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

ABSTRACT 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 Na 3 V 2 (PO 4 ) 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.