Initially anode-free sodium metal battery enabled by strain-engineered single-crystal aluminum substrate with (100)-preferred orientation

Constructing high cycling stability and rate performance under limited or ideally zero sodium excess, namely initially anode-free design, which can obtain the ultimate energy density of sodium metal batteries, is highly desired yet remains challenging. Here, highly ordered and regularly arranged Al(100) single crystal current collector is constructed based on the grain boundary migration theory through a simple high-temperature calcination method, which eliminates the diffusion resistance of Na⁺ migration at grain boundaries, reduces the nucleation overpotential and interface diffusion energy barrier, increases the Na⁺ transfer rate, and exhibits uniform reversible sodium deposition capability. Profiting from the modified current collector surface, the Al(100) electrode can be cycled stably for 500 cycles with a Coulombic efficiency of 99.9% (2 mA cm^-2/2 mAh cm^-2), and its symmetrical cell delivers an adequate Na plating/stripping stability over 2500 h (0.5 mA cm^-2/0.5 mAh cm^-2) and 1500 h (1 mA cm^-2/1 mAh cm^-2). The Al(100) current collector enhances the rate performance of the initially anode-free Al(100)‖Na₃V₂(PO₄)₃ full battery, enabling it to sustain 100 cycles at a high current density of 1.755 mA cm^-2 with a final discharge capacity of 68.0 mAh g^-1 (constant current-constant voltage charging protocol).