An Organic Flow Cathode for Flexible Lithium-Organic Redox Flow Batteries
Authors: Park Minjoon , Ryu Jaechan , Cho Jaephil 
Published: February 2020

Organic electrode materials have been considered as promising candidate for large-scale electrochemical energy storage systems (EES) due to low cost, safety, and even biomass orientation. Among various organic materials, the carbonyl-containing molecules have been highlighted to achieve high-performance redox couples with stable and reversible redox reactions. Recently, the Aziz’s group proposed an aqueous-type organic-inorganic redox flow battery using 9,10-antraquinone-2,7-disulphonic acid and bromine, which demonstrated high power density of 0.6 W cm−
2 at 1.3 A cm−
2 with stable capacity retention. However, the system has an intrinsic limitation on the operating potential below 1.5 V like other aqueous redox flow battery systems based on all-vanadium or zinc/bromine, which only allow a narrow selection for new redox-active species. Non-aqueous electrolyte systems appear to be a good choice for high-energy redox battery systems, which can provide higher operating potential (>2 V) and energy density. The researchers in Pacific Northwest National Laboratory (PNNL) studied the state-of-the-art non-aqueous lithium hybrid redox flow battery using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and ferrocene compound as liquid cathode materials in order to attain high energy density and cell voltage.

As for the organic cathode materials, various quinone-based organic materials have been widely studied for lithium ion batteries due to their abundant active sites and fast electron and ionic transfer kinetics. Due to low electrical conductivities of organic electrode materials, large amount of conductive materials and binders is added for the slurry coating process to assemble the coin-cell type batteries. Moreover, the dissolution of quinone based molecules into electrolyte places additional obstacles. Thus, current research has been focused on suppressing these issues in aprotic electrolytes. Interestingly, the synthesis of organic molecule with high solubility is one of the major research topics in the redox battery system. Thus, such features associated with solubility issues can make our tube-type redox batteries a novel flexible energy storage system. The construction of liquid type electrodes could be a new approach for the development of organic-based redox batteries including a novel flexible energy storage system. Herein, we report the first demonstration of liquid type flexible batteries. In addition the structure dependent electrochemical performances of two polycyclic quinone derivatives, 5,12-naphthacenequinone (NAQ) and 1,2-nenzanthraquinone (BAQ), will be studied. This constitutional isomers as liquid cathode materials can reversibly uptake two lithium ions in liquid phase after dissolving in tetraethylene glycol dimethyl ether (TEGDME) containing 1.3 M lithium bis(trifluoromethane sulfonyl) imide salt (LiTFSI). Importantly, BAQ with an asymmetric structure presented higher redox potential during charging and discharging steps than NAQ, showing two pronounced voltage plateaus with stable cycling performance. Tube-type redox batteries using the organic redox-active materials that dissolved in aprotic electrolyte can be easily scale-up and flexible. And the architecture injected with BAQ showed reliable mechanical flexibility under in-situ bending test of 50 mm min−
1, showing energy efficiency of ~80% and coulombic efficiency of ~99.5%.