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

A ternary P3-type K-0.5[Mn0.8Fe0.1Ni0.1]O-2 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 K-0.5[Mn0.8Fe0.1Ni0.1]O-2 exhibits a high reversible discharge capacity (similar to 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[Mn0.8Fe0.1Ni0.1]O-2 is attributed to the slight structural variation (similar to 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 similar to 438 meV predicted by the nudged elastic band (NEB) method.