P2- and P3-KxCoO2 as an electrochemical potassium intercalation host.

Reversible and topotactic potassium intercalation of P2- and P3-KxCoO2 electrodes is studied in non-aqueous K cells for the first time. They deliver a highly reversible capacity of 60 mA h g-1 in the voltage range 2.0-3.9 V accompanied by reversible phase transitions including potassium/vacancy orderings. The electrochemical properties and structural change of P2-KxCoO2 are compared to those of O2-LixCoO2 and P2-NaxCoO2.