(592a) Ionic and Electronic Charge Storage Mechanisms in Crystalline Positive Electrode Materials for Rechargeable Aluminum Batteries

Rechargeable aluminum metal batteries are an emerging energy storage technology with great promise: aluminum is high capacity, low cost, earth abundant, environmentally friendly, and inherently safe. Despite these opportunities, their technological development has been hindered by fundamental challenges associated with aluminum electrochemistry. Few electrolytes enable the reversible electrodeposition of aluminum metal at room temperature, while few positive electrode materials have been demonstrated that are both high performance and electrochemically compatible in those electrolytes.

Here, recent progress will be discussed in the development and characterization of crystalline transition metal compounds and organic structures as positive electrode materials for rechargeable aluminum metal batteries. Molecular-scale understanding of their ionic and electronic charge storage mechanisms will be elucidated, revealed by a combination of electrochemical, spectroscopic, and diffraction methods. In the crystalline chevrel phases Mo₆S₈ and Mo₆Se₈, we use solid-state nuclear magnetic resonance (NMR) spectroscopy to reveal that aluminum ions intercalate simultaneously within two cavities and that the chevrel structure undergoes reversible electrochemical anionic redox upon ion intercalation. Unusually, electrons are transferred exclusively to the anionic framework, which we resolve at the molecular scale and show that it occurs preferentially to a specific crystallographic chalcogen site, while the transition metal clusters (Mo) remain remarkably electronically insulated. In crystalline organic structures, we demonstrate for the first time a rechargeable aluminum battery using tetrathiobarbituric acid (TTBA) as the positive electrode and study its ionic and electronic charge storage mechanism. In particular, two-dimensional solid-state dipolar-mediated NMR correlation experiments, which probe sub-nanometer proximities, are used to elucidate interactions between the intercalating ions and specific moieties of the crystalline organic frameworks. Overall, the results suggest material design principles aimed at realizing improved transition metal and organic electrode materials for multivalent-ion batteries.