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

ABSTRACT 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 Na 3 V 2 (PO 4 ) 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.