Nanoconfinement Synthesis of Ultrasmall Bismuth Oxyhalide Nanocrystals with Size-Induced Fully Reversible Potassium-Ion Storage and Ultrahigh Volumetric Capacity

Bismuth oxyhalide (BiOX, X = Cl, Br, I) nanomaterials have attracted enormous attention because their unique layered structure and nano-size effect endows them with fascinating physicochemical properties and wide application potential. However, synthesis of ultrasmall BiOX nanocrystals with sizes down to sub-10 nm remains a considerable challenge and the investigation of such ultrasmall BiOX nanocrystals in alkali-metal ion batteries have not yet been explored so far. The fabrication of ultrasmall BiOX nanocrystals with an average size of 6 nm on reduced graphene oxide by a versatile nanoconfinement strategy is reported here. The obtained ultrasmall BiOCl nanocrystals-based monolithic composites are evaluated as free-standing anodes for potassium-ion batteries (PIBs) and exhibit the highest reversible capacity of 521 mAh g(-1) at 0.05 A g(-1) among all reported Bi-based PIB anodes and ultrahigh volumetric capacity of 1148 mAh cm(-3), along with excellent rate capability (205 mAh g(-1) at 5 A g(-1) ) and extraordinary cycling stability with 94.7% capacity retention after 3000 cycles. More importantly, a fully reversible potassium storage mechanism of BiOCl (BiOCl <-> Bi <-> K3Bi) induced by the ultrasmall nano-size effect is revealed for the first time by detailed characterizations, providing new fundamental insights for boosting the electrochemical performance of nanostructured materials.