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Batteries 2022, 8, 181 6 of 12 anodic diffusion coefficient of the Na ion (D, cm2/s, Figure 3e) that is derived by a linear relation based on peak current (Ip, mA) and scan rate (ν) based on the Randles–Sevcik equation (Equation (3)) from the cyclic voltammograms in Figure S4, as follows: [47,48] Ip F 1/2 1/2 m = 0.4463 RT AD Cν 1/2 (3) where m is the electrode mass, F is the Faraday constant (96,486 C/mol), R is the gas constant (8.314 J/mol K), T is the temperature, A is the electrode area per unit mass (in cm2/g), and C is the Na-ion concentration of the electrodes. The D values of the charging and discharging processes increased from the 0.5 g/L electrode to the 6.0 g/L electrode and then descended for the 8.0 g/L electrode (see Figure 3e and Table S1), which indicates that the open sponge-like structures tend to promote efficient delivery of Na ions to the Na0.44MnO2 active materials. In addition, the 6.0 g/L electrode (117.0 Ω) showed a decreased charge transfer resistance (Rct) as reflected from the smaller semicircle in the high medium-frequency region in the electrochemical impedance spectroscopy (EIS) spectra (see Figure 3f) when compared to the other electrodes (564.0 Ω for the 0.5 g/L electrode, 301.5 Ω for the 4.0 g/L electrode, and 222.3 Ω for the 8.0 g/L and 390.3–1000.22 Ω in the literature for slurry coated electrodes [49–51]), which also indicates excellent charge transfer ability for the open sponge-like structures. This can prove to relax resistance for transporting both Na ions and electrons within the electrode [52]. Figure 3g,h shows that the 6.0 g/L electrode exhibited the highest gravimetric energy and power densities at all C rates and the highest volumetric energy and power densities above 1 C, demonstrating its benefits, particularly at faster C rates. Figure 4a shows the superior cycling capability of the 6.0 g/L cathode compared with the 0.5 g/L electrode at 0.1 C for 10 cycles and then at a high rate of 5 C for 90 cycles. After 100 cycles, the 6.0 g/L electrode maintained a specific capacity value of 100.1 mAh/g (90.2% cycling retention) despite the high C-rate (5 C), whereas the capacity of the 0.5 g/L electrode considerably degraded to 32.5 mAh/g (34% cycling retention). The corresponding volumetric capacities are shown in Figure S3, showing doubling volumetric capacity for the 6.0 g/L cathode at 5 C after 100 cycles. The stable high-rate electrochemical behavior of the 6.0 g/L electrode is attributed to the unique structure that provides an efficient pathway to access and accept abundant Na ions in the high-rate electrochemical reactions [46]. This corroborates the excellent electrochemical kinetic properties shown by the highest Na-ion diffusion coefficient D and lowest Rct of the EIS spectra (see Figure 3e). In order to further investigate the unique effect of the open-pores networking structure on Na storage behavior at the electrochemical reaction, we carried out the calculation of the degree of capacitive effect of the electrodes via the relationship between measured current (i) and scan rate (v) of the CV curves (i = avb, where a and b are constants) [53,54]. In Figure 4b, the gradient of the log i−log v plot deducts the b value depending on the applied voltage to indicate the proportion of capacitance-controlled behavior, as shown in the detailed value in Figure S5. The b value determined from the gradient of the log i−log v plot is in the range between 0.5 and 1.0, where b = 0.5 indicates a diffusion-controlled behavior and b = 1 indicates a capacitance-controlled process. That is, the larger the b value, the greater the contribution of the faster surface redox reaction during charging/discharging [55]. It is noted that the b values of the 6.0 g/L electrode at the specific voltages generating the electrochemical reactions between the Na ions and electron electrode are higher than those of the 0.5 g/L electrode, which can mean generating more profound capacitive kinetics for the Na storage [56]. In the CV curve of the 6.0 g/L electrode at 0.5 mV/s (Figure 4c), there are six main pairs of redox peaks indicating the charge ordering during the electrochemical behavior of the Na0.44MnO2 electrodes [5,11] (the other CV curves for both 0.5 g/L and 6.0 g/L electrodes at various scan rates (ν) of 0.1, 0.5, and 1 mV/s are shown in Figure S4). We also obtained the quantification of ion diffusion and surface induced capacitance contributions for each of the peaks from the CV curve. It revealed two parts of the surfacePDF Image | Cathode Electrodes High-Rate Cycle-Stable Na-Ion Batteries
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