Interfacial Dirac-Modulated TiN/MXene Heterostructure Enables Decoupled Ion-Electron Transport for Ultrafast Ammonium-Ion Storage.

Aqueous ammonium-ion hybrid pseudocapacitors (AAI-HPCs) demand anodes that unify metallic conductivity, ultrafast NH4 + kinetics, and robust cycling, a feat unattainable with conventional 2D materials due to irreversible restacking, necessitating atomic-precision heterostructure design. Herein, in situ nitrogen-engineered TiN/MXene cascades are developed through hexamine-derived NH3 nitridation, inducing spontaneous N-vacancy formation and epitaxial TiN nucleation, simultaneously preventing MXene restacking while creating expanded ion diffusion highways. Polyvinylpyrrolidone (PVP)-directed interfacial confinement precisely integrates ultrathin Ag-Bi2Te3 nanoplates into the TiN/MXene matrix, where topological Dirac states ensure metallic conductivity while enhancing mechanical robustness. The resulting Ag-Bi2Te3@TiN/MXene heterostructure establishes dual hydrogen-bonded NH4 + coordination sites, combining stable Ti─N─H─N anchoring with Ag-enhanced Te─H─N interactions, collectively reducing diffusion barriers by 33.3% (from 36 to 24 eV). Ex-situ/operando analysis confirms reversible Bi3+/Bi0 and Ag+/Ag0 redox couples operating in concert with strain-adaptive MXene frameworks, achieving exceptional 98.1% capacity retention over 5,000 cycles. Full-cell Ag-Bi2Te3@TiN/MXene//AC (AAI-HPCs) deliver record energy density 79.2 Wh kg-1 (at 800 W kg-1) capable of powering commercial electronics for >100 s, with flexible pouch cells reaching 96.5 Wh kg-1 under mechanical stress-surpassing reported MXene-based NH4 + systems. This work establishes interfacial electron modulation as a universal design paradigm for decoupled ion-electron transport in next-generation AAI-HPCs.