Interplay\nBetween Ni and Fe-ions Substituted Mn-Rich\nPrussian Blue Analogues as a Promising Cathode for High-Energy K‑ion\nBatteries

The paradigm shift in energy demands has mandated the\ndevelopment\nof a feasible alternative and sustainable battery chemistry such as\nK-ion batteries that deliver the desired energy density and complement\nthe existing expensive Li-ion batteries. Therefore, identifying and\nimproving suitable cathodes with promising stability at high operating\nvoltage is necessary. In this context, a comprehensive analysis was\ncarried out to synthesize Ni and Fe-substituted manganese-based Prussian\nblue analogues. Structural characterizations revealed the formation\nof the solid solution by observing the slightly altered unit cell\nparameters. The simultaneous precipitation of the transition metal\nions has resulted in smaller primary particle sizes of ∼60\nnm, facilitating the facile diffusion of K-ions. Simultaneously, the\naltered local electron distribution in −CN– coordination\naround the M–N bonds in the transition metal (Ni and Fe) environment\nhas significantly improved the electrochemical performance. The solid\nsolution behavior during (de)potassiation under the competing electrochemically\nactive (Mn and Fe) and inactive (Ni) transition metal substitution\nhas mitigated the detrimental Jahn–Teller effect of Mn<sup>2+/3+</sup> and the lowered average discharge potential from 3.75\nto 3.72 V vs K/K<sup>+</sup>. This has ultimately improved the rate\ncapability in PBMNF-712 (K<sub><i>x</i></sub>Mn<sub>0.7</sub>Ni<sub>0.1</sub>Fe<sub>0.2</sub>[Fe(CN)<sub>6</sub>]) dramatically\nto 88 mAh/g, whereas PBM (K<sub><i>x</i></sub>Mn[Fe(CN)<sub>6</sub>]) delivered only 48 mAh/g at a current density of 1000 mA/g\nwith reduced charge transfer resistance. This approach has demonstrated\nrobust cycling stability at 500 mA/g, delivering a specific capacity\nof 93 mAh/g even after 300 cycles.