Employing energy from renewable sources, such as solar and wind, will become increasingly important for sustainable development.¹ Organic rechargeable batteries (ORBs) have been receiving a significant amount of attention in the literature for their versatile molecular structure design, sustainability, flexibility and availability from abundant natural sources.² These attributes will aid in the design of more sustainable technologies.

While recent studies have shown significantly improved electrochemical performances, most of the ORBs are not appropriate for practical applications on account of two reasons. The first is the low cycle life observed for organic electrodes, resulting from dissolution or side reactions with the electrolyte. In order to overcome the dissolution of active materials in the electrolyte, a number of investigations have been directed towards conjugating redox-active molecules with polymers or by incorporating them into gel-polymer electrolytes.³ Secondly, and more importantly, very few electrode candidates show the single well-defined plateau voltage profile which is essential for maximizing the energy density of the full-cell. When we compare conventional transition-metal based electrodes to organic ones, the redox-active organic molecules possess numerous redox states, often having multiple well-defined plateaus. Although several studies have shown that functional group substitution induces overall redox voltage shifts, obtaining a single well-defined plateau using multiple redox-states of organic molecules remains a major challenge.

Herein, we report a new strategic direction. By introducing pyromellitic diimide (PMDI) into a molecular triangle, cyclic through-space electron-sharing occurs. This property results in increased cycle life and, more importantly, a single well-defined output voltage. The PMDI triangle exhibits a single well-defined voltage at 2.2 V, compared to two peaks at 2.2 and 1.7 V, or 2 and 2.7 V, for reduction and oxidation, respectively of PMDI monomeric analogues. Our results suggest that an efficient delocalization of electrons within the triangle is important for obtaining a singly well-defined plateau, and hopefully can be extended to other redox-active organic materials.

1. Armand, M.;  Tarascon, J. M. Building Better Batteries. Nature 2008 451,652–657.

2. Morita, Y.; Nishida, S.; Murata, T.; Moriguchi, M.; Ueda, A.; Satoh, M.; Takui, T. Organic Tailored Batteries Materials Using Stable Open-Shell Molecules With Degenerate Frontier Orbitals. Nat Mater. 2008, 10, 947–951.

3. Liang, Y.; Tao, Z.; Chen, J. Organic Electrode Materials for Rechargeable Lithium Batteries. Adv. Energy Mater. 2012, 2, 742–769.