Unravelling kinetic and mass transport effects on two-electron storage in radical polymer batteries

Abstract

Although inorganic and metallic materials have demonstrated great success in energy storage, increasing concerns on resource depletion and potential environmental consequences associated with ore mining and processing have driven the search for sustainable and environmental-friendly alternatives. Organic redox molecules exhibiting multi-electron storage and fast electron transfer kinetics are ideal compounds for sustainable high-energy storage devices with high-power output. Nitroxide radical polymers (NRPs) are the representative materials that could achieve these functions. Nevertheless, most NRP batteries only demonstrated one-electron storage via the redox couple (I) NO⁺/NO˙. Two-electron storage through successive redox couple (I) NO⁺/NO˙ and (II) NO˙/NO⁻ has been observed rarely only under specific electrode and electrolyte conditions with no mechanism presented. Here, we realize two-electron storage in a NRP/Li battery by using polyether-based NRP gel with a common battery condition from which we unravel electron transfer kinetics and the ion transport process in the battery condition. Electrochemical analysis reveals a ten-fold higher heterogeneous electron transfer rate constant (k₀) for the redox couple (I) NO⁺/NO˙ in comparison to (II) NO˙/NO⁻, which agree with the value from Ab initio calculations and the Marcus–Hush theory's prediction. We find out that the high reorganization energy and slow diffusion of Li⁺ for redox couple NO˙/NO⁻ are major causes for its sluggish or absent electron transfer process, which could be improved by altering the chemical composition of NRPs. Our findings imply how the molecular design of polymer electrode materials facilitates high-energy and high-power density storage.