The materials chemistry and electrochemistry of cathode materials capable of anionic redox

Lithium-ion (Li-ion) batteries have been one of the great inventions from the 20th century, and have helped usher in a new age of portable electronics and electric vehicles. This technological revolution is key to a number of future applications, from the integration of renewable energy into the power grid, to the electrification of the transport system. As our dependence on batteries continues to grow, the pressure is on to push this technology further to increase energy storage capabilities, while also finding cheaper materials capable of energy storage. For example, sodium-ion (Na-ion) batteries involve similar chemistry to Li-ion, but are attractive due to the natural abundance of sodium.

Currently, the most common materials for Li-ion batteries include graphite (anode) and LiCoO2 (cathode). However, the energy density of the battery is limited by this cathode material, and new materials must be developed with increased energy storage. Over the last few years, a new family of materials know as Li-rich layered oxides have been found which exhibit significant increases in energy storage. This comes from the storage of charge on the oxygen within the framework material, as well as the transition metal centres, enabling an almost doubled energy storage capacity. This chemistry has been termed anionic redox chemistry, and has also been observed in sodium based compounds. While promising, there are major problems with these new systems, notably a drop in voltage of the battery after a few cycles due to structural changes in the material, as well as voltage hysteresis which limits the energy efficiency. This project will focus on understanding the anionic redox chemistry present in both Li-ion and Na-ion based systems, and how material design can help mitigate performance drops. The main aims of this research are:

- To investigate the structural changes in the cathode material that lead to a drop in performance. Such structural changes come from the migration of transition metal ions through the structure as it is de-lithiated (i.e.: the cell is charged), due to instability caused by the anionic redox chemistry. It is important to understand both why and how these changes occur, so new materials can be found with higher performance.

- To synthesise new Li and Na based layered oxide systems, which are capable of anionic redox. These materials will ideally show limited voltage drop and hysteresis. Synthetic procedures and materials design should be carried out by considering the findings from investigations into the anionic redox chemistry. Strategies for designing new materials could involve varying the transition metals present within the framework or changing the ratios of the metals present.

The research methodology will involve various synthesis methods, including hydrothermal and solid-state synthesis. Characterisation techniques will include the use of XRD, solid state NMR and electron microscopy to investigate material structures, while also using TGA analysis and ICP spectroscopy to analyse material composition. Importantly, high energy radiation sources will be used to carry out in-situ measurements on cells, to get a better understanding of the anionic redox chemistry in motion. The project will develop IP and new science in the field of Li-ion and Na-ion batteries, and is aligned with the EPSRC theme of Energy.