# Novel Q-Carbon Anodes for Sodium-Ion Batteries
## 3 Results and Discussion
Compared with a graphite-like structure, the higher ratio of the disordered carbon in Q1 is much easier for sodium intercalation/deintercalation, resulting in a higher capacity, although with a lower surface area [17; 31; 32]. It seems that although Q1, with a higher amount of disordered carbon, is much easier for sodium intercalation/deintercalation with a high ICE due to its low surface area, it has a high capacity but is less stable with cycling, resulting in a continual loss of capacity with cycling, probably due to the continual built up of the SEI layer. On the other hand, Q2, with a higher amount of graphite and a higher surface area, has a low ICE and low capacity, but its cycling stability is much better, probably due to the formation of a better SEI layer. To investigate the cause of the cycling difference between Q1 and Q2, cyclic voltammetry (CV) was performed at a scan rate of 0.05 mV\(\cdot\)s\({}^{-1}\) in the range of 0 to 3 V. Figure 2a,b shows the first three CV curves of the Q1 and Q2 carbon. For both Q1 and Q2, there is an irreversible reduction peak between 1.0 V and \(\sim\)0.2 V during the first cathodic scan, resulting from the irreversible reaction of the electrolyte and the formation of the SEI on the electrode surface. The following two cycles overlap well, leading to improved CEs. The CV curves show a pair of strong redox peaks near 0 V which are associated with the intercalation/de-intercalation of sodium-ion into/out of the graphitic layers or nanopores. The overlapping wide peaks between 0.25 and 1.75 V are associated with the surface adsorption/desorption of sodium ions. It is noted that Q1 has a better symmetric redox peak than Q2 and the current densities of Q1 are much higher those of Q2, which is consistent with the higher capacity of Q1 shown in Figure 1. As mentioned earlier, the higher ratio of disordered carbon in Q1 compared to Q2 is beneficial for sodium intercalation/deintercalation, which is further validated by the rate capability test shown in Figure 2c,d. Q1 has the highest capacity at the lowest current density of C/10 (37.2 mA\(\cdot\)g\({}^{-1}\)). Both Q1 and Q2 show a consistent capacity drop with an increasing current rate, as depicted in Table 3, with Q1 and Q2 decreasing from 293.25 mA\(\cdot\)g\({}^{-1}\) and 215.72 mAh\(\cdot\)g\({}^{-1}\) at 37.2 mA\(\cdot\)g\({}^{-1}\) to 5.21 mAh\(\cdot\)g\({}^{-1}\) and 4.38 mAh\(\cdot\)g\({}^{-1}\) at 3720 mA\(\cdot\)g\({}^{-1}\), respectively. From the data in both Figures 1 and 2, it is confirmed that Q1 indeed outperforms Q2 under all current densities. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**C Rate**} & **Current Density** & **Q1: Average Capacity** & **Q2: Average Capacity** \\  & **(mA\(\cdot\)g\({}^{-1}\))** & **(mAh\(\cdot\)g\({}^{-1}\))** & **(mAh\(\cdot\)g\({}^{-1}\))** \\ \hline C/10 & 37.2 & 293.25 & 215.72 \\ \hline C/5 & 74.4 & 277.93 & 201.15 \\ \hline C/3 & 124 & 247.84 & 171.85 \\ \hline
1C & 372 & 198.46 & 120.08 Figure 3a,b shows the C1 spectra with four signature peaks corresponding to moieties present in the SEI: (a) carbonate (CO\({}_{3}\)) moieties with a binding energy (BE) of 289.5 eV, O-C=O moieties at BE of 288 eV, C-O species at BE of 286.2 eV, and C species at BE of 284.0 eV, which is usually associated with hydrocarbon surface contamination in XPS spectra [33] and the presence of defects in the graphitic structure; [PERSON] et al. [34] claim this could be sp\({}^{3}\) sites. The higher carbonate peak in Q1 carbon (7.1%) than in Q2 (5.3%) indicates that the SEI layer is thicker in Q1 than in Q2, which is consistent with the continual decrease of capacity with cycling shown in Figure 1. Figure 3c,d show the F1s spectra for Q1 and Q2 carbon; it has two signature peaks for trace salt NaPF\({}_{\rm x}\) at BE of 686.3 eV and NaF at BE of 683.2 eV. Q1 shows a higher NaPF\({}_{\rm x}\) salt content of 6.5% on the surface of the SEI compared to Q2 with only 2.8%, whereas Q2 is the opposite, showing a high NaF (5.3%) content on the surface compared to the 3.2% in Q1.