Effect of sodium content on electrochemical performance of Prussian Blue analogues electrode materials for sodium ion batteries

Prussian blue analogues (PBAs) are considered as an economical electrode material for sodium-ion batteries, which can be synthesized on a large scale through a simple and low-cost co-precipitation method. The physical parameters of PBAs have a significant impact on their electrochemical performance, among which sodium content is considered one of the key factors affecting electrochemical performance. In this study, PBAs with different sodium contents are prepared using sodium citrate-assisted co-precipitation method, and the influence of sodium content on the electrochemical performance is systematically investigated. Furthermore, the morphological and structural changes of cubic and monoclinic PBAs samples after cycling are studied to explore the intrinsic mechanism of how sodium content affects the electrochemical performance. The study reveals that when the sodium content exceeds 1.5, the structure of PBAs will transfer from cubic phase to monoclinic phase. In terms of electrochemical performance, compared to cubic phase, monoclinic phase samples exhibit higher initial capacity, but lower rate capability and cycling performance, and their cycling stability deteriorates more severely with increasing sodium content. The cubic phase exhibits a more stable crystal structure compared to the monoclinic phase after cycling. During the charge-discharge process, the de-intercalation of excessive sodium ions in monoclinic PBAs leads to structural distortion, triggering the phase transition and resulting in the destruction of the crystal structure, which is the fundamental reason for the influence of sodium content on the electrochemical performance.