Self-Discharge Behavior of Graphitic Cathodes for Rechargeable Aluminum Batteries

Self-discharge, which is associated with energy efficiency loss, is a critical issue that hinders practical applications of rechargeable aluminum batteries (RABs). The self-discharge properties of two commonly-used RAB positive electrode materials, namely natural graphite (NG) and expanded graphite (EG), are investigated in this work. EG, which has a wider spacing between graphitic layers and a larger surface area, has a higher self-discharge rate than that of NG. After 12 h of rest, NG and EG electrodes retain 74% and 63% of their initial capacities, respectively, after charging up to 2.4 V at 0.3 A g⁻¹. Operando X-ray diffraction, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy are employed to study the self-discharge mechanism. The self-discharge loss is related to the spontaneous deintercalation of AlCl₄⁻ anions from the graphite lattice charge-compensated by Cl₂ gas evolution at the same electrode and can be restored (i.e., no permanent damage is caused to the electrodes) in the next charge-discharge cycle. It is found that the charging rate and depth of charge also affect the self-discharge properties. In addition, the self-discharge rates of NG in 1-ethyl-3-methylimidazolium chloride–AlCl₃ and urea–AlCl₃ electrolytes are compared.