Anode-free sodium metal batteries: optimisation of electrolytes and interphases

Anode-free sodium metal batteries (AFSMBs) represent a significant advancement in energy storage technology, offering high energy density and cost-effective solutions. However, their applications are impeded by the critical sodium deposition behavior, which poses safety risks and compromises battery performance. This review examines the recent progress in electrolyte and interphase optimization which is pivotal for the realization of dendrite-free sodium anodes in AFSMBs. We elucidate the mechanisms of sodium deposition, dendrite formation, and their impacts on battery performance, with the focus on electrolyte composition. A stable solid electrolyte interphase (SEI) is emphasized for preventing dendrite growth and improving Coulombic efficiency (CE). Also, recent strategies in interfacial design, such as the introduction of artificial SEI layers, the architectural design of current collectors, and the electrochemically interfacial kinetic modulations which have shown a great promise in regulating sodium deposition and enhancing battery performance are presented. Lastly, an outlook on the challenges and future directions is provided for achieving safer AFSMBs that are more durable, and capable of delivering higher energy densities, thereby facilitating their integration into practical applications.