Anode Design Concepts for Energy-Dense Sodium Batteries

Sodium-ion batteries (SIBs) are emerging as cost-effective complements to lithium-ion technology, yet their deployment is constrained by anode selection, electrolyte design, and interphase instabilities. This Focus Review highlights advances from hard-carbon (HC) anodes (commercial benchmark) to metallic-sodium anodes in both explicit (e.g., Na foil) and implicit (e.g., “anode-free”) configurations, emphasizing solid-electrolyte/cathode-electrolyte interphase (SEI/CEI) formations, mossy/dendritic growth, sodium orphaning, and mitigation strategies via current-collector engineering and protective interphases. We further analyze electrolyte design, examining how salt choice, solvent coordination, and additives regulate interphase chemistry and sodium deposition. Finally, we assess prospects and challenges of Na–O2/Na–air batteries, including reaction pathways, discharge products, and harsh anode environments. Remaining challenges include limited cycle life at realistic areal capacities, unresolved failure mechanisms, and the need for standardized evaluation protocols. Overall, the Review aims to provide a forward-looking, comprehensive perspective on opportunities for future research and innovations in the development of next-generation sodium batteries.