Polymer electrolytes are the subject of intensive study, in part because of their potential use as the electrolyte in all-solid-state rechargeable lithium batteries. These materials are formed by dissolving a salt (for example LiI) in a solid host polymer such as poly(ethylene oxide) (refs 2, 3, 4, 5, 6), and may be prepared as both crystalline and amorphous phases. Conductivity in polymer electrolytes has long been viewed as confined to the amorphous phase above the glass transition temperature, Tg, where polymer chain motion creates a dynamic, disordered environment that plays a critical role in facilitating ion transport. Here we show that, in contrast to this prevailing view, ionic conductivity in the static, ordered environment of the crystalline phase can be greater than that in the equivalent amorphous material above Tg. Moreover, we demonstrate that ion transport in crystalline polymer electrolytes can be dominated by the cations, whereas both ions are generally mobile in the amorphous phase. Restriction of mobility to the lithium cation is advantageous for battery applications. The realization that order can promote ion transport in polymers is interesting in the context of electronically conducting polymers, where crystallinity favours electron transport.

The crystal structures of the polymer electrolyte complexes PEO6:LiPF6 and PEO6:LiSbF6 have been obtained from powder diffraction data collected from deuterated molecules on the OSIRIS neutron powder diffractometer at ISIS, Rutherford Appleton Laboratory. The structures are similar to that recently reported for the PEO6:LiAsF6 complex and consist of rows of Li+ ions encapsulated within columns formed by pairs of nonhelical PEO chains. The Li+ ion is coordinated by five ether oxygens. The anions reside between the columns and are not coordinated to the Li+ ions. Despite broad similarities, the structures do exhibit differences, and these differences are discussed.

The structure of (CH3(OCH2CH2)2OCH3)2LiSbF6 was solved by single-crystal X-ray diffraction techniques. The compound crystallizes in the orthorhombic Pccn space group with a unit cell containing four lithium ions, each of which is coordinated by two CH3(OCH2CH2)2OCH3 or diglyme molecules. The SbF6 - anion does not directly interact with the cation, similar to its isolated environment in crystalline, high molecular weight P(EO)6LiSbF6. A comparative vibrational spectroscopic study of (CH3(OCH2CH2)2OCH3)2LiSbF6 and P(EO)6LiSbF6 demonstrated that the ethylene oxide vibrations in both systems were essentially decoupled and could be analyzed in terms of a single diglyme or PEO molecule, respectively. A spectroscopic comparison of the isostructural crystalline P(EO)6LiAsF6, P(EO)6LiPF6, and P(EO)6LiSb6 compounds demonstrated that the band frequencies in the former compound are consistently higher by a few wavenumbers than those of the latter two systems. This was attributed to the effect of the Li−O distances.