Formation of Mn-Ni Prussian Blue Analogue Spheres as a Superior Cathode Material for Potassium-Ion Batteries

Potassium manganese hexacyanoferrate shows great potential as a cathode material for potassium-ion batteries (PIBs) due to its impressive electrochemical performance, abundant elements, and easy synthesis. However, severe capacity fading and poor K+ diffusion kinetics greatly limit its large-scale application. Herein, we propose a facile anion exchange method to construct Mn-Ni Prussian blue analogue (denoted MnNi-PBA) spheres. The introduction of Ni can stabilize the structure to enhance the cycling performance, and rich active sites can be provided by the formation of a unique porous spherical structure, thus enabling shorter ion diffusion pathways during charge/discharge. Consequently, the MnNi-PBA sphere cathode delivers an initial discharge capacity of 130.6 mAh g(-1) at 10 mA g(-1), an enhanced rate capability of 66.3 mAh(-1) at 200 mA g(-1), and a long cycle life with 83.8% capacity retention after 500 cycles. When assembled with a pitch-derived soft carbon anode, a full cell exhibits excellent cycling stability and rate performance. In addition, ex situ X-ray diffraction demonstrates that the MnNi-PBA spheres undergo reversible structural changes (monoclinic <-> cubic) throughout the cycling process. Therefore, this work may offer a design strategy to synthesize Mn-based Prussian blue analogues for the application of PIBs.