Interplay of crystal phase and morphology in WO₃ for aqueous ammonium ion batteries: Decoupling their respective roles in NH₄⁺ storage performance

The development of high-performance aqueous ammonium-ion batteries (AAIBs) requires a deeper understanding of how a material's crystal phase and morphology govern its charge storage. This study decouples this interplay in WO₃, engineering distinct polymorphs through controlled synthesis. Through precise hydrothermal synthesis and quantitative Rietveld analysis, we engineered distinct WO₃ polymorphs, revealing that a monoclinic-dominant phase (86.4% monoclinic, 13.6% hexagonal) delivers superior performance over other crystal structures. The optimal performance of this monoclinic-dominant WO₃ was achieved in a 2 M (NH₄)₂SO₄ electrolyte, yielding a specific capacity of ~60 mAh/g and exceptional long-term stability with 96% capacity retention over 300 cycles. In a WO₃//graphite full-cell configuration, this material enabled an impressive energy density of 32.5 Wh/kg and a power density of 238 W/kg. The superior kinetics of the monoclinic phase were quantitatively confirmed by its high NH₄⁺ diffusion coefficient, with values of 2.49 × 10⁻⁹ cm² s⁻¹ (anodic) and 2.10 × 10⁻⁹ cm² s⁻¹ (cathodic), significantly outperforming the mixed-phase and hexagonal-rich samples. Furthermore, ex-situ FTIR analysis provided direct evidence of the NH₄⁺ intercalation process, revealing characteristic N-H bonding vibrations within the WO₃ host. Complementary X-ray photoelectron spectroscopy (XPS) further confirmed NH₄⁺ intercalation at the chemical-state level, revealing the emergence of N 1s signals associated with hydrogen-bonded ammonium species and a reversible W⁶⁺/W⁵⁺ redox transition that provides Faradaic charge compensation during cycling. This work definitively establishes the monoclinic phase as the optimal host for NH₄⁺ storage. It provides a critical design principle: long-term cyclability hinges more on structural integrity than on theoretical kinetics alone.