Molecular-Scale Elucidation of Ionic Charge Storage Mechanisms in Rechargeable Aluminum–Quinone 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.