Electrochemical Performance and Charge Storage Mechanism of Flavin-like Organic Electrodes for Rechargeable Aluminum Batteries
Authors: Gordon Leo , Miroshnikov Mikhail , Jadhav Ankur , John George , Messinger Robert J. 
Published: December 2020

Rechargeable aluminum batteries represent a beacon of possibilities for sustainable electrochemical energy storage. Aluminum metal is energy dense, exhibiting high volumetric and gravimetric capacities of 8.05 A h cm-3 and 2.98 A h g-1, respectively. These capacities approximate to 75% of the volumetric and 400% of the gravimetric capacities of lithium metal. Additionally, aluminum is the most abundant metal in the Earth’s crust (8.23 wt. %), non-flammable, and benefits from mature mining, refinement, and recycling industries, thus making aluminum a low-cost alternative to lithium anodes in batteries. However, rechargeable aluminum batteries currently suffer from a scarcity of positive electrode materials (cathodes) that are both high performance and compatible with commonly used ionic liquid electrolytes. Organic molecules are derived from renewable and sustainable sources and can be leveraged as cathode materials; when paired with aluminum metal anodes, they result in batteries with earth abundant electrodes that are environmentally viable at scale.

Here, we demonstrate a rechargeable aluminum battery using the organic molecule tetrathiobarbituric acid (TTBA) for the positive electrode and investigate its charge storage mechanism up from the molecular level through nuclear magnetic resonance (NMR) and electrochemical methods. For the electrolyte, a non-flammable, non-volatile ionic liquid was used, composed of AlCl3 and [EMIm]Cl (molar ratio: 1.5:1). The electrochemical performance of these cells was investigated through cyclic voltammetry and galvanostatic cycling techniques to determine reaction reversibility, cell capacity, and rate capability. Chiefly, the electrochemical results display >100 mA h g-1 for over 600 cycles at 100 mA g-1 and approximately 70 mA h g-1 at 500 mA g-1. The Al-TTBA batteries demonstrate long cycle lifetimes, yielding hundreds of cycles with significant capacity retention. This long cycle lifetime is attributed to TTBA’s low solubility in the electrolyte, a result of TTBA’s tetrameric structure.

Molecular-level understanding of the charge storage mechanism was probed through spectroscopic and diffraction analyses, particularly multi-dimensional solid-state NMR measurements of TTBA electrodes conducted at different stages-of-charge. Solid-state 1H single-pulse and 13C{1H} cross-polarization (CP) magic-angle-spinning (MAS) NMR measurements yielded insights into local chemical, electronic, and structural changes that the TTBA undergoes upon ion complexation, while solid-state 27Al single-pulse NMR was used to probe aluminum coordination environments. Two-dimensional (2D) 27Al{1H} dipolar-mediated NMR correlation techniques, which probe sub-nanometer through-space proximities between 27Al and 1H moieties, were used to unambiguously establish the presence of aluminum bound to the electrode structure. X-ray diffraction (XRD) studies demonstrated changes in crystalline structure upon cycling.

Overall, the results establish TTBA as a promising positive electrode material for rechargeable aluminum batteries, as well as yield molecular-level insights into its unique charge storage mechanism that are expected to aid the design of organic structures for multivalent-ion batteries.