Synergetic Effect of Block and Catalysis on Polysulfides by Functionalized Bilayer Modification on the Separator for Lithium-Sulfur Batteries

: Lithium-sulfur (Li-S) batteries have been regarded as one of the most 21 promising candidates for the next generation energy storage devices owing to high 22 energy density and low-cost characteristics. However, one crucial problem hindering 23 their practical application is the ceaseless shuttle of soluble lithium polysulfides (LiPSs) 24 between cathode and anode, which always leads to rapid capacity fade and serious self-25 discharge. Herein, a unique bilayer coating strategy designed to modify the 26 polypropylene (PP) separator was developed in this study, which consisted of a bottom 27 zeolite (SSZ-13) layer serving as LiPSs movement barrier, and a top ZnS layer used for 28 accelerating redox process of LiPSs. Benefitted from their synergetic effect, the bilayer 29 modified separator offers absolute block effect to LiPSs diffusion and moreover, 30 significant catalysis effect on sulfur species conversion, as well as outstanding Li + 31 conductivity, excellent electrolyte wettability and desirable mechanical property. 32 Consequently, the Li-S battery assembled with the SSZ-13/ZnS@PP separator 33 demonstrates excellent cycle stability and rate capability, showcasing a capacity decay 34 rate of only 0.052% per cycle at 1 C over 500 cycles.


Introduction
Lithium-ion batteries (LIBs) have played a significant role in facilitating the rapid growth of high-performing mobile devices and electric vehicles [1,2].However, further increase in energy density is still necessary due to the industry requirements.In this case, lithium-sulfur (Li-S) batteries are considered as one of the most promising nextgeneration energy storage systems, on account of their high specific capacity (1675 mAh g -1 ) and mass energy density (2600 Wh kg -1 ) [3,4].Nevertheless, different from www.energymaterj.com
These soluble LiPSs can migrate to the Li anode and deposit on its surface as solid Li2S2/Li2S, which further produces the notorious "shuttle effect" [8][9][10][11].Meanwhile, accompanied by the process of iterative Li plating/stripping, Li dendrites grow on the anode, causing increased polarization and even security incidents [12,13].
Massive strategies for settling obstinate problems mentioned above have been proposed such as electrolyte optimization [14,15], functional sulfur host [16,17] and separator coating [18].Among them, separators are advantageous platforms to address critical issues such as dendrite proliferation, "shuttle effect" and interfacial stability.At the same time, the modification of commercial separators is one of the preferred routes used nowadays due to its simplicity and effectiveness.Currently, researches of the surface coating on the separators concentrate on functional polymers [19][20][21][22][23] and other compounds with unique polar groups [24,25].These candidates can chemically interact with LiPSs meanwhile accelerate their redox reaction, however, always exhibited insufficient shuttle suppression and catalytic activity, particularly when the sulfur loading on the cathode is high.Based on above consideration, achievement of the synergetic effect of completely blocking the "shuttle effect" as well as effectively catalyzing LiPSs conversion still remains to be challenging.

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can totally block LiPSs on the cathode side without impacting the Li + transport and hence adequately restrain "shuttle effect".This is because the calculated S-S bond lengths of LiPSs (i.e.Li2S4, Li2S6 and Li2S8) range from 2.043 Å to 2.218 Å and the Li-S bond length is close to 2.255~2.389Å, which are well in excess of the pore size of SSZ-13 zeolite (0.38 nm) in any angle [34].Meanwhile, the diameter of Li + is about 0.076 nm [35] so that the porosity of SSZ-13 is able to satisfy the requests of transporting Li + and blocking LiPSs.On the other hand, we further modified the separator on the basis of the zeolite layer with ZnS, which is considered to be a good catalyst that promotes the adsorption and conversion on the LiPSs [36], eliminating the risk of accumulated LiPSs blocking the channels of the separator.Benefiting from the synergetic effect of block and catalysis provided by the SSZ-13/ZnS bilayer modifier on the separator (as shown in Fig. 1a), as a consequence, Li-S cell reaches to a distinguished initial specific capacity of 1364.22 mAh g -1 at 0.2 C, and an average capacity attenuation of only 0.052% per cycle at 1 C over 500 cycles, which is much superior to other cells with monolayer-modified and pristine separators.Even at a high sulfur loading of 5.2 mg cm −2 , the cell with bilayer modification still accomplishes brilliant cycle stability.

Modified Separator Fabrications
The ZnS powder was synthesized by a simple hydrothermal method [37], where CH4N2S (99%, Innochem) and Zn(Ac)2•2H2O (99%, Innochem) were mixed uniformly in ethylene glycol with the molar ratio equal to 1:1 under continuous stirring.Then the obtained solution was transferred and sealed in a Teflon-lined stainless autoclave and www.energymaterj.comEnergy Materials heated at 180 ℃ for 24 h.Finally, the product was cleaned with distilled water and dried at 60 ℃.
The SSZ-13 (Si/Al molar ratio of 25~30, Zhizhen), conductive carbon (Super P) and polyvinylidene fluoride (PVDF) were mixed in the 1-methyl-2-pyrrolidinone (NMP) with a mass ratio of 8:1:1.The obtained SSZ-13 slurry was loaded on a piece of commercialized PP separator (Celgard 2400) by vacuum filtrating and then drying at 50 ℃ overnight to produce the SSZ-13@PP separator.The same method was used to prepare the ZnS slurry.The obtained ZnS slurry was coated on the surface of pristine separator and SSZ-13@PP separator to fabricate ZnS@PP and SSZ-13/ZnS@PP modified separators, respectively.

Characterizations
The microcosmic morphologies of the materials and separators were observed by a Tescan Mira4 scanning electron microscope (SEM) equipped with an energy disperse spectrum (EDS) microanalyzer.The powder X-ray diffraction (XRD) measurement was conducted on a Rigaku Ultima IV X-ray diffractometer with Cu-Kα radiation in a range of 3-40°.The pore structure of the zeolite was measured on a ASAP 2460 analyzer by N2 adsorption-desorption experiment.The UV-Vis spectrum was recorded on a Shimadzu UV-3600 instrument.

Electrochemical Measurements
The sulfur cathode consisted of sulfur/mesoporous carbon (CMK-3, XF Nano) composite (prepared via melt-infiltration strategy at 155 ℃ for 12 h), Super P and PVDF with a mass ratio of 8:1:1.The coin-type Li-S cell was assembled in an argonor format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
The Li-S cell was cycled galvanostaticly on a Neware tester in the voltage range of 1.7~2.8V.The electrochemical workstation (PARSTAT MC) was used for the measurements of cyclic voltammetry (CV) at different scan rates and electrochemical impedance spectrum (EIS) in the frequency range from 10 5 to 0.01 Hz at perturbation amplitude of 5 mV.
For redox kinetic study, a symmetrical cell was employed with two identical electrodes in 0.05 M Li2S6/DME/DOL solution.The electrode was fabricated by mixing host material, Super P and PVDF in the NMP with a weight ratio of 8:1:1 and then the obtained slurry was coated onto the Al foil.CV measurement was performed at a scan rate of 10 mV s -1 in the voltage range of -0.8~0.8V [38].The galvanostatic intermittent titration technique (GITT) was conducted at a current density of 0.1 C for 15 min with the following rest for 2 h in the voltage range of 1.7~2.8V.The Li-S cell was charged to 2.35 V at 0.2 C and then switched to potentiostatic mode to monitor the shuttle current.

Results and Discussion
The XRD pattern of as-prepared ZnS was provided in Fig. 1b.There are three characteristic peaks located at 28.9°, 48.1° and 57.1°, corresponding to (111), ( 220

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with an average diameter of ~0.6 μm.The EDS elemental mappings in Fig. 1c demonstrate the uniform distribution of S and Zn elements in the spherical particles.
These are all in line with expectations.For quantitative analysis of pores structure of commercialized SSZ-13, the N2 absorption-desorption measurement was carried out.Fig. 1d shows that pore sizes of the SSZ-13 are mainly centered around 0.38 nm.
Additionally, the embedded SEM image reveals that SSZ-13 is composed of uniform spherical units with diameters ranging from 0.27 to 0.45 μm.The EDS mapping in  The above raw materials were coated on the pristine PP separator to create functionalized surface modification.Fig. 2a shows the SEM image of the pristine PP separator, in which the micron-sized pores can be observed.These pores provide convenient channels for the electrolyte penetration and Li ion movement.Nevertheless, they also allow LiPSs to pass through freely, leading to metal corrosion once they get www.energymaterj.com

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in touch with the Li anode.As contrast, Fig. 2b~2c provide the top and cross-sectional images of SSZ-13/ZnS@PP separator, respectively.It can be seen that the pristine separator is uniformly covered by the SSZ-13 and ZnS particles with scarce pores, fabricating a continuous and flat coating layer with the total thickness of approximately 9 μm.Moreover, the EDS elemental mappings in Fig. 2d distinctly exhibits that the elements of SSZ-13 (Al, Si) and ZnS (Zn, S) are separated into two layers, revealing an intact bilayer architecture on the pristine separator (The strong Al signal at the bottom is mainly attributed to the aluminum object platform).It needs to be emphasized that the monolayer-modified SSZ-13@PP and ZnS@PP separators are also dense and uniform without noticeable cracks, as shown in Fig. S1a~1b in Supporting Information.
The excellent mechanical property of the SSZ-13/ZnS@PP separator can be confirmed in Fig. 2e since it still remains intact after being folded several times, indicating ideal integration of the bilayer coating with the pristine separator substrate.
In addition, the contact angle test was carried out to evaluate the wettability of liquid electrolyte toward the modified separator.As depicted in Fig. 2f, the SSZ-13/ZnS@PP separator offers a lower contact angle of 12° in contrast to the pristine separator (26°).
The result suggests that the functionalized SSZ-13/ZnS modification is capable of facilitating the rapid electrolyte penetration and hence Li ion transport.Aiming at exploring the barrier capability of the SSZ-13/ZnS@PP separator, Hshaped glass device was applied to compare the permeability of LiPSs across the separator.As shown in Fig. 3a, 5 mL Li2S6 (25 mM) solution (left chamber) and 5 mL DOL/DME mixed solvent (right chamber) were separated by various separators.For the device composed of PP separator, the solution color becomes yellow after 12 h and turns brown at last, indicating a free Li2S6 permeation.For ZnS@PP case, the permeation did not be stopped, but the diffusion concentration decreases significantly owing to the chemisorption of ZnS on LiPSs.This can be confirmed in the UV-Vis spectrum in Fig. 3b, as the intensity of S6 2-peak decreases prominently and the solution color becomes lighter after the ZnS powder was added into the LiPSs solution.When SSZ-13@PP and SSZ-13/ZnS@PP separators were employed, the solutions in the right chamber of H-shaped device all remain colorless after 24 hours, indicating the exceptional physical blocking capability of SSZ-13 towards LiPSs.
DFT calculation was further implemented to reveal the block effect from the SSZ-13.The molecular structures of various LiPSs are provided in Fig. 3d.The monomer units of Li2S6 and Li2S8 molecules display varying S-S bond lengths ranging from 2.043 Å to 2.218 Å, in the meanwhile, the bond length is close to 2.113 Å in the Li2S4 molecule which is the typical S-S bond length of polysulfide ions (Sn 2-).Furthermore, the Li-S bond length in these species is close to 2.255~2.389Å.It is summarized that the sizes of LiPSs molecules (i.e.Li2S4, Li2S6 and Li2S8) are all well in excess of the pore size of SSZ-13 zeolite (3.8 Å) in any angle so that LiPSs diffusion could be blocked.The above results clearly indicate that the ZnS and SSZ-13 plays the distinctive role of chemical www.energymaterj.com

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interaction and physical barrier on LiPSs, respectively, which guarantees the synergetic function of the modified separator.
In addition, the calculated diameter of Li + is about 0.076 nm so that the porosity of SSZ-13 is able to satisfy the requests of transporting Li + .To affirm it, the bulk resistance extracted from the Nyquist plot of a symmetrical stainless-steel cell was used to calculate ionic conductivity of various separators according to the inserted equation in Fig. 3c.It can be seen that the ionic conductivity of SSZ-13/ZnS modified separator (1.14 mS cm -1 ) is higher than that of the pristine one, probably attributed to better wettability (as confirmed in Fig. 2f) and facilitated Li + transport pathway provided by the homogeneous pore size and rich pore structure of SSZ-13.As a solution, the dense SSZ-13 zeolite which was fabricated adhere to commercial PP separator plays a critical role in sieving LiPSs and Li ions.The redox kinetic behaviors of the restrained LiPSs by the functionalized modification on the separator were further investigated via various CV measurements.
The CV curves of the symmetrical cell in Fig. 4a show that the cell employing ZnS electrode has higher current response than other cases, revealing the latent catalytic activity of ZnS for accelerating the redox kinetics of those adsorbed LiPSs.The CV curves of the Li-S cells with modified and pristine separators at various scan rates are provided in Fig. 4b and Fig. 4c, respectively.The observed shifts of cathodic and anodic peaks (Oa, Ra and Rb) [39] stand for the increased polarization at high scan rate.The Liion diffusion coefficient (DLi + ) as a key parameter for redox kinetics to evaluate the diffusion characteristics of Li-ions, can be calculated according to the Randles-Sevcik equation below [40,41]  www.energymaterj.com

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exhibit a positive shift with significantly higher peak currents relative to other counterparts, verifying that the bilayer modification are conducive to the reversible conversion and then adequate utilization of sulfur species.The Tafel slope is further calculated derived from the oxidation peak located at ~2.39 V and reduction peak located at ~2.03 V.The cell with SSZ-13/ZnS@PP separator shows lower Tafel slope value of 56 mV dec -1 for the oxidation from Li2S to Li2Sx (Fig. 4h) and 38 mV dec -1 for the reduction from Li2Sx to Li2S (Fig. 4i), compared to the PP case, suggesting the accelerated redox kinetics owing to the bidirectional catalysis capability of the synergetic structured coating [42].
The geometrical configuration of the minimum energy path for Li2S4 migration on the facet of ZnS was investigated by density functional theory (DFT) calculation (For brevity, Li2S4 was used as the prototype for modeling).As provided in Fig. 4j~4k, the migration barrier for Li2S4 on the corresponding surface of ZnS was only 1.42 eV.This benefits the rapid diffusion of Li2S4 to the nearby conductive area (e.g., SP particle in the coating) [43] and then electrochemical conversion.Thus far, above results well support the notion that the facilitated entrapping-diffusion-conversion process of LiPSs can be realized mainly profited from the introduction of ZnS.The GITT test was also applied to study the effect of the functional coating on the conversion reaction process of the Li-S system.As shown in Fig. 5a~5b, compared to PP sample, the cell with SSZ-13/ZnS@PP separator displays smaller IR drops of QOCV (quasi-open-circuit voltage, 25.9 mV) and CCV (close-circuit voltage, 32.9 mV), suggesting the enhanced redox kinetics [44,45].At the same time, the S8 dissolution process of the SSZ-13/ZnS@PP cell and PP cell accounts for approximately 34% (8.93 or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Energy Materials h/25.9 h) and 37% (6.73 h/18.2 h) of the discharge phase, respectively.The relatively smaller proportion of the modified separator can be attributed to higher efficiency for Li2S nucleation [46,47].
The Li-S full cell was assembled for investigating the actual contribution of the various modified coatings on the electrochemical performance.The discharge curves of these cells all deliver two well-defined potential plateaus, exhibited in Fig. 5c.
However, the larger capacity at lower potential plateau (QL) and higher QL/QH value in the SSZ-13/ZnS@PP cell manifest favorable conversion of LiPSs to insoluble sulfides [48,49].A smaller polarization can also be observed with SSZ-13/ZnS@PP separator, indicating enhanced electrochemical reversibility.The SSZ-13/ZnS@PP cell produces a high initial specific capacity (1364.22mAh g −1 ) at 0.2 C, for comparison, the initial capacities of SSZ-13@PP, ZnS@PP and pristine cell decrease to 1085.25, 999.07 and 844.59 mAh g −1 , respectively.Furthermore, the highest reversible capacity of 992.28 mAh g −1 can be maintained in the SSZ-13/ZnS@PP cell after 140 cycles (Fig. 5d), accompanied by stable charge/discharge plateau during cycling (Fig. S3).It should be noted that the the shuttle current of the cell with SSZ-13/ZnS@PP separator shown in Fig. 5e is about 0.38 mA, which is apparently lower than that of PP separator (≈0.60 mA).This proves the synergetic block and catalysis effect on LiPSs brought by the bilayer SSZ-13/ZnS modification, effectively restraining the "shuttle effect" and hence improving the cell performance.The powerful restriction on the redox shuttle further contributes to the homogeneous deposition of metallic Li and prevents the dendrite growth.It can be noticed in Fig. S4 that the cycled Li with PP separator becomes rough with lots of cracks and particles.On the contrary, the Li surface with SSZ-13/ZnS@PP separator after cycling is apparently smooth without visible bulk aggregations.
or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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The rate tests of Li-S cells with different separators were conducted at current density ranging from 0.1 C to 2 C. The initial specific capacity of the cell with SSZ-13/ZnS@PP separator reaches 1383.47 mAh g -1 at 0.1 C (~82.7 % of the theoretical limit) and still sustains 904.36 mAh g -1 when the current density was raised to 2 C, all far beyond those with pristine and monolayer-coated separators (Fig. 5f).Additionally, the increased polarization at high rates is negligible (Fig. S5) and the recoverability of the cell capacity is proven when the discharge current returned to 0.2 C. EIS measurement of Li-S cells was used to evaluate the difference in charge transfer process at open circuit voltage, where the semicircle in the high-frequency region stands for the charge resistance (Rct).As exhibited in Fig. S6, the SSZ-13/ZnS@PP cell delivers lower Rct value (48.0 Ω) in comparison with the PP (107.9Ω), SSZ-13@PP (76.6 Ω) and ZnS@PP (82.8 Ω) cells, which indicates an enhanced charge transfer kinetics brought by the bilayer modification on the separator.The merit combined with high ionic conductivity of the modified separator (as mentioned above) together contributes to the outstanding rate capability of the cell.
Besides, the long-term cycle performances of Li-S cells at 1 C are exhibited in Fig. 5g.The SSZ-13/ZnS@PP cell exhibits a discharge capacity of 885.4 mAh g −1 and maintains at 655 mAh g -1 after 500 cycles with a capacity attenuation of only 0.052% per cycle.Subsequently, the acceptable cycling stability of the Li-S cell with SSZ-13/ZnS@PP separator under high sulfur loading of ~5.2 mg cm −2 with average coulombic efficiency of ~99.2% was demonstrated in Fig. 5h, demonstrating the great potential of the SSZ-13/ZnS@PP separator for practical applications.

Conclusions
In summary, we have successfully engineered a bilayer modification on the separator for Li-S cells by a facile suction filtration strategy, which builds up an excellent synergetic relationship between inhibiting the LiPSs diffusion and catalyzing their conversion.The experimental studies combined with DFT calculations reveal that the appropriate pore size of SSZ-13 zeolite physically blocks the movement of the LiPSs towards Li anode but ensuring free and rapid transport of Li-ions.Additionally, the introduced ZnS coating chemically interacts and then accelerates the redox kinetics of the confined sulfur species by providing a low surface migration barrier.
) and (311) lattice planes of ZnS (JCPDS No. 80-0020), respectively, indicating successful preparation of the material.The inserted SEM image reveals a clear sphere morphology www.energymaterj.com

Fig.1e displays
Fig.1e displays homogeneous distribution of Al and Si elements in the SSZ-13.

Figure 1 .
Figure 1.(a) Structure and function illustration of the SSZ-13/ZnS modified separator.(b) XRD pattern (inset is SEM image) and (c) EDS elemental mappings of the ZnS.(d) Pore size distribution (inset is SEM image) and (e) EDS elemental mappings of the SSZ-13.

Figure 3 .
Figure 3. (a) LiPSs diffusion tests in the H-shaped cells with different separators.(b) UV-Vis spectrum of LiPSs solution before and after being adsorbed by the ZnS.(c) The calculated ionic conductivity for different separators.(d) Molecular structures of various LiPSs monomer units. :

Where
Fig.4gdisplays the CV curves of Li-S cells with different separators at a scan rate of 0.1 mV s −1 .It is remarkably that the reduction peaks for SSZ-13/ZnS@PP separator

(Figure 4 .
Figure 4. (a) CV curves of the symmetrical cells using different electrodes.CV curves of Li-S cells with (b) SSZ-13/ZnS@PP and (c) PP separator at various scan rates.Linear relationship of Ip-v 0.5 for (d) peak Oa, (e) peak Ra and (f) peak Rb.(g) CV curves of Li-S cells with various separators at a scan rate of 0.1 mV s -1 .Tafel slopes derived from the (h) oxidation peak and (i) reduction peak.(j) Energy profile for Li2S4 migration along different adsorption sites on the ZnS facet.(k) Initial state, transition state and final state of Li2S4 migration on the ZnS {220} facet.

(
Figure 5. GITT measurements of the Li-S cells with (a) PP separator and (b) SSZ-13/ZnS@PP separator.(c) Comparison of charge/discharge profiles for different separators at 0.2 C. (d) Cycle stability of the Li-S cells with various separators at 0.2 C. (e) Shuttle currents of the Li-S cells with different separators.(f) Rate capabilities of the Li-S cells.(g) Long-term cycling performance of the Li-S cells at 1 C. (h) Cycle performance of the Li-S cell with SSZ-13/ZnS@PP at the sulfur loading of 5.2 mg cm −2 at 0.2 C.

Figure S3 .
Figure S3.Charge/discharge profiles of the Li-S cells with SSZ-13/ZnS@PP separator during cycling at 0.2C.

Figure S5 .
Figure S5.Charge/discharge profiles of the Li-S cells with SSZ-13/ZnS@PP separator at different discharge rates.

Figure S6 .Highlights 1 .
Figure S6.EIS plots of Li-S cells with various separators at open circuit voltage.