Alkali metal crystalline polymer electrolytes

Polymer electrolytes have been studied extensively because uniquely they combine ionic conductivity with solid yet flexible mechanical properties, rendering them important for all-solid-state devices including batteries, electrochromic displays and smart windows(1-3). For some 30 years, ionic conductivity in polymers was considered to occur only in the amorphous state above T-g. Crystalline polymers were believed to be insulators. This changed with the discovery of Li+ conductivity in crystalline poly(ethylene oxide)(6):LiAsF6 (refs 4, 5). However, new crystalline polymer electrolytes have proved elusive, questioning whether the 6: 1 complex has particular structural features making it a unique exception to the rule that only amorphous polymers conduct. Here, we demonstrate that ionic conductivity in crystalline polymers is not unique to the 6: 1 complex by reporting several new crystalline polymer electrolytes containing different alkali metal salts (Na+, K+ and Rb+), including the best conductor poly(ethylene oxide)(8):NaAsF6 discovered so far, with a conductivity 1.5 orders of magnitude higher than poly(ethylene oxide)(6):LiAsF6. These are the first crystalline polymer electrolytes with a different composition and structures to that of the 6: 1 Li+ complex.