However, even with the use of nanoscale materials, the large volume changes associated with charging and discharging cause the fragmentation of the anode material, [2] decreasing their capacities, and cycling lifetimes. The coating and doping of anode nanomaterials are among the various methods proposed to address this problem. [3] However, these methods are complex and expensive, and the overlayer coated on the anode nanomaterials may hinder the fast transport of carrier ions. Another approach based on the reaction between an anode material and electrolyte has also been reported. [4] This method induces the spontaneous restructuring of a bulk anode material into a 3D porous nanostructure during battery cycling with a glyme-based electrolyte [4b-e] . Although the short diffusion distance in thus-formed nanostructure is beneficial for improving the rate performance of the anode, its energy capacity still rapidly decreases with increasing current rate (C-rate). [3a,b,5] This indicates that a strategy based on reducing the diffusion distance alone is not sufficient to simultaneously improve the energy capacity, rate capability, and cycling stability of alloying anodes.To determine another variable governing the kinetics of carrier-ion diffusion at high C-rates, it is necessary to investigate why the practical capacity of the anode decreases with It is challenging to develop alloying anodes with ultrafast charging and large energy storage using bulk anode materials because of the difficulty of carrierion 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.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202201446.