Dissolution of molybdenum current collector as Crucial and Undesired process in aluminum batteries

Abstract

Rechargeable aluminum batteries (RABs) using Lewis acidic aluminum chloride–1-ethyl-3-methylimidazolium chloride (AlCl₃-EMImCl) ionic liquid electrolytes are promising alternative energy storage systems. Molybdenum (Mo), often used as a current collector, is typically considered stable with negligible redox activity in such electrolytes. However, this study shows that Mo reacts with AlCl₃-EMImCl (1.5:1) electrolyte. When Mo-foil or powder is immersed, the initially colorless/yellowish ionic liquid turns red, indicating Mo dissolution. Magnetometry confirms the presence of Mo species with localized unpaired electrons in the red liquid, not found in metallic Mo. UV–VIS spectroscopy reveals Mo³⁺ and Mo⁴⁺ species formation. ICP-OES shows 1.99 ± 0.06 mass-% of Mo dissolves in the electrolyte. Due to Mo's instability, cyclic voltammetry (CV) and galvanostatic cycling with potential limitation (GCPL) show increasing redox activity over cycles, similar to unstable platinum (Pt), with a discharge capacity of ∼136 mAh^.g^-1 at 20 mA^.g^-1 after 100 cycles. X-ray photoelectron spectroscopy (XPS) indicates three oxidation states of Mo^4+/5+/6+ on the aluminum negative electrode, due to Mo-cation migration and adsorption. Covering the Mo current collector with electrochemically inactive Co₃O₄ suppresses Mo reactivity by reducing active Mo surface area. These findings demonstrate Mo's significant impact on AlCl₃-based RABs' electrochemical performance, which is not negligible.