AbstractAqueous rechargeable microbatteries are promising on-chip micropower sources for a wide variety of miniaturized electronics. However, their development is plagued by state-of-the-art electrode materials due to low capacity and poor rate capability. Here we show that layered potassium vanadium oxides, K_xV₂O₅·nH₂O, have an amorphous/crystalline dual-phase nanostructure to show genuine potential as high-performance anode materials of aqueous rechargeable potassium-ion microbatteries. The dual-phase nanostructured K_xV₂O₅·nH₂O keeps large interlayer spacing while removing secondary-bound interlayer water to create sufficient channels and accommodation sites for hydrated potassium cations. This unique nanostructure facilitates accessibility/transport of guest hydrated potassium cations to significantly improve practical capacity and rate performance of the constituent K_xV₂O₅·nH₂O. The potassium-ion microbatteries with K_xV₂O₅·nH₂O anode and K_xMnO₂·nH₂O cathode constructed on interdigital-patterned nanoporous metal current microcollectors exhibit ultrahigh energy density of 103 mWh cm⁻³ at electrical power comparable to carbon-based microsupercapacitors.