Sulfhydryl‐functionalized COF‐based electrolyte strengthens chemical affinity toward polysulfides in quasi‐solid‐state Li‐S batteries

Abstract

For lithium‐sulfur batteries (Li‐S batteries), a high‐content electrolyte typically can exacerbate the shuttle effect, while a lean electrolyte may lead to decreased Li‐ion conductivity and reduced catalytic conversion efficiency, so achieving an appropriate electrolyte‐to‐sulfur ratio (E/S ratio) is essential for improving the battery cycling efficiency. A quasi‐solid electrolyte (COF‐SH@PVDF‐HFP) with strong adsorption and high catalytic conversion was constructed for in situ covalent organic framework (COF) growth on highly polarized polyvinylidene fluoride‐hexafluoropropylene (PVDF‐HFP) fibers. COF‐SH@PVDF‐HFP enables efficient Li‐ion conductivity with low‐content liquid electrolyte and effectively suppresses the shuttle effect. The results based on in situ Fourier‐transform infrared, in situ Raman, UV–Vis, X‐ray photoelectron, and density functional theory calculations confirmed the high catalytic conversion of COF‐SH layer containing sulfhydryl and imine groups for the lithium polysulfides. Lithium plating/stripping tests based on Li/COF‐SH@PVDF‐HFP/Li show excellent lithium compatibility (5 mAh cm⁻² for 1400 h). The assembled Li‐S battery exhibits excellent rate (2 C 688.7 mAh g⁻¹) and cycle performance (at 2 C of 568.8 mAh g⁻¹ with a capacity retention of 77.3% after 800 cycles). This is the first report to improve the cycling stability of quasi‐solid‐state Li‐S batteries by reducing both the E/S ratio and the designing strategy of sulfhydryl‐functionalized COF for quasi‐solid electrolytes. This process opens up the possibility of the high performance of solid‐state Li‐S batteries.

INTRODUCTION

Lithium-sulfur batteries (Li-S batteries) are based on a conversion reaction that allows them to exceed the energy density of lithium-ion batteries, which operate through intercalation mechanisms.^1–4 To ensure the high performance of Li-S batteries, a high ratio of electrolyte to sulfur (E/S; 20 µL mg⁻¹) is typically employed, which severely restricts their energy density.^5–8 However, due to the strong shuttle effect of soluble lithium polysulfides (SLPs) and the sluggish kinetics of conversion reaction, it is challenging to achieve low E/S ratios without compromising stability.^9–12 Additionally, lean electrolytes may increase the concentration of SLPs, causing a more pronounced shuttle effect. This would increase the electrolyte viscosity and reduce the ion transfer and conversion, further causing a rapid decline in cycling efficiency.^13–17

To tackle these issues, various composite cathode designs for Li-S batteries have been widely employed to minimize the polysulfide shuttle effect, including enhancing the high conductivity, designing closed and permeable structures, tailoring the catalytic conversion of SLPs, and increasing the quasi-solid conversion of S₈ to Li₂S₄.^18–21 Nevertheless, sulfur host design often requires a multistep synthesis process for composite cathode, making them complex and financially impractical.^2,22–25 Notably, separator modification is expected to improve the electrochemical performance of Li-S batteries by reducing SLP diffusion and increasing electrolyte affinity.^26–29 As a key component of Li-S batteries, the separator plays a critical role in preventing short circuits and providing Li⁺ migration channels. Recently, extensive research has been carried out into the functional design of the separators. Physically, a physical barrier based on the radius difference between Li⁺ and S_x²⁻ has been designed to optimize the pore size of the separator, and SLPs near the cathode are blocked by electrostatic repulsion.^30–32 Chemically, separators with functionalized dangling bonds can chemisorb polysulfides to intercept the SLPs.^33–35 These strategies of modified separators are effective in inhibiting SLPs. However, the conversion efficiency from SLPs to Li₂S remains low, and it is difficult to avoid high-content liquid electrolytes. Hence, it is still a crucial task to create electrolytes with strong catalytic capability and remarkable chemisorption, while taking into account the limited liquid electrolytes.

Gel polymer electrolytes (GPEs) are one of the promising materials in rechargeable batteries. In Li-S batteries, GPEs must fulfill the following conditions: (I) retain polar groups of the polymer matrix to dissociate lithium salts; (II) have a rich microstructure to ensure fast lithium ion conduction; (III) provide excellent electrochemical/thermal stability; (IV) ensure effective adsorption and barrier of SLPs.^36–40 Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) possesses a high dielectric constant, good electrochemical stability, and excellent thermal tolerance, making it ideal for GPEs.^41–44 However, the single PVDF-HFP chain has poor Li-ion conductivity.^45–47 Meanwhile, PVDF-HFP is not effective in adsorbing SLPs due to their penetration. Two-dimensional (2D) covalent organic frameworks (COFs) are porous crystalline structures composed of light elements (C, H, O, N, S, or B) that are linked together by covalent bonds.^48–50 High-porosity COFs are advantageous for electrolyte storage and Li-ion transfer.^51–53 Therefore, a synergistic design of composite electrolytes based on high-porosity PVDF-HFP and functionalized COFs is expected to inhibit the shuttling of SLPs and accelerate the conversion of SLPs while reducing the electrolyte content to achieve high-performance quasi-solid Li-S batteries.

Herein, sulfhydryl-functionalized COF-SH@PVDF-HFP membranes were developed to adsorb the SLPs and catalyze their conversion. COF-SH@PVDF-HFP effectively inhibits the diffusion of SLPs and ensures rapid lithium-ion transport. The assembled cells showed stable lithium plating/stripping, exhibiting excellent lithium compatibility (2 mA cm⁻² and 5 mAh cm⁻² for 1400 h). In situ Fourier-transform infrared (FTIR), in situ Raman, UV–Vis, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations confirmed the fast catalytic conversion of SLPs with the COF-SH barrier layer containing sulfhydryl and imine groups. As a result, the assembled Li-S batteries exhibited excellent electrochemical performance (initial capacity of 808.4 mAh g⁻¹ at 2 C) with a capacity retention rate of 77.3% after 800 times (a fading rate of only 0.03% per cycle). This work provides a deep understanding of the shuttle effect from the amount of electrolyte and functional design, which serves as a reference for preparing high-performance gel electrolytes of Li-S batteries.

RESULTS AND DISCUSSION

Synthetic route

Figure 1A shows the condensation model of COF-SH, where 1,3,5-homobenzotrialdehyde and 2,5-diamino-1,4-benzenedithiol dihydrochloride are catalyzed to COF-SH by glacial acetic acid at ambient pressure and temperature. The polar imine and sulfhydryl groups are interspersed throughout the smallest structural unit of COF-SH. When the imine and sulfhydryl groups are present in the gel electrolyte, the Lewis acid-based interaction promotes the dissociation of the lithium salt and acts as a binding site to facilitate the transport of Li⁺ and inhibit the movement of the anion in the electrolyte. In Li-S batteries, imine and sulfhydryl groups can effectively adsorb SLPs and inhibit the movement of polysulfide anions, while sites with catalytic activity can accelerate the conversion of SLPs. When COF-SH is grown on the PVDF-HFP fiber, the imine and sulfhydryl groups are evenly distributed throughout the chain segments of the polymer to achieve full functionalization.

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The three-dimensional (3D) view of COF-SH is presented based on the density functional theory calculation. Notably, the periodicity of the symmetric stacking for the two-dimensional (2D) layered structure induces a uniform distribution of honeycomb-like pores with abundant one-dimensional (1D) nanochannel, indicating long-range ordering along the ab-plane (Figure 1B). Subsequently, the adsorption energy of COF-SH on insoluble Li₂S and soluble Li₂S₄ was calculated, and results (Figure 1C) show that the adsorption energy of -SH-Li₂S and -N-Li₂S were −4.95 and −4.58 eV, respectively. Similarly, the adsorption energy for -SH-Li₂S₄ and -N-Li₂S₄ were −3.86 and −1.55 eV, respectively. The simulation results indicate that the sulfhydryl group and the imine atomic groups have good adsorption behavior to lithium polysulfide. The Li⁺ adsorption was assessed based on the COF-SH of sulfhydryl and imine groups (Figure 1D). For sulfhydryl groups, the Li⁺ is bound to S atoms with an adsorption energy of about −3.18 eV. Similarly, Li⁺ bound to N atoms with an adsorption energy of −2.97 eV for imine groups. Furthermore, the migration path of Li⁺ was investigated (Figure 1E), in which Li⁺ can migrate freely in the pores of COF-SH with an energy barrier of about 1 eV, suggesting that COF-SH can promote the diffusion of Li⁺. In addition, we also verify the catalytic effect of COF-SH on the catalytic conversion process of Li₂S (Figure 1F). S atom was adsorbed on the sulfhydryl group, and the Li atom was attached to the C atom to achieve the decomposition of Li₂S by overcoming an energy of about 0.71 eV, which confirms that COF-SH can induce the conversion of SLPs and accelerate the redox reaction kinetics (Figure 1G).

Characterization of the electrolyte membrane

Figures 2A and S1A show the scanning electron microscopy (SEM) and optical photograph (inset) of the PVDF-HFP electrospinning membrane. It can be seen that the surface of the electrospinning membrane is smooth in white. In addition, the PVDF-HFP membrane possessed a uniform and tunable nanofibrous structure with abundant voids. The PVDF-HFP electrospinning membrane has a good consistency with a thickness of about 59 μm (Figure S1B). As shown in Figure 2B, the diameter of the electrospinning fiber is ~472 nm. Figure S1C–H shows the elemental mapping of the cross-section of the PVDF-HFP electrospinning membrane. The elements C, F, and O are uniformly distributed. The presence of O is caused by slight oxidation during sample preparation.

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The top-view SEM image of COF-SH@PVDF-HFP is shown in Figure 2C. It is almost difficult to observe the nanofibrous structure on the surface of COF-SH@PVDF-HFP because of the attachment of COF-SH nanoparticles. In addition, the optical image (inset) shows that the surface of COF-SH@PVDF-HFP at the COF-SH side is yellowish brown. Figure 2D shows the SEM image of the fibers stripped from the COF-SH@PVDF-HFP surface. Clearly, COF-SH nanoparticles were grown in situ on the surface of PVDF-HFP fibers, and the micron-sized pores of PVDF-HFP membrane were filled by COF-SH nanoparticles. The cross-sectional SEM image (Figure 2E) shows that the thickness of COF-SH@PVDF-HFP is ~66.8 μm. COF-SH nanoparticles are located in the upper part, while the interwoven structure of nanofibers and COF-SH appears in the middle layer, and PVDF-HFP nanofibers are in the bottom layer, which shows a gradual composite construction. To further investigate the composite structure, the elemental distribution of the cross-section is shown in Figure 2F. In addition to the C, F, and O elements, the S and N are derived from COF-SH. Among them, the color distribution of S and N shows a gradual change from dark to light, which further confirms its stepwise composite structure. Transmission electron microscopy (TEM) image (Figure 2G) shows that the average particle size of the COF-SH particles is ~74.7 nm. A series of lattice fringes is observed in Figure 2H, which is related to COF-SH. The elemental distribution of COF-SH nanoparticles is presented in Figure 2I. The elements of C, N, S, F, and O are evenly distributed. These results proved that the COF-SH nanoparticles were grown on PVDF-HFP nanofibers and COF-SH@PVDF-HFP was prepared successfully.

Figure 3A displays the Raman spectra of COF-SH@PVDF-HFP and PVDF-HFP. The lattice vibrations induced by COF-SH can be detected in the pink region. The vibrations of C–S, C=N, and aromatic rings were also detected. The FTIR spectra of COF-SH@PVDF-HFP and PVDF-HFP are shown in Figure 3B. The peaks corresponding to the C–C, C=C, and C–N are found in COF-SH@PVDF-HFP, while the peak corresponding to the -SH is found at 2500–3500 cm⁻¹.⁵⁴ In addition, microscopic infrared tests were carried out to characterize the riveted COF-SH in detail. Forty-nine sets of microscopic infrared data were randomly collected on the surface of COF-SH@PVDF-HFP (Figure S2). Among them, five data were chosen (e.g., A–E), as shown in Figure 3C. All the data show a high degree of consistency in the 700–4000 cm⁻¹ region, suggesting a high homogeneity on the surface components. Specifically, the peaks at ~3320, ~2850, ~1690, and ~1400–1650 cm⁻¹ in Figure 3D can be indexed to -SH, -C–H-, -C=C-, and C–N groups of COF-SH, respectively. The schematic diagram of the electrolyte construction is shown in Figure 3E. After in situ riveting COF-SH on the PVDF-HFP nanofibers, the SLPs at the cathode side are effectively blocked, preventing the shuttle effect. The size of the electrospinning membrane of PVDF-HFP can be adjusted by the spinning and collecting device (Figure 3F). The thicknesses of PVDF-HFP and COF-SH@PVDF-HFP were 59 and 82 μm, respectively (Figure 3G). The optical photographs after absorbing the liquid electrolyte (1 mol L⁻¹ LiTFSI in 50 µL 1,3-dioxolnd/1,2-dimethoxyethane and 1% LiNO₃) are displayed in Figure 3H,I. PVDF-HFP showed a transparent gel-like structure, while COF-SH@PVDF-HFP showed a light yellow.

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SLPs adsorption and catalytic conversion

Figure S3 shows the Li₂S₆ blocking experiments of visible U-type electrolytic cells based on COF-SH@PVDF-HFP. The solution on the right side of the electrolytic cell gradually became deeper after standing for 24 h, indicating that the shuttle effect of SLPs was reduced. This is due to the dual effect of the effective adsorption of COF-SH on SLPs and the physical barrier of nano-COF-SH on the pores of the spinning membrane. To further verify the adsorption of COF-SH on SLPs, UV–Vis tests were carried out. Figure 4A shows the UV–Vis results of PVDF-HFP soaked in Li₂S₆ solution with a time interval of 10 min. It can be seen that the absorbance is almost unchanged. However, under the same condition, COF-SH@PVDF-HFP shows a significant decrease in absorbance in Figure 4B, which indicates that the adsorption and anchoring of Li₂S₆ has been realized by the COF-SH. In addition, XPS tests were conducted to further verify the surface bonding of the gel electrolyte after adsorbing SLPs. Figure 4C shows the Li 1s spectra after adsorbing the Li₂S₆. The strong Li 1s signal for COF-SH@PVDF-HFP suggests that the COF-SH promoted the immobilization action of SLPs. Figure 4D–F displays the N 1s, S 2p, and C 1s spectra of COF-SH@PVDF-HFP before and after the adsorption of Li₂S₆. In the N 1s spectrum, only the C–N peak in the imine atomic group existed before adsorption, and the Li–N peak appeared after adsorption.^55,56 In the S 2p spectrum, only the -SH signal can be detected before the adsorption, and the polythionate peak appears after the adsorption.⁵⁷ In the C 1s spectrum, the -CF₂ and -CF₃ peaks cannot be detected, which is due to COF-SH being covered by the surface and pores of PVDF-HFP.⁵⁸ These results indicate that SLPs can be adsorbed by the imine radicals and sulfhydryl groups of COF-SH, which further inhibits the shuttling of SLPs.

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For a more detailed adsorption process, the catalytic behavior and the chemical interaction between the COF-SH and the Li₂S_n could be further characterized by the in situ FTIR spectra, as shown in Figure 4G–I. The data collection is located on the Li-S cathode side, and the voltage range during the test is from the open circuit voltage to 1.7 V. The operando FTIR signals were exhibited at different discharging states. The FTIR intensities of three characteristic peaks of C–S, C=S/O=S, and C–N (1067, 1200, and 1350 cm⁻¹) were increased gradually when the cell was discharged at ~2.3 V. These results suggested that both Li₂S_n and COF-SH generated strong chemical interaction at continuous discharging process, which reduced the reaction energy barrier and accelerated the catalytic conversion of Li₂S_n.

Ionic migration and interface stability

Based on the alternating current (AC) impedance in Figure 5A, the bulk resistance (R_b) values of PVDF-HFP and COF-SH@PVDF-HFP are 4.984 and 4.062 Ω, corresponding to the ionic conductivity of 1.9 and 3.3 mS cm⁻¹, respectively. The enhanced ionic conductivity of COF-SH@PVDF-HFP may be due to the promotion of Li⁺ transport by the imine radicals and sulfhydryl groups of COF-SH in the composite electrolyte. The Li⁺ migration number can reflect the anchoring effect on the lithium salt anions and polysulfide anions. Figure 5B,C exhibits the results based on chronoamperometry tests and AC impedance. According to the calculations, the Li⁺ transfer numbers of PVDF-HFP and COF-SH@PVDF-HFP were 0.22 and 0.52, respectively. These results indicated that the imine radicals and sulfhydryl groups in COF-SH anchored both the anions dissociated from lithium salt and the SLPs, and accelerated the transport of lithium ions. Figure S4 shows the linear sweep voltammetry of gel electrolytes. The data suggest that the current mutation occurs at ~4.5 V for COF-SH@PVDF-HFP, indicating that COF-SH has no redox reaction below 4.5 V, and COF-SH@PVDF-HFP exhibits better electrochemical stability than the PVDF-HFP. The working voltage is 1.7–2.8 V for Li-S batteries, which suggests that the voltage window of COF-SH@PVDF-HFP is fully satisfied for its application.

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To further verify the stability of PVDF-HFP and COF-SH@PVDF-HFP, the lithium plating/stripping tests of lithium-blocking batteries were carried out. Figure S5 shows the lithium plating/stripping tests under a current density of 1 mA cm⁻² for 1 h. In the first 150 h, the polarization voltages of COF-SH@PVDF-HFP and PVDF-HFP are very close. When it was stabilized, the voltage of COF-SH@PVDF-HFP began to decrease gradually, implying a stable electrochemical process at low current density. To assess the long-life stability, a test at high current density/capacity under 2 mA cm⁻²/5 mAh cm⁻² was carried out, as shown in Figure 5D. The cell based on PVDF-HFP shows an obvious voltage polarization after ~10 h, implying poor cycling stability. PVDF-HFP showed a significant short-circuit phenomenon after 250 h. However, the cell based on COF-SH@PVDF-HFP maintained a stable cycling after a few hours of activation with a low polarization voltage of ~4 mV after 1400 h, which demonstrates that COF-SH@PVDF-HFP possesses a favorable stability for lithium plating/stripping.

In situ Raman spectroscopy was performed to investigate the conversion of SLPs during the initial discharging process. The results are shown in Figure 5E, and the corresponding initial discharge curve at the different discharging stages is shown in Figure 5F. The peaks at 146, 213, and 476 cm⁻¹ were detected at ~2.33 V, which correspond to S₈²⁻ and S₆²⁻ species.^18,59,60 With the continued discharging, two peaks disappear near 2.1 V, indicating that S₈ is reduced to S₈²⁻ and S₆²⁻, and further reduced to low-order lithium polysulfides.^18,61 Similarly, the peaks at 467 cm⁻¹ correspond to the transitions of S₆²⁻ and S₄²⁻, and they are further reduced to lower order S₂⁻ (~330, 394, and 515 cm⁻¹) as the voltage decreased to 1.7 V.^62,63 These results show that the SLPs are effectively confined to the cathode side, which is closely related to the functional modification of the COF-SH. Similarly, the dynamic transformation of SLPs (variation of peak intensity with discharge voltage) is more clearly observed in the contour plot in Figure 5G. These results show that modified COF-SH has successfully adsorbed and catalyzed the transformation of SLPs.

Electrochemical performance

AC impedance tests were carried out based on Li-S batteries to verify the kinetic process during the charge–discharge process. Two semicircles (R2 and R3) were observed both in COF-SH@PVDF-HFP and PVDF-HFP, representing the interfacial impedance with the Li-S cathode and Li anode, respectively, as shown in Figure S6 and Table S2. The low electrochemical impedance implies that COF-SH@PVDF-HFP-based batteries exhibit enhanced interface compatibility and superior transport dynamics of lithium ions. Figure 6A is the cyclic voltammetry (CV) curve based on the COF-SH@PVDF-HFP and PVDF-HFP batteries at a scan rate of 0.2 mV s⁻¹. During the oxidation process, the anodic peak-current voltage difference for anodic side (∆E_anodic based on COF-SH@PVDF-HFP and PVDF-HFP cells was 0.028 V, implying that COF-SH@PVDF-HFP possesses a lower energy barrier of Li₂S/Li₂S₂ oxidization. During the reduction process, the first reduction peak represents the solid-liquid conversion process of S₈ to long-chain SLPs (Li₂S_n, 4 ≤ n ≤ 8).⁶¹ For the PVDF-HFP, at the initial stage of discharge, the solid S₈ is transformed into long-chain Li₂S_n, and then is dissolved in the electrolyte as well as shuttled away from the cathode side with the concentration gradient. However, COF-SH@PVDF-HFP blocks the migration of long-chain Li₂S_n and promotes the accumulation of long-chain Li₂S_n near the cathode side, resulting in a slight voltage hysteresis (COF-SH@ PVDF-HFP/2.28 V vs. PVDF-HFP/2.26 V). In addition, the potential of reduction peak at ~1.96 V for the PVDF-HFP-based battery is lower than that of the COF-SH@PVDF-HFP-based battery (~2.00 V), implying that COF-SH@PVDF-HFP-based battery has a lower potential barrier for Li₂S reduction. The cathodic peak-current voltage differences of ∆E_cathodic1 and ∆E_cathodic2 are 0.02 and 0.03 V, respectively, indicating that COF-SH@PVDF-HFP has slightly fast ion migration kinetics.

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To further investigate the redox kinetics, the local discharge curves at 0.2 C are shown in Figure 6B. Evidently, the nucleation potential of Li₂S/Li₂S₂ for COF-SH@PVDF-HFP (2.08 V) is lower than that of PVDF-HFP (2.05 V). In the charging curve (Figure 6C), the nucleation potential of S₈ for COF-SH@PVDF-HFP is 2.26 V, lower than that of PVDF-HFP (2.19 V). These results show that COF-SH@PVDF-HFP-based batteries have faster reaction kinetics in the redox process. Figure 6D shows the cycling performance at 0.2 C and the corresponding initial charge/discharge curves (Figure S7). The COF-SH@PVDF-HFP and PVDF-HFP batteries delivered specific capacities of 1359.5 and 1213.1 mAh g⁻¹, respectively. The reversible capacities are 628.8 and 105.4 mAh g⁻¹ after 190 cycles, corresponding to the capacity retention of 46.3% and 8.6%, respectively. However, the PVDF-HFP modified by COF-SH promotes the closure of the pore structure of the spinning fiber, and meanwhile, a large number of imine radicals and sulfhydryl groups of COF-SH realize the adsorption and catalytic conversion of SLPs. Thus, a decent rate of performance was exhibited, as shown in Figure 6E. As the current density was increased from 0.1 to 2 C, the reversible capacities of COF-SH@PVDF-HFP-based batteries were 1286.3, 929.3, 816.8, 759, and 688.7 mAh g⁻¹, respectively. When the current density was reset to 0.1 C, the reversible capacity was 915.8 mAh g⁻¹. However, the reversible capacities of PVDF-HFP-based batteries were 594.3, 334.7, 226.3, 149.1, and 58.6 mAh g⁻¹ under the same conditions, respectively. When the current density was reset to 0.1 C, the rechargeable specific capacity was only 284.7 mAh g⁻¹. This large difference in rate performance indicates a severe shuttling effect in PVDF-HFP-based cells. Figure S8 shows the charge–discharge curves based on the rate tests. Under the same conditions, COF-SH@PVDF-HFP batteries have a lower polarization and longer charge–discharge plateau than that of PVDF-HFP, indicating that COF-SH@PVDF-HFP used as a gel electrolyte can effectively reduce the polarization, including the intrinsic polarization caused by the conductivity and the polarization related to the energy barrier of the electrochemical reaction. Figure 6F shows the cycling performance of mass high-loading (~5 mg cm⁻²) Li-S cells based on COF-SH@PVDF-HFP. Evidently, under a low E/S ratio (4 μL mg⁻¹), the cells exhibit excellent cycling stability at 0.3 C after 100 cycles with a capacity retention rate of 66.1%. Figure 6G further discloses their stability even at a high current density of 2 C. It exhibits an outstanding long-life performance of 568.8 mAh g⁻¹ after 800 cycles with a capacity retention of 77.3% (a decay rate of only 0.03% per cycle), showing an excellent long cycle life. Nevertheless, the PVDF-HFP-based battery had a rapid decline in capacity in subsequent tests, suggesting a serious shuttle effect. In addition, the capacity fading rates of Li-S batteries based on various modified separator/gel electrolytes were compared for long cycles (Figure 6H and Table S1). The COF-SH@PVDF-HFP cell holds a fading rate of only 0.03% per cycle at 2 C for 800 times, indicating excellent SLP inhibition and fast reaction kinetics.

CONCLUSION

A quasi-solid-state Li-S battery with a suitable E/S ratio has been developed. In this structure of quasi-solid-state electrolyte, the sulfhydryl and imine groups functionalized COFs were riveted in situ on the PVDF-HFP nanofibers, which improved the chemical affinity of COF-SH@PVDF-HFP to polysulfide, greatly reduced the electrolyte contents, and promoted the catalytic conversion of lithium polysulfide. This was proven by in situ FTIR, in situ Raman, UV–vis spectroscopy, and XPS spectra, together with DFT theoretical calculations. The quasi-solid-state Li-S battery shows a stable lithium plating/stripping behavior, and good cycling stability and rate capability. This research provides an effective design strategy for improving the redox kinetics of sulfur for the development of a quasi-solid-state Li-S battery.

ACKNOWLEDGMENTS

The authors thank the Shiyanjia Lab () for the TEM analysis. This research was supported by the National Natural Science Foundation of China (52202104), the Joint Funds of the Zhejiang Provincial Natural Science Foundation of China (LZY23B030002), the China Postdoctoral Science Foundation (2021T140433, 2020M683408), the Quzhou Science and Technology Bureau Project (2022D015, 2023D023), the International Cooperation Projects of Sichuan Provincial Department of Science and Technology (2021YFH0126), the Fundamental Research Funds for the Central Universities (ZYGX2020ZB016), the Key Research and Development Program of Yunnan Province China (202103AA080019), and Yunnan Major Scientific and Technological Projects (202202AG050003).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

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