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.