Sodium metal anodes for room-temperature sodium-ion batteries: Applications, challenges and solutions

Room-temperature (RT) sodium-ion batteries (SIBs) have gained much attention due to rich sodium resource and low cost for potential application in large-scale energy storage. To date, cathode materials have been well investigated, but anode materials still face long-standing challenges including low capacity and high cost, which have led to comprehensive research efforts in resolving these issues. Among the many candidate anode materials best suited to meet these challenges, Na metal possesses the most future potential: the lowest redox potential, highest theoretical capacity, earth-abundancy and a demonstrated feasibility in previous electrochemical systems. Yet several factors still hinder the applicability of Na metal anodes in future larger scale, such as unstable solid electrolyte interphase, large volume change upon cycling and safety concerns related to the uncontrolled dendritic Na growth. As such, this review summarizes the applications of Na metal anodes before providing an in-depth review of research efforts attempting to solve the aforementioned challenges. Specifically, recently-developed strategies to protect Na metal anodes by electrolyte optimization, artificial solid electrolyte interphase, and electrode structure design are outlined and analyzed in detail. We also highlight recent progresses on Na metal protection based on solid-state batteries and conclude by providing a future outlook on the development of Na metal anodes.