Sn 4 P 3 @C as a High‐Capacity Additive to Reinforce Hard Carbon Anodes for Practical High‐Performance Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) are promising for large‐scale energy storage, yet their energy density is constrained by anode materials. Commercially mature hard carbon (HC) suffers from low capacity, initial Coulombic efficiency (ICE), and tap density, whereas high‐capacity alloy‐type anodes are subjected to severe volume change and poor reversibility. Herein, we propose an additive engineering strategy to directly upgrade commercial HC by integrating Sn 4 P 3 @C (TPA) through a mild wet‐mixing process. This process preserves the intrinsic sodium‐storage framework of HC while enabling homogeneous incorporation of the additive. In the HC/TPA composite, TPA provides additional conversion/alloying sodium‐storage contribution and improves tap density and electronic transport, whereas the HC matrix maintains structural continuity, buffers the volume variation of Sn 4 P 3 , and stabilizes interfacial evolution. HC/TPA‐30% delivers a reversible capacity of 457.0 mAh g −1 with ICE of 87.9%, and retains a capacity ∼64% higher than HC after 300 cycles at 500 mA g −1 . With tap density increased to 1.1 g cm −3 , its electrode‐level volumetric capacity is 212.5% higher than HC. Moreover, a 1.0 Ah pouch cell delivers cell‐level gravimetric and volumetric energy densities of 135.7 Wh kg − 1 and 206.9 Wh L − 1 , demonstrating the feasibility of a high‐capacity additive for upgrading commercial HC platforms toward high‐volumetric‐energy and long‐life SIBs.