Development of Redox Flow Battery Based on Lithium Ion Secondary Battery System
Authors: Nariyama Honami , Ito Shuji , Okada Yuka , Inatomi Yuu , Fujimoto Masahisa 
Published: February 2020

Recently, in the context of concerning resource depletion and leveling electrical load, the demand for expanding stationary energy storages have been increasing to utilize natural resources and energy more effectively. Redox flow batteries are considered as promising ones other than lead-acid or sodium-sulfur ones for stationary use, while their energy densities should be raised for widespread use. The low energy densities of the conventional flow batteries are mainly due to the following two factors; one is that they employ aqueous electrolyte solutions with narrow potential window, leading to their low cell voltages. Another is that solubilities of active materials to the electrolytes are low, leading to their low specific capacities. As the solution of these problems, we developed a novel type of the redox flow batteries based on the original concept that can increase their energy densities dramatically. By using a nonaqueous electrolyte, the potential window becomes wider so that the cell voltage can be higher than 3V. Furthermore, the cell utilizes solid active material of Li-ion battery set in, not dissolved in, the electrolyte. Therefore, the energy density can be dramatically improved for general redox flow batteries whose capacity is limited by the solubility of the active material in the electrolyte. In our system, active materials are charged and discharged without direct contact with current collectors by introducing redox molecules in a non-aqueous electrolyte. There have been no reports of batteries having such a mechanism for both positive and negative electrodes and having cell voltages of 3V or higher. In this study, we report the results of the experiment that prove our concept of the new flow battery. We have to select redox molecule species, solid active materials, and a Li-ion conductive separator that can divides electrolytes in the positive electrode side and the negative one to demonstrate our concept. Furthermore, the redox molecule and the solid active material need to have different redox potential so that the redox molecule can mediate electron transfer between current-collector and active material. Such redox mediators are required to be dissolved to nonaqueous electrolyte and to be reduced and oxidized repeatedly. We focused on tetrathiafulvalene (TTF) as a positive redox mediator, which has been reported as an positive active material of organic batteries and can be expected to be durable for redox reactions because they have π-conjugated electron cloud on the molecular plane. Cyclic voltammetry confirmed the two-step reversible redox reactions（E1 = 3.3 V, E2 = 3.6 V vs. Li/Li+）. Since olivine type lithium iron phosphate has its equilibrium potential between the two-step redox potentials of TTF, we selected TTF and LiFePO4 as a positive-electrode system. On the other hand, in a negative electrode system has, the more negative potential enables the higher cell voltage. We employed biphenyl as a negative redox mediator, and graphite as a negative solid active material. This is because we found that biphenyl-Li solution has extremely low potential (E=0.03 V vs. Li/Li+), and can reduce (i.e. insert Li+ ions in) graphite at that potential. Note that it is known that an ether-based solution in which a certain aromatic compound is dissolved has the ability to dissolve Li metal and its redox potential is low. After confirming that the mediators reacted with the active materials in both electrode systems as designed, we prepared 1M LiBF4/propylene carbonate (PC) with 5mM TTF dissolved in and a LiFePO4 powder immersed in as the positive electrode electrolyte, and with 0.1M biphenyl, 1M LiPF6/2-methyltetrahydrofuran (2-MeTHF) that dissolved Li metal and with a graphite powder as a the negative electrode electrolyte. A garnet type LLZ solid electrolyte was used as the Li-ion conductive separator set between electrolytes in the positive and negative electrode systems. Charge-discharge tests for flow battery cells constructed as above showed that the charge/discharge capacities exceeding the capacity due to charge accumulation on mediator molecules. This suggests that the mediators reduced and oxidized the solid active materials, i.e., enabled charging and discharging of the solid active materials in the flow battery system. At this stage, it turned out that about 22% of the solid active materials was utilized. As a result, we demonstrated that it is possible to operate the new battery and that a basic part of our concept for the novel high-energy density redox flow battery.

Figure 1