*2.3. Electrochemical Measurements*

A Vigor glovebox filled with high-purity (≥99.999%) Ar-gas (H2O and O2 < 1.0 ppm) was used to fabricate 2032 type Na-ion half-cells. Metallic sodium foil was used for the counter and reference electrodes. The separator consisted of Whatman glass fiber (GF/B type) soaked in 1 M NaClO4 (≥98.0%, Sigma-Aldrich,), in propylene carbonate (PC, 99.7%, Sigma-Aldrich), and 3% vinylene carbonate (VC, 97%, Sigma-Aldrich). Working electrodes contained 70wt.% TiO2 nanosheets, 20wt.% acetylene black (MTI Chemicals), and 10wt.% of polyvinylidene fluoride (PVDF, MW: 600.000, MTI Chemicals) binder. Nmethylpyrrolidone (NMP, 99.9%, Aladdin Chemicals) solvent was used for preparing the slurry containing the active material, conductive additive, and binder, followed by coating on a 10 μm thick copper foil. Working electrodes consisted of an active material loading of 2–3 mg/cmA Neware BTS4000 model multichannel battery tester was employed to galvanostatically cycle the assembled half-cells between 0.01–3.0 V (vs. Na+/0) at 25 ◦C. Specific capacities were expressed, calculated according to the active material weight, within a 3% error limit. A ZIVE SP1 electrochemical workstation was used for electrochemical impedance spectroscopy (EIS) measurements (1 MHz to 10 mHz) and for cyclic voltammetry (CV) measurements (0.1 to 100 mV/s). EIS measurements were performed using a 3-electrode coin-cell configuration. Diffusion-controlled and diffusionindependent processes were calculated according to Equation (1).

$$i(V) = k\_1 v + k\_2 v^{\frac{1}{2}} \tag{1}$$

*k*1*v* and *k*2*v* represent current contributions for diffusion-independent and diffusioncontrolled processes, respectively. *k*<sup>1</sup> and *k*<sup>2</sup> coefficients are the slope and intercept from the

linear fit of the plot *i*(*V*) vs. *v*, respectively. Na-ion diffusion coefficients of TiO2 nanosheets were determined using Equation (2).

$$\mathcal{D} = \frac{1}{2} \left( \frac{RT}{AF^2 \sigma\_w \mathcal{C}} \right)^2 \tag{2}$$

where D is Na-ion diffusion coefficient at absolute temperature T, and R denotes the gas constant. Faraday's constant, Na-ion concentration in electrolyte, and electrode area are represented as F, C and A, respectively. Warburg impedance, *σw*, was calculated from the slope of the linear plot of Z' vs. ω−1/2.
