To increase the storage capacity and improve the sluggish ion diffusion in the titanium oxide negative electrode for aqueous asymmetric supercapacitors, a novel binder-free titanium supported K+ intercalated hollandite TiO2 electrode with large (2 x 2) atom tunnels is facilely synthesized via a phase transformation from anatase TiO2 to hollandite TiO2 in a high temperature (220 degrees C) alkaline hydrothermal system combining with a post ion exchange process to partially extract the intercalated K+ ions. The as-prepared K+ intercalated hollandite TiO2 electrode delivers a high areal capacitance of 526.6 mF cm(-2) due to that the (2 x 2) atom tunnels provide large space for the Li+ ion storage and suitable pathway for ionic transport. A novel 2.4 V flexible all-solid-state asymmetric supercapacitor, assembled with the as-prepared hollandite TiO2 negative electrode, carbon cloth supported MnO2 positive electrode and LiCl/polyvinyl alcohol gel electrolyte, exhibits high voltage window of 2.4 V, high volumetric energy density of 63.9 mWh cm(-3), good cycle-performance and excellent flexibility, suggesting its promising practical applications in energy storage. The studies offer a novel strategy to enhance the capacitance and rate-performance of the TiO2 based negative electrode for the flexible high voltage aqueous or all-solid-state asymmetric supercapacitors.