The electrocatalytic activity of BaTiO3 nanoparticles towards polysulfides enables high-performance lithium–sulfur batteries

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The electrocatalytic activity of BaTiO3 nanoparticles towards polysulfides enables high-performance lithium-sulfur batteries Hongcheng Gao , Shunlian Ning , Jiasui Zou , Shuang Men , Yuan Zhou , Xiujun Wang , Xiongwu Kang PII: DOI: Reference:

S2095-4956(20)30043-7 https://doi.org/10.1016/j.jechem.2020.01.028 JECHEM 1081

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Journal of Energy Chemistry

Received date: Revised date: Accepted date:

27 December 2019 18 January 2020 18 January 2020

Please cite this article as: Hongcheng Gao , Shunlian Ning , Jiasui Zou , Shuang Men , Yuan Zhou , Xiujun Wang , Xiongwu Kang , The electrocatalytic activity of BaTiO3 nanoparticles towards polysulfides enables high-performance lithium-sulfur batteries, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.028

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The electrocatalytic activity of BaTiO3 nanoparticles towards polysulfides enables high-performance lithium-sulfur batteries Hongcheng Gaoa,1, Shunlian Ninga,1, Jiasui Zoua, Shuang Mena, Yuan Zhoua, Xiujun Wangb, Xiongwu Kanga,* a

New Energy Research Institute, School of Environment and Energy, South China University

of Technology, Higher Education Mega Center, 382 East Waihuan Road, Guangzhou 510006, Guangdong, China b

School of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou 510640, Guangdong, China *Corresponding author. 1

These authors contributed equally to this work.

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

Abstract: The slow redox dynamics and dissolution of polysulfides in lithium-sulfur (Li-S) batteries result in poor rate performance and rapid decay of battery capacity, thus limiting their practical application. Ferroelectric barium titanate (BT) nanoparticles have been reported to effectively improve the electrochemical performance of Li-S batteries due to the inherent self-polarization and high adsorption capacity of the BT nanoparticles towards polysulfides. Here in this paper, BT nanoparticles, behave as highly efficient electrocatalyst and demonstrate much higher redox dynamics towards the conversion reaction of polysulfides and Li2S than TiO2, as shown by both electrochemical measurements and density functional theory calculation. The coupling of the sulfur host of the hollow and graphitic carbon flakes (HGCF) and the BT nanoparticles (HGCF/S-BT) enable excellent electrochemical performance of Li-S batteries, delivering a 0.047% capacity decay per cycle in 1000 cycles at 1 C, 788 mA h g–1 at 2 C and a reversible capacity of 613 mA h g−1 after 300 cycles at a 1

current density of 0.5 C at a S loading of 3.4 mg cm–2. HGCF/S-BT also shows great promise for practical application in flexible devices as demonstrated on the soft-packaged Li-S batteries. Keywords: Electrocatalysis; Redox reaction; Li-S battery; Polysulfide; DFT calculation 1. Introduction Lithium-sulfur (Li-S) batteries have attracted great attention as alternatives for the nextgeneration batteries due to the low cost, high abundance and energy density (2600 Wh kg−1) of sulfur [1–4]. However, the massive commercialization of Li-S batteries is highly hindered by the sluggish reaction kinetics and the dissolution of lithium polysulfides (Li2Sx, 4 ≤ x ≤ 8) into the electrolyte, which results in low capacity, poor rate and cycling performance [5–9]. Various strategies have been developed to overcome these problems and improve the electrochemical performance of Li-S batteries. For example, various transition metal oxides such as TiO2, SiO2, Al2O3 and MnO2 have been incorporated into the carbon matrix of sulfur host to trap the polysulfides via the strong physical/chemical adsorption [1,10–15]. Cui and coworkers utilized superparamagnetic γ-Fe2O3 nanoparticles to minimize the shuttle effect of polysulfides through external magnetic field [16]. Xu et al. constructed double-shelled NiONiCo2O4@C hollow nanocages as an efficient sulfur host to immobilize polysulfides [17]. Meanwhile, various materials have been shown to behave as electrocatalysts and promote the transformation of the polysulfides and Li2S2/Li2S, facilitating the redox reaction kinetics and improving the electrochemical performance [18–21]. For example, Ke and co-workers described a sulfur host material by subtly introducing conductive polypyrrole polymer into the pores of MIL53(Fe) and markedly enhanced the rate performance and specific capacity of LiS batteries by increasing the ion diffusion kinetics [22]. Ferroelectric materials have been demonstrated as an effective strategy in improving the electrochemical performance of energy conversion devices due to their built-in electric field 2

[23–26]. Lately, it has been reported that the electrochemical performance of Li-S batteries can be enhanced by incorporating the ferroelectric BT nanoparticles into the sulfur cathode, ascribing to the high adsorption capacity of BT nanoparticles induced by the polarized surface [27]. However, the roles of BT nanoparticles played in regulating the electrochemical performance of Li-S batteries remain far less explored. For example, the high rate performance of Li-S batteries requires fast reaction kinetics of polysulfide and Li2S on cathode materials, which cannot be fullfilled by the high adsorption capacity of such BT nanoparticles. Besides, the slow reaction kinetics inevitably results in the saturation of polysulfides on the adsorbent at high sulfur loading, preventing further adsorption of the polysulfides and leading to dissolution of the polysulfides, formation of dead sulfur and the decay of the battery capacity (scheme 1) [28]. Herein, barium titanate nanoparticles (BT) were prepared by a sol-gel/hydrothermal method and the electrocatalytic activity towards the electrochemical conversion of polysulfides and Li2S (Scheme 1) were demonstrated by linear sweep voltammetry (LSV), cyclic voltammetry (CV), density functional theory (DFT) calculation and galvanostatic intermittent titration technique (GITT). When coupled with hollow and graphitic carbon flakes (HGCF) sulfur host derived from a metal-organic-framework (MOF) MIL53(Fe), the high redox kinetics of the polysulfides and Li2S on HGCF/S-BT result in a 2 C rate performance, a reversible capacity of 613 mA h g–1 with a high sulfur loading of 3.4 mg cm–2 after 300 cycles at 0.5 C, much higher than that of TiO2 counterpart.

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Scheme 1. Schematic illustration of the HGCF-TiO2 electrode (left) displaying adsorption ability towards polysulfide and HGCF-BT electrode (right) behaving as electrocatalyst on the reaction of polysulfide and Li2S, effectively suppressing the dissolution of polysulfide and promote the uniform deposition of Li2S. 2. Experimental 2.1. Materials All chemicals were analytical grade and used without further purification. Barium acetate (99%), tetrabutyl titanate (98%), ferric chloride hexahydrate (99%), terephthalic (99%) and sulfur (99.99%) were purchased from Energy Chemical Co. Acetate, Nitric acid and dimethylformamide (DMF) were purchased from Damao Chemical Reagents Factory. Titanium dioxide (anatase, 5–10 nm in diamter) was purchased from Aladdin. Deionized (DI) water was supplied by a Barnstead Nano pure water system (18.3 MΩ∙cm). 2.2. Synthesis of BaTiO3 (BT) and MIL53(Fe) BT nanoparticles were prepared by a sol-gel/hydrothermal method [29]. Firstly, barium acetate was dissolved in acetic acid at 110 °C. After the solution was cooled to room temperature, tetrabutyl titanate and deionized water were added with a ratio according to the predetermined

compositions.

A

homogeneous

sol

was

formed

at

40

°C

by

continuously stirring and further added into 2 M KOH solution, which was further poured into a Teflon lined stainless steel-autoclave with a filling volume of 60% and annealed at 160 °C for 24 h. The deposits were filtered, washed three times with distilled water and absolute ethanol to remove the impurities and finally dried in a vacuum oven at 60 °C for 24 h, The obtained white powder was BT nanoparticles. Metal-organic framework MIL53(Fe) was synthesized through a solvothermal process reported previously [30,31]. Typically, ferric chloride hexahydrate (1 mmol) and terephthalic acid (1mmol) were dissolved in DMF (5 mL) solution, stirred at room temperature for 30 min and then transferred into a Teflon lined stainless steel-autoclave, which was kept in an oven at 150 °C for 15 h. After cooling to room temperature naturally, the solution was centrifuged 4

and the collected solid was dispersed into a 200 mL of distilled water and heated at 80 °C for overnight. The product was then collected by centrifugation and dried in vacuum oven at 150 °C for 24 h. 2.3. The preparation of hollow and graphitic carbon flakes (HGCF) and HGCF HGCF/Sulfur The as-prepared MIL53(Fe) was placed inside an alumina crucible and transferred into a tube furnace. The sealed tube furnace was heated to a target temperature of 800 °C with a heating rate of 3 °C/min and maintained for 2 h under continuous Ar flow. Then, the obtained products were cooled to room temperature and washed with 2 M HCl to remove the residual iron components. Subsequently, the sample was washed 3 times with deionized water and ethanol before drying in a vacuum oven at 60 °C for 24 h. Sulfur encapsulation was performed by a melt-diffusion method. The obtained HGCF and sulfur with a weight ratio of 2:8 were placed and sealed in a glass bottle under Ar condition, transferred to the tube furnace and annealed at 155 °C for 12 h (Fig. S1c). The optical image, XRD and SEM image of the as-prepared carbonized MIL53(Fe) and sulfur loaded HGCF are shown in Figs. S1 and S2. 2.4. Material characterization The powder X-ray diffraction was carried out on a Cu Kα radiation source (XRD, Bruker D8 Advanced Diffractometer, German). The morphologies of the products were observed using Field-emission scanning electron microscopy (FE-SEM, Hitachi SU8000, Japan) and transmission electron microscopy (TEM, FEI, Tecnai G2 F30). X-ray photoelectron spectroscopy (XPS, PHI X-tool, Japan) using Al Kα as X-ray source was applied to detect the chemical state of samples. The sulfur content was measured by using a thermogravimetric analysis (TGA, Mettler Toledo TGA/SDTA851, USA) under nitrogen atmosphere with a heating rate of 10 oC∙min-1. The Raman spectra of samples were collected using a LabRAM HR Evolution Laser Raman Spectrometer (HYJ, France) with the excitation wavelength of 325 nm. Brunauer-Emmett-Teller surface area measurements were performed on a Quantachrom Autosorb-1 instrument analyzer at 77 K. UV-Vis spectra were collected by UV5

2600 (Shimadzu, Japan). Fourier transformed infrared spectrum was carried out in a Nicolet 6700 (Thermo Fisher, USA) spectrometer with pure KBr. 2.5. The fabrication of the electrode and electrochemical measurements Sulfur cathode was prepared with a HGCF/S composite, Super P and the binder (PVDF) at a weight ratio of 80:10:10 or HGCF/S:BT:Super P:PVDF (80-x:x:10:10, x = 5, 15, 25) in Nmethyl-2-pyrrolidinone (NMP) to form a uniform slurry. The slurry was coated onto a carboncoated aluminum foil as the current collector and dried at 55 °C for 12 h and then cut into a disc with a sulfur loading about 1.3–1.5 mg cm–2. The sulfur-to-electrolyte ratio is set to 30 g L–1. Then, CR2016-type coin cells were assembled in an argon-filled glove box (VigorLG2400/750TS, China) at an oxygen and water contents less than 1 ppm with various cathodes as the working electrodes, metallic lithium as the reference/counter electrode and Celgard 2400 as a separator. The electrolyte consisted of 1.0 M solution of LiTFSI in DOL/DME (1:1 volume ratio, TFSI = bis(trifluoromethylsulfonyl)imide; DOL = 1,3dioxalane; DME = dimethyl ether) with 0.2 M LiNO3 as electrolyte additive. A soft-package Li-S battery (size: 3.0 cm×3.0 cm, msulfur = 8.1 mg, Fig. S19a) was assembled by using the same cathode materials, electrolyte and anodes with coin cells, and the sulfur loading was 0.9 mg cm–2. The package material was Al-plastic film. The Ni-strip and Al-strip were used to bring out the anode and cathode current collectors from the soft-package battery. The charge/discharge properties were performed on a CT2001A cell test instrument (LAND Electronic Co, Ltd, China) under a cut-off voltage of 1.8–2.7 V at 25 °C. Cyclic voltammetry (CV) tests with a scan rate of 0.1 mV∙s−1 from 1.7 to 2.7 V and the electrochemical impedance spectroscopy (EIS) were investigated in the frequency range of 0.01 Hz–100 kHz on a CHI660C electrochemical workstation (Chenhua instrument Co, China). During the galvanostatic intermittent titration technique (GITT) measurements, the coin cells were first charged or discharged by a constant current pulse of 0.05 C for 10 min and then an incremental charging/discharging steps by a relaxation time of about 5 h under a voltage of 6

1.8–2.7 V to allow the equilibrium potential of lithium storage at different points. All the specific capacity values were calculated according to the mass of sulfur. 2.6. Visualized adsorption measurements 5 mM Li2S6 solution were prepared by mixing sulfur and lithium sulfides (Li2S) with a molar ratio of 5:1 in DME:DOL = 1:1 volume ratio, followed by vigorous magnetic stirring for 12 h at 60 oC. BT and TiO2 particles with the same surface area were added into the same volume Li2S6 solution, respectively. The adsorption ability of BT and TiO2 particles was investigated by UV-Vis spectra measurement after remained stationary for 12 h. 2.7. Symmetrical cell measurements The electrodes were fabricated without sulfur for symmetrical cell measurement. BT, TiO2 and HGCF materials are dispersed into isopropanol by ultrasonication, then deposited onto the carbon paper disks dropwise and dried at 55 °C for 12 h. The two identical electrodes were assembled into a standard CR2016-type coin cells with a celgard-2400 membrane as the separator. The electrolyte consisted of 0.2 M LiNO3, 0.5 M Li2S6 and 1.0 M solution of LiTFSI in DOL/DME. For comparison, electrolyte without Li2S6 was used as a control. CV of symmetric cell was tested at a scan rate of 10 mV∙s−1 from −0.8 to 0.8 V. 2.8. Deposition and kinetics measurements The deposition behavior of Li2S on BT and TiO2 was probed by a CHI 660A electrochemical workstation in coin cells. A solution of Li2S8 in DME/DOL (1:1) (0.2 M) was used as electrolyte, which was prepared by dissolving sulfur and lithium sulfides (Li2S) in a molar ratio of 7:1 in DME/DOL under vigorous stirring at 60 °C for 24 h. BT or TiO2 loaded carbon paper (CP) disks were used as working electrode and a metallic lithium foil as counter electrode. 50 μL Li2S8 solution was first dropped onto the cathode, the separator film was applied and then 30 μL electrolyte without Li2S8 was dropped on the lithium anode side. The fabricated cells were galvanostatically discharged to 2.06 V under a current of 0.112 mA, and

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then kept at 2.05 V potentiostatically for Li2S nucleation and growth. The potentiostatic discharge was ceased when the current was below 10−5 A. The kinetics of electrochemical oxidation of polysulfides on BT, TiO2 and HGCF materials was evaluated by linear sweep voltammetry (LSV) on a three-electrode electrochemical cell. The working electrodes were prepared by sequential cast of host materials (BT, TiO2 and HGCF) and 0.5 wt% Nafion solution on glassy carbon. Lithium and Pt foils were used as a counter and reference electrode respectively [32]. A solution of 10 mM Li2S6, 1.0 M LiTFSI and 0.2 M LiNO3 in DOL/DME was used as polysulfides electrolyte. 2.9. Computational details All calculations were performed using CASTEP package in Material Studio 8.0 based on density functional theory [33]. The exchange-correlation potential is described by the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functions [34]. The structures were built based on materials visualize model. The BT (0 0 1) and TiO2 (1 0 1) surfaces were modeled by a 3-layers slab repeated in 3×3 and 3×4 surface unit cells, containing totally 117 and 144 atoms respectively. The periodic region of BT (x=3.997 Å, y=3.997 Å, z=4.0314 Å) and TiO2 was along X, Y and Z directions with an optimized vacuum space of 15 Å. The geometric optimization and other properties are performed with a 2×2×1 Monkhorst-Pack k-point mesh and the density of states (DOS) calculation was described by the double k-points meshes. The convergence criterion of optimal geometry and maximum force were set at 1.0×10−5 eV/atom and 0.005 eV/Å. The self-consistent field (SCF) tolerance, the maximum displacement and the energy cutoff were set to 5×10–6 eV, 0.002 Å and 400 eV respectively. In BT, the atoms in the two bottom-layers were fixed and those in the upper layers were set free in the calculations. No constraint was applied for the geometry optimization of the polysulfide molecules. The adsorption energies (ELi2Sn) of Li2Sn (n=1, 2, 4, 6 and 8) on BT (001) are calculated by ELi2Sn = E(surf+Li2Sn)-E(surf) – ELi2Sn, where E(surf+ Li2Sn) and E(surf) are the total energies of the surface with and without Li2Sn 8

adsorbates, and E(Li2Sn) is the energies of free Li2Sn molecules. The transition state is determined by a complete LST/QST approach and confirmed by the nudged elastic band (NEB) method. 3. Results and discussion

Fig. 1. (a) The XRD pattern and (b) SEM image of BT nanoparticles. The insets in (a) and (b) are diagram of the crystal structure and size distribution histogram of tetragonal BT nanoparticles respectively. (c) TEM images of HGCF, (d) HGCF/S composite, (e) the prepared cathode and the elemental mapping of Ba, Ti, O, C and S of HGCF/S-BT. BT nanoparticles of tetragonal phase was synthesized by a sol-gel/hydrothermal method, as described in the experiment part. XRD patterns in Fig. 1(a) show well-defined crystalline tetragonal phase of BT nanoparticles (PDF#: 81-2201) [35], as supported by the strong and broad IR band in 500–600 cm–l range in Fig. S3(a) and four peaks at 715, 515, 305 and 255 cm–1 in Raman spectrum in Fig. S3(b), which are ascrbed to the Ti-OI normal stretching vibration of a TiO6 octahedra of BT nanoparticles of tetragonal phase [36] and spontaneous 9

polarization of BT [37,38] respectively. The size of BT nanospheres is determined to be 90– 140 nm from SEM image in Fig. 1(b). The hollow and porous graphitic carbon flakes were prepared by carbonization of metalorganic-framework (MOF) MIL53(Fe) (Figs. S4-6), which were detailed in experiment part. Fig. S4 shows a typical FE-SEM images of HGCF and sulfur-loaded HGCF (HGCF/S), where a flake-like morphology was observed. The hollow structure of the carbon flake was demonstrated by the TEM images in Fig. 1(c), which is fully occupied by sulfur for HGCF/S as shown in Fig.e 1(d). The composite cathode HGCF/S-BT were prepared by coupling HGCF/S and BT nanoparticles with a certain mass ratio. As shown in TEM image and the elemental mapping of HGCF/S-BT in Fig. 1(e), the BT nanoparticles are homogeneously distributed on HGCF/S. Fig. S5(a) shows the XRD patterns of the as-prepared HGCF, pure sulfur and HGCF/S. A sharp diffraction peak at 26o for HGCF is indexed to (002) planes of highly graphitized carbon flake(PDF#: 89-8487) [39], which are favorable for the fast electron transfer [40]. The XRD peaks of orthorhombic sulfur (PDF#: 08-0247) in Fig. S5(a) and Raman peaks below 500 cm–1 [41,42] for HGCF/S in Fig. S5(b) suggest successful introduction of sulfur into HGCF materials. The Raman peaks at ~1590 and ~1400 cm–1 are ascribed to the G and D bands [2,33,43,44] due to the vibration of all pairs of sp2 carbon atoms of graphitic layers and the disorder of sp3 defects, respectively [45,46]. The intensity ratio of D and G bands (ID/IG) bands varies with the structure of carbon and amounts of defects [47,48], and increases from ≈ 0.31 for HGCF to ≈ 0.84 for HGCF/S, indicating the formation of numerous defects and the existence of interaction between sulfur and HGCF compartme [45,49]. The Brunauer-Emmett-Teller (BET) surface area, pore size distribution and total pore volume of HGCF are calculated to be 935.1 m2 g−1, 5–40 nm and 1.24 cm3 g−1 respectively, from the nitrogen adsorption/desorption isotherms in Fig. S6(a). Such extremely high surface area can provide large electrode/electrolyte interfaces for rapid electron and ion transfer while 10

the large pore volume as well as the hollow structure is apt to the high sulfur loading, facilitated electrolyte infiltration and good coordination to the volume variation during delithiation/lithiation process [40]. Upon sulfur loading of 78 wt%, as determined by TGA measurement (Fig. S7), the surface area and total pore volume of HGCF/S are largely reduced to 5.4 m2 g–1 and 0.02 cm3 g–1, as shown in Fig. S6(b), suggesting that the porous structure are fully occupied by sulfur.

HGCF and HGCF/S are further characterized by the XPS

spectroscopy, as shown in Fig. S8. The C 1s signals for HGCF and HGCF/S are quite consistent with each other (Fig. S8b,c), and the three deconvoluted peaks at 284.6, 285.6 and 288.9 eV are attributed to C-C/C=C, C-O/C-S and O-C=C bond, respectively [50–52]. As anticipated, no sulfur was observed for HGCF (Fig. S8a). However, two intense peaks are observed at 163.9 and 165.1 eV [51] for HGCF/S (Fig. S8d), due to the elementary S 2p3/2 and S 2p1/2. The two weak peaks at 168.4 and 168.6 eV are ascribed to S 2p3/2 and S 2p1/2 of SO42– or SO32– [51] due to the oxidation of sulfur during melt-diffusion process. The adsorption configurations and energies of polysulfides (Li2Sx, x =1, 2, 4, 6 and 8) on BT materials are further evaluated by DFT calculations on BT (001) plane. Several adsorption configurations of Li2Sx on BT were analyzed (Fig. S9), and the most stable ones are shown in Fig. 2(a). It is observed that the polysulfides prefer to adsorb on the BT (001) surface perpendicularly by Ti-S and Li-O bonds, indicating that polysulfides can be well confined on BT substrate. The absorption energies of Li2S, Li2S2, Li2S4, Li2S6 and Li2S8 species on BT (001) surfaces at the most stable configurations are calculated to be −7.39, −4.7, −4.9, −3.82 and −4.66 eV, respectively, which are much higher than that on TiO2 surface [53,54]. More insights into the adsorption of polysulfides on the BT (001) surface were analyzed by the electron charge transfer from Li2Sx to BT and the charge density difference, as shown in Fig. 2(b) and Fig. S10, which suggests apparent interfacial charge interaction between Li2Sx and BT and formation of Ti-S and Li-O chemical bonds. These analysis indicates that BT shows

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much stronger interactions with polysulfides and may more efficiently promote the redox reaction of polysulfides than TiO2 nanoparticles. The interaction of the polysulfides and BT are further evaluated by the total density of states (DOS) and partial density of states (PDOS) analysis. As shown in Fig. S11, the adsorption of polysulfides induces apparent change of the total DOS of BT, and the molecular orbitals between Li and O atoms (Fig. S12a) and S and Ti atoms (Fig. S12b) overlap very well with each other, further indicating strong interaction between polysulfides and BT materials [55]. In addition, the dissolution of polysulfide for HGCF/S-TiO2 and HGCF/S-BT electrodes during discharging at 0.2 C is shown in Fig. 2(c). For HGCF/S-TiO2 electrode, the color of the whole electrolyte changed from colorless to bright yellow in 1 h and became darker after 4 h due to the dissolution of polysulfides into the electrolyte. In contrast, it is barely observed even after 4 h discharge for HGCF/S-BT electrode, suggesting effective suppression of polysulfide possibly by the fast redox dynamics of Li2S6 and Li2S on BT nanoparticles. Morever, the UV-vis spectra and the optical image of Li2S6 solution (5 mM) in DOL/DME (1:1, volume ratio) and that after 12 h addition of BT and TiO2 particles of the same total surface area (Fig. S13). The absorption intensities of Li2S6 was much reduced by adsorption of BT and TiO2 particles. Possibly due to the inherent spontaneous polarization and the charged surface, BT particles demonstrate much higher adsorption ability on polysulfide than TiO2 particles and thus may more efficiently suppress the dissolution of polysulfide, improve the utilization of sulfur and enhance the electrochemical performance HGCF/S-BT.

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Fig. 2. (a) The adsorption configurations and energies of Li2Sx (x = 1, 2, 4, 6, 8) on BT (001) surface, (b) electrons transferred from Li2Sx to BT and the charge density differences at the most stable adsorption configurations, (c) digital photos of glass vials containing HGCF/STiO2 (left) and HGCF/S-BT (right) cathodes, electrolytes and anodes during discharging at 0.2 C.

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Fig. 3. (a) The CV curves of symmetric cells at a scan rate of 10 mV∙s–1; (b) LSV at a scan rate of 3 mV/s and Tafel plots (inset) measured in a three-electrode cell for HGCF/S, HGCF/S-TiO2 and HGCF/S-BT electrodes; (c) diffusion energy barriers of Li2S on BT and TiO2; (d) the potentiostatic discharge profiles of Li2S8 solution at 2.05 V on BT for evaluating the kinetics of Li2S deposition. Fig. 3(a) shows the CV tests of symmetrical cells of HGCF, HGCF-TiO2 and HGCF-BT electrodes from −0.8 to 0.8 V in a 0.5 M Li2S6 electrolyte at a scan rate of 10 mV∙s–1. Since the current profiles are mainly from the redox reaction of Li2S6 and Li2S, thus, the apparently higher current density on HGCF-BT electrode than that on HGCF and HGCF-TiO2 counterparts suggests highly promoted redox kinetics of polysulfide and Li2S on HGCF-BT electrode [40,56,57]. The catalytic performance of BT nanoparticles towards the redox reaction of polysulfides was further examined by linear scanning voltammetry (LSV) in a 10 mM Li2S6-based electrolyte in a three-electrode cell [32]. As shown in Fig. 3(b), the lower onset potential, higher peak current densities and lower Tafel slope (49.8 mV dec−1) than TiO2 (142.9 mV dec−1) are observed on BT electrode. All these merits suggests much faster redox kinetics of polysulfides and suppression of the formation of dead sulfur on BT than TiO2, as shown in Scheme 1. It should be noted that too strong adsorption of polysulfides on electrodes might block the active surface sites for further redox reaction and thus retard the reaction 14

kinetics. Thus, the diffusion energy barrier of Li2S on the surfaces of BT (001) and TiO2 (101) is also studied by the DFT calculation. As shown in Fig. 3(c), the diffusion energy barrier of Li2S on BT surface is much lower than that on TiO2 surface, indicating that Li2S has superior surface diffusion dynamics and can be more homogeneously dispersed on BT than TiO2 [3]. The excellent redox kinetics of polysulfide and Li2S is further demonstrated by the nucleation and growth of Li2S experiments on the surface of HGCF/S-BT and HGCF/S-TiO2 electrodes (Figs. 3(d) and S14). Obviously, the current from Li2S deposition on HGCF/S-BT electrode is much higher than that on HGCF/S-TiO2 electrode and the capacity of Li2S precipitation on BT electrode (192.7 mA h gs−1) is about 1.6 times that of TiO2 electrode (116.1 mA h gs−1), suggesting much reduced overpotential and excellent kinetics toward electrochemical conversion of polysulfides to Li2S on BT electrode than TiO2 [58–60]. Such superior catalytic performance and surface diffusion dynamics of polysulfides on BT might account for the much enhanced electrochemical performance of HGCF/S-BT with respect to HGCF/S-TiO2. The electrochemical performance of HGCF/S-BT composite was optimized by tuning the loading of BT nanoparticles, with the mass ratio of HGCF/S:BT:super P:PVDF is (80x):x:10:10 and the corresponding samples are denoted as HGCF/S-xBT (x = 5, 15 and 25). Among these samples, HGCF/S-15BT demonstrates the best electrochemical performance, (Fig. S15). Then the electrochemical performance of HGCF/S-BT, HGCF/S-TiO2 and HGCF/S are characterized and shown in Fig. 4. Fig. 4(a) shows the CV scans at a scan rate of 0.1 mV∙s−1. The two cathodic peaks at 2.23 and 1.97 V for HGCF/S are attributed to the reduction of sulfur to long-chain polysulfides (Li2Sx, 4 < x < 8) and further to short-chain polysulfides (Li2Sn, 1 ≤ n < 4) [61,62]. In the reverse process, the two anodic peaks at 2.42 and 2.49 V are associated with the reversible transformation of the short-chain polysulfides to the long-chain polysulfides and further to sulfur respectively [63,64]. Compared to HGCF/S and HGCF/S-TiO2, the apparently reduced potential differences between the anodic and cathodic peaks, the much better resolved and narrowed anodic peaks and the enhanced current 15

intensity for HGCF/S-BT electrode [41,65–67] indicate markedly reduced polarization and enhanced redox kineticson HGCF/S-BT electrode [68,69]. The cycling performance of HGCF/S-BT cathodes with sulfur loading of 1.3–1.5 mg cm– 2

at a scan rate of 0.2 C (1 C = 1675 mA h g−1) is shown in Fig. 4(b) and an initial discharge

capacity at a current rate of 0.05 C is determined to be 1110, 1322 and 1443 mA h g−1 for HGCF/S, HGCF/S-TiO2 and HGCF/S-BT respectively. After pre-activation, HGCF/S, HGCF/S-TiO2 and HGCF/S-BT deliver a capacity of 832, 913 and 941 mA h g−1 at 0.2 C, which retain 642, 726 and 858 mA h g−1 after 120 cycles, corresponding to a 77.2%, 79.5% and 91.2% capacity retention relative to that of Li-S batteries after pre-activation. The rate performance from 0.1 to 2 C rates are shown in Fig. 4 (c). The HGCF/S-BT cell delivers a discharge capacity of 732 mA h g−1 at 2 C, while HGCF/S and HGCF/S-TiO2 only shows a reversible capacity of 184 and 230 mA h g−1 at 2 C, respectively, much worse than that of HGCF/S-BT. The discharge/charge voltage profiles of these composite electrodes derived from Fig. 4(c) are presented in Fig. S16. Obviously, the two discharge plateaus are observed at 2.3 and 2.0 V for HGCF/S-BT electrode at 2 C, while no discharge plateau was observed for HGCF/S and HGCF/S-TiO2, suggesting much better rate performance for the former electrode. The polarization of HGCF/S, HGCF/S-TiO2 and HGCF/S-BT are analyzed through the voltage gap between the discharge/charge voltage profiles at 0.1C, which is determined to be 209, 180 and 138 mV respectively, as shown in Fig. 4(d). All these data suggest much enhanced redox dynamics of polysulfide on HGCF/S-BT electrode [70,71].

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Fig. 4. (a) The CV profiles at a scan rate of 0.1 mV∙s−1, (b) cycling performance at 0.2 C, (c) rate performance from 0.1 C to 2 C, (d) charge-discharge voltage profiles at 0.1 C and (e) cycling performance at 1 C of HGCF/S, HGCF/S-TiO2 and HGCF/S-BT electrodes. Fig. 4(e) shows the cycling performance of HGCF/S, HGCF/S-TiO2 and HGCF/S-BT cathodes at 1 C. HGCF/S-BT delivers an initial capacity of 896 mA h g−1 after the first five cycles at 0.05 C, which retains 466.1 mA h g−1 after 1000 cycles at 1 C, corresponding to a capacity retention of 52% and an average capacity decay of 0.047% per cycle. In contrast, the capacity of HGCF/S and HGCF/S-TiO2 only retains 151 mA h g−1 after 200 cycles and 210 mA h g−1 after 1000 cycles at 1 C. To rule out the contribution of BT nanoparticles to the capacity of HGCF/S-BT, the CV tests of HGCF-BT cathode without S loading in the potential window of 1.7–2.7 V were performed and shown in Fig. S17(b). It can be seen that ferroelectric BT materials do not react with lithium and provide no capacity. However, TiO2 17

exists weak react with lithium at 1.9 V. Such superior cycling stability of HGCF/S-BT cathode might be attributed to the high redox kinetics of HGCF-BT nanocomposite. It warrants attention that the electrochemical performance of HGCF/S-BT cathode is significantly superior to that of the oxide cathodes reported in the literature, as shown in Table S2.

Fig. 5. The cycling performance of HGCF/S-BT electrode at sulfur loading of (a) 3.4 and 4.5 mg cm–2 at 0.2 C, (b) at a sulfur loading of 3.4 mg cm–2 at 0.5 C, (c) discharge/charge voltages curves and photographs of LED lit (inset) for the first cycle of a soft package, (d) cycling performance of soft-packaged Li-S batteries at 0.05 C bent to various angles. HGCF/S-BT electrode with S loading of 3.4 and 4.5 mg cm–2 delivers an initial capacity of 1275 and 1075.1 mA h g−1 and an areal capacity of 4.2 and 4.9 mA h cm−2 at 0.05 C, respectively, which retains 739.8 and 641.5 mA h g−1 after 100 cycles at 0.2 C, corresponding to 86.3% and 83.3% of retention (Fig. 5a). The two typical charge/discharge plateaus are still well observed after such cycling test, indicating superior electrochemical kinetics on HGCF/S-BT (Fig. S18b). Importantly, HGCF/S-BT with S of 3.4 mg cm–2 still delivers a reversible capacity of 613 mA h g−1 after 300 cycles at a current density of 0.5 C, indicating

18

superior long cycling stability of HGCF/S-BT at high S loading and at high current rate (Fig. 5b). As shown in Fig. 5 (c), the soft-packaged Li-S battery with HGCF/S-BT cathodes (size: 3.0×3.0 cm2) delivers an initial capacity of 1216.7 mA h g−1 at 0.05 C and light 21 lightemitting diodes (LED). The two plateaus observed in the charge-discharge voltage profiles at 2.3 and 2.1 V suggest good redox kinetics of polysulfide. When bended by 90°, 180°, and back to 0°, no obvious brightness change (Fig. S19b) or capacity decrease was observed in the first 80 cycles (Fig. 5d), indicating excellent promise for practical application of HGCF/S-BT in flexible devices. The redox kinetics of polysulfide on HGCF/S, HGCF/S-TiO2 and HGCF/S-BT were further evaluated by galvanostatic intermittent titration technique (GITT) and shown in Fig. S20, from which the averaged reaction resistances for lithiation/delithiation process were obtained by dividing the overpotential with the constant current pulses, as presented in Fig. S20(d-e). Apparently, a lower reaction resistance was obtained for HGCF/S-BT than HGCF/S and HGCF/S-TiO2 during the discharge process, suggesting that the facilitated diffusion kinetics of lithium ions and the high redox dynamics of polysulfide and Li 2S on HGCF/S-BT electrodes. The EIS spectra of the as-prepared HGCF/S, HGCF/S-TiO2 and HGCF/S-BT cells and those before and after 5 and 100 charge/discharge cycles were also measured and shown in Fig. S21. By fitting to the Randles equivalent circuit in Fig. S21(c) the bulk electrolyte resistance (R0) and charge transfer resistance (Rct) at the electrode/electrolyte interface (Table S1). HGCF/S-BT showed the smallest and almost unchanged Rct of the three electrodes in the cycle processes, implying superior electronic and charge transfer at the electrode/electrolyte interface [72]. 3. Conclusions In summary, BT nanoparticles were prepared by a sol gel/hydrgothermal method and the catalytic performance of BT nanoparticles towards electrochemical conersion of polysulfides 19

in a cathode of Li-S batteries were explored. It is demonstrated that BT materials can highly facilitate the redox reaction of polysulfide by electrochemical measurements. DFT calculation and electrochemical measurements also demonstrate much lower surface diffusion energy barrier and superior surface diffusion dynamics of Li2S on BT than TiO2. Attributed to the highly electrocatalytic properties of BT materials, HGCF/S-BT composite cathode demonstrate a low capacity fading rate of 0.047% average capacity decay per cycle at 1C rate, and rate capability of 788 mA h g–1 at 2 C, and a stable areal capacity of 2.5 and 2.9 mA h cm−2 at sulfur loading of 3.4 and 4.5 mg cm–2. The HGCF/S-BT composite also demonstrate a very good potential for practical application as flexible devices. Supporting Information Samples characterizations and some electrochemical performances, Figs. S1-21, Tables S1 and S2. Acknowledgments This work was supported by the Fundamental Research Funds for Central Universities (SCUT No. 2019ZD22) and Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569) Conflict of Interest The authors declare no conflict of interest. References [1] X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Adv. Mater. 29 (2017) 1601759. [2] Z. Liu, L. Zhou, Q. Ge, R. Chen, M. Ni, W. Utetiwabo, X. Zhang, W. Yang, ACS Appl. Mater. Interfaces 10 (2018) 19311–19317. [3] J. Zhou, X. Liu, L. Zhu, J. Zhou, Y. Guan, L. Chen, S. Niu, J. Cai, D. Sun, Y. Zhu, J. Du, G. Wang, Y. Qian, Joule 2 (2018) 2681-2693. [4] F. Wu, S. Chen, V. Srot, Y. Huang, S.K. Sinha, P.A. van Aken, J. Maier, Y. Yu, Adv. Mater. 30 (2018) 1706643. [5] Y. Liu, X. Qin, S. Zhang, G. Liang, F. Kang, G. Chen, B. Li, ACS Appl. Mater. Interfaces 10 (2018) 26264–26273. [6] F. Pei, L. Lin, D. Ou, Z. Zheng, S. Mo, X. Fang, N. Zheng, Nat. Commun. 8 (2017) 482. [7] H. Pan, K.S. Han, M.H. Engelhard, R. Cao, J. Chen, J.-G. Zhang, K.T. Mueller, Y. Shao, J. Liu, Adv. Funct. Mater. 28 (2018) 1707234. 20

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TOC

The BT nanoparticles can be effectively suppress polysulfides shuttle, facilitate redox dynamics, and ensure uniform deposition of Li2S due to the inherent self-polarization and high adsorption capacity.

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