Ultrafast ion transport at a cathode–electrolyte interface and its strong dependence on salt solvation

Abstract

To access the full performance potential of advanced batteries, electrodes and electrolytes must be designed to facilitate ion transport at all applicable length scales. Here, we perform electrodynamic measurements on single electrode particles of ~6 nAh capacity, decouple bulk and interfacial transport from other pathways and show that Li intercalation into LiNi0.33Mn0.33Co0.33O2 (NMC333) is primarily impeded by interfacial kinetics when using a conventional LiPF6 salt. Electrolytes containing LiTFSI salt, with or without LiPF6, exhibit about 100-fold higher exchange current density under otherwise identical conditions. This anion group effect is explained using molecular dynamics simulations to identify preferred solvation structures, density functional theory calculations of their binding energies and Raman spectroscopy confirmation of solvation structure. We show that TFSI preferentially solvates Li+ compared to PF6, and yet its preferred solvation structures provide a lower Li+ binding energy, suggesting a lower desolvation energy consistent with ultrafast interfacial kinetics.

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Fig. 1: Design of experiments.
Fig. 2: Kinetics investigation of NMC333 single-particle electrodes in liquid electrolytes using EIS and PITT measurements.
Fig. 3: Kinetic limitations for NMC333 as a function of interfacial reaction rate.
Fig. 4: Macroscopic kinetics of NMC333 composite electrodes in selected electrolytes.
Fig. 5: XPS characterization of the solvent and salt interfacial compositions of cycled NMC333 composite electrodes.
Fig. 6: CN and solvation structures.
Fig. 7: Li binding energy and Raman spectra in various electrolytes.

Data availability

All relevant data are included in the paper and its Supplementary Information.

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Acknowledgements

This work was supported as part of the NorthEast Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award no. DE-SC0012583. Z.D. and S.P.O. acknowledge computational resources provided by the Triton Shared Computing Cluster at the University of California San Diego, and the Extreme Science and Engineering Discovery Environment supported by the National Science Foundation under grant no. ACI-1053575. Z. Du from Oak Ridge National Laboratory is acknowledged for kindly providing the casted NMC333 cathodes. Z.D. also acknowledges discussions with T. Hou, UC Berkeley, and help with MD from C. Jian, York University.

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B.W. and Y.-M.C. initiated and designed the research. B.W. conducted the experiments and electrochemical analysis. P.-C.T. assisted the nanofabrication with FIB. Z.D. and S.P.O. performed the MD simulations as well as DFT calculations. Z.W.L.-H. and L.F.J.P. assisted with XPS analysis. All authors contributed to writing the manuscript under the supervision of Y.-M.C.

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Correspondence to Yet-Ming Chiang.

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Supplementary Information

Supplementary Figs. 1–13, Tables 1–15, methods and refs. 1–4.

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Wen, B., Deng, Z., Tsai, P. et al. Ultrafast ion transport at a cathode–electrolyte interface and its strong dependence on salt solvation. Nat Energy 5, 578–586 (2020). https://doi.org/10.1038/s41560-020-0647-0

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