Chemical Activation of S/Li₂S in Li-S Batteries by a Bidirectional Organic Redox Mediator

The energy density and lifespan of prototype Li-S batteries under high sulfur loading and lean electrolyte have been mainly restricted by the incomplete interconversion between insulating S₈ and Li₂S. The introduction of an electrocatalyst has been preserved as an effective way to breakthrough the bottleneck of the interconversion rate. Herein, we demonstrate a novel bidirectional redox mediator, insoluble dithiobisphthalimide (DTPI), as the electrocatalyst for both S₈ reduction and Li₂S oxidation. Due to the dual-functional role of both electron/Li⁺ donor and acceptor, DTPI can efficiently accelerate the redox reactions during charge/discharge and significantly alleviate the incomplete conversion of sulfur species. Consequently, the Li-S batteries with DTPI deliver superior specific capacity and cycling stability in comparison with those without DTPI. Especially, the redox mediator is scalable for synthesis and the DTPI-based 5 A·h pouch cell delivers a specific discharge capacity of around 870 mA·h·g⁻¹ at 0.1 C (1 C=1675 mA/g) without capacity fading over 80 cycles. The bidirectional catalysis mechanism has been studied through theoretical calculation and ex-situ characterization of the cathode materials. This work approves the effectiveness of bidirectional organic redox mediator in the construction of practical Li-S batteries. (Figure presented.)