Exploring the Role of Redox-Shuttle Mediators in Lithium-Sulfur Batteries

Abstract

The lithium-ion battery (LIB) is widely acknowledged as the state-of-the-art battery technology today and is used in consumer electronics, stationary storage, and electrified transportation1,2. However, there is a drive towards developing technologies with higher energy densities. The lithium-sulfur battery is one of the most promising next-generation batteries for high-energy applications, due to its high theoretical gravimetric energy density of 2567 W h kg-1, low raw material costs, and reduced environmental impact3. Despite these potential advantages, lithium-sulfur batteries are still beset with difficulties, one of the most significant is the so-called “polysulfide shuttle effect.” Operation of the battery involves the conversion of sulfur to polysulfide species, which dissolve into the electrolyte and diffuse to the lithium metal electrode, leading to loss of active material. This results in poor cycle life, rapid capacity fading and low coulombic efficiencies4.The use of dissolved, electrochemically active redox mediators could mitigate these effects by catalysing sulfur redox reactions during cell cycling. While it is generally agreed that the redox potential of the mediators is a key factor in their operation, no report to date has conclusively demonstrated how mediators of varying potentials can target specific sulfur redox reactions. Herein, we report a study of the mediators 9-fluorenone (9-FO), duroquinone (DQ), 2,6-di-tert-butyl-1,4-benzoquinone (TBBQ) and 1,5-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)anthra-9,10-quinone (AQT) to understand their interactions with sulfur, lithium polysulfides, lithium sulfide and lithium metal5. We use liquid-chromatography mass spectrometry (LC-MS) to isolate various polysulfide chain lengths after reaction with mediators. It is observed that high potential redox mediators, such as TBBQ, are better able to drive Li2S oxidation to higher polysulfide chain lengths, and low potential redox mediators such as 9-FO push sulfur towards lower chain polysulfides. This offers a novel and effective approach to screening suitable redox-active molecules. Galvanostatic cycling is employed to relate these chemical interactions to enhanced cell performance. Developing a fundamental understanding of how redox mediators operate in a cell will aid in the molecular design of novel redox-active compounds.[1] J. Vetter, P. et al: Ageing mechanisms in lithium-ion batteries, J. Power Sources, 2005, 147, 269–281.[2] D. Castelvecchi: Electric cars: the battery challenge, Nature, 2021, 596, 336–339.[3] P. G. Bruce et al: Li-O2 and Li-S batteries with high energy storage, Nat. Mater., 2012, 11, 19–29.[4] S. Zhang et al: Recent advances in electrolytes for lithium-sulfur batteries, Adv. Energy Mater., 2015, 5, 1–28.[5] Y. Tsao et al: Designing a quinone-based redox mediator to facilitate Li2S oxidation in Li-S batteries, Joule, 2019, 3, 872–884.