The Synergetic Effects of Multifunctional Composite with More Efficient

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The Synergetic Effects of Multifunctional Composite with More Efficient Polysulfide Immobilization and Ultrahigh Sulfur Content in Lithium-Sulfur Batteries Manfang Chen, Shouxin Jiang, Cheng Huang, Jing Xia, Xianyou Wang, Kaixiong Xiang, Peng Zeng, Yan Zhang, and Sidra Jamil ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02029 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Materials & Interfaces

The Synergetic Effects of Multifunctional Composite with More Efficient Polysulfide Immobilization and Ultrahigh Sulfur Content in Lithium-Sulfur Batteries Manfang Chen, Shouxin Jiang, Cheng Huang, Jing Xia, Xianyou Wang,∗ Kaixiong Xiang, Peng Zeng, Yan Zhang, Sidra Jamil (National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, China)

ABSTRACT: A high-sulfur-loading cathode is the most crucial component for lithium-sulfur batteries (LSBs) to obtain considerable energy density for commercialization

applications.

The

major

challenges

associated

with

high-sulfur-loading electrode are poor material utilization caused via the nonconductivity of the charged product (S) and the discharged product (Li2S), poor stability arisen from dissolution of lithium polysulfides (LiPSs) into most organic electrolytes and pulverization and structural damage of the electrode caused by large volumetric expansion. A multifunctional synergistic composite enables ultrahigh sulfur content for advanced LSBs, which comprises the sulfur particle encapsulated with an ion-selective polymer with conductive carbon nanotubes and dispersed around Magnéli phase Ti4O7 (MS-3) by bottom-up method. The ion-selective polymer

∗

Corresponding author: Xianyou Wang Tel: +86 731 58293377; fax: +86 731 58292052.

E-mail address: [email protected] (X. Wang). 1

ACS Paragon Plus Environment

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provides physical shield and electrostatic repulsion against the shuttling of polysulfides with negative charge while it can permit the transmission of lithium ion (Li+) through the polymer membrane, and the carbon nanotubes twined around the sulfur promote electronic conductivity and sulfur utilization as well as strong chemical adsorption of LiPSs by means of Ti4O7. Due to this hierarchical construction, the cathode possesses lofty final sulfur loading of 72% and large sulfur areal mass loading of 3.56 mg cm-2, which displays the large areal specific capacity of 4.22 mAh cm-2. In the same time, it can provide excellent cyclic performance with the corresponding capacity attenuation ratio of 0.08% per cycle at 0.5 C after 300 cycles. Especially, while sulfur areal mass loading is sharply enhanced to 5.11 mg cm-2, MS-3 composite exhibits a large initial areal capacity of 5.04 mAh cm-2 and still keeps high reversible capacity of 696 mAh g-1 at 300th cycle even at a 1.0 C. The design of high-sulfur-content

cathode

is

a

viable

approach

for

boosting

practical

commercialized application of LSBs. KEYWORDS: high sulfur content, chemisorption, electrostatic repulsion, shuttle effect, lithium sulfur

1. INTRODUCTION Energy conversion and storage devices have attracted a great deal of attention for their rapidly rising commercial demand in daily life applications including transportation, grid storage, electric vehicles, and advanced portable electronics.1,2 Among the current battery systems, LSBs are regarded as one of the most prospective 2


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candidates for future generation electricity storage due to the large energy density and specific capacity on the basis of the electrochemical reaction of 16Li + S8 → 8Li2S.3 However, the commercialized application of LSBs is impeded by intractable issues. Primarily, the insulating sulfur at room temperature and its relevant discharge products Li2S2 or Li2S immensely hamper electrons transportation and decelerate batteries reaction kinetics.4 In addition, the large volume change during the cyclic process can cause the disruption of the electrode structure.5 The most important factor is the dissolution of LiPSs and their shuttle effect.6 In widely used ether-electrolyte system, while sulfur is lithiated gradually, the high solubility of long-chain LiPSs will be formed which will result in immense active material losing, poor cycling behavior and low coulombic efficiency (CE).7 Multifarious methods have been tried to solve the fore-mentioned intractable problems. The most commonly used strategy is to couple sulfur with carbonaceous materials or conductive polymer with fine structure.8-11 Commonly, the carbonaceous host material can not only enhance the electron transport during the electrode reaction but also absorb LiPSs owing to the probable pore size and large surface area.12 In our previous work, graded porous carbon from lotus seedpod shells was prepared and the high original capacity of 1138 mAh g-1 at 0.5 C was obtained.13 In addition, the conjugated polymers with unique mechanical properties such as bendability, flexibility, or even stretchability, not only act as a good sulfur host to accommodate the volume change but also tailor the synthetic tailorability and processability of conjugated polymers, which make them ideal candidates for electrochemical energy 3



storage.14 Zhou et al. synthesized yolk-shell S-polyaniline composite. The electrode manifested the reversible capacity of 628 mA h g-1 at a rate of 0.5 C at 200th cycle, respectively.15 Although physically composited sulfur with conductive host material can trap the dissolved LiPSs in some extent, it is obvious that the capacity loss could not be eradicated during a long cycle process, which could be assigned to the poor affinity between polar LiPSs and non-polar carbonaceous surface. On the other hand, the driving force for the shuttling of LiPSs is the concentration gradient of high-order and low-order LiPSs between two electrodes, which is difficult to be prevented by simple physical absorption methods. Considering the above problems, a new type of compound has been developed, which provides chemisorption on sulfur species through the “chemical anchor”, thereby inhibiting the dissolution and diffusion of LiPSs.16,17 As to chemical adsorption, the LiPSs can be held within the cathode region through the strong chemical bonds, thus the LiPSs shuttle effect can be effectively alleviated.18 A common strategy is to modify carbon material and graft hydrophilic functional groups onto carbonaceous material surface to anchor LiPSs through their chemical interactions. This method can hinder the dissolution and diffusion of LiPSs to some degree; Chen et al. investigated different surfactants decorated with porous carbon as sulfur host and concluded that sodium dodecyl sulfonate-anchored composite material revealed the strongest interactions between LiPSs and the oxygen-containing functional groups in the surfactants.19 Another efficient method is to use metal oxides with well-defined nanostructures, such as SiO2, TiO2, MnO2, V2O5.20-24 Recently, 4


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Nazar et al. studied Ti4O7 as the host for LSBs and found that it exhibited good affinity for LiPSs and excellent cycle stability due to its metallic conductivity and strong chemical binding ability with LiPSs.25 Magnéli-phase titanium oxides (e.g., Ti4O7) have unique crystal and electron structure, which can provide a good conductivity close to the conductivity of metal. Tao et al. also studied the Ti4O7 material as LSBs and delivered the discharge capacity of 623 mAh g-1 at 0.5 C.26 Although sulfur areal mass loading below 2.0 mg cm-2 has represented some good properties, the actual areal capacities are far less than the actual areal capacities of commercialized lithium ion batteries (LIBs).27 Even with outstanding electrochemical properties, the low sulfur content and low areal mass loading electrodes are still far from commercialization. In order to promote the commercialized application of LSBs, the development of high-mass-loading cathode with high sulfur content has currently become one of the most urgent topics in LSB fields. Herein, we report a multifunctional synergistic composite enabled ultrahigh sulfur content, which comprises the sulfur particle encapsulated with an ion-selective polymer with conductive carbon nanotubes and embraced Magnéli phase Ti4O7. Due to this hierarchical construction, the cathode possesses the sulfur content as high as 90 wt% via a simple chemical reaction deposition method for LSBs. Such design and construct have some virtues: (1) the ion-selective polymer membrane blocks the leak of LiPSs and allows the shuttling of Li+; (2) the hollow space of the polymer coating structures can store LiPSs and restrain volume change; (3) the carbon nanotubes twined around the sulfur promote the conductivity of electronic and the utilization of 5



sulfur, which can minimize the using of conductive carbon in the electrode; (4) the substoichiometric Ti4O7 possesses polar O-Ti-O units, which shows a strong affinity for LiPSs including solid Li2S2/Li2S via chemical interaction. Furthermore, the electrochemical properties don’t reply on the surface area but on the intensity of electrostatic repulsion and chemical interaction.28 The strategy of this method is not only to impede outward diffusion of LiPSs but also to capture the soluble LiPSs to facilitate the reformation of S8.29 When the multifunctional synergistic composites are used as the cathode materials in LSBs, they can conquer the problems related to maximizing sulfur loading without damaging electrical conductivity and reserve of LiPSs, thus the as-prepared cathode materials exhibit excellent sulfur utilization and outstanding cycling stability with high CE, suggesting that it will be potential cathode materials for the commercialized LSBs.

2. EXPERIMENTAL SECTION 2.1 Preparations of MS-n (n=1, 2 and 3) Composites. 1 mL poly(sodium p-styrenesulfonate) (PSS; molecular weight 500000) was dispersed in 400 mL deionized water (DIW) to generate a homogeneous aqueous solution, and then Na2S2O3▪5H2O (4.9636 g) was dispersed into the above dispersion solution and drastic stirred for 0.5 h. 50 mL dilute sulfuric acid solution (0.4 M) was dropwise mixed into the above solution, kept on mechanical stirring for 3 h. Then, 1 mL PSS was dispersed into the above-mentioned solution and drastic stirred for 1 h (solution A). The as-obtained mixture was centrifuged at 10000 rpm and washed twice with 6


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DIW. The yolk-shell S-PSS composite (named MS-1) was finally obtained via vacuum drying at 50 °C for 48 h. To further improve electronic conductivity of the MS-1 composite, the CNTs (from Cnano Technology, Beijing, China) were added into above-mentioned solution A and magnetic stirred for 3 h (solution B). The resultant mixture was centrifuged at 10000 rpm, washed three times with DIW and dried in drying oven at 50 °C for 24 h (named MS-2). Due to Ti4O7 possessing strong chemisorption of LiPSs and metallic conductivity, Ti4O7 (from Kela Material, Changsha, China) was joined to fore-mentioned solution B and magnetically stirred for 3 h. The morphology of Ti4O7 was irregularity bulk and the average particle size of Ti4O7 was about 1µm. The resulting mixture was filtered, washed three times with DIW and dried in drying oven at 50 °C for 24 h (named MS-3). To reduce the particle size of Ti4O7, the SEM image of Ti4O7 and particle size distribution after ball-milling were examined (Figure S1). A sample of relative to MS-3 without PSS polymer was tested as the control sample (named MS-0). All preparation processes are performed at room temperature. 2.2 Materials Characterization. The shapes of the as-synthesized composites were performed via scanning electron microscopy (SEM, Hitachi S-4800). To investigate the phase ingredient of samples, X-ray diffraction (XRD, Model LabX-6000, Shimadzu, Japan) was recorded in the 2θ range of 10°-80°. To investigate the sulfur loading in the as-prepared samples, thermogravimetric analyses (TGA) was conducted on a Series Q500 instrument (TA Instruments, USA) in an N2 atmosphere at 10 °C min-1 from indoor temperature to 600 °C. To research surface chemical 7



component and functional groups of the as-prepared samples, X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific) were examined. 2.3 Electrochemical Measurements. The cathode electrodes were prepared by active material, acetylene black and polyvinylidene fluoride binder and the mixed slurry of the weight ratio was 80:10:10. The slurry was casted on Al foil. Then, Al foil was dried at 60 °C for 12 h in the vacuum. The sulfur areal mass loading of 3.56 and 5.11 mg cm-2 were obtained by simple doctor blade-coating technology. The electrolyte was 1 mol L-1 lithium bis(trifluoromethane) sulfonimide (LiTFSI) in a mixture of equal volumes of 1,3 dioxolane (DOL) and 1,2-dimethoxyethane (DME) with the 3 wt% LiNO3 additives. The proportion of electrolyte to sulfur was about 20 µL mg-1. The anode was metal lithium and the separator was the Celgard 2400 membranes. The coin cells were assembled in the glove box in Ar atmosphere. The charge/discharge data were recorded on a battery testing system (CT-3008, Neware Co., Ltd.) between 1.7 and 2.8 V (vs Li+/Li). The cyclic voltammetry (CV) was tested on an electrochemical workstation (Princeton Applied Research VersaSTAT3, AMETEK, Inc.) with a low scan rate of 0.1 mV s-1. The electrochemical impedance spectroscopy (EIS) was researched by the same instruments over a frequency range from 100 kHz to 1 Hz with an alternating current voltage of 5 mV.

3. RESULTS AND DISCUSSION Figure 1a exhibits that pure sulfur suffers from severe volume change and polysulfide dissolution during discharge process. In order to solve above-mentioned 8


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intractable issues, a multifunctional synergistic composite is prepared by a simple bottom-up method which comprises the sulfur particle encapsulated with an ion-selective polymer (poly(sodium p-styrenesulfonate), PSS) with a conductive carbon nanotubes (CNTs) and dispersed around Magnéli phase Ti4O7, as shown in Figure 1b. Initially, the polymer coating provides a thin barrier to suppress the shuttling of LiPSs, and the rigid polymer membrane allows expansion inwards and maintains the stable sphere structure. The PSS polymer layer reciprocates abundant negatively charged -SO3- groups on the surface of the sulfur, which can efficiently limit the spread of polysulfides anions outside of the cathode owing to common ion electrostatic repulsion.30 In addition, the process does not restrict the free transmission of Li+. Conducting polymer PSS coating can allow electron transport and maintain structural stability.31 Secondly, the conductive CNTs twined around sulfur particles further promote electronic conductivity and decrease the addition of conductive carbon in the electrode manufacture procedure. Finally, Magnéli phase Ti4O7 possesses polar O-Ti-O units with strong affinity for LiPSs. Due to the surface electrostatic shielding effect of PSS and the high electric conductivity of CNTs as well as strong chemisorption of LiPSs by means of Ti4O7, the shuttling of LiPSs in the electrolyte can be effectually restrained and the sulfur utilization will be drastically enhanced.

9



Figure 1. Schematic diagram of discharge procedure of (a) pure sulfur and (b) as-prepared composite. Figure 2 exhibits SEM images of the as-synthesized composites. For the yolk-shell structured MS-1 composite, as shown in Figure 2a and b, the well-defined spherical morphology with the approximate size of 1-2 µm and an integrated polymer layer can be observed, which suggest that PSS is homogeneously coated on the surface of sulfur generating a yolk-shell structure (Figure S2a-c). The hydrophilic fields of this polymer dissolve in the aqueous solution while the hydrophobic fields support the growth of the sulfur. Such opposed effect leads to polymer deposit on the surface of sulfur spheres. Energy dispersive X-ray spectroscopy (EDS) analysis also further ascertains PSS successfully encapsulated sulfur particles. As shown in Figure 2c, the surface of the MS-1 composite demonstrates a high C/S weight ratio of 6.8. The SEM and EDS results exhibit that the sulfur is well encapsulated by PSS with a yolk-shell structure. The void space between the sulfur particle and polymer PSS shell is expected to impactful assuage the volume change of sulfur during cyclic process, 10


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thereby maintaining the integrality of structure and enhancing the cycling stability of composites. Furthermore, the polymer PSS is electronically conductive and is in favor of enhancing the rate performance. Figure 2d and e show the SEM images of MS-2 composite that CNTs uniformly dispersed on the surface of the MS-1 composite (Figure S2d-i). As shown in Figure 2f, the surface of the MS-2 composite manifests a higher C/S weight ratio of 10.9, indicating that CNTs are well dispersed into the MS-2 composite. Figure 2g and h demonstrate Ti4O7 is embedded with the MS-3 composite. The EDS results reveal the surface of the MS-3 composite with a large C/S weight ratio of 9.2 and confirm the existence of C, S, O and Ti, as shown in Figure 2i. To investigate the distribution of S, CNT and Ti4O7, the EDS elemental mapping for MS-1, 2 and 3 are investigated and all element presents a homogeneous distribution (Figure S3a-c).

11



Figure 2. (a, b) MS-1, (d, e) MS-2, and (g, h) MS-3 for SEM images; (c) MS-1, (f) MS-2, and (i) MS-3 for EDS spectra; the white scale bars are 1 µm. Figure 3a and b manifest that transmission electron microscopy (TEM) images incorporated with corresponding Fast Fourier Transform (FFT) diffraction patterns of commercial Ti4O7, which reveal that the interplanar distances can be assigned to Ti4O7 (104) with d=0.25 nm.32 The X-ray diffraction pattern indicates that Ti4O7 is the primary crystalline phase as shown in Figure 3c. Figure 3d shows the Fourier transform infrared (FT-IR) spectra of as-synthesized composites. It can be distinctly seen that the three composites show analogical FT-IT characteristic peaks. One characteristic peak at 2921 cm-1 is assigned to the stretching vibration of C-H. Two absorption bands centered at 1213 and 1014 cm-1 can be recognized as the absorption frequencies of the -SO3- moiety in PSS.33 Therefore, the FT-IR results further confirm the existence of PSS in three samples. Figure 3e displays the XRD profiles of the as-prepared composites. The positions of diffraction peaks for all samples are well in line with standard card (JCPSD no. 08-0247) of the Fddd orthorhombic phase of sulfur.34 To confirm the sulfur content in the as-prepared composite, all as-prepared samples are measured by TGA method. Based on the TGA results, the sulfur content in the MS-1, MS-2 and MS-3 composites is calculated to be approximately 93.1, 88.9 and 90.4 wt%, respectively, which is determined by both elemental analysis and EDS analysis as shown in Table S1.

12


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Figure 3. Ti4O7 for (a, b) TEM images (insets: FFT patterns), (c) XRD diffractrograms; the as-prepared composites for (d) FT-IR spectra, (e) XRD diffractrograms, (f) TGA traces. To verify surface chemical composition and functional groups of the as-prepared samples, XPS analysis is investigated. As illustrated in Figure 4a, the XPS survey spectra of the MS-1 and MS-2 composites show four binding energy peaks positioned at 285.08, 532.08, 164.08 and 228.08 eV corresponding to the C1s, O1s, S2p and S2s, respectively. For the MS-3 composite, two new binding energy peaks located at 458.8 and 464.5 eV correspond to Ti2p3/2 and Ti2p1/2, respectively, indicating that Ti4O7 is successfully added into composite. As shown in Figure 4b, in S2p spectra, two typical 13



binding energy peaks at 163.9 and 164.9 eV are ascribed to the signals of S2p3/2 and S2p1/2 owing to their spin-orbit coupling.35 The pronounced broad signal located at 168.3 eV is assigned to the sulfate species, such as SO42- or S2O32-, generated by the oxidation in air.36 As shown in Figure 4c, the C 1s spectra of MS-3 sample reveal three peaks at 284.8, 286.4 and 288.9 eV, respectively, corresponding to C-C/C=C, C-O/C-S and C=O.37 Figure 4d displays two obvious characteristic peaks of Ti4O7 corresponding to Ti-O binding at 458.8 and 464.5 eV. The small peak at 456.0 eV reveals Ti-S binding in the MS-3 composite. Apparently, above results demonstrate that the MS-3 composite contains some favorable groups on the surface, which are considered to effectively improve the binding ability with LiPSs.

Figure 4. XPS survey spectra of (a) the as-prepared composites, (b) S2p, (c) C1s, and (d) Ti2p of the MS-3 composite. 14


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Coin cells with lithium anode are assembled to investigate the electrochemical performances of the electrode (sulfur content: approximate 90 wt%; sulfur areal mass loading: 3.56 mg cm-2). The final sulfur loading in the electrode is calculated to be about 72%. The electrochemical performances of the MS-3 composite are studied by CV measurement. Figure 5a exhibits the initial seven cycles of the CV curves for the MS-3 electrode. Apparently, the CV profiles manifest alike shape, revealing one oxidation peak and two reduction peaks. The oxidation peak at approximate 2.5 V is associated with LiPSs’ formation.38 The first reduction peak at approximate 2.27 V is ascribed to the conversion of sulfur to LiPSs and another peak at 2.0 V is attributed to further reduction of LiPSs to Li2S2 or Li2S.39 In the succedent six cycles, the CV redox peaks current or potential are no obvious change, suggesting excellent electrochemical reversibility and cycling durability of the MS-3 electrode. Figure 5b presents the typical galvanostatic profiles of the MS-3 composite at 0.2 C. The results are in accordance with CV studies’ and two discharge plateaus and one charge plateau are observed for all cycles. Furthermore, the first and second cycle discharge curves are nearly overlapped each other, which further indicates the decreased potential polarization and enhanced reversibility of the MS-3 cathode by multifunctional synergistic interaction. Figure 5c exhibits the cycling properties of the MS-3 cathode in comparison with pure sulfur and MS-n (n=1, 2 and 3) at 0.2 C. The incipient specific capacity of pure sulfur is 1074 mAh g-1, indicating a poor sulfur utilization of 64.1%. Subsequently, pure sulfur sample shows a rapid capacity decrease and poor discharge capacity of 775 mAh g-1 at 80th cycle, corresponding to a high capacity fade 15



of 0.35% per cycle. These are the typical results of LiPSs shuttle effect. For the MS-n (n=1, 2 and 3) cathodes manifest high incipient capacities of 1157, 1255 and 1357 mAh g-1 at 0.2 C, suggesting high sulfur utilization of 69.0, 74.9, and 82.1%, respectively. After 100 cycles, the MS-n (n=1, 2 and 3) cathodes maintain high reversible capacities of 978, 1066 and 1204 mAh g-1, indicating high capacity retention of 84.5, 84.9 and 87.6%, respectively. Among all electrodes, the MS-3 cathode delivers the highest discharge capacity, suggesting the effectively suppressed LiPSs shuttle owing to multifunctional synergistic interaction. Besides, the strong chemical adsorption of Ti4O7 can promote close adhesion of LiPSs and uniform deposition of sulfur during the electrochemical process. Even after 200 cycles, the MS-3 cathode shows the excellent capacity stability and high CE. The reversible capacity remains at 1075 mAh g-1 and the average columbic efficiency is 98.7%. The performance of MS-2 is better than MS-1 because of the addition of high electric conductive CNTs. To confirm the effect of CNTs in the system, S/CNTs is prepared without PSS (Figure S4). The S/CNTs composite shows a high first capacity of 1052 mAh g-1 and a stable reversible capacity of 758 mAh g-1 at 140th cycle. The first specific capacity of S/CNTs is higher than that of the MS-1, however, the cycling stability of S/CNTs is lower than that of the MS-1. The results indicate that the PSS can maintain structure stability and the CNTs can enhance sulfur utilization due to high electrical conductivity (Figure S4). To reveal the MS-3 structure stability, the TEM and SEM images for the MS-3 before and after cycling are explored and the morphology of the MS-3 sample is well maintained (Figure S5), which suggests that 16


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combining the negatively charged PSS layer and conductive CNTs on the surface of sulfur act as a protective barrier with strong chemical adsorption of LiPSs by means of Ti4O7, which can validly control the dissolution and diffusion of long-chain LiPSs into electrolyte and promote the cyclic stability of MS-3 electrode. The enhanced electrochemical performances of the MS-n (n=1, 2 and 3) cathodes are also measured at 0.5 C. As displayed in Figure 5d, MS-3 exhibits high incipient capacity of 1184 mAh g-1. Even after 300 cycles, MS-3 composite still keeps a reposeful capacity of 892 mAh g-1 at an ultralow capacity fade of 0.08% per cycle. As illustrated in Figure 5e, the pure sulfur and MS-n (n=1, 2 and 3) cathodes show original specific capacities of 1001, 1022, 1131 and 1184 mAh g-1, respectively. Inversely, pure sulfur sample shows reversible capacity of 410 mAh g-1, revealing the relatively rapid capacity decay of 0.20% per cycle and poor electrochemical stability. The MS-1 and MS-2 cathodes exhibit reversible capacities of 688 and 830 mAh g-1, respectively, suggesting low capacity decay of 0.11 and 0.88% per cycle. These outcomes further manifest that the as-fabricated MS-n (n=1, 2 and 3) have improved specific capacity, cyclic stability and rate performance compared with pure sulfur, suggesting higher sulfur utilization and alleviated LiPSs shuttle effect. MS-3 electrode is also investigated without LiNO3 additive electrolyte. Figure S6 shows that the reversible capacity of MS-3 electrode exhibits a slight decrease without LiNO3 additive, while the reversible capacity will increase with LiNO3 additive. To further investigate visually the soluble LiPSs in the MS-n (n=1, 2 and 3) electrodes, ultraviolet/visible (UV/Vis) absorption spectroscopy is used. As shown in Figure 5f, compared with the 17



MS-1 and MS-2 electrodes, the MS-3 electrode exhibits the weakest peak intensities at 300 cycles, which reveals low content of LiPSs in the electrolyte. Accordingly, during the cyclic process, the loss of LiPSs is effectively alleviated by excellent conductivity, strong physical and chemical absorption for LiPSs.40,41

Figure 5. (a) Cyclic voltammograms and (b) charge/discharge profile of the MS-3 composite at 0.2 C, (c) cycling stability of pure sulfur and MS-n composites at 0.2 C, (d) charge/discharge profile of the MS-3 composite at 0.5 C, (e) cycling stability of pure sulfur and MS-n composites at 0.5 C, (f) UV/Vis adsorption spectra of 18


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DOL/DME solution with MS-n composites at 300th cycle. The color changes of the DOL/DME solutions of the MS-n (n=1, 2 and 3) electrodes are characterized to visually verify the minimized diffusion and dissolution of LiPSs. Figure 6a and b show the color changes of the DOL/DME solutions of the MS-1 and MS-2 electrodes as they changed from colorless to yellow with increasing cycle numbers, respectively. However, the MS-3 electrode doesn’t show evident color change as shown in Figure 6c, which is in agreement with the UV/Vis results. In order to explain clearly suppression effect of MS-3 for LiPSs, the UV/Vis adsorption spectra and optical photo of DOL/DME solution of the MS-0 and MS-n are investigated. The polysulfides/DOL-DME solution with the MS-3 turns from nigger-brown to colorless gradually. The results show that the MS-3 can strongly suppress the shuttle effect of polysulfides (Figure S7).

Figure 6. Typical color changes of the DOL/DME solutions with different cycles for (a) MS-1, (b) MS-2, and (c) MS-3. 19



The rate capabilities of MS-n (n=1, 2 and 3) are also measured at different charge/discharge ratios from 0.1 to 2 C and then compared with pure sulfur. As illustrated in Figure 7a, the MS-3 composite displays a good discharge capacity of 1416, 1375, 1186 and 1020 mAh g-1 at different current ratios from 0.1, 0.2, 0.5 and 1.0 C, respectively. Even at 2 C, MS-3 composite still displays a steady discharge capacity of about 931 mAh g-1. After 50 cycles, when current density finally backs to 0.1C, the specific capacity of MS-3 cathode can almost recover to its original level, obtaining as high as discharge capacity of 1298 mAh g-1. In contrast, pure sulfur composite displays an incipient specific capacity of 1133 mAh g-1 at 0.1 C and then the capacity reduces fast upon cycling. When the current density exceeds 1.0 C, reversible specific capacity of pure sulfur electrode is inferior to 532 mAh g-1. The results suggest that rate capability and the cyclic reversibility of the MS-3 electrode are superior to those of pure sulfur and other electrodes. Figure 7b shows the corresponding preliminary charge/discharge profiles of the MS-3 composite at various current densities. It is noteworthy that the three-steady charge/discharge plateaus are maintained at large current densities, indicating kinetically efficient charge transfer in the MS-3 electrode. The feasibility of using the MS-3 electrode for higher sulfur areal mass loading is further investigated since sulfur areal mass loading is a crucial parameter in determining the energy density of LSBs. By simple doctor blade-coating technology, high sulfur areal mass loading of 5.11 mg cm-2 is achieved and electrochemical performances are displayed in Figure 7c. The MS-3 electrode manifests a lofty incipient capacity of 986 mAh g-1 at 1.0 C, in accordance with a 20


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superb areal capacity of 5.04 mAh cm-2. After 300 cycles, a discharge capacity of 696 mAh g-1 is maintained with lofty capacity conservation of 70.5%. The CE remains higher

than

98%

during

the

extended

cycling

processes.

Although

the

charge/discharge profiles in Figure 7d show a slightly higher polarization due to the relatively sluggish electrode reaction kinetics with a higher areal sulfur loading, the two discharge plateaus are still flat and stable for the initial 10 cycles. Figure 7e exhibits EIS plots of the MS-n (n=1, 2 and 3) electrodes after 300 cycles. The as-prepared composites show the similar shape including two semicircles and one line. The depressed semicircles in low-frequency domain can be assigned to charge-transfer resistance (Rct) influenced by the electronic conductivity of electrode and electrode-electrolyte interface while the semi-circle in high-to-medium frequency domain is attributed to the diffusion resistance (Rf) of SEI film. The line represents the Warburg impedance (Wo). The equivalent circuit models for investigating impedance spectra are presented the inset of Figure 7e. CPE stands for constant phase element, which is used instead of capacitance. Re represents the ohmic resistance from the electrolyte, current collectors and cell connections.42 The value of Rct for the MS-3 composite is much lower than those of the MS-1 and MS-2 composites, which indicates that the MS-3 electrode shows the fastest polysulfide reaction kinetics and more efficient electric contact within the cathode structure than those of the MS-1 and MS-2 cathodes. Moreover, the electrical conductivities of Ti4O7, PPS, MS-1, MS-2 and MS-3 are measured and outcomes are displayed in Table S2. The enhanced kinetics can be assigned to the strong and valid LiPSs adsorption and high 21



Page 22 of 43

conductivity of the MS-3 composite. The equivalent circuit parameters and matching outcomes are tabulated in Table 1. To further verify above results, the Li+ spread coefficient is computed via following Equ. (1):43 DLi+ =

R 2T 2 2 A2 n 4 F 4C 2σ 2

(1)

where DLi+, R, T, A, n, F and C represent Li+ diffusion coefficient, gas constant, absolute temperature, electrode surface area, number of electron per molecule during reaction, Faraday constant and Li+ concentration, respectively. σ stands for the Warburg factor calculated from Equ. (2):44 Z ' = Re + Rf + Rct + σω −1/ 2

(2)

Figure 7f shows the slope for plots of Z’ versus the reciprocal root square of lower angular frequencies (ω-1/2). Analogously, the σ of the MS-3 electrode is far lower than those of the MS-1 and MS-2 electrodes, demonstrating the fastest Li+ diffusion kinetics. This is ascribed to valid fixation of LiPSs and improved conductivity for MS-3 composite.

22


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Figure 7. (a) Rate capability of pure sulfur and MS-n composites, (b) corresponding charge/discharge profiles at various rate of the MS-3 composite, (c) cycling performance and CE of the MS-3 composite at 1.0 C (d) corresponding charge/discharge curves at various cycles of the MS-3 cathode, (e) EIS after 300 cycles of the MS-n composites, (f) the dependence of Z’ on ω-1/2 for three electrodes. Table 1 EIS arguments of composites. Samples

Re (Ω)

Rf (Ω)

Rct (Ω)

σ (Ωs-1/2)

DLi+ (cm2 s-1)

MS-1

4.53

22.72

35.64

21.86

5.20×10-12

MS-2

4.18

16.55

25.01

12.20

1.67×10-11

23



MS-3

3.86

12.33

21.40

6.57

Page 24 of 43

5.75×10-11

In order to elucidate the relationships between sulfur areal mass loading and electrochemical performances, the changes of the areal capacities and capacity retentions with different sulfur areal mass loading for different cycles are studied, and the results are given in Figure 8a-c. The areal capacities of the MS-n composites are computed as discharge capacity (mAh g-1) of sulfur × sulfur mass loading (g cm-2). The pure sulfur and MS-n composites with sulfur areal mass loading of 3.56 mg cm-2 exhibit areal capacities of 3.56, 3.64, 4.03, and 4.22 mAh cm-2, respectively, as shown in Figure 8a. Even if the MS-3 composite possesses a higher sulfur areal mass loading of 5.11 mg cm-2, it also delivers superb areal capacity of 5.01 mAh cm-2 as illustrated in Figure 8b. Figure 8c manifests that after 100 cycles, capacity retentions are 53.5, 77.8, 80.4, and 83.0% for pure sulfur and the MS-n (n=1, 2 and 3) cathodes, respectively. As discussed previously, electrode loading has a significant influence on the energy density of the battery.45-47 According to the estimation from Wang and co-workers, an areal capacity of at least 4.0 mAh cm-2 is required for LSBs to be comparable to commercial LIBs.48 Majority of superb capacities reported in the papers for the fabrication of electrodes are obtained at a low sulfur content (less-than 70 wt%) and/or low sulfur mass loading (less-than 2 mg cm-2) whereas here we provide a sulfur areal mass loading of at least 3.56 mg cm-2 and an excellent sulfur content of 90 wt% with a high areal capacity of 4.22 mAh cm-2 to obtain an energy density superior to conventional LIBs cathodes. As illustrated in Figure 8d, areal capacity of MS-3 composite is much better than that of the recently reported data. 24


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Figure 8. Areal capacities for (a) pure sulfur and MS-n composites at 0.5 C, (b) the MS-3 composite at 1.0 C, (c) capacity preservation ratios of pure sulfur and MS-n composites at 0.5 C, (d) comparison of areal capacity in this work with those in the references. Based on the above results, the MS-3 composite enables an excellent electrochemical property with an excellent sulfur content and a superb sulfur areal mass loading, which can be assigned to the followings: (1) the advantages of PSS are featured with the abundant SO3- groups and branched structures, which can provide strong affinity to absorb LiPSs formed during discharge/charge process through electrostatic interaction and thus lead to the suppressed LiPSs shuttle effect, guaranteeing good cycling stability; (2) the interconnected CNTs form a good and robust conductivity framework that ensures highly efficient electron transport and 25



excellent rate capability; (3) the substoichiometric Ti4O7 possesses polar O-Ti-O units and high metallic conductivity, which shows an enhanced conductivity and strong affinity for LiPSs including solid Li2S2/Li2S via chemical interaction, contributing to high sulfur utilization and tolerant rate capability.

4. CONCLUSIONS A multifunctional synergistic composite comprises the sulfur particle encapsulated with an ion-selective polymer with conductive carbon nanotubes and dispersed around Magnéli phase Ti4O7 (MS-3) is prepared by a simple bottom-up method. The advantages of the design enabled high sulfur loading cathode are able to concurrently address sulfur hosting, electron conducting, and polysulfide migration issues. The MS-3 composite achieves excellent cycling performance and rate capability assigned to the high electric conductivity of CNTs and the electrostatic shield effect of the surface PSS as well as strong chemisorption of LiPSs by means of Ti4O7. As a result, the MS-3 composite shows high original specific capacity of 1375 mAh g-1 at 0.2 C and 1184 mAh g-1 at 0.5 C with excellent sulfur content of 90 wt% and sulfur areal mass loading of 3.56 mg cm-2. Even at a higher sulfur areal mass loading of 5.11 mg cm-2, the MS-3 composite manifests a high incipient areal capacity of up to 5.04 mAh cm-2, which is comparable to that of commercialized LIBs. The reversible capacity is still kept at 696 mAh g-1 at 300th cycle even at 1.0 C. Therefore, the effective strategy for achieving superb sulfur loading and immobilizing LiPSs by multifunctional synergistic interaction should be of great significance for the practical 26


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commercialization application of LSBs.

ACKNOWLEDGMENTS This work is supported financially by the National Natural Science Foundation of China under project No. 51272221, the Hunan Provincial Innovation Foundation for Postgraduate (CX2017B292) and the Key Project of Strategic New Industry of Hunan Province (No. 2016GK4005 and 2016GK4030). The authors thank Dr. Lixiao Miao at Sound Group Institute of New Eenergy and Soundon New Energy Technology Co., Ltd for his support in guiding the experiment in LSBs.

Supporting Information. Optical image and particle size distribution of Ti4O7 after ball-milling; TEM image of MS-1 and 2; SEM characterization and elemental mappings of MS-1, 2 and 3; TEM image and cyclic performance of S/CNTs; TEM and SEM images of MS-3 before and after cycles; cyclic performance of MS-3 without LiNO3 electrolyte; Polysulfide adsorption experiments of MS-0, 1, 2 and 3.

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TOC graphic:



Figure 1. Schematic diagram of discharge procedure of (a) pure sulfur and (b) asprepared composite.


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Figure 2. (a, b) MS-1, (d, e) MS-2, and (g, h) MS-3 for SEM images; (c) MS-1, (f) MS2, and (i) MS-3 for EDS spectra; the white scale bars are 1 μm.



Figure 3. Ti4O7 for (a, b) TEM images (insets: FFT patterns), (c) XRD diffractrograms; the as-prepared composites for (d) FT-IR spectra, (e) XRD diffractrograms, (f) TGA traces.


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Figure 4. XPS survey spectra of (a) the as-prepared composites, (b) S2p, (c) C1s, and (d) Ti2p of the MS-3 composite.



Figure 5. (a) Cyclic voltammograms and (b) charge/discharge profile of the MS-3 composite at 0.2 C, (c) cycling stability of pure sulfur and MS-n composites at 0.2 C, (d) charge/discharge profile of the MS-3 composite at 0.5 C, (e) cycling stability of pure sulfur and MS-n composites at 0.5 C, (f) UV/Vis adsorption spectra of DOL/DME solution with MS-n composites at 300th cycle.


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Figure 6. Typical color changes of the DOL/DME solutions with different cycles for (a) MS-1, (b) MS-2, and (c) MS-3.



Figure 7. (a) Rate capability of pure sulfur and MS-n composites, (b) corresponding charge/discharge profiles at various rate of the MS-3 composite, (c) cycling performance and CE of the MS-3 composite at 1.0 C (d) corresponding charge/discharge curves at various cycles of the MS-3 cathode, (e) EIS after 300 cycles of the MS-n composites, (f) the dependence of Z’ on ω-1/2 for three electrodes.


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Figure 8. Areal capacities for (a) pure sulfur and MS-n composites at 0.5 C, (b) the MS-3 composite at 1.0 C, (c) capacity preservation ratios of pure sulfur and MS-n composites at 0.5 C, (d) comparison of areal capacity in this work with those in the references.


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