Overview of Research Activities

Solid State Chemistry, Materials Science and Electrochemistry

Research does not recognise the traditional boundaries between subjects. We combine solid state chemistry, materials science and electrochemistry and by doing so are able to address the exciting scientific challenges that occur in the field of ionically conducting solids.

Although solid state ionics represents the starting point for our research, our interests extend beyond the confines of that subject to include synthesis of new nanomaterials (inorganic nanotubes and mesoporous transition metal oxides), rational synthesis of solids (oxides, sulfides etc) and new crystallographic methods. More details on our activities are given below.

Lithium Oxygen Battery

Energy storage in current Li-ion batteries is limited by the positive intercalation electrode, which does not have a sufficiently high charge to weight ratio for many applications. Although research on new intercalation materials is intense, such research can only hope to double the energy stored. Breaking through this barrier to obtain a step change in energy storage is a major challenge. One of but a few possible alternative technologies is the Li-O2 battery in which the positive intercalation electrode is replaced with an O2 electrode. Here, O2 from air combines with Li+ within a porous carbon matrix to form lithium peroxide. Significantly, the charge density of this material far exceeds that of standard positive electrodes, opening the door to significantly higher charge storage approaching ten times that of today's LiCoO2 based cells. We were the first group to demonstrate that the O2 reduction in the presence of Li+ is reversible, i.e. Li-O2 batteries are rechargeable. However, many challenges remain before a viable Li-O2 battery can be realised and we are working on a number of these, for example, electrolyte and cathode stability. Indeed, we recently demonstrated a Li-O2 battery that is capable of 100 cycles at high capacity. Our work on optimising the porous O2 electrode is complemented by fundamental studies of model systems to probe fully the mechanism of reversible lithium oxide formation.

The Sodium Ion Battery

Sodium-ion batteries (NIBs) have regained interest in the scientific community since first discovered in the 1980s due to the increasing worldwide energy demand for large-scale and low-cost batteries. Na-ion technology stands up as the perfect candidate to satisfy these requirements; it is highly abundant (6thmost abundant element in earth), accessible and widely distributed worldwide (and hence cheap).

The sodium chemistry per se is an exciting new field of research for materials scientists. Sodium exhibits a larger ionic radius than lithium which translates into robust materials that may not suffer as many transformations upon electrochemical cycling and therefore offer potentially improved stability. Furthermore, sodium research may broaden the materials design aspect. For example, novel lithium insertion materials with unusual Li coordination geometry can be synthesised by prior synthesis of the sodiated analogue and subsequent Na+/Li+ ion exchange.

Intercalation Compounds / Nanomaterials

Lithium intercalation into solid hosts is the fundamental mechanism underpinning the operation of electrodes in rechargeable lithium batteries. We seek to synthesise new lithium intercalation compounds with unusual properties or combinations of properties and to gain a fundamental understanding into the factors governing their performance as electrode materials. We are also interested in nanomaterials since the nanoscale can enhance intercalation properties.

Crystallography

In the absence of single crystals it is important to establish methods by which the entire crystal structure can be solved ab initio from powder X-ray or neutron diffraction. We have developed a powerful new method by which this can be achieved. The method uses a simulated annealing approach to minimise the difference between observed and calculated powder diffraction patterns, providing far reaching benefits beyond crystallography itself. We are developing a new approach to establishing the structure of nanomaterials based on a global-optimisation refinement using the fundamental diffraction equation of Debye.

Solid State Electrolytes

By combining salts and polyethers it is possible to synthesise thousands of metal-polyether complexes, alternatively known as polymer electrolytes. We have discovered ionic conductivity in crystalline polymer/salt complexes, when all such materials had been considered to be insulators for decades. We have gone on to show that it is possible to dope such crystalline complexes, thus raising the conductivity of these materials substantially to levels equalling that of the best amorphous polymers. We have reported 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.