P2‐K 0.75 [Ni 1/3 Mn 2/3 ]O 2 Cathode Material for High Power and Long Life Potassium‐Ion Batteries

Rationally designed P2‐K 0.75 [Ni 1/3 Mn 2/3 ]O 2 is introduced as a novel cathode material for potassium‐ion batteries (KIBs). P2‐K 0.75 [Ni 1/3 Mn 2/3 ]O 2 cathode material designed through electrochemical ion‐exchange from P2‐Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 exhibits satisfactory electrode performances; 110 mAh g −1 (20 mA g −1 ) retaining 86% of capacity for 300 cycles and unexpectedly high reversible capacity of about 91 mAh g −1 (1400 mA g −1 ) with excellent capacity retention of 83% over 500 cycles. According to theoretical and experimental investigations, the overall potassium storage mechanism of P2‐K 0.75 [Ni 1/3 Mn 2/3 ]O 2 is revealed to be a single‐phase reaction with small lattice change upon charge and discharge, presenting the Ni 4+/2+ redox couple reaction. Such high power capability is possible through the facile K + migration in the K 0.75 [Ni 1/3 Mn 2/3 ]O 2 structure with a low activation barrier energy of ≈210 meV. These findings indicate that P2‐K 0.75 [Ni 1/3 Mn 2/3 ]O 2 is a promising candidate cathode material for high‐rate and long‐life KIBs.