Facile migration of potassium ions in a ternary P3-type K_0.5[Mn_0.8Fe_0.1Ni_0.1]O₂ cathode in rechargeable potassium batteries

A ternary P3-type K_0.5[Mn_0.8Fe_0.1Ni_0.1]O₂ material is introduced, herein, as a promising cathode material for K ion batteries. The disadvantages associated with Mn-based layered cathode materials – structural degradation and capacity deterioration – are overcome by the partial replacement of Mn³⁺ with Fe³⁺ and Ni²⁺, which tends to increase the average oxidation state of Mn to 3.75+. First-principles calculation predicts the sequence of redox pairs from Mn^3+/4+, Fe^3+/4+, and Ni^2+/3+ with increasing the operating voltage to 3.9 V K_0.5[Mn_0.8Fe_0.1Ni_0.1]O₂ exhibits a high reversible discharge capacity (~120 mAh g⁻¹) and excellent structural integrity over 300 cycles (74% capacity retention) between 1.5 – 3.9 V at 50 mA g⁻¹. This outstanding performance of K_0.5[Mn_0.8Fe_0.1Ni_0.1]O₂ is attributed to the slight structural variation (~4.1%) of its P3–O3 phase transition predicted by the first principles calculation. Surprisingly, the obtained capacity reaches 76 mAh g⁻¹ at a rate of 2.5 A g⁻¹, in which the facile migration of K ions is explained by the low activation energy barrier of ~438 meV predicted by the nudged elastic band (NEB) method.