A Biomimetic Elastic SEI Film Enables Micron‐Sized Bismuth Anodes for Wide‐Temperature Sodium‐Ion Batteries

Micron‐sized bismuth (µ‐Bi) is a promising high‐capacity alloy anode for sodium‐ion batteries, but its ∼250% volume expansion causes particle pulverization and continuous SEI destruction, especially at low temperatures. Inspired by the alternating hard inorganic/soft organic layers of seashells, this work in situ constructs a biomimetic elastic SEI on µ‐Bi via microscopic solvation engineering. The G2/NaBF 4 electrolyte is designed to promote the formation of a flexible polyether‐like organic component and reinforcing NaF‐containing inorganic domains through complementary interfacial decomposition pathways, thereby constructing a mechanically adaptive organic–inorganic SEI. The biomimetic SEI has a low Young's modulus of 334 MPa and a broad elastic transition, efficiently dissipating volumetric stress. Consequently, the µ‐Bi anode delivers 100% capacity retention after 450 cycles at 5C and 25°C, stable cycling at 9.49 mg cm −2 , and excellent wide‐temperature rate/cycling performance at −40°C. This biomimetic interfacial design offers a new strategy to overcome volume‐expansion failure in micron‐sized alloy anodes.