Abstract
Al metal-organic batteries are a perspective high-energy battery technology based on abundant materials. However, the practical energy density of Al metal-organic batteries is strongly dependent on its electrochemical mechanism. Energy density is mostly governed by the nature of the aluminium complex ion and utilization of redox activity of the organic group. Although organic cathodes have been used before, detailed study of the electrochemical mechanism is typically not the primary focus. In the present work, electrochemical mechanism of Al metal-phenanthrenequinone battery is investigated with a range of different analytical techniques. Firstly, its capacity retention is optimized through the preparation of insoluble cross-coupled polymer, which exemplifies extremely low capacity fade and long-term cycling stability. Ex situ and operando ATR-IR confirm that reduction of phenanthrenequinone group proceeds through the two-electron reduction of carbonyl groups, which was previously believed to exchange only one-electron, severely limiting cathode capacity. Nature of aluminium complex ion interacting with organic cathode is determined through multiprong approach using SEM-EDS, XPS, and solid-state NMR, which all point to the dominant contribution of AlCl 2+ cation. Upon full capacity utilization, Al metal-polyphenanthrenequinone battery utilizing AlCl 2+ offers an energy density of more than 200 Wh/kg making it a viable solution for stationary electrical energy storage.
Highlights
Many developed countries are striving for transfer from fossil-based energy resources towards renewable energy
Nature of aluminium complex ion interacting with organic cathode is determined through multiprong approach using Scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS), X-ray photoelectrons spectroscopy (XPS), and solid-state nuclear magnetic resonance (NMR), which all point to the dominant contribution of AlCl2+ cation
In literature, cycling of phenanthrenequinone group (PQ) exhibited relatively low capacity utilization, well below 50% of the theoretical capacity (257 mAh/g) [10, 13], leading to the conclusion that PQ electroactive moiety can undergo only single-electron reduction. This was in stark contrast with our electrochemical cycling results of a similar AQ compound, which is an isomer of PQ
Summary
Many developed countries are striving for transfer from fossil-based energy resources towards renewable energy. Energy Material Advances difficult due to high charge density and consequent strong interaction with inorganic hosts [6] This can be effectively circumvented by the application of organic materials, which have so far been successfully applied to different multivalent battery systems like Mg, Ca, Al, and Zn [7,8,9,10,11,12]. There is a sevenfold difference between the anode capacity for AlCl4- and AlCl2+, the higher value is still an order of magnitude smaller than the capacity of the metal anode in a cell utilizing Al3+ cations (2980 mAh/g) This states a clear need to utilize monochloroaluminium or move beyond chloroaluminium species if high energy density Al rechargeable batteries are targeted. The electrochemical mechanism is investigated using a multitude of complementary spectroscopic techniques: IR spectroscopy, X-ray photoelectrons spectroscopy (XPS), 27Al solid-state magic-angle spinning (MAS) NMR, and energy dispersive X-ray spectroscopy (EDS)
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