Facile migration of potassium ions in a ternary P3-type K0.5[Mn0.8Fe0.1Ni0.1]O2 cathode in rechargeable potassium batteries

Abstract A ternary P3-type K0.5[Mn0.8Fe0.1Ni0.1]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 Mn3+/4+, Fe3+/4+, and Ni2+/3+ with increasing the operating voltage to 3.9 V. K0.5[Mn0.8Fe0.1Ni0.1]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 K0.5[Mn0.8Fe0.1Ni0.1]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.