<p>Alloying-type anodes hold promise for sodium-ion batteries (SIBs) due to their high theoretical capacities, yet they suffer from severe capacity fading caused by large volume expansion during sodiation. Identifying a universal structural descriptor that links lattice chemistry with stress resistance and ion transport is therefore critical. Here, we introduce the crystal packing factor (PF) as a predictive metric and validate its effectiveness in layered bismuth compounds (BiOCl, Bi₂O₂S, and Bi₂O₂NCN) spanning a broad PF range. Bi₂O₂NCN, characterized by an atomically sparse and electronically conjugated [Bi₂O₂]²⁺-NCN²⁻ layered framework with the lowest PF (0.613), markedly outperforms Bi₂O₂S (0.694) and BiOCl (0.763). Its open framework provides abundant interlayer free volume and weak steric constraints, thereby buffering mechanical strain and accelerating Na⁺ diffusion. First-principles calculations corroborate that Bi₂O₂NCN shows suppressed stress accumulation and a lower migration barrier of 0.18 eV compared to Bi₂O₂S and BiOCl. Experimentally, Bi₂O₂NCN delivers a high capacity of 486 mA·h·g⁻¹ at 0.3 C (1 C=656 mA·g⁻¹) and maintains 230 mA·h·g⁻¹ after 6600 cycles at 15 C with 93% capacity retention. Multimodal structural characterizations further confirm a reversible conversion-alloying mechanism. These findings establish the crystal PF as a generalizable guideline for designing stress-resistant alloying-type anodes, offering a powerful pathway toward durable, high-performance SIBs.</p>