Multinuclear Solid-State NMR, DSC, and Conductivity Studies of Solid Polymer Electrolytes Based on Polyurethane/Poly(dimethylsiloxane) Segmented Copolymers

Abstract

Solid polymer electrolytes based on polyurethane/poly(dimethylsiloxane) segmented copolymers (PS55) have been characterized by differential scanning calorimetry (DSC), ionic conductivity, and multinuclear solid-state NMR measurements. The results of DSC measurements indicate the formation of transient cross-links between Li+ ions and the ether oxygens on complexation with LiClO4, resulting in an increase in the soft segment T g. However, the soft segment T g remains almost invariant at high salt concentration. There is a conductivity jump at around 310−330 K that the behavior of ionic conductivity changes from Arrehnius- to Vogel−Tamman−Fulcher (VTF)-type behavior. Below this jump temperature, the conductivity follows Arrehnius-like behavior, implying a diffusing mechanism for transport of the charge carriers where the charge carriers are decoupled from the segmental motion of the polymer chain. By contrast, the diffusion of charge carrier is assisted by the segmental motions of the polymer chains above the jump temperature, suggested by the VTF-like behavior. At high salt concentration, the ionic conductivity decreases due to the formation of ion pairs and/or ion clusters. Solid-state 13C NMR results from cross-polarization time constant (T CH) measurements along with two-dimensional (2D) WISE NMR suggest that a significant decrease in the mobility of the soft segment as the salt is added. Polysiloxane backbone is not affected until at a higher salt concentration, as observed by the line width change in the 29Si NMR spectrum. The onset temperature of 7Li motional line narrowing is correlated with the soft segment T g. The activation energies obtained from ionic conductivity, 7Li line width, and T 1 measurements indicate that there is a strong correlation between the ionic conductivity of the solid polymer electrolyte and the mobile lithium cation.

Solid-State NMR Study of New Copolymers as Solid Polymer Electrolytes

“…Due to the very high mobility of lithium in our samples, 7 Li spectra consist of a single sharp peak with full-widths at half height as low as 12 Hz, which reach a maximum at low temperatures of 120 Hz (Figure 3). This is almost two orders of magnitude lower than the linewidths reported for polyurethane-poly(dimethylsiloxane) copolymers [17]. The highest ionic conductivity reported in Table 1 is 2.54 × 10 −4 Scm −1 at 60 • C, which is higher (about 3 times) than those reported earlier [17], so we can evaluate qualitatively the ionic conductivity of a polymer by this method.…”

“…This is almost two orders of magnitude lower than the linewidths reported for polyurethane-poly(dimethylsiloxane) copolymers [17]. The highest ionic conductivity reported in Table 1 is 2.54 × 10 −4 Scm −1 at 60 • C, which is higher (about 3 times) than those reported earlier [17], so we can evaluate qualitatively the ionic conductivity of a polymer by this method. Interestingly, 1 H decoupling appears to have no effect on the 7 Li linewidth in our samples: thus, the mobility of lithium ions is sufficient to completely eliminate 1 H-7 Li dipolar couplings.…”

“…Lin et al [17] calculated the activation energy (Ea) of lithium diffusion from the slope of the curve at low temperatures by the Arrhenius relationship. This information is relevant for sample 3 because While 7 Li linewidths are influenced by motions with correlation times shorter than µs, faster motions with correlation times in the nanoseconds will contribute to longitudinal relaxation (T 1 ) of the NMR signal.…”

“…Lin et al [17] calculated the activation energy (Ea) of lithium diffusion from the slope of the curve at low temperatures by the Arrhenius relationship. This information is relevant for sample 3 because Sample 3 shows the lowest lithium mobility (see Figure 4), despite the highest ratio of PEGMA.…”

“…The investigation of Li + ion mobility by different techniques appears to be a fascinating field of research. Several articles were published on the investigation of SPE by solid-state NMR [16][17][18][19], especially scrutinizing the mobility of lithium coordinated with the polymer chains; however, the examination of the mobility of the organic part has not been extensively studied. In this work, we describe the use of a combination of 13 C and 7 Li high-resolution solid-state NMR results to establish a complete portrait of high-performance comb-like copolymers.…”