Triarylmethyl cation redox mediators enhance Li–O2 battery discharge capacities

Abstract

Low discharge capacities resulting from electronically insulating Li2O2 film growth on carbon electrodes is a major impediment to Li-O2 battery commercialization. Redox mediation is an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li2O2 film growth and extending discharge lifetimes. However, to continue improving upon prior research, exploration of new classes of redox mediators and discovery of novel selection criteria is required. Herein, we report a new class of triarylmethyl cations which are effective at enhancing discharge capacities up to 26-fold. Surprisingly, we observe that redox mediators with more positive redox mediator reduction potentials, and thus more sluggish kinetics for reaction with oxygen, lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structureproperty relationships for future improvements in redox-mediated O2/Li2O2 discharge capacities. To aid future redox mediator discovery, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and concentrations needed to achieve efficient redox mediation at a given current density. This analysis is expected to guide future redox mediator exploration.

New Reaction Pathway of Superoxide Disproportionation Induced by a Soluble Catalyst in Li‐O2 Batteries

“…The typical heterogeneous reaction between solid Li 2 O 2 and carbon cathodes in Li-O 2 battery can be converted to the one between Li 2 O 2 and dissolved species by using redox mediators (RMs) with enhanced discharge capacities. [6] The dissolved RMs in electrolytes can be firstly reduced or oxidized through electrochemical processes at electrodes. The reduced RMs are oxidized to regenerate by O 2 with the formation of O 2 À , which undergoes disproportionation for Li 2 O 2 during ORR; on the other hand, the oxidized RMs at cathodes chemically convert Li 2 O 2 to O 2 during OER.…”

Recent Progress of Halide Redox Mediators in Lithium–Oxygen Batteries: Functions, Challenges, and Perspectives

“…Up to now, three types of RMsorganic (e.g., vitamin K1 and 2,2,6,6-tetramethyl­piperidinyl­oxyl), organometallic (e.g., iron phthalo­cyanine and heme), and halide redox mediators (HRMs)have been developed. − Among them, organic and organometallic RMs, even in their reduced state, are vulnerable to ROIs and the Li metal anode, while HRMs are relatively stable . Besides, the molecule sizes of HRMs are generally much smaller than those of organic and organometallic RMs, enabling their faster diffusion .…”

Nitrogen-Anchored Boridene Enables Mg–CO2 Batteries with High Reversibility

“…5−7 Nevertheless, their high activities, dendrite formation, and high costs have restricted practical application. 8,9 Comparatively, the Mg candidate is anticipated to offer substantial improvements in the volumetric energy density and battery affordability, due to the use of earth-abundant, highcapacity, and dendrite-resistant Mg−metal anodes. 10−12 However, there are still several issues, including high polarization rate, slow redox reaction kinetics, and poor reversibility, for the Mg−CO 2 batteries.…”