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 Na_0.9Ni_0.45Mn_0.55O₂ 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.