Hypostatic instability of aluminum anode in acidic ionic liquid for aluminum-ion battery

Aluminum-ion batteries are considered to be a promising post lithium-ion battery system in energy storage devices because aluminum is earth-abundant, has a high theoretical capacity, and is of low cost. We report on the chemical activities and stabilities of chloroaluminate anions [Al _n Cl _n+1]^- with aluminum metal using a different mole ratio of AlCl₃ and 1-ethyl-3-methylimidazolium chloride. The morphological changes in the Al metal surface are investigated as a function of dipping time in electrolyte, revealing that the Al metal surface is locally attacked by chloroaluminate anions followed by the formation of a new Al oxide layer with a specific lattice plane and a craterlike surface around the cracking site. The aluminum-ion battery exhibits outstanding cycle life and capacity even at the high C-rate of 3 A g^-1, with a high energy efficiency of 98%, regardless of the differences in the size of chloroaluminate anions.