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Rechargeable sodium metal batteries (SMBs) have emerged as promising alternatives to commercial Li-ion batteries because of the natural abundance and low cost of sodium resources. However, the overuse of metallic sodium in conventional SMBs limits their energy densities and leads to severe safety concerns. Herein, we propose a sodium-free-anode SMB (SFA-SMB) configuration consisting of a sodium-rich Na superionic conductor-structured cathode and a bare Al/C current collector to address the above challenges. Sodiated Na 3 V 2 (PO 4 ) 3 in the form of Na 5 V 2 (PO 4 ) 3 was investigated as a cathode to provide a stable and controllable sodium source in the SFA-SMB. It provides not only remarkable Coulombic efficiencies of Na plating/stripping cycles but also a highly reversible three-electron redox reaction within 1.0-3.8 V versus Na/ Na + confirmed by structural/electrochemical measurements. Consequently, an ultrahigh energy density of 400 Wh kg À1 was achieved for the SFA-SMB with fast Na storage kinetics and impressive capacity retention of 93% after 130 cycles. A narrowed voltage window (3.0-3.8 V vs. Na/Na + ) further increased the lifespan to over 300 cycles with a high retained specific energy of 320 Wh kg À1 . Therefore, the proposed SFA-SMB configuration opens a new

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Two-dimensional (2D) elemental metals have attracted considerable attention owing to their fascinating physical properties and diverse potential applications. The single-atom-thick phases of metallic elements in the periodic table had been...