Tin sulfide modified separator as an efficient polysulfide trapper for stable cycling performance in Li–S batteries

Abstract

Lithium–sulfur batteries (Li–S) are considered the most promising systems for next-generation energy storage devices due to their high theoretical energy density and relatively low cost. However, the practical applications of Li–S batteries are hindered by the poor electronic conductivity of sulfur and capacity degradation resulting from the shuttle effect of lithium polysulfides (LiPSs). Herein, we demonstrate use of a tin-sulfide (SnS₂) modified separator to facilitate the redox reaction involving LiPS intermediates and realize improved electrochemical performance in a Li–S battery. Density functional theory (DFT) calculations revealed that SnS₂ exhibits a strong affinity with LiPSs and induces a rapid conversion of trapped polysulfides. As a result, Li–S batteries with a SnS₂-modified separator exhibited an enhanced specific capacity of 1300 mA h g⁻¹ at 0.2C (corresponding to a high areal capacity of 4.03 mA h cm⁻²), which was maintained at 1040 mA h g⁻¹ after 150 cycles. Furthermore, an excellent rate capability is achieved with a capacity of 700 mA h g⁻¹ (2.17 mA h cm⁻²) at 5C. Additionally, the modified separator exhibited excellent cycling performance up to 500 cycles at 2C, with a low capacity decay rate of 0.0710% per cycle. The excellent performance of the sulfur electrode is mainly attributed to the incorporation of the SnS₂ coating layer on the separator, which effectively confines polysulfides via both chemical and physical interaction and rapidly improves lithium ion diffusion. Moreover, the SnS₂ coating layer greatly improves sulfur utilization and efficiently accelerates the kinetic conversion of trapped polysulfides.

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Conceptual insights

Controlling the shuttling effect of lithium polysulfides (LiPSs) and improving the active material utilization of sulfur electrode has been considered as an essential strategy to improve the energy density and cycle life of Li–S batteries. Tin sulfide (SnS₂) is a good conductive and polar material which can provide strong interactions with LiPSs and also has catalytic effect on the polysulfide conversion reaction. In this work, we demonstrated a simple approach using a SnS₂ modified separator for Li–S batteries, that can be used as an effective trapping agent to minimize the LiPSs shuttling and to enhance the LiPS conversion reaction. Modifying the regular separator has strongly improved the performance of Li–S batteries, as compared to the regular separator. DFT calculation has further confirmed the strong interactions of SnS₂ with LiPSs. This work provides an additional insight in the possibility of developing an efficient separator for high energy density Li–S batteries.

Introduction

Increasing demand for high-energy-density rechargeable batteries has been generated in various sectors for applications such as personal electronic devices, electric vehicles, and large-scale energy storage;^1–3 to meet this energy storage demand, several rechargeable batteries have been developed over the last few decades, including lithium ion batteries;³ flexible lithium ion batteries and sodium ion batteries.^4,5 Among such varied battery systems, the Li–S battery has attracted interest due to a high theoretical specific capacity of 1675 mA h g⁻¹ and high theoretical specific energy density of 2600 W h kg⁻¹.^6–9 Moreover, sulfur is inexpensive, abundant, and environmentally benign with low toxicity. However, the practical energy density and cycling stability of Li–S batteries are limited due to multiple issues, including the inherently poor electrical conductivity of sulfur, dissolution of intermediate LiPSs and large volumetric changes during charge–discharge cycling.⁶ To address these critical issues and improve the performance of Li–S batteries, research efforts have focused on developing composite sulfur electrodes utilizing integrated conductive carbon materials or conducting polymers^10–12 and modification of the electrode to confine lithium polysulfides by suppressing the shuttling effect.^13–15 Among these methods, the physical confinement of LiPSs within the sulfur electrode has been accomplished using various carbonaceous materials.^12,16–18 However, the cycling stability and energy storage capacity of Li–S batteries are not satisfactory for practical applications, since a nonpolar carbon-based material can only provide a weak physical interaction with LiPSs.¹¹

Various polar host metal oxides,¹⁹ ferroelectric nano particles,²⁰ and sulfiphilic metal sulfides^21–23 have also been added to the sulfur cathode to chemically trap the lithium polysulfide. In addition, other strategies have been explored for improving the performance of Li–S batteries, such as modification of the electrolytes,¹⁰ separators,^21,24,25 binders,¹⁵ and protection of the Li metal anode for control of the shuttle effect.²⁶ Recently, modified separators for use in Li–S cells have received much interest because they provide notable improvements in electrochemical performance. These modified separators can facilitate electron and ion transport and enhance electrolyte uptake. Moreover, they help to suppress polysulfide migration by trapping the dissolved polysulfides.²⁷ Conductive carbon, including ultra-lightweight MWCNT,²⁸ MWCNT@PEG,²⁹ MWCNT/SPANI,³⁰ carbonized eggshell membrane;³¹ and permselective graphene oxide membrane³² have been used to modify the separators to enhance the conductive layer and trap polysulfide. Furthermore, metal oxides such as Al₂O₃ and TiO₂ have been coated onto the polypropylene membrane separator to exploit their strong interaction with polysulfides.²⁴ Moreover, metal sulfides not only provide a strong interaction with polar LiPSs but also create enough active sites for the redox reactions due to their more conductive nature, ensuring rapid transport of electrons during the redox reaction.²³ Jeong et al. designed a bifunctional separator (MoS₂/CNT), which can significantly reduce the shuttle effect.²¹ Yao et al. reported that 2D Sb₂S₃/CNT coating on commercial separator can effectively entrap and recycle the lithium polysulfides in Li–S battery.²⁵ Therefore, utilizing metal–sulfides modified separator is an efficient strategy to improve the performance of Li–S batteries.

In an effort to understand the correlation between polysulfide adsorption capability and electrochemical performance in Li–S, a series of metal sulfides have been systematically investigated as hosts for Li–S batteries by the Yi Cui group.²² Recently, SnS₂/graphene had been proposed as an efficient sulfur host for Li–S batteries. Although such advanced cathode composite designs have shown admirable improvement in enhancing the Li–S performance by providing decent conductivity and sulfur confinement, complex and costly fabrication process of the sulfur cathode composites are not conducive to the commercial applications. In addition, high amount of trapping materials needed to effectively suppress lithium polysulfide shuttle in sulfur composite, which eventually reduces the overall energy density of Li–S battery.^21–23 It would be highly desirable to identify novel materials and advanced strategies, which can not only trap polysulfides but also increase the Li-ion conductivity to enhance the rate capability. Herein, we demonstrate an innovative and simple strategy to effectively trap polysulfides using a SnS₂-modified separator. A conceptual illustration of the SnS₂-modified separator with further elaboration of the role of each component in the electrode is provided (Scheme 1). This sulfide-modified separator enables the achievement of a battery system with high specific capacity, high areal capacity, and remarkable rate capability with significant cycle-life.

Scheme 1 Schematic illustrations of Li–S battery with SnS₂-modified separator.

Results and discussion

To confirm the phase purity and crystal structure of the synthesized materials, XRD analysis was carried out. Accordingly, Fig. 1a shows the powder XRD patterns for as-prepared SnS₂ nanosheets obtained from the hydrothermal reaction between SnCl₄·5H₂O and thioacetomide. It can be seen that no peaks are observed corresponding to Sn and S, indicating a pure phase of SnS₂ (JCPDS Card no. 23-677). All the hkl values were indexed to the hexagonal crystal phase, which is in good agreement with the reported values. Fig. 1b shows the morphology of the as-prepared SnS₂ products that are uniformly distributed nanosheets. The TEM images of SnS₂ in Fig. 1c clearly reveals the sheet like morphology of SnS₂ with an average particle size of ∼30 nm. The HR-TEM image in Fig. 1d clearly depicts the layered structure with a d-spacing spacing of ∼0.59 nm along with ∼10 layers of SnS₂. Additionally, the selected area electron diffraction (SAED) pattern in inset: Fig. 1d suggests the polycrystalline nature of SnS₂ nanosheets.³³Fig. 1e and f show SEM images of the pristine separator and SnS₂-modified separator, respectively, used to investigate the effect of the metal sulfide on the pore structures, and its morphological changes upon modification. As seen in Fig. 1e, the pristine separator exhibits a uniformly inter-connected submicron pore structure providing excellent ionic pathways and preventing the transfer of electrons between the cathode and anode. It can be clearly seen that there is a significant change in the morphology of the pristine separator compared to the modified separator. The surface of the modified Celgard separator was uniformly coated with SnS₂ and KB particles with a thickness of approximately 4 μm (ESI,† Fig. S1). The SnS₂ nanosheeets were mixed with KB particles to enhance the conductivity of the coating layer in the separator (referred as SnS₂-modified separator). After modification, a dense layer of sulfide nanosheets and KB covers the surface of the separator and the nanopores of the separator. Corresponding snapshots in the insert in Fig. 1e and f show a color change from white to black after surface coating with the SnS₂ nanosheets and KB nanoparticles. Although the modified separator shows an increased thickness and SnS₂ particles could decrease the porosity, the improved electrolyte wettability of SnS₂-modified separator than pristine separator (Fig. S2, ESI†) can overcome those negative effect. The enhanced wettability in modified separator reduces the interface resistance and enhance the transport of Li⁺, which is essential to improve the overall electrochemical performance of Li–S batteries overall electrochemical performance of Li–S batteries.³⁴

Fig. 1 (a) XRD pattern of the as-synthesized SnS₂. (b) SEM image of the as-synthesized SnS₂ nanosheets. (c) TEM image of as synthesized SnS₂ nanosheets. (d) HR-TEM image of SnS₂ nanosheets (inset is SAED pattern). (e and f) SEM image of the pristine separator and SnS₂-modified separator and their corresponding full-sized photographs (inset).

The electrochemical performance of the Li–S cells built using the pristine separator and SnS₂-modified separator is shown in Fig. 2. The sulfur cathodes are prepared simply by mixing elemental sulfur, carbon and binder, with an areal sulfur loading for the electrode of ∼3.1 mg cm⁻². In Fig. 2a, the CV curves recorded at 0.05 mV s⁻¹ show two cathodic peaks located at approximately 2.31 and 2.03 V, which are assigned to the conversion of sulfur to soluble high-order polysulfides (Li₂S_x, x ≥ 4) and the subsequent production of solid short-chain Li₂S/Li₂S₂. The peaks at 2.32 and 2.38 V result from the delithiation of low-order lithium sulfides and high order LiPSs, respectively, to form elemental sulfur.³⁴ Although the shapes of the CV curves remain the same, the difference in peak current density clearly indicates a change in capacity.

Fig. 2 (a) Cyclic voltammograms of Li–S cells with a pristine separator and a SnS₂-modified separator measured at a scan rate of 0.05 mV s⁻¹. (b) Discharge/charge in the first cycle of the Li–S cell with the pristine separator and the SnS₂-modified separator at a current rate of 0.1C. (c) Rate behaviour of the Li–S cell with the SnS₂-modified separator and pristine separator. (d and e) Galvanostatic charge–discharge profiles of the Li–S cell with the SnS₂-modified and pristine separator at varying current rates. (f) The capacity utilization rates are based on the theoretical capacity of sulfur.

With the aim of understanding the effect of the SnS₂-modified separator on the electrochemical performance of the Li–S batteries, Li–S cells were subjected to charge–discharge cycling. Fig. 2b shows the charge–discharge voltage profiles measured at a rate of 0.1C for the Li–S cells built using the pristine separator and the SnS₂-modified separator. The entire plateau for the charge–discharge processes exactly reflects the peak positions for the CV curves, which clearly indicates sulfur redox reactions between the sulfur and polysulfides. The first plateau at 2.36–2.27 V corresponds to the reduction of elemental sulfur to higher order LiPSs, while the second plateau at approximately 2.09 V is due to the conversion of higher order LiPSs to lower order LiPSs. Furthermore, the two plateaus at approximately 2.32 and 2.38 V are due to the oxidation of low order and high order LiPSs to form elemental sulfur. Interestingly, a difference in the voltage hysteresis is observed for charging and discharging at average capacity for both the pristine and modified separator cells (ΔV_{SnS2-modified separator} = 143 mV; ΔV_{Pristine separator} = 176 mV). This difference in voltage hysteresis is mainly attributed to the low polarization of the SnS₂-modified separator.²² The initial discharge capacities for the Li–S cells with a pristine and SnS₂-modified separator were estimated to be 897 and 1476 mA h g⁻¹ at 0.1C, respectively. The higher initial discharge capacity is indicative of greater active material utilization in the Li–S cell with a SnS₂-modified separator. Therefore, it is well validated that the SnS₂-modified separator facilitated efficient inhibition of polysulfide migration and the reactivation of trapped polysulfides.

To investigate the advantage of the SnS₂-modified separator and its feasibility for practical application, the rate performance of Li–S cells built using the SnS₂-modified separator and pristine separator were evaluated at different C rates range from 0.2 to 5C (Fig. 2c). The discharge capacities of the SnS₂-modified cell were found to be 1297, 1107, 965, 832 and 700 mA h g⁻¹ at C-rates of 0.2, 0.5, 1.0, 2.0 and 5.0, respectively. The discharge capacity of the SnS₂-modified separator cell decreased from 1297 to 1107 mA h g⁻¹ as the current density increased from 0.2C to 0.5C. When the cells were operated at 0.2C after being subjected to a high C-rate (5C), it was possible to recover the original capacity at 0.2C, thus demonstrating excellent rate capability and capacity retention. This performance is noticeably better than the cells with the pristine separator. The corresponding charge–discharge profile at different current rates are shown in Fig. 2d and e. The two discharge plateaus at 2.3 and 2.0 V can be still observed even at a high C-rate corresponding to the two-stage conversion reaction of sulfur to higher/lower order LiPSs in the charge–discharge profile of the cell with SnS₂-modified separator, while almost no plateaus can be noticed in that with pristine separator. In addition, the cell with SnS₂ modified separator shows significantly smaller voltage hysteresis at high rates compared to cell with pristine separator (Fig. S3, ESI†). The excellent electrochemical performance is mainly due to the incorporation of SnS₂ modified separator, which can inhibit the shuttling of LiPSs via both physical and chemical interaction. Furthermore, SnS₂ modified separator can provide fast transport path for electron and lithium ion during the charge–discharge processes, thus improving the utilization of active materials (Fig. 2f). The specific capacity obtained at 0.2, 0.5, 1, 2, and 5C corresponds to the initial capacity achieved based on the theoretical capacity of sulfur (1675 mA h g⁻¹). In contrast, the cell with pristine separator shows poor capacity utilization rate, indicating the low utilization of active material. The performance of Li–S batteries using the SnS₂-modified separator was also compared to several other batteries incorporating high-performance single modified separators or a bifunctional interlayer (Table 1). The areal capacity of the Li–S cell with the modified separator is 4.54 mA h cm⁻² at 0.1C, 4.03 mA h cm⁻² at 0.2C and 2.17 mA h cm⁻² at 5C.

Table 1 Comparison of the electrochemical performance demonstrated in this work with that obtained in previous studies involving use of a modified separator using sulfur cathodes in Li–S batteries

Modified separator	Sulfur content (%)	Sulfur loading (mg cm^−2)	Loading on the separator (mg cm^−2)	Cycle performance (mA h g^−1)	Cycles	Rate behavior (mA h g^−1)	Ref.
SnS 2 -Modified separator	70	3.1	0.6	1040 at 0.2C	150	700 at 5C	This work
				927 at 2C	500
Al2O3 coated separator	60	1.6		593 at 0.2C	50	453 at 1C	24
Acetylene black-CoS2 coated separator	77.8	1.5–2	0.5–0.7	380 at 2C	450	475 at 4C	27
MoS2 modified separator	65			401 at 0.5C	600	770 at 1C	34
Three layers of Ketjen black modified separator	70	1.5		770 at 2C	150	300 at 6C	35
				522 at 4C
MWCNT @PEG-modified separator	60	1.60	0.26	490 at 0.5C	500	657 at 5C	29
Inactive interlayer modified separator	58	2.0	0.30	644 at 0.5C	150	625 at 1C	36
Porous nitrogen and phosphorous doped graphene	70	1.5	1.0–1.1	638 at 1C	500	633 at 2C	37
BaTiO3 coated separator	60	5.0	2.4	929.5 at 0.1C	50		38
PVDF-C layer coated separator	64	1.1	0.068	669 at 0.5C	500	393 at 5C	39
MOF-based separator	70	0.6–0.8		1000 at 1C	1000	537 at 3C	40
PANiNF/MWCNT coated separator	60	1.4	0.01	1020 at 0.2C	100	612 at 1C	28
PEDOT:PSS coated separator	64			914 at 0.25C	1000	751 at 2C	41
Porous graphene coated separator	63	1.8–2.0	0.54	1165 at 0.5C	150	479 at 5C	42

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To the best of the authors’ knowledge, the SnS₂-modified separator exhibits a remarkable rate performance because of its unique polysulfide adsorption capability. For comparison, P. Zheng et al. previously reported an acetylene black (AB)–CoS₂ modified separator that showed a reasonably high rate and extended cycle life in Li–S batteries.²⁷ The AB–CoS₂ modified separator cell exhibited a specific capacity of 475 mA h g⁻¹ at 4C. Very interestingly; the present SnS₂-modified separator delivered 700 mA h g⁻¹ even at a higher C-rate (5C). The rate performance for the SnS₂-modified separator was also superior to that reported for a MoS₂-modified separator,³⁴ revealing the robust nature of the polysulfide adsorption for suppressing polysulfide migration. A SnS₂/S/C composite electrode delivered only 250 mA h g⁻¹ at 5C while the SnS₂-modified separator exhibited a much higher capacity even at the same rate, which clearly indicates a remarkable rate performance for the cell with the SnS₂-modified separator.⁴³

To determine feasibility for practical applications, cycle-life testing is considered as one of the primary data benchmarks. Fig. 3a demonstrates the cycling performance of the Li–S cells with the pristine and the SnS₂-modified separators at 0.2C. Both cells delivered initial capacities of 897 mA h g⁻¹ and 1300 mA h g⁻¹ corresponding to 54% and 78% of their theoretical capacity, respectively. After 50 cycles, the specific capacity of the cell with the SnS₂-modified separator was found to be approximately 1195 mA h g⁻¹, which is 91.9% of its initial capacity. However, the cells with the pristine separator exhibited a much lower capacity (500 mA h g⁻¹) after 50 cycles compared to the cells with the modified separator. Upon further cycling, the capacities of the pristine and SnS₂-modified separator decreased to 400 and 1050 mA h g⁻¹ respectively. The capacity of the SnS₂-modified separator stabilized at 1000 mA h g⁻¹ even after 150 cycles, indicating remarkable cycling stability due to polysulfide inhibition during charge–discharge cycling. The coulombic efficiency of the Li–S cells with the pristine separator and modified separator was 96% (after 100 cycles) and 99% (after 150 cycles), respectively. Fig. 3b shows the long-term cycling stability of the Li–S cell with the SnS₂-modified separator at a current rate of 2C. The cell with the modified electrode delivers a reversible capacity of 597 mA h g⁻¹ at the 500th cycle, with a high coulombic efficiency of 96%. The enhanced electrochemical performance of the Li–S cell with a SnS₂-modified separator suggests that the migration of LiPSs was greatly suppressed via physical and chemical interaction between the SnS₂-modified separator and the dissolved LiPSs.

Fig. 3 (a) Cycling performance of the Li–S cell with the pristine separator and SnS₂-modified separator at a current rate of 0.2C. (b) Cycling performance of the Li–S cell with the SnS₂-modified separator at a high current rate of 2C.

To understand the significant impact of the modified separator in our Li–S cells, the conductivity of the SnS₂-modified separator was measured by the four-probe test method. The electrical conductivity of the SnS₂-modified separator is 2.291 S cm⁻¹, demonstrating fast electron transfer at the interface. Additionally, ionic conductivity of the pristine and modified separators were determined from Nyquist plots of the SS/separator/SS cell.³⁴ The ionic conductivity of the pristine separator and SnS₂-modified separators were calculated to be 0.76 mS cm⁻¹ and 0.84 mS cm⁻¹, respectively (Fig. S4, ESI†). Although the SnS₂-coating layer changed the surface porosity of the pristine separator, the SnS₂-modified separator displayed higher ionic conductivity. This could be due to the enhanced electrolyte wettability of the SnS₂-modified separator. To further understand the significant impact of the modified separator in our Li–S cells, electrochemical impedance spectroscopic analysis (EIS) was carried out before and after cycling. Typical EIS data with equivalent circuit models were shown in Fig. 4 and the electrochemical impedance parameters are summarized in Table S1 (ESI†). The Nyquist plot shown in the intercept on the real axis at high frequency corresponds to the ohmic resistance (R_s) of the electrode and electrolyte. The Ohmic resistance for both pristine and SnS₂-modified separators were almost similar, indicating the same electrode and electrolyte environment. A single semicircle in the high-to-medium frequency region is related to charge-transfer resistance (R_ct), while the inclined line at low frequency region is associated with Warburg impedance (W_s), and constant phase elements (CPE) corresponds to double layer capacitance. As shown, R_ct values of the cell with SnS₂-modified separator (23.74 Ω) is lower than that for the pristine separator (45.45 Ω), indicating an enhanced the active material utilization to deliver a high capacity. Furthermore, the variation in charge transfer resistance clearly confirms that SnS₂-modified separator has more conductive and stable structure than pristine separator, resulting in superior electrochemical performance in the Li–S cell. Impedance data were also collected after cycling (Fig. 4b) to identify any changes in charge transfer resistance. As expected, the charge transfer resistance of the cell with SnS₂-modified separator showed a lower R_ct value (9.341 Ω) than the cell with pristine separator (28.6 Ω), which is mainly due to the high electrical conductivity and improved electrolyte wettability between the separator and sulfur cathode. The above results confirmed the benefits of SnS₂-modified separator in improving electrochemical performance Li–S cell.³⁵

Fig. 4 Electrochemical impedance spectra of Li–S batteries with a pristine separator and a SnS₂-modified separator (a) fresh cell (b) after 150 cycles at a rate of 0.2C.

To understand the lithium polysulfide interaction with SnS₂, an adsorption ability test was carried out. For the adsorption test, a 1 mM Li₂S₈ solution was prepared by chemically reacting elemental sulfur with Li₂S in a DOL/DME (1 : 1 by volume) solution. The Li₂S₈ solution was initially yellow in color. The addition of SnS₂ to the Li₂S₈ solution resulted in a concomitant color change from yellow to colorless (Fig. 5a). This strongly suggests that SnS₂ was strongly bound with the polysulfides.²³ The polysulfide interaction with SnS₂ is clearly reflected in the cycling ability (Fig. 3a and b). The UV-Vis spectra (Fig. 5b) for a bare Li₂S₈ solution showed a peak at approximately 620 nm (75% transmittance), which is characteristic of S₃˙⁻, resulting from the dissociation of S₆²⁻ (S₆²⁻ → 2S₃˙⁻). Moreover, the transmittance values decreased to 0% in the wavelength range of 400–500 nm, which indicates the existence of higher order polysulfides (S₈²⁻, S₆²⁻, S₄²⁻), respectively.³⁵ With the addition of SnS₂, the characteristic peaks in the wavelength range of 400–500 (∼90% transmittance) and 620 nm (∼98% transmittance) completely disappeared, which demonstrates the strong adsorption of polysulfides within our designed system.

Fig. 5 (a) Photographs of Li₂S₈ in DOL/DME solution and after adding SnS₂ powder into the Li₂S₈ solution. (b) UV/Visible spectra for the lithium polysulfide solution (Li₂S₈) before and after the addition of the SnS₂ powder. (c) X-ray photoelectron spectroscopy showing the 3d peaks for Sn before and after discharge. (d) Raman spectra measured of SnS₂ before and after cycling.

Furthermore, the interaction between LiPSs and SnS₂ was also investigated by XPS and Raman analysis. The SnS₂-modified separator (before and after discharge state) was used for the XPS analysis. The Sn 3d spectra of the SnS₂-modified separator (before discharging) are shown in Fig. 5c; the two peaks located at 487.34 and 495.84 eV, with an energy separation of 8.5 eV, are attributed to the Sn 3d_5/2 and Sn 3d_3/2 spin orbit levels of SnS₂.⁴⁴ After discharging, both Sn 3d peaks shift by approximately 0.63 to 0.79 eV towards a lower binding energy, confirming the strong adsorption capability for Sn–Sn²⁻ with LiPSs. Fig. 5d shows the Raman spectra measured for the SnS₂-modified separator (before and after cycling). The SnS₂-modified separator (before cycling) shows a strong peak at ∼313 cm⁻¹, which corresponds to the first-order, in-plane vibrational A_1g mode of SnS₂. The characteristic A_1g peak of SnS₂ at 313 cm⁻¹ shows a positive shift (after cycling), indicating polysulfide trapping in SnS₂ by providing electrons to surrounding LiPSs.⁴⁵ These results further verify the strong binding between the SnS₂ and polysulfide.

To further investigate the binding nature of the polysulfide on SnS₂, DFT calculations were carried out. In this study, graphene was chosen for comparison due to commonality in use as a conductive coating material in Li–S batteries. We investigated the adsorption of Li₂S_n species (n = 4, 6 and 8) on graphene and SnS₂. Since the chemical interaction mainly arises due to lithium ions,⁴⁶ we considered both 1- and 2-fold adsorption geometries for each case. The most stable adsorption configurations on graphene and SnS₂ substrates are shown in Fig. 6. Except for Li₂S₄ adsorption on SnS₂, 1-fold adsorption is preferred for all other cases. Table 2 lists the adsorption energies. The Li₂S_n adsorption energy on graphene is only approximately 0.1 eV, but that on SnS₂ is in the range of −0.2–−0.5 eV. Clearly, the adsorption of polysulfide on SnS₂ is significantly enhanced, especially for the Li₂S₄, which is stabilized by −0.5 eV compared to adsorption on graphene. This difference in the binding nature between SnS₂ and graphene can be easily explained by calculating the charge transfer between the two materials. Bader analysis^47,48 shows that 0.98e, 0.31e, 0.20e of charge is transferred from Li₂S₄, Li₂S₆, and Li₂S₈ to the SnS₂ substrate, respectively, but negligible charge transfer occurs during adsorption on graphene. Therefore, a strong chemical interaction exists between polysulfide and SnS₂, but only physical interaction, such as via the van der Waals force, dominates in the Li₂S_n–graphene system.

Fig. 6 Conformation for Li₂S_n species (n = 4, 6, 8) adsorption on SnS₂ (a–c) and graphene (d–f). The red, gray, and turquoise balls represent the Li, C, and Sn atoms and the yellow and orange balls symbolize S in the SnS₂ and Li₂S_n species, respectively.

Table 2 Calculated adsorption energy of Li₂S_n species on SnS₂ and graphene substrates

Model cluster	SnS2 [eV]	Graphene [eV]	Δ [eV]
Li2S4	−0.50	−0.14	−0.36
Li2S6	−0.19	−0.10	−0.09
Li2S8	−0.23	−0.11	−0.12

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To estimate the Li-ion diffusion coefficient (D_Li⁺) for the pristine and SnS₂-modified separators, a series of cyclic voltammograms at different scan rates were recorded. As indicated in Fig. 7, cathodic peaks are identified at approximately 2.3 V and 2.03 V, which are labeled A and B, respectively, corresponding to the reduction in elemental sulfur to long-order polysulfides (Li₂S_x, x ≥ 4) and the subsequent formation of short-order polysulfides (Li₂S₂/Li₂S). The anodic peaks at approximately 2.4 V, labeled C, result from the transition of Li₂S₂/Li₂S to polysulfides and elemental sulfur. The Randles–Sevick equation was used to obtain values for D_Li⁺, as given below.³²

I_p = 2.69 × 10⁵n^1.5AD_Li^+0.5C_Liν^0.5

where D_Li⁺ is the lithium ion diffusion coefficient (cm² s⁻¹), I_p is the peak current in amperes (A), n is the number of electrons involved in the reaction (n = 2 for Li–S battery), A is the area of the electrode (cm²), C_Li refers to the lithium ion concentration (mol L⁻¹) and ν is the scanning rate (V s⁻¹).

Fig. 7 Cyclic voltammograms measured at different scan rates and corresponding linear fits to the peak current of the Li–S cell. (a and b) With the pristine separator, (c and d) with the SnS₂-modified separator.

As shown in Fig. 7a–d, the linear relationship between the peak current (I_p) and square root of the scan rate (ν^0.5) indicates a diffusion controlled process. The diffusion coefficients (D_Li⁺) for the Li–S batteries with the pristine separator and SnS₂-modified separator are compiled in Table S2 (ESI†). The diffusion coefficients were calculated to be D_Li⁺ (A) = 3.176 × 10⁻⁸ cm² s⁻¹, D_Li⁺ (B) = 1.792 × 10⁻⁸ cm² s⁻¹, and D_Li⁺ = 1.499 × 10⁻⁷ cm² s⁻¹ for the Li–S cell with the pristine separator. For the SnS₂-modified separator, the calculated diffusion coefficient values were estimated to be D_Li⁺ (A) = 3.272 × 10⁻⁷ cm² s⁻¹, D_Li⁺ = 2.22 × 10⁻⁷ cm² s⁻¹, and D_Li⁺ = 9.811 × 10⁻⁷ cm² s⁻¹. It is clearly observed that the Li–S cell with the SnS₂-modified separator exhibits higher lithium ion diffusion coefficient values compared to the Li–S cell with the pristine separator, indicating that the incorporation of the SnS₂ host enables a highly efficient catalyzing conversion of the polysulfide redox.³⁴

To better understand the morphological changes in the separator before and after cycling, a postmortem analysis was carried out for the pristine, modified separator and Li anode after 150 cycles at 0.2C. Fig. 8a shows an SEM image for the pristine separator after cycling, which reveals that the pores in the pristine separator were completely blocked by the agglomeration of sulfur-related species. This blocking can retard the lithium ion transport, thus, resulting in poor electrochemical performance.^49–51 Compared to the morphology of the fresh SnS₂-modified separator in Fig. 1f, the morphology of the SnS₂-modified separator after cycling (Fig. 8b and Fig. S5, ESI†) shows deposition of sulfur species on the SnS₂ nanosheets, indicating that the SnS₂-modified separator acts as a matrix to trap and suppress polysulfide migration. An uneven passivation layer with cracks was observed on the Li anode surface of the Li–S cell with the pristine separator due to the side reaction between the migrated sulfur species and metallic Li (Fig. 8c). In contrast, the Li–S cell with the SnS₂ treated separator showed a much smoother Li metal anode surface, as shown in Fig. 8d, due to suppression of the LiPSs shuttle effect and effective conversion of the sulfur redox realized by the SnS₂. Further, The cells with pristine and separator were dissembled after cycling to find evident for LiPSs trapping (Fig. S6, ESI†). LiPSs deposition over lithium metal anode is observed for the cell with pristine separator, whereas no such deposition is noted with SnS₂-modified separator.

Fig. 8 SEM images of the separators after 150 cycles at 0.2C (a) pristine, (b) SnS₂-modified separator, and (c and d) corresponding SEM images on lithium metal anode surface.

Conclusion

In summary, we have successfully introduced a SnS₂-modified Celgard separator for Li–S batteries. This modified separator provides a highly efficient method for suppressing shuttling of LiPSs, leading to high Coulombic efficiency and long cycling stability. The Li–S cell with a SnS₂-modified separator achieved an excellent rate performance of 700 mA h g⁻¹ at 5C, with high areal capacities. The specially coated SnS₂ modified separator not only captured polysulfides efficiently by a strong chemical and physical interaction but also ensured the rapid diffusion of lithium ions. Moreover, the SnS₂ coating can potentially serve as an excellent current collector for facilitating electron and ion transport; thus, enhancing the utilization of sulfur and ensuring reactivation of the trapped active material. The approach demonstrated in this work provides an additional insight into the development of similar strategies for high-energy-density and stable Li–S batteries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project was supported by the National Research Foundation of Korea (NRF) (MSIP) (No. 2017R1A2B2010148) and the Climate Change Research Hub of EEWS from KAIST (Grant No. N11180108). P. R. is grateful to the Korean Federation of Science and Technology Societies for financial support through the Brain Pool Program and Dr V. K. Pillai, Director, CSIR-CECRI, for his support and granting him the sabbatical leave.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nh00172c

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