Smoothing the Sodium‐Metal Anode with a Self‐Regulating Alloy Interface for High‐Energy and Sustainable Sodium‐Metal Batteries

Abstract Because of the large abundance of sodium (Na) as a source material and the easy fabrication of Na‐containing compounds, the sodium (Na) battery is a more environmentally friendly and sustainable technology than the lithium‐ion battery (LIB). Na‐metal batteries (SMBs) are considered promising to realize a high energy density to overtake the cost effectiveness of LIBs, which is critically important in large‐scale applications such as grid energy storage. However, the cycling stability of the Na‐metal anode faces significant challenges particularly under high cycling capacities, due to the complex failure models caused by the formation of Na dendrites. Here, a universal surface strategy, based on a self‐regulating alloy interface of the current collector, to inhibit the formation of Na dendrites is reported. High‐capacity (10 mAh cm −2 ) Na‐metal anodes can achieve stable cycling for over 1000 h with a low overpotential of 35 mV. When paired with a high‐capacity Na 3 V 2 (PO 4 ) 2 F 3 cathode (7 mAh cm −2 ), the SMB delivers an unprecedented energy density (calculated based on all the cell components) over 200 Wh kg −1 with flooded electrolyte, or over 230 Wh kg −1 with lean electrolyte. The dendrite‐free SMB also shows high cycling stability with a capacity retention per cycle over 99.9% and an ultrahigh energy efficiency of 95.8%.