All-Solid-State Lithium–Sulfur Batteries Enhanced by Redox Mediators

Abstract

Redox mediators (RMs) play a vital role in some liquid electrolyte-based electrochemical energy storage systems. However, the concept of redox mediator in solid-state batteries remains unexplored. Here, we selected a group of RM candidates and investigated their behaviors and roles in all-solid-state lithium–sulfur batteries (ASSLSBs). The soluble-type quinone-based RM (AQT) shows the most favorable redox potential and the best redox reversibility that functions well for lithium sulfide (Li2S) oxidation in solid polymer electrolytes. Accordingly, Li2S cathodes with AQT RMs present a significantly reduced energy barrier (average oxidation potential of 2.4 V) during initial charging at 0.1 C at 60 °C and the following discharge capacity of 1133 mAh gs –1. Using operando sulfur K-edge X-ray absorption spectroscopy, we directly tracked the sulfur speciation in ASSLSBs and proved that the solid–polysulfide–solid reaction of Li2S cathodes with RMs facilitated Li2S oxidation. In contrast, for bare Li2S cathodes, the solid–solid Li2S–sulfur direct conversion in the first charge cycle results in a high energy barrier for activation (charge to ∼4 V) and low sulfur utilization. The Li2S@AQT cell demonstrates superior cycling stability (average Coulombic efficiency 98.9% for 150 cycles) and rate capability owing to the effective AQT-enhanced Li–S reaction kinetics. This work reveals the evolution of sulfur species in ASSLSBs and realizes the fast Li–S reaction kinetics by designing an effective sulfur speciation pathway.

A quasi-intercalation reaction for fast sulfur redox kinetics in solid-state lithium–sulfur batteries

“…There are mainly two kinds of solid-state electrolytes for Li-S batteries: ceramic electrolytes (mainly sulfide-based electrolytes) 13,14 and solid polymer electrolytes (mainly polyethylene oxide (PEO)-based electrolytes). 15,16 The sulfide-based electrolytes have a high Li + conductivity, while their interfacial stability between sulfur and electrolyte is poor, requiring a high stacking pressure which limits their practical application. The PEObased electrolytes have good interfacial stability, but their Li + conductivity is low at room temperature, requiring a high temperature operating environment (460 1C).…”

“…11 To improve the reaction kinetics of S and Li 2 S, introducing catalysts into the cathode of solid-state Li-S batteries has been proposed. 12,15 Although this strategy efficiently helps improve S to Li 2 S redox kinetics, the large volume change between S and the discharged Li 2 S and their poor electrical conductivities still lead to cathode failure. The development of suitable sulfur cathodes with a high electrical conductivity and the ability to accommodate the large volume change is important.…”

Challenges and advances of organic electrode materials for sustainable secondary batteries

“…For instance, Cui and co-workers designed a Li 2 S@AQT cell with all-solid-state electrolyte, and Li 2 S cathodes with AQT redox mediator presented a reduced energy barrier for activation and superior cycling stability. [213] AQT redox mediator promoted the charge transfer rate at the electrode-electrolyte interface, thus activating the redox reaction of insulting S/Li 2 S speciates. Another example is 2D polyimide COFs used as the S-loading host for Li-S batteries by Li's work, and the ultrathin COF nanosheets with redox-active carbonyls could not only combine polysulfides species through sulfiphilic/lithiophilic interactions but also physically confine polysulfides within well-defined pores.…”

Understanding Mechanisms of All-solid-state Lithium-Sulfur Batteries through Operando Raman and Ex-situ XAS Studying

“…Based on current findings, the overall reaction mechanism of Li-S batteries with both LEs and SEs is a phase change reaction between sulfur (S8) and Li2S. 10 The total redox reaction is shown in…”

“…[4][5][6] State-of-the-art researches in ASLSBs focus on optimizing carbon additives, developing high conductive SEs, designing advanced electrode structures, and employing catalysts to boost the electrochemical performance. [7][8][9][10] However, the performance of the ASLSBs is still far from expectations, especially the rate performance is much lower than in LEs. Meanwhile, the underlying mechanisms of the ASLSBs are critical in designing highperformance batteries but are not fully understood yet.…”