Mechanistic Insights into Fast Charging and Discharging of the Sodium Metal Battery Anode: A Comparison with Lithium.

Na metal anode receives increasing attention as a low-cost alternative to Li metal anode for the application in high energy batteries. Despite extensive research efforts to improve the reversibility and cycle life of Na metal electrodes, their rate performance, i.e. electrochemical plating and stripping of Na metal at high current, is underexplored. Herein, we report that Na metal electrodes, unlike the more widely studied Li metal electrodes which survive high current density up to 20 mA/cm2, cannot be fast charged or discharged in common ether electrolyte. The fast charging, namely metal plating, is comprised by severe side reactions that decompose electrolyte into electrochemically inactive Na(I) solid species. The fast discharging, namely metal stripping, is disabled by local Na removal that deteriorates the electrical contact with the current collector. While the fast charging failure is permanent, the capacity loss from fast discharging can be recovered through a restructuring process at a low discharging current which rebuilds the electrical connection. We further reveal that the unsatisfactory rate performance of Na metal electrodes is associated with intrinsic physicochemical properties of Na. This study delineates the mechanistic origins of Na's limitation in fast plating and stripping, and demonstrates the necessity of improving the charging and discharging rate performance of Na metal electrodes.