Exploring Aluminum-Ion\n(Al<sup>3+</sup>) Insertion\nin Ammonium Vanadium Bronze (NH<sub>4</sub>V<sub>4</sub>O<sub>10</sub>) for Aqueous Aluminum-Ion Rechargeable Batteries

Rechargeable aluminum-ion batteries (AIBs) are promising\nalternatives\nto lithium-based batteries due to their competitive energy densities.\nAqueous AIBs enable the use of electrode materials with open-framework\nstructures and large interlayer spacings, which facilitate aluminum\nion insertion and extraction. Herein, we explore ammonium vanadate\n(NH<sub>4</sub>V<sub>4</sub>O<sub>10</sub>; NVO) with a high interlayer\nspacing of ∼9.4 Å as a potential positive electrode for\nAIBs. The material demonstrates an initial high discharge capacity\nof 210 mA h g<sup>–1</sup> in 1 M AlCl<sub>3</sub> electrolyte\nbut degrades due to structural distortion from the strain induced\nby the intercalating [Al(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> cation. The effects of ammonium salt additives (NH<sub>4</sub>X:\nX = Cl, F, CH<sub>3</sub>CO<sub>2</sub>, HCO<sub>2</sub>) on the electrochemical\nperformance are investigated, with a detailed focus on NH<sub>4</sub>Cl, demonstrating notable improvements in structural stability over\n1 M AlCl<sub>3</sub>. Ex situ XRD, Fourier transform infrared, X-ray\nphotoelectron spectroscopy, and inductively coupled plasma-optical\nemission spectrometry analyses reveal partial stabilization of the\nNVO structure and enhanced cyclability over a few tens of cycles.\nSolvent composition adjustments with 1 M AlCl<sub>3</sub> as the salt\nshowed similar trends. This work, in addition to identifying optimal\nAl<sup>3+</sup> intercalating hosts, emphasizes the critical role\nof electrolytes in advancing aqueous AIB technologies.