Solid Electrolytes

Despite significant improvements since their first commercialization in 1991, modern rechargeable lithium ion batteries do not fully meet the growing demands of industry in terms of capacity, charge/discharge performance and safety. Higher capacity can be achieved with a lithium metal anode, but lithium dendrites are likely to grow through liquid or gel electrolytes, eventually leading to short circuits and battery failure. Electrochemical cells utilizing solid ceramic or polymer electrolytes can circumvent these issues. Many ceramic electrolytes provide a rigid pathway for lithium-ion migration and show comparatively high conductivity, but suffer from inflexible-interface problems that are inherent with solid ceramic electrolytes.

Compared to ceramic electrolytes, polymer electrolytes are more able to accommodate for changing interfacial structures occurring during cycling of the battery. By combining salts and polyethers such as polyethylene oxide (-CH2-CH2-O-)n, it is possible to synthesise thousands of metal-polyether complexes, alternatively known as polymer electrolytes. Such materials are in effect co-ordination compounds in the solid state.

The 6:1 complexes (6 ether oxygen's per lithium), poly(ethylene oxide)6:LiXF6, where X=P,As,Sb, have a structure composed of polymer tunnels within which the Li+ ions reside. We predicted that this structure should support ionic conductivity and went on to show that this is so. This work represented the discovery of ionic conductivity in crystalline polymer electrolytes when all such materials had previously been considered to be insulators. It represents a new direction in the study of ion transport in the solid state that is quite different from the conventional picture of ion transport in amorphous polymers that has dominated the field since the late 1970's.

We have gone on to show that it is possible to dope the 6:1 complexes thus raising the conductivity of these materials substantially to levels equalling and exceeding that of the best amorphous polymers and paving the way for the application of polymer electrolytes in devices such as all-solid-state rechargeable lithium batteries.

In addition, we have developed a new class of solid ionic conductors that are different from both ceramic and polymer electrolytes: small molecule electrolytes, in which cations are coordinated by discrete low molecular weight ligands such as glymes. Unlike ceramic electrolytes, they are soft solids, yet, unlike amorphous polymer electrolytes, they are highly crystalline, of low molecular weight, and have no polydispersity or chain entanglement. Due to their monodispersity and different crystal structures, the relation between structure and conductivity can be investigated.