Materials Compatibility in Rechargeable Aluminum Batteries: Chemical and Electrochemical Properties between Vanadium Pentoxide and Chloroaluminate Ionic Liquids

To demonstrate the importance of electrode/electrolyte stability in rechargeable aluminum (Al) batteries, we investigate the chemical compatibility between vanadium pentoxide (V2O5), a proposed positive electrode material for Al batteries, and the common chloroaluminate ionic liquid electrolytes. We reveal that V2O5 reacts with both the Lewis acidic (Al2Cl7-) and the Lewis neutral species (AlCl4-) within the electrolyte. The reaction products are identified using a combination of electrochemical analyses, Raman spectroscopy, liquid-state and solid-state nuclear magnetic resonance (NMR) spectroscopy, and density functional theory (DFT) calculations. The results establish that V2O5 chemically reacts with Al2Cl7- to form vanadium oxychloride (VOCl3) and amorphous aluminum oxide. V2O5 also chemically reacts with AlCl4- to produce dioxovanadium chloride (VO2Cl) and a new species of metavanadate anion coordinated with aluminum chloride (AlCl3VO3-). These products furthermore exhibit electrochemical redox activity between V5+ and V2+ oxidation states. Our results have significant implications when interpreting the electrochemical properties and mechanisms of rechargeable Al-V2O5 batteries.