Microporous Carbon Polyhedrons Encapsulated Polyacrylonitrile

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Microporous carbon polyhedrons encapsulated polyacrylonitrile nanofibers as sulfur immobilizer for lithium-sulfur battery Ye-Zheng Zhang, Zhen-Zhen Wu, Gui-Ling Pan, Sheng Liu, and Xue-Ping Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00389 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 23, 2017

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ACS Applied Materials & Interfaces

Microporous

carbon

polyhedrons

encapsulated

polyacrylonitrile nanofibers as sulfur immobilizer for lithium-sulfur battery Ye-Zheng Zhang†,Zhen-Zhen Wu†,Gui-Ling Pan*‡, Sheng Liu†, and Xue-Ping Gao*† †Institute of New Energy Material Chemistry, School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin 300350, China. Fax: +8622-23500876, E-mail: [email protected] ‡Key Laboratory of Functional Polymer Materials of the Ministry of Education, Nankai University, Tianjin 300071, China. E-mail: [email protected]

Keywords: Lithium–sulfur battery; Microporous carbon polyhedrons; Polyacrylonitrile; Electrospinning; SEI layer.

Abstract

Microporous carbon polyhedrons (MCPs) are encapsulated into polyacrylonitrile (PAN) nanofibers by electrospinning the mixture of MCPs and PAN. Subsequently, the as-prepared MCPs-PAN nanofibers are employed as sulfur immobilizer for lithium-sulfur battery. Here, the S/MCPs-PAN multi-composites integrate the advantage of sulfur/microporous carbon and sulfurized PAN. Specifically, with large pore volume, MCPs inside PAN nanofibers provide a sufficient sulfur loading. While PAN-based nanofibers offer a conductive path and matrix.

1 Environment ACS Paragon Plus


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Therefore, the electrochemical performance is significantly improved for the S/MCPs-PAN multi-composite with a suitable sulfur content in carbonate-based electrolyte. At the current density of 160 mA g−1sulfur, the S/MPCPs-PAN composite delivers a large discharge capacity of 789.7 mAh g−1composite, high coulombic efficiency of about 100% except in the first cycle, and good capacity retention after 200 cycles. In particular, even at 4 C rate, the S/MCPs-PAN composite can still release the discharge capacity of 370 mAh g−1composite. On the contrary, the formation of the thick SEI layer on the surface of nanofibers with a high sulfur content are observed, which is responsible for the quick capacity deterioration of the sulfur-based composite in carbonate-based electrolyte. This desigh of the S/MCPs-PAN multi-composite is helpful for the fabrication of stable Li-S battery.

1. Introduction Lithium–sulfur (Li–S) battery is one of the most appealing choices for the light-weight and high energy battery system. Theoretically, sulfur can present a large specific capacity of 1670 mA h g−1, and therefore Li–S battery affords a high energy density of 2600 W h kg−1.1-4 Furthermore, sulfur offers obvious advantages as low cost, eco-friendly and abundant electrodeactive materials. However, there are still several hurdles that need to be overcome before commercialization of Li–S battery even after more than half a century of research.5 The major problems include the low utilization, poor high-rate capability, and fast capacity fading of active materials, owing to the low electronic/ionic conductivity of sulfur and high solubility of intermediate polysulfides produced during the electrochemical reaction processes.6-8 Many efforts aren attempted to tackle the above-mentioned issues, including the fabrication of S/C composites,9-19 S/polymer composites20-23 and S/C/polymer multi-composites.24-29 These strategies can effectively immobilize sulfur active material within the electrodes and improve the


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conductivity of the composites. However, in most instances, the discharge capacity of the sulfur cathode still decay slowly in long term cycles because of the unavoidable dissolution of polysulfides into ether-based electrolyte (LiTFSI/ether-based solvents). In carbonated-based electrolyte (LiPF6/carbonate-based solvents), the dissolution loss of the polysulfides produced in the discharge process sulfur can be radically eliminated,30-43 leading to the excellent cycle stability of the sulfur cathode. However, only two types of cathode materials (S/PAN and sulfur/microporous carbon) are compatible with conventional carbonate-based electrolyte. The S/PAN composite, namely sulfurized polyacrylonitrile (PAN), prepared by heating the mixture of sulfur and PAN in a wider temperature region from 280 to 800 °C, as reported previously.30-35 The as-prepared S/PAN composite shows well compatibility with carbonate-based electrolyte, and generally presents large reversible capacity and long cycle life. However, the high-rate capability is unsatisfied for the S/PAN composite due to the poor electronic conductivity (10−4 S cm−1).36 To overcome the drawbacks of the low electronic conductivity and poor high-rate performance, conductive carbon37-39and metallic oxide40 are used to modify the S/PAN composites. Another candidate of sulfur cathode in carbonate-based electrolyte is the sulfur/microporous carbon composite, which is usually prepared by infiltrating sulfur into the micropores of carbon matrix.9-10,41-46 The stable cycle performance (referred to the second cycle afterwards) of the sulfur/microporous carbon can be obtained in carbonate-based electrolyte, more superior to that in ether-based electrolyte. The common feature of these sulfur-based composites in electrochemical reaction can be described as following: 1) the good dispersion of sulfur with suitable content inside micropores of carbon or PAN without the appearence of crystalline sulfur;10 2) large irreversible capacity and low discharge plateau in the initial cycle;9-10,41-45 3) the solid-solid reaction mechanism with



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a single step transformation between sulfur and sulfide, and without the dissolution of intermediate polysulfides.41,45 Therefore, it is deduced from the feature above that SEI layer could be formed as a protective layer on the surface of sulfur-based composites to insure the excellent cycle stability in carbonate-based electrolyte, although the SEI layer is never be observed directly by SEM or TEM in the past. Meanwhile, it should be noted that the formation of SEI layer in the first cycle results inevitably in the large initial irreversible capacity. In particular, when the crystalline sulfur exists in the composite, the utilization of the sulfur active material is extremely lower in carbonate-based electrolyte. As mentioned above, the sulfur/microporous carbon and sulfur/PAN composites show their individual advantage and common feature. If two components can be combined into a new sulfur-based composite, the desirable electrochemical performance and reaction mechanism could be explored in carbonate-based electrolyte. Therefore, in this work, we introduce electrospun MCPs-encapsulated PAN (MCPs-PAN) nanofibers as sulfur container, which can provide 3D ionic/electronic transport channels for allowing Li-ions /electrons to react with the confined sulfur. In MCPs-PAN nanofibers, sulfur can be dispersed uniformly inside PAN nanofibers and in micropores of MCPs, which are beneficial to enhance the structure stability and ionic/electronic conductivity of sulfur cathode in carbonate-based electrolyte. In addition, the morphology of nanofibers is good for observing SEI layer on the surface and exploring the reaction mechanism.

2. Experimental Section 2.1 Preparation of MCPs


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The microporous carbon polyhedrons (MCPs) were prepared as reported previously.47 Typically, zinc nitrate hexahydrate (2.97 g) was dissolved in MeOH (100 mL). Then, 2methylimidazole methanol solution (100 mL, 0.8 M) was rapidly added into the former solution under slow magnetic stirring for 2 h at ambient temperature. Afterwards, the white precipitates were obtained by means of centrifugation, washed repeatedly with methanol and then dried at 120 °C in a vacuum. Finally, the resulting products of ZIF-8 polyhedrons were put into a ceramic boat and heated to 900 °C for 6 h under argon atmosphere to get MCPs. 2.2 Preparation and characterization of the composites The fabrication procedure of S/MCPs-PAN multi-composite was illustrated in Scheme 1. Specifically, MCPs (0.2 g) were added into N, N-dimethylformamide (DMF, 4.6 g) and sonicated for 2 h. PAN (MW=150,000, 0.4 g) was dissolved in the above solution. Strong mechanical stirring was applied for 12 h to form a homogeneous precursor for electrospinning. Then, the solution was transferred into the syringe (10 mL) with 22-gauge blunt tip and spun by applying a work voltage of 16 kV. The feeding rate and needle-to-collector distance were fixed at 10 µL min−1 and 20 cm, respectively. For comparison, PAN nanofibers were also prepared under the same conditions in the absence of MCPs. The PAN or MCPs-PAN was tailored into several small pieces. Then the nanofiber paper (0.4 g) was ground with sublimed sulfur (2.4 g). Two mixtures were firstly heated to 155 °C for 3 h, followed by heating to 300 °C for 8 h and 4 h, respectively. After cooling down to room temperature, S/MCPs-PAN composites with different sulfur content were obtained, marked as S/MCPs-PAN-52 and S/MCPs-PAN-65, respectively. The numbers in the marks represent the sulfur content in the S/MCPs-PAN composites. The whole heat treatment process was conducted in a tube furnace with flowing Ar gas. The sulfur content of the S/MCPs-PAN composites cannot be calculated accurately by TG curves, since



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sulfurized PAN lose hydrogen and nitrogen together with sulfur in the temperature range of 320 to 800 °C. The sulfur content is calculated based on the mass change before/after the heat treatment process. The microstructure and morphology of the composites were characterized by X-ray diffraction (XRD, Rigaku MiniFlex II), Brunauer-Emmett-Teller analysis (BET, JW-BK 122W), scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI, Tecnai F20)

Scheme 1 Schematic illustration of the preparation process for S/MCPs-PAN multi-composites. 2.3 Electrochemical measurement The cathodes were prepared by casting homogeneous slurry containing S/MCPs-PAN composites (80 wt.%), Super P (10 wt.%) and LA-132 binder (10 wt.%) on an Al foil. After drying at 60 °C for 24 h, the electrodes were punched into round films with a diameter of 10 mm. The sulfur loading for each cathode is about 1 mg cm−2. Coin-type cells (CR2032) were assembled using the Celgard 2300 as separator and Li foil as negative electrode in an Ar-filled glove-box. The electrolyte was 1.0 M LiPF6 in carbonate mixed solvents of PC/EC/DEC (1:4:5 by volume, Jinniu, Tianjin). The galvanostatic charge/discharge measurements were tested


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between 1.0–3.0 V (vs Li/Li+) by the LAND battery tester (CT2001A, Wuhan Jinnuo, China). Cyclic voltammetry (CV) were carried out by using an electrochemical workstation (CHI 600A, Shanghai ChenHua, China) at a scan rate of 0.1 mV s−1 for the initial 5 cycles. After disassembling cells in an Ar-filled glove-box, the cycled electrodes were collected and rinsed with DEC to remove LiPF6. After drying, the structure and morphology of the tested electrodes were identified by SEM (Hitachi S-4800), Fourier transform infrared (FTIR) spectra (Tensor 27, Bruker) and Bruker AV 400 M Hz spectrometer .

3. Results and discussion XRD patterns of S/MCPs-PAN-52 (52 wt.% S), S/MCPs-PAN-65 (65 wt.% S) and elemental sulfur are presented in Fig. 1. For the S/MCPs-PAN-52 composite, no characteristic peaks of crystalline sulfur can be detected, indicating that the sulfur is uniformly distributed in micropores of MCPs and inside PAN nanofibers as amorphous state.25 Only a broad peak can be detected at around 24°, corresponding to the graphitic (002) plane.31 However, the weak diffraction peaks of crystaline sulfur can be identified with increasing the sulfur content to 65 wt.%, where the residual sulfur covers on the outer surface of nanofibers and exists in crystalline state. S-MCPs-PAN-52 S-MCPs-PAN-65 S

Intensity

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10

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2θ (deg.)

Fig. 1 XRD patterns of S/MCPs-PAN-52, S/MCPs-PAN-65 and sulfur.



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SEM and TEM images (Fig. 2) show that MCPs are composed of uniform polyhedrons with an average size of 80 nm. In addition, MCPs present the large micropore volume (0.91 cm3 g−1) and applicable pore size (Fig. S1). When MCPs are introduced into PAN by electrospinning, the long and straight MCPs-PAN nanofibers with diameter of 100-300 nm are obtained as shown in SEM image (Fig. 2b). In particular, MCPs encapsulated inside PAN nanofibers can be observed clearly as indicated by red arrows in TEM image (Fig. 2d). In the case of S/MCPs-PAN-52 composite (Fig. 2e), the nanofiber morphology with a smooth surface still remains, suggesting the sulfur is completely infiltrated into MCPs-PAN fibers, which is further confirmed by the HAADF-STEM image of S/MCPs-PAN-52 (Fig. 3a). In addition, it is demonstrated from the elemental mapping (sulfur and carbon) that sulfur is uniformly distributed inisde MCPs-PAN nanofibers, consistent with the XRD analysis.

Fig. 2 SEM and TEM images of MCPs (a and c), electrospun MCPs-PAN nanofibers (b and d), and S/MCPs-PAN-52 (e). The MOF-derived MCPs are indicated by red arrows in Fig. 2d.


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a

b

c

Fig. 3 HAADF-STEM image of S/MCPs-PAN-52 (a) and corresponding elemental mapping of sulfur (b) and carbon (c). The electrochemical characterization of the S/MCPs-PAN composites are investigated in carbonate-based electrolyte of 1.0 M LiPF6 in PC/EC/DEC (1:4:5 by volume). Cyclic voltammograms (CVs) of the S/MCPs-PAN-52 and S/MCPs-PAN-65 composites are presented in Fig. 4. For the S/MCPs-PAN-65 composite with higher sulfur content, two peaks can be observed at 1.86 and 1.16 V (vs Li/Li+) in the first cathodic process. While for the S/MCPsPAN-52 composite with lower sulfur content, the high potential peak above 1.86 V (vs Li/Li+) is almost invisible, and only one board peak at 1.25 V with a shoulder peak at 1.58 V (vs Li/Li+) can be observed in the first cathodic process. In the subsequent cycles, only one cathodic peak at 1.65 V (vs Li/Li+) is shown for two composites. However, the peak areas in the anodic and cathodic processes are much lower for the S/MCPs-PAN-65 composite with higher sulfur content. The same tendency can be also found in the charge-discharge curves of the composites, which is different from that in ether-based electrolyte.



0.8

1st 2nd 3rd 4th 5th

(a) S/MCPs-PAN-52

0.8

0.0

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(b) S/MCPs-PAN-65

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1.5

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Fig. 4 Typical cyclic voltammograms (CVs) of S/MCPs-PAN-52 (a) and S/MCPs-PAN-65 (b) at a scan rate of 0.1 mV s−1. The initial three charge/discharge curves of the composites are shown in Fig. 5. Obviously, the charge/discharge potential plateaus are consistent with the anodic/cathodic peak potentials as shown in CVs. The S/MCPs-PAN-52 composite shows a narrow discharge profile above 1.75 V (vs Li/Li+), and a long discharge plateau between 1.4 and 1.75 V (vs Li/Li+), and the later shifts up to above 1.75 V (vs Li/Li+) in the subsequent cycles. The initial coulombic efficiency is 78.2 % for the S/MCPs-PAN-52 composite. For the S/MCPs-PAN-65 composite, two discharge plateaus can be also observed in the first discharge process, with a short discharge plateau at about 2 V (vs Li/Li+), and a long and low discharge plateau at about 1.4 V (vs Li/Li+). However, the reversible capacity is lower with a poor initial coulombic efficiency of 47.7 %. 3.0

+ Potential (V, vs Li/Li )

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(a) S/MCPs-PAN-52

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Capacity (mAh g -composite)


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+ Potential (V, vs Li/Li )

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1st 2nd 3rd

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Capacity (mAh g -composite)

Fig. 5 The initial three charge-discharge curves of (a) S/MCPs-PAN-52 and (b) S/MCPs-PAN65 at a current density of 160 mA g−1sulfur. The cycle performance of the S/MCPs-PAN composites is indicated in Fig. 6. The S/MCPsPAN-52 composite delivers a large reversible capacity of 789.7 mAh g−1composite (1518.6 mA h g−1sulfur) in the second cycle and the discharge capacity is maintained at 666.2 mAh g−1composite after 200 cycles, indicating a high utilization (90.7 %) of sulfur active material, and a good capacity retention (84.4 %). The initial coulombic efficiency is Additionally, the coulombic efficiency of the S/MCPs-PAN-52 composite is almost identical to 100 % from the second cycle, showing the solid-solid reaction characteristics during the electrochemical reaction processes. In comparison, the S/MCPs-PAN-65 sample shows a low utilization of sulfur active material and low discharge capacity. 1200 100 1000 80 800 60

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Fig. 6 The cycle performance of the as-prepared S/MCPs-PAN multi-composites and the Coulombic efficiency of S/MCPs-PAN-52 at a current density of 160 mA g−1sulfur. The dramatic difference in CVs and charge-discharge curves for the S/MCPs-PAN composite with different sulfur contents is mainly related to the formation of SEI layer on the surface of nanofibers during the initial discharge process in carbonate-based electrolyte. In particular, the formation of SEI layer is sensitive to the crystalline or amorphous state of sulfur in the composite. It means the sulfur content in the composite should be controlled below a certain valve as reported previously in sulfurized PAN and sulfur/microporous carbon.9-10,34,43 To date, there are four possible reaction mechanisms to explain this phenomenon: 1) S8 shows no rechargeability in carbonate-based electrolyte;43 2) the lower solubility of polysulfides in carbonate-based electrolyte resulting in the passivation of sulfur active material;48 3) the irreversible reactions between LiPF6 and polysulfides;49 and 4) the irreversible reactions between carbonate solvents and polysulfides.50-51 To get an insight into the reaction mechanism of the S/MCPs-PAN composite with different sulfur content, more information should be collected on cathode material from disassembled Li-S battery after the initial cycle. To verify the reaction mechanism, SEM images of S/MCPs-PAN nanofibers with different sulfur contents in the charged/discharged state during the first cycle are presented in Fig. 7. Clearly, nanofibers with the smooth surface coexist with conductive carbon particles in pristine electrodes of S/MCPs-PAN-52 and S/MCPs-PAN-65 composites. In the discharged state, the surface of nanofibers are covered by rough and dense precipitates for both S/MCPs-PAN-52 and S/MCPs-PAN-65 composites. However, in the charged state, such rough precipitates disappear almost, leaving the clean surface of nanofibers for the S/MCPs-PAN-52 composite. Even after 200 cycles, the clean surface on PAN-based nanofibers is still observed as shown in Fig. S5. On the contrary, in the case of the S/MCPs-PAN-65 composite, such rough precipitate particles are


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turned back incompletely, and the surface of nanofibers is still covered by the thick and dense layer in the charged state. Therefore, the SEI layer is formed with carbonate solvents on the surface of nanofibers in the initial discharged state, leading to the irreversible capacity and potential polarization. It should be noted here that the good cycle stability of the S/C composites in carbonate-based electrolyte is mainly attributed to the formation of the SEI layer on account of the limited solubility of polysulfides in carbonate-based electrolyte,52 different from the high solubility of polysulfides in ether-based electrolyte. In the meantime, the content and existing form of sulfur in the S/C composites is very sensitive to the utilization and reversible capacity of sulfur active material in the carbonate-based electrolyte. In particular, the amorphous sulfur in microporous carbon or PAN is indispensable and compatible in the carbonate-based electrolyte.52 When amorphous sulfur with the low content is distributed uniformly inside MCPs-PAN nanofibers, the SEI layer formed in the discharged state can be turned back almost in the charged state. Here, the thin SEI layer as a protective layer on the surface is important and helpful to insure the utilization of sulfur active material and good cycle stability of the composite based on the solid-solid reaction mechanism. In contrast, the high sulfur content with appearance of the crystalline state results in the formation of the thick and dense SEI layer on the surface of MCPsPAN nanofibers in the discharged state. Especially, the formation of the SEI layer is initiated at the high potential plateau region (above 1.8 V) during the initial discharge process as shown in Fig. S3 and Fig. S4. Such thick and dense SEI layer is largely retained in the charged state, which is harmful to the utilization and reversible capacity of sulfur active material. This observation provides a strong evidence that the formation of thick and dense SEI layer leads to the corresponding reversible capacity fade of the S/MCPs-PAN-65 composite with the appearence of crystalline sulfur on the surface of nanofibers. While, the high distribution of



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sulfur as amorphous state inside micropores of MCPs or heterocyclic rings of dehydrogenated PAN nanofibers is responsible for the high utilization and reversible capacity of sulfur active material. Actually, it is regrettable that the sulfur content in the S/C composites is hard to increase after taking the sulfur utilization into consideration, which is a common feature in the carbonate-based electrolyte as reported previously.9-10,29-30,41,52

Fig. 7 High-magnification SEM images of S/MCPs-PAN nanofibers during the first cycle. S/MCPs-PAN-52 for pristine (a), discharged state (b), and subsequently charged state (c). S/MCPs-PAN-65 for pristine (d), discharged state (e), and subsequently charged state (f). It is demonstrated from above electrochemical performance and SEM observation that the poor utilization of sulfur and fast capacity fade of the S/MCPs-PAN-65 composite is originated from the formation of the thick and dense SEI layer on the surface on nanofibers. To identify the SEI layer, FTIR spectra are collected from the S/MCPs-PAN-65 cathode at the high potential plateau region (at 1.8 V) during the initial discharge process, as shown in Fig. 8. Some increasing assignments are marked on FTIR spectra.53-55 The peak at 842 cm−1 is related to the signal of δCO3 (LiCO3). The characteristic peak at 1634 cm−1 is ascribed to the C=O stretching vibration of thiocarbonate. In the mean time, there are some organic groups detected. It means that the components of the SEI layer is complex, composed of inorganic salts and organic products by


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irreversible side reactions between high-order polysulfides with carbonate solvents at the high

3500

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550(νLiO)

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pristine electrode discharged electrode

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2996(νCH)

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potential plateau region during the initial reduction process.

Transmitance (a.u.)

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500

)

Fig. 8 FTIR spectra of a pristine cathode and the cathode after initial discharge to 1.8 V. To further ascertain the chemical composition of the SEI layer on the surface of nanofibers, 1H NMR spectra are measured for the S/MCPs-PAN-65 composite after the initial discharge to 1.8 V and 100 cycles (Fig. 9). Clearly, more information can be detected. including thiocarbonate, ethanol, propylene glycol, ethylene glycol and –SnRSn–.51,56 It implys the cleavage at the CH2–O of carbonate solvents from the presence of –SnRSn– and Li2CO3, as well as the cleavage at the C–O bonds of OCO2 in carbonate solvents from the presence of ethanol, ethylene glycol and propylene glycol in the SEI layer. On the basis of the above analysis, possible mechanisms for the formation of the SEI layer on the cathode surface are described and presented in Scheme 2. Usually, the formation of the SEI layer could consume the sulfur active material, resulting in the irreversible capacity and poor initial coulombic efficiency. Badly, the utilization of the sulfur active material is lower due to the formation of the thick and dense SEI layer originated from



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crystalline sulfur on the surface, which could further hinder the subsequent electrochemical reactions of sulfur beneath the SEI layer. It is suggested that sulfur as amorphous state should be distributed uniformly inside micropores of MCPs and PAN nanofibers, which is crucial for constructing a stable sulfur cathode compatible with conventional carbonate-based electrolyte. It is noted that the short and high discharge plateau is almost finished at the initial discharge to 1.8 V, while the long and low discharge plateau is not started yet in the discharge curves. Especially, some irreversible reactions occur for the formation of SEI layer during the initial discharge process. Therefore, in this work, we use the cathode after the initial discharge to 1.8 V for the characterization of NMR, FTIR and SEM.

Fig. 9 1H NMR in D2O of the cathode after the initial discharge to 1.8 V and 100 cycles. The cathodes are extracted with D2O, and then the extracted solutions are subjected to 1H NMR spectra. O

O O

O

+

Sn2-

O-

O

CO32-

Sn O O

+

Sn2-

O H3C

CO32-

+ Sn 2-

Sn-

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CH3

+ S 2n

H3C

Sn -

+ H3C

+ H3C

O O

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-

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+

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+ Sn2O-

CO32-


+

2 H3C

Sn -

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O-

O

+ Sn2-

O

O

O

O

S nO

-O

OO O O

-O

-O

+

Sn-

-S n

Sn -

O CH3

+ Sn2-

-

+ Sn2O-

H3 C

O-

O

S nO

H3C O

O

+ Sn2Sn -

O

CH3 O

Sn-

-O

O

O

O

O-

O

+

-

S n-

Sn

H3C S n-

H3C O

O H3C

O

O

CH3

+ S 2n

H3 C

O

Sn-

+

H3C

O-

O

+ Sn 2-S

n

S n-

+

2 H3C

O-

Scheme 2 Possible mechanisms for the formation of thick SEI layer on the S/MCPs-PAN-65. Based on the above analysis, the S/MCPs-PAN-52 composite presents the optimized electrochemical performance. Here, the synergistic effect of MCPs and PAN nanofibers is obvious for improving the electrochemical performance of sulfur-based composites. For example, the S/MCPs (54 wt. % S) composite shows large initial discharge capacity of 1005.7 mA h g−1composite (1862.4 mA h g−1sulfur), but the discharge capacity is dropped sharply to 583.3 mAh g−1composite (1080.2 mAh g−1sulfur) in the second cycle (Fig. 10a), much lower as compared with that of the S/MCPs-PAN-52 composite. The initial coulombic efficiency of S/MCPs is poor, similar to the microporous carbon/sulfur composites in the carbonate-based electrolyte as shown in Table S1. Here, a partial sulfur exposed near the surface of MCPs reacts with the carbonate solvents in the initial discharge process. It means that the initial irreversible capacity of the S/MCPs (54 wt % S) composite is larger due to the formation of the thick SEI layer on the surface of MCPs with a large specific surface area to expose to carbonate-based electrolyte. It seems that multi matrix is effective to improve the initial coulombic efficiency, such as CNT/microporous carbon.42 The sulfur-contained PAN matrix, as a buffer layer, efficiently prevent the penetration of the carbonate solvents, reducing the initial irreversible capacity without harming electron and Li+ ion transportation. When only PAN nanofibers are used as the matrix of sulfur, the utilization of sulfur active material is still poor, and the high rate capability of the S/PAN (39 wt % S) composite is unsatisfied. It should be noted that the high-rate



capability of the sulfur-based composite is hard to enhance in carbonate-based electrolyte due to the the solid-solid reaction mechanism and the formation of SEI layer, different from the reaction mechanism of soluble polysulfides in ether-based electrolyte. Here, with the synergistic effect of MCPs and PAN nanofibers, the S/MCPs-PAN-52 composite presents an excellent high-rate capability. Noteworthily, the large discharge capacity of 370 mA h g−1composite is obtained at 4 C rate, suggesting the fast kinetics of the S/MCPs-PAN-52 composite. Moreover, the discharge capacity can be recovered when the rate is decreased from 4 C to 0.1 C. The excellent high-rate capability is mainly attributed to: 1) the introduction of MCPs into PAN nanofibers can provide a sufficient sulfur loading, and 2) such MCPs-encapsulated PAN nanofibers can insure the structure stability of the composite, and offer 3D ionic/electronic transport channels. S/MCPs-PAN-52 S/MCPs

(a)

800

-1


1000

600

400

200

0 0

20

40

60

80

100

Cycle number

1000

S/MCPs-PAN-52 S/PAN

(b) 0.1C

800

0.2C

-1


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0.1C 0.5C

1C

600

2C 3C 4C

400

200

0

0

10

20

30

40

50

Cycle number

Fig. 10 The cycle performances of S/MCPs and S/MCPs-PAN-52 at a current density of 160 mA g−1sulfur (a), and the high-rate performance of S/MCPs-PAN-52 and S/PAN (b).


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As reported previously, the sulfur/microporous carbon and sulfur/PAN composites show their individual intrinsic advantages in carbonate-based electrolyte, especially the good cycle stability (referred to the 2nd cycle afterwards). However, there are still many challenges for such composites in carbonate-based electrolyte, including the poor initial coulombic efficiency, low utilization of sulfur active material, and unsatisfied high-rate discharge capability. After introducing microporous carbon polyhedrons (MCPs) into PAN nanofibers, the high initial coulombic efficiency (78.2), good utilization (90.7%) of sulfur active material, and satisfied high-rate discharge capability (4C rate) are obtained in carbonate-based electrolyte. In other words,

S/MCPs-PAN

multi-composites

not

only

inherit

the

advantages

of

both

sulfur/microporous carbon and sulfur/PAN, but also overcome their individual intrinsic disadvantages. The synergistic effect of MCPs and PAN nanofibers is effective for achieving the desired electrochemical performance with high initial coulombic efficiency, high sulfur utilization, good capacity retention, and excellent high-rate capability, instead of a simple combination of MCPs and PAN nanofibers.

4. Conclusions In summary, MCPs are encapsulated into PAN nanofibers to fabricate MCPs-PAN nanofibers by electrospinning the mixture of MCPs and PAN. Here, S/MCPs-PAN nanofibers take the advantage of the complementarity of sulfur/microporous carbon and S/PAN nanofibers to overcome their individual and intrinsic disadvantages. Inparticular, MCPs inside PAN nanofibers provide a sufficient sulfur loading, while PAN-based nanofibers insure the structure stability of the composite, and offer fast channels for ionic and electronic transportation. Correspondingly, S/MCPs-PAN nanofibers, with a suitable sulfur content as amorphous state inside MCPs and



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PAN nanofibers, present the optimized electrochemical performance, including a high sulfur utilization, good capacity retention, and excellent rate capability. Here, the thin SEI layer formed on the surface of nanofibers is beneficial to the cycle stability of the composite, while the formation of the thick and dense SEI layer on the surface of nanofibers is detrimental for the utilization of sulfur active material in carbonate-based electrolyte. Therefore, the novel design of S/MCPs-PAN nanofibers is helpful to achieve the S-based composite with desirable comprehensive electrochemical performance for the fabrication of high energy Li-S battery.

Supporting Information. N2 adsorption/desorption isotherms/corresponding pore size distribution, SEM images, TEM images, charge-discharge curves, TG curves and characteristics of various sulfur/micropore carbon are included.

Acknowledgments Financial supports from the 973 Program (2015CB251100), and NSFC (21573114 and 51502145) of China are gratefully acknowledged. References 1. Gao, X. P.; Yang, H. X. Multi-Electron Reaction Materials for High Energy Density Batteries. Energy Environ. Sci. 2010, 3, 174−189. 2. Fan, Q.; Liu, W.; Weng, Z.; Sun, Y. M.; Wang, H. L. Ternary Hybrid Material for HighPerformance Lithium–Sulfur Battery. J. Am. Chem. Soc. 2015, 137, 12946−12953. 3. Osada, N.; Bucur, C. B.; Asoab, H.; Muldoon, J. The Design of Nanostructured Sulfur Cathodes using Layer by Layer Assembly. Energy Environ. Sci. 2016, 9, 1668−1673. 4. Kong, L. L.; Zhang, Z.; Zhang, Y. Z.; Liu, S.; Li, G. R.; Gao X. P. Porous Carbon Paper as Interlayer to Stabilize the Lithium Anode for Lithium–Sulfur Battery. ACS Appl. Mater.


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Table of Contents 1000 800

-1


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600 400 200 0

0

50

100 Cycle number


150

200

Microporous Carbon Polyhedrons Encapsulated Polyacrylonitrile

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