From Local to Diffusive Dynamics in Polymer Electrolytes: NMR Studies on Coupling of Polymer and Ion Dynamics across Length and Time Scales

Abstract

We combine 1 H, 7 Li, and 19 F NMR methods to selectively investigate polymer, cation, and anion dynamics in polymer electrolytes on various length and time scales and over broad temperature ranges. By mixing unentangled poly(propylene glycol) (PPG) with lithium perchlorate (LiClO 4 ) or lithium bis-(trifluoromethylsulfonyl)imide (LiTFSI), fully disordered samples are obtained at all studied concentrations. In static field gradient diffusometry, we observe that the longrange motion of all components slows down when the salt concentration is increased, but the effect is more prominent for PPG−LiClO 4 than PPG−LiTFSI electrolytes and, in general, differs for the respective components. The self-diffusion coefficients D of polymer and ions have essentially temperature-independent ratios, where cations are less mobile than anions and do not show Arrhenius temperature dependence. To ascertain short-range motions in broad dynamic ranges, spin−lattice relaxation studies, including field-cycling relaxometry, are combined with stimulated-echo experiments. We show that rate and heterogeneity of local lithium and polymer dynamics depend on the salt content. For intermediate salt concentrations, the segmental motion even exhibits bimodal distributions of correlation times τ, implying structures with salt-rich and salt-depleted regions. Relating diffusion coefficients D and correlation times τ, we find that the lithium ion transport is strongly coupled to polymer segmental motion even in polymer electrolytes with micro-heterogeneous salt distributions. Finally, field-cycling susceptibilities reveal that Rouse dynamics is important not only for the reorganization of polymer chains but also for the transport of lithium ions.

Ionic Conductivity in Polyfluorene-Based Diblock Copolymers Comprising Nanodomains of a Polymerized Ionic Liquid and a Solid Polymer Electrolyte Doped with LiTFSI

“…Indeed, the measured apparent activation volume closely resembles the ionic volumes of the smaller ions ([Li] + and [Br] − ), suggesting that these ions are responsible for ionic conduction (the ionic volume of [TFSI] − is much greater, Figure a). This finding is at odds with the results from the DFT calculations made in the gas phase as well as with 19 F and 7 Li NMR studies in PPG/LiTFSI and PEO/LiTFSI . The former revealed that [Li] + is always more strongly complexed to the PEO chain as compared to [TFSI] − , whereas the latter identified that [TFSI] − is more mobile than [Li] + with diffusion coefficient ratios of D F / D Li ≈ 4 .…”

“…51 The former revealed that [Li] + is always more strongly complexed to the PEO chain as compared to [TFSI] − , whereas the latter identified that [TFSI] − is more mobile than [Li] + with diffusion coefficient ratios of D F /D Li ≈ 4. 62 Based on the results from the pressure dependence, we can exclude that [TFSI] − has the major contribution to the ion conduction mechanism. Evidently, the contribution of [TFSI] − to the ionic conduction is diffusionlimited.…”

Ion Conductivity and Correlations in Model Salt-Doped Polymers: Effects of Interaction Strength and Concentration

“…This is similar to recent experimental results showing that, even in samples with inhomogeneous salt distributions, the product of lithium ion diffusion and polymer relaxation time is temperature-independent in lithium salt-doped poly(propylene glycol). 61 We then normalized cluster relaxation rate by polymer relaxation rate, as shown in Figure 6c. The normalized cluster relaxation rate versus concentration is flat at l B = 0σ, indicating that ion dynamics is highly dependent on polymer dynamics for ions with no electrostatic interactions.…”

Polymer Dynamics in Block Copolymer Electrolytes Detected by Neutron Spin Echo

“…2,5,6 Another popular approach is pulse-field gradient NMR wherein the Brownian motion of ions is quantified in the absence of an applied potential. 3,[7][8][9][10] In these cases, the translation of ions can be accommodated by segmental relaxation of the polymer chains. 11,12,21,[13][14][15][16][17][18][19][20] Thus, ionic conductivity of a well-studied polymer electrolyte, a mixture of poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt measured by ac impedance spectroscopy, can be explained entirely by the segmental relaxation quantified by quasi-elastic neutron scattering (QENS).…”

“…A popular approach for characterizing ion transport in electrolytes is ac impedance spectroscopy, which reflects the oscillation of ions in response to a small ac potential. ,, Another popular approach is pulse-field gradient NMR, wherein the Brownian motion of ions is quantified in the absence of an applied potential. ,− In these cases, the translation of ions can be accommodated by segmental relaxation of the polymer chains. − Thus, ionic conductivity of a well-studied polymer electrolyte, a mixture of poly­(ethylene oxide) (PEO) and lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) salt measured by ac impedance spectroscopy, can be explained entirely by the segmental relaxation quantified by quasi-elastic neutron scattering (QENS) . To our knowledge, no attempt has been made to study the relaxation processes that govern polymer electrolytes under large applied potentials.…”