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.

To demonstrate the importance of electrode/electrolyte stability in rechargeable aluminum (Al) batteries, we investigate the chemical compatibility between vanadium pentoxide (V2O5), a proposed positive electrode material for Al batteries, and the common chloroaluminate ionic liquid electrolytes. We reveal that V2O5 reacts with both the Lewis acidic (Al2Cl7 –) and the Lewis neutral species (AlCl4 –) within the electrolyte. The reaction products are identified using a combination of electrochemical analyses, Raman spectroscopy, liquid-state and solid-state nuclear magnetic resonance (NMR) spectroscopy, and density functional theory (DFT) calculations. The results establish that V2O5 chemically reacts with Al2Cl7 – to form vanadium oxychloride (VOCl3) and amorphous aluminum oxide. V2O5 also chemically reacts with AlCl4 – to produce dioxovanadium chloride (VO2Cl) and a new species of metavanadate anion coordinated with aluminum chloride (AlCl3VO3 –). These products furthermore exhibit electrochemical redox activity between V5+ and V2+ oxidation states. Our results have significant implications when interpreting the electrochemical properties and mechanisms of rechargeable Al–V2O5 batteries.

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.

Rechargeable aluminum–organic batteries are of great interest as a next-generation energy storage technology because of the earth abundance, high theoretical capacity, and inherent safety of aluminum metal, coupled with the sustainability, availability, and tunabilty of organic molecules. However, the ionic charge storage mechanisms occurring in aluminum–organic batteries are currently not well understood, in part because of the diversity of possible charge-balancing cations, coupled with a wide array of possible binding modes. For the first time, we use multidimensional solid-state NMR spectroscopy in conjunction with electrochemical methods to elucidate experimentally the ionic and electronic charge storage mechanism in an aluminum–organic battery up from the atomic length scale. In doing so, we present indanthrone quinone (INDQ) as a positive electrode material for rechargeable aluminum batteries, capable of reversibly achieving specific capacities of ca. 200 mAh g–1 at 0.12 A g–1 and 100 mAh g–1 at 2.4 A g–1. We demonstrate that INDQ stores charge via reversible electrochemical enolization reactions, which are charge compensated in chloroaluminate ionic liquid electrolytes by cationic chloroaluminous (AlCl2 +) species in tetrahedral geometries. The results are generalizable to the charge storage mechanisms underpinning anthraquinone-based aluminum batteries. Lastly, the solid-state dipolar-mediated NMR experiments used here establish molecular-level interactions between electroactive ions and organic frameworks while filtering mobile electrolyte species, a methodology applicable to many multiphase host–guest systems.

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.