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Beyond Conversion Chemistry: Unlocking a Cooperative Solid-Solution-Capacitive Sodium-Storage Mechanism in Nickel
Jiaqin Liu1,2, Tongzhen Wang3, Jie Yang3
1State Key Laboratory of Chemical Resource Engineering, College of Chemistry, Beijing University of Chemical Technology, Beijing, 100029, People's Republic of China. jqliu@buct.edu.cn.
None:
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.
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