Nano-Bi@Hard Carbon Composite Anode for Sodium-Ion Batteries with 385 Wh L^-1 Volumetric Energy Density and Durability

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries (LIBs) for sustainable energy storage, yet their adoption in compact applications is hindered by a low volumetric energy density (VED) or relatively poor structural instability. Herein, we present a nano-Bi@hard carbon (nano-Bi@HC) composite featuring densely packed microspheres with uniformly dispersed Bi nanoparticles (6-18 nm) embedded in a hard carbon matrix, achieving a remarkable VED of 385 Wh L^-1, with exceptional cycling stability (97% retention over 4000 cycles). Utilizing pectin's carboxyl functional groups, Bi³⁺ ions were homogeneously coordinated to enable a high Bi content (86.2 wt %) without agglomeration during carbonization. The nano-Bi@HC anode exhibits an unparalleled volumetric capacity of 1950 mAh cm^-3 under a high areal loading (12.3 mg cm^-2) and compaction density (4.8 g cm^-3). Paired with a Na₃V(PO₄)₃ cathode, the full cell demonstrates a superior performance across temperature ranges, highlighting its practical potential. This study establishes a scalable strategy for designing high-VED and durable alloy-based anodes, bridging critical gaps in SIB technology.