Enhancing cyclic stability of Mn-based Prussian blue cathode for Potassium Ion storage by High-spin Fe substitution strategy

Aqueous K-ion batteries (AKIBs) are being considered as promising candidates, although they still confront numerous problems. Here we propose a high-spin Fe substitution method to enhance the long-term cyclic stability of Mn-based Prussian blue analogue (Mn-PBA) cathode. The prepared K1.86Mn[Fe(CN)6]0.96·0.19H2O (KMF) are electrochemically activated for 20-cycle CV scan in the hybrid electrolyte containing KNO3+Fe(NO3)3. The modified cathode (KMF-100) achieves a high capacity of 185.8 mAh g−1 and high-capacity retention of 89.8% at 2 A g−1 after 1500 cycles (based on maximum capacity). The high-spin Fe substitution strategy not only activates the transition metal at high spin sites to contribute extra capacity, but also promotes the partial dissolved Mn2+ to return to the PBA in the hybrid electrolyte with Fe3+, relieving the Jahn-Teller effect of Mn(Ⅲ). In contrast, the KMF in the pure KNO3 electrolyte does not exhibit this behavior. The DFT calculation further demonstrate the reconstituted Prussian blue structure can increase the electrical conductivity and improve ion diffusion kinetics of the cathode material. Notably, it suppresses the change in C-Fe bond lengths during K-ion extraction, maintaining outstanding structural stability throughout cycle. This work provides a novel way to prepare high-stability manganese-based PBA cathode.