Nanoconfinement of Ultrasmall Bismuth in Hard Carbon Enables Ultrahigh-Capacity Sodium-Ion Batteries at Low Temperatures

Abstract Electrochemical energy storage systems that perform reliably under low temperatures are crucial for applications in transportation, renewable energy buffering, and devices in harsh climates. Alloy-type anodes, particularly bismuth (Bi), are promising for low-temperature sodium-ion batteries (SIBs) due to their high operating potentials and fast Na+ transport. However, challenges such as significant volume changes during cycling hinder their performance. To address this, we preemptively stabilize Bi by exploiting Bi3+-induced ionic bridging with xanthan gum to construct a homogeneous precursor network, followed by carbon locking of sub-10 nm Bi nanoparticles within a hard-carbon scaffold. This design reduces particle migration, mitigates alloying-induced pulverization, and preserves critical Bi–C interfacial contact. The resulting Bi@HC anode shows ultrahigh reversible specific capacity of 520 mAh g–1 at 0.1 A g–1 and long-term stability. When paired with a Na3V2(PO4)3 cathode, the full cell retains ∼250 mAh g–1 after 620 cycles at −40 °C. These results highlight carbon locking as an effective design strategy for durable alloy anodes, enabling sodium-ion batteries with fast charge capability and wide-temperature operation.