Enabling 100C Fast‐Charging Bulk Bi Anodes for Na‐Ion Batteries

It is challenging to develop alloying anodes with ultrafast charging and large energy storage using bulk anode materials because of the difficulty of carrier-ion diffusion and fragmentation of the active electrode material. Herein, a rational strategy is reported to design bulk Bi anodes for Na-ion batteries that feature ultrafast charging, long cyclability, and large energy storage without using expensive nanomaterials and surface modifications. It is found that bulk Bi particles gradually transform into a porous nanostructure during cycling in a glyme-based electrolyte, whereas the resultant structure stores Na ions by forming phases with high Na diffusivity. These features allow the anodes to exhibit unprecedented electrochemical properties; the developed Na-Bi half-cell delivers 379 mA h g^-1 (97% of that measured at 1C) at 7.7 A g^-1 (20C) during 3500 cycles. It also retained 94% and 93% of the capacity measured at 1C even at extremely fast-charging rates of 80C and 100C, respectively. The structural origins of the measured properties are verified by experiments and first-principles calculations. The findings of this study not only broaden understanding of the underlying mechanisms of fast-charging anodes, but also provide basic guidelines for searching battery anodes that simultaneously exhibit high capacities, fast kinetics, and long cycling stabilities.