PDF Publication Title:
Text from PDF Page: 008
Paper Journal of Materials Chemistry A literature,56 small sulfur allotropes with chain-like structure, S2–4, have at least one dimension #0.39 nm. Thus, this peak is consistent with the fact that small S2–4 molecules instead of large cyclo-S8 exist in the composite.56 Furthermore, the HRTEM image (Fig. S8b†) shows that the lattice spacing of the AC@S sample washed by CS2 is 0.39 nm, which is suitable to accom- modate small S2–4 molecules. EDS mapping results (Fig. S8c†) also vividly demonstrate that the sulfur signal is overlapped with carbon signal, implying the existence of sulfur and good distribution in AC matrices. It can therefore be concluded that a part of the small sulfur allotropes (S2–4) penetrated into the interlayers of carbon via the SC-CO2 method, which is favorable for alleviating the dissolution and shuttling problems of polysuldes. The rate performances of AC@S and AC/S-155 electrodes are also compared in this work. Fig. 4d clearly illustrates the charge and discharge proles of AC@S electrodes under various current densities from 0.1 to 2 A g1. All the charge and discharge curves of the AC@S sample exhibit well-dened voltage plateaus and small polarization along with the gradu- ally increased current density. This result vividly demonstrates that the AC@S sample has a superior electrochemical revers- ibility and fast electrochemical kinetics. Furthermore, as shown in Fig. 4e, the specic discharge capacities of the AC@S sample are 882, 697, 523, 381 and 242 mA h g1 at 0.1, 0.2, 0.5, 1 and 2 A g1, respectively, which are much higher than those of the AC/S-155 sample; the specic discharge capacity of the AC@S sample can be recovered well to 762 mA h g1 at 0.1 A g1 aer multi-rate tests. In contrast, the specic capacity of the AC/S-155 sample fades very fast with increasing current density, and only retains 392 mA h g1 aer recycling at 0.1 A g1. Obviously, both the specic capacity and coulombic efficiency of the AC@S samples are higher than that of AC/S-155 samples. This remarkable improvement in the rate performance of the AC@S sample can be greatly attributed to the strong interaction between the sulfur and carbon matrix with the aid of SC-CO2. The universality of the SC-CO2 method was validated by the MCMB@S and MWCNTs@S samples. Fig. 4f and S9† present the charge/discharge capacity and capacity retention of three different C@S cathodes. Aer 100 cycles at 0.1 A g1, the reversible discharge capacities of AC@S, MCMB@S and MWCNTs@S were 817, 715 and 441 mA h g1, corresponding to capacity retentions of 90.5%, 88.6% and 51.2%, respectively. Meanwhile, as shown in Fig. S9 and Table S2,† the specic capacities, capacity retentions and coulombic efficiencies of C/ S-155 samples prepared by melt-diffusion method were lower than that of C@S samples derived from the SC-CO2 method. These results conrm that the SC-CO2 method may be suitable for different carbon matrices to achieve various C@S cathodes with fascinating Li storage performance. Additionally, to better demonstrate the advantages of the SC-CO2 method, Table 1 provides a comparison of various synthesis methods for carbon–sulfur composites. Obviously, the SC-CO2 synthetic strategy signicantly surpasses other conventional synthesis methods of carbon–sulfur composites. The merits of the SC-CO2 method can be summarized as follows. (1) The reaction media used in the SC-CO2 method is SC-CO2 uid, which is a cheap, the microstructure of carbon matrices to create more space for sulfur storage, and synchronously guarantee the highly efficient sulfur transfer in carbon matrices. The lithium storage properties of AC@S sample were evalu- ated by CR2025 button cells. Fig. 4a displays the CV proles of AC@S sample, which exhibit the typical features of multi- electron redox reactions in sulfur cathodes.49 Two cathodic peaks at 2.3 V and 2.0 V can be assigned to the stepwise reduction from S8 to soluble long-chain polysuldes (Li2Sn, 8 $ n $ 4) and further to insoluble Li2S2/Li2S.23 In the subsequent anodic scanning, two asymmetric oxidation peaks were observed at 2.3 and 2.4 V, which correspond to the reversible conversion of Li2S2/Li2S to Li2S8 and S.23 Notably, the succeed- ing CV proles overlapped well with each other, suggesting a relatively good cycling stability. Moreover, the galvanostatic charge–discharge curves of the AC@S sample for the 1st, 30th, 50th and 100th cycles at 0.1 A g1 are presented in Fig. 4b. Two discharge plateaus and two charge plateaus can be clearly detected, which are consistent with the CV results (Fig. 4a). During the 1st cycle, a high specic capacity of 905 mA h g1 was obtained. Aer that, AC@S electrode still retained a stable specic capacity of 817 mA h g1 aer 100 cycles at 0.1 A g1, along with a good capacity retention of 90.5% and a superior coulombic efficiency of approximately 100%. In order to better clarify the merits of the AC@S sample, the long-term cycling performance comparison between AC@S and AC/S-155 was conducted. As shown in Fig. 4c, the AC@S elec- trode has a high reversible specic capacity and outstanding cyclic stability, compared to the AC/S-155 sample. Aer 100 cycles at 0.1 A g1, the reversible discharge/charge capacities of AC@S and AC/S-155 were 819/817 mA h g1 and 394/ 386 mA h g1, respectively. Moreover, the capacity retention of AC@S was 90.5%, which is much higher that of AC/S-155 (58.7%). This remarkable cycling stability of the AC@S sample can be attributed to two factors. Firstly, with the assistance of SC-CO2, the AC@S sample has a more homogenous sulfur dispersion, guaranteeing good contact between sulfur and the conductive carbon framework. Secondly, SC-CO2 aids sulfur to deeply inltrate the inner pores and interlayers of AC and form strong interactions between sulfur and AC, which could trap sulfur and polysuldes during cycling. To verify the existence of sulfur in the interlayers of the AC matrices, we designed the following experiment to get deep insight into the Li storage mechanism of the AC@S sample. The AC@S sample was repeatedly washed by CS2 solution to remove sulfur in the pores or on the surface of the AC matrices. It should be mentioned that if sulfur is stably encapsulated in the interlayers, it will be hardly removed by CS2. As shown in Fig. S8a,† three reduction peaks appeared at 2.3 V, 2.0 V and 1.9 V. Two peaks located at 2.3 V and 2.0 V were assigned to the stepwise reduction from S8 to soluble long-chain polysuldes (Li2Sn, 8 $ n $ 4) and further to insoluble Li2S2/Li2S, corre- sponding to the characteristic electrochemical reactions of S8. This result indicates that sulfur (S8) can reach into the deep pores of AC matrices with the assistance of SC-CO2 uid. Interestingly, a novel electrochemical behavior was observed, in which a reduction peak was detected at 1.9 V. According to the This journal is © The Royal Society of Chemistry 2017 J. Mater. Chem. A View Article Online Published on 24 November 2017. Downloaded by University of Texas Libraries on 08/12/2017 20:16:36.PDF Image | Supercritical CO2 Mediated Incorporation of Sulfur into Carbon Matrix
PDF Search Title:
Supercritical CO2 Mediated Incorporation of Sulfur into Carbon MatrixOriginal File Name Searched:
SupercriticalCO2mediatedincorporationofsulfurintocarbonmatrixascathodematerialstowardshigh-performancelithiumsulfurbatteries.pdfDIY PDF Search: Google It | Yahoo | Bing
Sulfur Deposition on Carbon Nanofibers using Supercritical CO2 Sulfur Deposition on Carbon Nanofibers using Supercritical CO2. Gamma sulfur also known as mother of pearl sulfur and nacreous sulfur... More Info
CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com (Standard Web Page)