Distinguishing the indistinguishable: Determining the oxygen and fluorine short-range ordering in disordered rocksalt lithium metal oxides through complex modelling

In order to move to a society not relying on fossil fuels for energy production, we need improved battery technologies, for e.g. electric vehicles and grid storage. Disordered rock salt lithium metal oxyfluorides have rapidly emerged as a promising candidate for the next generation of lithium-ion batteries because they have the potential to double the amount of energy stored compared to conventional lithium-ion batteries. Theoretically, the atomic structure of disordered rock salts can be described by two atomic positions. One position is randomly occupied by the positive transition metal ions and lithium ions, while the negative oxygen and fluorine ions randomly occupy the other. In reality, the positions are not random at all. Instead, there is a significant short-range ordering of the ions, meaning that on the atomic scale, the ions on each position become ordered. This short-range ordering is key to understanding and improving the properties of the disordered rocksalt batteries. It has been established that oxygen and fluorine are not randomly placed, but the two atoms are notoriously difficult to distinguish for almost all techniques. Therefore, is the degree and type of short-range ordering of oxygen and fluorine completely unknown. The answer to this fundamental question about the atomic structure will make it possible to optimize the next-generation of Li-ion batteries.

The project will be carried out in collaboration with assistant professor Lauren Marbella at Columbia University in New York City, USA. Lauren Marbella is an expert in ssNMR studies of energy materials, and ssNMR is crucial to this project since it is one of the only techniques capable of separating oxygen and fluorine from each other. Using very high-resolution pair distribution function analysis, oxygen and fluorine can also be separated based on the slight difference in the oxygen- and fluorine-metal bond length. However, separately these two techniques are not enough to establish the oxygen/fluorine short-range ordering. Therefore, these two techniques will be directly combined alongside different x-ray spectroscopic measurements to derive one atomistic model that shows the oxygen and fluorine ordering. The project is derived so that it will expand my “toolbox” of characterization techniques with ssNMR and combine it with my previous experience in neutron and x-ray techniques.