Grain‐Optimized Copper Current Collectors for Highly Stable Anode‐Free Sodium Batteries

Improving cycling stability and capacity retention is critical for the development of high-energy-density and low-cost anode-free sodium batteries (AFSBs). However, unstable Na plating/stripping and uncontrolled solid electrolyte interphase (SEI) evolution still hinder their practical application. In this work, we investigate the impact of grain-boundary density in the Cu substrate on the performance of AFSBs. The results reveal that higher grain-boundary density Cu exhibits a stronger affinity for sodium and can lower the sodium nucleation energy barriers, facilitating the formation of highly crystalline and densely packed sodium deposits. Moreover, the high surface energy at grain-boundaries strengthens anion adsorption, leading to an anion-rich interfacial solvation structure, which results in the formation of a thin, NaF-rich stable SEI film. Anode-free cells constructed with ultrahigh-grain-boundary density Cu (UGB-Cu) and Na₃V₂(PO₄)₃ (NVP) cathode achieved stable cycling for 800 cycles at 5C with an average coulombic efficiency (CE) of 99.97%. This work elucidates the dual role of grain-boundary in improving both substrate-sodium affinity and interfacial SEI chemistry, highlighting grain-boundary engineering of Cu foils as a practical and scalable pathway toward high-performance AFSBs.