A Sodium–Antimony–Telluride Intermetallic Allows Sodium‐Metal Cycling at 100% Depth of Discharge and as an Anode‐Free Metal Battery

Repeated cold rolling and folding is employed to fabricate a metallurgical composite of sodium-antimony-telluride Na2 (Sb2/6 Te3/6 Vac1/6 ) dispersed in electrochemically active sodium metal, termed "NST-Na." This new intermetallic has a vacancy-rich thermodynamically stable face-centered-cubic structure and enables state-of-the-art electrochemical performance in widely employed carbonate and ether electrolytes. NST-Na achieves 100% depth-of-discharge (DOD) in 1 m NaPF6 in G2, with 15 mAh cm-2 at 1 mA cm-2 and Coulombic efficiency (CE) of 99.4%, for 1000 h of plating/stripping. Sodium-metal batteries (SMBs) with NST-Na and Na3 V2 (PO4 )3 (NVP) or sulfur cathodes give significantly improved energy, cycling, and CE (>99%). An anode-free battery with NST collector and NVP obtains 0.23% capacity decay per cycle. Imaging and tomography using conventional and cryogenic microscopy (Cryo-EM) indicate that the sodium metal fills the open space inside the self-supporting sodiophilic NST skeleton, resulting in dense (pore-free and solid electrolyte interphase (SEI)-free) metal deposits with flat surfaces. The baseline Na deposit consists of filament-like dendrites and "dead metal", intermixed with pores and SEI. Density functional theory calculations show that the uniqueness of NST lies in the thermodynamic stability of the Na atoms (rather than clusters) on its surface that leads to planar wetting, and in its own stability that prevents decomposition during cycling.