a Chemistry thesis defense ft: Alexis Scida of the Ji Group

Renewable energy sources such as solar and wind power offer sustainable alternatives to fossil fuels but produce energy intermittently, making it challenging to meet peak societal demands for electricity. To bridge this gap, large-scale, long-duration energy storage systems are essential. Among the possible technologies available as candidates for these systems, aqueous zinc-metal batteries stand out due to the high abundance of zinc as a mining byproduct of copper, the inherent safety and affordability of water-based systems, and their environmental compatibility. However, the commercialization of these batteries is hindered by challenges such as limited electrochemical stability of aqueous electrolytes, poor reversibility of zinc metal,  and dendrite formation. Overcoming these obstacles requires a deep understanding of ion solvation and the mechanisms of solid electrolyte interphase (SEI) formation. This dissertation leverages nuclear magnetic resonance (NMR) spectroscopy and complementary techniques to elucidate these mechanisms, advancing the design of high-performance aqueous zinc-metal batteries.

The solvation of ions and their resulting influence on the electrochemical stability of aqueous electrolytes was explored using solution-state NMR spectroscopy. In a highly concentrated lithium chloride electrolyte at sub-zero temperatures, solvation energies were identified as critical to suppressing HER onset, challenging the prevailing hypothesis that disrupting hydrogen-bonding networks alone strengthens water’s O-H bonds. In a complementary investigation, Raman spectroscopy and solution-state NMR were used to examine the impact of ion charge density and hydration shells on proton and hydroxide ion solvation. This study demonstrated that anion-induced hydrogen-bond network disruption and the chaotropic nature of cations significantly influence susceptibility to HER and oxygen evolution reactions (OER), offering valuable insights for rational electrolyte design.

The role of organic additives in promoting SEI formation in aqueous electrolyte systems was investigated using a zinc chloride electrolyte modified with vanillin. The addition of 5 mg/mL vanillin effectively suppressed HER and zinc dendrite formation, achieving an impressive Coulombic efficiency of 99.34%. Solution-state NMR analysis revealed that vanillin undergoes acid-catalyzed chlorination and decomposition, followed by electrochemical reduction to form a protective organic SEI. This study provides an initial exploration into organic SEI formation in aqueous zinc metal batteries, offering promising strategies to enhance the performance and durability of zinc anodes.

Towards a deeper understanding of the influence of organic additives in aqueous zinc-metal battery systems, a detailed and mechanistic investigation of SEI formation was conducted in a hybrid organic water-in-salt electrolyte system comprised of 30 m ZnCl₂, 5 m LiCl, 10 m H-TMACl, and dimethyl carbonate (DMC). This study employed a suite of advanced spectroscopic techniques—including solution-state NMR, electrophoretic and diffusion NMR, and magic-angle spinning solid-state NMR (MAS ssNMR)—to trace the SEI formation process from the acid-catalyzed decomposition of 13C-labeled DMC into methanol and carbon dioxide to the emergence of new species on the electrode surface after electrochemical cycling. Electrophoretic NMR provided insights into solvation structure and ion diffusion, while MAS ssNMR revealed carbon-containing species within the SEI, formed via electrochemical reduction of CO₂ and subsequent reactions. This comprehensive analysis reveals the critical interplay between solvation chemistry, decomposition pathways, and electrode interface reactions, significantly advancing the understanding of SEI formation mechanisms in aqueous zinc-metal battery systems and guiding the design of next-generation hybrid electrolytes.