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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Random copolymers of the polyphosphazene [NPR 2 ] n have been synthesized via living ionic polymerization with mixed substituents at the phosphorus atoms (i.e., R ) bis(2-methoxy-ethyl)amino and n-propylamino). The polymers melt at 190 °C and start to decompose above 300 °C. Thin polymer electrolyte membranes were prepared by solution casting with dissolved lithium triflate (LiSO 3 CF 3 ) and with NaI. The transparent membranes showed favorable mechanical properties below 100 °C. T g values ranged between -50 and -36 °C. Membranes with 10 wt % LiSO 3 CF 3 (corresponding to the atomic ratio Li/(O + N) ) 1/30) showed rather low conductivities between 3.2 × 10 -7 S cm -1 at 30 °C and 1.9 × 10 -5 S cm -1 at 100 °C as determined from impedance measurements. The dispersion of 4 wt % Al 2 O 3 nanoparticles in the polyphosphazene membranes with 10 wt % LiSO 3 CF 3 , however, leads to an increase of the conductivities by 2 orders of magnitude, that is, 1.0 × 10 -5 S cm -1 at 30 °C and 1.5 × 10 -3 S cm -1 at 100 °C. The heterogeneously doped salt-in-polymer membranes thus combine good mechanical stability with a high ionic conductivity. The temperature dependence of the conductivity data was analyzed in terms of the MIGRATION model, which gives a consistent explanation of the non-Arrhenius dependence.

The effect of both nanoparticles and low molecular weight borate esters on the ionic conductivity of cross-linked polysiloxanes was systematically investigated by means of measuring conductivity spectra in the impedance regime at temperatures between -30 and 90 degrees C. Salt-in-polymer electrolytes were prepared by dissolving lithium triflate (LiSO(3)CF(3)) in comblike polysiloxanes bearing one methyl and one oligoether side group per silicon. An amount of 10 mol % of the oligoether side groups exhibited a terminal allytrimethoxysilane serving as a cross-linker moiety (T(0.1)OPS). Thus prepared polymer electrolyte membranes were completely amorphous and mechanically stable with an optimum conductivity value of 5.7 x 10(-5) S x cm(-1) at 15 wt % of lithium triflate (LiSO(3)CF(3)) at room temperature (T(0.1)OPS + 15 wt % LiSO(3)CF(3)). Further investigations concerned the influence of additives, i.e., nanosized ceramic fillers (alpha-Al(2)O(3) and SiO(2), up to 10 wt %) as well as two low molecular weight borate esters (tris(2-(2-methoxyethoxy)ethyl) borate (B2) and tris(2-(2-(2-methoxyethoxy)ethoxy)ethyl) borate (B3)) with maximum concentrations of 40 wt % as referred to polysiloxane T(0.1)OPS. The addition of borate esters resulted in a considerable increase of the conductivity, while still maintaining the mechanical stability. Optimum conductivities of 3.7 x 10(-5) and 1.6 x 10(-4) S x cm(-1) were measured for B2 and B3, respectively, at room temperature. A fit of the temperature-dependent DC conductivity by the empirical Vogel-Tammann-Fulcher (VTF) equation showed that there was an increased number density of mobile charge carriers in the case of borate esters as additives. However, the shape of the conductivity spectra in the dispersive regime changed considerably in going from nanoparticles as additives to borate esters. A careful and consistent modeling of the conductivity spectra and of the temperature dependence of the DC conductivity was done within the framework of the MIGRATION concept. The result was that the addition of borate esters to the polymer host most probably increased both number density of mobile charge carriers as well as their mobility.

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