Self-Healing Mechanism of Bi-Sn Foil Anode with Enhanced Cycling Stability for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted increasing attention as a cost-effective and sustainable alternative to lithium-ion batteries (LIBs) for large-scale energy storage owing to the abundance of sodium and its electrochemical similarity to lithium. However, the development of suitable anode materials remains a key challenge. Alloy-type metals are considered promising anode candidates because they offer high theoretical capacities and multiple electron transfer reactions. In this study, we investigate a Bi-Sn alloy foil anode prepared by rolling to a thickness of 36 μm and punching into 4 mm-diameter discs. The foil is employed directly as the anode without the addition of conducting agents or binders, enabling the intrinsic electrochemical behavior of the active material to be evaluated. Electrochemical tests were performed in Swagelok-type cells using sodium metal as the counter electrode and two different electrolytes: 1 M NaPF₆ in 1,2-dimethoxyethane (DME) and 1 M NaPF₆ in ethylene carbonate/diethyl carbonate (EC/DEC). The Bi-Sn foil demonstrates excellent cycling performance in DME, retaining a capacity of 530 mAh g^-1 (14.84 mAh cm^-2) after 100 cycles at 0.1 Cequivalent to 91.5% of its theoretical capacity. In contrast, rapid capacity fading is observed in EC/DEC, underscoring the critical role of electrolyte chemistry in alloy-type anodes. Morphological analyses reveal that during cycling in DME, the Bi-Sn foil undergoes significant mechanical deformation, including cracking and pulverization into nanoscale domains. However, the fragmented particles spontaneously reconstruct into a porous structurea phenomenon referred to as self-healing. This porous structure maintains electrical connectivity to the current collector, enabling capacity retention. These findings demonstrate that pulverization is not inherently detrimental to alloy-type anodes; rather, it can be mitigated by using an ether-type electrolyte to facilitate self-healing. This strategy offers a new pathway for the development of alloy-type anodes composed of low-melting-temperature metals, such as Bi, Sn, and Pb.