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 (C x [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.

Few materials are known to electrochemically intercalate trivalent aluminum cations, a charge storage mechanism central to rechargeable aluminum-ion battery electrodes. Here, using the chevrel phase Mo 6 S 8 as a model crystalline electrode material, we couple electrochemical and solid-state 27 Al NMR methods to understand quantitatively the aluminum-ion intercalation mechanism up from the molecular level. Unlike divalent Mg 2+ cations, trivalent Al 3+ cations intercalate simultaneously, as opposed to sequentially, into two cavities within the chevrel framework during galvanostatic discharge. Minimal Al 3+ cation trapping occurs upon deintercalation (<7%). The simultaneous ion intercalation mechanism, as well as slow solid-state ion diffusion, can both be understood in terms of the high charge density of Al 3+ cations. We also reveal that an amorphous surface layer forms upon aluminum-ion desolvation from molecular chloroaluminate anions in the ionic liquid electrolyte. The results yield quantitative molecular-level understanding of aluminum-ion intercalation in a model crystalline electrode material and establish solid-state 27 Al NMR as a powerful characterization tool for rechargeable aluminum-ion batteries.

Solid-state 27Al MAS NMR spectroscopy and DFT calculations reveal that intercalated AlCl4− anions exhibit a wide range of molecular geometries and environments, establishing that the intercalated graphite electrodes exhibit high extents of disorder.

Rechargeable aluminum-graphite batteries using chloroaluminate-containing electrolytes have been the focus of significant research, particularly due to their high-rate capabilities. Engineered graphite electrodes have been shown to exhibit supercapacitor-like rate performance, despite the fact they store charge via the electrochemical intercalation of polyatomic AlCl4 − anions. However, the origins of such rate capabilities are not well understood. Here, using electrochemical techniques, we disentangle quantitatively the diffusion-limited Faradaic, pseudocapacitive, and capacitive contributions to charge storage, revealing that AlCl4 − anions intercalate into graphite with significant pseudocapacitive characteristics due to low ion diffusion limitations. Pristine and mildly exfoliated graphites are compared, where exfoliation resulted in significantly higher pseudocapacitive AlCl4 − intercalation at the highest potential redox pair as well as higher galvanostatic capacity retention at faster discharge rates. The relationships between graphite structure, ion mass transport, and the overall rate of electrochemical AlCl4 − intercalation are discussed. Ion diffusion within the electrolyte phase of the porous electrode is shown to play a key role in controlling the rate of intercalation at higher potentials and faster rates, which can be enhanced by reducing electrode tortuosity. The results establish that chloroaluminate anion intercalation into graphite exhibits non-diffusion-limited pseudocapacitive contributions that are tunable by modifying the graphite structure.