As battery technology continues to improve and becomes cheaper, we rely more heavily on rechargeable battery technology in mobile electronics, healthcare and transport. For this purpose, lithium ion batteries are of interest due to the incredibly high volumetric and gravimetric energy densities compared to alternative battery technologies. Commercial lithium ion batteries rely on an organic liquid electrolyte which is often toxic, flammable and can lead to leakage if the battery is mistreated or disposed of incorrectly. One potential solution to this is the development of solid state electrolyte materials which can efficiently transport ions between the anode and cathode, but are also non-flammable, stable (both chemically and thermally) and compatible with the other battery materials.
This PhD project aims at understanding the structure and properties of a composite material formed from a sponge-like framework consisting of metal nodes and organic linker molecules, and an ionic liquid. These sponge-like frameworks are called Metal-Organic Frameworks and comprise a huge number (>70,000) of materials with variations in the chemical composition and structural arrangements. Guest molecules can be captured within the pores of such framework materials and in many cases, this can improve a particular property beyond that of the pure components. Incorporating a guest within the pores of the framework results in a pseudo solid-state material as the guest is trapped and there is an energy barrier which must be overcome in order to escape.
In particular, we are interested in the application of these composite materials as ionic conductors for solid electrolytes in batteries. The negligible volatility and non-flammability of the ionic liquids alleviate many of the safety concerns when compared with traditional organic electrolytes whilst maintaining high ionic conductivities required for operation. However, key questions still remain with regards to the nature of the interaction between the framework and the ionic liquid. To what extent do the electronic interactions disrupt the framework? Can this interaction be tuned to enhance specific properties such as ionic conductivity?
To answer these questions, novel composite materials will be synthesised to look at the effects of specific structural changes including: the size of the pores, the chemical composition of the framework, dynamic behaviour of the framework and the amount of pore filling to name a few. In order to confirm the successful synthesis of these novel composite materials, meticulous characterisation of the structure must be carried out via a multitude of techniques in order to prove that both the framework and ionic liquid are still intact and that the ionic liquid has been successfully incorporated into the pores of the framework. After this, it is then possible to measure the ionic conductivity properties of the composites and begin to understand trends between structural changes and the resulting properties of the material.