Entropy and Electronic Structure Modulation of a Prussian\nBlue Analogue Cathode with Suppressed Phase Evolution for Potassium-Ion\nBatteries

Severe\nstructural evolution and high content of [Fe(CN)<sub>6</sub>]<sup>4–</sup> defects drastically deteriorate K-ion storage\nperformances of Prussian blue-based cathodes. Herein, a potassium\nmanganese iron copper hexacyanoferrate (KFe<sub>2/3</sub>Mn<sub>1/6</sub>Cu<sub>1/6</sub>HCF), with suppressed anionic vacancies, eliminated\nband gap, and low K-ion diffusion barrier, is regarded as a cathode\nfor potassium-ion batteries. The entropy stabilization effect and\nrobust Cu–N bond induced by the inert Cu-ion with large electronegativity\nboost KFe<sub>2/3</sub>Mn<sub>1/6</sub>Cu<sub>1/6</sub>HCF to exhibit\ngreat phase state stability, thus inhibiting the structural transition\nof monoclinic ↔ cubic. Hence, KFe<sub>2/3</sub>Mn<sub>1/6</sub>Cu<sub>1/6</sub>HCF undergoes a zero-stress solid-solution reaction\nmechanism, where Fe and Mn serve as dual active sites for charge compensation.\nConsequently, KFe<sub>2/3</sub>Mn<sub>1/6</sub>Cu<sub>1/6</sub>HCF\ndisplays a high reversible capacity of 127.5 mAh·g<sup>–1</sup> with an energy density of 469.2 Wh·kg<sup>–1</sup> at\n10 mA·g<sup>–1</sup> and superior cyclic stability with\na high retention of 90.7% over 100 cycles. A high-energy-density K-ion\nfull battery is assembled, contributing an ultralong lifetime over\n1000 cycles with a low-capacity fading rate of 0.038% per cycle.