Hongli Zhu1
Northeastern University1
Sulfur, with its high theoretical specific capacity of 1675 mAh g-1, affordability, and earth abundance, stands as a promising candidate for cathodes. Lithium-sulfur (Li-S) batteries, boasting an ultrahigh theoretical energy density (2600 Wh kg-1)— a value ten times higher than that of contemporary commercial Lithium-ion batteries, hold immense potential. Yet, Li-S batteries employing ether-based liquid electrolytes face challenges such as the formation of long-chain polysulfides and the shuttle effect, leading to rapid capacity decay, low coulombic efficiency, and poor cycling life. All-solid-state lithium-sulfur batteries (ASLSBs), which substitute traditional organic liquid electrolyte with solid-state electrolytes (SEs), are considered to be a fundamental solution to these issues. SEs provide enhanced safety compared to their flammable and volatile organic counterparts and are assumed to eliminate the formation of polysulfides, thereby inherently addressing the shuttle effects.<br/>Nevertheless, the absence of clear experimental evidence to understand the chemistries and mechanisms of sulfur's electrochemical reaction in ASLSBs prompts further investigation. Existing literature suggests the chemistries in ASLSBs transition directly from S8 to Li2S. However, the overlooked formation of Li2S2, insoluble in liquid electrolytes, could potentially impact ASLSB performance. Considering Li2S2 is a thermodynamically unstable phase, the implementation of advanced in-operando characterization becomes crucial for a deeper understanding of the reaction mechanisms in ASLSBs.<br/>In this talk, we present four key findings that would be of interest to the readership of MRS audiences:<br/>1. We study the chemical transformation of sulfur in ASLSBs and elucidate the reaction mechanism via a combination of operando Raman spectroscopy and ex-situ X-ray absorption spectroscopy (XAS). This gives us the first empirical proof of electrochemical pathways in ASLSBs.<br/>2. We designed a unique cell for the operando Raman study, facilitating the characterization of the battery in an inert argon atmosphere within a cleanroom-in-a-glovebox system. This custom cell with a side window and a pressure-controlling stainless-steel framework allows internal battery material exposure post-assembly without further sealing. Moreover, its square design ensures a focused laser beam on a flat sample surface, eliminating background distractions.<br/>3. We have, for the first time, unveiled a two-step transformation in the Li-S redox reaction in ASLSBs. Our findings prove that the process does not generate any long-chain polysulfides, thereby inhibiting the shuttle effect in SE, a significant departure from the scenario with liquid electrolyte.<br/>4. We found that the transition among S8, Li2S2, and Li2S exhibits sluggish reaction kinetics, caused by the slow "point-to-point" ion diffusion at the interface in ASLSBs and high “solid to solid” reaction barrier, resulting in incomplete reactions during both discharging and charging processes.<br/>These insights will contribute to the continued research and development in the field of ASLSBs.