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Batteries 2022, 8, 108 12 of 16 , x FOR PEER REVIEW the SEI layer (Figure S18). Furthermore, the growing SEI layer reduced the specific capacity 1000 mA g−1 current density). Improved HC capacities for high current density operations at high temperatures may be due to the faster reaction kinetics and a higher Na+ diffusion coefficient. However, electrolyte degradation starts from 60 ◦C, which will affect battery performance for long cycling. When current density is dropped back to 100 mA g−1, the capacities of all batteries return to the same level as before, changing the current density, indicating high stability and reversibility of the HC over 50 cycles. The oxidation capacities during long-term cycling at 25 and 40 ◦C with 100 mA g−1 current density are exhibited in Figure 9b. At 25 ◦C, the capacity decreases slowly and continuously with 115 mA h g−1 oxidation capacity maintained after 200 cycles. EIS showed an increasing semi-circle diameter during 25 ◦C cycling, which indicated a rising of Rct due to continuous growth of ◦12 of 16 duetoincreasedoverpotentialsandshortenedintercalationprocesses(FigureS19).At40 C, thefasterkinetics,largerDNa+ andsmallerSEIlayercauseasmallerIRdrop(FigureS20), which boosts the HC specific capacity and cyclic stability with over 220 mA h g−1 specific capacity after 200 cycles at 40 ◦C, corresponding to 76% initial oxidation capacity. Even Moreover, large 𝐷 provides fa◦ster Na+ transport and ◦can enhance the capacity with a though the capacity at 40 C is higher than that at 25 C, it still displays a continuous large currentcarpaatceitaytdhroipg,hwtheicmhpmearyarteularte.to high electrolyte degradation kinetics. Figure 8. Dependence of natural logarithm of the average Na+ diffusion coefficient on reciprocal Figure 8. Dependence of natural logarithm of the average Na+ diffusion coefficient on temperature. reciprocal temperature. Figure 9a shows the HC electrode galvanostatic oxidation capacity with multiple cur- rent rates at 25, 40 and 60 °C (reduction in Figure S17). After stabilising at 100 mA g−1 current density for 20 cycles, it exhibits 240, 205, 112, 55 and 41 mA h g−1 specific capacity for 50, 100, 200, 500 and 1000 mA g−1 current density at 25 °C, respectively. The battery tested at 40 °C shows the highest capacity at 50 and 100 mA g−1 (260 and 240 mA h g−1) and increases of 80 and 10 mA h g−1 in the capacity at 200 mA g−1 and 500 mA g−1 compared with the battery at 25 °C (respectively). Moreover, even though the battery at 60 °C shows less specific capacity with 50 and 100 mA g−1 currents compared with at 40 °C, it displays −1 8 the highest capacity at a large current rate (200, 120 and 55 for 200, 500 and 1000 mA gPDF Image | Temperature Dependence of Hard Carbon Sodium Half-Cells
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