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Batteries 2022, 8, 108 10 of 16 Batteries 2022, 8, x FOR PEER REVIEW + kinetics and fast Na ◦ 10 of 16 C. For long cycling, the uninterrupted electrolyte suggesting an unstable SEI, and a large increase in Rs is displayed in the plots, indicating serious electrolyte breakdown as observed in the previously mentioned separator colour change. Furthermore, the small change in semicircle radius shows that a stable SEI layer is not formed on the HC surfaces. Even though the continuous formation of new SEI layers and electrolyte break down consume electrolyte and increase Rs, the cell still maintains high capacity and stability over 20 cycles due to its low overall resistance, high reaction diffusion at 80 degradation is likely to lead to fast cell degradation. eries 2022, 8, x FOR PEER REVIEW ◦ −1 −1 Figure 6. CV profile of HC at temperatures from 15 to 70 °C at 1 mV s scan rate between 3 and 0.01 11 o Figure 6. CV profile of HC at temperatures from 15 to 70 C at 1 mV s scan rate between 3 and V vs. Na+/Na +(a) 1st cycle and (b) 2nd cycle. 0.01 V vs. Na /Na (a) 1st cycle and (b) 2nd cycle. Impedance measurements were made at 2 V vs. Na+/Na, in a region where the CV shows little electrochemical activity. Nyquist plots at temperatures from 10 to 80 °C are shown in Figure 7, showing variations as the HC electrodes were cycled in sodium half- cells at 100 mA h g−1 current density. The semicircles in the Nyquist plots are formed by the superposition of two semicircles from the SEI layers, Cdl and Rct. All semicircles in- crease in diameter during the cycling, which indicates the growth of SEI layers during the cycling. The largest semicircle is seen in the 10 °C Nyquist plots, which relates to the larg- est charge transfer resistance. The semicircle diameter reduces with increasing tempera- ture indicating thinner SEI layers. The semicircles in the Nyquist plots at 25 °C are shifted right after 20 cycles, indicating an increasing solution resistance corresponding to electro- lyte degradation [34,40]. At 40 °C, more electrolyte decomposition is observed during the galvanostatic cycling compared with 25 °C. The increasing temperature increases both HC sodiation and SEI forming kinetics, with Na2CO3 produced at high potential and sodium alkyl carbonate at low potential [41,42]. At 80 °C, Nyquist plots exhibit the smallest semi- circle size and only a slight increase in diameter of the semicircle after 20 cycles, suggest- ing an unstable SEI, and a large increase in Rs is displayed in the plots, indicating serious electrolyte breakdown as observed in the previously mentioned separator colour change. Furthermore, the small change in semicircle radius shows that a stable SEI layer is not formed on the HC surfaces. Even though the continuous formation of new SEI layers and Figure 7. Cont. electrolyte break down consume electrolyte and increase Rs, the cell still maintains high capacity and stability over 20 cycles due to its low overall resistance, high reaction kinetics and fast Na+ diffusion at 80 °C. For long cycling, the uninterrupted electrolyte degradation is likely to lead to fast cell degradation. tfPDF Image | Temperature Dependence of Hard Carbon Sodium Half-Cells
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