Page 116 - Read Online

Page 2 of 13 Ahmed et al. Energy Mater. 2025, 5, 500079 https://dx.doi.org/10.20517/energymater.2024.209

INTRODUCTION
Organic ionic plastic crystals (OIPCs) are a promising class of ion-conducting materials that hold great
potential for different energy-related applications. They are found to be suitable for lithium and sodium
solid-state batteries , protic conductive materials , solar cells , in the formation of protective solid
[5,6]
[1-4]
[7,8]
[2]
electrolyte interphase , and many other applications [9,10] . One of the advantages of OIPCs in comparison to
regular ionic liquids is their ability to remain in solid phases even at very high temperatures and good
[11]
solubility of different doped salts in solid phases [12-20] . Rational design of new OIPCs with properties required
for different applications requires a good fundamental understanding of ion and charge transport in these
materials.
One of the most intriguing properties of some OIPCs is the strong decoupling of charge transport and ions
self-diffusion in solid phases. The DC conductivity, σ , (i.e., charge transport) of OIPCs significantly drops
DC
at solid-solid or liquid-solid phase transitions, while ion diffusion maintains smooth temperature behavior
and remains fast [11,21,22] . The origin of this decoupling is related to the strong ionic correlations in solid
phases of OIPCs and characterized by the parameter called ionicity or inverse Haven ratio, H -1[23,24] , which
shows the difference between experimentally measured σ , affected by ion-ion correlations and Nernst-
DC
Einstein (NE) conductivity, σ , corresponding to the uncorrelated ions motion [25-28]
NE

Here, n is ion concentration, q is the charge of the ion, ϕ is a fraction of the mobile ions, and and are
±
[21]
self-diffusion coefficients of cations and anions, respectively. In our previous paper , for OIPC [P ][PF ],
6
1224
we showed that in the solid phases, H = 0.01-0.1, indicating that ionic correlations suppress conductivity by
-1
10-100 times and implying the existence of a charge trapping mechanism at high ion mobility in OIPCs.
Among the various hypotheses describing the conductivity mechanisms in OIPCs [29-33] , we proposed that
[21]
in the [P ][PF ] system, charge transport is fast at short timescales; however, due to ion-ion correlations,
6
1224
charge transport becomes trapped at the length scale of the elementary crystalline unit cell. Our assumption
was based on experimental data obtained from a combination of different techniques working in broad
timescales, such as Broadband Dielectric spectroscopy (BDS), including GHz frequencies, Light Scattering
(LS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG-NMR) for diffusion measurements.
Unfortunately, [P ][PF ] has a high melting point (~about 150 °C), making it challenging to measure ion
6
1224
diffusion in a liquid state, and we assumed that ion diffusion is comparable in liquid and solid states based
on indirect evidence from BDS and LS spectra. To deepen our understanding of ionic correlations and
charge transport in OIPCs, we performed a comprehensive study of a different OIPC [P ][TFSI] system
12
using the same broad set of experimental techniques. This OIPC has a melting temperature of around 85 °C,
allowing detailed studies of the ion diffusion, charge transport and relaxation processes in liquid and solid
phases. Analysis of our results revealed that diffusion of many (but not all) ions and ionic rearrangements in
solid OIPC phases does remain very fast, while conductivity drops following our previous work with
[P ][PF ]. Hence, the similar patterns of charge transport in [P ][PF ] and [P ][TFSI], whose chemical
12
6
6
1224
1224
structures are largely different, indicate the occurrence of a general mechanism of conductivity suppression
for many OIPCs in solid phases. We speculate that the crystalline structure of OIPCs favors collective ion
hopping where ions with the same charge exchange their positions without contributing to the charge
transport.