Figure 8.
Impedance spectra registered at a fully sodiated state (at a potential of 0.01 V) during 1st and 50th cycles for analyzed materials (filled symbols denote the frequencies of 103, 102, and 10−2 Hz) with an electrical equivalent circuit (inset) applied for fitting (a), rate capability data (charge and discharge capacities are marked by filled and open symbols, Coulombic efficiencies are designated by semi-open symbols) after five initial formation cycles at 50 mA/g (b), and prolonged cycling performance at 350 mA/g (c) for the NTO-350 electrode.