Structural and Compositional Optimization of Sn–Bi Anodes for Sodium-Ion Batteries

The growing demand for energy storage solutions has increased efforts to identify rechargeable batteries that simultaneously deliver long cycle life, high energy density, cost-effectiveness, and safety. Although lithium-ion batteries dominate the market, their dependence on localized lithium resources creates sustainability concerns. Sodium-ion batteries are emerging as an alternative, taking advantage of sodium’s abundance and lower cost while operating on a working principle closely related to lithium-ion batteries. Their overall performance is determined by their components where the anode is a key element that influences reversible capacity, conductivity, ion diffusion, and structural integrity.

Tin-based anodes have attracted interest for sodium ion batteries because they offer a high theoretical capacity (847 mAh g⁻¹) but face a significant challenge, they suffer from huge volume expansion during cycling, leading to mechanical degradation and reduced lifespan. One effective strategy to mitigate this issue is combining tin with other elements to enhance structural stability and electrochemical performance.

In this work, we explore bismuth as a complementary element for tin-based anodes. Bismuth not only offer strong electrical conductivity but also poses additional advantages, including high theoretical capacity (385 mAh g⁻¹), and favorable ion diffusion kinetics. A Sn–Bi composition may improve mechanical stability by spreading stress throughout the formation and decomposition of sodium-tin and sodium-bismuth phases, thereby limiting cracking and preventing loss of electrical contact.

We synthesize thin films of Sn–Bi composite via electrodeposition. This method eliminates complications from binders, allowing for direct evaluation of the anode material’s electrochemical behavior without interference from additional components. Understanding bismuth’s structural evolution during cycling and its phase interaction mechanisms with tin is essential for designing durable, high-rate anodes. By applying these effects, we aim to develop anodes that combine the strengths of both materials for advanced sodium-ion batteries.