Polyelectrolyte multilayers are built up from ionically modified polyphosphazenes by layer-by-layer assembly of a cationic (poly[bis(3-amino-N,N,N-trimethyl-1-propanaminium iodide)phosphazene] (PAZ + ) and an anionic poly[bis(lithium carboxylatophenoxy)phosphazene] (PAZ -). In comparison, multilayers of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) are investigated. Frequency-dependent conductivity spectra are taken in sandwich geometry at controlled relative humidity. Conductivity spectra of ion-conducting materials generally display a dc plateau at low frequencies and a dispersive regime at higher frequencies. In the present case, the dispersive regime shows a frequency dependence, which is deviating from the typical behavior found in most ion-conducting materials. Dc conductivity values, which can be attributed to long-range ionic transport, are on the order of σ dc ) 10 -10 -10 -7 S‚cm -1 and strongly depend on relative humidity. For PAZ + /PAZ -multilayers σ dc is consistently larger by one decade as compared to PSS/ PAH layers, while the humidity dependence is similar, pointing at general mechanisms. A general law of a linear dependence of log(σ dc ) on relative humidity is found over a wide range of humidity and holds for both multilayer systems. This very strong dependence was attributed to variations of the ion mobility with water content, since the water content itself is not drastically dependent on humidity.

A range of different combinations of polyelectrolytes is employed to form multilayers by layer-by-layer assembly, which are investigated by impedance spectroscopy. In particular, the alkali counterion employed in the layer formation is varied. Impedance spectra of different multilayer systems are qualitatively similar, and they are changing in a similar way with relative humidity (RH). From the spectra, the dc conductivity σdc and its dependence on humidity are extracted. The humidity dependence of σdc follows a general law of log(σdc) = aRH + b, which is valid for all systems. Absolute values of σdc and slopes a depend on the type of polyelectrolytes employed but are independent of the type of alkali counterion involved. On the basis of these data, we discuss the contribution of different small ionic species, i.e., anions, alkali cations, or protons, to the conductivity and conclude that the differences between different polymer systems as well as the humidity dependence are consistent with the conduction of protons or hydronium ions, while the contribution of other cations or anions to σdc is negligible.

Summary: A series of comb‐like polysiloxanes was prepared as the base polymer for solvent‐free polymer electrolyte membranes. Hydrosilylation of poly(methylhydrosiloxane) (PMHS) was used to substitute hydrogen by the two types of side groups tetraethylene glycol allyl methyl ether and allyltrimethoxysilane (ATMS) with varying molar ratios between 5 and 40 mol‐% ATMS. The ATMS side groups served to cross‐link as‐prepared polymer electrolyte membranes after dissolving lithium trifluoromethylsulfonate (triflate) or lithium bis(trifluoromethylsulfonyl)imide. The ionic conductivities of these salt‐in‐polymer membranes prepared with a constant concentration of 10 wt.‐% lithium triflate showed a maximum conductivity of 4.6 × 10−5 S · cm−1 at 30 °C for 10 mol‐% ATMS substitution. In another series of experiments with the ATMS substitution held constant at 10 mol‐%, the salt concentration was varied yielding a maximum conductivity of 1.4 × 10−4 S · cm−1 at 30 °C for 12.5 wt.‐% lithium triflate. magnified image

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We have investigated a new improved lithium ion conducting salt-in-polymer electrolyte system consisting of a polysiloxane backbone with oligoether side chains and added LiCF(3)SO(3) (LiTf), which has a conductivity at 30 degrees C of up to 1.3 x 10(-4) S cm(-1) and up to 6.9 x 10(-5) S cm(-1) after cross-linking, which is employed to enhance mechanical stability. The mechanisms governing local dynamics and mass transport have been studied on the basis of temperature dependent spin-lattice relaxation time and pulsed field gradient diffusion measurements for (7)Li, (19)F and (1)H, respectively. The correlation times characterizing the local ion dynamics reflect the complexation of the cations by the polyether side chains of the polymer and show the anion as the more mobile species. In contrast, (7)Li and (19)F diffusion coefficients and their activation energies are rather similar, suggesting the formation of ion pairs with similar activation barriers for cation and/or anion long-range transport. In general, the activation energies describing local reorientation are significantly smaller than those characterizing long range diffusion, suggesting that the long-range transport of both cations and anions is a much more complex process than a simple succession of free ion jumps, and involves (1) the coupling of conformational side-chain reorientations to the cation movement, and (2) the correlated diffusion of cations and anions within dimers or clusters. An important practical conclusion from our results is that the relatively high ionic conductivity in polysiloxane-based polymer electrolytes could even be increased if salt dissociation could be enhanced further.