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Ahmed et al. Energy Mater. 2025, 5, 500079 https://dx.doi.org/10.20517/energymater.2024.209 Page 3 of 13

EXPERIMENTAL
Materials
1-Ethyl-1-methylpyrrolidinium bis (trifluoromethyl sulfonyl) imide [P ][TFSI] was purchased from Sigma
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Aldrich [Scheme 1A] and used as received. The structure of Diethyl(methyl)(isobutyl)phosphonium
Hexafluorophosphate [P 1,2,2,4 ][PF ] is shown in Scheme 1B for comparison purposes. The samples for all
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measurements were opened and loaded into cells inside a glove box in an inert atmosphere.

Broadband dielectric spectroscopy
Broadband dielectric spectroscopy (BDS) was used to perform conductivity spectra measurements in a
broad frequency range from 0.01 Hz up to 50 GHz. To cover the whole wide frequency range, we used the
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-2
Alpha-A analyzer from Novocontrol for measurements in the frequency range of 10 to 10 Hz; an Agilent
RF Impedance Material Analyzer, E4991A, with WinDETA Software from Novocontrol, was used in the
frequency range 10 to 3 × 10 Hz, and the Panoramic Network Analyzer, Agilent Technologies, E8364C
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6
with performance probe and the Agilent Electronic Calibration module for frequency measurements from
5 × 10 Hz up to 5 × 10 Hz. The samples were stabilized for 20 min at each temperature to reach a
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10
precision of ± 0.2 K. The details of sample cells and calibration procedure can be found in our previous
work .
[21]
Light scattering
Light scattering (LS) was measured using a Raman spectrometer and tandem Fabry-Perot (TFP)
interferometer. The experiments were performed in backscattering geometry using a laser wavelength of 532
nm. Three mirror spacings were used in the TFP interferometer: 0.4, 3, and 15 mm to cover the wide
frequency range. A T64000 Raman spectrometer from Horiba Jobin Yvon was employed to measure Raman
modes for normalization spectra at different temperatures. The temperature was stabilized by a JANIS
cryostat with a LakeShore temperature controlling unit. More details about LS can be found in our previous
work .
[21]

Pulsed field gradient nuclear magnetic resonance (PFG-NMR) measurements
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Self-diffusion NMR measurements of H and F in pure OIPCs were conducted using a 300 MHz NMR
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spectrometer at a magnetic field of 7T with H Larmor frequency at 300 MHZ and F Larmor frequency at
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1
282.2 MHz. The sample was packed in a 5 mm NMR tube in an Ar-filled glovebox and placed in a DOTY Z-
spec pulse field gradient (PFG) NMR probe. It was then melted in-situ at 100 °C and subsequently cooled to
room temperature to remove thermal history. Measurements were then performed as temperature was
increased to 65, 75, 90, and 95 °C. A spin-echo pulse sequence was used and self-diffusion coefficients were
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calculated by using the Stejskal-Tanner equation: S = S exp[-D(γgδ) (Δ - δ/3)]. Gradient field strength (g)
0
varied between 10-900 G/cm for 16 increments. Gradient pulse duration (δ) ranged from 2-3 ms and
diffusion time (Δ) covered a range from 10-20 ms. Relaxation delay times ranged from 8-17 s to
accommodate T relaxation times. Acquisition times spanned from 10-100 ms. To improve signal-to-noise
1
ratios, spectra were signal-averaged using 4-40 scans. Convection in the liquid phase at higher temperatures
was suppressed by limiting the sample height in the NMR tube to 1 cm and adding small pieces of quartz to
break up convection cells. It is worth noting that PFG-NMR measures diffusion on the micrometer scale.
See Supplementary Material for additional details about spectra analysis [Supplementary Figure 1A and B]
and estimation of mobile ions fraction in melted state and Phase I [Supplementary Figure 2A and B].

Wide angle X-ray scattering
Wide Angle X-Ray Scattering (WAXS) experiments were carried out on a Xenocs Xeuss 3.0 instrument with
both fresh and aged samples of [P ][TFSI]. The solid samples were ground, placed in hermetic Al pans, and
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aligned perpendicular to the direction of the X-ray beam (transmission mode). The samples were then
heated up to 50 °C and equilibrated for 10 min. Next, data were collected for 6 min followed by a 6-min
interval with the X-ray beam switched off to avoid overheating the samples. This procedure was repeated