TUNNELS Lithium cations (blue) are located inside cylindrical tubes formed by polymer chains, anions outside.
	© NATURE 2001

Chemistry professor Peter G. Bruce and colleagues at the University of St. Andrews prepared crystalline and amorphous forms of complexes of poly(ethylene oxide) and lithium salts such as LiAsF₆ and LiSbF₆. Ionic conductivities of the crystalline compounds are significantly higher than those of the corresponding amorphous compounds over a range of temperatures, they report [Nature, 412, 520 (2001)]. They also demonstrated, using NMR spectroscopy, that the ionic conductivity in the crystalline materials is dominated by transport of the lithium cations.

"This discovery may have important practical implications, for example in lithium batteries where the use of a single ion-conducting electrolyte may lead to improvements in electrode kinetics," comments Bruno Scrosati, professor of electrochemistry at the University of Rome. "Although the results are not of immediate application, they may open the route for the design of new, highly conductive, solvent-free polymer electrolytes, this being one of the key goals for the progress of lithium polymer battery technology."

The crystalline complexes prepared by Bruce's team are formed with six ether oxygen atoms per lithium ion. The polymer chains fold to form cylindrical tunnels inside of which the lithium ions are coordinated by the ether oxygens. The anions are located outside the tunnels in the spaces between the chains and do not coordinate the cations. The team suggests that the enhanced ionic conductivity of the crystalline materials results from the cations moving through the tunnels.

"Researchers have been working on polymer electrolytes consisting of a salt dissolved directly into a polymer for around 25 years," Bruce tells C&EN. "Until now, it has been widely believed that ionic conductivity occurs in amorphous materials above their glass-transition temperatures where the polymer chains are in motion, creating a dynamic, disordered environment that facilitates ion transport. Relatively little attention has been paid to crystalline polymer electrolytes because it was thought that they did not support ionic conductivity."

Both anions and cations are generally mobile in the amorphous phase, Bruce points out, whereas restricting the mobility to the lithium cations is desirable for battery applications.

"Our results define a different direction in the search for ionically conducting polymers, one which emphasizes order and structure as important features and challenges us to seek new crystalline polymer electrolytes with suitable structures and with partial occupancy of sites by potentially mobile ions," Bruce's team concludes.

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