Sepiolite-sulfur as a high-capacity, high-rate performance, and low-cost cathode material for lithium–sulfur batteries

Journal of Power Sources 293 (2015) 527e532

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Sepiolite-sulfur as a high-capacity, high-rate performance, and lowcost cathode material for lithiumesulfur batteries Junan Pan a, 1, Cheng Wu a, 1, Juanjuan Cheng a, Yong Pan a, Zengsheng Ma a, Shuhong Xie b, **, Jiangyu Li c, * a

Hunan Provincial Key Laboratory of Thin Film Materials and Devices, Xiangtan University, Xiangtan, Hunan 411105, China Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, School of Materials Science and Engineering, Xiangtan University, Xiangtan, 411105 Hunan, China c Department of Mechanical Engineering, University of Washington, Seattle, WA 98195, USA b

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


The first specific discharge capacity is about 1436 mAh g1 at 0.2 C current rate.
The discharge capacity remains 901 mAh g1 after 300 cycles at 0.2 C current rate.
The first discharge capacity is 1206 mAh g1 under 1 C current density.
The raw materials are abundant and low cost.
The manufacturing process is simple and easily scalable.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 January 2015 Received in revised form 22 May 2015 Accepted 26 May 2015 Available online 2 June 2015

Lithiumesulfur batteries have the theoretical energy density of up to 2600 Wh/kg, though its commercialization is limited by high material cost and poor cyclic performance. In this work, we show that sepiolite-sulfur is a high-capacity, high-rate performance, and low-cost cathode material for lithium esulfur batteries. Sepiolite is a porous mineral with specific structure, outstanding physical and chemical adsorption characteristics, and excellent ion exchange capability, making it an ideal matrix material for lithiumesulfur batteries. It is shown that the first specific discharge capacity of sepiolite-sulfur cathode is about 1436 mAh g1 at 0.2 C current rate, and it remains as high as 901 mAh g1 after 300 cycles. Under 1 C current density, the first discharge capacity is 1206 mAh g1, and maintains a high value of 601 mAh g1 after 500 cycles. The raw materials are abundant and low cost, and the manufacturing process is simple and scalable, making it promising for the commercialization of lithiumesulfur batteries. © 2015 Elsevier B.V. All rights reserved.

Keywords: Lithium-sulfur batteries Cathode Sepilolite-sulfur composite Low-cost Adsorption

1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Xie), [email protected] (J. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2015.05.108 0378-7753/© 2015 Elsevier B.V. All rights reserved.

Sepiolite [1e3], a hydrated magnesium silicate clay mineral with layered chain structure and fibrous morphology, is an abundant and low cost material with worldwide annual production around 1,300,000 tons. The major ingredient of sepiolite is

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Mg8Si12O30(OH)4(H2O)4
8H2O, which consists of two tetrahedral silica sheets with a sandwich layer of magnesium oxide hydroxide, and the cross section area of the tubular longitudinal channel is 1.06 nm  0.37 nm in the unit cell. It is also highly porous, with the pore volume up to over half of the fiber, resulting in high specific surface area and outstanding physical and chemical adsorption characteristics and ion exchange ability. Due to its excellent adsorbability [4], sepiolite has been widely used to absorb heavy metals [5,6], chloride [7,8], basic dyes [9,10] and cationic surfactants [11,12]. In this work, we seek to explore its application as a matrix material for lithiumesulfur batteries. Lithiumesulfur batteries [13e16] has the theoretical energy density of up to 2600 Wh/kg, which is 3e5 times more than that of the traditional lithium-ion batteries. However, polysulfide generated in the process of battery charging and discharging tends to dissolve in electrolyte [17e19], which decreases the activity of the active substances gradually, reduces the electrical conductivity, intensifies the corrosion and increases the inner resistance of the battery, resulting in poor cyclic performance. To overcome these difficulties, a variety of nanostructured metal oxide additives were added into the positive electrode materials, including Mg0.6Ni0.4O [20e22], lanthanum oxide [23], aluminum oxide [24,25], silicon oxide [26] and titanium dioxide [27,28], with an expectation to utilize their high specific surface area and surface groups to adsorb lithium polysulfide and inhibit its dissolution. For example, Song et al. [20] added nanosized Mg0.6Ni0.4O into sulfur cathode and increased the discharge capacity and cyclic durability. Nanosized Al2O3 particles with large specific surface area were added into sulfur electrode by Choi et al. [24]. Nazar et al. [26] also induced the mesoporous silica material into the porous carbon/sulfur composite, resulting in much improved cycling performance of the porous carbon/sulfur composite. However, owing to the complicated preparation process and high cost of the nanostructured metal oxides, mesoporous silica and other additives, it is difficult to realize the commercial development of the lithiumesulfur batteries. We speculated that sepiolite could be an excellent matrix and absorbing material for lithiumesulfur batteries due to its ion transmission channel and large pore volume. Indeed, we obtained high capacity and excellent cycling and rate performance of lithiumesulfur batteries by using the synergistic effect of natural porous fiber structure and strong absorptivity of sepiolite, which effectively restrained the polysulfide dissolution and thus improved the conversion efficiency of positive electrode active material. The first specific discharge capacity is about 1436 mAh g1 at 0.2 C current rate, and remains as high as 901 mAh g1 after 300 cycles. Under 1 C current density, the first discharge capacity is 1206 mAh g1, and maintains a high value of 601 mAh g1 after 500 cycles. The raw materials are abundant and low cost, and the manufacturing process is simple, making it promising for the commercialization of lithiumesulfur batteries with high performance and light weight. 2. Experimental 2.1. Material preparation and characterization Sepiolite powders used in this work were bought from a mineral processing factory in Liuyang county, Hunan Province, China. They were then screened through a 200-mesh sieve before mixed with sulfur (S), and the weight ratio of sepiolite/S is controlled at 3/2 by gravimetric method. Finally the mixed powders were heated under vacuum at 100  C for 3 h. The structure and composition of the sepiolite were examined by using X-ray diffraction (XRD, Rigaku D/max-2400) and X-ray

fluorescence (XRF, Bruker S4-P10VER). The morphologies before and after the sulfur injection were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F20) with energy dispersive X-ray spectroscopy (EDS). The specific surface area and pore size distribution were measured according to the BrunauereEmmette Teller (BET) method and BarretteJoynereHalenda (BJH) method, respectively. Infrared spectroscopy (IR, PerkineElmer Spectrum One spectrophotometer) was applied to verify the structures of sepiolite before and after the sulfur injection, using KBr pellets prepared with a pressed-disk technique (0.8 mg sample þ 200 mg KBr). 2.2. Electrochemical test Positive electrodes comprised of 70 wt% sepiolite/S composite, 20 wt% acetylene black and 10 wt% polyvinylidene fluoride binders were homogeneously mixed in an N-methyl-2-pyrrolidone (NMP) solvent under continuous magnetic stirring for 3 h, and then the slurry was coated uniformly on an aluminum foil. The electrodes were dried under vacuum at 60  C for 12 h. 2016 type coin cells were assembled in an argon-filled glove box, where lithium metal was used as a counter electrode and polypropylene membrane was used as the separator. The electrolyte was 1 M lithium bis (tri-fluoromethanesulfonyl) imide (LiTFSI) dissolved in a 1:1 volume ratio mixture of 1,3-dioxolane and dimethoxy ethane. Electrochemical chargeedischarge and cyclic performances were measured at room temperature using a BTS-5V3A battery test system (Neware, Shenzhen, China), with the potential range from 3.0 to 1.0 V (vs. Li/Liþ). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested using a CHI660D electrochemical workstation (Chenhua, Shanghai, China) over a frequency range of 1 MHz to 0.1 Hz, with an open-circuit AC voltage amplitude of 5 mV. 3. Results and discussion 3.1. Microstructure The images of sepiolite mineral and powders bought from the factory are shown in Fig. 1(a)e(b), where layered structure is observed in the mineral with yellowish colour. The XRD patterns of sepiolite and sepiolite/S samples are given in Fig. 1(c), from which we can see two strong diffraction peaks corresponding to crystalline quartz (SiO2) and calcite (CaCO3). The diffraction intensity of sepiolite (Mg8Si12O30(OH)4(H2O)4
8H2O) is relatively weak, because the used sepiolite powders were grinded by the raw ore directly, which have many impurities [29e31]. After mixed with sulfur, the diffraction peaks of sulfur appear while those of sepiolite remain. This is also confirmed by IR spectrum in Fig. 1(d), where no obvious difference is found before and after surlfur injection. The absorption peak at 3560 cm1 in the IR spectrum is attributed to the crystalline water of sepiolite (based on Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) data shown in Fig. S1 in the supplementary information, the crystalline water in sepiolite is decomposed between 160  C and 360  C, consistent with literature reports [32,33], and thus during our processing, these crystalline water is stable), bands in the range of 1200e400 cm1 are characteristic of silicate, the absorption peaks at 1030 cm1 and 469 cm1 are ascribed to the SieO stretching and bending of sepiolite, and the peaks at 789 cm1 and 681 cm1 correspond to the SieOeSi stretching and vibration of sepiolite [29,34,35]. In addition, the chemical analysis of sepiolite powder was carried out using XRF and the main components are found to be SiO2, MgO, CaO, Al2O3 and Fe2O3, as shown in Table 1. From the percentage of MgO, we estimate that

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Fig. 1. Images of (a) sepiolite ore and (b) sepiolite powders; (c) XRD patterns of sepiolite and sepiolite/S composite; and (d) FT-IR spectra of sepiolite and sepiolite/S composite.

the content of Mg8Si12O30(OH)4(H2O)4
8H2O is around 55%. The morphology of sepiolite was further examined by SEM and TEM. From Fig. 2(a), we can see that the sepiolite powders are composed of short microfibrous bundles with length in the range of 200 nme2 mm and diameter in the range of 50e200 nm. Fig. 2(b) shows the morphology after sulfur injection, and it appears that the surface of the sepiolite powders is covered by sulfur, which increases the average diameter of the fibers, though the fibrous morphology with high length-diameter ratio remains, suggesting no structure change in consistency with the XRD results. The TEM image in Fig. 2(c) shows that the sepiolite is composed of many needle-like fiberous clusters, and the diameter of a single needle is about 30e50 nm with length between 500 nm and 2 mm. After the sulfur injection, obvious aggregation appears due to thermal treatment as seen in Fig. 2(d). The presence of sulfur was proven by the EDS of TEM, as shown in Fig. S2 in the supplementary information. N2 adsorption and desorption isotherm was used to measure the changes of the specific surface area of the sepiolite before and after sulfur injection. As seen in Fig. 3(a), the sepiolite shows an IV type isotherm [36] of mesoporous material. After mixing with 40 wt% sulfur, the specific surface area of the sample decreases sharply from 33.289 m2 g1 to 9.134 m2 g1 and the pore volume decreases from 0.06 cm3 g1 to 0.02 cm3 g1, indicating that the sulfur has covered the sepiolite surface and some sulfur has injected into the interior pore of the sepiolite. Fig. 3(b) reveals the change of pole diameter before and after the sulfur injection. The majority pore size of sepiolite is about 2e3 nm, which belongs to mesopores, and the distribution of the pore diameter of sepiolite/S composite is Table 1 Chemical composition of sepiolite powders by XRF experiment (%). SiO2

MgO

CaO

Al2O3

Fe2O3

F

Na2O

K2O

P2O5

Total

53.8

13.71

12.4

3.53

1.42

0.83

0.627

0.359

0.15

86.826

more homogeneous than that of sepiolite due to the physical adsorption of sulfur. 3.2. Electrochemical properties The cycling performance and Coulombic efficiency of the sepiolite/S cathode over 300 cycles at a rate of 0.2 C (with 1 C ¼ 1675 mA g1) are shown in Fig. 4(a). It is observed that the first discharge capacity is about 1436 mAh g1, and discharge capacity as high as 901 mAh g1 remains after 300 cycles, with a high Coulombic efficiency of 96.9%. The capacity loss per cycle is approximately 0.12%. The rate performance of the sepiolite/S cathode is shown in Fig. 4(b), with the current density increased from 0.2 C to 2 C gradually in four steps, and then decreased to 0.2 C before final testing at 0.5 C. The discharge capacity is around 1500 mAh g1 under 0.2 C, and maintains a high value of 750 mAh g1 as the current density reached 2 C. When the current density reduced to 0.2 C, the discharge capacity recovers a high value of 1219 mAh g1. These discharge capacities indicate that the sepiolite-sulfur cathode has an excellent rate performance. Fig. 5(a) shows the cyclic voltammograms of sepiolite/S cathode in a voltage range between 0.1 and 3.0 V before cycle and after 500 cycles at 1 C with a scanning speed of 0.2 mV s1. It can be seen that the reaction between sulfur and lithium is a multi-step reaction. There are an anodic oxidation peak and two cathodic reduction peaks before the first cycle. In the course of negative scanning, there are two cathodic peaks appearing in around 2.3 V and 1.8 V, where the cathodic peak around 2.3 V is attributed to the elemental sulfur converted into long chain multi-lithium sulfide (Li2Sn 4  n < 8), while 1.8 V corresponds to the process of the long chain multi-lithium sulfide deoxidized into short chain multi-lithium sulfide (Li2Sn n < 4) and lithium sulfide (Li2S), respectively. The sharp oxidation peak around 2.55 V of forward scan indicates that the multi lithium sulfide and Li2S have oxidized into elemental sulfur [37,38]. It is obvious that after 500 cycles, the location of

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Fig. 2. The morphology of sepiolite and sepiolite/S composite; SEM image of (a) sepiolite and (b) sepiolite/S composite; TEM image of (c) sepiolite and (d) sepiolite/S composite.

cathodic peaks shift to higher potential, while the anodic peak shift to lower potential, which indicates the cathodic polarization decreased and the reversibility of battery increased. The chargeedischarge curves at different cycles for sepiolite/S cathode at 1 C are shown in Fig. 5(b). It is obvious that two discharge voltage platform appear in the 1st cycle curve, which demonstrates that the reaction between sulfur and lithium to form polysulfide or Li2S is a multi-step reaction. It is observed that the first discharge voltage platform shrinks while the second discharge voltage platform widens gradually with increased cycles, and the result is similar to that of cyclic voltammetry. As seen in Fig. 5(b), the discharge capacities are 1206, 920, 800, 731, 668, 601 mAh g1 at cycle number 1, 100, 200, 300, 400, 500, respectively. Fig. 5(c) shows the cyclic performance of sepiolite/S cathode under 1 C current density. The first discharge capacity is

1206 mAh g1, and the capacity maintains a high value of 601 mAh g1 after 500 cycles, corresponding to 0.1% capacity loss per cycle. In comparison, if we baked the raw sepiolite powders at 350  C for 3 h to remove the crystalline water, and then test the chargeedischarge performance of the sepiolite/S batteries, as shown in Fig. S3 in the supplementary information, it is observed that the first discharge capacity under 1 C is reduced to around 1100 mAh g1, indicating the positive effect of crystalline water on the electrochemical performance of sepiolite/S batteries. Furthermore, it is observed in the Coulombic efficiency curve that the sepiolite/S battery maintains an efficiency around 84.8% after 500 cycles, implying that the chargeedischarge is relatively reversible at high current density of 1 C, though there is small energy loss during charging. Such reduction is due to loss of active materials because of irreversible reaction processes as well as shuttle effect.

Fig. 3. (a) Nitrogen adsorptionedesorption isotherms at 77 K of sepiolite and sepiolite/S composite; (b) BJH pore size distribution of sepiolite and sepiolite/S composite.

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Fig. 4. Electrochemical characterization of sepiolite/S cathodes containing 40 wt% sulfur; (a) Cycling performance and the corresponding Coulombic efficiency for chargeedischarge at 0.2 C discharge rate and (b) rate capability under different discharge rate.

The experimental EIS data were fitted using the Multiple Electrochemical Impedance Spectra Parameterization (MEISPþ) software based on the planar finite length diffusion with general boundary conditions model, as shown in Fig. 5(d). The equivalent circuit is shown in Fig. S4, and the corresponding impedance values are summarized in Table S1 in the supplementary information. It is observed that both the electrolyte resistances (Rser) and resistance related to finite length diffusion (Rd) increase after 500 cycles, as expected, due to dissolution of polysulfide in the electrolyte and the irreversible loss of active materials [39e43]. The interface charge-

transfer resistance (Rct) and activated adsorption resistance (Rt), however, decrease after 500 cycles, and they are approximately proportional to each other. It has been reported that metal oxides may lead to the formation of an interfacial layer, favoring the electrochemical stability of the system and resulting in reduction in charge-transfer resistance as cycles increase [21,22]. Metal oxides in sepiolite may contribute to such charge-transfer resistance reduction we observed. The overall impedance is observed to be reduced, consistent with the decreased cathodic polarization shown in Fig. 5(a). These results suggests that the configuration of

Fig. 5. (a) Cyclic voltammetry curves of the sepiolite/S composite before the first cycle and after 500 cycles with a scan rate of 0.2 mV s1 in a voltage range between 3.0 and 1.2 V vs. Li metal reference electrode at 1 C discharge rate; (b) Galvanostatic dischargeecharge profiles for the sepiolite/S composite at 1C discharge rate; (c) cycling performance and the corresponding Coulombic efficiency at 1 C discharge rate; and (d) EIS of the sepiolite/S cathodes before the first cycle and after 500 cycles at 1 C discharge rate and fully charged to 3 V.

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sepiolite and the adsorbability of sepiolite provide efficient electron migration pathways for the active material, which improved the cycle performance of lithiumesulfur batteries. 4. Conclusions In conclusion, the sepiolite/S composite material was prepared by using natural sepiolite and sulfur with simple vacuum heat treatment. The sepiolite/S cathode shows excellent capacity, cyclic and rate performance, with first specific discharge capacity about 1436 mAh g1 at 0.2 C current density, and remaining as high as 901 mAh g1 after 300 cycles. Under 1 C current density, the first discharge capacity is 1206 mAh g1, and maintains a high value of 601 mAh g1 after 500 cycles. These results show that the sepiolitesulfur is a high capacity, high rate performance cathode material for lithiumesulfur batteries. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 11472236, 11372267 and 11372268) and Excellent Youth Foundation of Hunan Scientific Committee (No. 13JJ1019). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.05.108. References [1] E. Galan, Clay Min. 31 (1996) 443e454. [2] A. Alvarez, Dev. Sedimentology 37 (1984) 253e287. [3] L. Bokobza, A. Burr, G. Garnaud, M.Y. Perrin, S. Pagnotta, Polym. Int. 53 (2004) 1060e1065. [4] J. Hrenovic, D. Tibljas, T. Ivankovic, D. Kovacevic, L. Sekovanic, Appl. Clay Sci. 50 (2010) 582e587. [5] R. Celis, M.C. Hermosin, J. Cornejo, Environ. Sci. Technol. 34 (2000) 4593e4599. [6] S. Kocaoba, Desalination 244 (2009) 24e30. ~ a-Amate, A. Cantos-Molina, [7] E. Gonz
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