Heterogeneous Electron Transfer with Redox Active Insulating Solid
Authors: Mondal Soumyadip , Freunberger Stefan A 
Published: February 2024

For more than a century, electron transfer kinetics at electrode/electrolyte interface has been studied for dissolved redox active species at conductors (heterogeneous) or amongst dissolved redox species (homogeneous). However, a third major case is almost unexplored: dissolved redox species exchanging charge with redox active insulators. The electron transfer with redox active insulators such as Lithium peroxide (Li2O2), Lithium sulfide (Li2S), Sulfur (S8), Lithium carbonate (Li2CO3) are salient feature and central difficulty of next generation ‘beyond intercalation’ batteries such as metal-air (O2), metal-CO2 and metal-sulfur (Li-S) batteries.
Redox mediation is the key process in electrochemistry with insulators. Mediators shuttle charge between the electron-conducting electrode and the surface of the insulating storage material to drive the discharge or charge reaction by reducing or oxidizing the surface of the redox active insulator. Mediation is now widely accepted to be key to high efficiency, rate capability, and lifetime. It may on the one hand involve external mediators that are added as an electrolyte additive and which have been studied in a wide variety for metal-O2 cells and a few examples for metal-sulfur. On the other hand, there is inherent mediation, where soluble intermediates act as mediators, such as lithium polysulfides in lithium-sulfur batteries. However, surprisingly little attention has been devoted to quantitatively assessing heterogeneous reaction kinetics between mediator and storage material. Work so far clearly shows that the kinetics of this electron transfer step is overall rate limiting and decisive to understand and to control reaction mechanisms towards improving energy efficiency, rate capability, capacity, and cycle life, which underscores the urgency to measure them and to identify governing factors.
Here we systematically assess heterogeneous electron transfer at redox active insulators and identify governing factors. Particular systems we present to investigate are twofold. First, mediated kinetics of (su)peroxide oxidation. Our previous work has strikingly discovered decreasing electron transfer rate as the driving force (mediator redox potential) increases beyond a certain value, which represents a first example of Marcus inverted-region behaviour for heterogeneous reactions1. We are leveraging the predictive value of this finding regarding possibilities to impact maximum kinetics and the occurrence of a separate kinetic parabola for singlet oxygen evolution. Second, mediated kinetics of S8 reduction and Li2S oxidation2. Major tools to assess kinetics include following species concentration over time using UV-Vis-NIR spectroscopy and Scanning Electrochemical Microscope (SECM). Doing so requires deconvoluting spectra of mixtures of mediator oxidation/reduction states even if pure substance spectra cannot be obtained. Particularly, obtaining kinetics in S chemistry requires assessing dynamic equilibria of multiple species, for which we develop chemo-metrics tools. The results afford detailed insights into kinetics, thermodynamics, and reaction mechanisms, allowing them to influence in an informed way.
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