Complex Charge Compensation Mechanisms in Lithium-Rich Chalcogenide Cathodes

Citation

Zak, Joshua Joseph (2023) Complex Charge Compensation Mechanisms in Lithium-Rich Chalcogenide Cathodes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1k50-3811. https://resolver.caltech.edu/CaltechTHESIS:06022023-055053747

Abstract

Lithium-ion batteries have revolutionized the world by enabling long-lasting portable electronics, electrified transportation, and grid storage solutions for renewable energy implementation. However, current commercialized technologies are limited by the one electron transfer per transition metal paradigm utilized by cathode materials that operate with an intercalation-based charge storage mechanism. Finding ways to increase the charge storage capabilities of the cathode into the multielectron regime has long been a focus of research efforts, and involvement of structural anions in the redox has been demonstrated as a promising way to accomplish multielectron storage. Layered lithium-rich oxide materials have been shown to afford dramatic improvements to overall storage capacity but are plagued by complex mechanisms and unwanted side reactions that lead to poor cycling stability and characterization difficulties. This thesis expands upon previous understanding of oxide-based anion redox materials and extends the exploration into sulfide and selenide systems, which allow the study of anion redox without the side processes that affect oxides. First, a dynamic charge compensation mechanism of late group metal-poor, lithium-rich oxide, Li₂Ru_0.3Mn_0.7O₃, is uncovered and found to involve an irreversible anion oxidation that leads to involvement of redox states on transition metals previously thought to be unavailable. Second, active electrolyte additives are explored as a method of stabilizing the cathode-electrolyte interface of anion redox material, Li₂RuO₃. Third, reversible anion redox is demonstrated in alkali-rich sulfides, Li₂FeS₂ and LiNaFeS₂, and proven to occur through oxidation of sulfides (S^2-) to persulfides ([S₂]^2-). Understanding of the structural ramifications of anion oxidation in Li₂FeS₂ is further expanded through computational and experimental methods. Fourth, the role of metal-anion covalency is systematically investigated through anion substitution of Li₂FeS₂ with S^2-, highlighting the importance of a holistic understanding of changes to the electronic and physical structure of anion redox materials to predict long-term performance. Finally, detailed perspectives and future outlooks on sulfur redox in lithium battery systems are offered with an exhaustive survey of thermodynamically stable binary and ternary persulfide materials.

Thesis Files