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Highlights The unique wire-in-wire structure endows TiO2/C nanofibers film with superior mechanical flexibility.The wire-in-wire TiO2/C nanofibers (TiO2 ww/CN) film shows outstanding electrochemical performances as free-standing anodes for Li/K-ion batteries and full cells.The TiO2 ww/CN film shows an extremely high pseudocapacitance contribution ratio in K-ion batteries. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00632-4., Wearable and portable mobile phones play a critical role in the market, and one of the key technologies is the flexible electrode with high specific capacity and excellent mechanical flexibility. Herein, a wire-in-wire TiO2/C nanofibers (TiO2 ww/CN) film is synthesized via electrospinning with selenium as a structural inducer. The interconnected carbon network and unique wire-in-wire nanostructure cannot only improve electronic conductivity and induce effective charge transports, but also bring a superior mechanic flexibility. Ultimately, TiO2 ww/CN film shows outstanding electrochemical performance as free-standing electrodes in Li/K ion batteries. It shows a discharge capacity as high as 303 mAh g−1 at 5 A g−1 after 6000 cycles in Li half-cells, and the unique structure is well-reserved after long-term cycling. Moreover, even TiO2 has a large diffusion barrier of K+, TiO2 ww/CN film demonstrates excellent performance (259 mAh g−1 at 0.05 A g−1 after 1000 cycles) in K half-cells owing to extraordinary pseudocapacitive contribution. The Li/K full cells consisted of TiO2 ww/CN film anode and LiFePO4/Perylene-3,4,9,10-tetracarboxylic dianhydride cathode possess outstanding cycling stability and demonstrate practical application from lighting at least 19 LEDs. It is, therefore, expected that this material will find broad applications in portable and wearable Li/K-ion batteries. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00632-4.