Marcus Inverted Electron Transfer in Redox-Mediated Lithiation of Iron Phosphate

Redox-mediated reactions enable charge transfer processes beyond conventional heterogeneous electrode interfaces and have emerged as versatile strategies in energy storage and conversion systems. However, the interfacial electron transfer mechanisms governing these reactions remain elusive. Here, ferrocene derivatives are employed as redox mediators to investigate interfacial electron transfer using the FePO4/LiFePO4 intercalation reaction as a model system, enabling independent control of thermodynamic driving force and electron transfer distance. This approach allows direct experimental observation of a Marcus-type inverted region. More importantly, the dependence of reaction kinetics on driving force can be fundamentally altered by interfacial electronic coupling, such that electron transfer distance becomes a dominant factor governing the overall reaction rate. Molecular stability further emerges as a key factor governing the effectiveness of redox-mediated processes under practical conditions and high utilization of FePO4 (up to 97%) is achieved through rational mediator selection. These results provide a general framework for understanding interfacial electron transfer in complex electrochemical environments and reveal the key factors governing redox-mediated reactions.