Molecular-Level Understanding of Ionic & Electronic Charge Storage Mechanisms in Rechargeable Aluminum Batteries Using Sold-State NMR

Rechargeable aluminum-ion batteries are safe and low-cost alternatives to traditional lithium-ion batteries due to the high volumetric capacity (8040 mAh/cm3) of aluminum. However, very few electrode materials have shown reversible aluminum ion intercalation. Intercalation Al3+ ions in these structures leads to poor capacity retention, mainly due to slow solid-state diffusion of the highly charged tri-valent aluminum ions within these crystalline structures. In 2015, aluminum ions were shown to reversibly intercalate/de-intercalate into the Chevrel Mo6S8 phase, though the intercalation mechanism is not well understood. Alternatively, molecular chloroaluminate ions within the ionic liquid electrolyte have also been proven to intercalate within graphite, organic and polymer electrodes. A better understanding of the ion intercalation and electron charge transfer mechanisms into positive electrode materials, including the dynamics and populations of these intercalating ions, is important for the molecular-level design and optimization of next-generation intercalation electrodes for rechargeable aluminum batteries. Here, we investigate ionic and electronic charge storage mechanisms of the electroactive ions (trivalent Al3+, divalent Zn2+, monovalent H+ as well as the molecular chloroaluminate anions) in different rechargeable aluminum battery chemistries such as Al-Mo6X8 (X=S, Se), Al-MoO2.8F0.2, Al-sulfur, Al-polymer, and Al-organic batteries, etc. Our research is mainly focused on these positive electrode materials for use in rechargeable batteries to study the intercalation process of the electroactive ions into crystalline host structures with the help of electrochemical measurements, NMR, and other characterization techniques. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy yields atomic-scale understanding of local environments, structures and dynamics, properties that are intrinsically linked to charge transfer and ion intercalation mechanism. The sensitivity of NMR signals in both solid and liquid regime also makes it an ideal characterization tool for identifying local environments in conversion-based battery chemistries like Al-sulfur. Various multi-dimensional through-space dipolar correlation & dipolar-filtered experiments were established in the NMR facility and utilized for a thorough understanding of different battery chemistries. The reversible electrochemical intercalation of aluminum ions into a model crystalline battery electrode, the chevrel Mo6S8, was quantitatively analyzed for the first time up from the molecular level. We demonstrate a unique reversible electrochemical anionic redox as a charge storage mechanism, which preserves the crystalline framework structure of the Mo6Se8 and does not involve the breaking and forming of chemical bonds, which is fundamentally different than that observed for lithium-ion intercalation into transition metal oxides. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties. Finally, fluorine-doped MoO3 was synthesized to be used at an electrode material in rechargeable aluminum batteries. High reversible specific capacity of 360 mAh/g was achieved for Al-MoO2.8F0.2 system using an aqueous electrolyte. The intercalation mechanism was investigated in detail using 1H, 19F, 27Al ssNMR measurements. A thorough molecular understanding of Al-S batteries was achieved via the use of multi-dimensional ssNMR on the electrode as well as electrolyte samples at different states-of-charge. Overall, the results establish ssNMR as a molecular-level characterization tool to understand working mechanism of various battery electrodes for rechargeable aluminum batteries