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.