Impact of Hierarchical Nanoporous Architectures on Sodium Storage in Antimony-Based Sodium-Ion Battery Anodes

Recently, materials with hierarchical nanoporous architectures have been proposed to enhance the performance of alloy-type lithium-ion battery (LIB) and sodium-ion battery (SIB) anodes. However, the origin of this enhancement has not been elucidated. The present work is aimed at identifying the fundamental mechanism behind this enhanced performance using sodium storage in antimony as a model system. We have found that the amount of sodium reversibly stored in antimony is enhanced by roughly 27% when hierarchical nanoporous antimony with bimodal porosity is used as the anode instead of nanoporous antimony with unimodal porosity. Electron microscopy analysis based on energy-dispersive X-ray spectroscopy mapping and computational analysis based on Monte Carlo simulations reveal that the difference in performance originates from mass transport limitations associated with the transfer of sodium ions from the electrolyte to the bulk of antimony. Typically, in hierarchical nanoporous antimony electrodes with bimodal porosity, the diffusion of sodium ions through the bulk of the material is very favorable, resulting in full sodiation of antimony. In contrast, under similar experimental conditions (i.e., the same charge/discharge rate) nanoporous antimony with unimodal porosity is only partially sodiated. The full sodiation of hierarchical nanoporous antimony is favored by large pores, which facilitate the penetration of the electrolyte into the bulk of antimony, reducing the overall effective diffusion length inside the material. Interestingly, in terms of cycle-life, the capacities achieved in these two types of electrode architectures start to decay after 200 cycles with the same decay trend, suggesting that the hierarchical nanoporous architecture does not contribute to the cycling stability; that is, the large pores only improve the charge storage kinetics. These insights will contribute to the development of high-performance alloy-type SIB and LIB anodes.