Page 121 - Read Online

Ahmed et al. Energy Mater. 2025, 5, 500079 https://dx.doi.org/10.20517/energymater.2024.209 Page 7 of 13

where S and S are the scale factors for the crystalline and liquid phases, and Z, M, and V are the number of
c
L
chemical formulas per unit cell, the molecular weights of these unit cells, and their respective volumes,
respectively [37,38] . For simplicity, we consider that (ZMV) = (ZMV) . In the fresh sample, W ≈ 19%. Next,
C
L
D
the previously obtained standards for the liquid and crystalline phases were used to determine the weight
fractions of the phases in the aged sample [Figure 4B]. In this case, W ≈ 50%. One should notice that, with
D
the considerations made in this work, these weight fractions are not absolute values and must be analyzed as
a qualitative comparison between the samples, suggesting that the aged sample has a ~2.5 times higher
fraction of the disordered state.

DISCUSSION
Based on the presented experimental data of [P ][TFSI] and our previously published results obtained for
12
[P ][PF ], where we performed a similar study , we can derive the common pattern about ion dynamics
[21]
1224
6
in these OIPCs. Figure 5 presents the conductivity and LS spectra of the melted and solid phases. As
mentioned above, the AC-DC crossover or τ defines the timescale where ions escape from the cage of
σ
surrounding ions and start the normal diffusion regime. In the conductivity spectra, it results in formation
of the σ plateau, and the melt phase demonstrates this typical behavior (upper panels in Figure 5).
DC
However, in the solid phase, the initial high-frequency plateau is terminated by Process I, and the second
low frequency plateau is formed with lower σ . The relaxation process observed in the light scattering
DC
spectra corresponds to the structural relaxation and can be associated with the same time of local ion
rearrangements (escape from the cage), τ . The light scattering relaxation processes for solid and melt
LS
phases have almost the same positions and coincide with the high-frequency AC-DC crossover in both
OIPCs, which indicates that local ionic mobility is comparable in liquid and solid phases. Howver, unlike
the melt state, there is a mechanism of charge trapping in the solid phases, appearing as Process I. At the
same time, there is no signature of this process in LS spectra. It might be explained by charge trapping
leading to formation of large dipole moment. The dielectric response is proportional to the square of the
dipole moment fluctuation, and therefore, this charge trapping appears as strong Process I. However, this
process involves many individual ion jumps. In contrast, light scattering is sensitive to fluctuations in
polarizability and is insensitive to dipole reorientation. As a result, LS detects individual ion jumps, but not
the charge trapping process.

The high ionic mobility indicated by the BDS and LS spectra is validated by direct measurements of the
cation and anion diffusion coefficients in the liquid and solid phases by PFG-NMR. Both cation and anion
self-diffusion coefficients show no sharp changes at the melt-solid phase transition in both OIPCs, while
σ , defined by the low-frequency DC plateau (after Process I), drops significantly [Figure 2]. However,
DC
there are important differences between [P ][TFSI] and [P ][PF ] data. First, unlike the [P ][PF ], where
1224
6
1224
12
6
all ions are mobile in Phase I [11,21] , only around 20% of cations and anions are mobile
[Supplementary Figure 2B] in Phase I of [P ][TFSI]. Second, there are much stronger drops of high-
12
frequency DC and AC conductivity and amplitude of structural relaxation peak in LS spectra in the solid
phase of [P ][TFSI] in comparison with the same for [P ][PF ] [Figure 5]. The reduction in the fraction of
6
12
1224
mobile ions can only explain part of the drop in conductivity, indicating that there are additional
mechanisms of conductivity suppression in the [P ][TFSI] system. To understand these mechanisms, we
12
should discuss the key difference between ion diffusion and conductivity. Ion self-diffusion, measured by
PFG-NMR, presents only self-part of velocity correlation functions, as given in