Mechanically Differentiated Lithium Versus Sodium Ion Storage in an Alloying‐Type Bismuth Anode

Abstract Sodium‐ion battery (SIB) research benefits from lithium‐ion battery (LIB) studies due to Li and Na thermodynamic similarities. However, underneath these thermodynamic considerations lie kinetics factors leading to distinct Na + versus Li + storage mechanisms even within the same electrode materials, which is ascribed to either ionic size variations or diffusion path differences. Herein, the library of such non‐negligible factors is extended by targeting an alloying‐type bismuth anode well known for its high volumetric capacity for both Li + and Na + storage, showing that the mechanical property of key reaction intermediates also plays a role in differentiating its Na + versus Li + storage mechanisms. Applying spatially and temporally resolved in situ transmission electron microscopy to Li + and Na + storage within bismuth, it not only demonstrates the thermodynamic similarity in a step‐wise phase transition but also discloses its kinetic distinctions in morphological and structural integrities determined by Young's modulus of involving reaction intermediates and spatial configuration, which well accounts for the surprisingly observed superior Na + storage performance of Bi anode compared to the inferior Li + storage. Findings here highlight the role of reaction intermediates’ mechanical properties in regulating the overall cycling durability of electrode materials, and guide future material engineering toward performance enhancement, particularly for SIBs.