A Stable Sodium Metal Battery at −40°C: Multiphase Sodium‐Alloy Skeleton Guided Uniform Deposition and Fast Desolvation

Sodium metal batteries (SMBs) represent a promising candidate for high-energy-density storage. However, their practical implementation is hindered by dendrite growth and structural degradation of sodium metal anode (SMA). These challenges are exacerbated at ultralow temperatures, where sluggish Na⁺ transport and inefficient desolvation lead to severe performance decline. Herein, we introduce five distinct Na-M (M = Sn, Sb, Bi, In, and Ge) alloys into SMA via mechanical rolling, resulting in a multiphase sodium-alloy composite anode (NSSBIG). This design enhances compositional disorder and establishes a stable sodium-based alloy skeleton, which improves sodiophilicity and ion transport kinetics. Theoretical calculations reveal a multiphase synergistic effect among the alloys that accelerates charge transfer and promotes desolvation at -40°C. Benefiting from this tailored architecture, the NSSBIG symmetric cell achieves exceptional cycling stability over 1050 h at -40°C (0.1 mA cm^-2/0.1 mA h cm^-2). When coupled with a Na₃V₂(PO₄)₃ cathode, the full cell retains 97% of its capacity after 825 cycles at -40°C (0.5 C), and a pouch-cell configuration maintains 89.5% capacity retention over 300 cycles at -40°C (0.2 C). This work provides a feasible strategy for developing dendrite-free anodes with rapid desolvation kinetics, establishing a viable pathway toward commercial ultralow-temperature SMBs.