Design Rules for Multi-Electron Systems in Next-Generation Batteries: From Mg Electrode-Electrolyte Interface to Anion Redox Activation in Li-Rich Sulfides

Citation

Kim, Seong Shik (Steve) (2023) Design Rules for Multi-Electron Systems in Next-Generation Batteries: From Mg Electrode-Electrolyte Interface to Anion Redox Activation in Li-Rich Sulfides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/v52j-t589. https://resolver.caltech.edu/CaltechTHESIS:06032023-004803929

Abstract

Li-ion batteries (LIBs) have revolutionized the modern world, powering portable electronic devices and more recently realizing electrification of transportation. With more technological advancements that further improved the performance, LIBs also play an important role as one of the most promising energy storage systems in transforming into renewable energy sources and achieving net zero emissions. However, state-of-the-art intercalation-based LIBs are beginning to mature and reach their theoretical capacity limits. To further improve the electrochemical performance of batteries and meet growing demands of energy storage applications, there have been growing efforts to increase the energy density beyond the limits of conventional LIBs. In this thesis, we examine two examples of multi-electron systems–Mg electrolytes and Li-rich sulfide cathode materials–to gain insights and establish design principles.

First, we explore the magnesium aluminum chloride complex (MACC) electrolyte to study the role of the electrode-electrolyte interface in Mg charge transfer. We demonstrate that MACC electrolyte which normally requires electrolytic conditioning can be chemically activated by the small addition of Mg(HMDS)₂. Solution-phase characterization reveals that Mg(HMDS)₂ helps prevent the formation of passivating film on the Mg surface by scavenging trace amounts of H₂O. Mg(HMDS)₂ also reacts with MACC to form free Cl⁻ which decorates the Mg surface which facilitates Mg electrodeposition and stripping.

Next, we investigate three different alkali-rich sulfides-LiNaFeS₂, LiNaCoS₂, and Li_1.33-1.33zTi_{0.67+0.33zVacz}S₂ - to probe the role of electronic and physical structure in governing reversible anion redox. We demonstrate that cryomilling LiNaFeS₂ mitigates particle fracturing by increasing microstrain and reducing crystallite size. Isostructural LiNaCoS₂ exhibits more covalent interactions between the transition metal-d and S-p states compared to LiNaFeS₂, but undergoes an irreversible conversion reaction. Lastly, Li₂TiS₃ exhibits no electrochemical activity, but introducing cationic vacancies in Li_1.33-1.33zTi_{0.67+0.33zVacz}S₂ activates S oxidation. Li_1.33-1.33zTi_{0.67+0.33zVacz}S₂ is studied further to study first-cycle activation and voltage hysteresis in Li-rich sulfides.