Sodium metal batteries are attracting increasing attention as a low-cost alternative to lithium-based systems, owing to their high theoretical specific capacity (1165 mAh g−1) and sodium’s high natural abundance. Yet their practical deployment remains limited by poor reversibility during repeated plating and stripping, due to the high reactivity of sodium with conventional organic electrolytes. While advances in lithium metal batteries have shown that electrolyte design can enable highly efficient cycling, translating these strategies to sodium systems has proven far less straightforward. Now, Jason Phong, Yang Shao-Horn and colleagues from the Massachusetts Institute of Technology systematically investigate the factors that govern sodium metal reversibility, revealing a fundamentally different picture from lithium-based systems.
By examining a range of electrolyte chemistries, the researchers explore how ion–solvent interactions, ion transport and interfacial processes contribute to battery performance. The results show that commonly considered thermodynamic factors, such as redox potential and solvation properties, do not directly determine coulombic efficiency in sodium systems. This contrasts with lithium batteries, where these descriptors often play a central role. Instead, performance is strongly linked to how quickly charge can pass through the solid–electrolyte interphase (SEI) layer formed on the metal surface. This indicates that sodium cycling is mainly limited by interfacial reaction rates rather than ion transport. Further analysis shows that a balanced mix of inorganic and organic components in the SEI is key to enabling fast and stable interfacial reactions.
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