High energy density 3V-class redox flow battery using LiFePO4 and graphite with organic bifunctional redox mediators

Abstract

Recently, the demand for expanding stationary energy storages has been increasing to utilize natural resources and energy more effectively. Redox flow batteries are considered to be promising for stationary use. However, their energy densities need to be increased. Herein, we propose a novel 3 V-class redox-flow battery composition containing the solid active materials of lithium-ion secondary batteries with redox mediators in each anode- and cathode-side. In our system, the active materials are charged and discharged without direct contact with the electrodes by introducing redox mediators in a non-aqueous electrolyte. Since the capacity is not limited by the solubility of the active material in the electrolyte, this system makes it possible to dramatically improve the energy density. For the negative electrolyte, we focused on aromatic compounds and lithium solutions in ether solvents, because lithium can reduce graphite. Biphenyl exhibited the bifunctionality of charging and discharging graphite, and succeeded in realizing a 3 V-class battery. In this study, we report the results of the experiment that proves our concept of the novel flow battery using the bifunctional mediators tetrathiafulvalene (TTF) and biphenyl, and the solid active materials LiFePO4 and graphite.