Intercalation of Alkali Metal−Polyethylene Oxide Polymer Electrolytes in Layered CdPS3

Abstract

Polyethylene oxide (PEO) has been inserted into the interlamellar space of CdPS3 pre-intercalated with hydrated alkali cations (Li, Na, K, and Cs). The intercalated compounds have been characterized by X-ray diffraction, vibrational spectroscopy, and conductivity measurements. These materials are interesting since the corresponding alkali metal PEO salts are well known solid polymer electrolytes (SPEs). The lattice expansion on intercalation, Delta d similar to 8 Angstrom, and the ether oxygens to alkali metal ratio of the intercalated polymer electrolyte show only minor change for different alkali cations and are the same irrespective of the molecular weight of PEG. The corresponding polypropylene oxide (PPO) intercalated compounds showed a similar lattice expansion. The projected one-dimensional electron density calculated from the X-ray diffraction shows that the alkali cations in the interlamellar space are displaced by +/-1.7 Angstrom from the center of the gap. The mid-IR spectra show features arising from the (-O-CH2-CH2-O-) moiety of the PEO in both traits and gauche conformations. A helical conformation for the intercalated PEO maybe ruled out. The data suggests that the intercalated PEO adopts a planar zig-zag structure with at least two strands complexing the alkali cation. The ambient temperature conductivity of the intercalated polymer electrolyte is comparable with that of the corresponding SPEs. Conductivity measurements provide evidence for change in the conductivity mechanism with temperature. Below 200 K the conductivities, irrespective of the cation, are small. Above 200 K, the conductivity rises rapidly with temperature. For the K and Cs compounds, the conductivities show an Arrhenius temperature dependence, but for the Li and Na intercalated polymer electrolytes the temperature variation follows the Vogel-Tamann-Fulcher relation characteristic of ionic motion coupled to polymer segmental relaxation modes. The conductivity parameters of the intercalated polymer electrolytes have been compared with those of the corresponding solid polymer electrolytes.