Revealing the Importance of Phase and Morphology Dynamics in the Excellent Long-Term Cycling Stability of Flower-Like BiOCl Sodium-Ion Anodes

Bismuth oxychloride (BiOCl) is a low-cost, nontoxic anode material for sodium-ion batteries (SIBs), offering a theoretical capacity of 308 mAh g⁻¹. However, its practical viability is limited by significant capacity loss in early cycles, followed by a protracted recovery. Herein, a unique hierarchical flower-like morphology of BiOCl is employed to enhance the long-term cycling performance. This electrode delivers 306.6 mAh g⁻¹ after 1200 cycles at 200 mAh g⁻¹, while also exhibiting excellent rate capability, retaining 98.4% of its capacity when the current density is increased from 50 to 2000 mA g⁻¹. Operando X-ray diffraction, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, and transmission electron microscopy reveal a sequential sodiation mechanism involving Bi, NaBi, and Na₃Bi formation. Capacity loss is strongly correlated to incomplete NaBi formation. To address this, an electrochemical activation protocol is developed, introducing voltage holds during initial cycles. This accelerates the transformation of flower-like Bi to a stable nanostructured sponge-like morphology, decreasing the capacity drop and expediting capacity recovery. SEM confirms this morphology forms as early as cycle 10 under activation, compared to cycle 50 under standard cycling conditions. These findings provide insight into the phase and morphology dynamics governing the material's long-term behavior, providing a framework to realize high-performance, long-cycle-life BiOCl anodes for SIBs.