1. School of Chemistry, University of St Andrews, St Andrews KY16 9ST, UK

Correspondence to: Peter G. Bruce¹ Correspondence and requests for materials should be addressed to P.G.B. (Email: p.g.bruce@st-and.ac.uk).

Abstract

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, 2, 3, 4, 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 AgBr_1-xI_x ionic conductors⁸. The demonstration that doping strategies can enhance the conductivity of crystalline polymer electrolytes represents a significant advance towards the technological exploitation of such materials.

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