Interplay Between Ni and Fe-ions Substituted Mn-Rich Prussian Blue Analogues as a Promising Cathode for High-Energy K-ion Batteries

The paradigm shift in energy demands has mandated the development of a feasible alternative and sustainable battery chemistry such as K-ion batteries that deliver the desired energy density and complement the existing expensive Li-ion batteries. Therefore, identifying and improving suitable cathodes with promising stability at high operating voltage is necessary. In this context, a comprehensive analysis was carried out to synthesize Ni and Fe-substituted manganese-based Prussian blue analogues. Structural characterizations revealed the formation of the solid solution by observing the slightly altered unit cell parameters. The simultaneous precipitation of the transition metal ions has resulted in smaller primary particle sizes of ∼60 nm, facilitating the facile diffusion of K-ions. Simultaneously, the altered local electron distribution in −CN– coordination around the M–N bonds in the transition metal (Ni and Fe) environment has significantly improved the electrochemical performance. The solid solution behavior during (de)potassiation under the competing electrochemically active (Mn and Fe) and inactive (Ni) transition metal substitution has mitigated the detrimental Jahn–Teller effect of Mn2+/3+ and the lowered average discharge potential from 3.75 to 3.72 V vs K/K+. This has ultimately improved the rate capability in PBMNF-712 (KxMn0.7Ni0.1Fe0.2[Fe(CN)6]) dramatically to 88 mAh/g, whereas PBM (KxMn[Fe(CN)6]) delivered only 48 mAh/g at a current density of 1000 mA/g with reduced charge transfer resistance. This approach has demonstrated robust cycling stability at 500 mA/g, delivering a specific capacity of 93 mAh/g even after 300 cycles.