Upcycling of Degraded Prussian Blue into Layered Materials for Sodium-Ion Battery.

Prussian blue and Prussian blue analogs are widely used in sodium-ion batteries (SIBs). In this study, we upcycle the degraded Prussian blue directly into layered materials for SIBs through thermal treatment. Prussian blue thermally decomposes to form metal oxides, which then recrystallize into layered metal oxides with metal-nitrogen bond on their surface under suitable temperature conditions. This transformation method is similar to solid-state synthesis, allowing for the pre-addition of necessary components before material conversion to optimize the composition and integrity of the target materials. Based on in situ x-ray diffraction observations of the crystal structure changes of Prussian blue at different temperatures, we demonstrate 1,000 °C as the optimal temperature for converting to layered materials. These materials exhibit an initial discharge capacity of 122.3 mAh g^-1 and good rate and cycling stability. We hope that this research will promote the sustainable development of the SIB industry.