Self‐Discharge Behavior of Graphitic Cathodes for Rechargeable Aluminum Batteries

Abstract

AbstractSelf‐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.