solution with 1 M LiTFSI and 1 wt% LiNO 3 in a DOL/DME (1:1 v/v) mixture http://engine.scichina.com/doi/10.1016/j.jechem.2019.10.018 was prepared and used as the electrolyte. To fully wet the interlayer, an excess electrolyte/sulfur ratio of 35 μl mg −1 is chosen for all assembled cells [32] . Electrochemical cycle stability and rate capability were tested using a LAND instrument and the voltage range is 1.7~2.8 V. A VSP-300 multichannel workstation was used to measure the cyclic voltammetry (CV) profiles.…”

Ultrafine Co3Se4 Nanoparticles in Nitrogen‐Doped 3D Carbon Matrix for High‐Stable and Long‐Cycle‐Life Lithium Sulfur Batteries

“…[ 7,9,10 ] Evidenced by the fact that current pouch cells with energy density exceeding 300 Wh kg −1 in LSB systems afford shorter stable lifespan for less than 100 cycles. [ 12 ]…”