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Triarylmethyl cation redox mediators enhance Li–O2 battery discharge capacities

Abstract

A major impediment to Li–O2 battery commercialization is the low discharge capacities resulting from electronically insulating Li2O2 film growth on carbon electrodes. Redox mediation offers an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li2O2 film growth and extending discharge lifetimes. As such, the exploration of diverse redox mediator classes can aid the development of molecular design criteria. Here we report a class of triarylmethyl cations that are effective at enhancing discharge capacities up to 35-fold. Surprisingly, we observe that redox mediators with more positive reduction potentials lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structure–property relationships for future improvements in redox-mediated O2/Li2O2 discharge capacities. Furthermore, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and the concentrations needed to achieve efficient redox mediation at a given current density. We expect this analysis to guide future redox mediator exploration.

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Fig. 1: Chemical structures and proposed mechanistic overview of triarylmethy cation redox mediation.
Fig. 2: Computational screening of redox mediators.
Fig. 3: Cyclic voltammograms of various redox mediators.
Fig. 4: Battery discharge curves for Li–O2 cells prepared with 1 M LiOTF in TEGDME and an LFP anode.
Fig. 5: Correspondence of \(E_{{\mathrm{R}}^+/{\mathrm{R}}^{\bullet}}\) and effective outer-sphere redox mediation.

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Data availability

Source data for figures in the main text have been uploaded to the Figshare public data repository and can be accessed at https://doi.org/10.6084/m9.figshare.21719882 (ref. 61). Source data are provided with this paper.

Code availability

All computer codes used in this manuscript are available from the cited references.

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Acknowledgements

K.D.G. acknowledges the support of the National Science Foundation and grant NSF 1954298. K.A. and L.A.C. thank the US Department of Energy for support under contract no. DE-AC02-06CH11357 from the Vehicle Technologies Office, which helped support the work done at ANL. We gratefully acknowledge the computing resources provided on Bebop, a high-performance computing cluster operated by the Laboratory Computing Resource Center (LCRC) at ANL. We also acknowledge the Electron Microscopy Core (EMC) at UIC’s Research Resource’s Center (RRC), where a substantial amount of post-discharge characterization was performed.

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Authors

Contributions

E.J.A., M.R.Z., R.A., K.A. and K.D.G. helped with the conception of this manuscript. E.J.A., M.R.Z. and L.A.C. performed the DFT evaluations and analysis of redox mediators. E.J.A. performed the CV experiments and analysis. E.J.A. performed the battery discharge experiments and analysis. E.J.A. and R.A. performed the post-discharge product characterization. E.J.A. and M.L. performed the DEMS experiments and analysis. E.J.A. and K.D.G. analysed crucial data and wrote the manuscript.

Corresponding author

Correspondence to Ksenija D. Glusac.

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Nature Chemistry thanks Javier Carrasco, Kisuk Kang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary discussion, Figs. 1–17, scheme 1 and Tables 1–6.

Supplementary Data 1

Additional redox mediator CV data.

Supplementary Data 2

CV and UV–vis titration data for chemically and electrochemically derived RO–OR species.

Supplementary Data 3

CV data for outer-sphere redox mediation experiments.

Supplementary Data 4

CV data for the Li+-coupling of outer-sphere redox mediation.

Supplementary Data 5

Scan rate-dependent peak CV current data.

Supplementary Data 6

CV data fitting in Ar-purged solutions.

Supplementary Data 7

CV data fitting in O2-saturated solutions.

Supplementary Data 8

Battery discharge data for Li anodes.

Supplementary Data 9

R+ concentration-dependent battery discharge data with LFP anodes.

Supplementary Data 10

R+ concentration-dependent battery discharge data with Li anodes.

Supplementary Data 11

Baseline and redox-mediated DEMS data.

Supplementary Data 12

Post discharge Raman data.

Supplementary Data 13

Data for comparison between extracted bimolecular rate constants and redox-mediated battery discharge capacity.

Supplementary Data 14

Unprocessed SEM image from pristine electrode.

Supplementary Data 15

Unprocessed SEM image from discharged electrode.

Supplementary Data 16

Unprocessed SEM image from discharged electrode.

Supplementary Data 17

Unprocessed SEM image from discharged electrode.

Supplementary Data 18

Unprocessed SEM image from discharged electrode.

Supplementary Data 19

Unprocessed SEM image from discharged electrode.

Supplementary Data 20

Unprocessed SEM image from discharged electrode.

Supplementary Data 21

Unprocessed SEM image from discharged electrode.

Source data

Source Data Fig. 2

Output energies (Ha) from DFT calculations of R+, R and RO–OR.

Source Data Fig. 3

CVs presented in Fig. 3.

Source Data Fig. 4

Battery discharge curves presented in Fig. 4.

Source Data Fig. 5

Extracted kinetic rate constants and battery discharge values compared in Fig. 5.

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Askins, E.J., Zoric, M.R., Li, M. et al. Triarylmethyl cation redox mediators enhance Li–O2 battery discharge capacities. Nat. Chem. 15, 1247–1254 (2023). https://doi.org/10.1038/s41557-023-01268-0

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