Effect of Eliminating Water in Prussian Blue Cathode for Sodium‐Ion Batteries

Abstract

Prussian blue analogs (PBAs) are promising cathode materials for sodium‐ion batteries (SIBs) due to their low‐cost, similar energy density comparable with that of LiFePO4 in lithium‐ion batteries, and long cycle life. Nevertheless, crystal water (≈10 wt%) in PBAs from aqueous synthesis environments can bring significant side effects in real SIBs, especially for calendar life and high temperature storage performance. Therefore, it is of great importance to eliminate crystal water in PBAs for future commercial applications. Herein, a facile heat‐treatment method is reported in order to remove water from Fe‐based PBAs. Although the heat‐treated sample can be easily rehydrated in air, it still exhibits a stable cycling performance over 2000 times under controlled charge cut‐off voltage. In situ synchrotron high‐temperature powder X‐ray diffraction demonstrates that the as‐prepared sample is maintained at a new trigonal phase after dehydration. Moreover, the redox reaction of low‐spin Fe2+/Fe3+ is activated and the high‐temperature storage performance of as‐prepared sample is significantly improved after removal of water.

Water driven phase transitions in Prussian white cathode materials

“…From synchrotron XRD (figures S5 and S6), CP-PW undergoes the same phase transition from P2 1 /n to R 3 between 30 to 40 • C. Hence, the phase transition near RT for Na-rich PW is independent of the synthesis method. The existence of both structures for the same compound near to RT has not been reported previously, although the two structures have been reported independently at RT [7,13,15].…”

“…In this case, the ND experiment used a larger sample volume compared to the XRD experiment (∼2 g vs ∼2 mg) potentially leading to lower heating efficiency through the whole sample. Previous work on Na 1.76 Fe[Fe(CN) 6 ]•2.6H 2 O revealed that the cubic Fm 3m phase appears at 150 • C when heated at 3 • C min −1 and continues to coexist with a high-temperature R 3 phase until 300 • C [15].…”

“…Moreover, a rhombohedral R 3 structure has been reported for Na 1.76 Fe[Fe(CN) 6 ]•2.6H 2 O with Na + ions distributed over three different sites indicating Na + disorder [14]. Later, the same group carried out another study on a similar compound Na 1.76 Fe[Fe(CN) 6 ]•2.6H 2 O and found a rhombohedral R 3 structure with two oxygen sites randomly distributed over the six faces of the subcube and Na + ions located in the center of the subcube [15]. Thus, there is not yet consensus in the literature regarding the structure and, in particular, the position of water in hydrated PW.…”

Transition Metal Oxalate-Based Materials: An Emerging Material Family for Alkali-Ion Battery Cathodes

“…Crystal water in organic battery systems is generally considered to have a negative effect on electrochemical performance . However, a new discovery was found in K 2 [(VOHPO 4 ) 2 (C 2 O 4 )]·4.5H 2 O for MIBs.…”

Guest water hinders sodium-ion diffusion in low-defect Berlin green cathode material

“…In some cases studies have reported that reducing the water content in PBAs leads to more stable cycling performance, whereas others report that the presence of water leads to a higher capacity. [16][17][18][19] Thus, further work is required for clarity regarding the role of guest water species on sodium ion mobility and cycling reversibility. The present paper seeks to demonstrate the beneficial effect of water removal on the sodium mobility and improvement of the cycling stability using Berlin green (BG).…”