Ion and polymer dynamics in polymer electrolytes PPO–LiClO4.II. H2 and Li7 NMR stimulated-echo experiments

Abstract

We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation functions characterizing the polymer segmental motion in polymer electrolytes PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of the salt on the polymer dynamics, we compare results for different ether oxygen to lithium ratios, namely, 6:1, 15:1, 30:1, and infinity. For all compositions, we find nonexponential correlation functions, which can be described by a Kohlrausch function. The mean correlation times show quantitatively that an increase of the salt concentration results in a strong slowing down of the segmental motion. Consistently, for the high 6:1 salt concentration, a high apparent activation energy Ea=4.1 eV characterizes the temperature dependence of the mean correlation times at Tg<T<or approximately 1.1Tg, while smaller values Ea approximately 2.5 eV are observed for moderate salt contents. The correlation functions are most nonexponential for 15:1 PPO-LiClO4 whereas the stretching is reduced for higher and lower salt concentrations. This finding implies that the local environments of the polymer segments are most diverse for intermediate salt contents, and, hence, the spatial distribution of the salt is most heterogeneous. To study the mechanisms of the segmental reorientation, we exploit that the angular resolution of 2H NMR stimulated-echo experiments depends on the length of the evolution time tp. A similar dependence of the correlation functions on the value of tp in the presence and in the absence of ions indicates that addition of salt hardly affects the reorientational mechanism. For all compositions, mean jump angles of about 15 degrees characterize the segmental reorientation. In addition, comparison of results from 2H and 7Li NMR stimulated-echo experiments suggests a coupling of ion and polymer dynamics in 15:1 PPO-LiClO4.

Interpretation of 1H and 2H spin–lattice relaxation dispersions: Insights from molecular dynamics simulations of polymer melts

“…2 H SLR is, however, different for deuterons in the methyl groups. These results are consistent with findings in experimental work on perdeuterated PPO [33][34][35]. There, it was reported that 2 H SLR occurs in two steps, which are related to deuterons in the methyl groups and the polymer backbone.…”

Solid-state Li NMR with applications to the translational dynamics in ion conductors

“…Also, the applicability of the BPP approach was questioned in a 7 Li spin-lattice relaxation study on hybrid organic-inorganic ion conductors [438]. In addition, 7 Li stimulatedecho experiments were used to measure the lithium ionic correlation functions for PPO-LiClO 4 at lower temperatures near the glass transition [457]. The results indicate the existence of strongly non-exponential correlation functions at variance with the BPP approach.…”

“…Also intercalation compounds based on titanium dioxide (=anatase) and titanium disulfide, i.e., Li x TiO 2 [121] and Li x TiS 2 [497][498][499] were studied in the ultra-slow domain for various Li contents, x. Finally, 7 Li stimulated-echo experiments were performed to study lithium ionic motion in polymer electrolytes [457]. In the following we will review some of these results.…”

From Micro to Macro: Access to Long‐Range Li+ Diffusion Parameters in Solids via Microscopic 6, 7Li Spin‐Alignment Echo NMR Spectroscopy

“…[109] In general, t p can be used to set the sensitivity of an SAE NMR experiment, that is, the larger t p is chosen, and thus the larger the phase w Q t p , the better Li jumps between sites with similar EFGs can be resolved. [45,113] However, it is worth noting that in the particular case of 7 Li the preparation time t p generally should be chosen as short as possible to reduce the influence of interfering dipole-dipole interactions on S 2 (t m ). [37,52] This limitation does not necessarily hold for 6 Li (spin-1), which is subject to much smaller homonuclear dipole-dipole couplings.…”