From Squaric Acid Amides (SQAs) to Quinoxaline-Based SQAs─Evolution of a Redox-Active Cathode Material for Organic Polymer Batteries

Abstract

The search for new redox-active organic materials (ROMs) is essential for the development of sustainable energy-storage solutions. In this study, we present a new class of cyclobuta­[b]­quinoxaline-1,2-diones or squaric acid quinoxalines (SQXs) as highly promising candidates for ROMs featuring exceptional stability and high redox potentials. While simple 1,2- and 1,3-squaric acid amides (SQAs), initially reported by Hünig and coworkers decades ago, turned out to exhibit low stability in their radical cation oxidation states, we demonstrate that embedding the nitrogen atoms into a quinoxaline heterocycle leads to robust two-electron SQX redox systems. A series of SQX compounds, as well as their corresponding radical cations, were prepared and fully characterized, including EPR spectroscopy, UV–vis spectroscopy, and X-ray diffraction. Based on the promising electrochemical properties and high stability of the new ROM, we developed SQX-functionalized polymers and investigated their physical and electrochemical properties for energy-storage applications. These polymers showed remarkable thermal stability well above 200 °C with reversible redox properties and potentials of about 3.6 V vs Li+/Li. By testing the galvanostatic cycling performance in half-cells with lithium-metal counter electrodes, a styrene-based polymer with SQX redox side groups showed stable cycling for single-electron oxidation for more than 100 cycles. These findings render this new class of redox-active polymers as highly promising materials for future energy-storage applications.

Development of the Squaramide Scaffold for High Potential and Multielectron Catholytes for Use in Redox Flow Batteries

“…To test this theory, we synthesized SQA-3 (Figure , R = n- butyl) using a similar strategy as for the SQX molecules involving condensation followed by N-alkylation. Consistent with the work of Hünig and Hansmann, , this class of squaramide showed a reversible redox couple in cyclic voltammetry experiments with an improved oxidation potential of 0.67 V vs Fc/Fc + (Figure a). To test the long-term stability of the resulting radical cation, we subjected this molecule to an oxidative H-cell cycling experiment (cycling between SQA-3 and SQA-3 • + ) as previously described with the earlier class of SQX squaramides.…”

“…This result, showing high levels of stability for what was assumed to be the radical cation of squaramide SQA-3 , is seemingly at odds with the recent finding of Hansmann and coworkers, who found that the radical cation of a very similar squaramide (from Figure , R = Me) showed rapid decomposition in EPR studies with a half-life of around 300 min and which prompted those authors to end their exploration of this class of squaramides …”

“…At the time we initiated this work, these scaffolds had not been explored as active materials for electrochemical energy storage, and structure–property understanding was nonexistent. Very recently and near the conclusion of our efforts, an elegant 2023 report by the Hansmann group showed that SQXs were functional as the redox-active species in a polymeric cathode for Li/organic batteries . While these previously reported SQX derivatives indicate the promise of squaramide-derived materials for charge storage applications, their oxidation potentials remain substantially lower than reported best-in-class catholyte materials, the higher-oxidation potential SQA molecules investigated by the Hansmann group underwent rapid decomposition of the radical cation via an undefined mechanism, and multielectron cycling in conditions relevant to flow batteries has not been achieved.…”

Multivalent Cation Transport in Polymer Electrolytes – Reflections on an Old Problem

“…Moreover, there is plenty of room in the chemical space to tailor organic electrodes to the specific requirements of the desired electrochemical system. [145,[160][161][162][163][164][165]…”

Development of Modular Nitrenium Bipolar Electrolytes for Possible Applications in Symmetric Redox Flow Batteries

“…Even though the use of organic solvents offers an increased potential window, discovery of suitable electrolytes that allow access to extreme redox potentials (< −2 V for reduction − and >+1 V for oxidation, ,− vs Fc/Fc + ) has been challenging due to the general scaling relationship of stability to energy density (i.e., the more extreme the potential, the more reactive the charged species). Therefore, developing electrolytes that balance stability and energy density to optimize the cell voltage remains a significant aim of the community.…”