Demystifying Tunneled Niobium Molybdenum Oxide With Near-Zero-Strain for Hydrogen Bond-Assisted Ammonium Ion Storage

Aqueous ammonium-ion batteries (AAIBs) have emerged as compelling contenders for sustainable large-scale energy storage. However, the advancement is significantly impeded by the dearth of energetic electrode materials and unclear relationship between diffusion kinetics and channel size. Herein, we present a second-level prepared 3D tunnel-structured binary transition metal oxide-niobium molybdenum oxide (Nb₂Mo₃O₁₄)-as a high-performance anode material for AAIBs. Comparative electrochemical evaluations across various metal ion systems (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺) reveal that Nb₂Mo₃O₁₄ exhibits superior storage performance specifically for NH₄ ⁺ ions with a near-zero-strain characteristic. In ammonium acetate electrolyte, Nb₂Mo₃O₁₄ electrode delivers a high specific capacity of 210.9 mAh g^-1, at a current density of 0.2 A g^-1, accompanied by an exceptional capacity retention of 88.9% after 14,000 cycles at 1 A g^-1. Furthermore, experimental and theoretical results demonstrate that the promotion of NH₄⁺ storage under hydrogen bond chemistry is dependent on the matched size of the tunnels. The relatively weak hydrogen bonds promote the efficient motion of bulky NH₄ ⁺ in 3D tunneled Nb₂Mo₃O₁₄. These findings highlight 3D tunnel-like binary transition metal oxides as valuable models for high-performance ammonium-ion storage, paving the way for the development of advanced AAIBs.