Vibrational study of the crystalline phases of (CH3(OCH2CH2)(2)OCH3)(2)LiSbF6 and P(EO)(6)LiMF6 (M = P, As, Sb)

The structure of (CH3(OCH2CH2)(2)OCH3)(2)LiSbF6 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)(2)OCH3 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)(6)LiSbF6. A comparative vibrational spectroscopic study of (CH3(OCH2CH2)(2)OCH3)(2)LiSbF6 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)(6)LiAsF6, P(EO)(6)LiPF6, and P(EO)(6)LiSb6 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.</p

Vibrational study of the crystalline phases of (CH3(OCH2CH2)(2)OCH3)(2)LiSbF6 and P(EO)(6)LiMF6 (M = P, As, Sb)

The structure of (CH3(OCH2CH2)(2)OCH3)(2)LiSbF6 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)(2)OCH3 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)(6)LiSbF6. A comparative vibrational spectroscopic study of (CH3(OCH2CH2)(2)OCH3)(2)LiSbF6 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)(6)LiAsF6, P(EO)(6)LiPF6, and P(EO)(6)LiSb6 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.</p

Structure of the polymer electrolyte complexes PEO₆:LiXF₆ (X=P,Sb), determined from neutron powder diffraction data

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:LiASF(6) 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.</p

Increasing the conductivity of crystalline polymer electrolytes

Polymer electrolytes consist of salts dissolved in polymers ( for example, polyethylene oxide, PEO), and represent a unique class of solid coordination compounds. They have potential applications in a diverse range of all-solid-state devices, such as rechargeable lithium batteries, flexible electrochromic displays and smart windows(1-5). For 30 years, attention was focused on amorphous polymer electrolytes in the belief that crystalline polymer: salt complexes were insulators. This view has been overturned recently by demonstrating ionic conductivity in the crystalline complexes PEO6: LiXF6 (X=P, As, Sb); however, the conductivities were relatively low(6,7). Here we demonstrate an increase of 1.5 orders of magnitude in the conductivity of these materials by replacing a small proportion of the XF6- anions in the crystal structure with isovalent N(SO2CF3)(2)(-) ions. We suggest that the larger and more irregularly shaped anions disrupt the potential around the Li+ ions, thus enhancing the ionic conductivity in a manner somewhat analogous to the AgBr1-xIx ionic conductors(8). The demonstration that doping strategies can enhance the conductivity of crystalline polymer electrolytes represents a significant advance towards the technological exploitation of such materials.</p

Ionic conductivity in crystalline polymer electrolytes

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(1). These materials are formed by dissolving a salt (for example LiI) in a solid host polymer such as poly(ethylene oxide) (refs 2-6), 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, T-g, where polymer chain motion creates a dynamic, disordered environment that plays a critical role in facilitating ion transport(2,3,7-9). 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 T-g. 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(10). 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(11,12).</p