Accelerating S↔Li2 S Reactions in Li-S Batteries through Activation of S/Li2 S with a Bifunctional Semiquinone Catalyst

Abstract

The reaction rate bottleneck during interconversion between insulating S₈ (S) and Li₂ S fundamentally leads to incomplete conversion and restricted lifespan of Li-S battery, especially under high S loading and lean electrolyte conditions. Herein, we demonstrate a new catalytic chemistry: soluble semiquinone, 2-tertbutyl-semianthraquinone lithium (Li⁺ TBAQ⋅^- ), as both e^- /Li⁺ donor and acceptor for simultaneous S reduction and Li₂ S oxidation. The efficient activation of S and Li₂ S by Li⁺ TBAQ⋅^- in the initial discharging/charging state maximizes the amount of soluble lithium polysulfide, thereby substantially improve the rate of solid-liquid-solid reaction by promoting long-range electron transfer. With in situ Raman spectra and theoretical calculations, we reveal that the activation of S/Li₂ S is the rate-limiting step for effective S utilization under high S loading and low E/S ratio. Beyond that, the S activation ratio is firstly proposed as an accurate indicator to quantitatively evaluate the reaction rate. As a result, the Li-S batteries with Li⁺ TBAQ⋅^- deliver superior cycling performance and over 5 times higher S utilization ratio at high S loading of 7.0 mg cm^-2 and a current rate of 1 C compared to those without Li⁺ TBAQ⋅^- . We hope this study contributes to the fundamental understanding of S redox chemical and inspires the design of efficient catalysis for advanced Li-S batteries.

Keywords: Bifunctional; Catalyst; Polysulfide; Reaction Rate.