Isotropic Sodiation Behaviors of Ultrafast-Chargeable Tin Crystals

High-rate performance and mechanical stability of anode materials are the two important characteristics that are necessary to develop fast-charging batteries with longevity. In the present study, we demonstrate that both high rate performance and mechanical stability of the anode can be achieved with the Na-Sn battery system. Experiments show that the sodiation rate in crystalline Sn (c-Sn) is 2-3 orders of magnitude faster than that reported for the Li-Si system. Furthermore, this extraordinary rate is nearly the same regardless of the orientation of c-Sn, which can improve the cycle life by retarding the pulverization of c-Sn. Two main microstructural features responsible for the observed characteristics are identified: (1) a transformation from crystalline to amorphous phase occurring at thin layers of c-Sn near the interfacial front and (2) pipe diffusion of Na through sodiation-induced dislocations. In this study, the observed behaviors are explained by elucidating the diffusion kinetics, whereas the associated mechanistic origins are analyzed by resolving the diffusion process of Na⁺ near the Na/Sn interface using atomic simulations.