Effects of Graphite Structure and Ion Transport on the Electrochemical Properties of Rechargeable Aluminum–Graphite Batteries

Rechargeable aluminum–graphite batteries using chloroaluminate-containing ionic liquid electrolytes store charge when molecular chloroaluminate anions intercalate into graphite. However, the relationship between graphite structure and bulk electrochemical properties is not well understood. Here, we characterize the structure of natural, synthetic, and pyrolytic graphites and analyze their electrochemical performance in aluminum–graphite cells, revealing insights into their charge storage mechanisms, rate capabilities, Coulombic efficiencies, and extended cycling stabilities. Natural graphite exhibited the highest specific capacity at all potentials and the greatest capacity retention during variable-rate galvanostatic cycling. The compositions of the intercalated electrodes (Cx[AlCl4]) were determined coulometrically, and their stage numbers were rationalized by using a hard-sphere model. Variable-rate CV analyses establish that the rates of reversible electrochemical chloroaluminate intercalation in natural and synthetic graphites are effectively reaction-limited at potentials <2.1 V, and neither strongly diffusion- nor reaction-limited above 2.3 V, differing significantly from the diffusion-limited electrochemical intercalation of lithium cations into graphite. The results yield a new understanding of the relationships between graphite structure, ion transport, and electrochemical properties in rechargeable aluminum–graphite batteries and are expected to aid the rational design of graphite electrodes with enhanced electrochemical performance.