Sodium uptake in cell construction and subsequent in operando electrode behaviour of Prussian blue analogues, Fe[Fe(CN)6]1−x·yH2O and FeCo(CN)6

Abstract

The development of electrodes for ambient temperature sodium-ion batteries requires the study of new materials and the understanding of how crystal structure influences properties. In this study, we investigate where sodium locates in two Prussian blue analogues, Fe[Fe(CN)₆]_1−x·yH₂O and FeCo(CN)₆. The evolution of the sodium site occupancies, lattice and volume is shown during charge–discharge using in situ synchrotron X-ray powder diffraction data. Sodium insertion is found to occur in these electrodes during cell construction and therefore Fe[Fe(CN)₆]_1−x·yH₂O and FeCo(CN)₆ can be used as positive electrodes. Na_zFeFe(CN)₆ electrodes feature higher reversible capacities relative to Na_zFeCo(CN)₆ electrodes which can be associated with a combination of structural factors, for example, a major sodium-containing phase, ∼Na_0.5FeFe(CN)₆ with sodium locating either at the x = y = z = 0.25 or x = y = 0.25 and z = 0.227(11) sites and an electrochemically inactive sodium-free Fe[Fe(CN)₆]_1−x·yH₂O phase. This study demonstrates that key questions about electrode performance and attributes in sodium-ion batteries can be addressed using time-resolved in situ synchrotron X-ray diffraction studies.