Failure mechanisms in redox flow batteries: from apparent capacity decay to active-material loss

Redox flow batteries (RFBs) are promising for long-duration energy storage, but their capacity decay is often a system-level consequence of coupled crossover, electrolyte imbalance, chemical degradation, interfacial deterioration, deposition instability, and operating-condition perturbations. A major challenge in lifetime diagnosis is that observable capacity loss does not always correspond to irreversible active-material loss; in many cases, it first reflects recoverable imbalance or redistribution of active species. This review introduces the concept of the dominant degradation boundary to identify the constraint that first establishes a persistent capacity limitation under a given chemistry and operating window and that controls the subsequent transition from apparent capacity decay to true active-material loss. Using a boundary-amplification-conversion framework, the degradation pathways of all-vanadium, iron-chromium, zinc-bromine, and organic redox flow batteries are compared. The analysis shows that different RFB chemistries exhibit distinct front-end boundaries, amplification processes, recoverability windows, and true-loss pathways. This framework provides a diagnostic basis for distinguishing recoverable and irreversible capacity decay, comparing degradation behavior across chemistries, and prioritizing mitigation strategies for lifetime extension.