Rethinking sodium-ion anodes as nucleation layers for anode-free batteriesElectronic supplementary information (ESI) available. See DOI: 10.1039/c8ta05911j

Here we report a room-temperature sodium metal battery, where the sodium initially stored in a Na 3 V 2 (PO 4 ) 3 cathode is plated, upon charging, onto an aluminum current collector coated with a thin nucleation layer. To maximize the battery performance, conventional sodium-ion anode materials, including non-graphitized carbons and sodium-alloying metals, were evaluated as nucleation layers to facilitate stable electroplating of sodium metal. Among several materials studied, carbon black and bismuth showed the highest sodium plating-stripping coulombic efficiencies in half-cell testing, averaging 99.9% and 99.85%, respectively, over 50 cycles at 0.5 mA cm −2 . Building on these findings, anode-free cells with Na 3 V 2 (PO 4 ) 3 cathodes were assembled in a discharged state, demonstrating energy densities up to 318 W h kg −1 at 0.25 mA cm −2 (∼C/6), a first-cycle coulombic efficiency up to 92%, a stable discharge voltage at 3.35 V, an average round-trip energy efficiency of 98%, and a capacity retention of 82.5% after 100 cycles at 0.5 mA cm −2 (∼C/3). With its unique performance highlighted in this work, the anode-free sodium battery emerges as a low-cost, high-performance option for stationary electric storage. The anode-free sodium battery with a nucleation layer is presented as a low-cost, high-performance option for stationary electric storage.