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Inhibiting the shuttle effect of Li–S battery with a graphene oxide coating separator: Performance improvement and mechanism study
Journal of Power Sources 342 (2017) 929e938
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Inhibiting the shuttle effect of LieS battery with a graphene oxide coating separator: Performance improvement and mechanism study Yong Jiang a, Fang Chen a, Yang Gao b, Yanyan Wang a, Shanshan Wang b, Qiang Gao a, Zheng Jiao b, Bing Zhao a, *, Zhiwen Chen b, ** a b
School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China Shanghai Applied Radiation Institute, Shanghai University, Shanghai 201800, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
GO-separator is used as a shuttle inhibitor of the LieS battery. Excellent polysulfide-diffusion inhibiting effect and cycle life are observed. Much less sulfur and polysulfide species are tested in separator and electrolyte. GO film still maintains 3D flexible porous structure with a few insoluble particles. Less sulfate species, lithium salts and polysulfides are detected on the cathode.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2016 Received in revised form 13 December 2016 Accepted 3 January 2017
In this paper, graphene oxide (GO) is integrated on commercial polypropylene separator by tape casting method and sandwiched between a sulfur cathode and the separator as a shuttle inhibitor of the Li-S battery. The issues of lithium polysulfides dissolution and shuttle effect are inhibited distinctly, and significant improvements not only in the active material utilization but also in capacity retention are observed. What's more, the improvement mechanism is studied in detail. The results demonstrate that the sulfur and polysulfide species in separator and electrolyte for the cell with GO-coating separator are much less than that with the pristine separator. The GO membrane still maintains three-dimensional porous and flexible structure with a few lithium polysulfides and Li2S2/Li2S nanoparticles anchored on the surface and inter-layers of GO sheets after long cycles. And the active materials are significantly localized within the cathode structure after GO-coating. In addition, less sulfate species, lithium salts, polysulfides and other insoluble species are identified on the cathode and separator after long-term cycling. © 2017 Elsevier B.V. All rights reserved.
Keywords: Graphene oxide coating separator Lithiumesulfur battery Shuttle effect Oxygen-containing functional group Polysulfide Mechanism study
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Zhao), [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.jpowsour.2017.01.013 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Because of the increasing demand of high specific capacity storage device, lithiumesulfur (LieS) battery system has become the focus of attention of many researchers. Compared to
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lithiumeion battery, LieS battery has a higher theoretical energy density (2650 Wh kg1) [1e3], which is an order of magnitude higher than the currently used lithium-ion battery system. Moreover, sulfur is also abundant, nonetoxic and cheap. Although LieS battery has such great potential, the insulating nature of the sulfur [4], high solubility of polysulfide intermediate species (Li2Sn, 4 n 8) and insolubility of final discharge products (Li2S2/Li2S) significantly affect utilization of the active material, resulting in low discharge/charge efficiency and poor cycling performance of LieS battery. Moreover, the density of S (2.03 g cm3) and its discharge product Li2S (1.66 g cm3) is different within the cathode, the huge volume change (~80%) usually leads to structure destruction, mechanical integrity decrease and rapid capacity decay of the electrode [5e8]. In addition, the soluble polysulfides in the liquid electrolyte may shuttle between the cathode and the anode (shuttle effect) and form Li2S2 or Li2S solid on the lithium anode [9], resulting in low active sulfur utilization, large electrode impedance, low Coulombic efficiency and poor cycle life. To overcome these issues, extensive research efforts have been devoted to develop methods to improve the electrical conductivity of sulfur cathode and maintain/re-utilize the soluble polysulfides within the cathode structure (alleviate the shuttle effect) by wrapping small sulfur molecules in confined micro porous carbon shell, changing the compositions of the electrolytes or by adding carbon-based barrier layers to localize the active material within the cathode region of the cell [10,11]. Of these, adding a carbon-based barrier layer between the sulfur cathode and separator has shown to be a promising approach owning to its simple fabrication process, low cost, no complex composite synthesis process and especially the highly improved rate capability and cycling stability. According to the previous literature, the super P [10], micro porous carbon paper [12], multiwall carbon nanotubes (MWCNTs) [13,14], graphene [15], reduced graphene oxide (rGO) [16,17], were used as the carbon-based barrier layers to prevent the diffusion of polysulfides and enhance the cycling performances of LieS batteries. Among them, Manthiram and co-workers inserted a freestanding MWCNTs [13,14] and micro porous carbon paper [12] interlayer between the sulfur cathode and separator. Morphology variation and elemental mapping of the carbon-based interlayer after cycling were characterized, and it demonstrated that most of the polysulfides were obstructed on the surface of the carboncoating. Chen and Li demonstrated that the bi-functional rGOcoated separator could inhibit the corrosion of lithium plate after 100 cycles [16]. Cui and co-workers introduced a thin metal oxides nanoparticles and carbon black composite coating on the separator, and a dense layer (with thickness of ~500 nm) composed of inactive sulfur and lithium sulfide was accumulated at the separatorcathode interface [10]. All of these previous reports showed that the carbon-based interlayer could reduce the charge transfer resistance of the sulfur cathode, stabilize the polysulfide species within the cathode region of the cell, and thereby improve the electrochemical performances. However, the detailed influence mechanism of carbon-based interlayer on the microstructures and chemical compositions of sulfur cathode during cycling is unclear, nor has it demonstrated specifically the whereabout of active sulfur or the components of sulfur species. Moreover, most of the previous reports that use conductive carbon as the barrier layer are typically using their high surface area and electrical conductivity properties. As an important member of conductive carbon, two-dimensional GO with abundant oxygen-containing groups has not well used to trap sulfur and polysulfides species at all. We believe that exploring functional separator for LieS battery and studying its improving mechanism in-depthly is a worthwhile field of inquiry but that it is only in its infancy.
In this study, GO was integrated on commercial polypropylene separator by tape casting method and sandwiched between a sulfur cathode and the separator as a shuttle inhibitor. In this GO-coating separator, the abundant oxygenecontaining functional groups on the GO membrane can intercept/absorb the diffusing polysulfide intermediates and prevent them diffusing through the separator effectively [16], while the porous conductive skeleton serves as an upper current collector to facilitate electron transport, enhance the utilization of active material, and reserve sulfur species at the separator-cathode interface [18]. Additionally, the commercial polypropylene films function as an electrically insulating membrane and a flexible substrate to support the GO-coating layer on the cathode side [14]. With these inspiring merits, the issues of lithium polysulfides dissolution and shuttle effect are inhibited distinctly, and significant improvements not only in the active material utilization but also in capacity retention are observed. And on this basis, the improvement mechanism is studied in detail. The result shows that the sulfur and polysulfide species in separator and electrolyte for the cell with GO-coating separator are much less than that with the pristine separator. The active materials are significantly localized within the cathode structure after GOcoating. Moreover, the FT-IR, XPS, SEM and EDS analyses further demonstrate that less sulfate species, lithium salts, polysulfides and other insoluble species are identified on the cathode and separator after long-term cycling.
2. Experimental 2.1. Synthesis of GO Firstly, GO sheets were prepared by oxidization of natural graphite powder using the modified Hummers method reported earlier [19,20]. The detailed preparation process was shown in the Supporting information. SEM and TEM images were shown in Fig. S1. The concentration of the GO suspension was about 2.0 mg mL1, which was determined by drying the suspension at 110 C for 24 h and then weighing the dried GO. Then, the GO suspension was used as the raw materials for preparation of graphene aerogel/nanoesulfur composite (GAeS) cathode electrode and polypropylene separator coating film.
2.2. Preparation of GAeS cathode electrode Threeedimensional porous GAeS cathode was prepared via a hydrothermal reductioneassembly of GO with a sulfur-dissolving CS2 and alcohol solution as described in our previous work [21]. In a typical experiment, 50 mL GO suspension and 10 mL ethanol were mixed, and then 5 mL solution of sublimed S/CS2 (30 mg mL1) was added. After stirring for 1 h, the mixture suspension was then sealed in a 100 mL Teflonelined stainless steel autoclave for hydrothermal reaction at 180 C for 12 h. The asprepared hydrogel was washed by ethanol and distilled water several times, cut into small circular pellets with a thickness of about 1.0 mm and diameter of 10 mm, and then freeze-dried overnight. Morphologies of the GAeS hybrids demonstrate that the sample has well-defined and interconnected threedimensional porous network with open pores in the range from submicrometer to several micrometers (Figs. S2a and b). The obtained GAeS circular pellets with mass loading of about 2.0 mg cm2 and porosity of about 29.8% were used as the cathode electrode directly. The active sulfur content in the circular pellet was estimated to be about 1.10e1.25 mg according to the TGA measurement (Fig. S2c).
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2.3. Preparation of GO-coating separator (GO-separator) The GO-coating separator was prepared by surface coating GO slurry on one side of a commercial microeporous polypropylene film Celgard 2400 (~25 mm thick) by tape casting method [18]. Firstly, the as-prepared GO powder (0.20 g) and 0.02 g polyvinylidene fluoride (PVDF) were added into 100 mL of isopropyl alcohol (IPA). After ultrasonication for 30 min and magnetic stirring with a rate of 1000 rpm for 12 h, the GO-based slurry was pasted on the commercial Celgard 2400 separator by tape casting method with blade height of about 50 mm. After heat-treatment at 50 C under vacuum for 12 h, the GO-coated separator was punched into circular disks with diameter of 16 mm. As shown in Figs. S3a and b, the surface of the separator turns from white to brown after the slurry coating, the thickness of GO coating film after drying was about 18.3 mm (Fig. S4d). 2.4. Cell assembly The CR2032 coinetype cells were assembled in high purity argon glove box (Mikrouna, Super 2440/750) with the GAeS, GOcoating separator, pure lithium foil (4 ¼ 15 mm, 1.0 mm thick, Aldrich) as the cathode electrode, separator and anode electrode, respectively. Note that the separator with GO film side faced the GAeS cathode. The electrolyte was 1 mol L1 of LiTFSI in 1,3dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 by volume) with 0.1 mol L1 LiNO3 additive. The as-prepared cell system was denoted as GAeS þ GOeseparator. For comparison, the pristine polypropylene separator Celgard 2400 was used directly without GO coating, and the assembled cell was denoted as GAeS þ separator. 2.5. Electrochemical measurements and mechanism study The electrochemical performances of the as-prepared GAeS þ GOeseparator and GAeS þ separator cells were tested on a LAND CT2001A cell test system under current density range from 100 mA g1 to 1000 mA g1 in the voltage range of 1.5e3.0 V. Cyclic voltammetry (CV) measurements were performed on CH Instrument 660C at a scan rate of 0.5 mV s1. Electrochemical impedance spectroscopy (EIS) was performed by applying a sine wave with amplitude of 5.0 mV in the frequency range from 100 kHz to 0.01 Hz. In order to study the effects of GO-coating separator on microstructure, morphology, polysulfide ions dissolution, chemical compositions of the cathode electrode and its electrochemical improved mechanism, the GAeS þ GOeseparator and GAeS þ separator cells were continuously charged and discharged for 50 cycles. Then the sulfur contents in the cathode, separator and electrolyte were tested by inductively coupled plasma optical emission spectrometry (ICPeOES) with a Thermo Scientific ICAP 6300 Duo View Spectrometer. SEM images of the GAeS cathode electrodes and the separator were taken using fieldeemission scanning electron microscopy (FEeSEM, JEOL JSMe6700F) operated at an accelerating voltage of 5 kV. EDS was used for elemental analysis. The surface electronic states and chemical compositions of GAeS cathode electrodes were investigated by Fourier transform infrared spectroscopy (FTeIR, AVATAR 370) and Xeray photoelectron spectroscopy (XPS, ESCA LAB 250Xi instrument with Al Ka radiation, 1486.7 eV). 3. Results and discussion The schematic of two kinds of LieS battery configurations during charge and discharge process is illustrated in Fig. 1. As for the
Fig. 1. Schematic representation of LieS cells with GOecoating separator (a) and pristine polypropylene separator (b). The GO-coating barrier aims at impeding the free migration of the polysulfides and preventing them from shuttling from sulfur cathode to lithium anode through the separator.
conventional LiS battery configuration (Fig. 1b), highly soluble polysulfides formed during discharge can readily diffuse out of the composite cathode structure, dissolve in the electrolyte and shuttle between the cathode and anode, which would cause an irreversible loss of sulfur continually, lead to low Coulombic efficiency and high impedance, and thus significantly undermine the cycling stability of the cell. However, the flexible and conductive GO layers on the side of GAeS cathode (Fig. 1a) can significantly reduce the shuttle effect of dissolved polysulfides since that there are numerous of oxygen-containing (mainly carboxylic acid, hydroxyl, and epoxide) groups on the surface of GO (Fig. S5), which could bond with polysulfides by moderate binding and act as sulfur reservoir to reutilize the dissolved polysulfide intermediates [10,15,16]. In addition, the GO film can improve the electric conductivity and accommodate the large volumetric expansion of sulfur during lithiation because of its flexible structure and rough surface. Therefore, the polysulfide dissolved in the electrolyte can be reused largely and repeatedly, and thus high sulfur utilization and effective active material reutilization would be accomplished. The cyclic voltammetry (CV) and galvanostatic charge/discharge measurements of the Li-S batteries were evaluated in the voltage range of 1.5e3.0 V. Fig. 2a and b shows 1st, 2nd and 5th CV profiles of GAeS þ separator and GAeS þ GOeseparator at a scan rate of 0.5 mV s1. It can be observed that both samples show two cathodic peaks at around 2.25 and 1.83 V, indicating a multiple-step reduction mechanism of sulfur within lithium [22,23]. The
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Fig. 2. Cyclic voltammograms and galvanostatic charge/discharge profiles of GAeS þ separator (a, c) and GAeS þ GOeseparator (b, d).
reductive peak at ~2.25 V can be ascribed to the transformation of sulfur to soluble long-chain polysulfides (Li2Sx, 4 x 8), and the peak at ~1.83 V corresponds to the further conversion to shortchain polysulfides (Li2Sx, 2 x 4) and following insoluble Li2S2/ Li2S [24,25]. In the first anodic scan process, the distinct oxidation peak at ~2.55 V is caused by the complete oxidation of Li2S/Li2S2 to Li2S8/S8. As the cycle number increased, the cathodic peaks of the cell with pristine separator shift to low voltage and the intensities of all the peaks decrease sharply, revealing the electrochemical instability of this cell configuration (Fig. 2a). However, the anodic peak of the cell with GO-coating separator slightly decreases with cycling, suggesting good capacity retention (Fig. 2b). During the second to fifth circle scan process, the redox peaks are displayed in similar positions with the same intensity, indicating its excellent capacity retention [26]. In addition, the cell with GO-coating separator has a smaller voltage gap between oxidation and reduction peaks than that of the cell with pristine separator. And the oxidation and reduction peaks current (ipc) in GAeS þ GOeseparator cell configuration are the same (±1.25 mA), whereas the values for GAeS þ separator are þ1.1 and 0.7 mA, respectively, further demonstrating that the cell with GO-coating separator is more reversible in lithium storage than that of the pristine ones [16,27,28]. Fig. 2c and d shows the galvanostatic charge-discharge profiles of GAeS þ separator and GAeS þ GOeseparator at the 1st, 2nd, 5th and 50th cycle at the current density of 100 mA g1. One charge plateau and two discharge plateaus are displayed, which are consistent with the redox peaks appeared in the CV curves and is in good agreement with the previous reports [29,30]. The discharge capacities of both samples suffer different degrees of decay upon cycling since that the active sulfur cathode needs to take a few cycles to reach a steady state to form stable electrochemical property because of high solubility of the polysulfide intermediates in the liquid electrolyte [12]. Compared to that of the cell with pristine separator, the cell with GO-coating separator displays higher initial discharge capacity (1550.3 mAh g1, calculating based
on the weight of S) and lower capacity loss after 50 cycles. This significant performance enhancement indicates that the GOseparator could effectively inhibit the diffusion of polysulfide species and thereby eliminate the severe loss of active material/capacity during long-term cycling. The rate charge/discharge curves of GAeS þ separator and GAeS þ GOeseparator are shown in Fig. 3a and b. It can be seen that the plateaus are much flat and stable for GAeS þ GOeseparator, suggesting a kinetically efficient reaction process with a small potential barrier. Whereas the discharge plateaus obviously shift or even disappear in the case of the GAeS þ separator at high current rates, indicating its high polarization and slow redox reaction kinetics with inferior reversibility. The initial discharge capacities of GAeS þ GOeseparator are 1550.3, 1125.6, 910.2 and 751.3 mAh g1 at 100, 200, 500 and 1000 mA g1, respectively. When the current density is restored to 100 mA g1 after cycling under high rate, it can still deliver a reversible capacity of 1007.5 mAh g1 (Fig. 3c). In contrast, the initial discharge capacities of GAeS þ separator are 810.3, 605.7 and 285 mAh g1 at 200, 500 and 1000 mA g1, respectively. EIS measurements of the two configurations before cycling are showed in Fig. 3d. It can be seen that the Nyquist plots consist of a depressed semicircle (in the high-frequency region) and an inclined straight line (in the low-frequency region), which correspond to the charge-transfer resistance (Rct) and the Warburg impedance (related to the Li-ion diffusion within the electrode), respectively [16]. The equivalent circuit model is presented in the inset. The Rct for GAeS þ separator and GAeS þ GOeseparator are 75.1 and 100.5 U, respectively, indicating about 25.3% impedance decreases after the insertion of GO-coating layer. And they both present electrochemical and diffusion mixing control processes since that the entire reaction rate is controlled by the electrochemical reaction rate and ion migration rate at the initial stage. The relatively low Rct for GAeS þ GOeseparator means that the GO-separator can intrinsically increase the contact surface area between GA-S cathode and the separator, reduce the contact resistance and therefore
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Fig. 3. Rate charge/discharge profiles of GAeS þ separator (a) and GAeS þ GOeseparator configurations (b). Comparisons of the rate capability (c), first Nuquist plots (d), cycling performances (e) and the 50th Nuquist plots (f) of the GAeS þ separator and GAeS þ GOeseparator configurations. The inset in (d) is the equivalent circuit model.
reduce charge transfer impendence of the battery [31e36]. After 50 cycles, the specific capacity of GAeS þ GOeseparator sample is maintained at about 835.2 mAh g1, and the capacity loss slows down as the cycling upon. However, the GAeS þ separator sample delivers only 550.7 mAh g1 after 50 cycles with continuous capacity fading (Fig. 3e). Moreover, one additional semicircle is displayed for the GAeS þ separator after 50 cycles (Fig. 3f), which can be attributed to Liþ migration through the passivation film. Combining with the enlarged charge transfer resistance in the electrode, the EIS spectrum suggests dissolution of polysulfides followed by their deposition onto the electrode, demonstrating the continuous cycling increases the charge transfer resistance of the electrode. In strong contrast, the Rct is much low for GAeS þ GOseparator, indicating good electrolyte infiltration, charge transport, and effective entrapment/reutilization of dissolved polysulfide intermediate species of GO-separator. These results demonstrate that cell with GO-coating separator exhibits better electrochemical performance upon cycling compared to that with pristine separator. Therefore, threeedimensional structure damage and chargetransfer resistance of GA-S cathode can be reduced. And the formation of Li2Sx (x ¼ 1 or 2) deposition layer and the surface corrosion of lithium plate anode are expected to be significantly inhibited [16,37].
To further understand the function of GO-separator and reveal the mechanism of improved electrochemical performances, the batteries (in the fully charged state) were disassembled after long cycles in an argon-filled glove box. GA-S electrodes and the separators were removed, washed using 1,3-dioxolane and their morphologies, microstructure and chemical compositions were characterized by SEM, EDS, FT-IR, XPS and ICP-OES. Fig. 4aec shows the sulfur content (total sulfur) tests in the cathode electrode, separator and electrolyte, respectively. As can be seen in Fig. 4a, the sulfur content in GA-S cathode shows a consistent decrease during prolonged cycling. Obviously, the sulfur content in cathode for GAeS þ GOeseparator is noticeably higher than that in the GAeS þ separator sample, which represents steady sulfur retention of about 35.3 wt% after 25 cycles. Whereas, sulfur content in GAeS þ separator sample gradually decreases with the increasing of cycles, only 14.2 wt% sulfur is obtained after 50 cycles. The similar phenomenons are observed for the sulfur content tests in polypropylene film and electrolyte (Fig. 4b and c). Sulfur contents for GAeS þ GOeseparator sample in polypropylene film and electrolyte after 50 cycles become stable at about 6.2 wt% and 9.0 wt%, respectively. Whereas, values for GAeS þ separator sample are more than twice of those for the cell with GO-coating separator and reflect upward trends, which are 13.5 wt% and 21.3 wt%,
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Fig. 4. The sulfur content (total sulfur) tests in the cathode electrode (a), separator (b) and electrolyte (c). Digital images of the separators in two configurations after 50 cycles (d).
respectively. The digital images of separators in two configurations after 50 cycles further demonstrate that most of the sulfur and polysulfides are trapped within the cathode region and on the surface of the GO-coating film, rather than diffusing through or remaining in the polypropylene separator. The results demonstrate that GO-coating film serves as the barrier layer could obstruct and immobilize the migrating polysulfides, prevent them from shuttling from sulfur cathode to lithium anode, as well as effectively inhibiting the shuttle behavior and improving the utilization of the active material. SEM images and EDS spectra of the GAeS cathode electrodes in GAeS þ GOeseparator and GAeS þ separator configurations after 50 cycles were shown in Fig. 5. The fresh GAeS electrode before cycling was used as a comparison. It can be seen that the GAeS hybrid aerogel has well-defined and interconnected threedimensional porous network with open pores in the range from submicrometer to several micrometers. From the EDS analysis, the sulfur content on the GA-S cathode is 68.5 wt%, which basically corresponds to the TGA test (Fig. S2c). As for the cell with GOeseparator, the threeedimensional porous structure of the GAeS cathode has stayed basically the same as the fresh electrode without any dense layer, and no fracture occurs after long-time charge/discharge cycles (Fig. 5b). In contrast, an almost completely damaged structure with distinct cracking and pulverization is found on the GA-S cathode using the pristine separator (Fig. 5c). Additionally, the sulfur content in the cell with pristine separator is much lower (14.2 wt%) than that with GOeseparator (35.3 wt%). These results demonstrate that the GO-separator acts as strong absorbent and barrier could efficiently immobilize the polysulfides and prevent their diffusion from the GA-S cathode to the electrolyte during the charge/discharge process. Thus the polysulfides shuttle phenomenon and the corrosion damage occurred on three-dimensional porous structure can be reduced. It can also be seen that the F element is observed at 0.7 keV in these two cycled samples, and the signal in GAeS þ separator is much
apparent compared to that with GO-coating separator, suggesting more side reaction between lithium polysulfides and fluorinecontaining electrolyte during cycling, which will be further discussed in the FTeIR and XPS. Fig. 6 shows the morphology and elemental analysis of the pristine separator and the GO-coating separator after 50 cycles. As we all known, the fresh separator is a micro porous polypropylene film owning nanopores with diameter of 10e100 nm on the surface (as shown in Fig. S4a). After the GO coating, a dense GO film with thickness of about 18.3 mm was formed on the surface of the separator and covered the nanopores (Figs. S4bed). The high specific area (274.9 m2 g1) of GO paper and abundant oxygencontaining groups should contribute to physically and chemically adsorb/trap the polysulfides. After 50 continuous charge-discharge processes, the GO membrane on the separator still connects with the polypropylene film intimately and remains intact without cracks, indicating its good structural stability and normal function during cycling (Fig. 6c and d). There is no obvious structural change for the GO membrane due to its flexibility and robustness. The cross section of the GO layer shows that it still has a three-dimensional porous structure with a few lithium polysulfides and Li2S2/Li2S nanoparticles anchored on the surface and inter-layers of GO sheets, promoting the thickness increase to 33.9 mm after long cycles (Fig. 6h). There's no obvious layered boundary for the GO layer and the polypropylene Celgard film (Fig. 6g and h), suggesting that the dissolved sulfur species are probably absorbed by the pores between the interlayer rather than forming continuous insulating S/Li2S-inactive layers. This feature is very critical because the agglomeration of active materials would increase the chargetransfer and ion transport resistance and thus induce irreversible capacity loss of the cell [12]. Moreover, the maintaining of porosity structure of the GO paper after long cycles guarantees its superior liquid electrolyte infiltration. The elemental mapping results indicate the sulfur signal is found all over the GO interlayer and distributed homogeneously (Fig. 6g), further demonstrating the
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Fig. 5. The SEM images and the corresponding EDS spectra (unit: keV) of fresh GAeS cathode electrode (a), and the electrodes after 50 cycles in GAeS þ GO-separator (b) and GAeS þ separator (c) configurations.
dynamic re-utilization of the active material. Moreover, the distinct fewer sulfur species signal is detected in the polypropylene Celgard film filtered out by the porous GO-coating, demonstrating its excellent interception and absorption function (Fig. 6f and Table S1 in Supporting Information). Combined with the SEM, EDS of the GA-S cathode electrode and the separator, the results demonstrate that the polysulfide species are adsorbed/trapped by the oxygencontaining groups of GO-coating rather than diffusing to the lithium anode, therefore the polysulfide shuttling behavior is inhibited, material utilization of dissolved sulfur is improved, and three-dimensional structure of sulfur cathode during cycling is maintained efficiently. To further probe these differences, we conduct FTeIR on GAeS cathode electrodes after 50 cycles as presented in Fig. 7. Only a few weak absorption peaks corresponding to CeC/C]C and
oxygenecontaining functional groups (OeH, C]O and CeO) are observed in the fresh GAeS cathode (Fig. 7a). In comparison with the fresh cathode, the absorption peaks are mainly centered in the range of 500e1800 cm1 for the cycled electrodes, which can be ascribed to the lithium polysulfides and their reaction products with electrolyte. As described above in the experimental section, small amount of LiNO3 additive is used in the electrolyte, which has been declared to be able to ease the shuttle effect at some extent [36]. However, its strong oxidation characteristic would oxidize the sulfur element to different valence states. Additionally, the 1,3edioxolane in the electrolyte is the source of H$ radical. Then, parts of the reactions occurred in the electrolyte could be inferred as follows [38e40]: LiN(SO2CF3)2 þ nee þ nLiþ / Li3N þ Li2S2O4 þ LiF þ LiCF3
(1)
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Fig. 6. Morphology the pristine separator (a, b), GO-coating separator (c, d) and the polypropylene Celgard film after stripping GO layer after 50 cycles (e). EDS analysis of pristine separator and GO-coating separator (f). High magnification cross-sectional SEM images and element mapping of the polypropylene Celgard film (g) and the GO layer (h).
LiN(SO2CF3)2 þ 2ee þ 2Liþ / Li2NSO2CF3
(2)
Li2S2O4þ 6ee þ 6Liþ / 2Li2S þ 4Li2O
(3)
LiN(SO2CF3)2 þ nee þ nLiþ / Li3N þ LiOSO2 þ LiCF3
(4)
Fig. 7. FTeIR spectra of fresh GAeS cathode electrode (a), and the electrodes after 50 cycles in GAeS þ GOeseparator (b) and GAeS þ separator (c) configurations.
Based on the above perceptions, it can be seen that the GAeS cathode in GAeS þ separator sample (Fig. 7c) shows Li2O, LieO stretching vibrations at 500e700 cm1, CeF, eSO2 and S]O stretching bands at 1120, 1320 and 1370 cm1, respectively, which can be attributed to the reactions between Liþ and the electrolyte. The SeO and CeS bonds at 700e900 cm1 is due to the reaction of the oxidized products of sulfur with the TFSIe. The SeO, CeS and S]O bonds at 900e1000, 1050 and 1200 cm1 is ascribed to the reactions between NO 3 , S and polysulfides. C]O, eLiCO2 and ROeLi stretching vibrations in the region of 1500e1800 cm1 should be attributed to the reaction of Liþ with the intermediate products. While the eCH, eCH2 bonds at 1400 cm1 is assigned to the DOL in the electrolyte. The spectra also inevitably reflect the absorption peak of eOH bond at 3470 cm1, which is due to the tests being performed in the air. The above results show that the various reactions between S, polysulfide species and electrolyte render many sulfate species, lithium salts, polysulfides and other insoluble species depositing on the surface of electrode, which may hinder the electron and ion transport, increase the reaction resistance and induce significant loss of active material, resulting in poor rate and cycling performances. In contrast, almost all the intensities of vibration peaks for GAeS þ GOeseparator sample are lower than that without GO-coating excepting for the SeO bond at 1050 cm1 (Fig. 7b and c). This is due to that the sulfur molecule and the functional group of graphene aerogel in GAeS cathode are covalently bound by CeOeS bond [21], which is not severely destroyed during charge and discharge. Fewer side reactions for GAeS cathode in GAeS þ GOeseparator sample suggest that the
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Fig. 8. The S 2p and Li 1s spectra of GAeS cathode electrodes after 50 cycles in GAeS þ separator (a, c) and GAeS þ GOeseparator (b, d) configurations.
GO-coating could effectively inhibit the shuttle effect of lithium polysulfides, less reactions between sulfur species and electrolyte occur, and thus fewer insoluble species deposit on the surface of electrode. XPS are further carried to investigate the surface electronic states and chemical compositions of the GAeS cathodes after 50 cycles. The higheresolution S 2p spectra of the electrodes (Fig. 8a and b) demonstrate that peak intensities of the S2 ion (170.4 eV) and S]O group (167.1 eV) in GAeS þ separator, corresponding to insoluble Li2S and reduction of TFSIe, respectively, are much stronger than that in the GAeS þ GOeseparator. Whereas, the peak intensity of the SeS bond at 163.7 eV in GAeS þ separator is much weaker than that in the latter. From the Li 1s spectrum of the cathode in GAeS þ separator (Fig. 8c), it can be seen that the peaks at 52.6, 54.4, 55.2, 56.2 and 59.1 eV correspond to lithium species of Li2S, LiOH, Li2CO3, LiF/Li2O and ROeLi. Whereas, only one Li 1s peak at 55.5 eV corresponded to lithium species of LiF or Li2O is found in the GAeS þ GOeseparator sample (Fig. 8d). Note that very weak Li2S/Li2S2 signal is found in the Li 1s spectrum of the cycled electrode, suggesting that most of Li2S converts reversibly to soluble lithium salt in the electrolyte. Overall, the XPS results indicate that large amount of S8 active material and less insoluble Li2S and inactive sulfate species exist in the GAeS þ GOeseparator system, which agree well with the sulfur content tests by ICP-OES (Fig. 4a), EDS analysis (Fig. 5) and the FTeIR spectra (Fig. 7). All of these results demonstrate that the GO-coating separator can significantly enhance the electrochemical performances of Li-S batteries. Through research of the improvement mechanism, the roles of GO-coating can be simply analyzed as follows [41e43]. Firstly, abundant of oxygen-containing functional groups on GO film can physically and chemically adsorb/trap sulfur and polysulfides to effectively localize the soluble polysulfide species within its porous skeleton and prevent their diffusion to the electrolyte, which could minimize the shuttling effect and repeatedly use the polysulfides even during long
cycles. Secondly, the GO-coating layer has a porous structure with superior liquid electrolyte infiltration, which is beneficial for rapid ions transport and rate performances. Thirdly, the mechanical flexibility of GO film can accommodate the volume expansion and shrinkage of sulfur cathode and keep the integrity of the cathode structure during continuous charging and discharge processes. Finally, the GO-coating layer has high electrical conductivity, which could act as the role of an upper current collector to decrease the internal charge transfer resistance and increase the conducting surface between GA-S cathode and the pristine polypropylene separator. This can effectively prevent the accumulation of inactive polysulfide intermediates at the interface between the cathode and separator, as well as increasing the utilization of active material and inactive polysulfide species.
4. Conclusion In summary, the GO-coating separator is a very simple and effective strategy to substantially inhibit the shuttle effect of polysulfide and improve the electrochemical performances of Li-S battery. Based on the analysis of the microstructure, morphology and chemical composition variations of the sulfur cathode electrode, separator and electrolyte, the improvement mechanism of GO-separator is studied in detail. The results demonstrate that the sulfur and polysulfide species in separator and electrolyte for the cell with GO-coating separator are much less than that with the pristine separator. The GO membrane still maintains threedimensional porous and flexible structure with a few lithium polysulfides and Li2S2/Li2S nanoparticles anchored on the surface and inter-layers of GO sheets after long cycles. And the active materials are significantly localized within the cathode structure after GOcoating. In addition, less sulfate species, lithium salts, polysulfides and other insoluble species are identified on the cathode and separator after long-term cycling.
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Acknowledgement This work was supported by the National Natural Science Foundation of China (21501119, 11575105, 21671130), Science and Technology Commission of Shanghai Municipality (15DZ0501402), Shanghai Municipal Education Commission (Peak Discipline Construction Program, N.13-A302-15-L02), Program for Changjiang Scholars and Innovative Research Team in University (IRT13078). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.013. References , P.G. Bruce, B. Scrosati, J.-M. Tarascon, W.V. Schalkwijk, Nat. Mater. 4 [1] A.S. Arico (2005) 366e367. [2] X.L. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. [3] Q.W. Tang, Z.Q. Shan, L. Wang, X. Qin, K.L. Zhu, J.H. Tian, X.S. Liu, J. Power Sources 246 (2014) 253e259. [4] X. Jia, C. Zhang, J.J. Liu, W. Lv, D.W. Wang, Y. Tao, Z. Li, X. Zheng, Nanoscale 8 (2016) 4447e4451. [5] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [6] A. Manthiram, Y. Fu, Y. Su, Accounts Chem. Res. 46 (2012) 1125e1134. [7] H. Yamin, A. Gorenshtein, J. Penciner, Y. Sternberg, E. Peled, J. Electrochem. Soc. 135 (1988) 1045e1048. [8] B. Scrosati, J. Garche, J. Power Sources 195 (2010) 2419e2430. [9] J. Wang, Y.-S. He, J. Yang, Adv. Mater. 27 (2015) 569e575. [10] H. Yao, K. Yan, W. Li, G. Zheng, D. Kong, Z.W. Seh, V.K. Narasimhan, Z. Liang, Y. Cui, Energy Environ. Sci. 7 (2014) 3381e3390. [11] W. Zhou, Y. Yu, H. Chen, F.J. DiSalvo, H.D. Abruna, J. Am. Chem. Soc. 135 (2013) 16736e16743. [12] Y.S. Su, A. Manthiram, Nat. Commun. 3 (2012) 542e555. [13] Y.S. Su, A. Manthiram, Chem. Commun. 48 (2012) 8817e8819. [14] S.-H. Chung, A. Manthiram, J. Phys. Chem. Lett. 5 (2014) 1978e1983. [15] G. Zhou, S. Pei, L. Li, D.W. Wang, S. Wang, K. Huang, Adv. Mater. 4 (2014)
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Inhibiting the shuttle effect of LieS battery with a graphene oxide coating separator: Performance improvement and mechanism study Yong Jiang a, Fang Chen a, Yang Gao b, Yanyan Wang a, Shanshan Wang b, Qiang Gao a, Zheng Jiao b, Bing Zhao a, *, Zhiwen Chen b, ** a b
School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China Shanghai Applied Radiation Institute, Shanghai University, Shanghai 201800, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
GO-separator is used as a shuttle inhibitor of the LieS battery. Excellent polysulfide-diffusion inhibiting effect and cycle life are observed. Much less sulfur and polysulfide species are tested in separator and electrolyte. GO film still maintains 3D flexible porous structure with a few insoluble particles. Less sulfate species, lithium salts and polysulfides are detected on the cathode.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2016 Received in revised form 13 December 2016 Accepted 3 January 2017
In this paper, graphene oxide (GO) is integrated on commercial polypropylene separator by tape casting method and sandwiched between a sulfur cathode and the separator as a shuttle inhibitor of the Li-S battery. The issues of lithium polysulfides dissolution and shuttle effect are inhibited distinctly, and significant improvements not only in the active material utilization but also in capacity retention are observed. What's more, the improvement mechanism is studied in detail. The results demonstrate that the sulfur and polysulfide species in separator and electrolyte for the cell with GO-coating separator are much less than that with the pristine separator. The GO membrane still maintains three-dimensional porous and flexible structure with a few lithium polysulfides and Li2S2/Li2S nanoparticles anchored on the surface and inter-layers of GO sheets after long cycles. And the active materials are significantly localized within the cathode structure after GO-coating. In addition, less sulfate species, lithium salts, polysulfides and other insoluble species are identified on the cathode and separator after long-term cycling. © 2017 Elsevier B.V. All rights reserved.
Keywords: Graphene oxide coating separator Lithiumesulfur battery Shuttle effect Oxygen-containing functional group Polysulfide Mechanism study
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Zhao), [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.jpowsour.2017.01.013 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Because of the increasing demand of high specific capacity storage device, lithiumesulfur (LieS) battery system has become the focus of attention of many researchers. Compared to
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lithiumeion battery, LieS battery has a higher theoretical energy density (2650 Wh kg1) [1e3], which is an order of magnitude higher than the currently used lithium-ion battery system. Moreover, sulfur is also abundant, nonetoxic and cheap. Although LieS battery has such great potential, the insulating nature of the sulfur [4], high solubility of polysulfide intermediate species (Li2Sn, 4 n 8) and insolubility of final discharge products (Li2S2/Li2S) significantly affect utilization of the active material, resulting in low discharge/charge efficiency and poor cycling performance of LieS battery. Moreover, the density of S (2.03 g cm3) and its discharge product Li2S (1.66 g cm3) is different within the cathode, the huge volume change (~80%) usually leads to structure destruction, mechanical integrity decrease and rapid capacity decay of the electrode [5e8]. In addition, the soluble polysulfides in the liquid electrolyte may shuttle between the cathode and the anode (shuttle effect) and form Li2S2 or Li2S solid on the lithium anode [9], resulting in low active sulfur utilization, large electrode impedance, low Coulombic efficiency and poor cycle life. To overcome these issues, extensive research efforts have been devoted to develop methods to improve the electrical conductivity of sulfur cathode and maintain/re-utilize the soluble polysulfides within the cathode structure (alleviate the shuttle effect) by wrapping small sulfur molecules in confined micro porous carbon shell, changing the compositions of the electrolytes or by adding carbon-based barrier layers to localize the active material within the cathode region of the cell [10,11]. Of these, adding a carbon-based barrier layer between the sulfur cathode and separator has shown to be a promising approach owning to its simple fabrication process, low cost, no complex composite synthesis process and especially the highly improved rate capability and cycling stability. According to the previous literature, the super P [10], micro porous carbon paper [12], multiwall carbon nanotubes (MWCNTs) [13,14], graphene [15], reduced graphene oxide (rGO) [16,17], were used as the carbon-based barrier layers to prevent the diffusion of polysulfides and enhance the cycling performances of LieS batteries. Among them, Manthiram and co-workers inserted a freestanding MWCNTs [13,14] and micro porous carbon paper [12] interlayer between the sulfur cathode and separator. Morphology variation and elemental mapping of the carbon-based interlayer after cycling were characterized, and it demonstrated that most of the polysulfides were obstructed on the surface of the carboncoating. Chen and Li demonstrated that the bi-functional rGOcoated separator could inhibit the corrosion of lithium plate after 100 cycles [16]. Cui and co-workers introduced a thin metal oxides nanoparticles and carbon black composite coating on the separator, and a dense layer (with thickness of ~500 nm) composed of inactive sulfur and lithium sulfide was accumulated at the separatorcathode interface [10]. All of these previous reports showed that the carbon-based interlayer could reduce the charge transfer resistance of the sulfur cathode, stabilize the polysulfide species within the cathode region of the cell, and thereby improve the electrochemical performances. However, the detailed influence mechanism of carbon-based interlayer on the microstructures and chemical compositions of sulfur cathode during cycling is unclear, nor has it demonstrated specifically the whereabout of active sulfur or the components of sulfur species. Moreover, most of the previous reports that use conductive carbon as the barrier layer are typically using their high surface area and electrical conductivity properties. As an important member of conductive carbon, two-dimensional GO with abundant oxygen-containing groups has not well used to trap sulfur and polysulfides species at all. We believe that exploring functional separator for LieS battery and studying its improving mechanism in-depthly is a worthwhile field of inquiry but that it is only in its infancy.
In this study, GO was integrated on commercial polypropylene separator by tape casting method and sandwiched between a sulfur cathode and the separator as a shuttle inhibitor. In this GO-coating separator, the abundant oxygenecontaining functional groups on the GO membrane can intercept/absorb the diffusing polysulfide intermediates and prevent them diffusing through the separator effectively [16], while the porous conductive skeleton serves as an upper current collector to facilitate electron transport, enhance the utilization of active material, and reserve sulfur species at the separator-cathode interface [18]. Additionally, the commercial polypropylene films function as an electrically insulating membrane and a flexible substrate to support the GO-coating layer on the cathode side [14]. With these inspiring merits, the issues of lithium polysulfides dissolution and shuttle effect are inhibited distinctly, and significant improvements not only in the active material utilization but also in capacity retention are observed. And on this basis, the improvement mechanism is studied in detail. The result shows that the sulfur and polysulfide species in separator and electrolyte for the cell with GO-coating separator are much less than that with the pristine separator. The active materials are significantly localized within the cathode structure after GOcoating. Moreover, the FT-IR, XPS, SEM and EDS analyses further demonstrate that less sulfate species, lithium salts, polysulfides and other insoluble species are identified on the cathode and separator after long-term cycling.
2. Experimental 2.1. Synthesis of GO Firstly, GO sheets were prepared by oxidization of natural graphite powder using the modified Hummers method reported earlier [19,20]. The detailed preparation process was shown in the Supporting information. SEM and TEM images were shown in Fig. S1. The concentration of the GO suspension was about 2.0 mg mL1, which was determined by drying the suspension at 110 C for 24 h and then weighing the dried GO. Then, the GO suspension was used as the raw materials for preparation of graphene aerogel/nanoesulfur composite (GAeS) cathode electrode and polypropylene separator coating film.
2.2. Preparation of GAeS cathode electrode Threeedimensional porous GAeS cathode was prepared via a hydrothermal reductioneassembly of GO with a sulfur-dissolving CS2 and alcohol solution as described in our previous work [21]. In a typical experiment, 50 mL GO suspension and 10 mL ethanol were mixed, and then 5 mL solution of sublimed S/CS2 (30 mg mL1) was added. After stirring for 1 h, the mixture suspension was then sealed in a 100 mL Teflonelined stainless steel autoclave for hydrothermal reaction at 180 C for 12 h. The asprepared hydrogel was washed by ethanol and distilled water several times, cut into small circular pellets with a thickness of about 1.0 mm and diameter of 10 mm, and then freeze-dried overnight. Morphologies of the GAeS hybrids demonstrate that the sample has well-defined and interconnected threedimensional porous network with open pores in the range from submicrometer to several micrometers (Figs. S2a and b). The obtained GAeS circular pellets with mass loading of about 2.0 mg cm2 and porosity of about 29.8% were used as the cathode electrode directly. The active sulfur content in the circular pellet was estimated to be about 1.10e1.25 mg according to the TGA measurement (Fig. S2c).
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2.3. Preparation of GO-coating separator (GO-separator) The GO-coating separator was prepared by surface coating GO slurry on one side of a commercial microeporous polypropylene film Celgard 2400 (~25 mm thick) by tape casting method [18]. Firstly, the as-prepared GO powder (0.20 g) and 0.02 g polyvinylidene fluoride (PVDF) were added into 100 mL of isopropyl alcohol (IPA). After ultrasonication for 30 min and magnetic stirring with a rate of 1000 rpm for 12 h, the GO-based slurry was pasted on the commercial Celgard 2400 separator by tape casting method with blade height of about 50 mm. After heat-treatment at 50 C under vacuum for 12 h, the GO-coated separator was punched into circular disks with diameter of 16 mm. As shown in Figs. S3a and b, the surface of the separator turns from white to brown after the slurry coating, the thickness of GO coating film after drying was about 18.3 mm (Fig. S4d). 2.4. Cell assembly The CR2032 coinetype cells were assembled in high purity argon glove box (Mikrouna, Super 2440/750) with the GAeS, GOcoating separator, pure lithium foil (4 ¼ 15 mm, 1.0 mm thick, Aldrich) as the cathode electrode, separator and anode electrode, respectively. Note that the separator with GO film side faced the GAeS cathode. The electrolyte was 1 mol L1 of LiTFSI in 1,3dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 by volume) with 0.1 mol L1 LiNO3 additive. The as-prepared cell system was denoted as GAeS þ GOeseparator. For comparison, the pristine polypropylene separator Celgard 2400 was used directly without GO coating, and the assembled cell was denoted as GAeS þ separator. 2.5. Electrochemical measurements and mechanism study The electrochemical performances of the as-prepared GAeS þ GOeseparator and GAeS þ separator cells were tested on a LAND CT2001A cell test system under current density range from 100 mA g1 to 1000 mA g1 in the voltage range of 1.5e3.0 V. Cyclic voltammetry (CV) measurements were performed on CH Instrument 660C at a scan rate of 0.5 mV s1. Electrochemical impedance spectroscopy (EIS) was performed by applying a sine wave with amplitude of 5.0 mV in the frequency range from 100 kHz to 0.01 Hz. In order to study the effects of GO-coating separator on microstructure, morphology, polysulfide ions dissolution, chemical compositions of the cathode electrode and its electrochemical improved mechanism, the GAeS þ GOeseparator and GAeS þ separator cells were continuously charged and discharged for 50 cycles. Then the sulfur contents in the cathode, separator and electrolyte were tested by inductively coupled plasma optical emission spectrometry (ICPeOES) with a Thermo Scientific ICAP 6300 Duo View Spectrometer. SEM images of the GAeS cathode electrodes and the separator were taken using fieldeemission scanning electron microscopy (FEeSEM, JEOL JSMe6700F) operated at an accelerating voltage of 5 kV. EDS was used for elemental analysis. The surface electronic states and chemical compositions of GAeS cathode electrodes were investigated by Fourier transform infrared spectroscopy (FTeIR, AVATAR 370) and Xeray photoelectron spectroscopy (XPS, ESCA LAB 250Xi instrument with Al Ka radiation, 1486.7 eV). 3. Results and discussion The schematic of two kinds of LieS battery configurations during charge and discharge process is illustrated in Fig. 1. As for the
Fig. 1. Schematic representation of LieS cells with GOecoating separator (a) and pristine polypropylene separator (b). The GO-coating barrier aims at impeding the free migration of the polysulfides and preventing them from shuttling from sulfur cathode to lithium anode through the separator.
conventional LiS battery configuration (Fig. 1b), highly soluble polysulfides formed during discharge can readily diffuse out of the composite cathode structure, dissolve in the electrolyte and shuttle between the cathode and anode, which would cause an irreversible loss of sulfur continually, lead to low Coulombic efficiency and high impedance, and thus significantly undermine the cycling stability of the cell. However, the flexible and conductive GO layers on the side of GAeS cathode (Fig. 1a) can significantly reduce the shuttle effect of dissolved polysulfides since that there are numerous of oxygen-containing (mainly carboxylic acid, hydroxyl, and epoxide) groups on the surface of GO (Fig. S5), which could bond with polysulfides by moderate binding and act as sulfur reservoir to reutilize the dissolved polysulfide intermediates [10,15,16]. In addition, the GO film can improve the electric conductivity and accommodate the large volumetric expansion of sulfur during lithiation because of its flexible structure and rough surface. Therefore, the polysulfide dissolved in the electrolyte can be reused largely and repeatedly, and thus high sulfur utilization and effective active material reutilization would be accomplished. The cyclic voltammetry (CV) and galvanostatic charge/discharge measurements of the Li-S batteries were evaluated in the voltage range of 1.5e3.0 V. Fig. 2a and b shows 1st, 2nd and 5th CV profiles of GAeS þ separator and GAeS þ GOeseparator at a scan rate of 0.5 mV s1. It can be observed that both samples show two cathodic peaks at around 2.25 and 1.83 V, indicating a multiple-step reduction mechanism of sulfur within lithium [22,23]. The
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Fig. 2. Cyclic voltammograms and galvanostatic charge/discharge profiles of GAeS þ separator (a, c) and GAeS þ GOeseparator (b, d).
reductive peak at ~2.25 V can be ascribed to the transformation of sulfur to soluble long-chain polysulfides (Li2Sx, 4 x 8), and the peak at ~1.83 V corresponds to the further conversion to shortchain polysulfides (Li2Sx, 2 x 4) and following insoluble Li2S2/ Li2S [24,25]. In the first anodic scan process, the distinct oxidation peak at ~2.55 V is caused by the complete oxidation of Li2S/Li2S2 to Li2S8/S8. As the cycle number increased, the cathodic peaks of the cell with pristine separator shift to low voltage and the intensities of all the peaks decrease sharply, revealing the electrochemical instability of this cell configuration (Fig. 2a). However, the anodic peak of the cell with GO-coating separator slightly decreases with cycling, suggesting good capacity retention (Fig. 2b). During the second to fifth circle scan process, the redox peaks are displayed in similar positions with the same intensity, indicating its excellent capacity retention [26]. In addition, the cell with GO-coating separator has a smaller voltage gap between oxidation and reduction peaks than that of the cell with pristine separator. And the oxidation and reduction peaks current (ipc) in GAeS þ GOeseparator cell configuration are the same (±1.25 mA), whereas the values for GAeS þ separator are þ1.1 and 0.7 mA, respectively, further demonstrating that the cell with GO-coating separator is more reversible in lithium storage than that of the pristine ones [16,27,28]. Fig. 2c and d shows the galvanostatic charge-discharge profiles of GAeS þ separator and GAeS þ GOeseparator at the 1st, 2nd, 5th and 50th cycle at the current density of 100 mA g1. One charge plateau and two discharge plateaus are displayed, which are consistent with the redox peaks appeared in the CV curves and is in good agreement with the previous reports [29,30]. The discharge capacities of both samples suffer different degrees of decay upon cycling since that the active sulfur cathode needs to take a few cycles to reach a steady state to form stable electrochemical property because of high solubility of the polysulfide intermediates in the liquid electrolyte [12]. Compared to that of the cell with pristine separator, the cell with GO-coating separator displays higher initial discharge capacity (1550.3 mAh g1, calculating based
on the weight of S) and lower capacity loss after 50 cycles. This significant performance enhancement indicates that the GOseparator could effectively inhibit the diffusion of polysulfide species and thereby eliminate the severe loss of active material/capacity during long-term cycling. The rate charge/discharge curves of GAeS þ separator and GAeS þ GOeseparator are shown in Fig. 3a and b. It can be seen that the plateaus are much flat and stable for GAeS þ GOeseparator, suggesting a kinetically efficient reaction process with a small potential barrier. Whereas the discharge plateaus obviously shift or even disappear in the case of the GAeS þ separator at high current rates, indicating its high polarization and slow redox reaction kinetics with inferior reversibility. The initial discharge capacities of GAeS þ GOeseparator are 1550.3, 1125.6, 910.2 and 751.3 mAh g1 at 100, 200, 500 and 1000 mA g1, respectively. When the current density is restored to 100 mA g1 after cycling under high rate, it can still deliver a reversible capacity of 1007.5 mAh g1 (Fig. 3c). In contrast, the initial discharge capacities of GAeS þ separator are 810.3, 605.7 and 285 mAh g1 at 200, 500 and 1000 mA g1, respectively. EIS measurements of the two configurations before cycling are showed in Fig. 3d. It can be seen that the Nyquist plots consist of a depressed semicircle (in the high-frequency region) and an inclined straight line (in the low-frequency region), which correspond to the charge-transfer resistance (Rct) and the Warburg impedance (related to the Li-ion diffusion within the electrode), respectively [16]. The equivalent circuit model is presented in the inset. The Rct for GAeS þ separator and GAeS þ GOeseparator are 75.1 and 100.5 U, respectively, indicating about 25.3% impedance decreases after the insertion of GO-coating layer. And they both present electrochemical and diffusion mixing control processes since that the entire reaction rate is controlled by the electrochemical reaction rate and ion migration rate at the initial stage. The relatively low Rct for GAeS þ GOeseparator means that the GO-separator can intrinsically increase the contact surface area between GA-S cathode and the separator, reduce the contact resistance and therefore
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Fig. 3. Rate charge/discharge profiles of GAeS þ separator (a) and GAeS þ GOeseparator configurations (b). Comparisons of the rate capability (c), first Nuquist plots (d), cycling performances (e) and the 50th Nuquist plots (f) of the GAeS þ separator and GAeS þ GOeseparator configurations. The inset in (d) is the equivalent circuit model.
reduce charge transfer impendence of the battery [31e36]. After 50 cycles, the specific capacity of GAeS þ GOeseparator sample is maintained at about 835.2 mAh g1, and the capacity loss slows down as the cycling upon. However, the GAeS þ separator sample delivers only 550.7 mAh g1 after 50 cycles with continuous capacity fading (Fig. 3e). Moreover, one additional semicircle is displayed for the GAeS þ separator after 50 cycles (Fig. 3f), which can be attributed to Liþ migration through the passivation film. Combining with the enlarged charge transfer resistance in the electrode, the EIS spectrum suggests dissolution of polysulfides followed by their deposition onto the electrode, demonstrating the continuous cycling increases the charge transfer resistance of the electrode. In strong contrast, the Rct is much low for GAeS þ GOseparator, indicating good electrolyte infiltration, charge transport, and effective entrapment/reutilization of dissolved polysulfide intermediate species of GO-separator. These results demonstrate that cell with GO-coating separator exhibits better electrochemical performance upon cycling compared to that with pristine separator. Therefore, threeedimensional structure damage and chargetransfer resistance of GA-S cathode can be reduced. And the formation of Li2Sx (x ¼ 1 or 2) deposition layer and the surface corrosion of lithium plate anode are expected to be significantly inhibited [16,37].
To further understand the function of GO-separator and reveal the mechanism of improved electrochemical performances, the batteries (in the fully charged state) were disassembled after long cycles in an argon-filled glove box. GA-S electrodes and the separators were removed, washed using 1,3-dioxolane and their morphologies, microstructure and chemical compositions were characterized by SEM, EDS, FT-IR, XPS and ICP-OES. Fig. 4aec shows the sulfur content (total sulfur) tests in the cathode electrode, separator and electrolyte, respectively. As can be seen in Fig. 4a, the sulfur content in GA-S cathode shows a consistent decrease during prolonged cycling. Obviously, the sulfur content in cathode for GAeS þ GOeseparator is noticeably higher than that in the GAeS þ separator sample, which represents steady sulfur retention of about 35.3 wt% after 25 cycles. Whereas, sulfur content in GAeS þ separator sample gradually decreases with the increasing of cycles, only 14.2 wt% sulfur is obtained after 50 cycles. The similar phenomenons are observed for the sulfur content tests in polypropylene film and electrolyte (Fig. 4b and c). Sulfur contents for GAeS þ GOeseparator sample in polypropylene film and electrolyte after 50 cycles become stable at about 6.2 wt% and 9.0 wt%, respectively. Whereas, values for GAeS þ separator sample are more than twice of those for the cell with GO-coating separator and reflect upward trends, which are 13.5 wt% and 21.3 wt%,
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Fig. 4. The sulfur content (total sulfur) tests in the cathode electrode (a), separator (b) and electrolyte (c). Digital images of the separators in two configurations after 50 cycles (d).
respectively. The digital images of separators in two configurations after 50 cycles further demonstrate that most of the sulfur and polysulfides are trapped within the cathode region and on the surface of the GO-coating film, rather than diffusing through or remaining in the polypropylene separator. The results demonstrate that GO-coating film serves as the barrier layer could obstruct and immobilize the migrating polysulfides, prevent them from shuttling from sulfur cathode to lithium anode, as well as effectively inhibiting the shuttle behavior and improving the utilization of the active material. SEM images and EDS spectra of the GAeS cathode electrodes in GAeS þ GOeseparator and GAeS þ separator configurations after 50 cycles were shown in Fig. 5. The fresh GAeS electrode before cycling was used as a comparison. It can be seen that the GAeS hybrid aerogel has well-defined and interconnected threedimensional porous network with open pores in the range from submicrometer to several micrometers. From the EDS analysis, the sulfur content on the GA-S cathode is 68.5 wt%, which basically corresponds to the TGA test (Fig. S2c). As for the cell with GOeseparator, the threeedimensional porous structure of the GAeS cathode has stayed basically the same as the fresh electrode without any dense layer, and no fracture occurs after long-time charge/discharge cycles (Fig. 5b). In contrast, an almost completely damaged structure with distinct cracking and pulverization is found on the GA-S cathode using the pristine separator (Fig. 5c). Additionally, the sulfur content in the cell with pristine separator is much lower (14.2 wt%) than that with GOeseparator (35.3 wt%). These results demonstrate that the GO-separator acts as strong absorbent and barrier could efficiently immobilize the polysulfides and prevent their diffusion from the GA-S cathode to the electrolyte during the charge/discharge process. Thus the polysulfides shuttle phenomenon and the corrosion damage occurred on three-dimensional porous structure can be reduced. It can also be seen that the F element is observed at 0.7 keV in these two cycled samples, and the signal in GAeS þ separator is much
apparent compared to that with GO-coating separator, suggesting more side reaction between lithium polysulfides and fluorinecontaining electrolyte during cycling, which will be further discussed in the FTeIR and XPS. Fig. 6 shows the morphology and elemental analysis of the pristine separator and the GO-coating separator after 50 cycles. As we all known, the fresh separator is a micro porous polypropylene film owning nanopores with diameter of 10e100 nm on the surface (as shown in Fig. S4a). After the GO coating, a dense GO film with thickness of about 18.3 mm was formed on the surface of the separator and covered the nanopores (Figs. S4bed). The high specific area (274.9 m2 g1) of GO paper and abundant oxygencontaining groups should contribute to physically and chemically adsorb/trap the polysulfides. After 50 continuous charge-discharge processes, the GO membrane on the separator still connects with the polypropylene film intimately and remains intact without cracks, indicating its good structural stability and normal function during cycling (Fig. 6c and d). There is no obvious structural change for the GO membrane due to its flexibility and robustness. The cross section of the GO layer shows that it still has a three-dimensional porous structure with a few lithium polysulfides and Li2S2/Li2S nanoparticles anchored on the surface and inter-layers of GO sheets, promoting the thickness increase to 33.9 mm after long cycles (Fig. 6h). There's no obvious layered boundary for the GO layer and the polypropylene Celgard film (Fig. 6g and h), suggesting that the dissolved sulfur species are probably absorbed by the pores between the interlayer rather than forming continuous insulating S/Li2S-inactive layers. This feature is very critical because the agglomeration of active materials would increase the chargetransfer and ion transport resistance and thus induce irreversible capacity loss of the cell [12]. Moreover, the maintaining of porosity structure of the GO paper after long cycles guarantees its superior liquid electrolyte infiltration. The elemental mapping results indicate the sulfur signal is found all over the GO interlayer and distributed homogeneously (Fig. 6g), further demonstrating the
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Fig. 5. The SEM images and the corresponding EDS spectra (unit: keV) of fresh GAeS cathode electrode (a), and the electrodes after 50 cycles in GAeS þ GO-separator (b) and GAeS þ separator (c) configurations.
dynamic re-utilization of the active material. Moreover, the distinct fewer sulfur species signal is detected in the polypropylene Celgard film filtered out by the porous GO-coating, demonstrating its excellent interception and absorption function (Fig. 6f and Table S1 in Supporting Information). Combined with the SEM, EDS of the GA-S cathode electrode and the separator, the results demonstrate that the polysulfide species are adsorbed/trapped by the oxygencontaining groups of GO-coating rather than diffusing to the lithium anode, therefore the polysulfide shuttling behavior is inhibited, material utilization of dissolved sulfur is improved, and three-dimensional structure of sulfur cathode during cycling is maintained efficiently. To further probe these differences, we conduct FTeIR on GAeS cathode electrodes after 50 cycles as presented in Fig. 7. Only a few weak absorption peaks corresponding to CeC/C]C and
oxygenecontaining functional groups (OeH, C]O and CeO) are observed in the fresh GAeS cathode (Fig. 7a). In comparison with the fresh cathode, the absorption peaks are mainly centered in the range of 500e1800 cm1 for the cycled electrodes, which can be ascribed to the lithium polysulfides and their reaction products with electrolyte. As described above in the experimental section, small amount of LiNO3 additive is used in the electrolyte, which has been declared to be able to ease the shuttle effect at some extent [36]. However, its strong oxidation characteristic would oxidize the sulfur element to different valence states. Additionally, the 1,3edioxolane in the electrolyte is the source of H$ radical. Then, parts of the reactions occurred in the electrolyte could be inferred as follows [38e40]: LiN(SO2CF3)2 þ nee þ nLiþ / Li3N þ Li2S2O4 þ LiF þ LiCF3
(1)
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Fig. 6. Morphology the pristine separator (a, b), GO-coating separator (c, d) and the polypropylene Celgard film after stripping GO layer after 50 cycles (e). EDS analysis of pristine separator and GO-coating separator (f). High magnification cross-sectional SEM images and element mapping of the polypropylene Celgard film (g) and the GO layer (h).
LiN(SO2CF3)2 þ 2ee þ 2Liþ / Li2NSO2CF3
(2)
Li2S2O4þ 6ee þ 6Liþ / 2Li2S þ 4Li2O
(3)
LiN(SO2CF3)2 þ nee þ nLiþ / Li3N þ LiOSO2 þ LiCF3
(4)
Fig. 7. FTeIR spectra of fresh GAeS cathode electrode (a), and the electrodes after 50 cycles in GAeS þ GOeseparator (b) and GAeS þ separator (c) configurations.
Based on the above perceptions, it can be seen that the GAeS cathode in GAeS þ separator sample (Fig. 7c) shows Li2O, LieO stretching vibrations at 500e700 cm1, CeF, eSO2 and S]O stretching bands at 1120, 1320 and 1370 cm1, respectively, which can be attributed to the reactions between Liþ and the electrolyte. The SeO and CeS bonds at 700e900 cm1 is due to the reaction of the oxidized products of sulfur with the TFSIe. The SeO, CeS and S]O bonds at 900e1000, 1050 and 1200 cm1 is ascribed to the reactions between NO 3 , S and polysulfides. C]O, eLiCO2 and ROeLi stretching vibrations in the region of 1500e1800 cm1 should be attributed to the reaction of Liþ with the intermediate products. While the eCH, eCH2 bonds at 1400 cm1 is assigned to the DOL in the electrolyte. The spectra also inevitably reflect the absorption peak of eOH bond at 3470 cm1, which is due to the tests being performed in the air. The above results show that the various reactions between S, polysulfide species and electrolyte render many sulfate species, lithium salts, polysulfides and other insoluble species depositing on the surface of electrode, which may hinder the electron and ion transport, increase the reaction resistance and induce significant loss of active material, resulting in poor rate and cycling performances. In contrast, almost all the intensities of vibration peaks for GAeS þ GOeseparator sample are lower than that without GO-coating excepting for the SeO bond at 1050 cm1 (Fig. 7b and c). This is due to that the sulfur molecule and the functional group of graphene aerogel in GAeS cathode are covalently bound by CeOeS bond [21], which is not severely destroyed during charge and discharge. Fewer side reactions for GAeS cathode in GAeS þ GOeseparator sample suggest that the
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Fig. 8. The S 2p and Li 1s spectra of GAeS cathode electrodes after 50 cycles in GAeS þ separator (a, c) and GAeS þ GOeseparator (b, d) configurations.
GO-coating could effectively inhibit the shuttle effect of lithium polysulfides, less reactions between sulfur species and electrolyte occur, and thus fewer insoluble species deposit on the surface of electrode. XPS are further carried to investigate the surface electronic states and chemical compositions of the GAeS cathodes after 50 cycles. The higheresolution S 2p spectra of the electrodes (Fig. 8a and b) demonstrate that peak intensities of the S2 ion (170.4 eV) and S]O group (167.1 eV) in GAeS þ separator, corresponding to insoluble Li2S and reduction of TFSIe, respectively, are much stronger than that in the GAeS þ GOeseparator. Whereas, the peak intensity of the SeS bond at 163.7 eV in GAeS þ separator is much weaker than that in the latter. From the Li 1s spectrum of the cathode in GAeS þ separator (Fig. 8c), it can be seen that the peaks at 52.6, 54.4, 55.2, 56.2 and 59.1 eV correspond to lithium species of Li2S, LiOH, Li2CO3, LiF/Li2O and ROeLi. Whereas, only one Li 1s peak at 55.5 eV corresponded to lithium species of LiF or Li2O is found in the GAeS þ GOeseparator sample (Fig. 8d). Note that very weak Li2S/Li2S2 signal is found in the Li 1s spectrum of the cycled electrode, suggesting that most of Li2S converts reversibly to soluble lithium salt in the electrolyte. Overall, the XPS results indicate that large amount of S8 active material and less insoluble Li2S and inactive sulfate species exist in the GAeS þ GOeseparator system, which agree well with the sulfur content tests by ICP-OES (Fig. 4a), EDS analysis (Fig. 5) and the FTeIR spectra (Fig. 7). All of these results demonstrate that the GO-coating separator can significantly enhance the electrochemical performances of Li-S batteries. Through research of the improvement mechanism, the roles of GO-coating can be simply analyzed as follows [41e43]. Firstly, abundant of oxygen-containing functional groups on GO film can physically and chemically adsorb/trap sulfur and polysulfides to effectively localize the soluble polysulfide species within its porous skeleton and prevent their diffusion to the electrolyte, which could minimize the shuttling effect and repeatedly use the polysulfides even during long
cycles. Secondly, the GO-coating layer has a porous structure with superior liquid electrolyte infiltration, which is beneficial for rapid ions transport and rate performances. Thirdly, the mechanical flexibility of GO film can accommodate the volume expansion and shrinkage of sulfur cathode and keep the integrity of the cathode structure during continuous charging and discharge processes. Finally, the GO-coating layer has high electrical conductivity, which could act as the role of an upper current collector to decrease the internal charge transfer resistance and increase the conducting surface between GA-S cathode and the pristine polypropylene separator. This can effectively prevent the accumulation of inactive polysulfide intermediates at the interface between the cathode and separator, as well as increasing the utilization of active material and inactive polysulfide species.
4. Conclusion In summary, the GO-coating separator is a very simple and effective strategy to substantially inhibit the shuttle effect of polysulfide and improve the electrochemical performances of Li-S battery. Based on the analysis of the microstructure, morphology and chemical composition variations of the sulfur cathode electrode, separator and electrolyte, the improvement mechanism of GO-separator is studied in detail. The results demonstrate that the sulfur and polysulfide species in separator and electrolyte for the cell with GO-coating separator are much less than that with the pristine separator. The GO membrane still maintains threedimensional porous and flexible structure with a few lithium polysulfides and Li2S2/Li2S nanoparticles anchored on the surface and inter-layers of GO sheets after long cycles. And the active materials are significantly localized within the cathode structure after GOcoating. In addition, less sulfate species, lithium salts, polysulfides and other insoluble species are identified on the cathode and separator after long-term cycling.
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Acknowledgement This work was supported by the National Natural Science Foundation of China (21501119, 11575105, 21671130), Science and Technology Commission of Shanghai Municipality (15DZ0501402), Shanghai Municipal Education Commission (Peak Discipline Construction Program, N.13-A302-15-L02), Program for Changjiang Scholars and Innovative Research Team in University (IRT13078). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.013. References , P.G. Bruce, B. Scrosati, J.-M. Tarascon, W.V. Schalkwijk, Nat. Mater. 4 [1] A.S. Arico (2005) 366e367. [2] X.L. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. [3] Q.W. Tang, Z.Q. Shan, L. Wang, X. Qin, K.L. Zhu, J.H. Tian, X.S. Liu, J. Power Sources 246 (2014) 253e259. [4] X. Jia, C. Zhang, J.J. Liu, W. Lv, D.W. Wang, Y. Tao, Z. Li, X. Zheng, Nanoscale 8 (2016) 4447e4451. [5] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [6] A. Manthiram, Y. Fu, Y. Su, Accounts Chem. Res. 46 (2012) 1125e1134. [7] H. Yamin, A. Gorenshtein, J. Penciner, Y. Sternberg, E. Peled, J. Electrochem. Soc. 135 (1988) 1045e1048. [8] B. Scrosati, J. Garche, J. Power Sources 195 (2010) 2419e2430. [9] J. Wang, Y.-S. He, J. Yang, Adv. Mater. 27 (2015) 569e575. [10] H. Yao, K. Yan, W. Li, G. Zheng, D. Kong, Z.W. Seh, V.K. Narasimhan, Z. Liang, Y. Cui, Energy Environ. Sci. 7 (2014) 3381e3390. [11] W. Zhou, Y. Yu, H. Chen, F.J. DiSalvo, H.D. Abruna, J. Am. Chem. Soc. 135 (2013) 16736e16743. [12] Y.S. Su, A. Manthiram, Nat. Commun. 3 (2012) 542e555. [13] Y.S. Su, A. Manthiram, Chem. Commun. 48 (2012) 8817e8819. [14] S.-H. Chung, A. Manthiram, J. Phys. Chem. Lett. 5 (2014) 1978e1983. [15] G. Zhou, S. Pei, L. Li, D.W. Wang, S. Wang, K. Huang, Adv. Mater. 4 (2014)
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