Grain boundary engineered aluminum current collector for energy-dense initially anode-free sodium metal batteries

The initially anode-free sodium metal batteries represent promising candidates for high specific energy and safe battery systems, relying solely on cathodic sodium reservoirs. However, the irreversible accumulation of electrochemically inactive "dead" Na and unstable solid electrolyte interphases fundamentally constrains cyclability through rapid active Na depletion. Here, we utilize a grain-boundary gallium-rich polycrystalline aluminum current collector to trigger controlled dissolution of aluminum ions. The dissolved aluminum ions modify the local coordination environment of the electrolyte, thereby perturbing the solvation equilibrium of sodium ions and facilitating sodium ions migration toward the positive electrode for charge compensation. The gallium-rich aluminum current collector enables stable Na plating/stripping for over 2500 cycles. The pouch cell with Na₃V₂(PO₄)₃ delivers 88.7% capacity retention after 100 cycles at 35.1 mA g^-1 in 1 M NaPF₆ in diglyme, based on a nominal capacity of 117 mAh g^-1. In addition, the full cell with a high positive electrode loading (47.4 mg cm^-2) achieves a high specific energy of 201.5 Wh kg^-1, calculated based on all cell components (positive electrode, negative electrode, separator and electrolyte). This work proposes a viable current collector design concept that can be extended to other initially anode-free battery systems.