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process of the 1st cycle. This reduction peak could be assigned to the reduction reaction of S to Na2S4 and Na2S2.[5f] A small reduction peak is observed at 1.67 V, which is attributed to Na2S2 transferring to sulfide Na2S.[5f] During the anodic process of the 1st cycle, there are no obvious oxidation peaks shown up in the curve. From the following cathodic and anodic sweep in the 2nd and 4th cycles, there are no obvious reduction and oxidation peaks observed on the CV curves of NGNS/S-H86. This phenomenon might be due to the formation of irreversible sodium polysulfides which could not recover to sulfur during charge process or the dissolution of sodium polysulfides into the electrolyte.[5a] The galvanostatic charge/discharge profile of NGNS/S-H86 cathode is shown in Figure 4h. A typical plateau at 1.7 V is observed on the discharge curve of 1st cycle, which can be assigned to the formation of sodium sulfide Na2S.[5f, 14b] A relatively high discharge capacity of 375 mAh g-1 is obtained at a cutting voltage of 1.0 V. During the second cycle, there are plateaus observed on both charge and discharge curves at 2.63 V and 1.25 V, respectively. The discharge capacity reduces to 82 mAh g-1 at the 2nd cycle, and at the 10th cycle only limited capacity ~25 mAh g-1 still remains. There are small plateaus observed on the discharge curves of the following 4th, 5th and 10th cycles, which reveal that S is constantly reduced to polysulfides in each cycle. However, the voltages of the plateaus show lower and lower with capacity reduction during cycling. This voltage drop might be attributed to the formation of surface film and the ion transfer across the liquid/solid interface.[5f] From rate performance of NGNS/S-H86 in Figure 5d, it can be observed that the capacity at 0.05C of NGNS/S-H86 reduced from 375 mAh g-1 of the 1st cycle to ~25 mAh g-1 of the 10th cycle. At 0.1C, only ~5 mAh g-1 remains. The CV and charge/discharge results of pure sulfur are shown in Figure 4i and 4j. In Figure 4i, There is one strong reduction peak observed at 2.02V in the cathodic process of 1st cycle, representing S reduce to the polysulfides Na2S4 and Na2S2.[5f] In 16 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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