Suppressing the Polysulfide Shuttle Effect by Heteroatom-Doping for

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Suppressing Polysulfide Shuttle Effect by HeteroatomDoping for High-Performance Lithium-Sulfur Batteries Manfang Chen, Shu Zhao, Shouxin Jiang, Cheng Huang, Xianyou Wang, Zhenhua Yang, Kaixiong Xiang, and Yan Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00273 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Sustainable Chemistry & Engineering

Suppressing Polysulfide Shuttle Effect by Heteroatom-Doping for High-Performance Lithium-Sulfur Batteries Manfang Chena, Shu Zhaob, Shouxin Jianga, Cheng Huanga, Xianyou Wang∗a, Zhenhua Yangb, Kaixiong Xianga, Yan Zhanga (a 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 b Key Laboratory of Low Dimensional Materials &Application Technology (Ministry of Education), School of Materials Science and Engineering, Xiangtan University, Xiangtan, Hunan 411105, China)

Abstract: In order to restrict polysulfide shuttle effect and enhance sulfur utilization of the lithium-sulfur batteries (LSB) especially at low charge/discharge rates, a facile hydrothermal synthesis and subsequent heating melting treatment are used to synthesize the heteroatom-doped carbon nanotubes/sulfur composite cathode. The composition analysis and structure characteristics of samples are examined by X-ray photoelectron spectroscopy, X-ray powder diffraction and transmission electron microscopy. The electrochemical performances of samples are measured by cyclic voltammetry and charge/discharge experiments. The results show that N, B, S tri-doped ACNTs with abundant mesoporous structure enable fast Li+ transmit and provide strong polysulfide adsorption ability. More importantly, it offers enough ∗a

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

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

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mechanical strength to support high sulfur loading (77 wt%) that maximizes their chemical role and can accommodate large volume changes. The N, B, S tri-doped ACNTs/S composite exhibits a superb incipient capacity of 1166 mAh/g-S at 0.3 C and large reversible capacity of 881 mAh/g-S at 700th cycle. To further promote the cyclic lifespan of LSB, the as-prepared N, B, S tri-doped ACNTs acted as both sulfur matrix and spring functional layer achieve a large reversible specific capacity of about 713 mAh/g-S at 1400th cycle at lofty current density of 0.5 C with a slow capacity decay of 0.014% cycle-1 and a higher sulfur loading of 90 wt%. Accordingly, reasonable design for heteroatom doping element in carbon material and separator modification will be distinctly vital for enhancing the electrochemical performance of LSB and boosting its industrial application.

Keywords: Heteroatom-doping, Separator modification, Carbon nanotubes, Sulfur host, Lithium-sulfur batteries

Introduction Rechargeable lithium-sulfur batteries (LSB) have been already paid to substantial attention in the future large-scale stored energy field because of their superb energy density, theoretical capacity and abundant resources of sulfur.1 However, some problems originating from low conductivity and volume variation of sulfur and Li2S during circulation process remain still to be solved prior to large-scale industrialization application. Furthermore, LSB via a reversible conversion reaction mechanism involves a train of lithium polysulfides (LiPSs, Li2Sn, 2≤n≤8) 2


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accompanied by dissolution/deposition transformations because the LiPSs easily dissolve in the organic electrolytes.2,3 A secondary reaction occurs during cycling procedure, soluble high-order LiPSs through separator spread to the anode and react with lithium to be reduced into low-order LiPSs and then go back to the cathode, which is known as the polysulfide shuttle.4,5 The notorious shuttle effect leads to the active material loss, poor coulombic efficiency, and rapid capacity attenuation which is one of the prime problems that hinder the commercialized application of LSB.6,7 A lot of works have been conducted to depress the shuttle effect, which include cathode design and coating,8 separator modification9,10 and functional interlayer11,12 as well as electrolyte adjustment, e.g., solid electrolyte,13,14 “solvent-in-salt” strategy,15 and functional lithium salt additive16 etc. Among above all strategies, the integration of sulfur and various conductive carbon material, such as three-dimensional porous carbon materials, hollow carbon spheres, graphene oxides, carbon nanotubes (CNTs), microporous carbon and their hybrids,17-19 is a common strategy to obtain good electrical conductivity, excellent cycling stability and outstanding rate performance. Particularly, CNTs possess classic 1D structure and show a self-weaving behavior to construct an interwoven conductive network for fast transfer of electrons. Notably, owing to the weak absorption ability of CNTs with sulfur species, when the sulfur content is higher than 70 wt%, sulfur species precipitate mainly on outer surface, which leads to rapid spread into the electrolyte. Recently, Shao’s group20 reported 3D interlinked porous carbon nanosheets/carbon nanotubes to encapsulate sulfur, which exhibited a high specific capacity of 689 mAh/g-S under 0.5 C at 200th cycle. In our 3



previous work, N, S co-doped hierarchical porous carbons derived from biomass using lotus plumule as the precursor were prepared and then an excellent reversible capacity of 952 mAh/g-S with sulfur content of 86.5 wt% after 300 cycles at 0.5 C was obtained.21 Heteroatom doping is a facile method to decorate the surface of carbon material and provide more active sites, and thus it can lead to an enhanced discharge capacity and cyclic performances for LSB. Such heteroatom atom doped carbon has advanced tremendously in the past years, especially by proving their usefulness in energy related fields, e.g., rechargeable cells, supercapacitors, storing hydrogen or fuel cells.22 Typical heteroatom atoms such as boron (B), nitrogen (N), and sulfur (S) have been intensely investigated in more recent years since they were served as cathode materials for LSB by Zhou and Guo.23,24 These materials seem to be among the most promising candidates for up-to-date high-performance LSB owing to their ability to provide superb energy density and long-cyclic life. The extremely high specific capacity which produced by heteroatom atom doping derives from two reasons: one is the strong chemical interaction, and another is the good electrical conductivity. By heteroatom doping, the properties of the material are altered compared to the undoped carbon materials. Although only a limited content of heterogeneous atoms can be introduced into the carbon-based material, plenty of active sites are formed. In this way, the lithium polysulfide is greatly restricted upon the surface of the material by the active sites because of the electrostatic interactions. Therefore, to ensure the surface of carbon material possessing enough active sites to absorb LiPSs and limit 4


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polysulfide shuttle, the strategy of the carbon-based material such as CNTs incorporating some heterogeneous to achieve co-doping, tri-doping even multi-doping is put forward. Motivated by the considerations above, instead of relying solely on the co-doping, herein we develop a facile and controllable synthesis of N, B, S tri-doped ACNTs using boric acid, urea, and thiourea as dopants. The N, B, S tri-doped ACNTs are expected to promote the electron transfer, accommodate the volumetric expansion and trap the soluble polysulfide. Furthermore, to further enhance cyclic lifespan of LSB, N, B, S tri-doped ACNTs host with interlinked frameworks and suitable pore distribution acts as superb sulfur loading substrate and separator modification for progressive LSB. Briefly, this distinct construction may bring the cathode material a step closer to the high-energy-density requirements, excellent cycling performance and fast charging/discharging. The influences of the doping element kind and separator modification for the electrochemical properties of LSB are also systematically investigated in detail.

Experimental Section Theoretical calculations. First-principle calculation was used to analyze interaction between doped carbon and LiPSs, which was implemented using the Vienna Ab Initio Simulation Package (VASP).25 The framework general gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) was used to treat exchange-correlation functional.26 To ensure convergence, the plane wave cut-off energy was set to 400 eV. A graphene nanoribbon was imitated via a hexagonal 6×6 5



supercell. To avoid interactions between graphene and its periodic images, periodic boundary conditions were used in all three directions with a vacuum gap of 15 Å in the vertical direction. The 2×2×1 of K point mesh was chosen. All atoms were fully relaxed until the maximum residual force component was less than 0.03 eV/Å. To obtain more credible binding energies, the semi empirical modification means DFT-D227 for the traditional Kohn-Sham DFT was used to calculate the van der Waals (vdW) interaction energies. Synthesis of the active carbon nanotubes (ACNTs) and heteroatom-doped ACNTs. The multiwalled carbon nanotubes (CNTs) were original from Cnano Technology (Beijing, China). The CNTs were activated via KOH at mass ratio of 5:1. The mixed material was calcined at 400 °C for 0.5 h and then heated to 850 °C for 1 h with in an argon atmosphere. Finally, to remove residual KOH, the as-obtained mixture was washed with hydrochloric acid and DI water until pH value was neutrality. Afterwards the mixture was dried in drying oven at 60 °C for 12 h. The active CNTs (ACNTs) were obtained. For synthesis of heteroatom-doped ACNTs material, urea, boric acid, and thiourea (CN2H4S) were served as the sources of N, B and S. The mass ratio 7:1 of dopants and as-prepared ACNTs were added into 60 mL DI water, and stirred vigorously for 0.5 h. Subsequently, the mixture was shifted into a 100 mL Teflon-lined autoclave, and then encapsulated in a rustless steel to undergoing hydro-thermal process at 180 °C for 24 h. Afterwards, the samples were obtained by filtration, washed with DI water three times and then desiccated at 60 °C for 12 h to remove residual dopants. The different heteroatom-doped ACNTs were obtained. 6


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Synthesis of the ACNTs/S and heteroatom-doped ACNTs/S composites. To prepare the carbon/sulfur composite, the as-prepared carbon was mixed with sulfur at a ratio of 1:5 and 1:10, then the mixed powder encapsulated in a vial. The vial was encapsulated in Ar atmosphere and calcined at 155°C for 12 h. Afterwards, the temperature was enhanced to 300 °C for 1.5 h and cooled to room temperature, and then the carbon/sulfur composite was synthesized. Materials Characterization. The structures and morphologies of the as-synthesized composites were performed on field-emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). To investigate phase ingredient of samples, X-ray diffraction (XRD, Model LabX-6000, Shimadzu, Japan) was recorded in the 2θ range of 10°-80°. Thermogravimetric analyses (TGA, TA Instruments, USA) was implemented to confirm sulfur content in an N2 atmosphere from indoor temperature to 500 °C. The specific surface areas and pore structures were done on the nitrogen adsorption/desorption tests (JW-BK112). The surface areas were computed by traditional Brunauer-Emmett-Teller (BET) means. The pore volumes (Vtotal) and pore size distributions were determined from isotherm adsorption branch on the basis of non-local density functional theory (NLDFT) model. To research surface chemical ingredient and function groups of the as-prepared samples, X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific) were examined. Electrochemical measurements. The cathode electrodes were synthesized by active material, acetylene black and polyvinylidene fluoride binder and the mixed 7



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slurry of the weight ratio was 80:10:10. The mixed slurry was coated on aluminum foil. Then, the foil was dried in the vacuum at 60 °C for 12 h. The sulfur weight was 1.7-2.0 mg, which provided a large areal sulfur mass loading of 2.2-2.5 mg/cm2. The electrolyte was 1 mol/L 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. This additive plays a role in suppressing shuttle effect of soluble LiPSs on Li anode.28 0.025 mL electrolyte was added for per cell based on the electrolyte/sulfur rate was about 10 mL/g.29 The anode was metal lithium and the separator was the Celgard 2400 membranes. The coin cells were fabricated in the glove box under argon atmosphere. The galvanostatic charge/discharge (GCD) data were carried out 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. The electrochemical impedance spectroscopy (EIS) was researched by the identical instruments over a frequency range from 100 kHz to 1 Hz with an alternating current voltage of 5 mV.

Results and discussion The compounds of CNTs mixed with KOH are first carbonized under an inert atmosphere to obtain highly porous active multiwalled CNTs (ACNTs) based on the following reactions: 6KOH + 2C → 2K + 3H2 + 2K2CO3 8


(1)

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K2CO3 → K2O + CO2 CO2 + C → 2CO K2CO3 + 2C → 2K + 3CO K2O + C → 2K + CO

(2) (3) (4) (5)

The reaction of carbon with KOH and the resultant CO are supposed to create abundant micropores on the surface of CNTs.30,31 Then, the multiple-doped ACNTs are synthesized through a hydrothermal method as shown in Figure 1a. Boric acid, urea and thiourea are used as dopants. During the hydrothermal process, the dopants are disintegrated to N, S co-doping, N, B co-doping and N, B, S tri-doping in ACNTs materials. The doping amount of heteroatom atoms can be easily adjusted via tuning the concentration of dopants in solution. To control variable, the doping element concentration is unified, yielding at the doping level of 3.5 atom% in these materials. The fabrication of the multiple-doped ACNTs/S cathode is obtained by a heating melting method at 160 °C for 12 h, and calcined in the tube furnace at 300 °C for 90 min to introduce as many as possible sulfur into ACNTs. The morphology and microstructure of ACNTs are performed by field-emission scanning electron microscopy (FESEM). As illustrated in Figure 1b-c, ACNTs demonstrate tube morphology with interwoven wool-ball structure. As shown in Figure 1e-g, the transmission electron microscopy (TEM) images clearly exhibit the hollow tube structure, and high-resolution TEM images show high graphitization degree and present apparent pores on the surface of ACNTs, resulting from the KOH activation or hydrothermal doping. This graphitized porous structure greatly facilitates 9



the enhancement of conductivity, transmission of Li+ and penetration of electrolyte, and thus effectively limits polysulfide shuttle effect. Figure 1h-j show that the carbon/sulfur composite still maintains the interwoven porous morphology after the sulfur impregnation, which indicates that the sulfur is homogeneously distributed into carbon nanotubes host. As shown in Figure 1i, the interplanar distances with

d=0.38nm can be attributed to sulfur (222).32 As shown in Figure 1d, energy-dispersive spectroscopy (EDS) demonstrates the presence of carbon and sulfur in composite. This is in line with TEM results and also proves that sulfur is successfully entrapped into the ACNTs carbon network.

10


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Figure 1. (a) Schematic illustration showing the fabrication of heteroatom doping ACNTs/S composite, SEM images of (b, c) ACNTs, (d) EDS spectrum of ACNTs/S, (e, f) TEM and (g) HRTEM images of ACNTs, (h) TEM and (i, j) HRTEM images of ACNTs/S. In order to detect the heteroatom doping content and surface chemical composition of the as-prepared carbon material, X-ray photoelectron spectroscopy (XPS) is examined. As displayed in Figure 2a, XPS survey spectra of ACNTs show only two peaks located at about 284.8 and 532.7 eV corresponding to C1s and O1s respectively. After heteroatom doping, the peaks at about 400.0, 192.0 and 165.1 eV, 11



respectively, correspond to the nitrogen, boron and sulfur atoms, suggesting that reasonable amounts of N, B or S are successfully incorporated into the carbon framework of ACNTs. XPS survey spectra of N, S dual-doped ACNTs show the N1s, S2p peak respectively. As shown in Figure 2b, the C1s spectra of N, S dual-doped ACNTs exhibit two peaks centered on 284.9 and 286.6 eV, respectively, attributed to C-C/C=C and C-O/C-S/C-N. However, XPS spectra for N, B co-doped ACNTs suggest N1s and B1s two peaks. As illustrated in Figure 2c, the C1s peak can be disassembled into three ingredients at 282.4, 284.6, and 285.7 eV corresponding to C-B, C-C, and C-N bonds, respectively. As shown in Figure 2d, the C1s spectra of N, B, S tri-doped ACNTs exhibit much more heteroatom groups, which is capable of enhancing efficiently the binding ability of LiPSs. These functional groups are beneficial to forming high-efficient active sites for inhibiting the dissolution and diffusion of LiPSs. Furthermore, the atomic percentages of the heteroatom-doped ACNTs samples are summarized from the XPS survey spectra in Table 1. The total content in doping ACNTs is 3.35-3.59 at%. To further analyze the effect of heteroatom doping on the property of sulfur host at atomic-scale, density functional theory (DFT) calculations are used to examine the adsorption behavior of the LiPSs on the surface of a series of heteroatom-doped graphene nanoribbons (GNR). Here, a small LiSH molecule is chosen as the representatives of LiPSs. As illustrated in Figure 2e, the Li in LiSH is situated at GNR with a distance of d=2.43 Å, which is in well consistent with the reported DFT results.33 Figure 2f-g exhibit the adsorption allocation of LiSH at feasible binding sites from DFT compute. Compared with the 12


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case of GNR, the distance is decreased to 2.36 Å at N, B co-doped site (Figure 2f), indicating strong adsorbability. As shown in Figure 2g, the expected distance of 2.33 Å at N, B, S tri-doped GNR site is lower than that of N, B co-doped GNR and original graphene case (2.43 Å), which suggests that N, B, S tri-doping enhances further the surface affinity property of nonpolar carbon and shows the strongest chemisorption ability for polysulfides. Furthermore, the calculated binding energies with vdW correction of N, B co-doped and N, B, S tri-doped GNR are 1.68 and 3.43 eV, respectively, which are actually higher than that of the original graphene case (0.58 eV). The existence of N, B, S tri-doping can make full use of three elements synergistic effect compared with the cases of co-doping. Overall, the theoretical findings in regard to the synergistic effect of N, B, S tri-doping well guide the experimental results.

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Figure 2. XPS survey spectra of (a) the ACNTs and doping ACNTs, (b) N, S dual-doped ACNTs, (c) N, B dual-doped ACNTs, (d) N, B, S tri-doped ACNTs of C 1s, first-principle calculations presenting the adsorption behavior of the LiSH on the 14


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surface of the different heteroatom doping (e) undoping, (f) N, B dual-doping and (g) N, B, S tri-doping. The as-prepared ACNTs and heteroatom-doped ACNTs are further identified by N2 adsorption/desorption isotherm measurement. As illustrated in Figure 3a-h, the typical adsorption/desorption isotherms of resultant ACNTs and modified ACNTs show the similar trend with representative type-Ⅳ isotherm as well as H3-type hysteresis loop, suggesting the existence of mesopore raised from the hollow carbon nanotubes. The N2 adsorption/desorption isotherm measurement of the as-prepared material illustrates an apparent rise at low P/P0 assigned to the generation of plenty of micropores by KOH activation. The BET specific surface areas of heteroatom-doped ACNTs (N, S co-doping, N, B co-doping and N, B, S tri-doping) are 259, 264 and 253 m2/g respectively whereas the doped ACNTs are lower than that of undoped ACNTs (373 m2/g). Besides, the DFT pore size spread data of all samples are shown in Table 1, revealing the relatively narrow pore size ranging from 8 to 12 nm. The decrease of BET surface areas and total pore volumes are probably because doping material has much more structural defects caused by incorporation heteroatom into the carbon network and the collapse of the mesopores channels compared with the undoped material. Even so, the pore structure can still efficiently favor electrolyte penetration and sulfur loading.

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Figure 3. (a, c, e, g) N2 adsorption/desorption isotherm, (b, d, f, h) pore-size spread for as-prepared materials.

16


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The X-ray diffraction patterns (XRD) are executed to examine the structure and ingredient of heteroatom-doped ACNTs. As illustrated in Figure 4a, all samples show two obvious diffraction peaks at 2θ = 26.0° and 43.0°, which can be attributed to the (002) and (001) crystal plane of graphite, respectively. It is worth noting that the peak intensity of the heteroatom-doped ACNTs weakens compared with the original ACNTs, indicating that the structural defects of graphitization carbon layer are usually owing to the insertion of heteroatom. Figure 4b shows the Raman spectra of all as-prepared samples. The two obvious peaks about 1340 and 1580 cm-1 involve the D and G band respectively. The ratio of D to G band for heteroatom-doped ACNTs is higher than that of undoped ACNTs due to the increase of defects from heteroatom doping. This is in agreement with the BET specific surface areas results. Figure 4c is the XRD patterns of the carbon/sulfur composite and exhibits sharp and strong peaks at 23.1 and 27.8°, which are in well agreement with (222) and (040) of Fddd orthorhombic phase of sulfur (JCPDS No. 08-0247).34 The thermostability of the as-synthesize carbon/sulfur composite is explored by thermogravimetric analysis (TGA). As presented in Figure 4d, the sulfur content of ACNTs/S and heteroatom-doped ACNTs/S (N, S co-doping, N, B co-doping and N, B, S tri-doping) are determined to be 81.61, 78.15, 80.15 and 77.47 wt%, respectively.

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Figure 4. (a) XRD diffractrograms, and (b) Raman spectra of as-prepared carbon material, (c) XRD diffractrograms, and (d) TGA traces of as-prepared carbon/sulfur materials. Table 1. Texture characteristics originated from the N2 adsorption/desorption isotherm, composition content reckoned from XPS spectra and ID/IG of the as-prepared carbon material. XPS (at%)

SBET (m2/g)

Vtotal (cm3/g)

PDFT (nm)

C

N

S

B

ACNTs

373

1.68

13.3

90.15

-

-

-

1.0

N, S co-doped ACNTs

259

1.18

17.8

86.38

3.34

0.25

-

1.05

N, B co-doped ACNTs

264

1.24

16.2

84.42

3.02

-

0.33

1.04

N, B, S tri-doped ACNTs

253

1.15

18.9

81.27

2.91

0.14

0.26

1.1

Sample

ID/IG

18


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The N, B, S tri-doped ACNTs/S composite is first studied by cyclic voltammetry (CV). The first three CV profiles of N, B, S tri-doped ACNTs/S at a low scan rate of 0.1 mV/s with the potentials ranging from 1.7 to 2.8 V are shown in Figure 5a. The reduction peaks located at 2.3 and 2.0 V are assigned to the reduction of sulfur to long-chain LiPSs, and successive reduction to short-chain LiPSs. The oxidation peak situated at 2.35 V concerns a series of production of short-chain to long-chain LiPSs until LiPSs are absolutely depleted and sulfur is formed at 2.4 V.35 Apparently, in continuous circulations, there are not obvious changes for the CV peak currents or positions, suggesting good electrochemical stability of the N, B, S tri-doped ACNTs/S cathode. To investigate the electrochemical properties and influence of the doping element on the properties of the as-synthesized composites, cyclic stabilities of all as-prepared carbon/sulfur compounds are studied at 0.3 C. As displayed in Figure 5b, ACNTs/S cathode manifests the initial specific capacity of 1102 mAh/g-S while heteroatom-doped ACNTs/S cathodes (N, S dual-doping, N, B dual-doping and N, B, S tri-doping) show lofty original discharge capacities of 1129, 1150 and 1166 mAh/g-S, respectively. After 700 cycles, the ACNTs/S cathode maintains the discharge capacity of 365 mAh/g-S with relatively high capacity attenuation of 0.0956% cycle-1 (with capacity retention of 33%). In contrast, the heteroatom-doped ACNTs/S cathodes (N, S co-doping, N, B co-doping and N, B, S tri-doping) exhibit specific capacities of 576, 730 and 881 mAh/g-S, respectively with low capacity attenuation of 0.0699, 0.0521 and 0.0349% cycle-1 and high capacity retentions of 51, 63 and 76% as well as high coulombic efficiency (CE) which reveals the excellent 19



electrochemical behavior. To better understand the positive role of N, B, S tri-doping for the long cycling stability, N, B, S tri-doped ACNTs/S electrode without LiNO3 as electrolyte additive manifests the CE of 84% and still maintains a good cycling stability, suggesting the excellent cycling durability of N, B, S tri-doping for suppressing the shuttle effect of polysulfides (Figure S1). The heteroatom N, B dual-doping is in favor of rapid charge transmission and also enhances affinity between inherent nonpolar carbon network and intrinsic polar polysulfides. Figure 5c-f demonstrate the potential differences for all as-prepared samples between the 1st, 300th, and 700th GCD potential plateaus. The potential distinction for N, B, S tri-doped ACNTs/S cathode is only 0.1342 V at initial GCD, while the other three composites are triflingly increased corresponding to 0.2077, 0.1631 and 0.1461 V, respectively. Even at 300th and 700th cycles, the potential distinctions for N, B, S tri-doped ACNTs/S cathode are only 0.1789 and 0.2115 V, which are less than other three cathodes. It's worth noting that the decreasing order of voltage difference for GCD voltage platform is in agreement with the long-term cycling stability for the as-prepared cathode. Generally, the voltage difference for GCD voltage platform is related to the polarization of electrode during the electrochemical response procedure. Distinctly, the polarization of N, B, S tri-doped ACNTs/S electrode is lower than that of other three electrodes during the long-term cyclic procedure. Hence, rational heteroatom doping can not only enhance the sulfur utilization but also confine the polysulfide dissolution and diffusion. This can be assigned to heteroatom doping by introducing strong interactions between active sites and LiPSs. Similarly, the rate 20


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capabilities of all carbon/sulfur composites are tested, as shown in Figure 5g. When cycled at 0.1, 0.2, 0.3, 0.5 and 1.0 C, the ACNTs/S shows the initial capacities of 1240, 1105, 1039, 740 and 600 mAh/g-S, respectively. When current ratio returns to 0.2 C, the concrete capacity of 967 mAh/g-S is obtained. In sharp contrast, as illustrated in Figure 5h, the N, B, S tri-doped ACNTs/S manifests the high specific capacities of 1374, 1291, 1150, 1038 and 894 mAh/g-S, respectively. When the current ratio returns to 0.2 C, the reversible capacity of 1236 mAh/g-S is obtained, suggesting that N, B, S tri-doped ACNTs/S exhibits excellent rate capability. The absorbability of the porous active CNTs paired with the multiple doping is regarded to synergistically improve the interactions with LiPSs and enhance the overall electrochemical performance. Apparently, the rate and cyclic performances of the all as-prepared carbon/sulfur composites are ordered as follows: N, B, S tri-doped ACNTs/S > N, B co-doped ACNTs/S > N, S co-doped ACNTs/S > ACNTs/S. The improvement in specific capacity and cycle stability can be assigned to a unique architectural structure for rapid electron transmit and suitable doping for strong chemisorption LiPSs.

21



Figure 5. (a) Representative CV profiles of the N, B, S tri-doped ACNTs/S, (b) comparison of cyclic properties and CE at 0.3 C, (c-f) GCD curves at various circulations, (g) the rate capability of as-prepared compound, (h) original GCD profiles at various ratios of N, B, S tri-doped ACNTs/S. Furthermore, CV measurements at different scanning rates from 0.1 to 0.4 mV/s 22


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are conducted to investigate Li ions diffusion properties. As illustrated in Figure 6a-d, accompanying the enhancement of the scan ratios, the peak currents also increase, the Li ions diffusion coefficient can be calculated from the classical Randles-Sevcik equation (6):36

Ip = 2.69 ×105 n3/2 ADLi + 1/2CLi v1/2

(6)

Where Ip represents peak point current, n stands for electrons number in the reaction,

A represents electrode area (0.785 cm2), DLi+ represents Li+ diffusion coefficient, CLi+ is the Li+ concentration, and v stands for scan rate. Figure 6e shows that all cathodic and anodic peak currents are the linear connection with the square root of scan rates. The computed results are displayed in Table 2. Apparently, the order of Li+ diffusion is: N, B, S tri-doped ACNTs/S > N, B co-doped ACNTs/S > N, S co-doped ACNTs/S > ACNTs/S, which is in well accordance with their cyclability and rate capabilities. Furthermore, it is well-known that the rate capabilities are closely relevant to the conductivities of the as-obtained composite cathodes, thus electrochemical impedance spectroscopy (EIS) is performed to assess the conductivities of all sample. As illustrated in Figure 6f-g, before and after the cycles, N, B, S tri-doped ACNTs/S shows the lowest charge-transfer resistance (Rct) from high-frequency semi-circle in all cathodes, which suggests that the N, B, S tri-doped ACNTs possesses profitable electrical conductivity and rapid Li+ transmit rate. The Rct values of ACNTs/S are the highest referred to sulfur partially detaching from the conductive carbon structure. It’s worth noting that ohmic resistance (Re) values increase after cycles, which can be attributed to a fraction of polysulfide dissolved in 23



electrolyte. The equivalent circuit models are employed to analysis EIS in the insets of Figure 6g. The reciprocal equivalent circuit parameters are displayed in Table 3. To further investigate visually the soluble LiPSs in the as-prepared electrode, ultraviolet/visible (UV/Vis) absorption spectroscopy is used. As shown in Figure 6h, in comparison with ACNTs/S electrode, the N, B, S tri-doped ACNTs/S electrode suggests the lowest content of LiPSs in the electrolyte. The Li2S6/DOL-DME solution with N, B, S tri-doped ACNTs turns from nigger-brown to colorless step by step with 2 h (Figure S2). Accordingly, during GCD cyclic procedure, the shuttle of LiPSs is effectively alleviated by strong chemical adsorption of N, B, S tri-doping. In addition, the strong chemical adsorption can promote close adhesion of LiPSs and uniform deposition of sulfur during the electrochemical process. The importantly enhanced electrochemical properties of N, B, S tri-doped ACNTs/S are assigned to its excellent structural parameters: 1) sturdy mechanical characteristic to restrain the volumetric swell/shrink of electrode; 2) high BET surface area and hierarchical interlinked micro-/mesopores as ion channels; 3) enhanced electrochemical activity and electronic conductivity; 4) fast Li+ transfer and strong chemisorption of LiPSs with the N, B, S tri-doping.

24


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Figure 6. CVs at various scan ratios of LSB: (a) ACNTs/S, (b) N, S dual-doped ACNTs/S, (c) N, B dual-doped ACNTs/S, (d) N, B, S tri-doped ACNTs/S, and (e) linear matching of peak point currents for the batteries, Nyquist plots after (f) 1st and 25



Page 26 of 48

(g) 300th cycles for as-prepared composites (inset: equivalent circuit models), (h) UV/Vis adsorption spectra of DOL/DME solution with the as-prepared composites at 300th cycle. Table 2. List of computed Li+ spread coefficient. DLi+ (cm2/s) A (anodic peak at 2.4 V) B (cathodic peak at 2.0 V) C (cathodic peak at 2.3 V)

ACNTs/S

N, S co-doped ACNTs/S

N, B co-doped ACNTs/S

N, B, S tri-doped ACNTs/S

3.087×10-8

3.232×10-8

3.481×10-8

4.149×10-8

5.302×10-9

1.408×10-8

1.389×10-8

2.668×10-8

2.936×10-9

3.583×10-9

4.008×10-9

1.275×10-8

Table 3. Resistance parameters mimicked from equivalent circuits. Cycle number

Resistance (Ω)

ACNTs/S




After 1st cycle

Re

4.51

3.24

2.92

2.65

Rct

32.22

25.08

12.49

6.7

Re

5.77

4.38

3.76

3.13

Rct

31.93

19.89

8.75

9.29

Rs

54.42

31.55

23.41

15.62

After 300 cycle

th

In order to make full use of the advantages of N, B, S tri-doped ACNTs, there is a flexible strategy that N, B, S tri-doped ACNTs serve as both the sulfur matrix and the functional material coated on the separator to physical confine and chemical entrapment of LiPSs. Figure 7a-b manifest a digital photograph of separator modification. Figure 7c shows an interwoven conductive network of N, B, S tri-doped ACNTs, which is beneficial for grasping dissolved polysulfides. Figure 7d displays N, B, S tri-doped ACNTs layer with the approximately 10 µm thickness. LSB with N, B, 26


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S tri-doped ACNTs as both a sulfur host and a spring interlayer (named as Cell/MCNTs/MPP) have been fabricated. The long-term cycling stability of Cell/MCNTs/MPP is tested at 0.5 C. The sulfur content of N, B, S tri-doped ACNTs/S is about 90 wt%, as determined by TGA shown in Figure 7e. The CV curves show two pairs of redox peaks during first three cycles shown in Figure 7f. The Cell/MCNTs/MPP shows the initial capacity of 897 mAh/g-S. After 1400 cycles, the high reversible capacity of 713 mAh/g-S is still maintained for Cell/MCNTs/MPP at 0.5 C with acceptable capacity retentions of 79.5% and negligible capacity fading of 0.014% cycle-1 shown in Figure 7g. The results indicate that the Cell/MCNTs/MPP even at high sulfur content manifests a high specific capacity, stable cyclic performance and excellent rate capability. This demonstrates high sulfur utilization and suppressed polysulfide shuttle effect due to the moderate heteroatom doping providing much more active sites and modified nonpolar surface of the matrix. The electrochemical performance of Cell/MCNTs/MPP is apparently better than those of the lately reported work. For example, Kuo et al.35 prepared N-doped three-dimensional reduced graphene oxide (N-3D-rGO) through sulfur-injection means, which represented a first concrete capacity of 1042 mAh/g-S at 0.2 C. Kuang et al.42 reported B-doped unzipped carbon nanotubes/sulfur compound, which displayed a reversible capacity of 750 mAh/g-S at 0.2 C at 400th cycle. Moreover, comparisons of the electrochemical properties of the as-synthesized heteroatom-doped carbon/sulfur with some other similar composites37-42 for LSB are given in Table 4. Apparently, the as-prepared heteroatom-doped ACNTs/S composites show excellent 27



cycling stability and high sulfur utilization.

Figure 7. (a, b) Optical images, (c) top-down and (d) cross-section SEM images of N, B, S tri-doped ACNTs coated separator, (e) TGA curves, (f) CV, and (g) cycling performances and CE of Cell/MCNTs/MPP.

28


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Table 4. Electrochemical properties of LSB on the basis of various carbon/sulfur materials. Initial/ Reversible capacity

Capacity decay (cycle-1)

Ref.

1166/881 897/713

0.035% 0.015%

This work

700

1150/730

0.052%

This work

0.3

700

1129/576

0.070%

This work

80

0.5

500

916/632

0.062%

37

0.85

58.8

0.5

500

988/644

0.070%

38

TiO@C-HS/S

1.5

70

0.5

500

1058/630

0.081%

39

S@MnO2@GO

0.6

52

0.35

400

603/261

0.141%

40

S/GN-CNT

1.3-1.6

70

0.5

500

758/463

0.078%

41

BUCNTs/S

1.2

65-70

0.2

400

1251/750

0.1%

42

Areal mass loading of S (mg/cm2)

S content (wt%)

Rate (C)


2.2-2.5

77.47 90.46

0.3 0.5

700 1400


2.2-2.5

80.15

0.3


2.2-2.5

78.15

S@N-3D-rGO

0.9-1.2

TiN nanotube/S

Samples

Cycles

(mAh/g-S)

GN: graphene nanosheet; CNT: carbon nanotube; BUCNTs: boron-doped unzipped carbon nanotubes Undoubtedly, the excellent electrochemical performance of Cell/MCNTs/MPP can be attributed to its effective doping of active N, B and S elements, low resistance and high electrochemical active area as well, as shown in Figure 8. The active N, B species doped into the hollow carbon nanotubes could improve LiPSs adsorption by offering more active sites, which can be acted as the adsorption for the Li of LiPSs and finally confine the polysulfide shuttle effect. This is consistent with the reported literature43,44. Besides, N, B, S tri-doping illustrated the strongest chemisorption 29



ability and the best electrochemical properties. Apparently, the above results indicate that the rational heteroatom-doped carbon nanotubes renders the carbon/sulfur composite with following excellent advantages: 1) the active CNTs introduce micro-/mesoporosity as electrochemical reactors to hold sulfur and polysulfide; 2) interconnected porous framework offers accessibility for electrolyte to active material; 3) robust mechanical character of CNTs alleviates active materials’ volume swell/shrink during circulation; 4) lofty electrical conductivity for compounds; 5) rational heteroatom doping provides much more active sites. In addition, the N, B, S tri-doped ACNTs coated on the separator has also a strong ability to physically block polysulfide migration and chemically anchor LiPSs through electronegative heteroatom groups to obtain the high sulfur utilization and cyclic stability even at high sulfur content of 90 wt%. Furthermore, a promising interlayer design offers the potential for practical applications of advanced LSB in the future.

Figure 8. Schematic representation of LSB with electrode configuration for Cell/MCNTs/MPP.

Conclusions 30


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Heteroatom

doping modified

the

nonpolar

surface

of

active

carbon

nanotubes/sulfur composites have been favorably prepared by a hydrothermal means and successive heating melting process. Heteroatom-doped ACNTs/S offers the improved affinity between inherent polar polysulfides and nonpolar carbon surface to restrain the shuttling of polysulfide during electrochemical reactions. Furthermore, the controllable heteroatom doping can effectively enhance electrical conductivity and provide much more active sites. The suitable chemical interactions of electron donating N, B and S atoms with Li+ in LiPSs lead to superb cyclic stability. First-principles calculations confirm that the strong adsorption behavior of LiPSs on the surface of N, B, S tri-doped ACNTs. The N, B, S tri-doped ACNTs/S exhibits a high first specific capacity of 1166 mAh/g-S and capacity retention of 76% after 700 cycles at 0.3 C with the sulfur content of 77.5 wt%. When N, B, S tri-doped ACNTs act as sulfur substrate and functional material coated on the separator, the Cell/MCNTs/MPP delivers a large reversible specific capacity of about 713 mAh/g-S after 1400 cycles at 0.5 C with a negligible capacity decay of 0.014% cycle-1 and a higher sulfur loading of 90 wt%. This result further proves that the rational multiple doping is a promising strategy for the design of novel LSB cathode material, and membrane modification can pave a way towards LSB applications.

Acknowledgments This work is supported financially by the National Natural Science Foundation of China under project No. 51272221, the Hunan Provincial Innovation Foundation for 31



Postgraduate (CX2017B292) and the Key Project of Strategic New Industry of Hunan Province (No. 2016GK4005 and 2016GK4030). The authors thank Dr. Lixiao Miao at National Center for Nanoscience and Technology for his support in guiding the experiment in LSB.

Supporting Information. Cyclic performance of N, B, S tri-doped ACNTs/S without LiNO3 electrolyte; Polysulfide adsorption experiments of ACNTs and N, B, S tri-doped ACNTs.

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TOC graphic:

Synopsis: N, B, S tri-doping in carbon material and separator modification enhance distinctly electrochemical performance of Li-S battery.



Figure 1. (a) Schematic illustration showing the fabrication of heteroatom doping ACNTs/S composite, SEM images of (b, c) ACNTs, (d) EDS spectrum of ACNTs/S, (e, f) TEM images and (g) HRTEM images of ACNTs, (h) TEM images and (i, j) HRTEM images of ACNTs/S.


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Figure 2. XPS survey spectra of (a) the ACNTs and doping ACNTs, (b) N, S dual-doped ACNTs, (c) N, B dual-doped ACNTs, (d) N, B, S tri-doped ACNTs of C 1s, first-principle calculations presenting the adsorption behavior of the LiSH on the surface of the different heteroatom doping (e) undoping, (f) N, B dual-doping and (g) N, B, S tri-doping.



Figure 3. (a, c, e, g) N2 adsorption/desorption isotherm, (b, d, f, h) pore-size distributions for the as-prepared materials.


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Figure 4. (a) XRD diffractrograms, and (b) Raman spectra of the as-prepared carbon material, (c) XRD diffractrograms, and (d) TGA traces of the as-prepared carbon/sulfur materials.



Figure 5. (a) Representative CV profiles of the N, B, S tri-doped ACNTs/S, (b) comparison of cyclic properties and CE at 0.3 C, (c-f) GCD curves at various circulations, (g) the rate capability of as-prepared compound, (h) original GCD profiles at various ratios of N, B, S tri-doped ACNTs/S.


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Figure 6. CVs at various scan ratios of LSB: (a) ACNTs/S, (b) N, S dual-doped ACNTs/S, (c) N, B dual-doped ACNTs/S, (d) N, B, S tri-doped ACNTs/S, and (e) linear matching of peak point currents for the batteries, Nyquist plots after (f) 1st and (g) 300th cycles for as-prepared composites (inset: equivalent circuit models), (h) UV/Vis adsorption spectra of DOL/DME solution with the as-prepared composites at 300th cycle.



Figure 7. (a, b) Optical images, (c) top-down and (d) cross-section SEM images of N, B, S tri-doped ACNTs coated separator, (e) TGA curves, (f) CV, and (g) cycling performances and CE of Cell/MCNTs/MPP.


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Figure 8. Schematic representation of LSB with electrode configuration for Cell/MCNTs/MPP.



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Suppressing the Polysulfide Shuttle Effect by Heteroatom-Doping for

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