NMR studies of ionic mobility in PVDF based polymer gel electrolytes

The pulsed-field gradient nuclear magnetic resonance (PFG-NMR) method of measuring self diffusion has been employed to understand the ionic mobility of liquid electrolytes and polymer gel electrolytes based on poly(vinylidene)
uoride (PVDF), propylene carbonate (PC) and lithium tetra
uoroborate (LiBF4). Self diffusion measurements were carried out using the resonant frequencies of hydrogen (1H), lithium (7Li) and uorine (19F) to track the solvent molecules, lithium cation and uorinatedBF4 anion, respectively. The order of diffusion constants was D1H > D19F > D7Li, since all entities are moving through the same medium the lithium had the largest radius, which was attributed to a large solvation shell. The NMR-PFG measurements of the polymer gel electrolytes using the lithium and hydrogen resonant frequencies revealed two distinct diffusive species which were attributed to a solvated amorphous polymer phase and pure solvent liquid phase of the polymer gel electrolytes. Ionic conductivities were measured using impedance spectroscopy for the liquid and polymer gel electrolytes. It was found that increasing the polymer concentration significantly decreased the ionic conductivity. The conductivity was observed to undergo a maximum with salt concentration due to the competition between the increase in ion carriers and viscosity as well as ionic association between the anion and cation. Peaks in the conductivity were analysed using the semi-empirical
Casteel-Amis equation which revealed that the conductivity mechanisms were similar in the liquid and polymer gel electrolytes. The ionic conductivity was predicted
using the Nernst-Einstein equation with the NMR-PFG diffusion measurements. The predicted values were observed to be significantly greater than the directly measured
conductivity, suggesting a high level of ionic association between the cation and anion. For the liquid electrolytes it was observed that ionic association increased with
temperature and salt concentration which was attributed to the lowering of the free energy of ion-pair formation at high temperatures. The ionic association for the gels
was observed to be significantly higher than the corresponding liquid electrolytes. The conductivity measurements of the polymer gel electrolytes showed that there are multiple phases contributing to the conductivity, corresponding most likely to a solvated amorphous phase and pure liquid electrolyte phase. It was observed that the
total conductivity was an average from these phases which were observed to converge with an increase in temperature, revealed by ratios of gel and liquid conductivities.
The NMR longitudinal (T1) and transverse (T2) relaxation times were measured. The liquid and polymer gel electrolytes exhibited similar T1 and T2 values, suggesting
that the systems were in the extreme narrowing regime (tumbling regime) on the high temperature side of the T1 minimum. By comparing the longitudinal relaxation times with the NMR diffiusion measurements it was possible to predict the degree of translational and rotational motion of the molecules in solution. It was concluded that the hydrogen and lithium relaxations were predominantly due to translational motion and the uorine relaxation was predominantly due to rotational motion. The transverse relaxation measurements of the polymer gel electrolytes revealed at least three different phases containing hydrogen ions and two phases containing lithium ions. These were attributed to interlamellar amorphous PVDF, a sol-
vated amorphous PVDF phase and a pure liquid electrolyte phase for the hydrogen measurements, where the lithium interlamellar amorphous polymer phase was not observed, implying no noticeable association with the polymer.
Viscosity measurements were taken for the liquid electrolyte at varying temperatures and salt concentrations. The ionic radius of each nucleus for the liquid electrolytes were determined using the Stokes-Einstein equation. The hydrogen and lithium
e�ective radii were observed to decrease with salt concentration, whereas the uorine radii increased. This was attributed to increased ionic association causing loss of the lithium solvation shell at higher salt concentrations.