Tailoring the electrochemical properties of catholytes for solid-state sodium batteries
Sodium-based all-solid-state batteries (ASSBs) represent a promising alternative to lithium-based systems due to their inherent safety and the abundance and low cost of sodium resources. However, their large-scale adoption is hindered by limited solid electrolyte ionic conductivity, poor interfacial compatibility with layered-oxide cathodes, and reduced cathode-level energy density resulting from inactive components within the composite electrodes. This dissertation investigates the electrochemical properties of solid electrolytes functioning as catholytes in ASSBs and establishes fundamental design principles for achieving high ionic conductivity, interfacial stability, and cathode-level energy density. The first part of this work focuses on polycarbonate-based solid polymer electrolytes incorporating sodium bis(fluorosulfonyl)imide (NaFSI) salts. These studies clarify the role of polymer–salt coordination in governing ionic transport and interfacial stability. Optimized formulations exhibit improved sodium-ion conductivity and cycling stability with layered-oxide cathodes and Na–Sn alloy anodes, supported by analysis of gas evolution and interfacial degradation pathways through online electrochemical mass spectrometry and post-mortem microscopy. The second part explores low-cost aluminum halide-based catholytes with tunable structural and electrochemical characteristics. Mechanochemical synthesis of NaAlCl₄ and its oxygen- and fluorine-modified derivatives reveals the critical influence of anion substitution and surface phase formation on sodium-ion transport and electrochemical performance. Oxygen incorporation produces composite electrolytes with conductivities exceeding 0.1 mS cm⁻¹ ; fluorination yields AlF₃–NaAlCl₄ mixtures with improved ionic conductivity, but no change in oxidative stability, so artificial cathode coatings are found to be necessary for high-voltage cycling stability. These findings are further explored by elucidating the chemical decomposition and gas generation mechanisms of NaAlCl₄ and oxygen-doped NaAlCl₄ under high-voltage conditions. Through combined spectroscopic and gas analysis, it is shown that the decomposition proceeds through redox-driven anion exchange, contributing to kinetic hindrance of high-voltage phase transitions. Building on these insights, the final stage of this work introduces redox-active halide catholytes to overcome the energy density limitations of traditional redox-inactive systems. Composites of NaNbCl₆ and NaAlCl₄ exhibit simultaneous ionic transport and redox activity, enabling up to 79% improvement in cathode-level energy density relative to conventional catholytes. Altogether, this research establishes a comprehensive understanding of the structural, electrochemical, and interfacial factors that govern catholyte performance in ASSBs. The findings provide a foundation for the rational design of multifunctional solid electrolytes that combine high conductivity, chemical stability, and redox activity, advancing the development of safe, high-energy, low-cost solid-state sodium batteries for large-scale energy storage.
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