Using polyoxometalates to enhance the capacity of lithium oxygen batteries

Abstract

Over recent years, lithium oxygen batteries have received significant interest due to their very high theoretical specific energy of 3500 Wh kg-1. This comfortably surpasses the performance of state-of-the-art Li-ion batteries, and because of this, makes them promising candidates to power electric vehicles. However, commercialisation of the lithium oxygen system remains a long way off, chiefly due to instability of the electrolyte and passivation of the electrode surface.

The aims of this project are to establish and examine redox mediators which can facilitate the oxygen reduction or evolution reactions in a lithium oxygen battery. Soluble redox mediators provide a means of moving the reaction interface away from the electrode surface, mitigating passivation of the working electrode with lithium peroxide, the desired discharge product.

This work explores the use of polyoxometalates, which offer the potential to be excellent electrolcatalysts due to their chemical stability. The electrochemistry of polyoxometalates were investigated, as was the influence of the electrolyte on their corresponding redox processes. The Keggin-type polyoxometalate TBA3PMo12O40 exhibited redox processes at potentials which were suitable to perform as a bifunctional catalyst. However, it demonstrated instability at potentials below 2.7 V vs. Li+/Li. In operando pressure measurements and XRD characterisation of the electrode surface confirmed that the Keggin–type polyoxometalate TBA4SiW12O40 demonstrated successful mediation of the oxygen reduction reaction. An increased discharge capacity and improved overpotential were observed. In operando pressure measurements with the addition of a charge mediator, tetrathiafulvalene, highlighted that the formation of decomposition products during galvanostatic cycling significantly impaired the cell cyclability. UV-vis measurements indicated that TBA4SiW12O40 displayed good stability against superoxide when compared to other organic redox mediators.

More information

Identifiers

ORCID for Nuria Garcia-Araez: orcid.org/0000-0001-9095-2379

Catalogue record

Export record