Initial Electrode Kinetics of Anion Intercalation and De-intercalation in Nonaqueous Al-Graphite Batteries^†

Graphitic materials with intercalated sites are considered as the mostly used positive electrode materials in nonaqueous Al batteries. Unlike the small-size cations, the intercalation/de-intercalation of large-size anions into/out of graphite would induce large volume expansion and micro-structure reconfiguration, leading to unexpected coulombic efficiency in the full cells (<95% within initial several cycles). For understanding the irreversible processes induced by anion intercalation/de-intercalation (AlCl₄^–), here the kinetics of first two cycles for the Al-graphite batteries have been systematically studied. To study kinetics behaviors at representative states, a combined method upon galvanostatic intermittent titration technique and electrochemical impedance spectroscopy has been carried out. The achieved diffusion coefficients of the positive electrodes assembled with different graphite sizes suggest that size effect also plays a critical role in determining the electrochemical kinetics in the mass transport in both electrolyte and graphitic layers as well as in interface reaction. The morphologies and micro-structures of the post-cycled graphite electrodes have been also experimentally studied, which also well supports the irreversible intercalation/de-intercalation behaviors in graphite electrodes. The results offer a significant platform to well understand the essential factors in tailoring coulombic efficiency from a kinetic view, which would be helpful in promoting the graphite electrodes in Al batteries.