Beyond Conversion Chemistry: Unlocking a Cooperative Solid-Solution–Capacitive Sodium-Storage Mechanism in Nickel Phosphide

Nickel-rich nickel phosphide (Ni₂P) has emerged as a promising sodium-ion battery anode owing to its high theoretical capacity and intrinsic electronic conductivity, yet its charge storage chemistry remains controversial and is often oversimplified as a conversion reaction. Herein, we design a freestanding Ni₂P composite electrode composed of ultrasmall Ni₂P nanocrystals embedded within a phosphorus-doped, graphene-like porous carbon matrix. Comprehensive in-situ and ex-situ analyses unequivocally demonstrate an interstitial solid-solution mechanism, wherein Na⁺ ions reversibly occupy lattice interstitials via (111)-oriented interplanar channels, inducing reversible lattice breathing without phase transformation. This bulk intercalation process is synergistically coupled with a substantial pseudocapacitive contribution, establishing a cooperative dual-mode storage mechanism. Benefiting from this solid-solution-capacitive chemistry, the electrode delivers a high reversible capacity (≈560 mAh g^-1), outstanding rate capability (135 mAh g^-1 at 10 A g^-1), and exceptional long-term stability (263 mAh g^-1 after 2000 cycles). When paired with a Na₃V₂(PO₄)₃@C cathode, the full cell achieves a high-energy density of 245 Wh kg^-1. This work establishes solid-solution-capacitive coupling as a general paradigm for designing high-rate and durable sodium-ion battery anodes.