Exploring Aluminum-Ion (Al3+) Insertion in Ammonium Vanadium Bronze (NH4V4O10) for Aqueous Aluminum-Ion Rechargeable Batteries

Rechargeable aluminum-ion batteries (AIBs) are promising alternatives to lithium-based batteries due to their competitive energy densities. Aqueous AIBs enable the use of electrode materials with open-framework structures and large interlayer spacings, which facilitate aluminum ion insertion and extraction. Herein, we explore ammonium vanadate (NH₄V₄O₁₀; NVO) with a high interlayer spacing of ∼9.4 Å as a potential positive electrode for AIBs. The material demonstrates an initial high discharge capacity of 210 mA h g^–1 in 1 M AlCl₃ electrolyte but degrades due to structural distortion from the strain induced by the intercalating [Al(H₂O)₆]³⁺ cation. The effects of ammonium salt additives (NH₄X: X = Cl, F, CH₃CO₂, HCO₂) on the electrochemical performance are investigated, with a detailed focus on NH₄Cl, demonstrating notable improvements in structural stability over 1 M AlCl₃. Ex situ XRD, Fourier transform infrared, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectrometry analyses reveal partial stabilization of the NVO structure and enhanced cyclability over a few tens of cycles. Solvent composition adjustments with 1 M AlCl₃ as the salt showed similar trends. This work, in addition to identifying optimal Al³⁺ intercalating hosts, emphasizes the critical role of electrolytes in advancing aqueous AIB technologies.