Ionically conducting polymer membranes (polymer electrolytes) might enhance lithium-battery technology by replacing the liquid electrolyte currently in use and thereby enabling the fabrication of flexible, compact, laminated solid-state structures free from leaks and available in varied geometries. Polymer electrolytes explored for these purposes are commonly complexes of a lithium salt (Lix) with a high-molecular-weight polymer such as polyethylene oxide (PEO). But PEO tends to crystallize below 60 °C, whereas fast ion transport is a characteristic of the amorphous phase. So the conductivity of PEO-LiX electrolytes reaches practically useful values (of about 10-4S cm-1) only at temperatures of 60-80 °C. The most common approach for lowering the operational temperature has been to add liquid plasticizers, but this promotes deterioration of the electrolytes mechanical properties and increases its reactivity towards the lithium metal anode. Here we show that nanometre-sized ceramic powders can perform as solid plasticizers for PEO, kinetically inhibiting crystallization on annealing from the amorphous state above 60 °C. We demonstrate conductivities of around 10-4 S cm-1 at 50 °C and 10-5 S cm-1 at 30 °C in a PEO- LiClO4 mixture containing powders of TiO2 and Al2O3 with particle sizes of 5.8-13 nm. Further optimization might lead to practical solid-state polymer electrolytes for lithium batteries.

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