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Ahmed et al. Energy Mater. 2025, 5, 500079 https://dx.doi.org/10.20517/energymater.2024.209 Page 11 of 13

CONCLUSIONS
We investigated ion dynamics in the plastic crystal [P ][TFSI] across a broad frequency and temperature
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[21]
range, and compared it with [P ][PF ] from our previous study . Despite the completely different
1224
6
chemical structures of these OIPCs, we observed strong similarities in ion dynamics in both systems and
hypothesize that the proposed scenario of the charge trapping might be common across all OIPCs. In
melted states, both systems present the behavior of regular ionic liquids with a single AC-DC crossover in
conductivity spectra and one relaxation process in light scattering corresponding to the ion rearrangement.
The conductivity of the melts appears to be ~2 times lower than expected from the NE equation, which is
consistent with typical ionic liquids. However, in the solid phases, both systems exhibit significant
suppression of σ , despite the small changes in cation and anion diffusion between phases. Although NMR
DC
data also suggest a drop in mobile fraction of ions in the solid phase of [P ][TFSI], this drop is not sufficient
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to explain the observed drop in conductivity. Meanwhile, the conductivity and light scattering spectra
indicate that at short timescales, ion dynamics are similar to those in ionic liquids, and conductivity
suppression happens at nm length scale. We suggested that observed drop of σ in solid phases of OIPCs is
DC
related to strong ion-ion correlations. Crystalline structures of OIPCs (although disordered) lead to the
effect that anions and cations can transfer only through the specific ion sites resulting in circulating ion
motions without significant charge transport. In turn, it leads to high ion mobility but reduces long-range
charge transport and low σ . Additionally, the ion transport occurs only through the disordered fractions.
DC
In the low-temperature solid phases, the portion of the ordered crystalline fractions increases, leading to the
reduction of the mobile ions and additional suppression in ionic conductivity. Thus, to increase the overall
conductivity of the plastic crystal, the crystallinity and ionic correlations should be suppressed. One possible
[17]
approach is doping OIPCs with a small amount of salt, as demonstrated in . However, further
comprehensive studies are needed to identify the exact mechanism by which salt doping increases
conductivity.

DECLARATIONS
Authors’ contributions
Conductivity measurements: Ahmed, M. D.; Abdullah, M.; Popov, I.
Light scattering measurements: Abdullah, M.; Singh, H.; Popov, I.
PFG-NMR measurements: Zheng, A.; Greenbaum, S.
WAXS measurements: Martins, M. L.; Ahmed, M. D.
Idea development and research supervision: Popov, I.; Sokolov, A. P.
Discussion of the results: Ahmed, M. D.; Martins, M. L.; Abdullah, M.; Singh, H.; Zheng, A.; Greenbaum, S.;
Sokolov, A. P.; Popov, I.

Availability of data and materials
The data supporting the plots and findings in this study are available from the corresponding author upon
reasonable request.

Financial support and sponsorship
This work was supported by the National Science Foundation (awards CHE-2102425 and CHE-2417963).
The NMR measurements at Hunter College were supported as part of the Breakthrough Electrolytes for
Energy Storage (BEES), an Energy Frontier Research Center funded by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences under Award #: DE-SC0019409.

Conflicts of interest
All authors declared that there are no conflicts of interest.