High Coulombic efficiency aluminum-ion battery using an AlCl₃-urea ionic liquid analog electrolyte

In recent years, impressive advances in harvesting renewable energy have led to a pressing demand for the complimentary energy storage technology. Here, a high Coulombic efficiency (∼99.7%) Al battery is developed using earth-abundant aluminum as the anode, graphite as the cathode, and a cheap ionic liquid analog electrolyte made from a mixture of AlCl₃ and urea in a 1.3:1 molar ratio. The battery displays discharge voltage plateaus around 1.9 and 1.5 V (average discharge = 1.73 V) and yielded a specific cathode capacity of ∼73 mAh g^-1 at a current density of 100 mA g^-1 (∼1.4 C). High Coulombic efficiency over a range of charge-discharge rates and stability over ∼150-200 cycles was easily demonstrated. In situ Raman spectroscopy clearly showed chloroaluminate anion intercalation/deintercalation of graphite (positive electrode) during charge-discharge and suggested the formation of a stage 2 graphite intercalation compound when fully charged. Raman spectroscopy and NMR suggested the existence of AlCl₄ ^-, Al²Cl₇ ^- anions and [AlCl₂·(urea)_n]⁺ cations in the AlCl₃/urea electrolyte when an excess of AlCl₃ was present. Aluminum deposition therefore proceeded through two pathways, one involving Al₂Cl₇ ^- anions and the other involving [AlCl₂·(urea)_n]⁺ cations. This battery is a promising prospect for a future high-performance, low-cost energy storage device.