1. School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK
2. School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK

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

Polymer electrolytes are the subject of intensive study, in part because of their potential use as the electrolyte in all-solid-state rechargeable lithium batteries¹. 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, T_g, where polymer chain motion creates a dynamic, disordered environment that plays a critical role in facilitating ion transport^{2, 3, 7, 8, 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 phase¹⁰. 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}.