Nano-sized Mo-doped titania (MoTiO) and Nb-doped titania (NbTiO) 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 TiO 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 MoTiO/AC hybrid supercapacitor showed the highest energy densities of 51 Wh kg at a power of 180 W kg with energy densities rapidly declining with increasing applied specific current. In comparison, the NbTiO/AC hybrid supercapacitor maintained its energy density of 45 Wh kg at 180 W kg better, showing 36 Wh g at 3200 W kg, 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 MoTiO/AC hybrid supercapacitor giving energy densities of 12 Wh kg and 2.5 Wh kg at power densities of 6700 W kg and 14 000 W kg, respectively.
Synchrotron high‐energy X‐ray diffraction computed tomography has been employed to investigate, for the first time, commercial cylindrical Li‐ion batteries electrochemically cycled over the two cycling rates of C/2 and C/20. This technique yields maps of the crystalline components and chemical species as a cross‐section of the cell with high spatiotemporal resolution (550 × 550 images with 20 × 20 × 3 µm3 voxel size in ca. 1 h). The recently developed Direct Least‐Squares Reconstruction algorithm is used to overcome the well‐known parallax problem and led to accurate lattice parameter maps for the device cathode. Chemical heterogeneities are revealed at both electrodes and are attributed to uneven Li and current distributions in the cells. It is shown that this technique has the potential to become an invaluable diagnostic tool for real‐world commercial batteries and for their characterization under operating conditions, leading to unique insights into “real” battery degradation mechanisms as they occur.
Nano-sized anatase (TiO 2) and doped anatase (Mo 0.1 Ti 0.9 O 2 and Nb 0.25 Ti 0.75 O 2) 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 halfcells (vs. Na metal counter electrodes) for the Mo-doped titania (Mo 0.1 Ti 0.9 O 2) and Nb-doped titania (Nb 0.25 Ti 0.75 O 2) 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 TiO 2 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 Mo 0.1 Ti 0.9 O 2 /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).
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