Sn₄P₃@C as a High-Capacity Additive to Reinforce Hard Carbon Anodes for Practical High-Performance Sodium-Ion...

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 Sn₄P₃@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 Sn₄P₃, 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.