English

• Lithium-sulfur batteries (LSBs), with a high theoretical capacity of 1 675 mAh·g^-1 and an energy density of 2 600 Wh·kg^-1, are considered a promising next-generation energy storage system^[1-6]. However, the commercialization of LSBs is significantly hindered by electronic/ionic transport inertia of sulfur, the notorious "shuttle effect" of polysulfide (LiPSs, Li₂S_x, 4≤x≤8) intermediates, and lithium dendrite formation caused by non-uniform lithium deposition^[7-8]. The LiPSs shuttling is found to be one of the main reasons for the rapid capacity decline, low Coulombic efficiency (CE), and poor rate property. For this reason, various strategies have been proposed to address this issue, including the design of sulfur cathodes, the introduction of an interlayer, and the addition of electrolyte additives^[9-14]. Among them, inserting an interlayer between the commercial separator and the sulfur cathode is considered a reliable and simple method to suppress the shuttle effect^[15].

  An ideal interlayer should exhibit strong adsorption for LiPSs, the ability to reduce the energy barriers for LiPSs conversion, excellent electronic conductivity, and rapid ion transport pathways^[16-20]. Two-dimensional (2D) materials have large specific surface areas, atomic thickness, and easily altered properties, which make them a promising candidate for the construction of thin and light-weight interlayers^[21-25]. Previously, our group developed a novel porous 2D material with rich surface silanol (Si—OH) via a selective etching treatment on vermiculite precursors, and the rich pores could help guide the uniform distribution of lithium ion flux in lithium metal batteries^[26-29]. It is revealed that such a porous 2D material (porous modified vermiculite nanosheets, PVS) mainly consists of elements Si and O as well as trace amounts of elements Al and Mg, and thus can be viewed as a silicon oxide compound. Based on the above-mentioned unique structure and properties, such a novel 2D material has great potential as a building block for interlayer preparation, and thus, it is necessary to systematically investigate the influence of the pore structure and the specific surface area of PVS on the LiPSs shuttling suppression.

  Herein, the PVS with varied pore structure was mixed with reduced graphene oxide (RGO) and polyvinylidene fluoride (PVDF) to prepare a slurry for the interlayer. With the extension of the acid treatment, the specific surface area of PVS increased, and the as-prepared interlayer exhibited better adsorption performance and higher lithium ion migration number, effectively mitigating the shuttle effect and enhancing the utilization of active materials. The LSBs with such a PVS-based interlayer delivered a high initial discharge capacity of 1 386 mAh·g^-1 at 0.1C and good cycling performance with a capacity of 437 mAh·g^-1 remaining after 500 cycles at 1C. Overall, this work provides us with a further understanding of the influence of pore structure on the performance of 2D material-based interlayer in suppressing LiPSs shuttle, and provides a theoretical basis for promoting the practical application of LSBs.

  1.   Experimental

  1.1   Chemicals

  Commercial PVDF was purchased from Aladdin and it was desiccated in a vacuum oven at 60 ℃ before use. N-methyl-2-pyrrolidone (NMP) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Hydrochloric acid was purchased from Chongqing Chuandong Chemical (Group) Co., Ltd. Vermiculite was purchased from Sigma-Aldrich. LS electrolyte [1.0 mol·L^-1 lithium bis (trifluoromethane sulfonyl) imide (LiTFSI) dissolved in 1, 2-dimethoxyethane/1, 3-dioxolane (DME/DOL, 1∶1, V/V) with LiNO₃ (mass fraction of 2%)], Li₂S₈ (0.2 mol·L^-1), and Li₂S₆ (0.2 mol·L^-1) were purchased from DuoDuo Chemical Reagent. All chemicals were analytically pure and used directly.

  1.2   Preparation of vermiculite nanosheets (VS) and PVS

  Two-dimensional VS were prepared from layered vermiculite crystals by a two-step ion exchange method. Specifically, vermiculite crystals (20 g) were stirred and peeled in saturated sodium chloride solution for 24 h (oil bath at 80 ℃). The mixture was vacuum filtered with deionized (DI) water and washed several times. The rinsed and drained samples were added to 2 mol·L^-1 lithium chloride (LiCl) solution. The mixture was stirred and heated for 24 h in an oil bath at 80 ℃, then vacuum filtered, thoroughly washed with DI water, and subsequently dialyzed until no chloride ions were detected using a silver nitrate solution. Suction filtration was then performed to remove excess water. The sample was dried to obtain 2D VS. The drained VS samples were placed in a conical flask filled with hydrochloric acid (90 mL, 36%) and the mixture was continuously stirred at 80 ℃ for varying durations (6, 12, and 18 h). The samples were then rinsed with DI water until neutral, and the aqueous solution was centrifuged (1 000 r·min^-1, 10 min). The supernatant was collected, freeze-dried to obtain a white powder, and finally yielded 2D PVS. The PVS samples etched for different times were named PVS-6, PVS-12, and PVS-18, respectively.

  1.3   Preparation of RGO

  Graphene oxide (GO) was synthesized from natural graphite by a modified Hummers method reported previously^[30]. To prepare RGO, 0.8 g of the previously prepared GO was dispersed in 400 mL DI water and sonicated for 2 h, followed by heating to 80 ℃ with adding 5 mL hydrazine hydrate, and kept at 80 ℃ for 24 h. When cooling to room temperature, the reaction solution was subjected to vacuum filtration with distilled water. The product was dried at 60 ℃ and denoted as RGO.

  1.4   Preparation of VS-PP (PP: porous polypropylene) and PVS-PP

  The interlayers were obtained using a typical blade-coating technique. Typically, a homogeneous slurry was produced by mixing VS, RGO, and PVDF with a mass ratio of 7∶2∶1 in NMP. Then, the slurry was coated onto a commercial PP (Celgard 2500) separator and dried in a vacuum oven for 24 h at 60 ℃. The coated PP was then cut into circular pieces with a diameter of 16 mm, and the VS-PP separator was yielded. Similarly, PVS-6-PP, PVS-12-PP, and PVS-18-PP separators were prepared for comparison. The average mass loading of the interlayer material was ca. 0.4 mg·cm^-2.

  1.5   Preparation of S/CNT cathodes

  The S/CNT (CNT: multi-walled carbon nanotubes) composite was synthesized using a melt-diffusion method. Specifically, it was prepared by mixing CNT and commercial sulfur powder with a mass ratio of 1∶2 and then the mixture was heated at 155 ℃ for 12 h. To fabricate the cathode, PVDF, Ketjen black (KB), and S/CNT were mixed in NMP with a mass ratio of 1∶1∶8, and then ground to form a homogeneous slurry, followed by coating onto a carbon-coated aluminum foil. The coating was vacuum-dried at 60 ℃ for 12 h and cut into circular pieces with a diameter of 12 mm. The areal mass loading of the sulfur cathode was 1.0-1.3 mg·cm^-2.

  1.6   LiPSs adsorption test

  20 mg of VS, PVS-6, PVS-12 or PVS-18 was added into 4 mL of Li₂S₆ (5 mmol·L^-1) solution. The color change of these solutions was compared, and the adsorption capacity of the samples for LiPSs was calculated according to the ultraviolet-visible (UV-Vis) spectroum test results on the solutions, and a Li₂S₆ solution was used as the blank group.

  1.7   Assembly of symmetric cells

  Five types of symmetric cells were assembled based on PP, VS-PP, PVS-6-PP, PVS-12-PP, or PVS-18-PP separators. Lithium metal sheets were the anode and cathode electrodes, respectively. The symmetric lithium cell (Li||PVS-18-PP||Li) was used for evaluating the interface resistance of the PVS-18-PP separator.

  1.8   Li₂S nucleation and dissociation tests

  The actual process of Li₂S nucleation and dissolution during the charge/discharge of LSBs was simulated by assembling half-cells for constant-voltage discharge/charging at specific voltages. A slurry of VS, RGO, and PVDF with a mass ratio of 7∶2∶1 was coated on carbon paper to prepare the cathode for the nucleation and dissolution cells, with a Li metal sheet as the anode and PP as the separator. On the cathode side, 20 μL of 0.2 mol·L^-1 Li₂S₈ solution was used as the electrolyte, while LS electrolyte was used on the anode side. The assembled cells were tested using a Neware battery tester, with specific parameters as follows:

  Li₂S nucleation: First, the cells were galvanostatically discharged to 2.06 V at 0.112 mA, followed by constant-voltage discharge at 2.05 V. At this voltage, Li₂S was generated and nucleated on the electrode surface. The current-time curve was recorded until the current dropped below 0.01 μA.

  Li₂S dissolution: The cells were galvanostatically discharged to 1.7 V at 0.1 mA to ensure complete reduction of long-chain LiPSs into Li₂S in the cell, followed by discharging to 1.8 V at 0.01 mA. Then, the cells were charged at a constant voltage of 2.4 V, where Li₂S was dissolved and decomposed into lithium ions and LiPSs. The current-time curve was recorded until the current dropped below 0.01 μA.

  The effects of the material on Li₂S nucleation and dissolution were analyzed based on the onset time of Li₂S nucleation/dissolution as well as the nucleation and dissolution capacities.

  1.9   Electrochemical measurements

  The electrochemical performance tests were executed using CR2032 coin-type cells with S/CNT cathode as the working electrode, VS-PP, PVS-6-PP, PVS-12-PP, or PVS-18-PP as the separators, and lithium metal as the counter-electrode. The electrolyte amount in each cell was 40 μL. The cells were assembled in an argon-filled glove box and the volume fractions of oxygen and water were less than 10^-6. The electrochemical workstation (CHI 604E, Chenhua, Shanghai) was used to test the electrochemical impedance spectra (EIS) (frequency range of 0.1 to 10⁶ Hz). The CT2001A battery test system (NEWARE Electronic Co., China) was used to test the cycling and rate performance.

  1.10   Characterization

  Field emission scanning electron microscopy (SEM, ZEISS SUPRA 40) and atomic force microscope (AFM, Bruker, Dimension Icon) were used to observe the micro-morphology of the 2D vermiculite. The morphological properties of these separators were observed by field emission scanning electron microscopy (SEM, ZEISS Gemini 300 from Germany). The N₂ adsorption-desorption isotherms of the samples were recorded on a gas adsorption apparatus (American Mike 2460) to produce the specific surface area and pore size distribution. The X-ray diffraction (XRD) patterns of the samples were determined by a Bruker D8 (Germany) instrument with Cu Kα radiation (λ=0.154 nm) and 2θ=5°-90° and the voltage was 40 kV, and the current was 100 mA. The X-ray fluorescence spectroscopy (XRF) was characterized by Zetium Panaco from the Netherlands. The molecular structure and chemical bond information of the materials were detected on the Fourier transform infrared spectroscopy analyzer. UV-Vis spectra were measured on a MAPADA UV-6300 spectrophotometer.

  2.   Results and discussion

  2.1   Phase structure analysis

  Via ion exchange and dialysis purification, the vermiculite crystal precursors were exfoliated into 2D PVS. SEM and TEM were first used to observe the morphology and microstructure of the samples, including VS, PVS-6, PVS-12, and PVS-18. In the SEM images (Fig. 1a-1d), compared with VS, the 2D PVS nanosheets with longer acid etching duration became smaller and smaller in lateral dimension. The TEM images (Fig. 1e-1h) confirmed the 2D morphology of the as-obtained PVS, and the results showed that the longer the acid etching time, the smaller the size of PVS. The morphology of the 2D PVS was further characterized by AFM. As shown in Fig. 1i-1l, the AFM images demonstrated the 2D morphologies of VS and PVS-18, and their thicknesses were found to be about 7 and 5 nm, respectively.

  Figure 1

  

  Figure 1.  (a-c) SEM images and (e-h) TEM images of VS, PVS-6, PVS-12, and PVS-18; (i, k) AFM images and (j, l) the corresponding height profiles of VS and PVS-18

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  The XPS survey spectrum of VS (Fig. 2a) reveals characteristic peaks corresponding to O, Si, Al, Mg, and C elements. In contrast, the XPS spectra of PVS-6, PVS-12, and PVS-18 showed no detectable signals for Al and Mg. Deconvolution of the XPS O1s peak in VS (Fig. 2b) identifies three distinct chemical bonds: Al—O (529.9 eV), Mg—O (531.6 eV), and Si—O (532.7 eV). However, PVS-6, PVS-12, and PVS-18 only had one Si—O peak with peaks of 532.8, 533.0, and 533.1 eV in their O1s spectra, respectively, indicating that Mg and Al have been largely removed. At the same time, the O1s peak shifted towards higher binding energies. Previous studies have demonstrated that in aluminosilicate systems, the binding energies of Al2p, Si2p, and O1s XPS spectra correlate with the Si/Al ratio, showing higher binding energies with increasing ratios^[31-32]. The elimination of Mg and Al alters the chemical environment surrounding Si and O atoms. In the XPS Si2p spectrum (Fig. 2c), the binding energy of the transition region from Si²⁺ to Si⁴⁺ in VS was 102.49 eV, while the binding energies of Si⁴⁺ in PVS-6, PVS-12, and PVS-18 were 102.49, 103.50, 103.69, and 103.70 eV, respectively. The results showed that with the increase of acid etching time, the Si2p and O1s peaks of PVS shifted upwards. To further understand the changes in chemical bonds in PVS, FTIR testing was conducted. Compared with the FTIR spectra of VS (Fig. 2d), all the PVS samples (PVS-18, PVS-12, and PVS-6) exhibited an additional absorption peak at 956 cm^-1, which is attributed to the stretching vibration of Si—OH^[33]. As shown in Fig. 2e, the longer the etching duration with hydrochloric acid, the stronger the Si—O peak, indicating a more abundant Si—O group on PVS-18. The XPS and FTIR results showed that as the etching time increased, there was more Si—O in PVS. It has been found that the combination of the porous structure of 2D PVS-18 and its rich surface active sites can help capture LiPSs through both physical adsorption and chemical interactions^[23].

  Figure 2

  

  Figure 2.  (a) XPS survey spectra, (b) O1s and (c) Si2p XPS spectra, (d, e) FTIR spectra, (f) N₂ adsorption-desorption isotherms, (g) pore size distribution curves, and (h) XRD patterns of VS, PVS-6, PVS-12, and PVS-18

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  N₂ adsorption-desorption measurement was conducted to study the pore structure of VS, PVS-6, PVS-12, and PVS-18 (Fig. 2f and 2g). The N₂ adsorption-desorption isotherms of PVS-18 (Fig. 2f) showed a typical combined feature of type Ⅰ and type Ⅳ isothermal behavior^[34]. The specific surface area (S_BET) of PVS-18 was calculated to be 427 m²·g^-1, much higher than that of VS (11 m²·g^-1). The specific surface areas of PVS-12 and PVS-6 were 330 and 311 m²·g^-1, respectively. The pore size distribution curves (Fig. 2g) indicated the presence of micro-/meso-/macropores in PVS-18, while VS only showed mesopores structure, and PVS-18 had a larger pore volume (0.39 cm³·g^-1) than VS (0.03 cm³·g^-1). It is worth noting that there were abundant mesopores and macropores in PVS-18, which is beneficial to the trapping of LiPSs by the physical adsorption of the porous structure. According to the experimental results, we speculate that the larger the specific surface area of the material, the more nucleation sites it has, which is conducive to nucleation.

  The XRD results of PVS-18, PVS-12, PVS-6, and VS were compared in Fig. 2h. As we can see, the VS had a different diffraction pattern compared to the other three samples. The (101) plane peaks assigned to the interlayer spacing of VS were located at 22.225°, 22.251°, and 22.435° for PVS-18, PVS-12, and PVS-6, respectively, indicating that the interlayer spacing of the 2D VS was almost the same (0.396 8, 0.399 1, 0.399 6 nm, respectively). The content of various components in VS, PVS-6, PVS-12, and PVS-18 was further analyzed using XRF, and the results are shown in Table 1. As the acid treatment duration increased, the mass fractions of elements Mg, Al, and Fe decreased.

  Table 1

  

  Table 1.  Mass fractions of VS, PVS-6, PVS-12, and PVS-18 from XRF test results  %

  下载: 导出CSV

  Sample	SiO2	MgO	Al2O3	Fe2O3	TiO2	CaO	K2O	P2O5	Trace element
  VS	41.27	29.56	11.83	10.67	1.44	0.29	4.24	0.10	0.08
  PVS-6	99.46	0.14	0.09	0.06	0.19	—	—	—	0.06
  PVS-12	99.59	0.09	0.04	0.06	0.14	0.02	—	—	0.06
  PVS-18	99.58	0.08	0.06	0.09	0.10	0.02	—	—	0.08

  2.2   Electrochemical performance

  The adsorption capacity for LiPSs is a critical factor for evaluating the effect of interlayer material in suppressing LiPSs. To investigate the affinity of PVS toward LiPSs, visual adsorption tests and UV-Vis absorption measurements were conducted. PVS-18, PVS-12, PVS-6, and VS were added to Li₂S₆ solutions in sealed vials (Inset of Fig. 3a), respectively. The solution containing PVS-18 became transparent after 2 h, whereas the solutions with PVS-12, PVS-6, and VS remained distinctly yellow. This indicated that PVS-18 exhibited the strongest adsorption capability for LiPSs. These experimental results were also consistent with the UV-Vis spectra (Fig. 3a). These results indicated that PVS-18 exhibited excellent adsorption capacity for LiPSs, which can be attributed to its largest S_BET and richest surface Si—OH group sites. Symmetric cells were assembled to investigate the lithium-ion transfer number (t_Li⁺) of the PVS-18, PVS-12, PVS-6, and VS modified separators, as well as the commercial PP separator. The t_Li⁺ (Fig. 3b-3f) of PVS-18-PP, PVS-12-PP, PVS-6-PP, VS-PP, and PP were calculated to be 0.72, 0.69, 0.65, 0.50, and 0.44, respectively, indicating that the PVS-18-based interlayer was the most conducive to the migration of Li^{+ [29, 35, 36]}. It is worth noting that the t_Li⁺ of PP is close to that of VS-PP, much smaller than PVS-18-PP, PVS-12-PP, and PVS-6-PP, which hints that the high t_Li⁺ of PVS-18-PP should come from the large pore volume and S_BET.

  Figure 3

  

  Figure 3.  (a) UV-Vis spectra of the Li₂S₆ solutions with VS, PVS-6, PVS-12, and PVS-18; Chronoamperometry profiles of the symmetric cell with (b) PVS-18-PP, (c) PVS-12-PP, (d) PVS-6-PP, (e) VS-PP, and (f) PP

  Inset in (a): digital photos of these Li₂S₆ solutions after 2 h of static adsorption; (b-f): Nyquist plots of the symmetric cell before and after polarization.

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  Simulate the actual nucleation and dissolution of Li₂S during the charging and discharging process of LSBs by assembling half cells for constant voltage discharge/charging at a specific voltage^[37-39]. Then Li₂S₈ was used as the active material, and the liquid-solid conversion efficiency of LiPSs was evaluated via the Li₂S nucleation tests on carbon fiber paper coated with PVS-18, PVS-12, PVS-6, and VS. The constant-potential discharge curves (Fig. 4a-4d) revealed that the cell based on PVS-18 exhibited a higher Li₂S precipitation capacity (307.499 mAh·g^-1) and an earlier peak rise time (1 392 s) than the other cells based on PVS-12 (275.732 mAh·g^-1, 1 508 s, Fig. 4b), PVS-6 (235.711 mAh·g^-1, 1 537 s, Fig. 4c), and VS (214.938 mAh·g^-1, 1 479 s, Fig. 4d), which is attributed to the rich pores and large S_BET of PVS-18 significantly shortens the ion transport distance and facilitates the rapid precipitation/dissolution kinetics of Li₂S. To further investigate the kinetics of Li₂S decomposition on PVS-18, PVS-12, PVS-6, and VS, the constant-potential charging tests were performed at 2.4 V. The PVS-18-based cell exhibited the highest oxidation current density, the earliest dissolution time (294 s), and the largest dissolution capacity (Fig. 4e and 4f). This indicates that PVS-18 endows the cell with a low oxidation overpotential and enables rapid conversion from Li₂S to LiPSs, which is crucial for the achievement of long-life and high-rate LSBs.

  Figure 4

  

  Figure 4.  Potentiostatic discharge profile of Li₂S₈ discharged at 2.05 V on carbon fiber papers coated by (a) VS, (b) PVS-6, (c) PVS-12, and (d) PVS-18; (e) Potentiostatic charge profile of VS, PVS-6, PVS-12, and PVS-18 at 2.40 V for evaluating dissolution kinetics of Li₂S in the cells; (f) Calculated dissolution capacities of the Li₂S dissolution test based on different electrodes

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  To explore the effect of PVS-18 on the electrochemical performance in-depth, the LSBs with the PVS-18-PP, PVS-12-PP, PVS-6-PP, and VS-PP were then assembled, and systematic electrochemical measurements were conducted. The EIS test was used to explore the charge transfer process inside the cell. The Nyquist plots of the cells were obtained by the EIS measurement, and the fitted curves were also displayed in Fig. 5a by an equivalent circuit (Inset of Fig. 5a) including ohmic resistance (R_s), charge transfer resistance (R_ct), and the Warburg impedance (Z_W). Compared with PVS-12-PP (56.2 Ω), PVS-6-PP (86.2 Ω), VS-PP (82.0 Ω), and PP (100.0 Ω), the cell using PVS-18-PP had the smallest charge transfer resistance (40.5 Ω, Fig. 5a), which is attributed to the high electrolyte wettability and fast lithium ion transport endowed by PVS-18. The cell using PVS-18-PP had outstanding rate performance compared with PVS-12-PP, PVS-6-PP, VS-PP, and pristine PP (Fig. 5b). The discharge capacities of the cell with PVS-18 were 1 079, 900, 799, 722, and 677 mAh·g^-1 at 0.2C, 0.5C, 1.0C, 2.0C, and 3.0C respectively, and a discharge capacity of 911 mAh·g^-1 was retained when the current density switched back to 0.2C. It can be seen that the discharge specific capacity of the PVS-18-based cell at the same rate was much higher than that of pristine PP. This is because the PVS-18 had a rich mesoporous structure that allowed rapid migration of ions and electrons, while the high S_BET also facilitated the absorption of electrolytes. The good reversibility rate of the PVS-18 separator indicated that the 2D vermiculite nanosheet had good structural stability. These results indicated that the PVS-18-based interlayer endowed the cell with high reversibility and rapid redox reaction kinetics.

  Figure 5

  

  Figure 5.  (a) Nyquist plots of LSBs with various separators from EIS tests before cycling; (b) Rate performance at various current densities; (c) Galvanostatic charge/discharge profiles at 0.1C, and (d) the corresponding Q_L and Q_H; Cycling performance with sulfur loading of 1.13 mg•cm^-2 at (e) 0.2C and (f) 1C

  Inset: the corresponding equivalent circuit, where CPE1 is the constant phase angle element.

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  The galvanostatic charge-discharge (GCD) profiles of these cells at 0.1C were shown in Fig. 5c. The initial capacity of the cell using PVS-18-PP was 1 386 mAh·g^-1, much higher than those using PVS-12-PP (1 297 mAh·g^-1), PVS-6-PP (1 163 mAh·g^-1), VS-PP (1 122 mAh·g^-1), and PP (1 014 mAh·g^-1), demonstrating higher S utilization. The cell using PVS-18-PP showed a larger ratio (Q_L/Q_H=2.85, Fig. 5d) of the discharge capacity (Q_L) at the lower plateau (1.7-2.1 V) to the discharge capacity (Q_H) at the higher plateau (2.1-2.4 V), compared with those using PVS-12-PP (2.71), PVS-6-PP (2.61), VS-PP (2.71), and PP (1.61), indicating the faster redox reaction from LiPSs to Li₂S as well as less LiPSs dissolution. These results suggest that PVS-18 improves sulfur utilization and promotes the LiPSs conversion reaction.

  Furthermore, the long-term cycling performances of these cells were studied under different current densities (Fig. 5e and 5f). The cell using PVS-18-PP showed an initial discharge specific capacity of 1 013 mAh·g^-1 at 0.2C and maintained a capacity of 700 mAh·g^-1 after 80 cycles (Fig. 5e). The cycling performance of these cells was investigated at 1C with the sulfur content of 1.13 mg·cm^-2 (Fig. 5f). The cell with PVS-18-PP delivered an initial discharge capacity of 753 mAh·g^-1, and a capacity of 438 mAh·g^-1 could be retained after 500 cycles, which was significantly superior to the cells using PVS-12-PP, PVS-6-PP, and VS-PP (retaining only 369, 286, and 212 mAh·g^-1, respectively). The average attenuation rates per cycle for PVS-18, PVS-12, PVS-6, VS, and PP were 0.080%, 0.10%, 0.12%, 0.14%, and 0.14%, respectively. The average capacity decay rate per cycle of PVS-18 was the smallest, indicating that it effectively reduces the impact of the "shuttle effect". The LSBs with the PVS-18 separator can significantly suppress the shuttle of LiPSs and demonstrate the best cycling performance. This indicates that the PVS-18-PP separator can effectively confine the shuttle effect through strong chemical adsorption of LiPSs.

  3.   Conclusions

  In conclusion, this study successfully developed a functionalized separator based on 2D PVS to address the LiPSs shuttle effect. The PVS-18 nanosheets obtained through an optimized acid-etching process exhibited an exceptionally high specific surface area of 427 m²·g^-1 and abundant polar active sites, beneficial for anchoring LiPSs effectively. The PVS-18-based interlayer had a large lithium-ion transfer number (t_Li⁺=0.72), and the PVS-18 could promote the nucleation and dissolution kinetics of Li₂S, bringing about well-suppressed LiPSs shuttle and enhanced utilization of active material. Electrochemical tests demonstrated that the LSBs employing the PVS-18-based interlayer had a low charge transfer resistance and thus facilitated rapid ion transport. Ultimately, the LSBs delivered an impressive initial discharge capacity of 1 386 mAh·g^-1 at 0.1C, superior cycling stability, and rate performance. This work provides a systematic analysis of the effect of the pore structure and surface chemistry of 2D porous material on the inhibition of LiPSs shuttling, to provide a better design scheme for 2D material-based interlayer used in high-performance LSBs.

  
  Acknowledgments: This work was financially supported by National Natural Science Foundation of China (Grants No.51972070, 52372185), Guizhou Provincial High Level Innovative Talents Project (Grant No.QKHPTRC-GCC[2022]013-1), Innovation Team for Advanced Electrochemical Energy Storage Devices and Key Materials of Guizhou Provincial Higher Education Institutions (Grant No.QianJiaoJi[2023]054), Advanced Electrochemical Energy Storage Devices and Key Materials Technology Innovation Talent Team Construction of Guizhou Province (Grant No.QKHPTRC-CXTD[2023]016), and Guizhou Province Basic Research Program (Natural Sciences) Talent Team Support Project (QianKeHeJiChu QNB[2025]003).
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  Figure 1  (a-c) SEM images and (e-h) TEM images of VS, PVS-6, PVS-12, and PVS-18; (i, k) AFM images and (j, l) the corresponding height profiles of VS and PVS-18

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  Figure 2  (a) XPS survey spectra, (b) O1s and (c) Si2p XPS spectra, (d, e) FTIR spectra, (f) N₂ adsorption-desorption isotherms, (g) pore size distribution curves, and (h) XRD patterns of VS, PVS-6, PVS-12, and PVS-18

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  Figure 3  (a) UV-Vis spectra of the Li₂S₆ solutions with VS, PVS-6, PVS-12, and PVS-18; Chronoamperometry profiles of the symmetric cell with (b) PVS-18-PP, (c) PVS-12-PP, (d) PVS-6-PP, (e) VS-PP, and (f) PP

  Inset in (a): digital photos of these Li₂S₆ solutions after 2 h of static adsorption; (b-f): Nyquist plots of the symmetric cell before and after polarization.

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  Figure 4  Potentiostatic discharge profile of Li₂S₈ discharged at 2.05 V on carbon fiber papers coated by (a) VS, (b) PVS-6, (c) PVS-12, and (d) PVS-18; (e) Potentiostatic charge profile of VS, PVS-6, PVS-12, and PVS-18 at 2.40 V for evaluating dissolution kinetics of Li₂S in the cells; (f) Calculated dissolution capacities of the Li₂S dissolution test based on different electrodes

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  Figure 5  (a) Nyquist plots of LSBs with various separators from EIS tests before cycling; (b) Rate performance at various current densities; (c) Galvanostatic charge/discharge profiles at 0.1C, and (d) the corresponding Q_L and Q_H; Cycling performance with sulfur loading of 1.13 mg•cm^-2 at (e) 0.2C and (f) 1C

  Inset: the corresponding equivalent circuit, where CPE1 is the constant phase angle element.

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  Table 1.  Mass fractions of VS, PVS-6, PVS-12, and PVS-18 from XRF test results  %

  Sample	SiO2	MgO	Al2O3	Fe2O3	TiO2	CaO	K2O	P2O5	Trace element
  VS	41.27	29.56	11.83	10.67	1.44	0.29	4.24	0.10	0.08
  PVS-6	99.46	0.14	0.09	0.06	0.19	—	—	—	0.06
  PVS-12	99.59	0.09	0.04	0.06	0.14	0.02	—	—	0.06
  PVS-18	99.58	0.08	0.06	0.09	0.10	0.02	—	—	0.08

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