English

• 0.   Introduction

  The variety of electronic devices and electrical facilities drives the application of high-energy storage lithium-ion batteries^[1-3]. Due to the potential performance and abundance, research on Li-S batteries has witnessed its prosperity in recent decades though the progress of commercialization is still hindered by the obstacles that exist in the cycle process^[4-8]. Among all the challenges, the following ones have been extensively studied. The nearly insulating nature of sulfur would cause a poor utilization of electrode material and limit the electron transfer during the electrochemical process. The "shuttle effect" caused by intermediate lithium polysulfides (LIPs, Li₂S_n, 4 ≤ n ≤ 8) could lead to fast capacity fading^[9-10]. Meanwhile, the growth of lithium dendrites has become another important factor limiting battery safety.

  Vast efforts have been made to seek a suitable measure to solve the problems in Li-S batteries^[11], like choosing effective sulfur host materials^[12-15], modification of the lithium metal electrode^[16-18], investigation of electrolyte additive effects^[19-21], and surface modification of membranes^[22-26]. Due to their large specific surface area and excellent conductivity, carbon materials are appropriate hosts for sulfur which enables the adsorption of polysulfides, suppression of shuttle effect, facilitation of rapid electron transfer, and enhancement of active material utilization^[27-30]. Ishikawa et al. applied polyglycerol-treated porous carbon for anchoring sulfur cathode and the batteries achieved 75.2% capacity retention after 50 cycles^[13]. Lin et al. adopted reduced graphene as a "lithiophilic" anode composite, which exhibited good mechanical flexibility and low overpotential^[31]. Zhang et. al. combined cyclic carbonate solvents and anions to regulate the solvation sheath of lithium ions, which obtained uniform and dendrite-free lithium deposition^[21]. Recent theoretical research on fast Li-ion transportation in carbon nanotubes (CNTs) also sheds light on strategies to design solid electrolytes^[32]. As one of the components of Li-S battery, the separator plays an important role not only in isolating the positive and negative electrodes but also in inhibiting the notorious polysulfide shuttling^[33-34]. Separator decoration is another effective strategy for solving the above problems, the functional separator with active materials can absorb the LIPs to improve the high electrochemical performance of Li-S batteries.

  The conventional separator modification is mainly by coating a thin layer of active material on the separator to block the migration of LIPs^{[33, 35-36]}. Functional active materials such as metal oxides, carbon material, and metal-organic frameworks (MOFs) are used to anchor LIPs by chemisorption and electrocatalysis reactions. Due to its various structures and sources, the carbon material is also adopted to modify the separator in more and more cases. The super P, Ketjen black, graphene, carbon nanotubes (CNTs), and biomass carbon materials are used as separator modification materials to enhance the sulfur fixation of the cathode^[37-42]. Chung and co-worker^[37] used multi-wall carbon nanotubes (MCNTs) to modify the polypropylene (PP) separator, the cathode obtained a high specific capacity of 300 mAh·g^-1 at the current density of 1C rate. The MCNTs modification provides an excellent conductivity which accelerates the Li⁺ transport and the coating can hinder the shuttle effect of LIPs. Chen and coworkers reported a PP separator coated by Al₂O₃ and CNTs layers to highly improve the cycling stability and rate performance of Li-S batteries^[20]. The multifunctional separator with various methods can significantly improve the safety and electrochemical performance of Li-S batteries.

  Herein, a bifunctional separator decorated by hollow carbon (HC) material was facilely prepared and the electrocatalysis performance of HC was deeply tested. The HC-modified separator provides multichannel for ion transportation, which improves the dynamic process of electrochemical reaction. The Li-S batteries assembled with the bifunctional separator showed outstanding cycling and rate performance. The origin-specific capacity of 1 035 mAh·g^-1 was realized and a higher specific capacity of 500 mAh·g^-1 was kept after 700 reversible cycles. A high reversible specific capacity was still kept at 500 mAh·g^-1 with a sulfur content of 3.2 mg·cm^-2. The double-side modified separator can not only adsorb the LIPs but also plays a robust physical barrier to prevent the growth of lithium dendrites. This work may indicate the scraping strategy to be a useful method for designing high-performance Li-S battery systems.

  1.   Experimental

  1.1   Synthesis of HC nanospheres

  The active material (HC nanospheres) were synthesized according to the previous report with some adjustments^[43]. 7.2 mmol·L^-1 resorcinol (Shanghai Macklin Biochemical Co., Ltd.) and 6.6 mmol·L^-1 hexadecyltrimethylammonium bromide (CTAB, Shanghai Titan Scientific Co., Ltd.) were vigorously stirred in 120 mL ethanol/water solution (V_water∶V_ethanol=2∶1). After mixing well, 0.8 mL NH₃·H₂O was added into the solution and continued to stir for 20 min. Then, 4 mL tetraethoxysilane (TEOS, Shanghai Titan Scientific Co., Ltd.) and 1.12 mL formaldehyde solution (Shanghai Titan Scientific Co., Ltd.) were added into the mixed solution and the mixture kept stirring for 24 h before transferring to 200 mL stainless steel Teflon-lined autoclave for another 24 h at 80 ℃. Further, the precursors were annealed for 3 h at 850 ℃ in a tubular furnace (Ar atmosphere, 3 ℃·min^-1). Then, HC was obtained by etching SiO₂ templates with 10% HF.

  1.2   Synthesis of HC interlayer and HC sulfur loader

  The flexible interlayers were built up by HC and the binder (polyvinylidene fluoride, PVDF) with a mass ratio of 9∶1. The slurry was dispersed in the solvent of N-methyl-2-pyrrolidinone (NMP) and then coated on a PP separator (Celgard 3501) by a medical scraper with a single side with 50 μm thickness. The modified interlayers were dried in a vacuum at 40 ℃ for 24 h to obtain a single-side modified membrane (PP/HC). Double-side modified membrane (PP/HCD) was prepared by scraping the PP/HC separator with slurry again, with a thickness of 100 μm. The separators were punched into 19 mm diameter disks to obtain the modified interlayers. HC spheres and pure sulfur (mass ratio of 3∶7) were mixed and heated in an argon atmosphere at 155 ℃ for 24 h to make the HC sulfur loader (HCS).

  1.3   Structure characterization

  The morphologies of the HC, HCS, PP, and modified separators were tested by scanning electron microscopy (SEM) (Carl Zeiss Ultra 55). The microstructure of these materials was examined by transmission electron microscope (TEM, TALOS F200X) at acceleration voltages of 200 kV. The structure information of the material mentioned above was performed by X-ray diffraction (XRD, D8 Advance, Bruker Corporation) with Cu Kα radiation (λ=0.154 16 nm) at an operating voltage of 40 kV and current of 40 mA at a scanning rate of 5 (°)·min^-1, and the 2θ ranged from 10° to 80°. The S contents in the composites were confirmed by a thermogravimetric analysis (TGA) with a temperature range of 50-600 ℃ under a nitrogen atmosphere at a heating rate of 5 ℃·min^-1. The specific surface area and pore size distribution were characterized with the Micromeritics ASAP 2020 surface analyzer. The specific surface area was obtained by the Brunauer-Emmett-Teller (BET) method. The pore size distributions were derived from the adsorption branches of isotherms using the Barrett-Joyner-Halenda (BJH) model.

  1.4   Preparation of Li₂S₆ and Li₂S₈ solution

  The Li₂S₆ and Li₂S₈ solutions were chosen to explore the catalytic and Li₂S nucleation properties of HC. The Li₂S and S powders (Shanghai Macklin Biochemical Co., Ltd.) were mixed with a molar ratio of 1∶5 for the Li₂S₆ solution and 1∶7 for the Li₂S₈ solution, respectively. The solvents of 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) solution (1∶1 in volume) were added into the mixture and then vigorously stirred at 60 ℃ for 24 h. All preparations and tests were operated in the glove box full of argon to avoid contact air.

  1.5   Assemble of cells and the electrochemical performance tests

  The symmetrical cells consisted of cathode and lithium cells. The cathodes were prepared by mixing HC, a conductive additive of carbon black, and a binder of PVDF with a mass ratio of 8∶1∶1. A homogeneous slurry was obtained after a vigorous stir process, and then the active materials were scraped on aluminum foil. After that, the aluminum foil with active material was cut into disks for further assembly process. The cathode symmetrical cells were assembled by using a CR2016 coin cell with a PP separator and Li₂S₆ electrolyte. Similarly, the lithium symmetrical cells were assembled by the same step with Li metal as electrodes. By using Li metal as anode and HCS as cathode, Li-S batteries using CR2016 coin cells were assembled with different separators of PP, PP/HC, and PP/HCD and labeled by PP-HCS, PP/HC-HCS, and PP/HCD-HCS. The electrolyte used in lithium symmetrical cells and Li-S cells was 1 mol·L^-1 lithium bistrifluoromethanesulfonyl imide (LiTFSI) dissolved in a mixture of DME/DOL (1∶1 in volume) and 1.0% LiNO₃ as additive. All the assembled processes were operated in an Ar-filled glove box (ρ_H2O < 0.08 mg·m^-3, ρ_O2 < 0.1 mg·m^-3). The Land battery testing equipment was managed for galvanostatic charge-discharge cycling and rate performance. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on the Chenhua CHI 760E electrochemical workstation (Shanghai, China). The EIS was conducted under open circuit potential conditions with frequencies ranging from 10⁵-10^-2 Hz. The CV curves and the cycle process were performed at a potential range of 1.7-2.8 V.

  2.   Results and discussion

  2.1   Morphology and contact angle test

  The SEM morphological characterization of HC and HCS is shown in Fig. 1. The abundant signals of C and S elements in Fig. 1d-1f indicated that S was successfully loaded into HC. In Fig.S1 (Supporting information), the TEM test results were added to the supporting information. The diameter of HC was between 500 nm-1 μm, the shell thickness was ca. 100 nm, this result is consistent with SEM. In addition, the HC spheres were filled with micropores. After molten sulfur loading, the structure of HC remained unchanged. The tolerance of high temperature can be a strong characteristic of the property for bearing the variation of temperature. The digital comparison changes of PP and the functional separators of PP/HC and PP/HCD under the temperature variation are displayed in Fig.S2. The surface of functional coating separators obtained by scraping was uniform, with no powder loss after folding and unfolding, which indicates that the modified layer had a tight connection with the PP separator. The photos clearly showed that the state of the original PP separator became smaller with shrinkage at 90 ℃, and the edge started melting at 110 ℃. For the PP/HC separator, no significant changes were found under 90 ℃, but curls were found at 110 ℃. This phenomenon may be caused by the uneven mass distribution on both sides. For the PP/HCD separator, only a little curly edge at 110 ℃ emerged, which demonstrated a stable high-temperature tolerance.

  Figure 1

  

  Figure 1.  SEM images of (a, b) HC and (c, d) HCS; (e, f) EDS (energy dispersive spectroscopy) mappings of C and S elements for HCS; (g, h) Dynamic contact angle measurements with electrolyte and Li₂S₆

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  To study the effect of a modified separator on the wettability of the electrolyte, the contact angle was investigated at the interface intersection between the electrolyte and separators, as shown in Fig. 1g-1h. The conventional PP separator formed a 34.8° contact angle, PP/HC performed 7.6° while the contact angle of the PP/HCD separator approached zero. Both indicate better affinity to the electrolyte than the bare PP separator. The low surface tension of the PP separator due to the non-polar interaction of the polymer itself causes a relatively large contact angle with the polar electrolyte^[44]. By modifying the surface, the surface roughness was increased which results in a larger surface tension and thus a better wettability. Compared with the conventional PP separator, the electrolyte is easier to permeate the modified separator, which facilitates rapid Li⁺ transportation during the charge and discharge process. The results in the Li₂S₆ solution were similar to those in the electrolyte, that is, the PP separator also presented poorer wettability. The permeability of Li₂S₆ for each separator could be weaker than that of the electrolyte, which indicates a general delaying of the soluble LIPs and more evidence can be found in the permeation tests.

  2.2   Structure characterization

  The surface and the cross-section morphology of the PP separator and modified separator are shown in Fig. 2a-2d. The original PP separator showed a porous state, and PP/HC presented a uniform modification layer. As the cross-sectional view of the HC separator in Fig. 2c and 2d, the thickness layer of the modified part was about 15 μm. The SEM results showed that the HC successfully coated on the surface of the PP separator with a flat surface condition. To investigate the structure and composition of the cathode materials, XRD and nitrogen adsorption-desorption tests were implemented. Peaks for sulfur can be located from XRD patterns of HCS which indicates a successful load of sulfur in HC (Fig. 2e). The N₂ adsorption-desorption isotherms and pore distribution of the samples are displayed in Fig. 2f, the results illustrated the coexistence of micro-, meso-, and macropores. The specific surfaces of HC and HCS were 1 127 and 109 m²·g^-1, respectively. The characteristic sulfur element peaks indicate a successful loading of S. Further, the inset characterization of HC showed two distinctive characteristic peaks located at 23.8° and 43.4° corresponding to (002) and (101) crystal planes of HC, respectively^[43]. As shown in Fig.S3, the TGA test of HCS exhibited a high sulfur content of 67%.

  Figure 2

  

  Figure 2.  Surface (a, b) and section (c, d) SEM images of the PP separator (a, c) and the PP/HC separator (b, d); (e) XRD patterns and (f) N₂ adsorption‐desorption isotherms of HC and HCS

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  2.3   Electrochemical performance

  For investigating the electrochemical performance based on different separators, the coin-type Li-S cells with sulfur loading of 1.8-2.0 mg·cm^-2 were assembled. The CV was executed to explore the electrochemical process. In the cathodic process (Fig. 3a), the CV curves of the HCS cathode delivered two negative peaks at 2.23 and 2.05 V, which correspond to the reduction process of S element to high-order soluble polysulfides (Li₂S_n, 4 ≤ n ≤ 8) and low-order insoluble Li₂S/Li₂S, respectively^[45]. A shoulder peak located at ca. 2.40 V of HCS is related to the oxidation process of Li₂S/Li₂S₂ to high-order polysulfides Li₂S_n (4 ≤ n ≤ 8). PP/HC-HCS and PP/HCD-HCS had similar location reaction peaks as PP-HCS. However, a large polarization voltage of PP/HC-HCS may be caused by the instability state of the material during the initial charge-discharge process. The charge-discharge curves of the first three cycles in Fig.S4 were consistent with the peak positions of the CV curves. By comparison, the voltage plateau in charge-discharge curves of PP-HCS was inconspicuous. The plateau was almost absent at ca. 2.30 V, indicating that the product's polysulfide quickly dissolved in the electrolyte causing rapid capacity degradation. For the PP/HCD-HCS cell with an obvious voltage plateau, the comparison of charging curves reveals that the Coulombic efficiency of the first three cycles was lower, which may indicate the ineffective release of the reversible capacity induced by the higher resistance during the infiltration process of the electrolyte and more thickness of the electrode. As the reaction proceeded, the electrolyte was completely infiltrated for stabilized Coulombic efficiency.

  Figure 3

  

  Figure 3.  (a) CV curves at the scanning rate of 0.1 mV·s^-1 and (b) cycle performance at a current density of 0.2C of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells; Reversible charge‐discharge curves of the (c) PP‐HCS, (d) PP/HC‐HCS, and (e) PP/HCD‐HCS cells at different cycles

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  The cycling performance of the Li-S cells with modified separators was tested by constant current charge-discharge at the current density of 0.2C (0.2C=335 mA·g^-1). The result after 700 cycles of the test is listed in Fig. 3b. To sufficiently activate the electrode materials, the current density with 0.05C was operated at the initial ten cycles. The overall trend was similar for PP-HCS and PP/HC-HCS, with the initial specific capacity showing a status of decreasing, then increasing, and finally decreasing. For the PP/HCD-HCS cell, the starting specific capacity reached 1 035 mAh·g^-1, which rapidly decreased to 978 mAh·g^-1 in the second cycle, and the Coulombic efficiency was low during this process. This may imply the consumption of a large amount of lithium ions during the formation of the solid electrolyte interface (SEI) film in the early stage, which causes a certain irreversible capacity. The specific capacity of PP/HC-HCS and PP/HCD-HCS cells with modified separators were higher than PP-HCS, and the PP/HCD separator showed the highest specific capacity of ca. 500 mAh·g^-1 after 700 cycles. As for PP-HCS and PP/HC-HCS, the initial activation delivered a high specific capacity, but the specific capacity decreased sharply when the current density increased to 0.2C, indicating that the cells cannot adapt to a rapid increase in current.

  The detailed charge-discharge properties of PP-HCS, PP/HC-HCS, and PP/HCD-HCS with 1st, 10th, 20th, 50th, 100th, 200th, 300th and 600th cycles are displayed in Fig. 3c-3e. The polarization voltage (the voltage window between the charging and discharging curve platform) of PP/HCD-HCS was the minimum, which is consistent with the CV test. The specific capacity of the batteries at the voltage higher plateau (Q_H) and lower plateau (Q_L) are presented in Fig.S5. The PP-HCS cell showed a low Q_H and Q_L with capacity retention rates (CRR) of 52% and 59% based on the 100th cycle. The CRR of PP/HC-HCS had higher values of 61% and 69%, and that of PP/HCD-HCS was 77% and 76%, respectively. As determined by the capacity of the battery, a high Q_L reflects the fast reaction kinetics for LiPSs to Li₂S/Li₂S₂. Therefore, the modified separator can effectively reduce the shuttle effect of LIPs during cycling.

  The CV test can provide insight into the redox kinetic properties inside the electrode structure. Fig. 4a and Fig.S6 show the CV curves of the cell assembled with PP/HCD-HCS at different sweep speeds (v) and the variation of the CV curve at different cycle times, respectively. In Fig. 4b, a good linearity plot of the peak current (I_p) against v^0.5 at different sweep speeds indicated that the main kinetics of ions inside the cell were controlled by a diffusion process. The CV curves had similar shapes at different cycles, with the oxidation peaks at the 1st, 5th, 10th, and 50th cycles showing two shoulder peaks, while a single peak appeared after 100 cycles (Fig. 4c). The CV plots at different cycles were decomposed and analyzed by multiple peaks fitting in Fig.S6. The oxidation peak at ca. 2.35 V could be decomposed and fitted into three peaks, of which two consecutive oxidation peaks were located at ca. 2.33 and 2.40 V corresponding to the conversion of Li₂S₂/Li₂S to Li₂S_n and S₈. The reduction peak at ca. 2.00 V can be fitted to ca. 1.94 and 2.03 V, which mainly correspond to the conversion of Li₂S_n to Li₂S₂/Li₂S. Usually, the magnitude peak current and area correspond to the change of electrode material-specific capacity^[46-47]. The peak current and area at 1st, 5th, 10th, and 50th were large and close to each other in size, which indicates the stability in the origin cycle process. The width of the oxidation peak decreased, and the position of the reduction peak showed a positive shift as the cycles increased. It is found that the oxidation peak is attributed to the process of conversion of Li₂S₂/Li₂S to S₈, and the LIPs conversion runs through the whole reaction process. Therefore, accelerating the conversion process of LIPs can effectively reduce capacity decay. The rate-specific capacity of the PP-HCS, PP/HC-HCS, and PP/HCD-HCS cells are displayed in Fig. 4d. The PP/HCD-HCS cell with PP/HCD separator had a good rate of performance and excellent recovery after charge-discharge process at a high current density. Furthermore, the cell with a PP/HCD separator had a high specific capacity of 708 mAh·g^-1 at a current density of 1C (1C=1 675 mA·g^-1) and 630 mAh·g^-1 after 100 cycles (Fig. 4e), and a high specific capacity of 505 mAh·g^-1 at the current density of 2C (Fig. 4f).

  Figure 4

  

  Figure 4.  (a) CV curves of PP/HCD‐HCS cells at different scan rates and (b) the relation between I_p and v^0.5 according to a; (c) CV curves of PP/HCD‐HCS under different cycles; (d) Rate performance of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells; Cycle performance of PP/HCD‐HCS at the current density of (e) 1C and (f) 2C; (g) GITT curves and (h‐j) detailed GITT description curves of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells

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  To investigate the diffusion behavior of Li⁺ of the electrode material during the cycling, GITT (galvanostatic intermittent titration technique) tests in Fig. 4g-4j were performed on the batteries assembled with different separators. The detailed enforcement process in our previous work^[29], and the diffusion coefficient of Li⁺ (D_Li⁺) can be calculated by the formula: D_Li⁺=4n_m²V_m²(ΔE_s)²/[πτS²(ΔE_t)²], where n_m is the amount of substance of the active material (mol), V_m is the molar volume of the electrode (cm³·mol^-1), ΔE_s is the change in voltage due to pulses and ΔE_t is the change in voltage due to the constant current charging (discharging) process, S is the contact area (cm²) and τ is the relaxation time (s). The lg D_Li⁺ distribution of the sample indicated that the value in the charging process was larger than that in the discharging process, and the value was larger in the pre-discharge phase than in the end-discharge phase. This change in D_Li⁺ is mainly due to the conversion of LIPs during the solid-liquid-liquid-solid interconversion process. After averaging, the D_Li⁺ of PP-HCS, PP/HC-HCS, and PP/HCD-HCS were 8.64×10^-13, 7.64×10^-12, and 8.20×10^-12 cm²·s^-1. The modification of the separator plays a positive role in improving lithium diffusion. HC with electrocatalytic performance can accelerate the conversion of LIPs, thus the cells with decorated separators have more cycling stability.

  The polysulfide permeation tests were operated by H-type cast in Fig. 5, where the PP, PP/HC, and PP/HCD separators were put into the middle of the mold with Li₂S₆ electrolyte on the left and the same volume of the base electrolyte on the right. The shuttle of polysulfide can be judged by the color change of the base electrolyte on the right side. As for the PP separator, the base electrolyte became darker yellow after 100 min, the PP/HC separator was slightly yellow while the PP/HCD separator was nearly colorless and transparent. In contrast, the modified separator plays an important role in delaying the shuttle of polysulfides. The hollow porous structure can provide more adsorption sites and ion transport channels to firmly bind the polysulfides. The double-side modified separator is favorable to be the LIPs barrier, thus reducing the shuttle effect of LIPs.

  Figure 5

  

  Figure 5.  Polysulfide permeation tests using the (a‐d) PP, (e‐h) PP/HC, and (i‐l) PP/HCD separators

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  However, the long-time penetration experiments have shown that separators modified with HC can provide a certain blocking effect on LIPs in a short period, but after long-term testing, the blocking effect of this coating has been weakened. Based on performance testing of the battery and analysis of the catalytic performance of electrode materials, it can be concluded that the catalytic effect of the coated HC can accelerate the conversion of long-chain LIPs during the charging and discharging process of the battery, which is the main reason for improving the stability performance of the battery. The catalytic performance of HC was explored by assembling the CR2016 cells with symmetrical cathodes of HC for work and counter electrodes.

  To explore the electrocatalytic performance of HC, the current response test of the cathode symmetrical batteries was implemented with and without Li₂S₆ in the electrolyte. As in Fig. 6a, a poor current response was presented without Li₂S₆ in the electrolyte, however, the obvious redox peaks at 0.033, 0.125, 0.402, -0.033, -0.125, and -0.402 V were obtained with Li₂S₆ in the electrolyte. The result indicates that HC has a certain catalytic effect on the reduction of polysulfides. The deposition of Li₂S in Fig. 6b clearly showed that HC could accelerate the nucleation process of Li₂S to promote the conversion of polysulfide. The current curve fell at the initial state which can be accounted for in the reduction process of long-chain LiPS to short-chain LiPS. The current increased since the nucleation of Li₂S, and the calculated precipitation capacity according to Faraday′s law of HC was 52.3 mAh·g^-1. The result demonstrates that the HC substrate delivers a positive effect in promoting the nucleation and growth of Li₂S.

  Figure 6

  

  Figure 6.  (a) CV curves of cathode symmetric cells with and without Li₂S₆; (b) Potentiostatic discharge profiles at 2.06 V; Galvanostatic cycling of symmetric cells with different separators at (c) 0.5 and (d) 1 mA·cm², respectively

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  The voltage-time profile of the symmetric lithium batteries with different separators was displayed in Fig. 6c, 6d, and S7, in which the current density was 0.5 and 1 mA·cm^-2, respectively. The voltage curve for symmetric cells with PP separators was stable at the origin cycle, but a sudden fluctuation in later cycles indicated poor stability of the cell. In contrast, the modified separators showed a stable state throughout the test. There was a slight fluctuation in the symmetric cells with PP/HCD separators in the origin process, which can be explained by the unstable SEI formation process^[48]. Such a process could be understood through the SEM images in Fig.S8-S10. The comparison reveals that the modified separator of PP/HCD can effectively reduce the growth of lithium dendrites. The double side decoration of the separator can form a bifunctional role, absorbing LIPs and preventing the growth of lithium dendrites, thus improving the electrochemical performance of the battery.The Nyquist plots of PP-HCS, PP/HC-HCS, and PP/HCD-HCS cells before cycling were obtained at the open circuit voltage in the frequency range of 10⁵-10^-2 Hz as displayed in Fig. 7a. The Nyquist curves of the samples were similar, consisting of two semicircles in the mid-high frequency region and a sloping line in low frequency. The relation between impedance and low frequency is shown in Fig. 7b. The D_Li⁺ can be predicted by the slope of the fitting line through the equation^[49]: D_Li⁺=R²T²/(2A²n_e⁴F⁴c²σ²), while R is the gas constant (8.314 J·K^-1·mol^-1), T is the electrode working temperature (25 ℃), A is the area of the active material, n_e is the number of the electrons transferred in the redox process, F is the Faraday constant (96 485 C·mol^-1), c is the Li⁺ concentration (c=n/V, where n is the amount of substance of active material and V is the volume of active material), and σ is the slope (σ=Z′/ω^-1/2, where Z′ is the real part of impedance and ω is the frequency of external signal) (Fig. 7b and 7e). The D_Li⁺ of PP-HCS, PP/HC-HCS, and PP/HCD-HCS before cycling was 1.02×10^-13, 8.03×10^-14, and 5.24×10^-12 cm²·s^-1, respectively. The result is consistent with the GITT method. The PP/HCD separator possesses the fastest ion transport process, which may be caused by the numerous channels of the HC coating. The physical retard and electrochemical catalyzation of LIPs through the double-side modification separator can improve the performance of the cells. The fitting lines of Nyquist plots were treated by ZSimpWin software to identify the equivalent circuit in Fig. 7c. The interface impedance (R_int), the charge transfer impedance (R_ct), and the impedance of SEI film (R_SEI) are associated with the two semicircles in Fig. 7a, and the Warburg diffusion impedance (W) is related with the straight line in the low-frequency range in Fig. 7a. The constant phase angle elements (CPE1, CPE2, and CPE3) in the equivalent circuit indicates the capacitive behavior in the charge-discharge process. All fitted values are shown in Table S1. The R_s of PP-HCS before cycling were small, while the R_s of modified separators were larger, which may be due to the slow wetting of the electrolyte caused by the modified separators.

  Figure 7

  

  Figure 7.  (a) Nyquist curves and (b) Z'‐ω^-1/2 curves of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells; (c) Equivalent circuit according to the Nyquist curves; (d) Nyquist curves of PP/HCD‐HCS cell at different cycles and (e) Z'‐ω^-1/2 curves; (f) Illustration for the diffusion of polysulfides with PP and HC decorated separators in the cells

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  To investigate the change of the electrode material during the cycling process, the Nyquist plots of the PP/HCD-HCS cell after the 1st, 5th, 10th, 50th, and 100th cycles are displayed in Fig. 7d, 7e, and Table S2. The R_SEI after 1st cycle was 226.7 Ω. The high impedance in the origin process may be caused by the formation of SEI. Through calculation, D_Li⁺ of 1st, 5th, 10th, 50th, and 100th cycles were 5.02×10^-12, 8.64×10^-12, 7.07×10^-12, 9.68×10^-12, and 7.96×10^-12 cm²·s^-1, respectively. The similar D_Li⁺ indicates a stable state of ion transport, and the PP/HCD separator provides a multichannel for ensuring the stable diffusion behavior of lithium ions. Fig. 7f schematically shows the inhibition of LIPs by the modified separator of HC. As the LIPs pass through the HC-modified separator coating in the electrochemical process, the adsorption and electrocatalysis by HC can accelerate ion transport and anchor LIPs. Therefore, the modified separator can reduce the shuttle effect of LIPs, thus making the specific capacity and cycling stability of the electrode material have high improvement.

  To develop the high-loading performance of the electrode material, the cell was assembled with 3.2 mg·cm^-2 active material loading and a PP/HCD separator. The cycle ability was examined at the current density of 0.2C in Fig.S11. A reversible specific capacity of ca. 500 mAh·g^-1 can be obtained after high loading, and the specific capacity of ca. 360 mAh·g^-1 was still available after 100 cycles. This indicates that the battery assembled with the PP/HCD separator has excellent cycling performance at high loading, which offers a commercial application potential.

  3.   Conclusions

  In this work, the PP/HC and PP/HCD separators modified by HC were obtained and applied in Li-S batteries. The double-side decoration can effectively adsorb the lithium polysulfide and resist the growth of lithium dendrites. The multichannel and electrocatalysis of HC can also accelerate ion transportation, which is beneficial to improving its electrochemical performance. With the PP/HCD separator, the starting specific capacity of 1 035 mAh·g^-1 was achieved at a sulfur content of 1.8 mg·cm^-2, and the high specific capacity of 500 mAh·g^-1 was achieved after 700 reversible cycles. The reversible specific capacity was still kept at 500 mAh·g^-1 with a sulfur content of 3.2 mg·cm^-2. The double-side modification of the separator with HC can both adsorb polysulfides and inhibit the growth of lithium dendrites, which is dual insurance to enhance the cycle capacity and reversible stability of the electrode material. Importantly, the simple scraping method utilized in this work is appropriate for large-scale manufacturing to industrialize the Li-S batteries.

  Supporting information is available at http://www.wjhxxb.cn

  
  
• 1. [1]

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  Figure 1  SEM images of (a, b) HC and (c, d) HCS; (e, f) EDS (energy dispersive spectroscopy) mappings of C and S elements for HCS; (g, h) Dynamic contact angle measurements with electrolyte and Li₂S₆

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  Figure 2  Surface (a, b) and section (c, d) SEM images of the PP separator (a, c) and the PP/HC separator (b, d); (e) XRD patterns and (f) N₂ adsorption‐desorption isotherms of HC and HCS

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  Figure 3  (a) CV curves at the scanning rate of 0.1 mV·s^-1 and (b) cycle performance at a current density of 0.2C of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells; Reversible charge‐discharge curves of the (c) PP‐HCS, (d) PP/HC‐HCS, and (e) PP/HCD‐HCS cells at different cycles

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  Figure 4  (a) CV curves of PP/HCD‐HCS cells at different scan rates and (b) the relation between I_p and v^0.5 according to a; (c) CV curves of PP/HCD‐HCS under different cycles; (d) Rate performance of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells; Cycle performance of PP/HCD‐HCS at the current density of (e) 1C and (f) 2C; (g) GITT curves and (h‐j) detailed GITT description curves of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells

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  Figure 5  Polysulfide permeation tests using the (a‐d) PP, (e‐h) PP/HC, and (i‐l) PP/HCD separators

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  Figure 6  (a) CV curves of cathode symmetric cells with and without Li₂S₆; (b) Potentiostatic discharge profiles at 2.06 V; Galvanostatic cycling of symmetric cells with different separators at (c) 0.5 and (d) 1 mA·cm², respectively

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  Figure 7  (a) Nyquist curves and (b) Z'‐ω^-1/2 curves of the PP‐HCS, PP/HC‐HCS, and PP/HCD‐HCS cells; (c) Equivalent circuit according to the Nyquist curves; (d) Nyquist curves of PP/HCD‐HCS cell at different cycles and (e) Z'‐ω^-1/2 curves; (f) Illustration for the diffusion of polysulfides with PP and HC decorated separators in the cells

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