New insights into sodium-ion cathode material could help improve battery lifetime for large-scale energy storage

By gaining a better understanding of the structures that form in the most nickel-rich possible sodium-ion cathode material, the researchers hope to design cathodes that do not degrade as quickly. Their study has been published in the Journal of the American Chemistry Society (JACS).

Sodium-ion batteries are being explored as a cheaper, more sustainable alternative to lithium-ion ones, which are commonly used in everyday objects such as laptops or phones. The materials used in lithium-ion batteries are considered critical because demand is high and they are becoming more expensive and harder to source. Sodium, by contrast, is far more abundant, can be sourced all over the world and costs a fraction of the price, making it an attractive option for large-scale energy storage.

“Not a lot is known about the crystal structures present in many sodium-ion cathodes, and lessons from lithium-ion batteries do not always carry across,” said James Steele, who led this research as a PhD student at the Cavendish Laboratory at the University of Cambridge. “Because sodium-ions are bigger, they repel each other more strongly and tend to arrange themselves in special patterns, or ‘superstructures’, in a phenomena we call sodium-ion vacancy-ordering.”

Nickel ions exist in different charge states, also arranging themselves through ‘charge-ordering’. Counterintuitively, ordering is usually bad for battery stability: when a material has lots of stable, ordered structures, continuously changing between them can accelerate degradation. Understanding what these structures are, and how they are stabilised, could help researchers design longer-lasting batteries through dopants or improved processing techniques.

For this study, the team identified the structures of the phases that form as sodium is electrochemically removed from sodium nickelate. To investigate how the cathode, or positive electrode, changes as sodium moves in and out during charging and discharging, the researchers used neutron diffraction on Wish, and X-ray diffraction at Diamond Light Source to work out the crystal structures of the material and to see how those structures changed while the battery was charging. These phases show special arrangements of sodium-ions and different nickel oxidation states that result in stable structures. The live, in operando measurements also allowed the team to track how the structure evolves in real time and to identify additional ‘metastable’ phases: unstable states that can’t be found by standard diffraction measurements, which are usually taken after a cell is charged and disassembled (giving it time to relax into stable structures).