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Incorporating Sulfur Inside the Pores of Carbons for Advanced Lithium–Sulfur Batteries: An Electrolysis Approach Bin He, Wen-Cui Li, Chao Yang, Si-Qiong Wang, and An-Hui Lu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07340 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 8, 2016
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Incorporating Sulfur Inside the Pores of Carbons for Advanced Lithium–Sulfur Batteries: An Electrolysis Approach Bin He, Wen-Cui Li, Chao Yang, Si-Qiong Wang, and An-Hui Lu* State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian116024, P. R. China E-mail:
[email protected] KEYWORDS: monolithic carbon, electrolysis approach, microporous carbon nanosheets, high electrochemical activity, lithium-sulfur battery
ABSTRACT: We have developed an electrolysis approach (ELA) that allows effective and uniform incorporation of sulfur inside the micropores of carbon nanosheets for advanced lithium sulfur batteries. The sulfur-carbon hybrid can be prepared with a 70 wt% sulfur loading, in which no nonconductive sulfur agglomerations are formed. Because the incorporated sulfur is electrically connected to the carbon matrix in nature, the hybrid cathode shows excellent electrochemical performance including a high reversible capacity, good rate capability and good cycling stability, as compared to one prepared using the popular melt-diffusion method.
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Sulfur is a promising cathode material for lithium-sulfur (Li-S) batteries.1-4 However, the insulating nature of sulfur and the high solubility of long-chain polysulfides (Li2Sx, 4≤ x≤ 8) severely limit the practical use of sulfur in an electrode.5-7 To improve the electron conductivity of the cathode and suppress the active material dissolution, sulfur is often incorporated into a carbon matrix.8-10 So far, the major reported approaches for fabricating sulfur-carbon hybrids include: mechanical mixing, a melt-diffusion strategy, and solution-based synthesis.11 The sulfur-carbon hybrid cathodes prepared by these methods generally contain some nonconductive sulfur agglomerations due to insufficient incorporation of sulfur inside the pores of carbons, and suffer from low utilization of the active material and severe capacity fade.12,13 Moreover, significant volume expansion of sulfur during charge-discharge process can cause damage of the electrode structure.14-17 To address this problem, an effective method would be to incorporate sulfur into the micropores of carbons to accommodate volume expansion and strain compared with those large sulfur particles encapsulated in mesopores and macropores.18-23 However, it is difficult for the sulfur to enter the micropores of the carbon matrix due to a high diffusion resistance. Hence, an effective approach for the preparation of sulfur-carbon hybrids with micropore-trapped and electrically connected sulfur in a uniform distribution throughout the entire carbon matrix is urgently needed. Herein, we report an electrolysis approach (ELA) to incorporate sulfur inside the pores of carbons for advanced lithium-sulfur batteries. The sulfur-carbon hybrids obtained with a high sulfur content of ~70 wt % exhibit excellent electrochemical performance including a high initial reversible capacity, good rate capability and good cycling stability. The good performance is attributed to the high dispersity and electrochemical activity of the sulfur.
Results and Discussion
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The incorporation of sulfur by ELA at room temperature is illustrated in Figure 1. First, a self-supporting and binder-free monolithic carbon was prepared (Figure 1a, experimental details see the experimental section), which was used as the working electrode for electrolysis. As illustrated in Figure 1b, H2S was bubbled into the solution and was electrolyzed at a constant voltage, during which polysulfide ions were formed. The polysulfide ions in the electrolyte can access the pores and active sites under the electric field and capillary forces, and once polysulfide ions reaches these places, they were converted into sulfur by losing two electrons. The sulfur gradually filled the pore space with increasing time of electrolysis (Figure 1c). Simultaneously, H2 and OH- were generated at the cathode. It is noted that the OH- react with H2S can increase H2S absorption and keep the pH relatively stable. Cyclic voltammogram (CV) prior to electrolysis is conducted. From the CV curves (Figure S1), it exhibited one broad anodic peak at 0.6-0.7 V, which corresponds to the conversion of sulfide or low polysulfide to higher polysulfides and sulfur. The anodic peak potential was moved to lower potential (0.4-0.5 V) at second cycle, corresponding to the conversion of higher polysulfides to sulfur. The overall reactions can be expressed as follows.24 Electrolysis reactions: Anode :
Cathode : Chemical reaction:
S2- → S↓+ 2e-
(1)
Sx2- → xS↓+ 2e-
(2)
xSx-12- → (x-1) Sx2- + 2e-
(3)
xSx-12- + S2- → Sx2- + 2e-
(4)
2H2O + 2e- → H2↑ + 2OH-
(5)
2OH- + H2S + xS→ 2H2O + Sx+12-
(6)
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Figure 1. (a) Synthesis process of the monolithic carbon used for electrolysis. (b) Illustration of the electrolysis approach (ELA) to the preparation of sulfur-carbon hybrids. (c) The process by which sulfur is incorporated inside the pores. The monolithic carbon (named as MLC-1) used as working electrode has good mechanical strength with a compressive strength measured under uniaxial compression up to 0.7 MPa (Figure S2a). The MLC-1 is self-supporting, with a continuous conductive carbon framework and an open pore system. This is significantly different from shaped carbon monoliths which possibly have a partially closed pore system due to the use of binders. The good mechanical strength and good conductivity allow MLC-1 to be formed as a working electrode, retaining its original structure without damage (Figure S2b). A typical scanning electron microscopy (SEM) image of MLC-1 is shown in Figure 2a. It can be seen that it consists of homogeneously interconnected nanosheet units, which produce abundant unblocked macropores in the carbon framework. The Hg intrusion analysis of MLC-1 (Figure S2c) shows that a plateau characterized
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with a saturation Hg uptake of 6.3 cm3 g-1 was reached when the pores are close to 250 nm in diameter. This indicates that MLC-1 has abundant and fully interconnected macropores, allowing easy diffusion of the electrolyte for electrolysis, which is in good agreement with the results of the SEM image. It should be noted that the advantages of the MLC-1 (e.g. high mechanical strength, interconnected structure, etc.) are beneficial for the advanced electrolysis. During the preparation of the Li-S electrode, the obtained monolith was ground into powder for use.
Figure 2. (a) SEM image of MLC-1 before electrolysis. (b) N2 sorption isotherms of MLC-1 and MLC-2, and the inset is the pore size distribution of MLC-1. (c) TGA curves of MLC-2, MLC-3 and pure sulfur. (d) XRD patterns of MLC-1, MLC-2 and MLC-3. The nitrogen sorption isotherm of MLC-1 is shown in Figure 2b. A significant nitrogen uptake at a relative pressure P/P0
