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polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries
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Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries Jiaohui Xu, Bo Jin*, Huan Li, Qing Jiang Key Laboratory of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun, 130022, PR China
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abstract
Article history:
Recently, lithium-sulfur batteries (LSBs) have received extensive attention due to its high
Received 28 March 2017
energy density of 2600 Wh kg1. At the same time, sulfur is earth-abundant, economical
Received in revised form
and non-poisonous. Nevertheless, the poor electrochemical performance restricts its
26 June 2017
commercial application, including the inferior cycling stability caused by the significant
Accepted 28 June 2017
dissolution of lithium polysulfides and the low specific capacity because of the poor elec-
Available online xxx
trical conductivity of sulfur. In this work, we adopt a simple and amicable process to prepare sulfur/alumina/polypyrrole (S/Al2O3/PPy) ternary hybrid material to overcome
Keywords:
these defects. In this strategy, each composition of the ternary hybrid material plays an
Sulfur/alumina/polypyrrole
essential role in cathode: alumina and PPy can provide strong adsorption for the dissolved
Cathode
intermediate polysulfides. Meanwhile, PPy also works as a conductive and flexible additive
Lithium-sulfur batteries
to expedite electron transport, and is coated on the surface of the as-prepared SeAl2O3 composite by in situ chemical polymerization. The sulfur is encapsulated uniformly and perfectively by the two components, which is confirmed by field emission scanning electron microscope. The ternary hybrid material manifests good electrochemical performance as expected, and displays high initial discharge capacity of 1088 mA h g1 and a discharge capacity of 730 mA h g1 after 100 cycles at a current density of 200 mA g1. Besides, S/ Al2O3/PPy also shows good rate capability. The synergy between alumina and PPy is the decisive factor, which gives rise to good electrochemical performance of cathode for highperformance LSBs. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, with the fast development of portable electronic devices such as smart phones, cameras, and notebook computers, the energy storage technologies have been in unprecedented demand [1]. Rechargeable lithium-ion batteries have occupied the main market for a long time due to their advantages such as overlong cycle life, relatively high energy
density as well as environmental friendliness [2,3]. However, it is urgent to find other alternatives with high-density and high-capacity to meet the ever-increasing energy requirements such as electric vehicles and hybrid electric vehicles. In recent years, lithium-sulfur batteries (LSBs) have received extensive attention due to its high energy density of 2600 Wh kg1 and high theoretical specific capacity of 1675 mA h g1 [4e10]. At the same time, sulfur is earthabundant, economical, and non-poisonous [11e13].
* Corresponding author. E-mail address: [email protected] (B. Jin). http://dx.doi.org/10.1016/j.ijhydene.2017.06.205 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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However, despite of these merits, there are still many challenges. LSBs are plagued with three serious defects, thus restricting its commercial application as follows, (i) the intrinsic insulating nature of sulfur (5 1030 S cm1 at 25 C) and LiS2, which leads to poor cycle stability and low utilization of the active material; (ii) the flagrant “shuttle effect” caused by the dissolution of lithium polysulfide intermediates (Li2Sx, 3 x 8) in the electrolyte; (iii) the obvious volume expansion (~80%) during cycle process because the sulfur has a much greater density than LiS2 [14,15]. To overcome the above-mentioned shortcomings, a great variety of methods have been carried out to enhance the cycling stability and rate capacity of LSBs. There are several common approaches applied in researches such as enlarging porousness, creating hollow morphology, nanocrystallization, carbon coating, and the combination of above methods. Over the decades, most attempts have been mainly dedicated on finding host materials. Various conductive carbonaceous nanomaterials (carbon nanotubes [14,16,17], carbon nanofibers [18], micro/mesoporous carbon [19,20], and graphene-based nanomaterials [21]), conductive polymers [22e25], and metal oxides [26e37] have been used to mitigate the “shuttle effect” and the volume expansion, and improve low conductivity. It should be pointed out that the nonpolar conductive carbon materials only show physical confinement of the polysulfide intermediates because the polysulfides are polar materials. However, the polar metallic oxides such as TiO2 [26,27], CeO2 [28], ZnO [29], MnO2 [30,31] and Al2O3 [32,33] can offer stronger chemical reactions with polysulfides, and can effectively minimize “shuttle effect”. Choi et al. fabricated the sulfur electrode decorated with nano-sized Al2O3 particles through the ultra-sonication and mechanical ball milling methods. The sulfur electrode with nano-sized Al2O3 exhibits higher discharge capacity of 660 mA h g1 [34]. The results showed that nano-sized Al2O3 particles could be of benefit to improving discharge capacity, but cycle performance still needs to be improved. He et al. reported TiO2-sulfur-carbon aerogel (TiO2eS-CA) composite electrode with the initial capacity of 858.5 mA h g1 and specific capacity of 639.2 mA h g1 after the 79th cycle at a current density of 120 mA g1 [35]. The cell displays superior electrochemical performance compared to electrode without TiO2, but the cathode delivers relatively lower rate performance. Dong et al. employed micron-sized flaky alumina as an adsorbent for the sulfur cathode materials, and the sulfur-acetylene black-Al2O3 (S-AB-Al2O3) electrode obtains the initial discharge capacity of 1171 mA h g1, and decreases to 585 mA h g1 after 50 cycles at a current density of 0.25 mA cm2 [36]. The composite material confirms that the addition of micron-sized flaky alumina plays an important role in reducing the dissolution of polysulfides. However, the long cycling stability has not been explored. Liu et al. discussed various nanostructured metal oxides/sulfur hosts, and convincingly validated the potential of oxides for LSBs [37]. On the other hand, our research group has indicated that polypyrrole (PPy) can provide multiple beneficial effects on the electrochemical properties of LSBs [38,39]. PPy can not only improve the electrical conductivity but also possess the high ability to absorb polysulfides. However, the researches on S/Al2O3/PPy ternary hybrid material as cathode for highperformance LSBs have never been investigated.
In this contribution, we combine polar Al2O3 and conductive PPy with sublimed sulfur as a cathode material by adopting a facile and environmental process for LSBs. The as-prepared S/Al2O3/PPy ternary hybrid material gives rise to good electrochemical properties including high specific capacity, stable cycling stability, and desirable rate capacity. In this strategy, Al2O3 and PPy can provide strong adsorption for the dissolved intermediate polysulfides. Meanwhile, the conductive PPy also works as a conductive and flexible additive to expedite electron transport, and is coated on the surface of SeAl2O3 composite by in-situ chemical polymerization. The dual layers of Al2O3 and PPy can not only strongly protect from active material loss but also effectively restrict the diffusion of lithium polysulfides to electrolyte. Consequently, it is reasonable to expect that the elective suitable system has splendid performance for LSBs.
Experimental Preparation of S/Al2O3 composite S/Al2O3 composite was prepared by two-step. Firstly, the sublimed sulfur and Al2O3 was mixed uniformly in a weight ratio of 7:3, and the mixture was ball-milled for 12 h with a revolving speed of 150 rpm using a ball grinder. Secondly, the as-prepared mixture was sealed in a 50 mL Teflon-lined stainless-steel autoclave and maintained at 155 C for 12 h to finish melt diffusion process of sulfur. After cooling down to ambient temperature naturally, the resulting S/ Al2O3 composite was collected after grinding in an agate mortar.
Preparation of S/Al2O3/PPy composite PPy was coated on S/Al2O3 composite by in situ chemical polymerization of pyrrole using FeCl3 as the oxidant. The asobtained S/Al2O3 composite was dispersed into 450 mL aqueous solution by sonication at room temperature for 30 min. 2.62 g sodium p-toluenesulfonate was added into the above aqueous suspension and stirred. Then 0.03 g purified pyrrole monomer was added dropwise, and constantly stirred in ice-water bath at 0e5 C. After that, 30 mL of 0.45 mol/L FeCl3 aqueous solution was added by dropping and stirred for 12 h at the same temperature. Finally, the resulting black aqueous suspension was centrifuged and washed several times with deionized water and alcohol. The resulting product was dried under vacuum oven at 60 C for 12 h. Moreover, the calculations show that the final composite contains 63 wt% sulfur, 27 wt% Al2O3, and 10 wt% PPy.
Preparation of Li2S6 solution for UV/Vis measurement In order to measure UV/Vis adsorption spectroscopy, Li2S6 solution was synthesized. Sulfur and Li2S with a mole ratio of 5:1 were added to a mixed solution of 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) (1:1, by volume). 0.05 M Li2S6 in DOL/DME solution was obtained after stirring at 90 C under argon atmosphere overnight.
Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Materials characterization Fourier transform infrared (FTIR) spectrum was performed with a Bruker TENSOR 27 spectrometer. X-ray diffraction (XRD) patterns were obtained on a Rigaku diffractometer with Cu Ka radiation (l ¼ 1.5406 A). In order to measure the sulfur content in samples, thermal gravimetric (TG) analysis was carried out by a Pyris Diamond TG (PerkinElmer Inc., America) under nitrogen atmosphere from 50 to 800 C at a heating rate of 10 C min1. Field emission scanning electron microscope (FESEM, JSM-6700F), transmission electron microscope (TEM, TECNAI-F20, FEI Inc., America) and a high-resolution TEM (HRTEM, TECNAI-F20, America) were used to observe the morphology and structure of the as-prepared samples. In order to evaluate the adsorption ability of PPy and Al2O3 for lithium polysulfides, UV/Vis adsorption spectrum analysis was carried out by UV/Vis spectrometer (Shimadzu UV-3000).
Electrochemical characterization To investigate the electrochemical properties of the composite, the working electrode was composed of 70 wt% active material, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride, which were dissolved in N-methyl-2-pyrrolidone to form uniform slurry. The slurry was casted onto a Al foil as a current collector and dried in a vacuum oven at 60 C for 12 h, and the amount of slurry is about 0.8 mg cm2. LIR2025 coin cells were assembled in an Ar-filled glove box with the lithium metal as the counter electrode as well as the reference electrode, and the polypropylene membrane as the separator. The electrolyte was 1 M lithium bis(triuoromethanesulfone)imide dissolved in a solvent mixture of DME and DOL (50:50, v/v) with 0.1 M LiNO3 as an additive. For comparison, the pristine sulfur and SeAl2O3 cathode electrodes were also prepared under the same condition. The electrochemical performance of the cells was investigated by a battery test system (LAND CT2001A) at a voltage range of 1.5e3.0 V. Cyclic voltammograms (CV) were tested using a CHI650D electrochemical workstation between 1.5 and 3 V with a scan rate of 0.1 mV s1 at ambient temperature. Electrochemical impedance spectra (EIS) measurement was operated on CHI660E electrochemical workstation (Shanghai Chenhua Instruments Ltd., P.R. China), and carried out in a frequency range of 100 kHze0.01 Hz with an amplitude of 5 mV.
Results and discussion Fourier transform infrared (FTIR) spectroscopy was applied to determine the presence of PPy in S/Al2O3/PPy composite. As shown in Fig. 1, it is remarkable that S/Al2O3/PPy displays the absorption spectra of both PPy and Al2O3. The spectrum of PPy has five characteristic peaks at 1544, 1460, 1301, 1167, and 1038 cm1, which is in accordance with the results reported in literature [40]. Peaks at 1544 and 1460 cm1 stem from the pyrrole ring fundamental vibrations, and the peak at 1167 cm1 results from CeN stretching vibration. Moreover, the peaks at 1301 and 1038 cm1 are ascribed to ]CeH inplane vibrations, respectively [23], indicating the molecular structure of PPy as inset in Fig. 1. Different from FTIR spectrum
Fig. 1 e FTIR spectra of S, PPy, Al2O3, and S/Al2O3/PPy. Inset: the molecular picture of PPy. of PPy, the characteristic peak intensities of S/Al2O3/PPy composite weaken. Consequently, a conclusion may be draw from this evidence that we have successfully synthesized PPy by experimentation. XRD patterns of S, Al2O3, PPy, S/Al2O3 and S/Al2O3/PPy are displayed in Fig. 2. As shown in Fig. 2, the diffraction peaks of sulfur and alumina can be well-indexed to rhombic sulfur (JCPDS No. 08-0247) and alpha-alumina (JCPDS No. 11-0661), respectively, disclosing the typical orthorhombic system of sulfur structure and trigonal system of alumina structure. The peaks of both sulfur and alumina are sharp and strong, manifesting that they are highly crystallized. However, the as-prepared PPy presents broad peak at around 25 , which is described to the amorphous structure. XRD patterns of S/ Al2O3 and S/Al2O3/PPy embody all the characteristic peaks of sulfur and alpha-alumina, suggesting that the addition of PPy does not change the crystal structure of sulfur and alumina. Conversely, the peak of PPy cannot be clearly observed, it is because of the amorphous structure, the trace amount, and high dispersion of PPy in the ternary composite. It is also observed that XRD peak intensity of S/Al2O3/PPy is lower than that of S/Al2O3, which could be caused by the well dispersion of sulfur and alumina in the ternary composite. The sulfur contents of the as-prepared composites were confirmed by thermal gravimetric (TG) analysis. TG analysis testing of pure sulfur, Al2O3, PPy, S/Al2O3 and S/Al2O3/PPy manifested in Fig. 3 was performed under nitrogen atmosphere from 50 to 800 C. Apparently, the mass loss of elemental sulfur begins at 150 C and ends at 300 C. The weight change of the two as-prepared composites has same tendency as that of the pristine sulfur at 150e300 C, indicating that the weight loss of the binary and ternary composites is assigned to the evaporation of sulfur within this range of temperature. Because the mass of alumina remains unchanged during the operating range (50e800 C), the weight loss of S/Al2O3 is equal to sulfur content of S/Al2O3, and determined to be 70 wt%. In addition, according to the curve of S/Al2O3/PPy, the sulfur loading is approximately 63 wt%, which well coincides with the percentage of the added
Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 2 e XRD patterns of S, Al2O3, PPy, S/Al2O3, and S/Al2O3/PPy.
Fig. 3 e TG curves of S, Al2O3, PPy, S/Al2O3, and S/Al2O3/ PPy. content. The further mass loss after 300 C might be related to the loss of PPy. The morphology and microstructure of pristine sulfur, S/ Al2O3 and S/Al2O3/PPy were observed by field emission scanning electron microscope (FESEM), and the results are shown in Fig. 4. Fig. 4a indicates that sublimed sulfur shows anomalous particle morphology with inhomogeneous particle size distribution of about 0.1e2 mm. Fig. 4b displays that alumina has homogeneous spherical-like morphology, and the length and breadth are roughly 300 and 100 nm, respectively. As shown in Fig. 4c, alumina is uniformly coated on the surface of sulfur with comparatively flat surface. Obviously, Fig. 4d demonstrates that the surface of S/Al2O3/PPy is rough, suggesting that the hybrid material is successfully decorated with
PPy. The covering of PPy is beneficial to enhancing the conductivity and lithium-ion transmission of the composite. Transmission electron microscopy (TEM) and high angle annular dark field scanning TEM (STEM) images (Fig. 5) are also displayed to further discern the microstructure of these samples. The morphology of sulfur and Al2O3 consists with FESEM results (Fig. 4). As presented in the insets of Fig. 5aeb, the interplanar spacing of 0.333 nm is in line with the (311) facet of sulfur, and the interplanar spacing of 0.351 nm corresponds to the (012) plane of Al2O3. Fig. 5d clearly reveals that PPy is evenly adhered to the surface of sulfur and alumina particles. HRTEM image of S/Al2O3/PPy shows lattice fringes of sulfur as well as Al2O3, as observed in Fig. 5e, the interlayer spacings of 0.208 and 0.333 nm correspond to the (113) and (311) lattice planes of Al2O3 and sulfur phase, respectively. The elemental mappings of sulfur, nitrogen and aluminum further confirm the existence of PPy and uniform distribution of these elements. The electrochemical performance of S, S/Al2O3 and S/ Al2O3/PPy was obtained by the measurement of cyclic voltammograms and constant-current charge/discharge testing. In order to comprehend the redox reaction, CV behavior of S, S/Al2O3, and S/Al2O3/PPy for the initial three cycles was measured at a scan rate of 0.1 mV s1 within a potential window of 1.5e3.0 V. As displayed in Fig. 6, all the samples show two characteristic cathodic peaks and one anodic peak. The two typical cathodic peaks are associated with two reduction steps of sulfur: the former located at about 2.3 V can be ascribed to the reduction of elemental sulfur to highordered lithium polysulfides (Li2Sx, 4 < x < 8), and the latter at about 2.0 V results from further reduction of high-ordered lithium polysulfides to low-ordered lithium polysulfides (LiS2 or Li2S2). During the reverse anodic scan, the only oxidation peak located at around 2.5 V can be attributed to the conversion of LiS2/Li2S2 into lithium polysulfides, and finally to
Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 4 e FESEM images of (a) pristine sulfur, (b) Al2O3, (c) S/Al2O3, and (d) S/Al2O3/PPy.
Fig. 5 e TEM images of (a) pristine sulfur (the insert is the corresponding HRTEM image), (b) Al2O3 (the insert is the corresponding HRTEM image), (c) S/Al2O3, and (d) S/Al2O3/PPy. (e) HRTEM image of S/Al2O3/PPy. (f) High angle annular dark field STEM image of S/Al2O3/PPy, and EDX elemental mappings of (g) S, (h) Al, and (i) N. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 6 e CV profiles of (a) S, (b) S/Al2O3, and (c) S/Al2O3/PPy for the first three cycles at a scan rate of 0.1 mV s¡1. elemental sulfur [41,42]. Fig. 6b shows CV profile of the binary composite. We find that the reduction peak is much broader in the first cycle. The reason may be that the electrical insulation of alumina on the surface could impede active sulfur to contact with conductive additive (acetylene black), leading to poor electrical contact and electron transfer, which could cause
lower redox kinetics and serious polarization. However, it is interesting that the broader peak disappears after the initial cycle due to the realignment of sulfur [43]. Throughout all the curves in Fig. 6, it is clear that the cathodic peaks in the first sweep have deviation compared to that of the second and the third sweep, which is mainly caused by the incomplete
Fig. 7 e Charge/discharge curves of (a) S, (b) S/Al2O3, and (c) S/Al2O3/PPy for selected cycles at a current density of 200 mA g¡1. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 8 e (a) Cycling performance of S, S/Al2O3, and S/Al2O3/ PPy at a current density of 200 mA g¡1. (b) Cycling performance of S/Al2O3/PPy at a current density of 1 C (1 C ¼ 1675 mA g¡1). activation of the cathode and the inferior sulfur utilization. In comparison with S/Al2O3, S/Al2O3/PPy displays higher reduction peaks (2.19 vs. 2.22 V) and lower oxidation peaks (2.51 vs. 2.48 V), and possesses higher and sharper current peaks, manifesting that the ternary composite has higher reaction kinetics and faster electron/ion transport. Moreover, starting
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from the second cycle, both the positions and intensities of the reduction and oxidation peaks vary less than those in the cases of S and S/Al2O3, which demonstrates the preferable reversibility of the ternary composite. The charge/discharge profiles of the three electrodes for the 1st, 10th, 50th, and 100th cycles at a current density of 200 mA g1 are disclosed in Fig. 7. As present in Fig. 7, all these three electrodes show two discharge plateaus and one charge plateau, which is in accordance with CV curves (Fig. 6). However, compared with S and S/Al2O3, S/Al2O3/PPy reveals larger discharge capacity and lower capacity degradation, manifesting that the ternary composite is more profitable for utilization of active material. Moreover, the gap potential between the charge and principal discharge plateaus is narrow, and the charge and principal discharge plateaus always keep well parallelism during cycling, suggesting that the system possesses smaller polarization and higher reaction kinetics. This is because that sulfur is effectively confined to alumina and PPy, which provides double protection to restrict the diffusion of intermediate polysulfides to the electrolyte. Fig. 8a contrasts cycling performance of S, S/Al2O3, and S/ Al2O3/PPy at a current density of 200 mA g1. The discharge capacity of S/Al2O3 and S is 988 and 738 mA h g1 in the first cycle, respectively, and only retains 482 and 147 mA h g1 after 100 cycles, respectively. Evidently, S/Al2O3/PPy composite cathode delivers more outstanding initial discharge capacity of 1088 mA h g1 and maintains 730 mA h g1 after 100 cycles. Even at high current density of 1 C (1 C ¼ 1675 mA g1), the ternary hybrid material has an initial capacity of 683 mA h g1 while maintaining 565 mA h g1 after 100 cycles with 82.7% capacity retention (Fig. 8b). These results suggest that S/Al2O3/ PPy cathode delivers the best electrochemical performance compared to S and S/Al2O3. This can be assigned to the cooperative effect between alumina and PPy, enhancing the utilization of sulfur and suppressing the diffusion of lithium polysulfides to the electrolyte during the cycling process. Furthermore, it is necessary to mention that the moderate
Fig. 9 e Rate performance of S, S/Al2O3, and S/Al2O3/PPy at various current densities. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 10 e Electrochemical impedance spectra of S, S/Al2O3, and S/Al2O3/PPy before cycling. addition of LiNO3 in the electrolyte also plays a significant role in the electrochemical performance through promoting to form a stable passivation film. With the consumption of LiNO3, the growth of passivation film is infinite, and the formed passivation film is stable. Sequentially, the formation of stable passivation film effectively restrains the dissolution of polysulfides on Li anode, and improves the electrochemical performance of LSBs [44]. Rate capacity of S, S/Al2O3 and S/Al2O3/PPy is presented in Fig. 9. The three cathodes were cycled at different current densities of 150, 400, 800, 1600, 3000, and 8000 mA g1. It is clearly seen that S/Al2O3/PPy reveals the initial capacity of 1287 mA h g1 at 150 mA g1, the subsequent specific discharge capacities are 1189, 900, 588, 383, and 146 mA h g1, respectively. On the contrary, S/Al2O3 and S exhibit inferior initial discharge capacities of 980 and 826 mA h g1, and bad rate capacity under the same situations. In addition, when the current density is recovered to 150 mA g1, the discharge capacity of 1001 mA h g1 is recovered, suggesting good electrochemical reversibility of the ternary composite. This good rate performance further confirms that the introduction of PPy enhances the redox kinetics and utilization of sulfur, and accelerates the ionic diffusion. In order to further comprehend the charge transfer resistance of three samples, EIS measurements were employed on the freshly assembled LSBs. As revealed in Fig. 10, all the three cells possess oblate semicircle in the high frequency rage and approximately straight line in the low frequency rage. The former results from the charge-transfer resistance, and the latter can be ascribed to Warburg impedance [45e47]. It is noticeable that the ternary hybrid composite possesses the smallest resistance compared to pure sulfur and S/Al2O3 electrodes, which is in agreement with the electrochemical performance (Fig. 8). This can be put down to the good electrical conductivity of PPy in S/Al2O3/PPy composite. We further performed UV/Vis adsorption spectrum analysis to compare the adsorption ability of S/Al2O3 binary composite and S/Al2O3/PPy ternary composite. The band at 265 nm corresponds to S2 6 , reflecting the existence of polysulfides. As shown in Fig. 11, S/Al2O3/PPy þ Li2S6 in DME/DOL solution
Fig. 11 e UVeVis adsorption spectra of Li2S6 in DME/DOL, S/ Al2O3 þ Li2S6 in DME/DOL, and S/Al2O3/PPy þ Li2S6 in DME/ DOL. Inset: digital pictures of polysulfides adsorption in sealed vials by adding different samples, (a) Li2S6 in DOL/ DME after standing for 2 h, Li2S6 in DOL/DME after being in contact with (b) S/Al2O3 and (c) S/Al2O3/PPy for 2 h.
shows the lowest peak intensity compared to other two samples, which is primarily ascribed to the cooperative adsorption capacity of Al2O3 and PPy. What's more, the inset also shows the polysulfides adsorption capacity of different samples, and the vial containing S/Al2O3/PPy þ Li2S6 in DME/DOL solution displays the lightest colour, indicating the improved polysulfides adsorption of Al2O3 and PPy due to the chemical and physical adsorption.
Conclusions In summary, we have successfully fabricated S/Al2O3/PPy ternary hybrid material as cathode for LSBs by utilizing a simple, low-budget, and environmental method, manifesting more good electrochemical performance compared to the counterparts of S and S/Al2O3. S/Al2O3/PPy composite cathode delivers an initial discharge capacity of 1088 mA h g1 at a current density of 200 mA g1, and still maintains 730 mA h g1 after 100 cycles. Furthermore, S/Al2O3/PPy shows good rate capability. In this strategy, Al2O3 and PPy can provide strong adsorption for the dissolved intermediate polysulfides. Meanwhile, the conductive PPy also works as a conductive and flexible additive to expedite electron transport, and is coated on the surface of SeAl2O3 composite by insitu chemical polymerization. The sulfur is effectively confined to alumina and PPy, which provides double protection to restrict the diffusion of intermediate polysulfides to the electrolyte. The synergy between alumina and PPy is the decisive factor, which gives rise to good electrochemical performance of cathode for high-performance LSBs. Nevertheless, there may be other exact specifics, which probably exert an influence on the electrochemical performance of LSBs. For example, the effect of PPy content in the ternary composite on the electrochemical properties of LSBs should be further investigated.
Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Acknowledgements [15]
This work was supported by the National Natural Science Foundation of China (Nos. 51631004 and 21673095), the Project of Science and Technology Development Plan of Jilin Province (No. 20170414010GH), and the Special Fund for Industrial Innovation in Jilin Province (No. 2016C039).
references
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Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries Jiaohui Xu, Bo Jin*, Huan Li, Qing Jiang Key Laboratory of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun, 130022, PR China
article info
abstract
Article history:
Recently, lithium-sulfur batteries (LSBs) have received extensive attention due to its high
Received 28 March 2017
energy density of 2600 Wh kg1. At the same time, sulfur is earth-abundant, economical
Received in revised form
and non-poisonous. Nevertheless, the poor electrochemical performance restricts its
26 June 2017
commercial application, including the inferior cycling stability caused by the significant
Accepted 28 June 2017
dissolution of lithium polysulfides and the low specific capacity because of the poor elec-
Available online xxx
trical conductivity of sulfur. In this work, we adopt a simple and amicable process to prepare sulfur/alumina/polypyrrole (S/Al2O3/PPy) ternary hybrid material to overcome
Keywords:
these defects. In this strategy, each composition of the ternary hybrid material plays an
Sulfur/alumina/polypyrrole
essential role in cathode: alumina and PPy can provide strong adsorption for the dissolved
Cathode
intermediate polysulfides. Meanwhile, PPy also works as a conductive and flexible additive
Lithium-sulfur batteries
to expedite electron transport, and is coated on the surface of the as-prepared SeAl2O3 composite by in situ chemical polymerization. The sulfur is encapsulated uniformly and perfectively by the two components, which is confirmed by field emission scanning electron microscope. The ternary hybrid material manifests good electrochemical performance as expected, and displays high initial discharge capacity of 1088 mA h g1 and a discharge capacity of 730 mA h g1 after 100 cycles at a current density of 200 mA g1. Besides, S/ Al2O3/PPy also shows good rate capability. The synergy between alumina and PPy is the decisive factor, which gives rise to good electrochemical performance of cathode for highperformance LSBs. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, with the fast development of portable electronic devices such as smart phones, cameras, and notebook computers, the energy storage technologies have been in unprecedented demand [1]. Rechargeable lithium-ion batteries have occupied the main market for a long time due to their advantages such as overlong cycle life, relatively high energy
density as well as environmental friendliness [2,3]. However, it is urgent to find other alternatives with high-density and high-capacity to meet the ever-increasing energy requirements such as electric vehicles and hybrid electric vehicles. In recent years, lithium-sulfur batteries (LSBs) have received extensive attention due to its high energy density of 2600 Wh kg1 and high theoretical specific capacity of 1675 mA h g1 [4e10]. At the same time, sulfur is earthabundant, economical, and non-poisonous [11e13].
* Corresponding author. E-mail address: [email protected] (B. Jin). http://dx.doi.org/10.1016/j.ijhydene.2017.06.205 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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However, despite of these merits, there are still many challenges. LSBs are plagued with three serious defects, thus restricting its commercial application as follows, (i) the intrinsic insulating nature of sulfur (5 1030 S cm1 at 25 C) and LiS2, which leads to poor cycle stability and low utilization of the active material; (ii) the flagrant “shuttle effect” caused by the dissolution of lithium polysulfide intermediates (Li2Sx, 3 x 8) in the electrolyte; (iii) the obvious volume expansion (~80%) during cycle process because the sulfur has a much greater density than LiS2 [14,15]. To overcome the above-mentioned shortcomings, a great variety of methods have been carried out to enhance the cycling stability and rate capacity of LSBs. There are several common approaches applied in researches such as enlarging porousness, creating hollow morphology, nanocrystallization, carbon coating, and the combination of above methods. Over the decades, most attempts have been mainly dedicated on finding host materials. Various conductive carbonaceous nanomaterials (carbon nanotubes [14,16,17], carbon nanofibers [18], micro/mesoporous carbon [19,20], and graphene-based nanomaterials [21]), conductive polymers [22e25], and metal oxides [26e37] have been used to mitigate the “shuttle effect” and the volume expansion, and improve low conductivity. It should be pointed out that the nonpolar conductive carbon materials only show physical confinement of the polysulfide intermediates because the polysulfides are polar materials. However, the polar metallic oxides such as TiO2 [26,27], CeO2 [28], ZnO [29], MnO2 [30,31] and Al2O3 [32,33] can offer stronger chemical reactions with polysulfides, and can effectively minimize “shuttle effect”. Choi et al. fabricated the sulfur electrode decorated with nano-sized Al2O3 particles through the ultra-sonication and mechanical ball milling methods. The sulfur electrode with nano-sized Al2O3 exhibits higher discharge capacity of 660 mA h g1 [34]. The results showed that nano-sized Al2O3 particles could be of benefit to improving discharge capacity, but cycle performance still needs to be improved. He et al. reported TiO2-sulfur-carbon aerogel (TiO2eS-CA) composite electrode with the initial capacity of 858.5 mA h g1 and specific capacity of 639.2 mA h g1 after the 79th cycle at a current density of 120 mA g1 [35]. The cell displays superior electrochemical performance compared to electrode without TiO2, but the cathode delivers relatively lower rate performance. Dong et al. employed micron-sized flaky alumina as an adsorbent for the sulfur cathode materials, and the sulfur-acetylene black-Al2O3 (S-AB-Al2O3) electrode obtains the initial discharge capacity of 1171 mA h g1, and decreases to 585 mA h g1 after 50 cycles at a current density of 0.25 mA cm2 [36]. The composite material confirms that the addition of micron-sized flaky alumina plays an important role in reducing the dissolution of polysulfides. However, the long cycling stability has not been explored. Liu et al. discussed various nanostructured metal oxides/sulfur hosts, and convincingly validated the potential of oxides for LSBs [37]. On the other hand, our research group has indicated that polypyrrole (PPy) can provide multiple beneficial effects on the electrochemical properties of LSBs [38,39]. PPy can not only improve the electrical conductivity but also possess the high ability to absorb polysulfides. However, the researches on S/Al2O3/PPy ternary hybrid material as cathode for highperformance LSBs have never been investigated.
In this contribution, we combine polar Al2O3 and conductive PPy with sublimed sulfur as a cathode material by adopting a facile and environmental process for LSBs. The as-prepared S/Al2O3/PPy ternary hybrid material gives rise to good electrochemical properties including high specific capacity, stable cycling stability, and desirable rate capacity. In this strategy, Al2O3 and PPy can provide strong adsorption for the dissolved intermediate polysulfides. Meanwhile, the conductive PPy also works as a conductive and flexible additive to expedite electron transport, and is coated on the surface of SeAl2O3 composite by in-situ chemical polymerization. The dual layers of Al2O3 and PPy can not only strongly protect from active material loss but also effectively restrict the diffusion of lithium polysulfides to electrolyte. Consequently, it is reasonable to expect that the elective suitable system has splendid performance for LSBs.
Experimental Preparation of S/Al2O3 composite S/Al2O3 composite was prepared by two-step. Firstly, the sublimed sulfur and Al2O3 was mixed uniformly in a weight ratio of 7:3, and the mixture was ball-milled for 12 h with a revolving speed of 150 rpm using a ball grinder. Secondly, the as-prepared mixture was sealed in a 50 mL Teflon-lined stainless-steel autoclave and maintained at 155 C for 12 h to finish melt diffusion process of sulfur. After cooling down to ambient temperature naturally, the resulting S/ Al2O3 composite was collected after grinding in an agate mortar.
Preparation of S/Al2O3/PPy composite PPy was coated on S/Al2O3 composite by in situ chemical polymerization of pyrrole using FeCl3 as the oxidant. The asobtained S/Al2O3 composite was dispersed into 450 mL aqueous solution by sonication at room temperature for 30 min. 2.62 g sodium p-toluenesulfonate was added into the above aqueous suspension and stirred. Then 0.03 g purified pyrrole monomer was added dropwise, and constantly stirred in ice-water bath at 0e5 C. After that, 30 mL of 0.45 mol/L FeCl3 aqueous solution was added by dropping and stirred for 12 h at the same temperature. Finally, the resulting black aqueous suspension was centrifuged and washed several times with deionized water and alcohol. The resulting product was dried under vacuum oven at 60 C for 12 h. Moreover, the calculations show that the final composite contains 63 wt% sulfur, 27 wt% Al2O3, and 10 wt% PPy.
Preparation of Li2S6 solution for UV/Vis measurement In order to measure UV/Vis adsorption spectroscopy, Li2S6 solution was synthesized. Sulfur and Li2S with a mole ratio of 5:1 were added to a mixed solution of 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) (1:1, by volume). 0.05 M Li2S6 in DOL/DME solution was obtained after stirring at 90 C under argon atmosphere overnight.
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Materials characterization Fourier transform infrared (FTIR) spectrum was performed with a Bruker TENSOR 27 spectrometer. X-ray diffraction (XRD) patterns were obtained on a Rigaku diffractometer with Cu Ka radiation (l ¼ 1.5406 A). In order to measure the sulfur content in samples, thermal gravimetric (TG) analysis was carried out by a Pyris Diamond TG (PerkinElmer Inc., America) under nitrogen atmosphere from 50 to 800 C at a heating rate of 10 C min1. Field emission scanning electron microscope (FESEM, JSM-6700F), transmission electron microscope (TEM, TECNAI-F20, FEI Inc., America) and a high-resolution TEM (HRTEM, TECNAI-F20, America) were used to observe the morphology and structure of the as-prepared samples. In order to evaluate the adsorption ability of PPy and Al2O3 for lithium polysulfides, UV/Vis adsorption spectrum analysis was carried out by UV/Vis spectrometer (Shimadzu UV-3000).
Electrochemical characterization To investigate the electrochemical properties of the composite, the working electrode was composed of 70 wt% active material, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride, which were dissolved in N-methyl-2-pyrrolidone to form uniform slurry. The slurry was casted onto a Al foil as a current collector and dried in a vacuum oven at 60 C for 12 h, and the amount of slurry is about 0.8 mg cm2. LIR2025 coin cells were assembled in an Ar-filled glove box with the lithium metal as the counter electrode as well as the reference electrode, and the polypropylene membrane as the separator. The electrolyte was 1 M lithium bis(triuoromethanesulfone)imide dissolved in a solvent mixture of DME and DOL (50:50, v/v) with 0.1 M LiNO3 as an additive. For comparison, the pristine sulfur and SeAl2O3 cathode electrodes were also prepared under the same condition. The electrochemical performance of the cells was investigated by a battery test system (LAND CT2001A) at a voltage range of 1.5e3.0 V. Cyclic voltammograms (CV) were tested using a CHI650D electrochemical workstation between 1.5 and 3 V with a scan rate of 0.1 mV s1 at ambient temperature. Electrochemical impedance spectra (EIS) measurement was operated on CHI660E electrochemical workstation (Shanghai Chenhua Instruments Ltd., P.R. China), and carried out in a frequency range of 100 kHze0.01 Hz with an amplitude of 5 mV.
Results and discussion Fourier transform infrared (FTIR) spectroscopy was applied to determine the presence of PPy in S/Al2O3/PPy composite. As shown in Fig. 1, it is remarkable that S/Al2O3/PPy displays the absorption spectra of both PPy and Al2O3. The spectrum of PPy has five characteristic peaks at 1544, 1460, 1301, 1167, and 1038 cm1, which is in accordance with the results reported in literature [40]. Peaks at 1544 and 1460 cm1 stem from the pyrrole ring fundamental vibrations, and the peak at 1167 cm1 results from CeN stretching vibration. Moreover, the peaks at 1301 and 1038 cm1 are ascribed to ]CeH inplane vibrations, respectively [23], indicating the molecular structure of PPy as inset in Fig. 1. Different from FTIR spectrum
Fig. 1 e FTIR spectra of S, PPy, Al2O3, and S/Al2O3/PPy. Inset: the molecular picture of PPy. of PPy, the characteristic peak intensities of S/Al2O3/PPy composite weaken. Consequently, a conclusion may be draw from this evidence that we have successfully synthesized PPy by experimentation. XRD patterns of S, Al2O3, PPy, S/Al2O3 and S/Al2O3/PPy are displayed in Fig. 2. As shown in Fig. 2, the diffraction peaks of sulfur and alumina can be well-indexed to rhombic sulfur (JCPDS No. 08-0247) and alpha-alumina (JCPDS No. 11-0661), respectively, disclosing the typical orthorhombic system of sulfur structure and trigonal system of alumina structure. The peaks of both sulfur and alumina are sharp and strong, manifesting that they are highly crystallized. However, the as-prepared PPy presents broad peak at around 25 , which is described to the amorphous structure. XRD patterns of S/ Al2O3 and S/Al2O3/PPy embody all the characteristic peaks of sulfur and alpha-alumina, suggesting that the addition of PPy does not change the crystal structure of sulfur and alumina. Conversely, the peak of PPy cannot be clearly observed, it is because of the amorphous structure, the trace amount, and high dispersion of PPy in the ternary composite. It is also observed that XRD peak intensity of S/Al2O3/PPy is lower than that of S/Al2O3, which could be caused by the well dispersion of sulfur and alumina in the ternary composite. The sulfur contents of the as-prepared composites were confirmed by thermal gravimetric (TG) analysis. TG analysis testing of pure sulfur, Al2O3, PPy, S/Al2O3 and S/Al2O3/PPy manifested in Fig. 3 was performed under nitrogen atmosphere from 50 to 800 C. Apparently, the mass loss of elemental sulfur begins at 150 C and ends at 300 C. The weight change of the two as-prepared composites has same tendency as that of the pristine sulfur at 150e300 C, indicating that the weight loss of the binary and ternary composites is assigned to the evaporation of sulfur within this range of temperature. Because the mass of alumina remains unchanged during the operating range (50e800 C), the weight loss of S/Al2O3 is equal to sulfur content of S/Al2O3, and determined to be 70 wt%. In addition, according to the curve of S/Al2O3/PPy, the sulfur loading is approximately 63 wt%, which well coincides with the percentage of the added
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Fig. 2 e XRD patterns of S, Al2O3, PPy, S/Al2O3, and S/Al2O3/PPy.
Fig. 3 e TG curves of S, Al2O3, PPy, S/Al2O3, and S/Al2O3/ PPy. content. The further mass loss after 300 C might be related to the loss of PPy. The morphology and microstructure of pristine sulfur, S/ Al2O3 and S/Al2O3/PPy were observed by field emission scanning electron microscope (FESEM), and the results are shown in Fig. 4. Fig. 4a indicates that sublimed sulfur shows anomalous particle morphology with inhomogeneous particle size distribution of about 0.1e2 mm. Fig. 4b displays that alumina has homogeneous spherical-like morphology, and the length and breadth are roughly 300 and 100 nm, respectively. As shown in Fig. 4c, alumina is uniformly coated on the surface of sulfur with comparatively flat surface. Obviously, Fig. 4d demonstrates that the surface of S/Al2O3/PPy is rough, suggesting that the hybrid material is successfully decorated with
PPy. The covering of PPy is beneficial to enhancing the conductivity and lithium-ion transmission of the composite. Transmission electron microscopy (TEM) and high angle annular dark field scanning TEM (STEM) images (Fig. 5) are also displayed to further discern the microstructure of these samples. The morphology of sulfur and Al2O3 consists with FESEM results (Fig. 4). As presented in the insets of Fig. 5aeb, the interplanar spacing of 0.333 nm is in line with the (311) facet of sulfur, and the interplanar spacing of 0.351 nm corresponds to the (012) plane of Al2O3. Fig. 5d clearly reveals that PPy is evenly adhered to the surface of sulfur and alumina particles. HRTEM image of S/Al2O3/PPy shows lattice fringes of sulfur as well as Al2O3, as observed in Fig. 5e, the interlayer spacings of 0.208 and 0.333 nm correspond to the (113) and (311) lattice planes of Al2O3 and sulfur phase, respectively. The elemental mappings of sulfur, nitrogen and aluminum further confirm the existence of PPy and uniform distribution of these elements. The electrochemical performance of S, S/Al2O3 and S/ Al2O3/PPy was obtained by the measurement of cyclic voltammograms and constant-current charge/discharge testing. In order to comprehend the redox reaction, CV behavior of S, S/Al2O3, and S/Al2O3/PPy for the initial three cycles was measured at a scan rate of 0.1 mV s1 within a potential window of 1.5e3.0 V. As displayed in Fig. 6, all the samples show two characteristic cathodic peaks and one anodic peak. The two typical cathodic peaks are associated with two reduction steps of sulfur: the former located at about 2.3 V can be ascribed to the reduction of elemental sulfur to highordered lithium polysulfides (Li2Sx, 4 < x < 8), and the latter at about 2.0 V results from further reduction of high-ordered lithium polysulfides to low-ordered lithium polysulfides (LiS2 or Li2S2). During the reverse anodic scan, the only oxidation peak located at around 2.5 V can be attributed to the conversion of LiS2/Li2S2 into lithium polysulfides, and finally to
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Fig. 4 e FESEM images of (a) pristine sulfur, (b) Al2O3, (c) S/Al2O3, and (d) S/Al2O3/PPy.
Fig. 5 e TEM images of (a) pristine sulfur (the insert is the corresponding HRTEM image), (b) Al2O3 (the insert is the corresponding HRTEM image), (c) S/Al2O3, and (d) S/Al2O3/PPy. (e) HRTEM image of S/Al2O3/PPy. (f) High angle annular dark field STEM image of S/Al2O3/PPy, and EDX elemental mappings of (g) S, (h) Al, and (i) N. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 6 e CV profiles of (a) S, (b) S/Al2O3, and (c) S/Al2O3/PPy for the first three cycles at a scan rate of 0.1 mV s¡1. elemental sulfur [41,42]. Fig. 6b shows CV profile of the binary composite. We find that the reduction peak is much broader in the first cycle. The reason may be that the electrical insulation of alumina on the surface could impede active sulfur to contact with conductive additive (acetylene black), leading to poor electrical contact and electron transfer, which could cause
lower redox kinetics and serious polarization. However, it is interesting that the broader peak disappears after the initial cycle due to the realignment of sulfur [43]. Throughout all the curves in Fig. 6, it is clear that the cathodic peaks in the first sweep have deviation compared to that of the second and the third sweep, which is mainly caused by the incomplete
Fig. 7 e Charge/discharge curves of (a) S, (b) S/Al2O3, and (c) S/Al2O3/PPy for selected cycles at a current density of 200 mA g¡1. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 8 e (a) Cycling performance of S, S/Al2O3, and S/Al2O3/ PPy at a current density of 200 mA g¡1. (b) Cycling performance of S/Al2O3/PPy at a current density of 1 C (1 C ¼ 1675 mA g¡1). activation of the cathode and the inferior sulfur utilization. In comparison with S/Al2O3, S/Al2O3/PPy displays higher reduction peaks (2.19 vs. 2.22 V) and lower oxidation peaks (2.51 vs. 2.48 V), and possesses higher and sharper current peaks, manifesting that the ternary composite has higher reaction kinetics and faster electron/ion transport. Moreover, starting
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from the second cycle, both the positions and intensities of the reduction and oxidation peaks vary less than those in the cases of S and S/Al2O3, which demonstrates the preferable reversibility of the ternary composite. The charge/discharge profiles of the three electrodes for the 1st, 10th, 50th, and 100th cycles at a current density of 200 mA g1 are disclosed in Fig. 7. As present in Fig. 7, all these three electrodes show two discharge plateaus and one charge plateau, which is in accordance with CV curves (Fig. 6). However, compared with S and S/Al2O3, S/Al2O3/PPy reveals larger discharge capacity and lower capacity degradation, manifesting that the ternary composite is more profitable for utilization of active material. Moreover, the gap potential between the charge and principal discharge plateaus is narrow, and the charge and principal discharge plateaus always keep well parallelism during cycling, suggesting that the system possesses smaller polarization and higher reaction kinetics. This is because that sulfur is effectively confined to alumina and PPy, which provides double protection to restrict the diffusion of intermediate polysulfides to the electrolyte. Fig. 8a contrasts cycling performance of S, S/Al2O3, and S/ Al2O3/PPy at a current density of 200 mA g1. The discharge capacity of S/Al2O3 and S is 988 and 738 mA h g1 in the first cycle, respectively, and only retains 482 and 147 mA h g1 after 100 cycles, respectively. Evidently, S/Al2O3/PPy composite cathode delivers more outstanding initial discharge capacity of 1088 mA h g1 and maintains 730 mA h g1 after 100 cycles. Even at high current density of 1 C (1 C ¼ 1675 mA g1), the ternary hybrid material has an initial capacity of 683 mA h g1 while maintaining 565 mA h g1 after 100 cycles with 82.7% capacity retention (Fig. 8b). These results suggest that S/Al2O3/ PPy cathode delivers the best electrochemical performance compared to S and S/Al2O3. This can be assigned to the cooperative effect between alumina and PPy, enhancing the utilization of sulfur and suppressing the diffusion of lithium polysulfides to the electrolyte during the cycling process. Furthermore, it is necessary to mention that the moderate
Fig. 9 e Rate performance of S, S/Al2O3, and S/Al2O3/PPy at various current densities. Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Fig. 10 e Electrochemical impedance spectra of S, S/Al2O3, and S/Al2O3/PPy before cycling. addition of LiNO3 in the electrolyte also plays a significant role in the electrochemical performance through promoting to form a stable passivation film. With the consumption of LiNO3, the growth of passivation film is infinite, and the formed passivation film is stable. Sequentially, the formation of stable passivation film effectively restrains the dissolution of polysulfides on Li anode, and improves the electrochemical performance of LSBs [44]. Rate capacity of S, S/Al2O3 and S/Al2O3/PPy is presented in Fig. 9. The three cathodes were cycled at different current densities of 150, 400, 800, 1600, 3000, and 8000 mA g1. It is clearly seen that S/Al2O3/PPy reveals the initial capacity of 1287 mA h g1 at 150 mA g1, the subsequent specific discharge capacities are 1189, 900, 588, 383, and 146 mA h g1, respectively. On the contrary, S/Al2O3 and S exhibit inferior initial discharge capacities of 980 and 826 mA h g1, and bad rate capacity under the same situations. In addition, when the current density is recovered to 150 mA g1, the discharge capacity of 1001 mA h g1 is recovered, suggesting good electrochemical reversibility of the ternary composite. This good rate performance further confirms that the introduction of PPy enhances the redox kinetics and utilization of sulfur, and accelerates the ionic diffusion. In order to further comprehend the charge transfer resistance of three samples, EIS measurements were employed on the freshly assembled LSBs. As revealed in Fig. 10, all the three cells possess oblate semicircle in the high frequency rage and approximately straight line in the low frequency rage. The former results from the charge-transfer resistance, and the latter can be ascribed to Warburg impedance [45e47]. It is noticeable that the ternary hybrid composite possesses the smallest resistance compared to pure sulfur and S/Al2O3 electrodes, which is in agreement with the electrochemical performance (Fig. 8). This can be put down to the good electrical conductivity of PPy in S/Al2O3/PPy composite. We further performed UV/Vis adsorption spectrum analysis to compare the adsorption ability of S/Al2O3 binary composite and S/Al2O3/PPy ternary composite. The band at 265 nm corresponds to S2 6 , reflecting the existence of polysulfides. As shown in Fig. 11, S/Al2O3/PPy þ Li2S6 in DME/DOL solution
Fig. 11 e UVeVis adsorption spectra of Li2S6 in DME/DOL, S/ Al2O3 þ Li2S6 in DME/DOL, and S/Al2O3/PPy þ Li2S6 in DME/ DOL. Inset: digital pictures of polysulfides adsorption in sealed vials by adding different samples, (a) Li2S6 in DOL/ DME after standing for 2 h, Li2S6 in DOL/DME after being in contact with (b) S/Al2O3 and (c) S/Al2O3/PPy for 2 h.
shows the lowest peak intensity compared to other two samples, which is primarily ascribed to the cooperative adsorption capacity of Al2O3 and PPy. What's more, the inset also shows the polysulfides adsorption capacity of different samples, and the vial containing S/Al2O3/PPy þ Li2S6 in DME/DOL solution displays the lightest colour, indicating the improved polysulfides adsorption of Al2O3 and PPy due to the chemical and physical adsorption.
Conclusions In summary, we have successfully fabricated S/Al2O3/PPy ternary hybrid material as cathode for LSBs by utilizing a simple, low-budget, and environmental method, manifesting more good electrochemical performance compared to the counterparts of S and S/Al2O3. S/Al2O3/PPy composite cathode delivers an initial discharge capacity of 1088 mA h g1 at a current density of 200 mA g1, and still maintains 730 mA h g1 after 100 cycles. Furthermore, S/Al2O3/PPy shows good rate capability. In this strategy, Al2O3 and PPy can provide strong adsorption for the dissolved intermediate polysulfides. Meanwhile, the conductive PPy also works as a conductive and flexible additive to expedite electron transport, and is coated on the surface of SeAl2O3 composite by insitu chemical polymerization. The sulfur is effectively confined to alumina and PPy, which provides double protection to restrict the diffusion of intermediate polysulfides to the electrolyte. The synergy between alumina and PPy is the decisive factor, which gives rise to good electrochemical performance of cathode for high-performance LSBs. Nevertheless, there may be other exact specifics, which probably exert an influence on the electrochemical performance of LSBs. For example, the effect of PPy content in the ternary composite on the electrochemical properties of LSBs should be further investigated.
Please cite this article in press as: Xu J, et al., Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.205
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Acknowledgements [15]
This work was supported by the National Natural Science Foundation of China (Nos. 51631004 and 21673095), the Project of Science and Technology Development Plan of Jilin Province (No. 20170414010GH), and the Special Fund for Industrial Innovation in Jilin Province (No. 2016C039).
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