Molten-Alloy-Derived Na–Sn Interpenetrating Framework as Ultra-stable Anodes for High-Energy Sodium Metal Batteries

Sodium metal provides exceptionally high theoretical capacity but suffers from poor cycling stability due to uncontrollable sodium deposition. Alloy-type anodes with defined structural frameworks offer a promising route to stabilize Na plating. Here, we report a molten-alloy-derived Na-Sn composite anode (NaSn-10) composed of a continuous Na matrix interpenetrated by a three-dimensional Na15Sn4 network. A simple melt alloying and rolling process induces spontaneous formation of finely dispersed Na15Sn4 domains, which act as a mechanically robust and highly sodiophilic framework. Selective Na extraction reveals an interconnected Sn-rich skeleton, confirming the internal topology of the composite structure. The strong coupling between the Na matrix and the embedded Na15Sn4 network accelerates Na nucleation, increases the exchange current density by nearly 1 order of magnitude relative to pristine Na, and induces uniform Na plating. Symmetric cells with NaSn-10 exhibit stable cycling for hundreds of hours at 0.5-1.0 mA cm-2 with low polarization. In situ optical observation further demonstrates compact, laterally uniform Na deposition, in contrast to the filament-type growth observed on bare Na. Full cells paired with high-loading Na0.9Ni0.45Mn0.55O2 deliver ∼120 mAh g-1 and retain capacity over 200 cycles at 200 mA g-1. This study establishes a structurally engineered Na-Sn alloy architecture as a scalable pathway toward stable and practical sodium metal anodes.