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Batteries 2022, 8, 108 9 of 16 (respectively) in the first reduction. The first cycle irreversible capacities from the two mech- anisms are similar to those at 25 ◦C. In the second reduction, the capacity of 90 mA h g−1 in the high potential region exceeds the 70 mA h g−1 found at 25 ◦C, which may be due to the faster adsorption kinetics and electrolyte degradation. Faster intercalation kinetics also add 25 mA h g−1 capacity in the low potential region for the second reduction. Even though the oxidation capacity falls by 24 mA h g−1 over the first 20 cycles, a larger drop than that at 25 ◦C, the cell still displays a high stability after 20 cycles with 89% reversible reduction capacity in the first cycle. The cell at 40 ◦C shows a similar curve to the 60 ◦C cell, with 45 mA h g−1 more capacity in the first reduction process than observed at 25 ◦C, and around 30 mA h g−1 capacity drop over the first 20 cycles (Figure S10). The cell tested at 80 ◦C has the highest initial reduction in capacity and shows low initial Coulombic efficiency. The majority of the irreversible capacity is from the high potential region with over 100 mA h g−1, which relates to electrolyte degradation when forming the SEI layer. The shape of the first reduction scan (Figure 5d) reveals the extra charge consumed during SEI layer formation alongside electrolyte degradation at 80 ◦C. In the low potential region, only 195 mA h g−1 capacity is observed in the first reduction process, which is less than that at 25 ◦C. Electrolyte degradation may be affecting access to sites inside the HC fibres, with a resulting loss in capacity due to the intercalation mechanism. In the second reduction, the 90 mA h g−1 adsorption capacity is similar to that found at 60 ◦C, but intercalation capacity is only 183 mA h g−1. After the first cycle, the cell became stable and only lost a further 20 mA h g−1 of specific capacity after 20 cycles, which indicates the electrolyte degradation mainly causes capacity loss in the first cycle. Furthermore, the observed capacity at this temperature is already close to the highest values observed with this type of HC. Figure 6a shows the first CV cycle at a HC electrode at temperatures from 15 to 70 ◦C in the range of 3 to 0.01 V at a scan rate of 1 mV s−1. There are three cathodic peaks at 1, 0.5 and circa 0 V vs. Na+/Na in the first cycle, which correspond to the SEI formation, adsorption and intercalation mechanisms, respectively [8,10,13]. Furthermore, the electrolyte was tested with CV by Eshetu and co-workers, who showed stability between 0 and 3 V vs. Na+/Na [24]. The SEI formation peaks near 1 V are at higher potential in cells studied above 25 ◦C, suggesting that high temperature leads to quicker formation of the SEI layer. All CV curves have one narrow oxidation peak at 0.1 V and one broad peak between 0.4 and 1 V. The 15 ◦C CV curve displays the smallest oxidation peak, which means lower HC activity. The oxidation peak size increases with temperature until 40 ◦C and then further heating decreases its intensity. The smallest potential of oxidation peaks is present at 40 ◦C, which indicates the lowest IR drop is present at 40 ◦C. In the second cycle, the 40 and 60 ◦C oxidation peaks are around the same size as in the first cycle, but the peak intensity from 70 ◦C drops to around the level seen at 25 ◦C. Hence, this HC/electrolyte combination has less stability at 70 ◦C (Figures S11 and S12). 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 increase 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 largest charge transfer resistance. The semicircle diameter reduces with increasing temperature 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 electrolyte 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 semicircle size and only a slight increase in diameter of the semicircle after 20 cycles,PDF Image | Temperature Dependence of Hard Carbon Sodium Half-Cells
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