Metal fluoride conversion cathodes for Li-metal batteries

Lithium ion batteries (LIBs) are the ubiquitous devices that power our phones, computers and an increasing number of our automobiles. However, the development of more demanding electric vehicles, and electric aircraft in particular, requires batteries with significantly higher energy densities. Current LIBs are unable to meet this challenge due to their fundamentally limited capacity of conventional layered transition metal oxide cathode materials. In addition, these oxygen containing cathode materials are prone to decompose in organic electrolytes at elevated temperatures, resulting in violent, and often televised, exothermic reactions. In applications such as aircraft, where battery failure can lead to loss of life, such thermal instability is unacceptable.

Transition metal fluoride cathodes offer gravimetric capacities three to five times greater than conventional materials, exhibit greater thermal stability than LiFePO4, and have even been shown to quench exothermic reactions related to the thermal decomposition of electrolytes. Recent work from our group has demonstrated exceptionally stable, high-capacity cycling of the prototypical FeF2 at high active material loading, using an ionic liquid electrolyte. In addition to enabling excellent performance at room temperature, this electrolyte compounds the safety advantages of the cathode material. Unlike conventional electrolytes, ionic liquids exhibit negligible vapor pressures, little to no flammability, and are thermally stable to between 200 degree C and 450 degree C.

In this project, the student will perfrom a rigorous examination of the thermal and electrochemical stability of various transition metal fluoride - ionic liquid combinations to inform the design of high performance, high temperature lithium-metal batteries. The aim is to define temperature limitations, understand thermal degradation mechanisms, and follow the long-term, high-temperature evolution of the cathode, electrolyte, and SEI. The project will involve the use of surface/interfacial characterization techniques including impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman, in-situ and ex-situ transmission electron microscopy and scanning electron microscopy with energy disperse X-ray spectroscopy (SEM-EDX).

This project falls within the EPSRC Energy research area. The aim of this theme is for the UK to meet its environmental and energy targets.