Sodium-rich NASICON-structured cathodes for boosting the energy density and lifespan of sodium-free-anode sodium metal batteries

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 Na3V2(PO4)(3) in the form of Na5V2(PO4)(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(-1) 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(-1). Therefore, the proposed SFA-SMB configuration opens a new avenue for fabricating next-generation batteries with high energy densities and long lifetimes.