Hierarchical Flux Competition Governs Electrochemical-Chemical Coupling in Redox-Mediated Batteries

Redox-mediated processes offer an effective strategy to accelerate sluggish kinetics and improve active-material utilization across diverse battery chemistries. However, rational optimization of redox-mediated systems remains challenging due to the lack of a quantitative framework describing the interplay among competing kinetic processes. Here, we establish a kinetic framework for electrochemical-chemical coupling by defining three effective flux descriptors corresponding to interfacial electron transfer, solid-state ion transport, and mediator diffusion. These descriptors, integrated with electrochemical parameters, reveal the hierarchy of competing fluxes and enable analytical determination of current-matching boundaries. The resulting framework provides a systematic approach to identify and manipulate rate-limiting steps, achieving flux-balanced operation with enhanced utilization and reduced polarization. Guided by this framework, a redox-targeting flow battery based on a [Fe(CN)6]3-/4-/Prussian blue catholyte delivers a volumetric capacity of 48 Ah/L at 100 mA/cm2 over 6,500 h, and achieves 27.6 Wh/L in a full-cell configuration. The demonstrated correspondence between kinetic descriptors, regime transitions, and device performance highlights the general applicability of the framework to redox-mediated electrochemical systems. This work reframes battery optimization as a problem of flux hierarchy and provides a generalizable pathway toward simultaneous enhancement of energy density and rate capability.