Layered K 0.28 MnO 2 ·0.15H 2 O as a Cathode Material for Potassium-Ion Intercalation.

Here, we present K _0.28 MnO ₂ ·0.15H ₂ O, which has a two-dimensional open framework, as an intercalation host for potassium ions. K _0.28 MnO ₂ ·0.15H ₂ O has a layered structure consisting of edge-sharing MnO ₆ octahedra with a large basal spacing of ∼7.3 Å, which facilitates K ⁺ -ion mobility. Water molecules in the interlayers between the MnO ₂ layers play an important role as a pillar to support the structure during repetitive de/potassiation cycles, as confirmed by an operando X-ray diffraction study. As a result, the large K ⁺ ions readily migrate into the crystal structure, resulting in satisfactory electrochemical performance in K-cells. With the aid of the structural pillar, the K _0.28 MnO ₂ ·0.15H ₂ O cathode delivers a high reversible capacity of 150 mA h g ^-1 over 100 cycles at a rate of 0.1 C (15 mA g ^-1 ), with acceptable power capability up to 5 C-rates.