Growth of NaO₂ in Highly Efficient Na–O₂ Batteries Revealed by Synchrotron In Operando X-ray Diffraction

The development of Na–O 2 batteries requires understanding the formation of reaction products, as different groups reported compounds such as sodium peroxide, sodium superoxide, and hydrated sodium peroxide as the main discharge products. In this study, we used in operando synchrotron radiation powder X-ray diffraction (SR-PXD) to (i) quantitatively track the formation of NaO 2 in Na–O 2 cells and (ii) measure how the growth of crystalline NaO 2 is influenced by the choice of electrolyte salt. The results reveal that the discharge could be divided into two time regions and that the formation of NaO 2 during the major part of the discharge reaction is highly efficient. The findings indicate that the cell with NaOTf salt exhibited higher capacity than the cell with NaPF 6 salt, whereas the average domain size of NaO 2 particles decreases during the discharge. This fundamental insight brings new information on the working mechanism of Na–O 2 batteries.