Cation and Anion Transport in Lithium Gel Electrolytes by Electrophoretic NMR
Andrew G. Korovich, Yongzheng Huang, Zhiyang Zhang, and Louis A. Madsen
Department of Chemistry and Macromolecules Innovation Institute
Virginia Tech, Blacksburg, VA 24061
Characterization of ion transport within electrolytes such as those used in lithium-ion batteries commonly involves the use
of dielectric impedance spectroscopy (IS). As a bulk electrochemical measurement, IS lacks chemical specificity, and
therefore does not allow individual cation and anion specific contributions to the total ionic conductivity (σ). Ideally, the total
ionic conductivity of a lithium battery would come entirely from the electrophoretic motion of lithium cations in the electrolyte,
with no motion of the anion, however in real salt-based electrolytes this is never the case. Without a direct measurement
of these individual ion contributions to conductivity, rational development of new electrolytes to improve lithium-ion battery
performance is difficult. Measurement of the individual cation and anion electrophoretic mobility allows us to explore both
the ion associations and the individual ion contributions to the total conductivity
In order to assess the electrophoretic mobility
for separate cationic and anionic species in a
lithium salt solution, one can use pulsed-fieldgradient (PFG) NMR diffusometry.1 However,
due to ion association interactions that act on
timescales (~1 ps) much shorter than the PFGNMR timescale (ms - s)2, NMR diffusometry
provides an overestimate of the true mobility.
These overestimates result from neutral ion
pairs and higher order neutral ionic aggregates
that do not contribute to electrophoretic motion,
but are still averaged into the measured selfdiffusion coefficient.2 As ion-ion interactions
increase with increasing ion density in a fluid,
measurement of conductivity and diffusion
performed on salt solutions typically used in
batteries (> 0.5 M) do not accurately represent
the active transport processes.
Figure 1: A comparison of our new polymer gel convection cell (left) and
our earlier capillary cell (right).2 We use both cells in order to prevent
large convection circulations from forming and obscuring the signal
phase shifts induced by electrophoretic motion.
By introducing an electric field pulse into a PFG pulse sequence, it is possible to directly measure the electrophoretic mobility
of individual charged species in solution.2,3 Our group has previously demonstrated measurement of electrophoretic mobility
values for cations and anions in pure ionic liquids utilizing a capillary cell and a convection-compensated PFG spin echo
pulse sequence to counteract the effects of convection generated in the ENMR experiment.2 Building on this work, we are
using a polyvinylidene fluoride (PVDF) fiber mat (Figure 1) in order to minimize convection currents. In addition, we have
developed a convection-compensated stimulated echo sequence to further reduce convection artifacts. Thus, for the first
time, we can measure the individual cation and anion electrophoretic mobilities in a lithium-ion battery electrolyte solution
(1M LiOTf in ethylene carbonate/dimethyl carbonate) absorbed into the PVDF matrix, without seeing effects from convective
motion. We have found the mobility of the triflate and lithium ions to be 2.0 x 10-9 m2/s*V and 3.7 x 10-10 m2/s*V, respectively,
at room temperature with an estimated error of ± 8%. We will discuss our progress to map cation and anion mobility vs.
salt composition and understand ion transport mechanisms in order to design, e.g., new battery electrolytes. We will also
discuss plans and progress toward understanding new ion gel electrolytes recently created in our lab.4
References:
1.
Hayamizu, K.; Aihara, Y.; Arai, S.; Martinez, C. G., Pulse-Gradient Spin-Echo 1H, 7Li, and 19F NMR Diffusion and
Ionic Conductivity Measurements of 14 Organic Electrolytes Containing LiN(SO2CF3)2. The Journal of Physical Chemistry
B 1999, 103 (3), 519-524.
2.
Zhang, Z.; Madsen, L. A., Observation of separate cation and anion electrophoretic mobilities in pure ionic liquids.
The Journal of Chemical Physics 2014, 140 (8), 084204.
3.
Holz, M.; Müller, C., Direct Measurement of Single Ionic Drift Velocities in Electrolyte Solutions. An NMR Method.
Berichte der Bunsengesellschaft für physikalische Chemie 1982, 86 (2), 141-147.
4.
Wang, Y., Chen Y., Gao, J., Yoon H.G., Jin, L., Forsyth, M., Dingemans, T. J. and Madsen, L.A., Highly
Conductive and Thermally Stable Ion Gels with Tunable Anisotropy and Modulus. Advanced Materials 2016, doi:
10.1002/adma.201505183.