Chelation-induced anti-Ostwald ripening: Ultrafine bismuth nanocrystals for ultrastable aqueous sodium storage

Aqueous sodium-ion batteries (ASIBs) are gaining attention for their inherent safety and the use of abundant sodium resources. Bismuth (Bi) anode, with its high theoretical capacity and low cost, enhances the performance and competitiveness of ASIBs in energy storage applications. However, as a conversion-type material, Bi inevitably undergoes dramatic volume changes during cycling, limiting the structural stability and calendar life of the electrode. Herein, we present a Bi-carbon composite electrode with ultrafine Bi nanocrystals (< 10 nm) uniformly integrated into nitrogen-doped carbon nanofibers (UF Bi@NCF). Despite Bi's low melting point (271 °C), Ostwald ripening of metallic Bi during carbonization (750 °C) is effectively suppressed by incorporating polyacrylic acid as a chelating polymer in the electrospun Bi(III)/polyacrylonitrile precursor solutions. The high dispersity of Bi nanocrystals at elevated temperature is attributed to the strong coordination and electrostatic interactions between carboxyl groups and Bi³⁺. This structural refinement significantly reduces localized stress concentrations during sodiation/desodiation. The UF Bi@NCF anode demonstrates a reversible capacity of 237.5 mAh g^-1 at 0.5 C, and negligible capacity decay even after 5,700 cycles at an extremely high current rate of 20 C for ASIBs. These findings highlight the potential of the anti-Ostwald ripening effect in enhancing the stability and performance of metal-carbon composite electrodes, providing valuable insights into the design of advanced materials for next-generation aqueous batteries.