Elaborating the Crystal Water of Prussian Blue for Outstanding Performance of Sodium Ion Batteries

Prussian blue (PB) is one of the main cathode materials with industrial prospects for the sodium ion battery. The structural stability of PB materials is directly associated with the presence of crystal water within the open 3D framework. However, there remains a lack of consensus regarding whether all forms of crystal water have detrimental effects on the structural stability of the PB materials. Currently, it is widely accepted that interstitial water is the stability troublemaker, whereas the role of coordination water remains elusive. In this work, the dynamic evolution of PB structures is investigated during the crystal water (in all forms) removal process through a variety of online monitoring techniques. It can be inferred that the PB-130 degrees C retains trace coordination water (1.3%) and original structural integrity, whereas PB-180 degrees C eliminates almost all of crystal water (similar to 12.1%, including both interstitial and coordinated water), but inevitably suffers from structural collapse. This is mainly because the coordinated water within the PB material plays a crucial role in maintaining structural stability via forming the -N equivalent to C-Fe-LS-C equivalent to N- conjugate bridge. Consequently, PB-130 degrees C with trace coordination water delivers superior reversible capacity (113.6 mAh g(-1)), high rate capability (charge to >80% capacity in 3 min), and long cycling stability (only 0.012% fading per cycle), demonstrating its promising prospect in practical applications.