Lithium batteries could have electrolytes made from a combination of the polymeric equivalents of crown ethers and cryptands and ionic compounds, according to Peter Bruce of the University of St Andrews, UK.

Ion-polyether complexes are created with the same CH₃CH₂-O units present in crown ethers or cryptands. The units can wrap around ions to form coordination complexes. When these complexes form long chains they can be considered 'ion-polymer complexes', where the cations are coordinated to the ether oxygen atoms and the anions are within the polymer matrix.

Until recently it was thought that ion transport only occurred in amorphous ion-polymer complexes. However, recent work has shown that crystalline ion-polymer complexes, which can be prepared at specific cation to oxygen ratios, can also exhibit conductivity. 

This conductivity is due to the lithium ions in the continuous tunnels formed by the interlocking polymer chains. Conductivity can be increased by 'doping', where defects in the crystal are introduced by, for example, swapping a small percentage of the anions in the complex. 

This is 'likely to define a future direction for research in the field and may lead to yet higher levels of ionic conductivity,' said Bruce.

Richard Kelly