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Ahmed et al. Energy Mater. 2025, 5, 500079 Energy Materials
DOI: 10.20517/energymater.2024.209

Article Open Access

The ion-ion correlations in organic ionic plastic

crystal

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Md. Dipu Ahmed , Murillo L. Martins , Mohanad Abdullah , Harmandeep Singh , Allen Zheng , Steven
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Greenbaum , Alexei P. Sokolov 1,4 , Ivan Popov 6,*
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Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA.
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Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
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Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA.
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Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
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Department of Physics and Astronomy, Hunter College of the City University of New York, New York, NY 10065, USA.
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The University of Tennessee - Oak Ridge Innovation Institute, University of Tennessee, Knoxville, TN 37996, USA.
*Correspondence to: Prof./Dr. Ivan Popov, The University of Tennessee - Oak Ridge Innovation Institute, University of Tennessee,
1508 Middle Dr, Ferris Hall, Knoxville, TN 37996, USA. E-mail: ipopov@utk.edu
How to cite this article: Ahmed, M. D.; Martins, M. L.; Abdullah, M.; Singh, H.; Zheng, A.; Greenbaum, S.; Sokolov, A. P.; Popov, I.
The ion-ion correlations in organic ionic plastic crystal. Energy Mater. 2025, 5, 500079. https://dx.doi.org/10.20517/
energymater.2024.209
Received: 10 Oct 2024 First Decision: 10 Jan 2025 Revised: 21 Jan 2025 Accepted: 25 Jan 2025 Published: 28 Mar 2025
Academic Editor: Yuping Wu Copy Editor: Fangling Lan Production Editor: Fangling Lan
Abstract
Organic ionic plastic crystals (OIPCs) are emerging as promising electrolyte materials for solid-state batteries.
However, despite the fast ionic diffusion, OIPCs exhibit relatively low DC conductivity in solid phases caused by
strong ion-ion correlations that suppress charge transport. To understand the origin of this suppression, we
performed a study of ion dynamics in the OIPC 1-Ethyl-1-methylpyrrolidinium bis (trifluoromethyl sulfonyl) imide
[P ][TFSI] utilizing dielectric spectroscopy, light scattering, and Nuclear Magnetic Resonance diffusometry.
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Comparison of the results obtained in this study with the published earlier results on an OIPC with a completely
different structure (Diethyl(methyl)(isobutyl)phosphonium Hexafluorophosphate [P 1,2,2,4 ][PF ]) revealed strong
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similarities in ion dynamics in both systems. Unlike DC conductivity, which may drop more than ten times between
melted and solid phases, diffusion of anions and cations remains high and does not show strong changes at phase
transition. The conductivity spectra in the broad frequency range demonstrate unusual shapes in solid phases with
an additional step separating fast local ion motions from suppressed long-range charge diffusion controlling DC
conductivity. We suggested that in solid phases, anions and cations can jump only between the specific ion sites
defined by the crystalline structure. These constraints lead to strong cation-cation and anion-anion correlations
strongly suppressing long-range charge transport.

Keywords: Organic ionic plastic crystal, ion conductivity, ion-ion correlations

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