A 3D Hierarchical Host with Enhanced Sodiophilicity Enabling Anode-Free Sodium-Metal Batteries.

Sodium-metal batteries (SMBs) are considered as a compliment to lithium-metal batteries for next-generation high-energy batteries because of their low cost and the abundance of sodium (Na). Herein, a 3D nanostructured porous carbon particle containing carbon-shell-coated Fe nanoparticles (PC-CFe) is employed as a highly reversible Na-metal host. PC-CFe has a unique 3D hierarchy based on sub-micrometer-sized carbon particles, ordered open channels, and evenly distributed carbon-coated Fe nanoparticles (CFe) on the surface. PC-CFe achieves high reversibility of Na plating/stripping processes over 500 cycles with a Coulombic efficiency of 99.6% at 10 mA cm -2 with 10 mAh cm -2 in Na//Cu asymmetric cells, as well as over 14 400 cycles at 60 mA cm -2 in Na//Na symmetric cells. Density functional theory calculations reveal that the superior cycling performance of PC-CFe stems from the stronger adsorption of Na on the surface of the CFe, providing initial nucleation sites more favorable to Na deposition. Moreover, the full cell with a PC-CFe host without Na metal and a high-loading Na 3 V 2 (PO 4 ) 3 cathode (10 mg cm -2 ) maintains a high capacity of 103 mAh g -1 at 1 mA cm -2 even after 100 cycles, demonstrating the operation of anode-free SMBs.