Dynamic ion-buffering gradient bilayer anode realizes 200 Wh kg^-1 dendrite-free sodium battery

The development of sodium batteries is hindered by dendrite growth and sodium ion loss in conventional anodes under practical operating conditions. We engineer a gradient sodium-tin alloy/sodium bilayer anode through in situ chemical displacement. The upper gradient alloy phase serves as an ion-buffering interlayer that synergistically regulates thermodynamic driving forces and ion-transfer kinetics to achieve dendrite-free morphology. The underlying metallic sodium layer functions as an ion reservoir dynamically compensating for sodium ions to maintain the structural stability of the gradient phase and mitigate the consumption of sodium during long-term cycling. The resulting symmetric cells demonstrate ultralong cyclability exceeding 7000 h at a current density of 3 mA cm^-2. Encouragingly, when paired with a high-loading Na₃V₂(PO₄)₃ cathode (30 mg cm^-2), the full cell cycles stably for nearly 1000 cycles and delivers an unprecedented energy density of 200 Wh kg^-1. This work establishes a materials design paradigm to address fundamental challenges in post-lithium battery systems.