(457a) Electrochemical Intercalation of Protons into Fluorine-Doped Molybdenum Oxide: A Positive Electrode for Rechargeable Aqueous Aluminum Metal Batteries

The global demand for low cost, energy dense electrochemical energy storage technologies has increased significantly in recent years due to the need to integrate renewable energy sources into the electrical grid and electrify transportation systems. Researchers have been particularly interested in batteries using aqueous electrolytes because of their low cost, safety, environmental friendliness, and ability to scale. However, one key issue with aqueous batteries is that their specific energy densities are lower compared to state-of-the-art non-aqueous systems, a consequence of the low electrochemical stability window of water (1.23 V). Strategies to overcome this challenge include “water in salt electrolytes” (WiSE), which permit higher cell voltages, though high electrolyte costs for WiSE-based batteries remain a challenge. An alternative approach to enhancing energy density while maintaining the advantages of traditional aqueous electrolytes is to pair the aqueous electrolyte with a high-capacity metal anode whose corresponding metal cation is multivalent, enabling multiple electron transfers per electroactive ion. Yet other than zinc, the majority of earth abundant multivalent metal anodes are not electrochemically compatible with water in rechargeable battery systems. Recently, it has been shown that artificial interfaces on aluminum metal anodes can enable reversible cycling in aqueous electrolytes, though much remains to be understood regarding the reversible electrodeposition of aluminum metal in such systems. In addition, few positive electrodes have been successfully paired with them to date, a consequence of the challenges associated with electrochemically intercalating highly charged Al³⁺ cations into electrode host structures.

Here, we report fluorine (F)-doped α-MoO₃ nanowires as a high-capacity positive electrode material for rechargeable aqueous aluminum batteries and study its ion charge storage mechanism. Using a combination of electrochemical measurements, solid-state nuclear magnetic resonance (NMR) spectroscopy, and X-ray diffraction (XRD), we investigate the nature of the electroactive ion, crystal structure changes upon ion (de)intercalation, and electrochemical effects of F-doping the α-MoO₃ electrodes. A high discharge capacity of 350 mAh/g was achieved upon first discharge, which faded upon continued cycling due to a combination of both cathode and anode effects. Solid-state ¹H, ¹⁹F, and ²⁷Al single-pulse and -dipolar-mediated NMR experiments performed at different states-of-charge establish proton intercalation upon discharge, while possible co-intercalation of hydrated aluminum cations was investigated. Quantitative solid-state ¹H NMR measurements were correlated with Coulomb-counting electrochemical measurements to yield further insights into the ion intercalation processes. F-doping is also shown to enhance solid-state ion transport, rate performance, and cyclability of the α-MoO₃ electrodes. Overall, the results establish F-doped α-MoO₃ as a promising proton intercalation electrode material for aqueous batteries, highlight the role of proton intercalation in aqueous electrolytes containing metal salts, and help lay the scientific groundwork towards rechargeable aqueous aluminum metal batteries.