Organic Multiple Redox Semi-Solid-Liquid Suspension for Li-Based Hybrid Flow Battery

Li-based hybrid flow batteries are very promising in the energy storage market for their high cell voltage and scale-up flexibility. However, the low volumetric capacity of catholyte has limited their practical application. A novel concept of organic multiple redox semi-solid-liquid (MRSSL) suspension was proposed and demonstrated by taking advantage of active materials in both liquid and solid phases in the suspension. In this study, high solubility of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in the liquid phase and high reversibility of 10-methylphenothiazine (MPT) composite in the solid phase were employed to develop a high-performance and low-cost organic MRSSL Li-based hybrid flow battery. It achieved a small voltage gap (<0.1 V) between liquid and solid phase, high cell voltage (≈3.4 V) and high energy density (260 Wh L^-1 ). Due to the synergistic interactions between the liquid-phase TEMPO and the solid-phase MPT, the viscosity of the MRSSL suspension was significantly reduced. An intermittent-flow-mode test of TEMPO-MPT MRSSL suspension was conducted, which proved that the suspension had an applicable cycling performance with high volumetric capacity (50 Ah L^-1 ). The organic MRSSL suspension concept offers a new approach to increase the volumetric capacity and energy density of Li-based hybrid flow batteries by combining various low-cost solid and liquid organic active materials.