Novel organic redox-active molecules for flow batteries show stable cycling performance

Organic redox-active molecules (ORAMs) are abundant and diverse, offering significant potential for low-cost and sustainable energy storage, especially in aqueous organic flow batteries (AOFBs). However, ensuring the stability of ORAMs during the charge and discharge process is critical, as side reactions can deactivate them and eliminate their redox activity. Air stability remains a challenge for many ORAMs, complicating their practical use.

Recently, a research group led by Prof. Li Xianfeng and Prof. Zhang Changkun from the Dalian Institute of Chemical Physics (DlCP) of the Chinese Academy of Sciences (CAS) developed novel naphthalene derivatives with active hydroxyls and dimethylamine frameworks that were stable in air and served as effective catholytes for AOFBs. This study, which was published in Nature Sustainabilityshows that these novel ORAMs can achieve long-term stable cycling even under atmospheric air conditions.

ORAMs are unstable and expensive, especially when used without inert gas protection. This can lead to irreversible capacity loss and shortened battery life.

In this study, the researchers synthesized active naphthalene derivatives using a scalable approach that combined in situ chemical and electrochemical methods. This approach simplified the purification process and significantly reduced the cost of molecular synthesis.

In addition, the researchers were able to demonstrate specific structural changes in the naphthalene derivatives during the electrochemical process. The naphthalene derivatives produced in this way have a multiply substituted framework with hydrophilic alkylamine frameworks, which not only protects against possible side reactions but also improves their solubility in aqueous electrolytes.

The 1.5 mol/L naphthalene-based AOFB showed stable cycling performance for 850 cycles (about 40 days) with a capacity of 50 Ah L^-1. Remarkably, even with continuous air flow in the catholyte, the naphthalene-based AOFB could run smoothly for approximately 600 cycles (about 22 days) without degradation in capacity and efficiency, demonstrating that the naphthalene-based catholyte had excellent air stability.

In addition, the researchers scaled up the production of naphthalene derivatives to the kilogram scale (5 kg per pot). Pilot-scale battery stacks containing these naphthalene derivatives achieved an average system capacity of about 330 Ah. They demonstrated remarkable cycling stability over 270 cycles (about 27 days) with a capacity retention of 99.95% per cycle.

“This study is expected to open a new field in the development of air-stable molecules. [technology] for sustainable and air-stable electrochemical energy storage,” said Prof. Li.