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

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 Na₃ V₂ (PO₄ )₂ F₃ 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%.