Synergetic effect of block and catalysis on polysulfides by functionalized bilayer modification on the separator for lithium-sulfur batteries

INTRODUCTION

Lithium-ion batteries (LIBs) have significantly boosted rapid growth of high-performing mobile devices and electric vehicles^[1]. However, further increase in energy density remains necessary for the industry requirements. In this case, lithium-sulfur (Li-S) batteries are considered as one of the most attractive energy storage devices on account of a high specific capacity (1,675 mAh g^-1) and mass energy density (2,600 Wh kg^-1)^[2,3]. Nevertheless, different from the intercalation mechanism in LIBs, Li-S chemistry undergoes a complex solid-liquid-solid conversion procedure^[4,5], generating long-chain lithium polysulfides (LiPSs, Li₂S_n, 4 ≤ n ≤ 8) during the cell operation. These soluble LiPSs can migrate to the lithium anode and deposit on its surface as solid Li₂S₂/Li₂S by corrosion reaction, further producing the notorious “shuttle effect”^[6]. Meanwhile, accompanied by iterative lithium plating/stripping, lithium dendrites grow on the anode, increasing polarization and even security incidents^[7,8].

Massive strategies for settling obstinate problems mentioned above have been proposed such as electrolyte optimization^[9], sulfur host design^[10] and separator coating^[11]. Among them, separators are advantageous platforms to address critical issues such as dendrite proliferation, “shuttle effect”, and interfacial instability. At the same time, modifying commercial separators is one of the preferred routes due to its simplicity and effectiveness. Currently, research of the surface coating on the separators concentrates on various functional polymers^[12,13] or polar compounds^[14,15]. For instance, Chen et al. developed a Fe,N co-doped mesoporous carbon sphere (Fe-N-MCS) as a coating material of polypropylene (PP) separators, which not only created a physical block to restrain LiPSs but also contributed to abundant adsorption sites for LiPS conversion^[16]. Yu et al. prepared hexagonal MnO nanoflakes with rich oxygen vacancies incorporated with N-doped carbon nanotubes (NCNTs) to modify the separator. The NCNTs served as a physical barrier and conductive matrix, and MnO provided chemical adsorption and catalytic capability towards LiPSs^[17]. Most of these candidates can physically/chemically interact with LiPSs while accelerating their redox reaction; however, they always exhibited insufficient shuttle restriction and catalytic activity, particularly when the sulfur loading on the cathode is high. Based on the above consideration, achieving the synergetic effect of completely suppressing the “shuttle effect” and effectively catalyzing LiPS conversion still remains challenging.

Zeolite molecular sieve is a kind of aluminosilicate with very regular nanoscale pore sizes between 0.3 and 1 nm^[18,19], high surface area, conspicuous mechanical strength, and superior thermal stability^[20,21]. The zeolite with unique internal channels is promising to totally block LiPSs on the cathode side without affecting the Li⁺ transport as a functionalized coating on the separator, consequently adequately restraining the “shuttle effect”. Meanwhile, polar metal sulfides (e.g., ZnS, CoS₃, and MoS₂)^[22-24] have been extensively studied in Li-S batteries as excellent catalysts for sulfur reaction owing to their strong adsorption capability and catalytic activity. On the one hand, they are highly sulfurophilic, providing good chemical affinity/compatibility for the sulfur species. On the other hand, the sulfur atoms in metal sulfides possess high electronegativity, hence capturing electrons from the metals and offering active sites to stabilize the intermediates^[25]. In addition, these sulfides generally exhibit better electrical conductivity than corresponding metal oxides, preferable for the redox reaction of sulfur species. Therefore, various metal sulfides can be employed on the separator to achieve strong adsorption and rapid conversion of the LiPSs^[26] and eliminate the risk of accumulated LiPSs blocking the channels of the separator.

Herein, we propose a novel bilayer modification on the separator via a facile coating method, which mainly consists of a bottom zeolite layer and a top ZnS layer. Benefiting from the synergetic effect of block and catalysis provided by the bilayer modifier mentioned above [Figure 1A], the Li-S cell reaches a distinguished initial specific capacity of 1,364.22 mAh g^-1 at 0.2 C, and an average capacity attenuation of only 0.052% per cycle at 1 C upon 500 cycles, which is much superior to other cells with monolayer-modified and pristine separators. Even at a high sulfur loading and lean electrolyte condition, the cell with the bilayer coating separator maintains brilliant cycle stability. This work provides a fresh idea to design a synergistically modified separator used for constructing shuttle-free and highly stable Li-S batteries.

Figure 1. (A) Structure and function illustration of the SSZ-13/ZnS modified separator. (B) XRD pattern (inset is SEM image) and (C) corresponding elemental mappings of overlap, Zn and S. (D) Pore size distribution (inset is SEM image) and (E) corresponding elemental mappings of overlap, Al and Si.

EXPERIMENTAL

RESULTS AND DISCUSSION

The XRD pattern of as-prepared ZnS was provided in Figure 1B. Three characteristic peaks located at 28.9°, 48.1°, and 57.1° correspond to (111), (220), 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 with the diameter distribution concentrated in the range of 50~100 nm [Supplementary Figure 1], matching the crystalline size calculated by the Debye-Scherrer equation^[28] based on the XRD result. The EDS elemental mappings in Figure 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. Figure 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 comprises uniform spherical units with diameters ranging from 0.27 to 0.45 μm. The EDS mapping in Figure 1E displays homogeneous distribution of Al and Si elements in the SSZ-13.

The above raw materials were coated on the pristine PP separator to create functionalized surface modification. Figure 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⁺ movement. Nevertheless, they also allow LiPSs to pass through freely, leading to metal corrosion once they get in touch with the lithium anode. In contrast, Figure 2B and C provides the top and cross-sectional images of SSZ-13/ZnS@PP separators, respectively. It is observed that the pristine separator is uniformly covered by the SSZ-13 and ZnS particles with tiny pores, fabricating a continuous and flat coating layer with a total thickness of approximately 9 μm. Moreover, the EDS elemental mappings in Figure 2D distinctly exhibit that the elements of SSZ-13 (Al, Si) and ZnS (Zn, S) are separated into two layers, revealing a compact 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 Supplementary Figure 2A and B in Supplementary Material.

Figure 2. SEM images of (A) PP separator and (B) SSZ-13/ZnS@PP separator. (C) Cross-sectional image and (D) EDS elemental mappings of the SSZ-13/ZnS@PP separator. (E) Flexibility and (F) contact angle measurements of the SSZ-13/ZnS@PP separator.

The excellent mechanical property of the SSZ-13/ZnS@PP separator can be confirmed in Figure 2E since it 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 evaluated the wettability of liquid electrolyte toward the modified separator. As depicted in Figure 2F, the SSZ-13/ZnS@PP separator offers a lower contact angle of 12° in contrast to the pristine separator (26°). It is considered that the polar ZnS particles on the top layer facilitate the rapid electrolyte penetration and, hence, Li⁺ transport.

To explore the barrier capability of the SSZ-13/ZnS@PP separator, an H-shaped glass device was applied to compare the permeability of LiPSs across the separator. As shown in Figure 3A, 5 mL Li₂S₆ (25 mM) solution (left side) and 5 mL DOL/DME mixed solvent (right side) were separated by different separators. For the device composed of PP separators, the right solution color becomes yellow after 12 h and turns brown at last, indicating a free Li₂S₆ permeation. For the ZnS@PP case, the permeation did not stop, but the diffusion concentration decreases owing to the chemisorption of LiPSs on the surface of ZnS. When SSZ-13@PP and SSZ-13/ZnS@PP separators were employed, the solutions in the right chamber of the H-shaped device all remain colorless after 24 h, indicating the exceptional physical blocking capability of SSZ-13 towards LiPSs.

Figure 3. (A) LiPS diffusion tests in the H-shaped cells with different separators. (B) Molecular structures and calculated sizes of various LiPSs monomer units. (C) The calculated ionic conductivity for different separators. (D) UV-Vis spectra of Li₂S₆ solution before and after being adsorbed by the ZnS. (E) Zn 2p, (F) S 2p XPS spectra of ZnS before and after Li₂S₆ adsorption.

DFT calculation was implemented to investigate the three-dimensional structures of various LiPSs. As provided in Figure 3B, the sizes of several LiPS molecules are all well in excess of the pore size of SSZ-13 (3.8 Å) in any orientation (except the thickness of Li₂S₄/Li₂S₆) so that LiPS diffusion can be blocked completely by the zeolite. At the same time, the calculated diameter of Li⁺ is about 0.076 nm so that the porosity of SSZ-13 satisfies the requests of transporting Li⁺. Also, Li⁺ prefers to be solvated by DME over DOL in the electrolyte, since the binding energies of Li⁺-DME and Li⁺-DOL ranged from 2.84 to 2.65 eV and from 1.71 to 1.69 eV, respectively^[29,30]. The calculated diameter of Li⁺(DME)₁ is 3.4 Å^[31], smaller than the pore size of SSZ-13, implying the feasibility of Li⁺ passing through the SSZ-13 pores without desolvation. To affirm it, the bulk resistance extracted from the Nyquist plot of a symmetrical stainless-steel cell calculated ionic conductivity of various separators according to the inserted equation in Figure 3C. It can be seen that the ionic conductivity of SSZ-13/ZnS-modified separators (1.14 mS cm^-1) is higher than that of the pristine one, attributed to facilitated Li⁺ transport pathway provided by the porous SSZ-13 with suitable pore size and enhanced electrolyte wettability contributed by the polar ZnS. As a solution, the dense SSZ-13 zeolite, fabricated to adhere to the commercial separator, plays a critical role in sieving LiPSs and Li⁺.

The chemical affinity of ZnS to LiPSs can be confirmed in the UV-Vis spectrum in Figure 3D, where the intensity of S₆^2- peak decreases prominently and the solution color becomes lighter after the ZnS powder was added into the LiPS solution. The latent interaction between LiPSs and ZnS was further investigated via XPS analyses. As shown in the Zn 2p spectra in Figure 3E, both Zn 2p_3/2 and Zn 2p_1/2 peaks shift towards lower binding energies after the ZnS was soaked in Li₂S₆ solution, suggesting the increased electron density at the metal center due to the polarization of electrons away from the terminal sulfur atoms (S_T^-1) in the Li₂S₆ to the electropositive Zn^[32]. In the S 2p spectra [Figure 3F], two characteristic peaks located at 162.2 and 163.7 eV ascribed to S^2- in the ZnS^[32] show negative shifts after capturing Li₂S₆; meanwhile, the additional peaks relating to bridge sulfur (S_B⁰) and S_T^-1 in Li₂S₆^[26] deliver positive shifts in the ZnS-Li₂S₆ compared to the pristine Li₂S₆ reported previously^[33]. These confirm the strong interaction between ZnS and Li₂S₆ by forming chemical bonds. Overall, the above results clearly indicate that the SSZ-13 and ZnS play distinctive roles of physical barrier and chemical interaction on LiPSs, respectively, which guarantees the synergetic function of the modified separator.

The redox kinetic behaviors of the restrained LiPSs by the functionalized modification on the separator were investigated via various CV measurements. The CV curves of the symmetrical cell in Figure 4A show that the cell employing a ZnS electrode has higher current response than the SP electrode, 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 Figure 4B and C, respectively. The observed shifts of cathodic and anodic peaks (O_a, R_a and R_b)^[34] stand for the increased polarization at high scan rates. The lithium-ion diffusion coefficient ($$D_{{\mathrm{L}\mathrm{i}}^{+}}$$) as a key parameter for redox kinetics was calculated by the Randles-Sevcik equation below^[35,36]:

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 I_p-v^0.5 for (D) peak O_a, (E) peak R_a and (F) peak R_b. (G) CV curves of Li-S cells with various separators. Tafel slopes derived from the (H) oxidation peak and (I) reduction peak.

Where I_p, n, A, $$C_{{\mathrm{L}\mathrm{i}}^{+}}$$, and v represent the peak current, electron transfer number in the reaction (n = 2), electrode area (1.54 cm²), Li⁺ concentration, and scan rate, respectively. So, the slopes of the curves (I_p-v^0.5) are positively correlated with the $$D_{{\mathrm{L}\mathrm{i}}^{+}}$$. As shown in Figure 4D-F and Supplementary Figure 3, the slope values and corresponding $$D_{{\mathrm{L}\mathrm{i}}^{+}}$$ representing the SSZ-13/ZnS@PP separator are higher than the counterparts in each conversion step, demonstrating faster Li⁺ diffusion rate and better sulfur redox kinetics in the entire charge/discharge processes.

The CV curves of Li-S cells using different separators at a scan rate of 0.1 mV s^-1 are displayed in Figure 4G. Remarkably, the reduction peaks for SSZ-13/ZnS@PP separators exhibit a positive shift with significantly higher peak currents relative to other counterparts, verifying that the bilayer modification is conducive to the reversible conversion and then adequately utilizing sulfur species. The Tafel slope is calculated from the oxidation peak at ~2.39 V and reduction peak at ~2.03 V. The cell with the SSZ-13/ZnS@PP separator shows a lower Tafel slope value of 56 mV dec^-1 for the oxidation from Li₂S to Li₂S_x [Figure 4H] and 38 mV dec^-1 for the reduction from Li₂S_xto Li₂S [Figure 4I], compared to the PP case, suggesting the accelerated redox kinetics owing to the bidirectional catalysis capability of the synergetic structured coating^[37].

To further confirm the catalytic effect of the SSZ-13/ZnS coating, the discharge products of sulfur cathodes matched with different separators were examined by XPS (the cells were discharged designedly to the low-potential plateau). As observed in Figure 5A, the characteristic peaks of S 2p_3/2 at ~162.1 eV in the S 2p spectra are assigned to the generated Li₂S₂ during discharging^[38]. The stronger Li₂S₂ signal on the cathode surface matched with the modified separator compared to the pristine one illustrates the accelerated sulfur reduction process. Besides, the Li₂S deposition experiment results are provided in Figure 5B and C for monitoring the formation of the discharge products. It can be seen that the accumulative capacity of deposited Li₂S in the modified separator far exceeds that of the PP separator (165.93 vs. 105.14 mAh g^-1), accompanied by higher current density and earlier precipitation time. This reveals that the SSZ-13/ZnS coating reduces the initial overpotential for the Li₂S nucleation and realizes faster growth of Li₂S^[39].

Figure 5. (A) S 2p XPS spectra of sulfur cathodes matched with different separators after discharging to 2.11 V. Potentiostatic discharge profiles of Li₂S deposition with (B) SSZ-13/ZnS@PP separator and (C) PP separator. (D) Energy profile for Li₂S₄ migration along the different adsorption sites on the ZnS (220) facet. (E) Li₂S₄ migration path on the ZnS (220) facet.

The LiPS migration usually depends on the exposed facets of the catalyst due to high surface affinity and reactivity. It was reported that the lowest energy shape of zinc blende nanoparticles is a rhombic dodecahedron, enclosed entirely by stoichiometric (220) facets^[40]. Therefore, the geometrical configurations of the minimum energy path for LiPS migration on the (220) facet of ZnS were investigated by DFT calculation (Li₂S₄ as the prototype for modeling). As provided in Figure 5D and E, the migration barrier for Li₂S₄ on the ZnS surface was only 1.42 eV. This benefits the rapid diffusion of Li₂S₄ to the nearby conductive area (e.g., SP particles in the coating)^[41] and then electrochemical conversion.

Thus far, the above results well support the conclusion that the favorable entrapping-diffusion-conversion process of LiPSs can be realized by the separator modification strategy. It needs to be emphasized that the original concept of the SSZ-13/ZnS bilayer coating is chemically anchoring LiPSs and accelerating their conversion earlier to the physical block. In other words, the zeolite layer was set as a final defense for resisting LiPS diffusion. Mixing the zeolite and ZnS together in a monolayer can simplify the coating procedure. However, the block and catalytic capability towards LiPSs could be simultaneously weakened due to the incomplete coating of the SSZ-13 and ZnS on the separator. Thus, the bilayer design is essential for better exerting their synergetic effects.

The GITT test explored the effect of the functional coating on the conversion reaction process of the Li-S system. As shown in Figure 6A and B, compared to the PP sample, the cell with the 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^[42,43]. At the same time, the S₈ dissolution process of the SSZ-13/ZnS@PP and PP cells accounts for approximately 34% (8.93/25.9 h) and 37% (6.73/18.2 h) of the discharge phase, respectively. The relatively smaller proportion in the modified separator can be attributed to higher efficiency for Li₂S nucleation^[44,45].

Figure 6. 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 separator at high sulfur loading at 0.2 C.

The Li-S full cell was assembled to investigate 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, as exhibited in Figure 6C. However, larger capacity at low-potential plateau (Q_L) and higher Q_L/Q_H value in the SSZ-13/ZnS@PP cell manifest favorable conversion of LiPSs to insoluble sulfides^[46,47]. A smaller polarization can also be observed with the SSZ-13/ZnS@PP separator, indicating enhanced electrochemical reversibility. The SSZ-13/ZnS@PP cell produces a high initial specific capacity (1,364.22 mAh g^-1) at 0.2 C; for comparison, the initial capacities of SSZ-13@PP, ZnS@PP, and pristine cells decrease to 1,085.25, 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 [Figure 6D], accompanied by a stable charge/discharge plateau during cycling [Supplementary Figure 4]. The shuttle current of the cell with the SSZ-13/ZnS@PP separator shown in Figure 6E is about 0.38 mA, apparently lower than that of a 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 Li deposition and prevents the dendrite growth. It can be noticed in Supplementary Figure 5A that the cycled Li with a PP separator becomes rough with lots of cracks and particles. On the contrary, the Li surface with an SSZ-13/ZnS@PP separator after cycling [Supplementary Figure 5B] is apparently smooth without visible bulk aggregations.

The rate tests of Li-S cells with different separators were conducted at current densities ranging from 0.1 C to 2 C. The initial specific capacity with the SSZ-13/ZnS@PP separator reaches 1,383.47 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 [Figure 6F]. Additionally, the increased polarization at high rates is negligible [Supplementary Figure 6] and the recoverability of the cell capacity is proven after the discharge current back 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 transfer resistance (R_ct). As exhibited in Supplementary Figure 7, the SSZ-13/ZnS@PP cell delivers a lower R_ct value (48.0 Ω) compared with the PP (107.9 Ω), SSZ-13@PP (76.6 Ω), and ZnS@PP (82.8 Ω) cells, indicating an enhanced charge transfer kinetics brought by the bilayer modification on the separator. This merit and high ionic conductivity of the modified separator together contribute to the outstanding rate capability of the cell.

Besides, the long-term cycle stabilities of the Li-S cells at 1 C are exhibited in Figure 6G. The SSZ-13/ZnS@PP cell delivers a discharge capacity of 885.4 mAh g^-1 and maintains at 655 mAh g^-1 after 500 cycles with an attenuation rate of only 0.052% per cycle. The acquired cell performances with the separator modification strategy in this work are competitive and even superior to some recent reports adopting other coating materials (as summarized in Supplementary Table 1). Subsequently, the acceptable cycling capability of the Li-S cell with an SSZ-13/ZnS@PP separator at a higher sulfur loading of ~5.2 mg cm^-2 and a lower E/S ratio of 10 uL mg^-1 is also indicated in Figure 6H, reflecting 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, establishing an excellent synergetic relationship between inhibiting the diffusion of LiPSs and catalyzing their conversion. The experimental studies and DFT calculations reveal that the appropriate pore size of SSZ-13 zeolite physically blocks the movement of the LiPSs towards the lithium anode but ensures free and rapid transport of Li⁺. 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. Consequently, the SSZ-13/ZnS@PP separator endows the Li-S cell with a high discharge capacity of 1,364.22 mAh g^-1 at 0.2 C, preferable capacity decay of 0.052% per cycle over 500 cycles at 1 C, and satisfactory rate capability. This work provides a simple but effective strategy of separator structure engineering to develop advanced Li-S batteries for commercial applications.