Molecular-Scale Understanding of Charge Storage Mechanisms in Positive Electrode Materials for Rechargeable Aluminum Metal Batteries

Rechargeable aluminum metal batteries are an emerging energy storage technology with great promise: aluminum has among the highest capacities of common metal electrodes and is low cost, earth abundant, environmentally friendly, and inherently safe. Despite these opportunities, their technological development has been hindered due to 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 exhibit high energy density and cycle life in those electrolytes. Here, recent progress will be discussed in the development and characterization of positive electrode materials for rechargeable aluminum metal batteries, including graphites, organic materials, sulfur, and crystalline transition metal compounds. Molecular-scale understanding of their charge storage mechanisms will be elucidated, revealed by a combination of electrochemical, spectroscopic, diffraction, imaging, and theoretical methods, such as multi-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, in operando X-ray diffraction (XRD), electron microscopy, and density functional theory (DFT). The diversity of electrochemical charge storage mechanisms possible for aluminum battery chemistries will be highlighted. Overall, the results are aimed at developing next-generation rechargeable aluminum metal batteries for diverse energy storage applications.