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The role of the electrolyte in non-conjugated radical polymers for metal-free aqueous energy storage electrodes

Nature Materials volume 22, pages 495–502 (2023)Cite this article

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Abstract

Metal-free aqueous batteries can potentially address the projected shortages of strategic metals and safety issues found in lithium-ion batteries. More specifically, redox-active non-conjugated radical polymers are promising candidates for metal-free aqueous batteries because of the polymers’ high discharge voltage and fast redox kinetics. However, little is known regarding the energy storage mechanism of these polymers in an aqueous environment. The reaction itself is complex and difficult to resolve because of the simultaneous transfer of electrons, ions and water molecules. Here we demonstrate the nature of the redox reaction for poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl acrylamide) by examining aqueous electrolytes of varying chao-/kosmotropic character using electrochemical quartz crystal microbalance with dissipation monitoring at a range of timescales. Surprisingly, the capacity can vary by as much as 1,000% depending on the electrolyte, in which certain ions enable better kinetics, higher capacity and higher cycling stability.

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Fig. 1: Schematic of ions’ chao-/kosmotropic character and their effect on the redox reaction of PTAm.
Fig. 2: Rate capability of PTAm composite electrodes with various aqueous electrolytes.
Fig. 3: Mass profiles and transferred water molecules for PTAm during CV with various aqueous electrolytes.
Fig. 4: Apparent molecular weight (Mw′) of the transferred species with various aqueous electrolytes during CV.
Fig. 5: In situ EIS/EQCM-D of a PTAm electrode.
Fig. 6: Coupled mass–charge responses for a partial sine cycle (0 to +10 mV or one-fourth period of the EIS cycle) for the oxidation of PTAm in various electrolytes during EIS.

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All data generated and analysed during this study are included in this Article and its Supplementary Information.

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Acknowledgements

The experimental work was supported by grant DE-SC0014006 funded by the US Department of Energy, Office of Science (T.M., R.M.T. and J.L.L.). The MD simulation work was supported by grant NSF-DMR-2119672 funded by the National Science Foundation, and the Texas A&M Institute for Data Science Career Initiation Fellowship (C.-H.L. and D.P.T.). The use of the Texas A&M University Soft Matter Facility (RRID:SCR_022482) and contribution of P. Wei are acknowledged.

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Authors and Affiliations

  1. Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX, USA

    Ting Ma, Ratul Mitra Thakur & Jodie L. Lutkenhaus

  2. Department of Chemistry, Texas A&M University, College Station, TX, USA

    Cheng-Han Li & Daniel P. Tabor

  3. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA

    Jodie L. Lutkenhaus

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Contributions

J.L.L. and T.M. conceived the study. T.M. developed the experimental procedures, carried out the experiments and analysed the data. T.M. and J.L.L. discussed the results and wrote the paper. R.M.T. performed the electron paramagnetic resonance and gel permeation chromatography tests. C.-H.L and D.P.T. conducted the MD simulation.

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Correspondence to Jodie L. Lutkenhaus.

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Nature Materials thanks Michel Armand 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 Tables 1–4, Figs. 1–45, discussions, references, PTAm synthesis, DFT computational details, MD computational details, kinetic investigations, Jones–Dole B coefficient, EQCM-D details, current collector|polymer film|electrolyte model, equations for anion and cation transfer in the PTAm reaction mechanism, and EIS calculations.

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Ma, T., Li, CH., Thakur, R.M. et al. The role of the electrolyte in non-conjugated radical polymers for metal-free aqueous energy storage electrodes. Nat. Mater. 22, 495–502 (2023). https://doi.org/10.1038/s41563-023-01518-z

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