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

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.