Activated clay of nest structure encapsulated sulfur cathodes for lithium–sulfur batteries

Abstract

Activated clay (AC) with a nest-like structure and a large surface area was employed to support sulfur as the cathode for lithium–sulfur batteries. The special structure may have a similar effect to small filter screens for entrapping sulfur and restricting the diffusion of polysulfides during cycling. A high capacity of 959.6 mA h g⁻¹ was achieved at a rate of 0.1 C in the first cycle for the AC/S composite with a sulfur content of 57 wt% and the reversible capacity remained high at up to 700.9 mA h g⁻¹ even after 50 cycles.

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1. Introduction

In the area of energy storage, interest has grown considerably in lithium–sulfur (Li–S) batteries, as sulfur offers a high theoretical specific capacity of 1675 mA h g⁻¹ at a safe operating voltage and low cost.^1–3 However, although this system has been attracting attention for more than two decades, it has not been commercialized on a large scale due to several unsolved problems, such as the inherent poor electrical conductivity of S (5 × 10⁻³⁰ S cm⁻¹ at 25 °C), and a poor cycling performance caused by the highly electrolyte-soluble lithium polysulfide intermediates formed during repeated discharge–charge processes. To overcome these problems effectively, studies of well-designed cathodes for Li–S batteries are of great importance.^4–7

In recent years, sulfur has been combined with porous substrates through proper structure design (surface coating and/or sulfur loading) to effectively confine sulfur on the cathode side and limit the dissolution of polysulfides in the electrolyte. These substrates include porous carbons/polymers/metal oxides and functionalized graphenes, and limit polysulfide dissolution through both physical and chemical interactions.^8–19

Here, we propose to use clays as the confined substrate for sulfur impregnation. Clays (Al₂O₃·4SiO₂·nH₂O) are very common in the natural environment and are non-toxic and low-cost materials. They play vital roles in terrestrial biogeochemical cycles and the containment of toxic waste materials.²⁰ As a result of their high surface area and abundant surface functional groups, they have been discovered to have good catalytic/support properties, especially for many organic reactions and bleaching purposes. With a large pore volume and small windows limiting the diffusion process, this material appears to be a suitable candidate for the encapsulation of sulfur and its corresponding reduced state species in spite of their low electrical conductivities. The confinement effect of a porous material is more important than its conductivity for Li–S batteries.²¹ Furthermore, the overall electronic conductivity of the electrodes can be improved by using enough Super P to build a conductive network.²² So, activated clay (AC) with a three-dimensional nest-like structure has been employed to design a novel composite of sulfur/activated clay (Fig. 1). After a simple acidification treatment, the clays are exfoliated into unsystematic nanorods which look like a nest. Compared with traditional sulfur cathode materials, the sulfur/activated clay (AC/S) composite with this special structure has several advantages.

Fig. 1 Scheme of the sulfur/activated clay (AC/S) composite.

Firstly, the nest-like structure can provide more active sites and reduce the loss of active materials due to its high specific surface area. Most of the embedded sulfur is located in micropores or under the filter screen through capillary action and filtration, which makes the sulfur interact with the oxide nanorods intensively. Secondly, besides the confinement effect, the surface activity of the AC appears to also have a pronounced effect on reducing lithium polysulfide dissolution, and tremendously alleviates the shuttle effect. In addition, the composite can also accommodate the volume expansion of sulfur generated during the electrochemical reaction. At last, the nest structure in the composite is designed to retain pathways for the mass transport of Li ions and endow the cathode with a high ionic conductivity.

2. Experimental

2.1 Material preparation

The clay impurities were removed by treatment with HCl and distilled water and the clay was dried at 120 °C for 24 h in a vacuum drying oven. The as-prepared activated mixture of clay (AC) was thoroughly mixed with sublimed sulfur in an agate mortar at different mass ratios. The mixture was then sealed in an air-free glass container and heated at 155 °C for 6 h to obtain the sulfur/activated clay (AC/S) composite.

2.2 Material characterization

The sulfur content of the composites was confirmed using a thermogravimetric analyzer (TG, STA 449 F3, NETZSCH). The microstructure and morphology of the AC and AC/S composites were detected using the Brunauer–Emmet–Teller method (BET, Tristar-II, Micromeritics), X-ray diffraction (XRD, Bruker AXS D8), transmission electron microscopy (TEM, JEM-2100, JEOL), and field emission scanning electron microscopy (FE-SEM, SU-8010, HITACHI).

2.3 Electrochemical measurements

In order to prepare the working electrode, the AC/S working electrodes were made by mixing 70 wt% of the active material, 20 wt% Super P and 10 wt% PVDF with N-methyl-2-pyrrolidine (NMP). Lithium metal was used as the counter and Celgard 2400 was used as the separator. The electrolyte used was 1 M LiTFSI in a mixture of dimethoxyethane (DME) and 1,3-dioxolane (DOL) (2 : 1 volume).

Galvanostatic charge–discharge experimental data were collected using a LAND cell test system (CT2001A, Wuhan, China). The AC impedance measurements and CV (cyclic voltammetry) were carried out using an electrochemical workstation (CHI660B, Shanghai, China).

3. Results and discussion

The content of sulfur in the AC/S composites was measured by thermogravimetric analysis (TG) under a N₂ atmosphere as shown in Fig. 2. The weight loss of the composite can be observed with increasing temperature from about 150 °C, and the curve becomes stable when the composite is heated over 350 °C due to the evaporation of sulfur from the AC substrate material. The sulfur contents in the as-prepared composites were 63 wt%, 57 wt% and 41 wt%, respectively, in comparison with sublimed sulfur, calculated from the TG curves.

Fig. 2 TG curves of the AC/S composites recorded under a N₂ atmosphere.

In order to prove the existence of a hierarchical porous structure in the AC substrate, we measured the nitrogen adsorption–desorption isotherm (Fig. 3a) at 77 K and the corresponding density functional theory pore size distribution curve (Fig. 3b). From Fig. 3, we should be able to prove the above conclusion.

Fig. 3 (a) N₂ sorption isotherms and (b) pore size distributions of the AC, and the AC/S composites with 63 wt%, 57 wt% and 41 wt% sulfur.

The general shape of the isotherm is defined as type III according to IUPAC and suggests the existence of pore sizes ranging from micro- to macropores. The slow increase at a relatively low pressure of P/P₀ (0.01–0.1) reveals the existence of few micropores. The following slope at medium relative pressure and the hysteresis loop in the adsorption–desorption isotherm (as type H3 according to IUPAC) illustrate the presence of developed mesoporosity, while the final abrupt increased tail at the relative pressure from 0.9 to 1.0 indicates the existence of seam type pores. The AC substrate has a pore size distribution mainly in three regions: 1.79–2.0 nm, 2.13–47 nm and >100 nm, representing the three levels of small micropores, abundant mesopores and some macropores. The Brunauer–Emmett–Teller (BET) surface area of the AC reduced dramatically from 122.01 m² g⁻¹ to 2.66 m² g⁻¹, 1.34 m² g⁻¹ and 0.95 m² g⁻¹ after loading different sulfur contents, and the total pore volume reduced from 0.2565 cm³ g⁻¹ to 0.0338 cm³ g⁻¹, 0.0122 cm³ g⁻¹ and 0.0096 cm³ g⁻¹ (Table 1).

Table 1 Specific surface area and pore volume of the AC and the AC/S composites^a

Sample	SBET (m^2 g^−1)	Vmicro (cm^3 g^−1)	Vt (cm^3 g^−1)
a SBET is the specific surface area, and Vt is the total pore volume.
AC	122.01	0.0133	0.2565
AC/S with 41% S	2.66	0.0023	0.0338
AC/S with 57% S	1.34	0.0006	0.0122
AC/S with 63% S	0.95	<0.0001	0.0096

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The morphology and structure of the AC and the prepared AC/S composites were characterized by FE-SEM and TEM as shown in Fig. 4. Tens of thousands of nanorods cluster into one large particle which looks like a nest structure. That special structure may have a similar effect as the small filter screens (Fig. 4a and c). In Fig. 4b, the nest structure is less obvious as it is overlaid by sulfur. Evidence of S containment and the identification of an embedded entrapped sulfur structure within the AC/S composite are provided by the TEM images (Fig. 4c and d) and XRD analysis (Fig. 5a), respectively. Fig. 4d shows the corresponding high resolution TEM (HRTEM) images of the AC/S composite with 57 wt% sulfur. The dispersed S at its crystalline state was around 0.38 nm with d-spacing (222). Fig. 5a shows the XRD patterns of the pristine sulfur powder and the as-prepared AC/S composite. All diffraction peaks of sulfur match very well with the standard diffraction lines of sulfur (JCPDS card no. 08-0247), showing that the entrapped sulfur microparticles can be indexed to orthorhombic crystal type. These XRD results are in good agreement with the TEM results. Fig. 5b is the FTIR spectra of the AC, S and AC/S. After sulfur impregnation, two peaks at 465 and 438 cm⁻¹ are observed for the AC/S, which is different from that of the pristine AC with only one broad peak. The weak absorption at 465 cm⁻¹ could be undoubtedly assigned to the S–S vibration mode of elemental sulfur (S₈), while the newly emerged peak at 438 cm⁻¹ could be explained by the weak interactions between the sulfur and the activated clay.

Fig. 4 FESEM images of (a) the activated clay and (b) the AC/S composite with 57 wt% sulfur. TEM images of (c) the activated clay and (d) the AC/S composite with 57 wt% sulfur.

Fig. 5 (a) X-ray diffraction patterns of the S, AC and AC/S, and (b) FTIR spectra of the S, AC and AC/S ranging from 420–520 cm⁻¹.

The electrochemical performance of the AC/S composite was investigated using galvanostatic charge–discharge measurements. The C rate specified in this study is based on the mass and theoretical capacity of sulfur (1 C = 1675 mA g⁻¹). The discharge capacities of the AC/S composites are shown in Fig. 6a. The AC/S composite with 57 wt% S shows a high capacity of 959.6 mA h g⁻¹ at a rate of 0.1 C in the first cycle and the reversible capacity remains high at up to 700.9 mA h g⁻¹ even after 50 cycles. The capacity of the AC/S composite with 57 wt% S falls for the first ten cycles then rises gradually due to slow kinetics. Also, the AC/S composite with 63 wt% S has a similar tendency with a slightly lower capacity. In contrast, the capacity of the AC/S composite with 41 wt% S drops rapidly even though it has the largest initial capacity. The initial capacity is closely related to the amount of sulfur loading. A higher sulfur content results in a lower initial capacity, and a lower sulfur content results in a poor cycling stability, which are in agreement with previous findings using a bimodal pore structure carbon for sulfur impregnation.²³

Fig. 6 (a) Cycling stability of the AC/S composites with sulfur contents of 63 wt%, 57 wt% and 41 wt% at a rate of 0.1 C for the voltage range of 1.2–2.8 V, (b) charge–discharge curves for different cycling numbers of the AC/S composite with 57 wt% sulfur at a rate of 0.1 C, (c) FTIR spectra of the AC and the AC/S composite before and after cycling ranging from 420–520 cm⁻¹.

The rapid initial capacity loss is also observed in the sulfur/activated carbon composite. When both the capacity and cycling stability are considered, the AC/S composite with 57 wt% S exhibits the best electrochemical performance. Although active clays and sulfur both have a poor conducting nature, the initial discharge capacity is comparable to that delivered by typical C/S composites, as seen in this case. It proves that the AC can provide a special and stable structure for sulfur entrapment, which is indicative of the greater importance of electrode confinement over electrode conductivity in some cases. It also shows that the overall electronic conductivity of the sulfur cathode can be effectively improved by using enough Super P to form a conductive network. The excellent cycling stability may be owed to the special structure of the AC/S composite. The special structure of the composite can restrict the diffusion of polysulfides during the charge–discharge process. In addition, AC can present more polarized surfaces than carbon so it is able to interact strongly with charged species, such as Li₂S_x. Thus, one would expect the onset of such surface interactions between the host structures and polysulfides to slow down the migration of the polysulfide species. Hence, the better capacity retention of the composite electrodes is what we experimentally observed.

Fig. 6b presents typical discharge–charge profiles of the AC/S composite with 57 wt% S for the first, second and fiftieth cycles at a rate of 0.1 C between 1.2–2.8 V. There are two plateaus presented in the discharge profile: one short higher potential plateau at about 2.3 V and another prolonged lower potential plateau at about 2.0 V. The charge–discharge curves are well maintained during extended cycling. To explore the effect of this special structure during the charge–discharge process, the FTIR spectrum of the AC/S composite was measured after the charge–discharge process was conducted. As shown in Fig. 6c, compared with the pristine AC, there is a newly emerged peak around 438 cm⁻¹ which is explained by the weak interactions between the sulfur and the activated clay, and the peak also appears, with a slight blue shift, after 50 cycles. The result indicates that the nest-like structure favors the confinement of sulfur and imparts the excellent cell performance in the cycling.

Fig. 7 shows the cyclic voltammograms of the cell for the first three cycles. In the initial cathodic process, the two broad reduction peaks that appeared at 2.0 and 2.25 V (vs. Li/Li⁺) are due to the two-step reduction of sulfur with metallic lithium. In the following cycles, the cathodic peak potentials move up to about 2.05 and 2.3 V (vs. Li/Li⁺), which is ascribed to the polarization of the electrode in the first cycle. In particular, the cathodic peak areas remain almost unchanged, and the anodic peaks have a slight negative shift, which confirms a relatively good reversibility and stability after the initial electrochemical process for the AC/S electrode.^24,25

Fig. 7 Cyclic voltammograms of the AC/S composite with 57 wt% S at a sweep rate of 0.05 mV s⁻¹.

To further analyze the attenuation mechanism of the AC/S composite electrode, the electrochemical impedance spectroscopy (EIS) results are displayed in Fig. 8. The impedance behavior of the composite without electrochemical activation is very different from that of the composite experiencing many cycles. The former is composed of two semicircles at high frequency and a nearly straight line at low frequency, while at high frequency, the latter has only one semicircle. As we know, the semicircle in the high to medium frequency region relates to the resistances of charge transfer and the SEI film and the oblique straight line in the low frequency region corresponds to the diffusion process within the cathodes. It is obvious that the AC/S composite electrode exhibited a low and stable charge transfer resistance upon cycling. The decreased resistance of the AC/S composite electrode along with cycling may be because the nest-like clay can chemically or physically adsorb polysulfide anions and, thereby, partially restrain the migration of polysulfides.

Fig. 8 EIS spectra of the AC/S composite with 57 wt% S.

4. Conclusions

In summary, the nest structure of activated clay with numerous meso- and microsized pores has been discovered to improve the electrochemical performance of Li–S batteries. The special nest structure may have a similar effect to small filter screens for entrapping sulfur and restricting the diffusion of polysulfides during cycling. Our test results indicate that once sulfur is loaded within the porous framework with a proper ratio of AC/S, the poor cycling stability issue of the sulfur electrode can be partially mitigated because of the confinement effect of the porous framework.

Acknowledgements

This work was financially supported by the Foundation of He’nan Educational Committee (15A150012).

Notes and references

1. W. Wei, J. L. Wang, L. J. Zhou, J. Yang, B. Schumann and Y. N. NuLi, Electrochem. Commun., 2011, 13, 399.
2. P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J. M. Tarascon, Nat. Mater., 2012, 11, 19.
3. X. Ji, K. T. Lee and L. F. Nazar, Nat. Mater., 2009, 8, 500.
4. B. C. Duan, W. K. Wang, A. B. Wang, Z. B. Yu, H. L. Zhao and Y. S. Yang, J. Mater. Chem. A, 2014, 2, 308.
5. S. Evers and L. F. Nazar, Acc. Chem. Res., 2013, 46, 1135.
6. H. Kim, H. D. Lim, J. Kim and K. Kang, J. Mater. Chem. A, 2014, 2, 33.
7. A. Manthiram, Y. Z. Fu and Y. S. Su, Acc. Chem. Res., 2013, 46, 1125.
8. G. Xu, B. Ding and J. Pan, J. Mater. Chem. A, 2014, 2, 12662.
9. M. M. Rao, W. S. Li and E. J. Cairns, Electrochem. Commun., 2012, 17, 1.
10. B. Zhang, X. Qin, G. R. Li and X. P. Gao, Energy Environ. Sci., 2010, 3, 1531.
11. L. Ji, M. Rao, H. Zheng, L. Zhang, Y. Li, W. Duan, J. Guo, E. J. Cairns and Y. Zhang, J. Am. Chem. Soc., 2011, 133, 18522.
12. D. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H. Cheng, I. R. Gentle and G. Q. M. Lu, J. Mater. Chem. A, 2013, 1, 9382.
13. G. Zheng, Y. Yang, J. J. Cha, S. S. Hong and Y. Cui, Nano Lett., 2011, 11, 4462.
14. E. Frackowiak, Phys. Chem. Chem. Phys., 2007, 9, 1774.
15. L. Ji, M. Rao, S. Aloni, L. Wang, E. J. Cairns and Y. Zhang, Energy Environ. Sci., 2011, 4, 5053.
16. S. Dorfler, M. Hagen, H. Althues, J. Tubke, S. Kaskel and M. J. Hoffmann, Chem. Commun., 2012, 48, 4097.
17. J. Guo, Y. Xu and C. Wang, Nano Lett., 2011, 11, 4288.
18. J. Schuster, G. He, B. Mandlmeier, T. Yim, K. T. Lee, T. Bein and L. F. Nazar, Angew. Chem., Int. Ed. Engl., 2012, 51, 3591.
19. Y. Yin, C. Ma, Z. Cao, Z. Sun, Y. Jia and S. Yang, RSC Adv., 2014, 4, 28871.
20. U. C. Ugochukwu, M. D. Jones, I. M. Head, D. A. C. Manning and C. I. Fialips, Int. Biodeterior. Biodegrad., 2014, 88, 185.
21. R. Demir-Cakan, M. Morcrette, F. Nouar, C. Davoisne, T. Devic, D. Gonbeau, R. Dominko, C. Serre, G. Ferey and J. M. Tarascon, J. Am. Chem. Soc., 2011, 133, 16154.
22. B. Guo, T. Ben, Z. Bi, G. M. Veith, X. G. Sun, S. Qiu and S. Dai, Chem. Commun., 2013, 49, 4905.
23. G. He, X. Ji and L. Nazar, Energy Environ. Sci., 2011, 4, 2878.
24. Y. Fu and A. Manthiram, J. Phys. Chem. C, 2012, 116, 8910.
25. Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 2004, 151, A1969.

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