1-s2.0-S0167273815002532-main

A functional carbon layer-coated separator for high performance lithium sulfur batteries
Zhiyong Zhang,Yanqing Lai,Zhian Zhang ⁎,Jie Li
School of Metallurgy and Environment,Central South University,Changsha,Hunan 410083,China
a b s t r a c t
a r t i c l e i n f o Article history:
Received 18November 2014
Received in revised form 24April 2015Accepted 12June 2015Available online xxxx
Keywords:
Separator modi fication
Functional conductive coating layer Polysul fides
Lithium sulfur batteries
A composite separator consisting of a functional layer of conductive carbon on the cathode-side of a Celgard sep-arator has been investigated to improve the electrochemical performance of lithium sulfur batteries.The cell with functional carbon layer-coated separator can reach a high initial capacity of 1070mAh g −1at the rate of 0.5C,and maintain a high capacity retention ratio with the capacity of 778mAh g −1after 100th charge/discharge cycle.Be-sides,the coulombic ef ficiency of the cell with functional carbon layer-coated separator rises from 80%to nearly 90%without the addition of LiNO 3additive,which indicates the reduction of shuttle effect.The improved cell per-formance is attributed to the functional carbon-coating layer,which serves as an upper current collector to facil-itate electron transport for high active-material utilization and a conductive network for trapping and depositing dissolved sulfur-containing active materials,as con firmed by scanning electron microscopy (SEM)and energy-dispersive X-ray spectroscopy (EDS).
©2015Elsevier B.V.All rights reserved.
1.Introduction
Advanced energy storage systems are highly desired to meet the in-creasing demands of high energy density batteries [1,2].Accordingly,lithium sulfur (Li –S)batteries have attract great attention as one of the most promising systems for the next generation high energy density rechargeable lithium batteries because of their high theoretical speci fic capacity (1675mAh g −1)and energy density (2600Wh kg −1)[3,4].As a cathode active material,sulfur also has advantages of non-toxicity and abundance in nature [5].However,the practical applications of Li –S bat-teries are still hindered by some major basic obstacles.Sulfur and its final discharge products (Li 2S 2,Li 2S)are electrical insulators,which can cause poor electrochemical accessibility,leading to a low utilization of active materials.In addition,polysul fides (Li 2S n ,4≤n ≤8)produced in discharge/charge processes can dissolve into organic electrolyte and be reduced to lower-order polysul fides at the interface of the lithium anode.These reduced products will migrate back to the cathode where they may be reoxidized.This process takes place repeatedly,creating polysul fides shuttle,which can cause loss of active materials and the low coulombic ef ficiency of Li –S batteries,eventually resulting in rapid capacity fading [6,7].
In order to prevent polysul fides shuttling in organic electrolyte,various approaches have been proposed by research teams over the lat-est three decades.One of the most effective strategies is to con fine sulfur into porous frameworks,such as porous carbon materials [8–10],metal
oxide matrix [11],and polymer matrix [12,13].The porous networks can signi ficantly improve the capacities of the cathodes by limiting the ploysul fide diffusion,while the shuttle effect has not been fully addressed.Another promising approach is the modi fication of cell con-figuration,which means building a physical barrier to prevent the mi-gration of polysul fides.The cell modi fication can be concluded into three aspects,that is,surface coating of the sulfur cathode [14,15],inser-tion of free-standing interlayer (carbon interlayer [16–18],polypyrrole interlayer [19]),and modi fication of separator.The first two methods of cell modi fication have been proven to be effective to enhance the per-formance of lithium –sulfur batteries,while the study on the separation modi fication of lithium –sulfur batteries is still at a very early stage.
Separator,the basic component of the lithium –sulfur battery,is a po-rous membrane (e.g.polypropylene,polyethylene,glass fibers),which serves solely as an electronic insulator and does not in fluence the trans-portation of ions through the membranes [20,21].Polysul fides can dif-fuse freely through the membranes and react with the anode,which can cause the degradation of the battery.Therefore,the inhibition of the shuttle effect by modi fication of separator will be an effective meth-od to improve the performance of lithium –sulfur batteries.A na fion coated separator was used as an ion selective membrane to block the diffusion of polysul fide anions across the membrane to the anode side,which greatly suppresses the shuttle effect and improve the cycling performance with an ultralow capacity degradation rate of 0.08%per cycle [22].Recently,we prepared the Al 2O 3-coated separator and found that the lithium-ion conductive Al 2O 3layer can block the diffu-sion of polysul fides and enhance the electrochemical performance of lithium –sulfur batteries [23].
Solid State Ionics 278(2015)166–171
⁎Corresponding author.Tel./fax:+8673188830649.E-mail address:zza75@ (Z.
Zhang)./10.1016/j.ssi.2015.06.018
0167-2738/©2015Elsevier B.V.All rights
reserved.
Contents lists available at ScienceDirect
Solid State Ionics
j o ur n a l h o m e p a g e :w ww.e l s e v i e r.c om /l o c a t e /s s i
In this paper,we were motivated to prepare a functional carbon layer coated separator via a simple and cheap slurry-coating technique.All the raw materials of the slurry are commonly used in laboratory and commercially available,for instance,super P carbon and PVDF.The electrochemical performance of lithium sulfur batteries with functional carbon layer-coated separator can be greatly improved,with a speci fic capacity of 778mAh g −1after 100cycles at 0.5C,which is higher than that of lithium sulfur batteries with routine separator.These results indicate that the functional carbon layer-coated separator is more suit-able in lithium sulfur battery applications.
2.Experimental
2.1.Preparation and characterization of a functional carbon layer-coated separator
Commercial conductive carbon powder (Super P Timcal)was added to 8wt%polyvinylidene fluoride (PVDF 6020Solef)solution with N-methyl-2-pyrrolidinone solution (NMP)as solvent where a ratio of SP/PVDF was fixed at 75/25(wt%/wt%).And the slurry was dispersed by ball milling for 1h.This slurry was coated on the cathode side of
a
Fig.1.Schematic illustration of functional carbon layer-coated separator preparation (a),con figuration of a Li –S cell with functional carbon layer-coated separator
(b).
Fig.2.SEM photographs of (a)routine PP/PE/PP separator (surface);(b)carbon-coated PP/PE/PP separator (surface);(c)carbon-coated PP/PE/PP separator (cross-section);and (d)carbon coating layer at high magni fication (cross-section).
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routine separator(Celgard2320),and dried at55°C under vacuum for 24h.At last,the functional carbon layer-coated separator was densified by rolling machine.The thickness of the functional carbon layer was ap-proximately13μm,almost the same with thickness of Celgard2320 separator before densification and the mass of the functional carbon layer is0.53mg cm−2.A schematic illustration of the fabrication of the functional carbon layer-coated separator is shown in Fig.1a.For brevity,the separator with a functional carbon coating layer will be re-ferred to as carbon-coated separator.
2.2.Cell assembly and characterization
The sulfur cathodes were prepared by conventional slurry-coating method with a doctor blade.The cathode slurry was prepared by mixing 60wt%element sulfur(99.98%,Aldrich),30wt%carbon black(Super P Timcal)and10wt%PVDF(6020Solef)in NMP solution.Then,the slurry was spread onto aluminum foil(20μm),and dried at60°C under vacuum overnight.The cathode is punched into10mm disks.The elec-trode area is0.785cm−2.The sulfur loading density was1.6mg cm−2. Coin-type(CR2025)cells were assembled in an argon-filled glove box (Universal2440/750)in which oxygen and water contents were less than1ppm.Sulfur electrode was used as the cathode,lithium foil as the counter and reference electrode,Celgard2320or the functional car-bon layer-coated membrane as the separator,and1.5mol L−1lithium bis(trifluoromethane sulfonel)imide(LiTFSI,99.95%,Aldrich)in a solvent mixture of1,3-dioxolane and1,2-dimethoxyethane(1:1,in vol-ume)as the electrolyte.It should be noted here that a LiNO3additive,a widely reported shuttle inhibitor,was not added to the electrolyte in order to fully evaluate the physical shielding role of the functional carbon layer-coated separator.The battery configuration of a Li–S cell with the functional carbon layer-coated separator is displayed in Fig.1b.
Cyclic voltammetry(CV)and electrochemical impedance spectros-copy measurements were conducted using Solartron1470E cell test. The cyclic voltammograms(CV)and the electrochemical impedance spectroscopy(EIS)of the working electrode were carried out in three-electrode system and the sulfur cathode was used as the working elec-trode,and metallic lithium was used as the reference electrode and the counter electrode.CV tests were performed at a scan rate of0.2mV s−1 in the voltage range of1.8to2.8V.EIS measurements were carried out at open-circuit potential in the frequency range between100kHz and 10mHz with a perturbation amplitude of5mV.EIS measurement of the cycled cell was conducted in charged cathode.The galvanostatic charge/discharge tests were carried out at a constant current density of838mA g−1(0.5C)in the potential range of1.8to2.8V under a LAND CT2001A charge–discharge system.All experiments were con-ducted at room temperature.Morphology characterization and elemen-tal mapping of the functional carbon layer-coated separator before and after cycling were observed with scanning electron microscopy(FEI Quanta-200)and(FEI Nanolab600i),respectively.
3.Results and discussion
Scanning electron micrographs of the surface of the routine separa-tor and the functional carbon layer-coated separator are presented in Fig.2.The routine separator(Fig.2a)exhibits a uniformly interconnect-ed submicron pore structure,which maintains the ionic pathway and blocks the transfer of electrons between the cathode and anode[22, 24].And the size of the pores is around100nm.In contrast to
the
Fig.3.Cyclic voltammogram curves of the Li–S cells with a carbon-coated separator at a scan rate of0.2mV s−1(a);Cycle performance and coulombic efficiency of Li–S cells with routine separator and carbon-coated separator at the current density of838mA g−1(b);Charge–discharge profiles of Li–S cell with routine separator(c)and carbon-coated separator(d)at a current density of838mA g−1(0.5C).
168Z.Zhang et al./Solid State Ionics278(2015)166–171
routine separator,the carbon-coated separator(Fig.2b)has a unique coating layer comprising close-packed carbon nanoparticles intercon-nected by PVDF binder.Meanwhile,another distinct feature of the car-bon coating layer is their porous structures,that is,well-connected interstitial voids formed between the carbon nanoparticles.Fig.2c pre-sents the cross sectional image of the carbon-coated separator,which indicates the carbon porous layer adheres well to the surface of routine separator.It can be found that the thickness of the conductive carbon layer was approximately13μm.Fig.2d presents the cross sectional image of the carbon coating layer at high magnification.These porous structures(shown in Fig.2b and d)will befilled with liquid electrolytes and may provide a facile pathway for ion movement.And the tortuous pores in the functional carbon coating layer may localize the polysulfide species diffusing from the cathode to the anode side.Also the functional carbon coating layer will serve as an upper current to facilitate electron transport and enhance the utilization of sulfur.
The cyclic voltammogram(CV)profiles were measured to identify the redox reactions for the cell with carbon-coated separator.Fig.3a shows thefirstfive CV cures of the cell,and two pairs of redox peaks were observed,similar to the previous work by Yu and Chen[25,26]. During the cathodic scan,the peaks at2.3V and2.0V are attributed to the formation of long-chain lithium polysulfides(Li2S n,4≤n≤8),and short-chain insoluble lithium sulfides(Li2S2or Li2S).The anodic peaks at around2.32V and2.38V are related to the reverse process.The cell exhibits rather stable profiles,revealing the high reversibility of the cathode reactions.And the highly consistent overlap of the cathodic peaks indicates that the electrochemical process is highly stable with the carbon-coated separator.Therefore,it is reasonable to conclude that the lithium–sulfur batteries with functional carbon coating layer show good electrochemical stability.
In order to demonstrate the advantageous electrochemical proper-ties of the cell using the carbon-coated separator,cycle performance test was carried out at the current density of838mA g−1(0.5C)be-tween1.8V and2.8V,as shown in Fig.3b.Considering that the content of sulfur in the cell with carbon-coated separator decreases to52%,we also test the cycle performance of lithium sulfur cell with the same sul-fur content for comparison by using carbon–sulfur composite materials. It can be seen that the capacity of both the sulfur cathodes decreases with an increased number of cycles.The discharge capacity of the cell with60%sulfur content using the routine separator decreases from 810mAh g−1to330mAh g−1after100cycles,which is similar to the result reported by previous literature[27,28].The specific capacity of lithium sulfur cell with52%sulfur content using the routine separator is450mAh g−1after100cycles.Whereas the initial discharge capacity of the cell with the carbon-coated separator is1090mAh g−1and a high reversible capacity of778mAh g−1is retained after100cycles,showing a great improvement in cyclability,especially relatively higher specific capacity and higher capacity retention rate.The carbon-coated separa-tor provides a large conductive surface for the transformation of polysulfides to solid Li2S/Li2S2and thus enhances the active material uti-lization in electrolyte,which result in an improvement in specific capac-
ity.Fig.3c and d presents typical charge/discharge voltage profiles of the Li–S cells with or without the carbon coating layer,respectively.Consis-tent with the result from CV measurement,two plateaus are observed in discharge curve for both of the Li–S cells,which can be ascribed to the two step of reaction of elemental sulfur with metallic lithium during the discharge process.It is found that the cell using the carbon-coated separator shows lower charge plateau potential and higher discharge plateau potential than that of the cell using the routine separator.The smaller potential separation between the charge and discharge plateaus indicates better kinetic characteristics of the cell using the carbon-coated separator[8].
To identify whether the introduction of carbon-coated separator would influence the rate performance of lithium–sulfur cells,the rate performances were also evaluated as shown in Fig.4a.The discharge ca-pacity gradually decreases as the current rate rising from0.5C to2C for both of the pared with the cell using the routine separator with a specific capacity of350mAh g−1at2C,similar to the research by Tao[29],a satisfactory capacity of700mAh g−1can be obtained for the cell with the carbon-coated separator.Moreover,the cell with the carbon-coated separator can recover most of the capacity when the current rate was reduced back to1C.Such improved electrochemical performance of the cell with the carbon-coated separator can be attribut-ed to that the polysulfides are limited in the area between the cathode and the carbon-coated separator by physical obstruction and electro-chemical deposition.Fig.4b and c presents typical charge/discharge volt-age profiles of the lithium–sulfur cells with or without the functional coating layer,respectively,at different current densities.It can be seen that the specific capacity and discharge voltage declined with an increase in the current density.As shown in Fig.4c,the cell with the carbon-coated separator can maintain the higher and longer discharge
plateau Fig.4.Rate performance of Li–S cells with routine separator and carbon-coated separator (a);charge–discharge profiles of Li–S cell with routine separator(b)and carbon-coated separator(c)at different rates.
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than that of the cell with the routine separator.This suggests that the po-larization of the lithium –sulfur cell decreases with the carbon-coated separator.
To understand the improved electrochemical performance of the cells using the carbon-coated separator,electrochemical impedance spectroscopy (EIS)analysis was carried out.The Nyquist plots of the cells with carbon-coated separator and routine separator before the
first discharge and after cycling are presented in Fig.5.For the cells before the first discharge,as shown in Fig.5a,the impedance plots are composed of a depressed semicircle at high frequency,which corre-sponds to the charge transfer resistance (R ct )of the sulfur electrode [27,30],and an inclined line at low frequency,which re flects the Li ion diffusion into the active mass [31].Besides,the intercept at real axis Z,corresponding to the combination resistance R e ,associates with the ionic conductivity of electrolyte,the intrinsic resistance of the cathode,separator,and anode,and the contact resistance at the active material/current collector interface.As shown in Fig.5b,the impedance plots of the cathodes after 100cycles are composed of two depressed semicir-cles at high frequency and middle frequency,respectively,and an in-clined line at low frequency,which agree with previous reports [32,33].The semicircle in the high frequency region corresponds to the charge transfer resistance (R ct ),and the semicircle at middle frequency is considered as the resistance of the SEI film (R s )formed on
the
Fig.5.Nyquist plots for the Li –S cells with routine separator and carbon-coated separator:(a)before cycling and (b)after cycling.
Table 1
Fitting results of EIS plots in Fig.5.Separator
Before cycling After cycling R e
R s R ct R e R ct R s Routine separator
1.891135
2.316 4.3Carbon-coated separator
4.7
32.3
3.7
8
2.3
Fig.6.Morphologies and elemental mapping results of carbon-coated separator:SEM image of carbon-coated separator after cycling (a)(b);(c)(d)elemental mapping of the cycled car-bon-coated separator (a).
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electrodes'surface[34,35].In addition,the inclined line at high frequen-cy reflects the Li ion diffusion into the active mass.
In order to understand the results better,the relevant equivalent cir-cuit models was given(as shown in Fig.5),and the related resistance data was obtained(as shown in Table1).Before cycling,it is clear that the charge transfer resistance(Rct)of the cell using carbon-coated sep-arator(32Ω)is smaller than that of the cell using routine separator (35Ω).It can be attributed to the conductive network on the separator and the better adhesion property of PVDF-carbon coating layer towards the sulfur cathode.After cycling,the SEIfilm resistance(Rs)of the cell using carbon-coated separator(2.3Ω)is smaller than that of the cell using routine separator(4.3Ω),which to some extent,confirms the blocking effect of the carbon-coated separator to polysulfi-pared with the resistance of the cells before cycling,the resistance of both cells decreases after cycling,which is due to a chemical activation process of the dissolution and redistribution of the active materials[33].
To further investigate the reason why the carbon-coated separator can considerably advance the electrochemical performance of the Li–S cells,a scanning electron microscope analysis was carried out on cycled pared to the microstructure image of the fresh carbon-coated separator in Fig.2b,the SEM images of the cycled carbon-coated separator in Fig.6a and b show deposited materials within the interspaces and on the carbon-coating layer,indicating that the carbon-coating layer acts as a matrix to capture and retain the polysulfide species by electrochemical deposition at the end of discharge–charge reaction.Fig.6c and d indicates the elemental mapping of the cycled carbon-coated separator,where sulfur-rich compounds are distributed homogeneously throughout the carbon-coating layer,which confirms the trapping of sulfur-rich com-pounds within the carbon-coated separator.
4.Conclusion
In summary,we prepared a functional carbon layer-coated separator by a cheap and simple slurry-coating method for lithium–sulfur batteries. The functional carbon layer consisting of commonly used super P carbon/ PVDF serves as not only afilter to absorb the migrating polysulfides, but also an upper current collector to facilitate electron transport and reactivate the trapped active material.Consequently,the lithium–sulfur cell employing the functional carbon layer-coated separator displays a high specific capacity with778mAh g−1at0.5C after100cycles,and excellent rate performance from0.5C to2C.Additionally,the coulombic efficiency of the cell was also improved obviously after using the carbon-coated separator.The excellent results prove that the modification of the separator will be a promising strategy to promote the development of the lithium–sulfur batteries.Acknowledgments
The authors acknowledge thefinancial supports from the Teacher Research Fund of Central South University(2013JSJJ027)and the Natu-ral Science Foundation of China(51474243).
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