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 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 (EC)-chemical coupling by defining three flux descriptors for interfacial electron transfer, solid-state ion transport, and mediator diffusion. Integrated with EC parameters, these descriptors reveal the hierarchy of competing fluxes and enable analytical determination of current-matching boundaries. The framework identifies and regulates rate-limiting steps to achieve 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 48.6 Ah/L at 100 mA/cm² over 6500 h in a symmetric-cell configuration and achieves 27.6 Wh/L in a full-cell configuration. A ferrocene/LiFePO₄ redox system also achieves 93.91% capacity retention at a practical areal capacity of 3 mAh/cm² over 2800 h. This work establishes a general framework for flux-balanced battery design, enabling simultaneous enhancement of energy density and rate capability.