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

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 Mo4+/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.