Root-Growth-Inspired Self-Morphology-Evolution of Microsized Bismuth Surrounded by Microsized Hard Carbon for Stabilized Sodium-Ion Storage.

Alloy-type materials are desirable for high-energy sodium-ion batteries. Different from nanoengineering with pre-reserving void space and confined carbon coatings, microsized particles promise high specific/volumetric capacities, easy manufacturing, and low cost but are prone to rapid capacity loss. Herein, inspired by the process of "root growth in soil", microsized Bi particles (µm-Bi, as "seeds") surrounded by microsized hard carbon particles (µm-HC, as "soil") are ingeniously dispersed through a simple mixing approach. This design utilizes the morphological self-evolution of µm-Bi into Bi-nanonetworks between dispersed µm-HC during repeated (de)sodiations, leading to a stable capacity retention of 99.8% for 2000 cycles, higher than that of the µm-Bi electrode (7.2%) at a high mass loading of 5.5 mg cm. The interconnected Bi-nanonetworks and µm-HC particles provide continuous electron pathways and facilitate electrolyte infiltration, which effectively boosts electrical contact, stable cycling, and high-rate capability. Especially, the hybrid BiHC (optimized weight ratio) thick-film electrode shows boosted comprehensive electrochemical performance, superior to HC and µm-Bi electrodes. The BiHC||NaV(PO) full cell, assembled without any pre-treatment, delivers 4500 stable cycles. This nature-inspired strategy provides a simple yet practical approach for employing the electrochemically driven evolution of micro-sized active materials and realizing high specific/volumetric capacities, fast kinetics, and long-term cycling stability.