Rhombohedral Potassium-Zinc Hexacyanoferrate as a Cathode Material for Nonaqueous Potassium-Ion Batteries

Rhombohedral potassium-zinc hexacyanoferrate K_1.88Zn_2.88[Fe(CN)₆]₂(H₂O)₅ (KZnHCF) synthesized using a precipitation method is demonstrated as a high-voltage cathode material for potassium-ion batteries (PIBs), exhibiting an initial discharge capacity of 55.6 mAh g^-1 with a discharge voltage of 3.9 V versus K/K⁺ and a capacity retention of ∼95% after 100 cycles in a nonaqueous electrolyte. All K ions are extracted from the structure upon the initial charge process. However, only 1.61 out of 1.88 K ions per formula unit are inserted back into the structure upon discharge, and it becomes the reversible ion of the second cycle onward. Despite the large ionic size of K, the material exhibits a lattice-volume change (∼3%) during a cycle, which is exceptionally small among the cathode materials for PIBs. The distinct feature of the material seems to come from the unique porous framework structure built by ZnN₄ and FeC₆ polyhedra linked via the C≡N bond and a Zn/Fe atomic ratio of 3/2, resulting in high structural stability and cycle performance.