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Lock of sulfur with carbon black and a three-dimensional graphene@carbon nanotubes coated separator for lithium-sulfur batteries
Accepted Manuscript Lock of sulfur with carbon black and a three-dimensional graphene@carbon nanotubes coated separator for lithium-sulfur batteries Haiwei Wu, Ying Huang, Weichao Zhang, Xu Sun, Yiwen Yang, Lei Wang, Meng Zong PII:
S0925-8388(17)30816-2
DOI:
10.1016/j.jallcom.2017.03.047
Reference:
JALCOM 41085
To appear in:
Journal of Alloys and Compounds
Received Date: 14 January 2017 Revised Date:
21 February 2017
Accepted Date: 5 March 2017
Please cite this article as: H. Wu, Y. Huang, W. Zhang, X. Sun, Y. Yang, L. Wang, M. Zong, Lock of sulfur with carbon black and a three-dimensional graphene@carbon nanotubes coated separator for lithium-sulfur batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.047. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT Lock of sulfur with carbon black and a three-dimensional graphene@carbon nanotubes coated separator for lithium-sulfur batteries Haiwei Wua, Ying Huanga,*1, Weichao Zhanga, Xu Suna, Yiwen Yanga, Lei wangb, Meng
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Zonga Department of Applied Chemistry and The Key Laboratory of Space Applied Physics and
Chemistry ,Ministry of Education, School of Natural and Applied Sciences, Northwestern
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’
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b
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Polytechnical University, Xi’an 710072, PR China
an 710021, China
Abstract
Three-dimensional graphene@carbon nanotube (G@CNT) composite was
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coated on polypropylene separator as an interlayer for high performance of lithium-sulfur batteries. In this study, the novel G@CNT nanotubes coated separator offers not only fast electron pathway for insulating sulfur, but also outstanding
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restriction of dissolving polysulfides. The heat impregnation of sulfur in carbon black
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also plays an auxiliary role on the cycling performance of the G@CNT interlayer assisted cathodes. As the dual sulfur locking strategy by heat impregnation and G@CNT interlayer, the prepared Li-S cells exhibited high rate capacity and sustainably enhanced cycling stability. Excellent cycling performance with a high capacity 935.1mAh g-1 at 0.2C and 755.6mAh g-1 at 1C after 200 cycles is achieved. Moreover, the cell delivers reversible discharge capacities up to 669.7mAh g-1 and 1∗
Corresponding author. Tel.: +86 29 88431636 E-mail address: [email protected]; [email protected]; 1
ACCEPTED MANUSCRIPT 555.1mAh g-1 after 200 cycles even at 2C and 4C. Keywords: lithium-sulfur batteries, interlayer, graphene, carbon nanotube 1.
Introduction
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Lithium-sulfur batteries have received more and more attention because of their high theoretical capacity (1675 mAh g-1), natural abundance, non-toxic and inexpensive compared to state of the lithium-ion batteries (LIBs) [1-5]. However,
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lithium-sulfur batteries still suffer from severe capacity fading, poor rate stability and
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low active material utilization, which greatly inhibit its further commercialization. The common performance-limiting factors include: (i) the poor conductivity of sulfur and Li2S2/Li2S that limits its electrochemical kinetics; (ii) the dissolution of lithium polysulfide intermediates Li2Sn (2
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loss of active material, fast capacity decay and corrosion of Li anode [6]. The ordinary solution is impregnating sulfur into conductive frameworks. Many types of sulfur hosts have been developed, such as ordered mesoporous carbon [7],
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microporous carbon [8], carbon nanotubes [9-11], graphene [12-15], metal organic
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frameworks [16], porous nano metal oxides and conductive polymers [17-21]. The functions of these sulfur hosts can be briefly characterized as follows: (i) provide macroporous/mesoporous/microporous pores for sulfur loading and restricting the loss of soluble polysulfide intermediates; (ii) contribute conductive layers to wrapping sulfur, forming a core/shell or yolk shell structure that inhibiting the dissolution of polysulfide intermediates; (iii) chemically adsorb sulfur atoms by introducing chemical bonds with sulfur that promoting effective confinement of polysulfides. 2
ACCEPTED MANUSCRIPT Indeed, these methodes have been proven to be beneficial for utilization of active sulfur. However, as per our information, complete confinement of polysulfides is difficult and those structural designs are often complex and high cost. Such problems
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limit the subsequent mass production of these materials, which inhibits the practicality of Li-S batteries. Therefore, developing low cost, stable, scalable and more effective approaches for high performance of Li-S batteries is still an urgent work with special
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significance.
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Recent studies have shown that the dissolution of polysulfides in ether-based electrolytes is difficult to completely avoid, the formation of soluble short chain polysulfides is essential and the dissolved polysulfides in electrolyte can be recycled at a certain degree t in Li-S batteries [22]. Considering these facts, there has been a
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newly developed method coming into sight [23]. Introducing a conducting interlayer between a cathode and a separator has been an effective strategy to regulate the polysulfides shuttle in recent years. For instance, conductive carbon black [24]，
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free-standing multiwalled carbon nanotubes [25-27], reduced graphene oxide [28-31],
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polypyrrole [32], and various carbonized materials have been proposed to coated on the one side of separator or as an free-standing interlayer for improving the electrochemical performance of Li-S batteries [33-36]. As an important part in the Li-S batteries, the separators are typically made of polypropylene (PP), polyethylene (PE) or glass fiber. Modifying the separator with these conductive interlayers could not only offer better electronic pathways through the insulating sulfur and discharging product, but also prevent the “shuttle effect” [24].With these modified 3
ACCEPTED MANUSCRIPT separators, Li-S batteries have shown improved cycling stability and rate capacity. However, the long-term cycle performance still needs to be improved and the electrochemical phenomenon of these new designed cells need to be detailed,
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especially the charging-discharging behavior. Moreover, more works keep focus on the interlayers, thereby neglecting the influence of the preparation of sulfur cathodes. The general discussion of the effect of sulfur cathodes in these works is rarely
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mentioned. Indeed, the heat impregnation of sulfur in carbon materials helps to
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improve the performance of Li-S batteries, and it is worth studying the electrochemical behavior of the double sulfur locking system based on carbon/sulfur cathode and added interlayer.
In this present work, we prefer modifying PP membrane rather than glass fiber
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reported accordingly because the large weight of the membrane would reduce the energy density of the battery and the cost is much higher [24, 25]. Also, since the production of these self-standing interlayers is much complex, we try to use a simple
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coating method. As seen in fig.1(a), graphene@carbon nanotube (G@CNT)
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composite has been prepared using a facile one-pot pyrolysis strategy using urea as the carbon source as we reported earlier [37].Then ,the G@CNT coating was applied onto one side of the PP separator (toward the cathode side). The three-dimensional G@CNT composite can act as a conductive interlayer to enhance the active material utilization and inhibit the migration of the lithium polysulfides, which in turn protects the Li anode and the structure of sulfur cathode from being damaged. In our previous report, the Potassium hydroxide (KOH) activated Super p (Ksp) was 4
ACCEPTED MANUSCRIPT prepared and showed increased specific surface areas and the pore volume for loading sulfur [38]. This kind of carbon was adopted again to fabricate carbon/sulfur composites by heat impregnation. SEM image of Ksp/S composite was shown in fig.
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1(b). It is clear that the Ksp and sulfur compound together, and the yellow arrow's part shows the smooth sulfur surface. Fig.1(c) shows an optical photograph of our G@CNT coated PP separator with scanning electron microscope (SEM)
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image of the interlayer surface at upper right of it. It can be seen from the SEM
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image that G@CNT bulk particles are uniformly dispersed on the surface of PP separator. The modified separator measures about 16cm long and 8cm wide with only 0.33mg/cm2 G@CNT composite loading, which is quite less than the reported
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researches [24, 28, 34, 35].
Figure1 (a) Schematic of the procedure for fabricating Li-S cells with G@CNT coated PP separator as an interlayer. (b) SEM image of carbon/sulfur cathode. (c) 5
ACCEPTED MANUSCRIPT Photograph of G@CNT coated PP separator (the inset is its SEM image)
2. Experimental
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All chemicals used in this study were purchased from Sinopharm Chmical Reagent Co., Ltd, China as an analytical reagent grade and used without further purification. Deionized water was used in all experiments.
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2.1. Preparation of the sulfur cathodes
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The potassium hydroxide (KOH) activated Super P (Ksp) was prepared as previous reported [38]. In a typical procedure, Super P powders were added to KOH (10 M) aqueous solution and stirred for 12 h at 40℃. After drying the extra water in the oven at 65 °C for 24 h, the as prepared intermediate product was placed in a
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horizontal tube furnace and heated to 800 °C and kept for 1 h at 3 °C min-1 under Ar atmosphere. The product was thoroughly washed with 0.5M HCl, then washed with distilled water until the pH value reached 7 and finally dried in vacuum at 120 °C for
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24 h. The as prepared product was marked as Ksp.
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Elemental sulfur and Ksp particles were firstly mixed together with a 3:1 weight ratio. Then the mixture was heated to 159℃ with a heating rate of 5℃ min-1 and kept for 12 h in a sealed stainless steel vessel with argon gas protected. The final product was collected and marked as Ksp/S. To fabricate the cathodes, 80 wt. % of the Ksp/S composites were mixed with (10 wt. %) Super P and (10 wt. %) PVDF dissolved in 1-methyl-2-pyrrolidinone (NMP) solution. The slurries were then coated onto aluminum foils and dried at 60 ℃ for 12h to forming working electrodes. The 6
ACCEPTED MANUSCRIPT mass of sulfur in the cathode was around 1.4 mg cm-2 and 60 wt. % percent. For comparison, sulfur cathode without heat impregnation was also fabricated in the same sulfur loading ratio.
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2.2. Preparation of the G@CNT interlayer Graphene oxide (GO) was prepared from natural graphite by the Hummers’ method [39]. The graphene@carbon nanotube (G@CNT) composite has been
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prepared using a facile one-pot pyrolysis strategy as we reported earlier [37]. Briefly,
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0.58 g Co(NO3)26H2O was added into 400 ml GO solution(1mg/ml) with followed by stirring for 4h at room temperature. Then, 30.0 g urea was added into the above suspension solution and the mixed solution was stirred continuously at 80℃ until a gray powder was obtained. The gray product was annealed to 900℃ with a heating
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rate of 3℃ min-1 and kept for 1 h in a tube furnace with argon gas protected. The final product was collected and marked as G@CNT composite. Then, the G@CNT/PVDF slurry was prepared by mixing 70 wt. % G@CNT composite with 30
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wt. % PVDF in NMP solvent, which was coated onto one side of a PP membrane by
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the doctoral blade casting method. The G@CNT-coated separator was dried in a vacuum oven at 40 ℃ for 12 h. 2.3 Materials characterization The surface morphology of the composites was performed by transmission
electron microscopy (TEM, Tecnai F30 G2, FEI, USA) and scanning electron microscope (SEM, Supra 55, German ZEISS)) equipped with an energy dispersive spectroscopy (EDS) system. The crystal structure of the composites was determined 7
ACCEPTED MANUSCRIPT by X-ray diffraction (XRD, Rigaku, model D/max-2500 system at 40 kV and 100 mA of Cu Ka). Electrochemical measurements were carried out using CR2016-type coin cell
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cells assembled in an argon-filled glovebox. Sulfur and Ksp/S cathodes were applied as working electrode. Li foil was used as counter electrode, polypropylene (PP) film (Celgard 2400) and G@CNT coated PP as separators, respectively. The organic
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electrolytes comprised solutions of lithium bis(trifluoromethanesulfonyl) (LiTFSI) in
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a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with a volume ratio of 1 : 1 and 0.1mol l-1 LiNO3 as an additive.
A multi-channel current static system Land (LAND CT200IA) was used to evaluate the electrochemical performance. The discharge/charge measurements were
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conducted at a voltage interval of 1.5 to 3.0 V at 0.2C, 1C, 2C and 4C. Specific capacity values were calculated based on the mass of sulfur in the samples. Cyclic voltammograms
(CVs)
were
recorded
on
a
Series
G750TM
Redefining
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Electrochemical Measurement (USA GMARY Co.) between 1.5V-3.0V at a scan rate
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of 0.2mV s-1 to characterize the redox behavior and the kinetic reversibility of the cell. 3. Results and Discussion The microstructure of the prepared Ksp particles and G@CNT composite are
shown in fig. 2. It can be seen Ksp is moniliform like spherical particles, which are less than 100 nm as shown in fig. 2(a) and fig. 2(b). The SEM images of the as-prepared G@CNT composite are shown in fig. 2(c) and fig. 2(d). It can be seen that intertwined carbon nanotubes are grown on the graphene sheets with a dense and 8
ACCEPTED MANUSCRIPT uniform distribution. Only small part of graphene sheet was found in fig. 2(c), indicating that large amounts of CNT almost totally wrapped the graphene sheets, forming a three three-dimensional (3D) nanostructured hybrid. TEM is further used to
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confirm the nanostructure of the hybrid. It is clear that the CNT are grown on the top of smaller Co nanoparticles and the diameter is determined by the size of Co
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nanoparticles as previous reported [37].
Figure 2 (a) SEM and (b) TEM images of Ksp particles. (c) SEM and (d) TEM images of G@CNT composite. In order to investigate electrochemical properties of Li-S cells using the G@CNT
interlayer and the heat treatment of the Ksp/S composite, we characterized four 9
ACCEPTED MANUSCRIPT different Li-S cell types: (1) pristine sulfur cathode without interlayer and heat impregnation, (2) heat treated Ksp/S cathode without interlayer, (3) sulfur cathode with G@CNT interlayer and (4) Ksp/S cathode with G@CNT interlayer as shown in
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fig. 3. The cyclic voltammetry (CV) plots for the cells after the 5th cycle are shown in fig. 3(a). It can be seen that the oxidization peaks of sulfur and Ksp/S cathode are broad and centered around 2.5V, which are caused by the transformation of lithium
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polysulfides to S8. When the G@CNT interlayer is added, the oxidization peaks of
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sulfur and Ksp/S cathode with G@CNT interlayer are divided into two sharp peaks around 2.4V and 2.5V. The current densities increase greatly, indicating an enhanced reaction kinetics for conversion of Li2S to soluble Li2Sx, 4 < x < 8) (2.4V) and Li2Sx to elemental S (2.5V) [40]. These enhanced reaction kinetics are also confirmed by
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further comparing reduction peaks of these cells. For sulfur and Ksp/S cathode, the reduction peaks of S8 to higher-order polysulfides (Li2Sx, 2 < x ≤ 8) and lower-order polysulfides (Li2S2 and Li2S) are around 2.26V and 1.91V, respectively. However,
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these reduction peaks of sulfur and Ksp/S cathode with G@CNT interlayer increase to
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around 1.98V and 2.3V. Electrochemical impedance spectra (EIS) plots of Li-S cells were performed on cells after 5th cycle to further study the transfer state of Li+ inside the cells. The collected data were analyzed using a fitting program with an equivalent circuit model shown in the inset to fig. 3(b). The resistance of all cells is composed of electrolyte resistance (Re) at very high frequency, the charge transfer (Rct) and the interfacial resistance (Rsf) corresponding to two semicircles from high to middle frequency, and a slope line corresponds to warburg impedance [33]. It can be seen 10
ACCEPTED MANUSCRIPT that the Ksp/S cathode shows lower electrochemical resistance than sulfur cathode which can be attributed to the well diffussion of sulfur in carbon host after heat treated. The sulfur cathode with G@CNT interlayer cathode exhibits a lower charge
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transfer resistance than that of pristine sulfur and Ksp/S cathodes. Furthermore, the Ksp/S cathode with G@CNT interlayer shows the lowest Rct and Rsf, indicating
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treated and G@CNT interlayer modified cathode.
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enhanced electrode/electrolyte interface and promoted charge transfer in both the heat
Figure 3 (a) The CV profile at the 5th cycle of the pristine sulfur, Ksp/S, pristine
sulfur with G@CNT interlayer and Ksp/S with G@CNT interlayer cathodes over a voltage range of 1.5-3.0 V at a scan rate of 0.2mV s-1. (b) Electrochemical impedance spectra plots of as prepared sulfur based cathodes. The enhanced transformation of Li+ is further confirmed in the galvanostatic 11
ACCEPTED MANUSCRIPT charge-discharge profiles and cycling performance at 0.2C in fig. 4(a) and fig. 4(b). The discharge profiles consist of two plateaus, which correspond to the reduction of elemental sulfur (S8) to long-chain lithium polysulfides at the initial slope part and to
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the formation of short-chain Li2S2/Li2S at the later platform [41]. The discharge capacity of the sulfur, Ksp/S, sulfur with G@CNT interlayer and Ksp/S with G@CNT interlayer cathodes is 629.9 mAh g-1, 860.9 mAh g-1, 1229.9 mAh g-1 and 1231.3 mAh
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g-1, respectively. The △ E in the figure is the potential difference of the flat
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discharge-charge platform, which implies the energy barrier of lithium-sulfur reaction process [42-44]. For pristine S cathode, though the △E is low, the capacity of its two plateaus are much lower than other cells, implying an activation and incomplete reaction process of the cell. The plateaus of Ksp/S cathode, sulfur cathode with
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G@CNT interlayer and Ksp/S cathode with G@CNT interlayer are flat and stable after 5 cycles. The △E of these cells decrease as the heat impregnation and G@CNT interlayer are applied in fig. 3(c), indicating an enhanced kinetically efficient reaction
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process of the corresponding Li-S cell. In particular, The Ksp/S cathode with
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G@CNT interlayer exhibited better cycling performance compared to Li-S cells with other configurations at 0.2 C, 1C and 2C for 200 cycles as shown in fig. 4(b), fig. 4(c) and fig. 4(d). In fig. 4(b), the Ksp/S cathode shows larger capacities than sulfur cathode at the initial cycles, but suffers rapid decline at the longer cycles and the capacity curve tends to overlap with the curve of sulfur cathode. The overlapping phenomenon indicates that the dissolving and loss of active sulfur species in the Ksp assisted sulfur cathodes are more and more severe that the Ksp particles cannot bear 12
ACCEPTED MANUSCRIPT in longer cycles as previous reported [38]. In contrast, the G@CNT assisted Li-S cells exhibit greatly enhanced cycle performance at 0.2C. After 200 cycles, the sulfur and Ksp/S cathodes with G@CNT interlayer exhibited the discharge capacity of 935.1
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mAh g-1 and 809.1 mAh g-1 whereas the cells without G@CNT interlayer showed lower discharge capacities around 300 mAh g-1. It is worth mentioning that we also found the Ksp/S cathode with G@CNT interlayer exhibit rather lower coulombic
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efficiency than other cells in the middle of the test. We laid the cathode for one week
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to further check the relationship of the coulombic efficiency and discharge capacity. It can be seen that though the cathode exhibit gradually declining of coulombic efficiency than other cells, the discharging capacity is still relatively higher. This phenomenon probably can be explained as follows. During the cycling process,
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polysulfides in the Li-S cells tend to aggregate near the sulfur electrode, then gradually spread from the electrode and dissolved in the electrolyte, forming a liquid like battery. Since sulfur is dual locked by heat impregnation and G@CNT interlayer,
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the Ksp/S cathode with G@CNT interlayer provides rather slower polysulfides
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diffusion rate than sulfur cathode with G@CNT interlayer in the discharge-charge process. Partial of polysulfides in the dual locked Li-S cell tend to aggregate near the electrode rather than totally diffuse into the electrolyte that the polysulfides shuttle and inactive polysulfides formation are both alleviated accordingly, thus high discharging capacity of the dual locked cell is obtained. On the other hand, as the G@CNT interlayer could absorb polysulfides and prevent its migration, it is suggested the soluble polysulfides near the sulfur electrode gradually increase with 13
ACCEPTED MANUSCRIPT the discharge-charge cycling going on. However, it may be limited by the high polarization of the kinetically slow reduction of Li2S2 to Li2S and the worse contact between the current collector and the polysulfides, that partial of soluble polysulfides
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near the sulfur electrode seems easy to be charged but hard to be discharged, which further lead to the overcharge of the cell. Therefore, though the cell shows enhanced capacity, the coulombic efficiency of the cell is much lower than 82.0% after 200
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cycles. We suggest that the above mentioned overcharge phenomenon and the
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traditional shuttle effect could be the main reasons for the loss of coulombic efficiency in the interlayer assisted Li-S cells.
The advantage of the G@CNT interlayer assisted Li-S cell is further verified by the cycling performance of the cells at 1C, 2C and 4C. As shown in fig. 4(c), Li-S
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cells all experience an activation process in the initial cycles at 1C. A maximum discharge capacity up to 408.9 mAh g-1, 599.7 mAh g-1, 1008.6 mAh g-1 and 1016.5 mAh g-1 can be obtained for the sulfur, Ksp/S, sulfur with G@CNT interlayer and
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Ksp/S with G@CNT interlayer cathodes, respectively. After 200cycles, the discharge
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capacity of 755.6 mAh g-1 for the Ksp/S cathode with G@CNT interlayer is still retained, which is much higher than the capacities of 597.4mAh g-1, 256.9 mAh g-1 and 198.4 mAh g-1 for sulfur, Ksp/S and sulfur with G@CNT interlayer cathodes, respectively. Furthermore, fig. 4(d) displays the high rate performance of sulfur and Ksp/S cathodes with G@CNT interlayer. The Ksp/S cathode with G@CNT interlayer still shows high cycling performance compared to the S cathode with G@CNT interlayer at 2C. Maximum discharge capacities up to 939.6 mAh g-1 and 917.1 mAh 14
ACCEPTED MANUSCRIPT g-1 can be obtained for the Ksp/S and sulfur cathodes with G@CNT interlayer. After 200 cycles, the discharge capacity of 669.7 mAh g-1 for the Ksp/S cathode is still retained, which is much higher than the capacity of 583.9 mAh g-1 for S cathode.
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When the current density increases to 4 C, the Ksp/S cathode with G@CNT interlayer still delivers reversible capacities of 555.1 mAh g-1 at 200th cycle with high coulombic efficiencies of 97.1 %. It is apparent that the high rate performance of Li-S cells is
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improved by double locking of sulfur by heat treatment and the introduction of
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G@CNT interlayer.
Figure 4 Electrochemical properties of Li-S cells: (a) Charge-discharge profiles of cells with different configurations at 5th cycle. (b) Cycling performance of cells at 0.2C (about 0.47 mA/cm2, 1C = 1675 mAh g-1). (c) Cycling performance of cells at 1C (about 2.35mA/cm2). (d) Cycling performance of sulfur cathode with G@CNT interlayer and Ksp/S cathode with G@CNT interlayer at 2C (about 4.70 mA/cm2) and cycling performance of Ksp/S cathode with G@CNT interlayer at 4C (about 9.40 15
ACCEPTED MANUSCRIPT mA/cm2). To further investigate the blocking function of the G@CNT interlayer, surface morphologies of G@CNT coated PP separator before and after 200th cycle are
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observed in fig. 5. In fig. 5(a), the pristine G@CNT coated PP separator shows that bulk G@CNT particles are glue knotted by PVDF tightly. After 200 cycles, the morphology observed in fig. 5(b) is quite different, and it seems that a uniform and
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smooth substance is coated onto the G@CNT particles. Only tiny of gluey CNTs can
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be seen and the G@CNT particles are probably covered with redistributed sulfur species as shown in fig.5 (b) and fig. 5(c). The effective inhibition of the lithium polysulfides is further confirmed by the EDS images in fig.5 (d), fig.5 (e) and fig.5 (f). It can be seen that C, S and F elements have the same distribution in the G@CNT
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interlayer, which suggests that there are elemental sulfur or lithium polysulfides coating onto the surface of G@CNT particles, which is similar to previous reports
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Figure 5 (a) SEM surface images of pristine G@CNT coated PP separator. (b, c) G@CNT coated PP separator after 200cycles at 0.2C. EDS images of corresponding 16
ACCEPTED MANUSCRIPT elemental mapping images of carbon (d), sulfur (e) and fluorine (f). The obvious variation in the surface morphologies of lithium anodes before cycling and after 200 cycles at 0.2C is also presented in fig. 6. In fig. 6(a), the surface
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of pristine lithium is smooth whereas obvious corrosion damage on the lithium anode surface can be observed in Ksp/S based pristine cell as shown in fig. 6(b) and fig. 6(e). As G@CNT interlayer added, the surface corrosion of the lithium anode is improved
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obviously in G@CNT interlayer assisted cells as shown in fig. 6(c) and fig. 6(d). In
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particular, the lithium anode in G@CNT interlayer modified Li-Ksp/S cell suffers less corrosion than the anode in G@CNT interlayer assisted pristine Li- pristine S cell, suggesting that the dual locking system with heat infiltration and added G@CNT interlayer promotes a synergistic inhibition of the polysulfides shuttle in Li-S batteries.
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Furthermore, the improvement of the lithium anodes is also good agreement with the S-mapping analysis by EDS, as showing in fig.5 (f), fig.5 (g) and fig.5 (h). The optical images of Li anodes of the Ksp/S cathode, S cathode with G@CNT interlayer
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and Ksp/S cathode with G@CNT interlayer were also performed to illustrate the
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corrosion part of Li anodes. As shown in fig.6 (i), fig.6 (k) and fig.6 (l), the black circles represent the middle part of the Li plate which is virtually opposite the sulfur cathode as discussed in the above section. It can be seen that the inside part of the black circle labeled on the Li plate, which is opposite the sulfur cathodes, has been damaged and appears dark grey in pristine Ksp/S cell from fig.6 (i) whereas the same parts in the G@CNT interlayer assisted cells are corroded little and appears bright, which is well agreed with the SEM and EDS analyses mentioned above. However, the 17
ACCEPTED MANUSCRIPT outside parts of the black circle labeled on the Li plate in G@CNT interlayer assisted cells, which are against G@CNT interlayer coated PP separator, seem to be damaged at some degree as shown in fig. 6(k) and fig. 6(l), and especially in the Li-pristine S
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cell with G@CNT interlayer. The SEM image of the Li anode near the black circle border further confirms this phenomenon as shown in fig.6 (j), indicating more lithium sulfide compounds, which are just against the G@CNT coated PP separator
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that not virtually opposite the sulfur cathode, are dissolved, spread from the electrode,
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forming unreactive sulfur that cannot be fully discharged to Li2S2 and Li2S. These unreactive lithium sulfide compounds then tend to migrate to the Li anode that results in loss of active materials and corrosion damage of the lithium anode. As the heat infiltration of sulfur, the Li anode in Ksp/S cell with G@CNT interlayer suffers the
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minimum damage than the other cathodes, indicating the effective inhibition of the polysulfides shuttle. Based on these facts, we can conclude the “shuttle effect” or the dissolving of polysulfides still exists in functional interlayer assisted Li-S cells, but
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high performance of Li-S cell can be achieved by suppressing the migration of lithium
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polysulfides to Li anode with the so called dual locking of sulfur strategy.
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Figure 6 SEM images of Li metallic plate before cycling (a) and after 200 cycles at 0.2C with different items: (b, e and f) lithium anode of Ksp/S cathode and
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corresponding sulfur mapping image; (c, g) lithium anode of S cathode with G@CNT interlayer and corresponding sulfur mapping image; (d, h) lithium anode of Ksp/S
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cathode with G@CNT interlayer and corresponding sulfur mapping image. (i, k, l) the optical Li anode of the Ksp/S cathode, S cathode with G@CNT interlayer and Ksp/S
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cathode with G@CNT interlayeafter 200cycles, respectively. (j) SEM image of the Li anode of S cathode with G@CNT interlayer. 4. Conclusion
In summary, we designed and prepared a G@CNT coated PP separator and heat treated carbon/sulfur composite for high performance of lithium-sulfur batteries. During the electrochemical process, the G@CNT interlayer provides the effective inhibition of the polysulfides migration and promotes transfer of Li+ ions. The heat 19
ACCEPTED MANUSCRIPT impregnation of sulfur plays an important role on the cycling performance of the G@CNT interlayer assisted cathodes. As the dual sulfur locking strategy, the prepared Li-S cell exhibited high rate capacity and sustainably enhanced cycling stability. We
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also analyzed the electrochemical behaviors of G@CNT interlayer assisted Li-S cells. Based on the above discussion, the loss of capacity and coulombic efficiency of the G@CNT assisted Li-S cells can be greatly attributed to the gradually dissolving and
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spreading of the lithium polysulfides from electrode. The unreactive lithium
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polysulfides in electrolyte cannot be fully discharged to Li2S2 and Li2S and tend to migrate to the Li anode, causing the corrosion of the Li plate even in the protection of G@CNT interlayer. The strategy and the discussion mentioned above may help us in developing high performance of interlayer assisted Li-S cell.
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Acknowledgements
This work was supported by the Natural Science Foundation of Shaanxi Province
References
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(Grant No. 2015JZ014).
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ACCEPTED MANUSCRIPT Highlights 1 Researched three-dimensional graphene@carbon nanotube coated separator for lithium-sulfur batteries.
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2 An insight into the dual sulfur locking strategy by heat impregnation and interlayer. 3 The loss of capacity and coulombic efficiency of the interlayer assisted Li-S cells
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polysulfides detached from the electrode.
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can be greatly attributed to the gradually dissolving and spreading of the lithium
S0925-8388(17)30816-2
DOI:
10.1016/j.jallcom.2017.03.047
Reference:
JALCOM 41085
To appear in:
Journal of Alloys and Compounds
Received Date: 14 January 2017 Revised Date:
21 February 2017
Accepted Date: 5 March 2017
Please cite this article as: H. Wu, Y. Huang, W. Zhang, X. Sun, Y. Yang, L. Wang, M. Zong, Lock of sulfur with carbon black and a three-dimensional graphene@carbon nanotubes coated separator for lithium-sulfur batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.047. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Lock of sulfur with carbon black and a three-dimensional graphene@carbon nanotubes coated separator for lithium-sulfur batteries Haiwei Wua, Ying Huanga,*1, Weichao Zhanga, Xu Suna, Yiwen Yanga, Lei wangb, Meng
a
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Zonga Department of Applied Chemistry and The Key Laboratory of Space Applied Physics and
Chemistry ,Ministry of Education, School of Natural and Applied Sciences, Northwestern
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’
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b
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Polytechnical University, Xi’an 710072, PR China
an 710021, China
Abstract
Three-dimensional graphene@carbon nanotube (G@CNT) composite was
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coated on polypropylene separator as an interlayer for high performance of lithium-sulfur batteries. In this study, the novel G@CNT nanotubes coated separator offers not only fast electron pathway for insulating sulfur, but also outstanding
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restriction of dissolving polysulfides. The heat impregnation of sulfur in carbon black
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also plays an auxiliary role on the cycling performance of the G@CNT interlayer assisted cathodes. As the dual sulfur locking strategy by heat impregnation and G@CNT interlayer, the prepared Li-S cells exhibited high rate capacity and sustainably enhanced cycling stability. Excellent cycling performance with a high capacity 935.1mAh g-1 at 0.2C and 755.6mAh g-1 at 1C after 200 cycles is achieved. Moreover, the cell delivers reversible discharge capacities up to 669.7mAh g-1 and 1∗
Corresponding author. Tel.: +86 29 88431636 E-mail address: [email protected]; [email protected]; 1
ACCEPTED MANUSCRIPT 555.1mAh g-1 after 200 cycles even at 2C and 4C. Keywords: lithium-sulfur batteries, interlayer, graphene, carbon nanotube 1.
Introduction
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Lithium-sulfur batteries have received more and more attention because of their high theoretical capacity (1675 mAh g-1), natural abundance, non-toxic and inexpensive compared to state of the lithium-ion batteries (LIBs) [1-5]. However,
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lithium-sulfur batteries still suffer from severe capacity fading, poor rate stability and
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low active material utilization, which greatly inhibit its further commercialization. The common performance-limiting factors include: (i) the poor conductivity of sulfur and Li2S2/Li2S that limits its electrochemical kinetics; (ii) the dissolution of lithium polysulfide intermediates Li2Sn (2
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loss of active material, fast capacity decay and corrosion of Li anode [6]. The ordinary solution is impregnating sulfur into conductive frameworks. Many types of sulfur hosts have been developed, such as ordered mesoporous carbon [7],
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microporous carbon [8], carbon nanotubes [9-11], graphene [12-15], metal organic
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frameworks [16], porous nano metal oxides and conductive polymers [17-21]. The functions of these sulfur hosts can be briefly characterized as follows: (i) provide macroporous/mesoporous/microporous pores for sulfur loading and restricting the loss of soluble polysulfide intermediates; (ii) contribute conductive layers to wrapping sulfur, forming a core/shell or yolk shell structure that inhibiting the dissolution of polysulfide intermediates; (iii) chemically adsorb sulfur atoms by introducing chemical bonds with sulfur that promoting effective confinement of polysulfides. 2
ACCEPTED MANUSCRIPT Indeed, these methodes have been proven to be beneficial for utilization of active sulfur. However, as per our information, complete confinement of polysulfides is difficult and those structural designs are often complex and high cost. Such problems
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limit the subsequent mass production of these materials, which inhibits the practicality of Li-S batteries. Therefore, developing low cost, stable, scalable and more effective approaches for high performance of Li-S batteries is still an urgent work with special
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significance.
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Recent studies have shown that the dissolution of polysulfides in ether-based electrolytes is difficult to completely avoid, the formation of soluble short chain polysulfides is essential and the dissolved polysulfides in electrolyte can be recycled at a certain degree t in Li-S batteries [22]. Considering these facts, there has been a
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newly developed method coming into sight [23]. Introducing a conducting interlayer between a cathode and a separator has been an effective strategy to regulate the polysulfides shuttle in recent years. For instance, conductive carbon black [24]，
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free-standing multiwalled carbon nanotubes [25-27], reduced graphene oxide [28-31],
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polypyrrole [32], and various carbonized materials have been proposed to coated on the one side of separator or as an free-standing interlayer for improving the electrochemical performance of Li-S batteries [33-36]. As an important part in the Li-S batteries, the separators are typically made of polypropylene (PP), polyethylene (PE) or glass fiber. Modifying the separator with these conductive interlayers could not only offer better electronic pathways through the insulating sulfur and discharging product, but also prevent the “shuttle effect” [24].With these modified 3
ACCEPTED MANUSCRIPT separators, Li-S batteries have shown improved cycling stability and rate capacity. However, the long-term cycle performance still needs to be improved and the electrochemical phenomenon of these new designed cells need to be detailed,
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especially the charging-discharging behavior. Moreover, more works keep focus on the interlayers, thereby neglecting the influence of the preparation of sulfur cathodes. The general discussion of the effect of sulfur cathodes in these works is rarely
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mentioned. Indeed, the heat impregnation of sulfur in carbon materials helps to
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improve the performance of Li-S batteries, and it is worth studying the electrochemical behavior of the double sulfur locking system based on carbon/sulfur cathode and added interlayer.
In this present work, we prefer modifying PP membrane rather than glass fiber
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reported accordingly because the large weight of the membrane would reduce the energy density of the battery and the cost is much higher [24, 25]. Also, since the production of these self-standing interlayers is much complex, we try to use a simple
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coating method. As seen in fig.1(a), graphene@carbon nanotube (G@CNT)
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composite has been prepared using a facile one-pot pyrolysis strategy using urea as the carbon source as we reported earlier [37].Then ,the G@CNT coating was applied onto one side of the PP separator (toward the cathode side). The three-dimensional G@CNT composite can act as a conductive interlayer to enhance the active material utilization and inhibit the migration of the lithium polysulfides, which in turn protects the Li anode and the structure of sulfur cathode from being damaged. In our previous report, the Potassium hydroxide (KOH) activated Super p (Ksp) was 4
ACCEPTED MANUSCRIPT prepared and showed increased specific surface areas and the pore volume for loading sulfur [38]. This kind of carbon was adopted again to fabricate carbon/sulfur composites by heat impregnation. SEM image of Ksp/S composite was shown in fig.
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1(b). It is clear that the Ksp and sulfur compound together, and the yellow arrow's part shows the smooth sulfur surface. Fig.1(c) shows an optical photograph of our G@CNT coated PP separator with scanning electron microscope (SEM)
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image of the interlayer surface at upper right of it. It can be seen from the SEM
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image that G@CNT bulk particles are uniformly dispersed on the surface of PP separator. The modified separator measures about 16cm long and 8cm wide with only 0.33mg/cm2 G@CNT composite loading, which is quite less than the reported
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researches [24, 28, 34, 35].
Figure1 (a) Schematic of the procedure for fabricating Li-S cells with G@CNT coated PP separator as an interlayer. (b) SEM image of carbon/sulfur cathode. (c) 5
ACCEPTED MANUSCRIPT Photograph of G@CNT coated PP separator (the inset is its SEM image)
2. Experimental
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All chemicals used in this study were purchased from Sinopharm Chmical Reagent Co., Ltd, China as an analytical reagent grade and used without further purification. Deionized water was used in all experiments.
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2.1. Preparation of the sulfur cathodes
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The potassium hydroxide (KOH) activated Super P (Ksp) was prepared as previous reported [38]. In a typical procedure, Super P powders were added to KOH (10 M) aqueous solution and stirred for 12 h at 40℃. After drying the extra water in the oven at 65 °C for 24 h, the as prepared intermediate product was placed in a
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horizontal tube furnace and heated to 800 °C and kept for 1 h at 3 °C min-1 under Ar atmosphere. The product was thoroughly washed with 0.5M HCl, then washed with distilled water until the pH value reached 7 and finally dried in vacuum at 120 °C for
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24 h. The as prepared product was marked as Ksp.
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Elemental sulfur and Ksp particles were firstly mixed together with a 3:1 weight ratio. Then the mixture was heated to 159℃ with a heating rate of 5℃ min-1 and kept for 12 h in a sealed stainless steel vessel with argon gas protected. The final product was collected and marked as Ksp/S. To fabricate the cathodes, 80 wt. % of the Ksp/S composites were mixed with (10 wt. %) Super P and (10 wt. %) PVDF dissolved in 1-methyl-2-pyrrolidinone (NMP) solution. The slurries were then coated onto aluminum foils and dried at 60 ℃ for 12h to forming working electrodes. The 6
ACCEPTED MANUSCRIPT mass of sulfur in the cathode was around 1.4 mg cm-2 and 60 wt. % percent. For comparison, sulfur cathode without heat impregnation was also fabricated in the same sulfur loading ratio.
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2.2. Preparation of the G@CNT interlayer Graphene oxide (GO) was prepared from natural graphite by the Hummers’ method [39]. The graphene@carbon nanotube (G@CNT) composite has been
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prepared using a facile one-pot pyrolysis strategy as we reported earlier [37]. Briefly,
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0.58 g Co(NO3)26H2O was added into 400 ml GO solution(1mg/ml) with followed by stirring for 4h at room temperature. Then, 30.0 g urea was added into the above suspension solution and the mixed solution was stirred continuously at 80℃ until a gray powder was obtained. The gray product was annealed to 900℃ with a heating
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rate of 3℃ min-1 and kept for 1 h in a tube furnace with argon gas protected. The final product was collected and marked as G@CNT composite. Then, the G@CNT/PVDF slurry was prepared by mixing 70 wt. % G@CNT composite with 30
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wt. % PVDF in NMP solvent, which was coated onto one side of a PP membrane by
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the doctoral blade casting method. The G@CNT-coated separator was dried in a vacuum oven at 40 ℃ for 12 h. 2.3 Materials characterization The surface morphology of the composites was performed by transmission
electron microscopy (TEM, Tecnai F30 G2, FEI, USA) and scanning electron microscope (SEM, Supra 55, German ZEISS)) equipped with an energy dispersive spectroscopy (EDS) system. The crystal structure of the composites was determined 7
ACCEPTED MANUSCRIPT by X-ray diffraction (XRD, Rigaku, model D/max-2500 system at 40 kV and 100 mA of Cu Ka). Electrochemical measurements were carried out using CR2016-type coin cell
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cells assembled in an argon-filled glovebox. Sulfur and Ksp/S cathodes were applied as working electrode. Li foil was used as counter electrode, polypropylene (PP) film (Celgard 2400) and G@CNT coated PP as separators, respectively. The organic
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electrolytes comprised solutions of lithium bis(trifluoromethanesulfonyl) (LiTFSI) in
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a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with a volume ratio of 1 : 1 and 0.1mol l-1 LiNO3 as an additive.
A multi-channel current static system Land (LAND CT200IA) was used to evaluate the electrochemical performance. The discharge/charge measurements were
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conducted at a voltage interval of 1.5 to 3.0 V at 0.2C, 1C, 2C and 4C. Specific capacity values were calculated based on the mass of sulfur in the samples. Cyclic voltammograms
(CVs)
were
recorded
on
a
Series
G750TM
Redefining
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Electrochemical Measurement (USA GMARY Co.) between 1.5V-3.0V at a scan rate
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of 0.2mV s-1 to characterize the redox behavior and the kinetic reversibility of the cell. 3. Results and Discussion The microstructure of the prepared Ksp particles and G@CNT composite are
shown in fig. 2. It can be seen Ksp is moniliform like spherical particles, which are less than 100 nm as shown in fig. 2(a) and fig. 2(b). The SEM images of the as-prepared G@CNT composite are shown in fig. 2(c) and fig. 2(d). It can be seen that intertwined carbon nanotubes are grown on the graphene sheets with a dense and 8
ACCEPTED MANUSCRIPT uniform distribution. Only small part of graphene sheet was found in fig. 2(c), indicating that large amounts of CNT almost totally wrapped the graphene sheets, forming a three three-dimensional (3D) nanostructured hybrid. TEM is further used to
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confirm the nanostructure of the hybrid. It is clear that the CNT are grown on the top of smaller Co nanoparticles and the diameter is determined by the size of Co
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nanoparticles as previous reported [37].
Figure 2 (a) SEM and (b) TEM images of Ksp particles. (c) SEM and (d) TEM images of G@CNT composite. In order to investigate electrochemical properties of Li-S cells using the G@CNT
interlayer and the heat treatment of the Ksp/S composite, we characterized four 9
ACCEPTED MANUSCRIPT different Li-S cell types: (1) pristine sulfur cathode without interlayer and heat impregnation, (2) heat treated Ksp/S cathode without interlayer, (3) sulfur cathode with G@CNT interlayer and (4) Ksp/S cathode with G@CNT interlayer as shown in
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fig. 3. The cyclic voltammetry (CV) plots for the cells after the 5th cycle are shown in fig. 3(a). It can be seen that the oxidization peaks of sulfur and Ksp/S cathode are broad and centered around 2.5V, which are caused by the transformation of lithium
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polysulfides to S8. When the G@CNT interlayer is added, the oxidization peaks of
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sulfur and Ksp/S cathode with G@CNT interlayer are divided into two sharp peaks around 2.4V and 2.5V. The current densities increase greatly, indicating an enhanced reaction kinetics for conversion of Li2S to soluble Li2Sx, 4 < x < 8) (2.4V) and Li2Sx to elemental S (2.5V) [40]. These enhanced reaction kinetics are also confirmed by
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further comparing reduction peaks of these cells. For sulfur and Ksp/S cathode, the reduction peaks of S8 to higher-order polysulfides (Li2Sx, 2 < x ≤ 8) and lower-order polysulfides (Li2S2 and Li2S) are around 2.26V and 1.91V, respectively. However,
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these reduction peaks of sulfur and Ksp/S cathode with G@CNT interlayer increase to
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around 1.98V and 2.3V. Electrochemical impedance spectra (EIS) plots of Li-S cells were performed on cells after 5th cycle to further study the transfer state of Li+ inside the cells. The collected data were analyzed using a fitting program with an equivalent circuit model shown in the inset to fig. 3(b). The resistance of all cells is composed of electrolyte resistance (Re) at very high frequency, the charge transfer (Rct) and the interfacial resistance (Rsf) corresponding to two semicircles from high to middle frequency, and a slope line corresponds to warburg impedance [33]. It can be seen 10
ACCEPTED MANUSCRIPT that the Ksp/S cathode shows lower electrochemical resistance than sulfur cathode which can be attributed to the well diffussion of sulfur in carbon host after heat treated. The sulfur cathode with G@CNT interlayer cathode exhibits a lower charge
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transfer resistance than that of pristine sulfur and Ksp/S cathodes. Furthermore, the Ksp/S cathode with G@CNT interlayer shows the lowest Rct and Rsf, indicating
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treated and G@CNT interlayer modified cathode.
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enhanced electrode/electrolyte interface and promoted charge transfer in both the heat
Figure 3 (a) The CV profile at the 5th cycle of the pristine sulfur, Ksp/S, pristine
sulfur with G@CNT interlayer and Ksp/S with G@CNT interlayer cathodes over a voltage range of 1.5-3.0 V at a scan rate of 0.2mV s-1. (b) Electrochemical impedance spectra plots of as prepared sulfur based cathodes. The enhanced transformation of Li+ is further confirmed in the galvanostatic 11
ACCEPTED MANUSCRIPT charge-discharge profiles and cycling performance at 0.2C in fig. 4(a) and fig. 4(b). The discharge profiles consist of two plateaus, which correspond to the reduction of elemental sulfur (S8) to long-chain lithium polysulfides at the initial slope part and to
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the formation of short-chain Li2S2/Li2S at the later platform [41]. The discharge capacity of the sulfur, Ksp/S, sulfur with G@CNT interlayer and Ksp/S with G@CNT interlayer cathodes is 629.9 mAh g-1, 860.9 mAh g-1, 1229.9 mAh g-1 and 1231.3 mAh
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g-1, respectively. The △ E in the figure is the potential difference of the flat
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discharge-charge platform, which implies the energy barrier of lithium-sulfur reaction process [42-44]. For pristine S cathode, though the △E is low, the capacity of its two plateaus are much lower than other cells, implying an activation and incomplete reaction process of the cell. The plateaus of Ksp/S cathode, sulfur cathode with
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G@CNT interlayer and Ksp/S cathode with G@CNT interlayer are flat and stable after 5 cycles. The △E of these cells decrease as the heat impregnation and G@CNT interlayer are applied in fig. 3(c), indicating an enhanced kinetically efficient reaction
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process of the corresponding Li-S cell. In particular, The Ksp/S cathode with
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G@CNT interlayer exhibited better cycling performance compared to Li-S cells with other configurations at 0.2 C, 1C and 2C for 200 cycles as shown in fig. 4(b), fig. 4(c) and fig. 4(d). In fig. 4(b), the Ksp/S cathode shows larger capacities than sulfur cathode at the initial cycles, but suffers rapid decline at the longer cycles and the capacity curve tends to overlap with the curve of sulfur cathode. The overlapping phenomenon indicates that the dissolving and loss of active sulfur species in the Ksp assisted sulfur cathodes are more and more severe that the Ksp particles cannot bear 12
ACCEPTED MANUSCRIPT in longer cycles as previous reported [38]. In contrast, the G@CNT assisted Li-S cells exhibit greatly enhanced cycle performance at 0.2C. After 200 cycles, the sulfur and Ksp/S cathodes with G@CNT interlayer exhibited the discharge capacity of 935.1
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mAh g-1 and 809.1 mAh g-1 whereas the cells without G@CNT interlayer showed lower discharge capacities around 300 mAh g-1. It is worth mentioning that we also found the Ksp/S cathode with G@CNT interlayer exhibit rather lower coulombic
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efficiency than other cells in the middle of the test. We laid the cathode for one week
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to further check the relationship of the coulombic efficiency and discharge capacity. It can be seen that though the cathode exhibit gradually declining of coulombic efficiency than other cells, the discharging capacity is still relatively higher. This phenomenon probably can be explained as follows. During the cycling process,
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polysulfides in the Li-S cells tend to aggregate near the sulfur electrode, then gradually spread from the electrode and dissolved in the electrolyte, forming a liquid like battery. Since sulfur is dual locked by heat impregnation and G@CNT interlayer,
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the Ksp/S cathode with G@CNT interlayer provides rather slower polysulfides
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diffusion rate than sulfur cathode with G@CNT interlayer in the discharge-charge process. Partial of polysulfides in the dual locked Li-S cell tend to aggregate near the electrode rather than totally diffuse into the electrolyte that the polysulfides shuttle and inactive polysulfides formation are both alleviated accordingly, thus high discharging capacity of the dual locked cell is obtained. On the other hand, as the G@CNT interlayer could absorb polysulfides and prevent its migration, it is suggested the soluble polysulfides near the sulfur electrode gradually increase with 13
ACCEPTED MANUSCRIPT the discharge-charge cycling going on. However, it may be limited by the high polarization of the kinetically slow reduction of Li2S2 to Li2S and the worse contact between the current collector and the polysulfides, that partial of soluble polysulfides
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near the sulfur electrode seems easy to be charged but hard to be discharged, which further lead to the overcharge of the cell. Therefore, though the cell shows enhanced capacity, the coulombic efficiency of the cell is much lower than 82.0% after 200
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cycles. We suggest that the above mentioned overcharge phenomenon and the
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traditional shuttle effect could be the main reasons for the loss of coulombic efficiency in the interlayer assisted Li-S cells.
The advantage of the G@CNT interlayer assisted Li-S cell is further verified by the cycling performance of the cells at 1C, 2C and 4C. As shown in fig. 4(c), Li-S
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cells all experience an activation process in the initial cycles at 1C. A maximum discharge capacity up to 408.9 mAh g-1, 599.7 mAh g-1, 1008.6 mAh g-1 and 1016.5 mAh g-1 can be obtained for the sulfur, Ksp/S, sulfur with G@CNT interlayer and
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Ksp/S with G@CNT interlayer cathodes, respectively. After 200cycles, the discharge
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capacity of 755.6 mAh g-1 for the Ksp/S cathode with G@CNT interlayer is still retained, which is much higher than the capacities of 597.4mAh g-1, 256.9 mAh g-1 and 198.4 mAh g-1 for sulfur, Ksp/S and sulfur with G@CNT interlayer cathodes, respectively. Furthermore, fig. 4(d) displays the high rate performance of sulfur and Ksp/S cathodes with G@CNT interlayer. The Ksp/S cathode with G@CNT interlayer still shows high cycling performance compared to the S cathode with G@CNT interlayer at 2C. Maximum discharge capacities up to 939.6 mAh g-1 and 917.1 mAh 14
ACCEPTED MANUSCRIPT g-1 can be obtained for the Ksp/S and sulfur cathodes with G@CNT interlayer. After 200 cycles, the discharge capacity of 669.7 mAh g-1 for the Ksp/S cathode is still retained, which is much higher than the capacity of 583.9 mAh g-1 for S cathode.
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When the current density increases to 4 C, the Ksp/S cathode with G@CNT interlayer still delivers reversible capacities of 555.1 mAh g-1 at 200th cycle with high coulombic efficiencies of 97.1 %. It is apparent that the high rate performance of Li-S cells is
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improved by double locking of sulfur by heat treatment and the introduction of
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G@CNT interlayer.
Figure 4 Electrochemical properties of Li-S cells: (a) Charge-discharge profiles of cells with different configurations at 5th cycle. (b) Cycling performance of cells at 0.2C (about 0.47 mA/cm2, 1C = 1675 mAh g-1). (c) Cycling performance of cells at 1C (about 2.35mA/cm2). (d) Cycling performance of sulfur cathode with G@CNT interlayer and Ksp/S cathode with G@CNT interlayer at 2C (about 4.70 mA/cm2) and cycling performance of Ksp/S cathode with G@CNT interlayer at 4C (about 9.40 15
ACCEPTED MANUSCRIPT mA/cm2). To further investigate the blocking function of the G@CNT interlayer, surface morphologies of G@CNT coated PP separator before and after 200th cycle are
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observed in fig. 5. In fig. 5(a), the pristine G@CNT coated PP separator shows that bulk G@CNT particles are glue knotted by PVDF tightly. After 200 cycles, the morphology observed in fig. 5(b) is quite different, and it seems that a uniform and
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smooth substance is coated onto the G@CNT particles. Only tiny of gluey CNTs can
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be seen and the G@CNT particles are probably covered with redistributed sulfur species as shown in fig.5 (b) and fig. 5(c). The effective inhibition of the lithium polysulfides is further confirmed by the EDS images in fig.5 (d), fig.5 (e) and fig.5 (f). It can be seen that C, S and F elements have the same distribution in the G@CNT
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interlayer, which suggests that there are elemental sulfur or lithium polysulfides coating onto the surface of G@CNT particles, which is similar to previous reports
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[25].
Figure 5 (a) SEM surface images of pristine G@CNT coated PP separator. (b, c) G@CNT coated PP separator after 200cycles at 0.2C. EDS images of corresponding 16
ACCEPTED MANUSCRIPT elemental mapping images of carbon (d), sulfur (e) and fluorine (f). The obvious variation in the surface morphologies of lithium anodes before cycling and after 200 cycles at 0.2C is also presented in fig. 6. In fig. 6(a), the surface
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of pristine lithium is smooth whereas obvious corrosion damage on the lithium anode surface can be observed in Ksp/S based pristine cell as shown in fig. 6(b) and fig. 6(e). As G@CNT interlayer added, the surface corrosion of the lithium anode is improved
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obviously in G@CNT interlayer assisted cells as shown in fig. 6(c) and fig. 6(d). In
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particular, the lithium anode in G@CNT interlayer modified Li-Ksp/S cell suffers less corrosion than the anode in G@CNT interlayer assisted pristine Li- pristine S cell, suggesting that the dual locking system with heat infiltration and added G@CNT interlayer promotes a synergistic inhibition of the polysulfides shuttle in Li-S batteries.
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Furthermore, the improvement of the lithium anodes is also good agreement with the S-mapping analysis by EDS, as showing in fig.5 (f), fig.5 (g) and fig.5 (h). The optical images of Li anodes of the Ksp/S cathode, S cathode with G@CNT interlayer
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and Ksp/S cathode with G@CNT interlayer were also performed to illustrate the
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corrosion part of Li anodes. As shown in fig.6 (i), fig.6 (k) and fig.6 (l), the black circles represent the middle part of the Li plate which is virtually opposite the sulfur cathode as discussed in the above section. It can be seen that the inside part of the black circle labeled on the Li plate, which is opposite the sulfur cathodes, has been damaged and appears dark grey in pristine Ksp/S cell from fig.6 (i) whereas the same parts in the G@CNT interlayer assisted cells are corroded little and appears bright, which is well agreed with the SEM and EDS analyses mentioned above. However, the 17
ACCEPTED MANUSCRIPT outside parts of the black circle labeled on the Li plate in G@CNT interlayer assisted cells, which are against G@CNT interlayer coated PP separator, seem to be damaged at some degree as shown in fig. 6(k) and fig. 6(l), and especially in the Li-pristine S
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cell with G@CNT interlayer. The SEM image of the Li anode near the black circle border further confirms this phenomenon as shown in fig.6 (j), indicating more lithium sulfide compounds, which are just against the G@CNT coated PP separator
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that not virtually opposite the sulfur cathode, are dissolved, spread from the electrode,
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forming unreactive sulfur that cannot be fully discharged to Li2S2 and Li2S. These unreactive lithium sulfide compounds then tend to migrate to the Li anode that results in loss of active materials and corrosion damage of the lithium anode. As the heat infiltration of sulfur, the Li anode in Ksp/S cell with G@CNT interlayer suffers the
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minimum damage than the other cathodes, indicating the effective inhibition of the polysulfides shuttle. Based on these facts, we can conclude the “shuttle effect” or the dissolving of polysulfides still exists in functional interlayer assisted Li-S cells, but
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high performance of Li-S cell can be achieved by suppressing the migration of lithium
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polysulfides to Li anode with the so called dual locking of sulfur strategy.
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Figure 6 SEM images of Li metallic plate before cycling (a) and after 200 cycles at 0.2C with different items: (b, e and f) lithium anode of Ksp/S cathode and
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corresponding sulfur mapping image; (c, g) lithium anode of S cathode with G@CNT interlayer and corresponding sulfur mapping image; (d, h) lithium anode of Ksp/S
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cathode with G@CNT interlayer and corresponding sulfur mapping image. (i, k, l) the optical Li anode of the Ksp/S cathode, S cathode with G@CNT interlayer and Ksp/S
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cathode with G@CNT interlayeafter 200cycles, respectively. (j) SEM image of the Li anode of S cathode with G@CNT interlayer. 4. Conclusion
In summary, we designed and prepared a G@CNT coated PP separator and heat treated carbon/sulfur composite for high performance of lithium-sulfur batteries. During the electrochemical process, the G@CNT interlayer provides the effective inhibition of the polysulfides migration and promotes transfer of Li+ ions. The heat 19
ACCEPTED MANUSCRIPT impregnation of sulfur plays an important role on the cycling performance of the G@CNT interlayer assisted cathodes. As the dual sulfur locking strategy, the prepared Li-S cell exhibited high rate capacity and sustainably enhanced cycling stability. We
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also analyzed the electrochemical behaviors of G@CNT interlayer assisted Li-S cells. Based on the above discussion, the loss of capacity and coulombic efficiency of the G@CNT assisted Li-S cells can be greatly attributed to the gradually dissolving and
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spreading of the lithium polysulfides from electrode. The unreactive lithium
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polysulfides in electrolyte cannot be fully discharged to Li2S2 and Li2S and tend to migrate to the Li anode, causing the corrosion of the Li plate even in the protection of G@CNT interlayer. The strategy and the discussion mentioned above may help us in developing high performance of interlayer assisted Li-S cell.
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Acknowledgements
This work was supported by the Natural Science Foundation of Shaanxi Province
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ACCEPTED MANUSCRIPT Highlights 1 Researched three-dimensional graphene@carbon nanotube coated separator for lithium-sulfur batteries.
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2 An insight into the dual sulfur locking strategy by heat impregnation and interlayer. 3 The loss of capacity and coulombic efficiency of the interlayer assisted Li-S cells
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polysulfides detached from the electrode.
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can be greatly attributed to the gradually dissolving and spreading of the lithium