High Energy Density Asymmetric Tetrathiafulvalene Derivative as a Catholyte for Non-Aqueous Redox Flow Batteries

Abstract

The inherent intermittency challenge associated with renewable energy sources prompts the rapid development of cost-effective, grid-scale energy storage technologies. Redox flow batteries (RFBs) represent a promising solution to this problem owing to their remarkable potential for scalability and the tunability offered by a wide range of available solution-based chemistries. However, to-date, RFB commercialization has been challenged by a lack of long-lived, energy-dense, and earth abundant charge storage materials. Many of these challenges can be addressed with the development of non-aqueous organic redox flow batteries (NAORFBs). Such batteries leverage the wide potential window (> 5.5 V) offered by organic solvents with the synthetic tunability of organic redox cores to create highly energy dense battery systems with electrolytes based on earth abundant materials (i.e., carbon, nitrogen, sulfur, oxygen, etc.). The energy density of NAORFBs can be optimized with strategies such as increasing the solubility of redoxmers, increasing the number of electrons transferred by redoxmers, and increasing the magnitude of the potential of employed redox materials [1]. Of these, solubility can be enhanced through the grafting of polar side groups to a redoxmer, while redox potential can be increased through the attachment of either electron-withdrawing or donating substituents, in the case of catholyte and anolytes, respectively, near or in conjugation with the redox core [2]. Recently, our group has demonstrated tetrathiafulvalene (TTF) as a robust, two-electron active catholyte material in NAORFBs [3]. Namely, TTF exhibits reversible oxidation events at 0.03 and 0.43 V vs. Ag/Ag+ (Figure 1A). Underivatized TTF, which is highly insoluble in many polar organic solvents, can be made miscible in MeCN, carbonates, and ethereal solvents, with the attachment of four PEG3 chains to the TTF core ((PEG3)2-TTF) (Figure 1B). When paired with a low potential Li metal anode, an RFB incorporating 0.5 M (PEG3)2-TTF (1.0 M e- concentration) in 1.0 M LiPF6-EC/EMC supporting electrolyte exhibits overall operating cell potentials of 3.44 and 3.64 V vs. Ag/Ag+. Here we describe our most recent efforts to develop a further optimized, rationally designed asymmetric TTF derivative – (PEG3/PerF)-TTF – incorporating both a perfluorophenyl ring fused directly to the TTF core and two PEG3 side chains (Figure 1B). The electron-withdrawing perfluorophenyl group functions to substantially raise the potential of both redox events to 0.43 V and 0.65 V vs. Ag/Ag+, while the two PEG3 chains allow the material to retain miscibility in polar solvents. The performance of (PEG3/PerF)-TTF has been successfully demonstrated in a battery at an even higher concentration of 0.75 M (1.5 M e- concentration) (Figure 1C). When also paired with a Li metal anode in 1.0 M LiPF6-EC/EMC supporting electrolyte, the battery exhibits exceptional operational energy density of 96 Wh/L at 6 mA/cm2 current density and capacity retention of 83.1% over 16.8 d of continuous operation (99.0% per d). These results further demonstrate TTF as a highly versatile catholyte with remarkable potential for application in NAORFBs.