Nano-sized Mo- and Nb-doped TiO₂ as anode materials for high energy and high power hybrid Li-ion capacitors

Nano-sized Mo-doped titania (Mo_0.1Ti_0.9O₂) and Nb-doped titania (Nb_0.25Ti_0.75O₂) were directly synthesized via a continuous hydrothermal flow synthesis process. Materials characterization was conducted using physical techniques such as transmission electron microscopy, powder x-ray diffraction, x-ray photoelectron spectroscopy, Brunauer-Emmett-Teller specific surface area measurements and energy dispersive x-ray spectroscopy. Hybrid Li-ion supercapacitors were made with either a Mo-doped or Nb-doped TiO₂ negative electrode material and an activated carbon (AC) positive electrode. Cells were evaluated using electrochemical testing (cyclic voltammetry, constant charge discharge cycling). The hybrid Li-ion capacitors showed good energy densities at moderate power densities. When cycled in the potential window 0.5-3.0 V, the Mo_0.1Ti_0.9O₂/AC hybrid supercapacitor showed the highest energy densities of 51 Wh kg^-1 at a power of 180 W kg^-1 with energy densities rapidly declining with increasing applied specific current. In comparison, the Nb_0.25Ti_0.75O₂/AC hybrid supercapacitor maintained its energy density of 45 Wh kg^-1 at 180 W kg^-1 better, showing 36 Wh g^-1 at 3200 W kg^-1, which is a very promising mix of high energy and power densities. Reducing the voltage window to the range 1.0-3.0 V led to an increase in power density, with the Mo_0.1Ti_0.9O₂/AC hybrid supercapacitor giving energy densities of 12 Wh kg^-1 and 2.5 Wh kg^-1 at power densities of 6700 W kg^-1 and 14 000 W kg^-1, respectively.