High Power Sodium-Ion Batteries and Hybrid Electrochemical Capacitors Using Mo or Nb-Doped Nano-Titania Anodes

Nano-sized anatase (TiO2) and doped anatase (Mo0.1Ti0.9O2 and Nb0.25Ti0.75O2) materials (ca. 5 nm) were synthesized using continuous hydrothermal flow synthesis and evaluated as negative electrodes in Na-ion batteries and hybrid capacitors. Na-ion half-cells (vs. Na metal counter electrodes) for the Mo-doped titania (Mo0.1Ti0.9O2) and Nb-doped titania (Nb0.25Ti0.75O2) electrodes both showed significantly higher specific discharge capacities than undoped anatase (ca. 75 mAh g−1 compared to only 30 mAh g−1 for undoped TiO2 at 1 A g−1). This improved performance was attributed to higher pseudocapacitive contributions to charge storage, as well as improved sodium ion diffusion and lower charge transfer resistance. Na-ion hybrid electrochemical capacitors (Na-HECs) were made from the electrodes with activated carbon positive electrodes. As expected, Na-HECs using doped titania showed superior performance to the undoped anatase, with power densities up to 10.5 kW kg−1 or energy densities of over 60 Wh kg−1 (based on the weight of active material in both anode and cathode). The Mo0.1Ti0.9O2/AC Na-ion hybrid capacitor also showed excellent specific capacitance retention of ca. 75% over 3000 cycles at 5 mA cm−2 (1 A g−1). © The Author(s) 2018.