PhD Chemistry: New Hybrid Superionic Conductors for All-Solid-State Batteries

Global warming and the increasingly problematic search for new fossil fuel reserves are two drivers for the major global challenge of the 21st Century; the development of new ways of generating and storing energy. Lithium batteries can be considered well-established in the field of consumer electronics and are gaining leverage for automotive use. Further development, however, is constrained by the absence of key enabling materials. The recent high profile recall of certain mobile devices has demonstrated the potential hazards associated with liquid electrolytes in high voltage Li-ion batteries. Such dangers could be avoided if a solid electrolyte with appreciable thermodynamic stability and Li+ conductivity can be designed.

It has recently been shown that several complex hydrides, previously considered mainly in terms of hydrogen storage, can exhibit fast ion diffusion. In fact, the release of hydrogen from materials such as LiBH4, LiAlH4 and Li3AlH6 is linked to a superionic transition to a fast Li+-conducting crystalline state. If these superionic phases can be stabilised at room temperature, then the prospect of new families of solid state electrolytes for Li+ ion batteries could become a reality.

Stabilising LiBH4 can be achieved by substituting halides for borohydride, enabling an improvement in Li+-ion conductivity of orders of magnitude at room temperature. Building on this principle and our previous work, in this project we will construct new hybrid materials containing ionic (borohydride/halide) and molecular components with structures that can be assembled as “building blocks” to design solids with prescribed Li-ion diffusion pathways. With appropriate choice of molecular component, we will be able to tune the thermodynamic stability of the electrolytes (to allow construction of high voltage cells). Our preliminary work has shown how a new family of fast Li+-conducting hybrid solids can be built from LiBH4 and molecular hydrogen-containing units. Replacing these molecular units with other selected species of varying size and shape will create open structures (and improved Li+ diffusion pathways) with increased thermal and chemical stability. By partial substitution of BH4- by other spherical and complex anions in the ionic sections of the structure, we will directly influence the degree of anion disorder, charge carrier defect concentration and the activation energy for Li+ hopping. These new materials will be characterised and tested as electrolytes in Li+ ion cells.

The aims of the project are to:

• Prepare and characterise new hybrid ionic-molecular fast Li+-ion conductors
• Modify crystal structure and nanostructure and study the effects on ionic conductivity
• Understand the mechanisms of Li+ ion conductivity and therefore propose how performance might be improved.
• Explore the scope for the synthesis of families of new hybrid superionic conductors designed by “building block” methods.
• Determine the thermal and chemical stability of the materials and how they might be stabilised.
• Test the stability of materials against reduction during operation in a battery.

This project will involve solid state, solution state and mechanochemical synthesis together with other methods of preparing and processing solids. Materials will be characterised by powder X-ray diffraction, electron microscopy, diffuse reflectance UV-Vis spectroscopy and IR and Raman spectroscopy among others. The new hydrides will also be tested for their thermal stability, ionic conductivity and ultimately in all-solid-state batteries. We will also apply for experiment time at national facilities.

The project will be performed in collaboration with Dr Eddie Cussen at the University of Strathclyde within the WestCHEM Joint Research School.

Applicants should be UK nationals with a First Class or Upper Second Class Honours degree or equivalent in Chemistry, Materials Science or related disciplines. The successful candidate will be highly self-motivated, be goal oriented and have good English writing and communication skills. An enthusiasm for innovation and speculative thinking is particularly encouraged. A master’s degree in a relevant subject would be advantageous but is not essential.