Polypyrrole-Modified Prussian Blue Cathode Material for Potassium Ion Batteries via In Situ Polymerization Coating

Potassium-ion batteries (PIBs) have received significant attention because of the abundant potassium reserves and similar electrochemistry of potassium to that of lithium. Because of the open framework and structural controllability, Prussian blue and its analogues (PB) are considered to be competitive cathodes of PIBs. However, the intrinsic lattice defects and poor electronic conductivity of PBs induce poor cycling performance and rate capability. Herein, we propose a polypyrrole-modified Prussian blue material (KHCF@PPy) via an in situ polymerization coating method for the first time. KHCF@PPy possesses a low defect concentration and improved electronic conductivity, and the electrode was found to exhibit 88.9 mA h g^-1 discharge capacity at 50 mA g^-1, with 86.8% capacity retention after 500 cycles. At a higher current density of 1000 mA g^-1, the initial discharge capacity was 72.1 mA h g^-1, which dropped slightly to 61.8 mA h g^-1 after 500 cycles. The capacity decay rate was 0.03% per cycle. Detailed characterization showed a lack of phase transition during the charge and discharge processes and determined that K ions were not completely extracted from the monoclinic structure, possibly contributing to the excellent cycling stability. This simple surface modification method represents a promising means of mitigating issues currently associated with PB-based cathodes for PIBs.