High Voltage Charge Storage Processes in Beyond Li Batteries

Rechargeable batteries are critical to de-carbonising our economy. They enable vehicles that are free from fossil fuels and offer a way to integrate intermittent renewable energy sources into our electricity grid. Li-ion is the most advanced rechargeable battery technology currently available, however, alternative chemistries that do not depend on critically resource-constrained elements Li, Ni and Co, are highly sought after.

Na-ion batteries can be based solely on earth-abundant elements, offering a lower cost and more sustainable substitute. However, they fall short of Li-ion batteries on performance, particularly energy density, due to a lack of suitable high energy Na-ion cathode materials.

One of the few routes available to increase the energy density of cathode materials is to store extra charge through oxidation and reduction of the oxide ions in the structure, so-called oxygen redox. However, while the oxidation of oxide ions gives a desirable, constant high voltage (4.5 V vs Li+/Li) on charge, this high voltage is lost on discharge due to irreversible structural changes that the material undergoes.

This ultimately reduces the potential gains in energy density that oxygen-redox materials could offer.

Mastering the oxygen redox reaction is a major focus of the Faraday Institution's CATMAT research project. The CATMAT team have reported a mechanism of oxygen redox in next generation cathode materials involving the formation of O2 which is trapped within the structure. Furthermore, some routes to suppress the structural rearrangement and promote stable oxygen redox have been discovered, such as through controlling the in-plane ordering within layered Na-ion cathodes, such as Na0.6Li0.2Mn0.8O2.

This project aims to build on this foundation of understanding to develop further strategies to manipulate the properties of oxygen redox cathodes. We will employ combined experimental (SQUID magnetometry, neutron scattering techniques, RIXS) and computational (ab initio molecular dynamics) approaches to understand the pathways which lead to the formation of trapped O2 molecules with a particular emphasis on studying their magnetic properties and behaviour.

Gaining a deeper understanding of the magnetic properties of Li-rich oxygen redox cathode materials, which have so far been overlooked, could unlock novel routes to against the detrimental structural changes and stabilise reversible high energy density cathodes.

This research is complementary to existing Faraday Institution CATMAT project activities as well as to Na-ion batteries and the NEXGENNA project and other beyond Li battery systems. This project falls under the EPSRC energy storage research area within the energy and decarbonisation theme. It fits with the EPSRC's strategic priority engineering net zero.