Redox mediators for lithium-air batteries

The need for better batteries has never been greater. Many automotive manufacturers now believe that limitations in the performance of current lithium-ion batteries represent the greatest barrier to the electrification of transport. Few battery chemistries have the potential to exceed the performance of lithium-ion, but of these lithium-air offers the highest theoretical specific energy (energy per unit weight) making it an ideal candidates for automotive applications. Battery operation is as follows: on discharge, O2 from the atmosphere enters the porous cathode where it is reduced and ideally forms Li2O2, which can then be oxidised upon charging. This is balanced by the oxidation and reduction of metallic lithium at the anode. Oxygen redox at the cathode is the defining reaction in the battery and the focus of this work. The lithium-air battery is known to undergo detrimental decomposition reactions during cycling due to large voltage polarization. The introduction of redox mediators limits polarization during charge in these devices and may allow for stable battery cycling with less decomposition. This project will focus on the development, implementation and opportunities resulting from the use of redox mediators in the lithium-air battery. The primary aims of the research are as follows;

1) To investigate the relationship between chemical structure and performance in the battery. It is important to optimise the operating voltage and reaction kinetics of the mediators within the battery. This project will explore the mechanism of mediator operation, considering the relationships between mediator chemistry and relevant properties such as heterogeneous electron transfer rate constant, standard potential and catalytic activity during oxidation of the discharge product Li2O2.

2) Build on our understanding of mediator operation/chemistry to design and synthesis new mediators. As our understanding of the mediator operation increases, opportunities to tune structure-performance relationships will emerge and be subsequently exploited. Such studies will allow for the design and synthesis of advanced mediators that combine good battery performance with enhanced component stability and device lifespan.

3) Establish the origin of the positive effect of mediators on the battery lifespan. Side-reactions with and without mediators will be identified and quantified to determine the positive effect, if any, of redox mediators in these systems. A typical analysis will include in situ gas analysis via mass spec and ex situ product and component analysis using XRD, NMR and FTIR. Isotopic labelling will be used to determine reaction mechanisms.
The project will develop IP and new science in the field of metal air batteries, and as such is fully aligned with the EPSRC theme of Energy and Physical Sciences. The Bruce group is world-leading in this field and the use of mediators as an additive for high performance metal-air batteries, while demonstrated, is poorly understood and offers numerous opportunities.