Electrochemical kinetic evolution of electrically neutral redox mediator in electrolyte toward advanced electrochemical energy storage device

Abstract

As a novel energy storage strategy, redox electrolytes are promising for the high-performance electrochemical energy storage devices with high energy density and power density simultaneously. In comparison to redox mediators in the form of ion-pair reactions, electrically neutral redox mediators with excellent stability exhibit distinctive electrochemical behaviors, which have rarely been systematically explored. In this study, we utilized a typical nitrogen-oxygen radical, 4-hydroxy-2,2,6,6-tetramethyl-piperidinooxy (TEMPO-OH) as an example, combining a novel quasi-steady electrochemical measurement and in-situ Raman spectrum to systematically characterize the distribution, diffusion and reaction of radicals during electrochemical processes, finally establish a model for the evolution of electrochemical kinetics. The diffusion of radicals is driven by concentration polarization, whereby a high radical concentration can effectively enhance the capacity and rate performance. Nevertheless, the charge transfer process based on radical exchange and oxidation persists, gradually extending outwards from the electrode/electrolyte interface. As a result, the radical diffusion path is prolonged, and irreversible capacity loss occurs during long-term electrochemical processes, which is detrimental to the enhancement of electrochemical performance. Our research facilitate the development of advanced electrochemical energy storage devices.