An Electronegative Modified Separator with Semifused Pores as a

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General Research

An Electronegative Modified Separator with Semi-Fused Pores as Selective Barrier for Highly Stable Lithium–Sulfur Batteries Junxiao Wang, Mengjia Li, Chong Liu, Yong Liu, Tongkun Zhao, Pengfei Zhai, and Jingtao Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01727 • Publication Date (Web): 11 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Industrial & Engineering Chemistry Research

An Electronegative Modified Separator with SemiFused Pores as Selective Barrier for Highly Stable Lithium–Sulfur Batteries Junxiao Wang,a Mengjia Li,b Chong Liu,a Yong Liu,* c Tongkun Zhao,a Pengfei Zhai,a Jingtao Wang* a

c

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School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R. China

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School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China

National Center for Research & Popularization on Calcium, Magnesium, Phosphate and Compound Fertilizer Technology, Zhengzhou University, Zhengzhou 450001, P. R. China

S Supporting Information ●

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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The rapid capacity fading and poor cycling life caused by polysulfide shuttle severely limit the development of high-energy-density lithium–sulfur batteries. Herein, a new type of electronegative modified separator with semi-fused pores is designed to suppress polysulfide shuttle and simultaneously elevate Li+ conduction. This separator is facilely prepared by lithiation of Nafion/polyacrylic acid electrospun fiber mat on commercial separator. Abundant electronegative groups on the separator strongly repel anionic polysulfides, effectively inhibiting the shuttle effect. Moreover, these groups provide numerous sites for Li+ transfer, and meanwhile the semi-fused pores work as low-barrier transport pathways, jointly facilitating Li+ conduction. Therefore, the battery exhibits outstanding cycling stability (decay rate of 0.023% per cycle during 1000 cycles at 1.0 C) and decent rate capability (discharge capacity of 730 mAh g−1 even at 3.0 C). Additionally, the sulfonated poly (ether ether ketone) is taken as an example to verify the universality of this design strategy. KEYWORDS: electrospun nanofiber, semi-fused pore, ion selective separator, polysulfide shuttle, lithium−sulfur batteries

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1. INTRODUCTION As the requisition for large-scale energy storage system is increasing constantly, exploring and utilizing high-energy-density battery systems are already in the extremely urgency.1−5 Lithium– sulfur (Li–S) batteries have gained extensive attention due to the appealing theoretical energy density (2600 Wh kg−1), low cost, and high abundance of sulfur.4−7 However, polysulfide shuttle in electrolyte causes rapid capacity fading and poor cycling life, severely limiting their development and commercialization.7−9 Enormous research efforts have been devoted to addressing this issue, including functionalization of cathode host materials,10,11 modification of separators,12,13 and exploration of solid electrolytes.14,15 Among them, designing cathode composites for anchoring polysulfides is an effective approach to retard the diffusion of polysulfide into electrolyte. Nevertheless, utilization of these materials generally compromises battery energy density.16 Doping heteroatoms to cathode would ruin the ideal electron-transporting system and thus reduce electrical conductivity.17 Besides, the shuttle effect still exists during the charge/discharge processes.18 The separator, an important component of battery system, is located between cathode and anode to prevent short-circuiting while allowing Li+ conduction.19 A strategy of surface modification of separators offers potential to overcome the shuttle effect by inserting a polysulfide barrier layer between cathode and separator.20 Inorganic materials with strong polarity, such as metal oxides,21,22 metal sulfides,23 and Ti3C2,24 have been applied to modify the separators owing to their strong interaction with polysulfides. For instance, Cui and co-workers employed a black phosphorus modified separator to capture polysulfides on separator surface.25 And the assembly battery exhibited an initial discharge capacity of 930 mAh g−1 and a retention rate of 86% after 100 cycles at 0.4 A g−1. Unfortunately, for most of these inorganic materials, their introduction would increase

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battery internal resistance and lower electrolyte wettability, thus having limited battery performances.26 Another noteworthy strategy is to coat polymers with anion groups (e.g., −SO3H or −CO2H) on separator surface.27 Based on electrostatic repulsion, Zhang et al. coated a Nafion layer as ionic sieves to repel polysulfides, which achieved an initial discharge capacity of 781 mAh g−1 and over 60% capacity retention after 500 cycles at 1.0 C.28 However, sufficient thickness is required to obtain a uniform anionic polymer layer, which significantly enhances battery internal resistance. Moreover, Li+ transport through the thick and dense polymer layer is sluggish as compared to that through porous separator.29 Therefore, it is still a significant challenge for Li–S batteries to design a modified separator with efficient inhibition for polysulfide shuttle and simultaneously high conduction for Li+. Recently, electrospinning, as an advanced technique to prepare nanofibers, has been widely used in host materials.30,31 Nanofiber structure has remarkable advantages, including excellent Li+ conduction and electrolyte wettability.19 Unfortunately, the application of electrospinning in modifying separators is rarely reported because the large interspaces between nanofibers cannot effectively prevent polysulfide shuttle. Lee and co-workers designed a new class of spiderweb separators with sandwich-type structures to suppress polysulfide shuttle.29 Although the electrospun fiber layer was covered with another layer of carbon nanotubes, the polysulfides were still able to pass through the spiderweb separator due to large interspaces. Consequently, the electrochemical performances were barely satisfactory as testified by the limited initial discharge capacity (819 mAh g−1) and cycling retention (72% at 2.0 C after 300 cycles). Herein, we report a new type of the Nafion/polyacrylic acid (PAA) modified separator based on electrospinning, in order to suppress polysulfide shuttle and simultaneously promote Li+

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conduction. Through a simple lithiation treatment, the fibers are fused with each other, forming relatively compact network with semi-fused pores. Based on synergistic effect of physical barrier and electrostatic repulsion, the modified separator highly inhibits polysulfide shuttle and gives the battery outstanding cycling stability. Furthermore, abundant anion groups and unique semi-fused pores contribute to enhanced Li+ conduction and electrolyte wettability, which reduce contact resistance and afford high rate capability. Moreover, sulfonated poly (ether ether ketone) (SPEEK) modified separator is also fabricated for addressing these issues, and similar results are obtained. The finding indicates that this is a universal and repeatable strategy for improving the performance of Li–S batteries, opening up broad prospects towards application. 2. EXPERIMENTAL 2.1 Materials and Chemicals Aqueous Nafion solution (5 wt% in a 1/1 mass ratio of 1-propanol/water, DE520) was supplied by Shanghai Hesen 17 Electric Co., Ltd. PAA (99%) (Mw=450000). 2,2'-benzidinedisulfonic acid (BDSA) (70 wt% in H2O), and LiOH·H2O were purchased from Aldrich and used directly. Poly (ether ether ketone) (PEEK, VictrexPEEK, grade 381G) was supplied by Nanjing Yuanbang Engineering Plastics Co., Ltd. N, N-dimethylformamide (DMF), tetrahydrofuran (THF), 1propanol, and ethanol were all purchased from Kewei Chemistry Co., Ltd., and used without further purification. Li2S6 solution was prepared according to the literature.32 Deionized water was used throughout the experiment. 2.2 Preparation of Modified Separators Electrospinning was performed with Nafion/PAA blend solution and SPEEK solution. Nafion/PAA electrospinning solution was obtained by adding PAA and BDSA to Nafion solution

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and stirring at 60 °C until a homogeneous solution. The mass fractions of PAA and BDSA were controlled at 5% and 0.5%, respectively. Next, the solution was caught inside a 5-mL plastic disposable syringe, and then electrospun at a rate of 0.030 mL min−1 and a voltage of 10 kV. The commercial polypropylene (PP) separator was fixed at the collector with the aluminum foil. The distance between the needle and the collector was kept at 20 cm. Finally, the Nafion/PAA nanofiber mat (NP mat/PP) modified separator was obtained by heat treatment at 100 °C for 12 h. The areal mass loading of Nafion/PAA mat was controlled to be about 0.30 mg cm−2 to balance the energy density and modification effect. SPEEK was produced from PEEK according to the procedure in literature.33 The sulfonation degree was kept at 78% with sulfonation time of 10 h. Subsequently, the SPEEK was dissolved in DMF at room temperature under stirring for 12 h to get a 20 wt% solution. The solution was applied at a voltage of 25 kV with a rate of 0.020 mL min−1. The needle-to-collector distance was 20 cm. The PP separator with SPEEK nanofiber mat was peeled off from the aluminum foil and then dried at 60 °C for 12 h to obtain the SPEEK nanofiber mat (SP mat/PP) modified separator with an areal mass loading of ~ 0.30 mg cm−2. Afterwards, the fiber-mat-modified separators were lithiated as follows: certain amount of LiOH·H2O was dissolved in the mixture of water and ethanol (9:1 w/w) to give a 0.1 M LiOH solution. Then, the NP mat/PP separator was immersed in the solution at room temperature for 6 h. Finally, Nafion/PAA-Li network (NP-Li/PP) modified separator was achieved by rinsing with water and drying at 60 °C for 24 h. The areal mass loading of the Nafion/PAA-Li became ~ 0.32 mg cm−2. The Li+ content was detected by acid-base titration.19 The sample deprotonation was performed in 2 M NaCl solution for 48 h. Then, 0.01 M standard NaOH solution was used to titrate

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the solution. Similarly, SPEEK-Li network (SP-Li/PP) modified separator with an areal mass loading of ~ 0.32 mg cm−2 was made from the same synthetic method. In addition, previous two electrospinning solutions were re-used to prepare Nafion/PAA-Li coated (NP coated/PP) separator and SPEEK-Li coated (SP coated/PP) separator, respectively. The Nafion/PAA blend solution was drop-casted on PP separator and dried at 60 °C for 6 h. The area mass of Nafion/PAA coated layer was controlled about 0.40 mg cm−2. Then, the modified separator with Nafion/PAA coating was soaked into aqueous 0.1 M LiOH at room temperature. The NP coated/PP separator was obtained after rinsing with water and drying at 50 °C for 12 h. The SP coated/PP separator was fabricated by the same process. 2.3 Characterizations The morphology and structure of the samples were observed by scanning electron microscopy (SEM, Philips XL30SFEG) and atomic force microscopy (AFM, Bruker Dimension FastScan™). Elemental maps were analyzed by an energy dispersive spectrometer device affiliated with the SEM. Contact angles were examined on a FACE (model OCA 25, Germany) at room temperature. Fourier transform infrared (FTIR) spectra of the modification layers were recorded with the wavelength of 400–4000 cm−1 by using a Nicolet MAGNA-IR560 instrument. Thermogravimetric analysis (TGA-50 SHIMADZU) was conducted under a nitrogen atmosphere from 30 to 800 °C with a heating rate of 10 °C min−1. 2.4 Electrochemical Measurements The mixture of super P and sulfur at the weight ratio of 1:3 was heated under vacuum at 155 °C for 12 h to obtain the C/S composite (Figure S1). The sulfur content is determined as 74.1 wt% (Figure S2). The cathode slurry was obtained by mixing C/S composites, super P, and LA133 with

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a mass ratio of 7:2:1 in the mixture of water and ethanol. The slurry was scraped onto aluminum foil by scraper and then dried 12 h at 60 °C under vacuum. Consistent with previously reported works,16,27 the slurry-coated C/S cathode was cut into 12 mm diameter disks with the sulfur loading of ~ 2.0 mg cm-2 for better evaluating our contribution. The lithium metal disk with a diameter of 16 mm was used as the anode. The electrolyte consisted of 1 M bis(trifluoromethanesulfonyl)imide lithium in a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane (1:1, v/v) with LiNO3 (2 wt%). The electrolyte/sulfur ratio was controlled to be around 15 μL mg−1. CR2032 coin cells were assembled in the glove-box filled with argon using the various modified separators and these electrode materials. The cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS) were measured by Princeton Electrochemical Workstation (PARSTATMC). The CV was controlled at a scan rate of 0.1 mV s−1 from 1.7 to 2.8 V. The EIS was commanded in the frequency range between 0.01 Hz and 100 kHz. Differently, as for the measurement of Li+ conductivity, these separators were soaked in electrolyte and sandwiched between stainless steel electrodes to perform EIS measurement at the frequency range from 1 Hz to 100 kHz. A constant potential of 10 mV was applied to the Li/Li symmetrical cells in order to determine the Li+ transference number of separators by chronoamperometry. The charge/discharge performance of cells was completed in the potential range from 1.7 to 2.8 V by LAND CT2001A measurement system. All tests are carried out in an incubator at 25 °C. 3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterizations Scheme S1 illustrates the synthetic process of NP-Li/PP separator. Pure Nafion solution cannot

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be directly used to prepare nanofibers through electrospinning owing to the lack of sufficient chain entanglement.34 The addition of PAA to Nafion solution addresses this problem and meanwhile brings more anion groups. BDSA is used as cross-linking agent to enhance the structural stability of nanofibers by reacting with −CO2H groups on PAA. Nafion/PAA nanofibers are directly prepared using PP separator as support, followed by heat treatment at 100 °C. Unlike PP separator, NP mat/PP separator exhibits improved thermal dimensional stability. As shown in Figure S3, no visible thermal shrinkage is observed after the heat treatment.19 Finally, the relatively compact NPLi/PP separator with semi-fused pores is obtained through the simple lithiation treatment of NP mat/PP separator. The surface structure of these modified separators was characterized by SEM and AFM. As shown in Figure 1a, the prepared nanofibers with diameter of ~ 200 nm are uniformly attached on PP separator to form a fiber mat. Cross-sectional SEM image (Figure 1c) reveals that the thickness of this fiber mat is ~ 900 nm. Note that the nanofibers interweave with each other and form micronscale interspaces. Such large interspaces might not effectively prevent polysulfide shuttle. Therefore, in this study, lithiation treatment was conducted to fuse the nanofibers for suturing these large interspaces (Figure S4). Suturing degree was controlled by treatment time (Figure S5). According to pre-experiment, the treatment time of 6 h was selected for an optimized structure. The porous fiber mat becomes a relatively compact fiber network with semi-fused pores (Figure 1d). Such structure conversion can also be observed by their AFM images, as depicted in Figure 1b and 1e. Meanwhile, the fusion of nanofibers in this process causes the contraction of interspaces, and therefore reduces the thickness of fiber mat to 600 nm (Figure 1f).

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Figure 1. The structural characterization and contact angle of separators. (a) Top-down SEM image and (b) AFM image of NP mat/PP. (c) Cross-sectional SEM image of NP mat. (d) Top-down SEM image and (e) AFM image of NP-Li/PP. (f) Cross-sectional SEM image of NP-Li. (g) Contact angles of separators.

The modification of polymer nanofibers obviously improves the wetting property of separator as proved by contact angle results (Figure 1g). The contact angle of electrolyte on the surface of PP separator is 44.1°,17 yet the contact angle of NP coated/PP separator is only 24.3°. The remarkably improved wettability is associated with the affinity of polar groups on modification layer towards electrolyte.35 Compared with NP coated/PP separator, NP mat/PP separator with fiber structure presents a lower contact angle of 14.7°, verifying that fiber structure is beneficial to the absorption and spread of electrolyte. Furthermore, the lithiation treatment of fiber mat results in the full exposure and uniform distribution of polar groups, which further improves the wetting property of separator. NP-Li/PP separator displays a contact angle of only 9.8°.

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Figure 2. (a) FTIR spectra and (b) TGA curves of modification layers. (c) Diffusion of the polysulfides in the Ushaped glass bottles with different separators. (d) UV–vis absorption spectra of polysulfide solutions on the permeated side after 24 h.

The chemical structure of these modification layers is determined by FTIR. As shown in Figure 2a, Nafion gives rise to characteristic bands for C−F (1159 cm−1), C−O−C (981 cm−1), and O=S=O (1229 and 1056 cm−1).36,37 PAA brings the characteristic band of C=O at 1707 cm−1. These results confirm the existence of –SO3H and –CO2H groups on modified separators. Besides, NP mat displays two new bands at 1648 and 1530 cm−1 after thermal crosslinking, which are ascribed to amide I of carbonyl stretching and amide II of N−H bending, respectively.19 This implies that the PAA chains have been successfully crosslinked by BDSA. Further evidence is provided by TGA curves (Figure 2b). Owing to the water escape during amidation reaction of PAA and BDSA, uncrosslinked NP mat shows more weight loss than crosslinked NP mat in the range of 30–120 °C. And the DTG curve shows a new peak at 90 °C, accordingly. The crosslinking significantly enhances solvent-resistant and water-resistant abilities, and therefore NP-Li/PP separator can maintain fiber structure after lithiation. For the NP-Li modification layer, the absorption bands at 11



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1707 and 1229 cm−1 generate obvious red shift due to the introduction of Li+ (Figure 2a), which indicates the formation of −CO2Li and −SO3Li, respectively.27,37 The exchange of H+ and Li+ further inhibits the dehydrating action of PAA, and therefore NP-Li modification layer undergoes different weight loss process, especially before 430 °C (Figure 2b). 3.2. Polysulfide-Inhibiting Property and Ions Conductivity Effectively preventing polysulfide diffusion is an essential requirement for separator in Li–S battery system. As depicted in Figure 2c, a visible permeation of polysulfides is detected for PP separator only after 10 min. By comparison, NP mat/PP separator displays enhanced polysulfideinhibiting property due to abundant anion groups on NP mat. However, the large interspaces in the mat allow part polysulfides to pass through, and yellowish polysulfides are observed across this separator after 12 h. This can be verified by the performance of NP-Li/PP separator, which possesses close chemical components but no large interspaces. The compact structure and uniform electronegative groups efficiently inhibit polysulfide diffusion, and no polysulfide permeation is observed even after 24 h. Such a performance is comparable to that of NP coated/PP separator, which has a thick and defect-free modification layer. Further evidence is provided by the UV–vis absorption spectra of permeated polysulfide solutions after 24 h (Figure 2d).13 It can be seen that the absorbance intensity of polysulfide solutions through NP mat/PP separator is almost half of PP separator, while polysulfide solutions through NP-Li/PP and NP coated/PP separators display the lowest absorbance intensity. Besides the polysulfide-inhibiting property, Li+ conduction is another key function for separator.38 Here, Li+ conductivities of separators are calculated from their impedance plots (Figure S6).39,40 As depicted in Figure 3a, PP separator attains a conductivity of 9.5×10−4 S cm−1 due to its

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porous structure (Figure S7), which can allow Li+ transport with the electrolyte. By comparison, the NP mat carries with plentiful –SO3H and –CO2H groups. These negatively charged anion groups serve as sites for the hopping of positively charged Li+ through the Coulombic interactions,20,28 and the unshared electron pairs in anion groups interact with Li+ in the electrolyte to contribute to Li+ transport.19,41 Besides, the presence of interspaces and the improvement of electrolyte affinity also promote Li+ conduction. Li+ conductivity of NP mat/PP separator therefore reaches 1.4×10−3 S cm−1. As for NP-Li/PP separator, the lithiation process introduces additional Li+ and increases the exposed anion groups. In addition, the semi-fused pores can serve as lowbarrier pathways for Li+ transport. The Li+ conductivity is further elevated up to 1.7×10−3 S cm−1. Notably, with the prolongation of lithiation time, although the Li+ content continues to increase, the structure of the separator becomes more compact, which is not favorable for improving the Li+ conductivity. Therefore, an optimal treatment time of 6 h was selected by pre-experiment (Figure S8). By comparison, NP coated/PP separator possesses the lowest Li+ conductivity (6.8×10−4 S cm−1) because the dense polymer modification layer could block the transfer pores on PP separator. This is a common problem for polymer-coated separators. For another, the Li+ transference number of separators is further evaluated by chronoamperometry (Figure S9). NP-Li/PP separator possesses the highest Li+ transference number of 0.61, superior to that of pristine separator (0.42), confirming the outstanding Li+ transport property. Collectively, NP-Li/PP separator exhibits both high polysulfide-inhibiting and Li+ conduction properties. These results highlight the necessity of lithiation process and the superiority of electronegative separator with semi-fused pores. 3.3. Electrochemical Performance To assess the characteristic of the cells in electrochemical reaction, the CV curves are measured

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at a potential range of 1.7−2.8V with scan rate of 0.1 mV s−1. As shown in Figure 3b, all the CV curves have two typical reduction peaks at 2.05 and 2.30 V in the cathodic scan, which correspond to the multistep electrochemical reaction of sulfur: from S8 to soluble polysulfides and then to insoluble Li2S2/Li2S. During the anodic scan, the overlapping oxidation peaks at ~ 2.30 V are observed, indicating the conversion of Li2S2/Li2S to intermediate polysulfides and finally to S8.42 Compared with the other three cells, the cell with NP-Li/PP separator presents redox peaks with the highest peak current and the biggest peak area, that is, fast transfer kinetics and high capacity.18 Such results are associated with the enhanced Li+ conduction and polysulfide-inhibiting properties of separator. Based on similar reasons, the performances of cells with NP/PP and NP-coated/PP separators are also superior to that of the cell with PP separator. To further confirm the advantages of NP-Li/PP separator, CV curves for the first five cycles are shown in Figure S10. A slight deviation of the peaks between the first curve and the subsequent cycles is detected because of the redistribution of active sulfur. Then, the cell presents better curve coincidence than the cell with PP separator after second cycle, which indicates superior electrochemical reversibility.43,44 PP NP coated/PP NP mat/PP NP-Li/PP

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Figure 3. (a) Temperature-dependent ionic conductivities. (b) CV curves, (c) EIS profiles, and (d) rate performances of the cells with different separators. (e) Charge/discharge profiles of the cell with NP-Li/PP 14


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separator at various rates. (f) Polarization voltages of the cells at 0.1, 0.5, and 1.0 C.

EIS of the fresh cells are collected and shown in Figure 3c. Each impedance profile displays a semicircle at high frequency, corresponding to charge transfer resistance (Rct).11,26,44 It can be seen that the cell with NP-Li/PP separator displays the lowest Rct of 32 Ω, only 34.7% of that with PP separator (92 Ω). This is ascribed to the enhanced Li+ conduction and improved electrode/separator interface. Likewise, the Rct of cell with NP mat/PP separator is also decreased to 45 Ω. Conversely, the cell with NP coated/PP separator shows the highest Rct of 101 Ω due to the increased resistance of Li+ diffusion. Rate performances of the cells are performed at the current rates from 0.1 to 3.0 C (Figure 3d). The cell with PP separator delivers capacities of 909, 720, 591, 479, 217, and 197 mAh g−1 at 0.1, 0.2, 0.5, 1.0, 2.0, and 3.0 C, respectively, which are the lowest among all cells at the same rate. These low capacities are ascribed to the loss of active sulfur caused by the severe polysulfide shuttle. By comparison, the cell with NP-Li/PP separator delivers significantly higher capacities of 1321, 1150, 1042, 945, 849, and 730 mAh g−1 at the corresponding rates, respectively. Notably, at high rate of 3.0 C, the capacity of this cell is 3.7 times of that of the cell with PP separator. With the current rate back to 0.2 C, 90% of capacity is recovered, which also exceeds the cell with PP separator (83%). The remarkably improved rate performance is mainly resulted from the functions of NP-Li/PP separator in preventing polysulfide shuttle and facilitating Li+ conduction. This can also be confirmed by the performance enhancement of cell with NP mat/PP separator. For the cell with NP coated/PP separator, despite the slightly higher Rct, the discharge capacities are superior to the cell with PP separator due to the excellent polysulfide-inhibiting property of NP coated/PP separator. However, compared with the cells with NP mat/PP and NP-Li separators, a rapid capacity fading is observed with the stepwise increase of current rate. Particularly, the capacity is only 423 15



mAh g−1 at 3.0 C, which is 33% of the initial capacity at 0.1 C. The reduction of capacities at high rates is ascribed to the inferior Li+ conductivity and high impedance for the cell. Furthermore, the corresponding galvanostatic charge−discharge (GCD) profiles are shown in Figure 3e and S11. The cell with NP-Li/PP separator displays well-defined GCD profiles with two discharge plateaus and one charge plateau. The electric potential difference between discharge plateau and charge plateau represents the potential polarization (Figure 3f).45 As expected, the cell with NP-Li/PP separator displays the lowest potential polarization among the four cells at each rate. For the cell with PP separator, its polarization voltage rapidly increases with the increase of the current density, reaching up to 0.44 V at 1.0 C. This value is even higher than that of the cell with NP-Li/PP separator at 3.0 C (0.40 V). Moreover, the charge−discharge plateaus entirely disappear at 2.0 and 3.0 C as a result of overlarge potential polarization (Figure S11). Such results suggest that NP-Li/PP separator can endow cells with low electrochemical impedance and fast kinetic reaction.

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0 1000

Cycle number

Figure 4. Long cycling performance of the cells with different separators at 1.0 C. (a) Charge/discharge profiles of first cycle. (b) Discharge capacities and capacity retention ratios after test. (c) Long-term cycling stability. 16


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The long-term cycling stability of the cells is conducted at 1 C with a sulfur loading of 2.0 mg cm−2. The cells with PP, NP coated/PP, NP mat/PP, and NP-Li/PP separators deliver initial capacities of 608, 730, 870, and 977 mAh g−1, respectively (Figure 4a). With the increase of cycle number, the cell with PP separator undergoes a rapid capacity fading and eventually fails at the 605th cycle, resulting from the rampant polysulfide shuttle (Figure 4c). And only a capacity of 220 mAh g−1 is retained, corresponding to a cyclic decay rate of 0.106% per cycle. Meanwhile, Coulombic efficiency is decreased to 90% at the cell failure. In sharp contrast, the cell with NPLi/PP separator achieves significant cycling stability. A reversible capacity of 748 mAh g-1 with a Coulombic efficiency above 99% is achieved after 1000 cycles. The capacity retention is as high as 77% (Figure 4b), and the corresponding capacity decay is 0.023% per cycle. This finding is attributed to the superior ability of NP-Li/PP separator in suppressing polysulfide shuttle. By comparison, the cell with NP mat/PP separator also attains a well-pleasing initial capacity of 870 mAh g-1, yet the occurrence of polysulfide shuttle due to large interspaces results in rapid capacity fading and Coulombic efficiency decline. The cyclic decay rate reaches up to 0.038%, 65% higher than that of the cell with NP-Li/PP separator. For the cell with NP coated/PP separator, although the overall capacity is restricted due to the sacrifice of Li+ conduction, an impressive cycling stability is achieved owing to the outstanding polysulfide-inhibiting property of separator. After 1000 cycles, the cell delivers a capacity retention of 66.7% (Figure 4b) and Coulombic efficiency of above 99%, which are higher than the cell with NP mat/PP separator. Additionally, Figure 4b further shows the discharge capacities of the cells at different plateaus after test. High voltage plateau corresponds to the conversion from S8 to soluble polysulfides, reflecting the behavior of polysulfide shuttle to some extent.18,26,46 The cell with NP mat/PP separator exhibits higher

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discharge capacities at high voltage plateaus when compared with other cells (Figure S12). This again confirms that NP mat/PP separator can effectively suppress the shuttle effect and reduce the loss of active materials during cycling. Considering the energy density of batteries in practical application, a high-sulfur-loading cathode of 5.6 mg cm−2 is chosen as representative to assess the feasibility of NP mat/PP separator (Figure S13). With a stable cycling, the cell delivers a discharge capacity of 710 mAh g−1 at 0.5 C after100 cycles. The corresponding areal capacity is as high as ~ 4.0 mA h cm−2. The results indicate that NP-Li/PP separator is also of great significance and prospect for high-sulfur-loading Li–S batteries.18,39 With the NP-Li/PP separator, the assembled cell can readily power a “ZZU”-shaped light-emitting diode panel for more than 1 h (Figure S14). To further reveal the function of NP-Li/PP separator, the cells are disassembled after the longterm cycling test (Figure S15). Benefited from the improved solvent-resistant ability after crosslinking, the NP-Li/PP separator can be easily detached from the disassembled cell, which implies that NP-Li/PP separator can maintain structural stability during the repeated cycling. Furthermore, the slight discoloration is observed on the surface of NP-Li/PP separator. The cycled lithium anode maintains relatively smooth morphology and displays weak S signal in the corresponding elemental maps. By contrast, for the cell with PP separator, the surface of separator is heavily contaminated with the dissolved polysulfides after cycling. Due to the rampant polysulfide shuttle, the lithium anode undergoes severe corrosion and strong S signal is detected on the surface of the lithium anode. These differences further corroborate that NP-Li/PP separator can effectively suppress the shuttle effect and enhance the cell stability. Such excellent cycling stability is superior to most Li–S batteries reported so far, including polymer coating,28,35 carbon material,16–18,47 and inorganic oxide modified separators (Table S1),25,40,42 highlighting the structure

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advantages of NP-Li/PP separator. 3.4. General Applicability of Modified Strategy b

c

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Figure 5. Characterization and performance of SPEEK system. Top-down SEM image of (a) SP mat/PP and (b) SP-Li/PP. (c) FTIR spectra of SP mat and SP-Li. (d) Temperature-dependent ionic conductivities of separators. (e) Cycling stability of the cells at 1.0 C.

The above section descripts the improvement of overall performance for cells with Nafion/PAAmodified separators. To verify the universality of this design strategy, SPEEK as an example is used to modify separators through the same procedures. Similar to NP mat/PP separator, SP mat/PP separator presents typical fiber structure (Figure 5a). After lithiation treatment, the swelling of SPEEK nanofiber sutures the large interspace between neighboring nanofibers and forms a relatively compact network with many semi-fused pores (Figure 5b). The chemical structures of these modification layers are further detected by FTIR (Figure 5c). The presence of characteristic bands at 1225, 1080, and 1023 cm-1 suggests the existence of −SO3H groups.47 After the lithiation, the peak around 1225 cm-1 shifts to 1217 cm-1 owing to the exchange of H+ with Li+. Furthermore, the difference in the thermal property between SP mat and SP-Li is proved by TGA curves (Figure S16). These results indicate that the SPEEK-modified separators possess similar structural features 19



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to Nafion/PAA-modified separators. Expectedly, SP-Li/PP separator displays excellent polysulfide-inhibiting property in polysulfide diffusion test, comparable to the dense SP coated/PP separator (Figure S17). Such property obviously outperforms SP mat/PP and PP separators. Similarly, the Li+ conductivities are calculated from impedance plots in Figure S18. As depicted in Figure 5d, SP-Li/PP separator delivers the highest Li+ conductivity of 1.3×10−3 S cm−1 due to the abundant –SO3H groups, unique semi-fused pore structure, and the introduction of additional Li+. Likewise, the Li+ conductivity of SP mat/PP separator is increased to 1.1×10−3 S cm−1. However, SP coated/PP separator displays a poor Li+ conductivity of 6.8×10−4 S cm−1 since the dense SPEEK layer blocks the pathways for Li+ transfer. The cycling stability of cells is tested at 1.0 C (Figure 5e). The cell with SP-Li/PP separator displays a higher initial specific capacity of 817 mAh g−1. After 200 cycles, a capacity of 678 mAh g−1 is maintained with a low capacity decay rate of 0.085%, which is far superior to other three cells as a result of enhanced polysulfide-inhibiting ability of SP-Li/PP separator. Coincidentally, fast capacity decay for the cell with SP mat/PP and low capacity for the cell with SP coated/PP are observed during the cycle test. These findings, quite similar to that in Nafion/PAA-modified separators, corroborate the superiority of the designed electronegative separator with semi-fused pores. Although these two series of modified separators achieve similarly enhanced electrochemical performances, Nafion/PAA-modified separators are superior to SPEEK-modified separators. This should be ascribed to the more anion groups in Nafion/PAA-modified separators. 4. CONCLUSION In summary, an electronegative NP-Li/PP separator with semi-fused pores is prepared by lithiation of electrospun nanofiber mat for Li–S battery. The modified separator with relatively

20


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compact structure and abundant –SO3H and –CO2H groups immensely suppresses polysulfide shuttle based on physical barrier and electrostatic repulsion. Meanwhile, the plentiful anion groups provide countless carrier sites for the hopping of positively charged Li+, and the characteristic semifused pores act as low-barrier transport pathways. Accordingly, Li+ conductivity is elevated to about 10−3 S cm−1 at room temperature. With NP-Li/PP separator, the Li−S cell displays outstanding electrochemical performances, including high rate performance (730 mAh g−1 at 3.0 C), excellent cycling stability, and ultralong cycling life (cyclic decay rate of 0.023% during 1000 cycles at 1.0 C). Besides, as an example, the SPEEK-modified separators are also fabricated, confirming the universality of this modification strategy. This study offers a new horizon for rational material selection and targeted structure design of battery separator, opening up an avenue toward highly stable Li–S batteries.

■ ASSOCIATED CONTENT S Supporting Information ●

Fabrication process of NP-Li/PP separator. SEM images, TGA curves, and impedance plots of separators. CV curves and charge/discharge profiles of the cells. Digital photograph of a homemade light-emitting diode panel. Polysulfide diffusion test. Comparison of the cycling stability.

■ AUTHOR INFORMATION Corresponding Author *Fax: +86-371-63887135; Tel: +86-371-63887135; E-mail: [email protected](Y. Liu) and [email protected] (J.T. Wang) Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS 21



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This work is supported by the National Natural Science Foundation of China (21576244, U1804127) and Training Plan for Young Backbone Teachers in Universities of Henan Province (2018GGJS003). Fok Ying Tung Education Foundation (161065) is also highly acknowledged. This invited contribution is part of the I&EC Research special issue for the 2019 Class of Influential Researchers.

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For Table of Contents Only

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Author's Biography and Picture

Junxiao Wang received his bachelor's degree in School of Chemical Engineering and Energy from Zhengzhou University in 2017. He proceeded to successive postgraduate at School of Chemical Engineering and Energy, Zhengzhou University. His research interests mainly focus on the design and fabrication of novel separator and solid-state electrolyte for high energy-density lithium–sulfur batteries.

Yong Liu received her bachelor and Ph.D. degree in Engineering from Tianjin University in 1998 and 2008, respectively. As a visiting scholar, she studied in University of Houston of USA from 2012 to 2013, with Prof. William S. Epling as a cooperation supervisor. Currently, she works at the School of Chemical Engineering and Energy of Zhengzhou University. Her current research focuses on ecological and environmental engineering.

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Jingtao Wang received his PhD degree in Chemical Technology from Tianjin University in 2012. Now he is a professor at the School of Chemical Engineering and Energy of Zhengzhou University. He attended a visiting scholar program at the University of Adelaide with Prof. Shizhang Qiao in 2017. His research interests encompass the development of advanced materials for chemical separation, membrane process intensification and electrochemical energy storage system.

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An Electronegative Modified Separator with Semifused Pores as a

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