Abstract Unlike carbon or inactive matrix‐supported systems, bimetallic Sn‐based materials with active/active elements can potentially mitigate the severe volume expansion of alloying‐type anodes, while maintaining high energy densities. However, the mechanisms governing the buffering effect and long‐term electrochemical stability in such systems are poorly understood. To address this gap, a bimetallic Sn–Bi alloy (SnBi N ) with fine grains of homogeneously distributed Sn and Bi alloys (≈50 nm) is synthesized via cooling rate control. This fine‐scale microstructure effectively alleviates mechanical stress during the full (de)lithiation of Sn and Bi, resulting in a pronounced buffering effect and enhanced structural stability during prolonged cycling. When employed as an anode in lithium‐ion batteries, SnBi N demonstrates a reversible capacity of 542 mAh g −1 at 0.1 A g −1 and long‐term cycling stability (capacity of 650 mAh g −1 after 300 cycles at a discharge/charge of 0.1/0.5 A g −1 ). SnBi N ‐based full cell with a LiNi 0.6 Co 0.2 Mn 0.2 O 2 cathode achieves high gravimetric (520 Wh kg −1 ) and volumetric (1128 Wh L −1 ) energy densities. Finite‐element simulations reveal that the uniform distribution of grain boundaries in SnBi N promotes homogeneous plastic strain and damage distribution, effectively relieving internal energy buildup and suppressing crack initiation. These mechanical insights underscore the importance of interfacial engineering in designing durable alloy anodes.