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Electrochemical evaluation of NGNS/S-H25, NGNS/S-H45, NGNS/S-H65 and NGNS/S- H86 nanocomposites along with two control samples of S and NGNSH was conducted at room temperature. Let’s first take a look at the case of S loading less than 50%. From the cyclic voltammetry (CV) profiles of NGNS/S-H25 in Figure 4a, it can be observed that there is a sharp cathodic peak at potential of 2.18 V during the reduction process in the 1st cycle. This peak could be attributed to the formation of long chain polysulfides Na2Sn (4 ≤ n ≤ 8).[5i] Another small broad cathodic peak at ~1.5 V with the following sweeping until the cut-off voltage corresponds to the reduction process of long chain polysulfides to Na2S.[14] In the reverse oxidation process in the 1st cycle, there is an anodic peak centered at 1.85 V, indicating that Na2S has been oxidized to short chain polysulfides Na2Sn (2 ≤ n < 4) and Na2S4.[5i] Another small anodic peak observed at ~2.23V is assigned to the oxidation process of Na2S4 to Na2S8 and S.[5j] After the current sweeps back to the reduction process of 2nd cycle, the long chain polysulfides related cathodic peak can barely be observed. This disappearance of cathodic peak may be due to the soluble polysulfides Na2Sn (4 ≤ n ≤ 8) dissolve and dissipate in the electrolyte during the first cycle or the precipitation of insoluble Na2Sn (1 ≤ n < 4) as solid phases which may have a detrimental effect on the kinetics of Na-ion diffusion into the electrode.[15] From the oxidation curves of the 2nd to 4th cycles, the anodic peaks can be observed with less intensity change. Figure 4b exhibits the charge/discharge profiles of NGNS/S-H25 at 0.05C. There is no obvious plateau observed from the discharge curves. Wang et al. reported similar discharge behavior of S/polymer composite in RT Na-S batteries.[5b] From the charge curves, an oxidation plateau shown at 1.85 V can be observed, which is consistent with the CV performance. The discharge/charge capacities have been reached 212 mAh g-1 and 181 mAh g-1 in the 1st cycle, respectively. The rate capability of NGNS/S-H25 from 0.05C to 5C is shown in Figure 5a. At 13 This article is protected by copyright. All rights reservedPDF Image | Nitrogen-Doped Graphene Nanosheets Composites as Cathode in Sodium-Sulfur Batteries
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