Morphology‐Regulated Structural Dynamics and Stress Mitigation Enable Stable Chloride Storage for Bismuth Oxychloride Cathodes

Layered metal oxychlorides, featuring robust M-O frameworks and high theoretical capacities from conversion-type Cl^- storage, represent a compelling class of cathode materials for Chloride-ion batteries (CIBs). Nevertheless, the large volume fluctuations inherent to conversion reactions often cause structural degradation and poor cycling reversibility. Here, we introduce a morphology-regulated strategy to modulate the structural dynamics of bismuth oxychloride (BiOCl) cathodes. BiOCl materials with pH-dependent morphologies ranging from nanoplates to submicron spheres were synthesized via hydrothermal crystallization, where the evolution was controlled by a dynamic balance between self-assembly and recrystallization. Nanoplates (BiOCl-N) exhibit superior Cl-storage capability, enabled by a high surface area, open mesoporous architecture, and accelerated ion diffusion. In situ XRD and ex situ XPS confirm a highly reversible Bi³⁺/Bi⁰ redox process in BiOCl-N, accompanied by dynamic Bi-O bond reconfiguration coupled with Cl^- migration. In contrast, bulk BiOCl-B and spherical BiOCl-S suffer from sluggish kinetics and incomplete reoxidation. Consequently, BiOCl-N maintains 158.5 mAh g^-1 after 200 cycles at 200 mA g^-1, over twofold the retention of bulk BiOCl-B. Finite element analysis reveals the lowest interfacial stress during cycling, indicating that morphology control effectively mitigates stress-induced collapse. This work underscores morphology engineering as an intrinsic route to stabilize phase transitions and enable high-performance CIBs.