Pyridinium electrolytes are promising candidates for flow-battery-based energy storage^1,2,3,4. However, the mechanisms underlying both their charge–discharge processes and overall cycling stability remain poorly understood. Here we probe the redox behaviour of pyridinium electrolytes under representative flow battery conditions, offering insights into air tolerance of batteries containing these electrolytes while providing a universal physico-chemical descriptor of their reversibility. Leveraging a synthetic library of extended bispyridinium compounds, we track their performance over a wide range of potentials and identify the singlet–triplet free energy gap as a descriptor that successfully predicts the onset of previously unidentified capacity fade mechanisms. Using coupled operando nuclear magnetic resonance and electron paramagnetic resonance spectroscopies^5,6, we explain the redox behaviour of these electrolytes and determine the presence of two distinct regimes (narrow and wide energy gaps) of electrochemical performance. In both regimes, we tie capacity fade to the formation of free radical species, and further show that π-dimerization plays a decisive role in suppressing reactivity between these radicals and trace impurities such as dissolved oxygen. Our findings stand in direct contrast to prevailing views surrounding the role of π-dimers in redox flow batteries^{1,4,7,8,9,10,11} and enable us to efficiently mitigate capacity fade from oxygen even on prolonged (days) exposure to air. These insights pave the way to new electrolyte systems, in which reactivity of reduced species is controlled by their propensity for intra- and intermolecular pairing of free radicals, enabling operation in air.

吡啶鎓电解质是基于液流电池的储能的有希望的候选者^1,2,3,4 。然而，人们对它们的充放电过程和整体循环稳定性的机制仍然知之甚少。在这里，我们探讨了吡啶鎓电解质在代表性液流电池条件下的氧化还原行为，深入了解含有这些电解质的电池的空气耐受性，同时提供其可逆性的通用物理化学描述符。利用扩展双吡啶鎓化合物的合成库，我们跟踪了它们在各种电势下的性能，并将单线态-三线态自由能隙确定为描述符，可以成功预测先前未识别的容量衰减机制的发生。使用耦合原位核磁共振和电子顺磁共振波谱^5,6 ，我们解释了这些电解质的氧化还原行为，并确定了电化学性能的两种不同状态（窄能隙和宽能隙）的存在。在这两种情况下，我们将容量衰减与自由基物种的形成联系起来，并进一步表明π-二聚化在抑制这些自由基与溶解氧等微量杂质之间的反应性方面起着决定性作用。我们的研究结果与围绕π-二聚体在氧化还原液流电池^{1,4,7,8,9,10,11}中的作用的普遍观点形成鲜明对比，使我们能够有效地减轻氧气造成的容量衰减，即使是在长时间（数天）暴露的情况下到空气中。 这些见解为新的电解质系统铺平了道路，其中还原物质的反应性由自由基分子内和分子间配对的倾向控制，从而能够在空气中运行。