High-Performance Non-Aqueous All-Organic Redox Flow Batteries

Redox flow battery has been widely considered as a promising large-scale stationary energy storage technology to stabilize the power grid and promote integration of renewables. The unique cell structure enables separation of energy and power thus offering tremendous design flexibility for different E/P applications. The limited voltage window in traditional aqueous flow batteries narrows down the intrinsic energy capacity. Therefore, nonaqueous redox flow batteries are being pursued because of the wider voltage window of nonaqueous electrolytes.

The current nonaqueous flow battery research is focused on the flow chemistry study and redox materials development.¹ Among them, all-organic flow batteries is an important type and use organic electroactive materials at both positive and negative sides, taking advantage of the structural diversity and tunability of organic materials.²However, the current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. Especially, development of suitable negative side redox materials is considered more challenging.

Here we report several high performance all-organic redox flow chemistries with cell voltages higher than 2V. These flow cell tests demonstrated remarkable cell efficiencies and high operational current densities that exceed those of most reported non-aqueous flow chemistries. A mechanistic study was carried out to gain fundamental understanding of performance degradation in these flow batteries, indicating that the chemical stability of the charged species in various supporting solvents and salts plays a critically important role in achieving decent flow cell cycling stability. This finding can serve as an important guidance for the selection and design of stable redox electrolyte systems. 

Figure 1. (a) Cyclic voltammetry (CV) and (b) cycling capacity of an all-organic flow chemistry based on 9-fluorenone (negative) and 2,5-di-tert-butyl-1-methoxy-4-[2’-methoxyethoxy]benzene (positive).³