Understanding the Morphological, Structural and Redox Behavior of Metal Sulfides as Cathode Active Materials in Solid-State Batteries

Understanding the charge storage process and material changes in solid-state batteries (SSBs) under realistic operating conditions is challenging due to the high stacking pressures applied during cycling when evaluating new materials. We have recently been developing a variety of incisive tools that enable us to evaluate the morphological, structural, and redox behavior of our CAMs under operating conditions in SSBs through computed X-Ray tomography, X-Ray diffraction, and X-Ray absorption and photoemission spectroscopies. In this talk I will discuss our recent work on understanding charge storage and structural changes in metal sulfide-based cathode active materials (CAMs) for SSBs through a combination of in-situ and ex-situ analysis. We are investigating sulfide-based CAMs because oxide-based CAMs are quickly approaching their limits in terms of capacity. Sulfides offer an intriguing direction for further research because they are high-capacity conversion-type cathodes that can help enable lithium metal anodes in SSBs, while simultaneously offering additional benefits by their reversible charge storage through stable anion redox (2S²⁻ → (S₂)²⁻ + 2e⁻), which can help further boost capacity despite their low operating voltage windows. I will detail our findings on CuS, which undergoes a macroscopic displacement reaction during lithiation, whereby micron-sized Cu networks form, which we were able to follow by in-situ synchrotron-based X-Ray tomography.[1] Despite the large volume expansion of 75% and unique displacement mechanism, CuS-based cells show surprisingly stable cycling behavior, maintaining a capacity of 305 mAh/g over 100 cycles.[2] We investigate further structural stabilization through implementation of ternary compositions such as Cu₃PS₄ and CuFeS₂ and find that in both cases we are able to promote stable cycling behavior (maintaining a capacity of 508 mAh/g over 60 cycles for Cu₃PS₄ and 436 mAh/g over 150 cycles for CuFeS₂) through favorable chemo-mechanical properties and the formation of finely-dispersed redox centers.[3]<br/><br/>References:<br/>[1] Z. Zhang, et al. Adv. Energy Mater. 2023, 13, 2203143.<br/>[2] A. L. Santhosha, et al. Adv. Energy Mater. 2020, 10, 2002394.<br/>[3] Z. Zhang, et al. Energy Technol. 2023, 11, 2300553.