Coexistence of two coordinated states contributing to high-voltage and long-life Prussian blue cathode for potassium ion battery

Abstract Prussian blue analogues (PBAs) are promising cathode materials for potassium-ion batteries (PIBs) due to the large interstitial voids to accommodate K+ with large size. Mn-based PBA (MnHCF) shows high redox potential but considerable capacity degradation. By contrast, Fe-based PBA (FeHCF) exhibits good cyclic stability but relatively low redox potential. The different electrochemical performance is mainly due to different coordinated states, i.e., Fe-C≡N-Fe in FeHCF and Fe-C≡N-Mn in MnHCF. In this work, those two coordinated states are incorporated to construct Fe/Mn coexistent PBA (FeMnHCF), which shows not only high average discharge voltage (3.82 V vs. K+/K), but also excellent cyclic stability (the capacity can maintain 90 mAh g-1 after 600 cycles) in commonly used ester electrolyte. The results demonstrate that the coexistent coordinated structure not only improves the redox potential of N-coordinated Fe, but also increases the capacity contribution at high voltage (above 3.5 V). Furthermore, the coexistent structure can effectively inhibit the dissolution of transition metal (TM) elements and maintain structural integrity during K+ insertion/extraction, contributing to excellent cyclic stability. This work provides a promising strategy to design stable high-voltage PBA cathode materials.