A new 3D open-framework potassium ferrite, K₂Fe₄O₇ (1), has been successfully synthesized under hydrothermal conditions. The framework of 1 consists of both FeO₆ octahedral and FeO₄ tetrahedral units with shared vertices and edges where there are 6- and 3-ring channels parallel to the trigonal a- and c-axes, enabling many highly mobile potassium ions to move quickly through the channels. The K⁺ ion conductivity of the polycrystalline sample was 5.0 × 10⁻² S cm⁻¹ at room temperature. Interestingly, conductivities along the a-axis direction in single crystal K₂Fe₄O₇ reached 3.5 × 10² S cm⁻¹ at 25 °C and 5.0 × 10² S cm⁻¹ at 500 °C with an activation energy of 0.08 eV. The conductor possessed a wide voltage stability up to 5 V vs. K/K⁺. Using the K₂Fe₄O₇ electrolyte, we demonstrate an all-solid-state potassium metal battery employing a Prussian blue cathode with a high charge/discharge rate of 250C (charge/discharge in 14 s, 1C = 87 mA h g⁻¹).

Metal-ionic-conductor K₂Fe₄O₇ (KFO) with instinct superhydrophilic properties was hydrothermally grown on nickel foam (NF), the formed binder-free nano-KFO/NF can deliver extremely large current density for water splitting.