BN-C composite interlayer with enhanced performance for LiS batteries

Journal Pre-proof A sandwich-structured TiN/BN-C composite interlayer with enhanced performance for Li-S batteries

Jinghui Zhu, Caiming Jiao, Tuo Kang, Liubiao Zhong, Sanfei Zhao, Yejun Qiu PII:

S1572-6657(20)30146-6

DOI:

https://doi.org/10.1016/j.jelechem.2020.113963

Reference:

JEAC 113963

To appear in:

Journal of Electroanalytical Chemistry

Received date:

31 October 2019

Revised date:

13 February 2020

Accepted date:

15 February 2020

Please cite this article as: J. Zhu, C. Jiao, T. Kang, et al., A sandwich-structured TiN/ BN-C composite interlayer with enhanced performance for Li-S batteries, Journal of Electroanalytical Chemistry(2020), https://doi.org/10.1016/j.jelechem.2020.113963

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© 2020 Published by Elsevier.

Journal Pre-proof

A sandwich-structured TiN/BN-C composite interlayer with enhanced performance for Li-S batteries Jinghui Zhu1, Caiming Jiao1, Tuo Kang1, Liubiao Zhong1,2, Sanfei Zhao3, Yejun Qiu1,* 1

Shenzhen Engineering Lab of Flexible Transparent Conductive Films, School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen, 518055, China.

2

State Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources Co. Ltd., Zunyi, Guizhou 563003, China. 3

China Ship Development and Design Center, Wuhan, 430064, China.

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Abstract: Aiming to commercialize Li-S battery, it is very important to achieve high

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sulfur loading alongside superior electrochemical performance. In this work, a novel composite material with sandwich structure constructed by titanium nitride

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nanoparticles and boron nitride-carbon nanofibers (BNC-TiN-BNC), has been

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successfully prepared through electrospinning and spraying method, and applied as the interlayer in Li-S batteries. Compared with the BNCNF interlayer, the

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sandwich-like BNC-TiN-BNC interlayer shows more excellent chemical adsorption

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for soluble lithium polysulfides (LiPSs) via the polar effects of the Ti-S and S-N bonds, and furthermore, the uniform distribution of TiN nanoparticles, filling in the

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pores between the nanofibers as packing layer of the BNC-TiN-BNC interlayer, can retard the migration of LiPSs more effectively by physical barriers. By introducing the BNC-TiN-BNC interlayer, the electron transfer, capacity and cycling life of Li-S batteries have been improved significantly, especially at high current density. With the sulfur loading of 2.5 mg cm-2, the cell with BNC-TiN-BNC interlayer delivers a high initial discharge capacity of 642.3 mA h g-1 at high current density of 2.0 C, and the capacity remains 594 mA h g-1 with only the decay rate of 0.025% over 300 cycles.

Keywords: Sandwich structure; Electrospinning; High sulfur loading; Titanium nitride; Boron nitride-carbon nanofibers 1. Introduction

*

Corresponding author. Tel.: +86-755-26032462; fax: +86-755-26033504. E-mail addresses: [email protected]

Journal Pre-proof Rechargeable lithium-ion batteries (LIBs) have been proven as a great successful energy storage system over the past few decades, but with the urgent demand of high energy density and long cycling lifespan, the LIBs are getting closer to the upper limit of their theoretical energy density[1, 2]. Lithium sulfur (Li-S) batteries have attracted extensive attentions owing to their prominent theoretical energy density (2600 W h kg-1) and specific capacity (1675 mA h g-1). Furthermore, some advantages of sulfur, such as low cost, environment amity and abundant reserves in the earth, make the Li-S batteries more potential candidates than LIBs for the next generation of energy

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storage system[3, 4]. However, some inherent defects of Li-S batteries still hamper

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their practical applications. Firstly, both of the sulfur and lithium sulfide (Li2S) as its discharge products are insulative, which leads to the low utilization of sulfur.

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Secondly, the repeated “solid to liquid to solid” phase transition and the large volume

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change of over 80% in the cathode would both have a great influence on the cathode

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structure stability. Moreover, the reaction between S and Li would generate a series of intermediate lithium polysulfides (LiPSs), which are soluble in the ether-group

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containing electrolyte solution to trigger the “shuttle effect” and further lead to the severe capacity loss and a short cycling lifespan[5-7]. To solve these shortcomings,

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many efforts have been carried out to restrict and reuse soluble LiPSs by improving the materials and structure of S cathode, for instance, employing the carbonaceous materials[8, 9], metal-organic frameworks (MOFs)[10, 11], metal oxides[12, 13] and metal sulfides[14, 15] to form composites with sulfur which would further trap the LiPSs via physical limitation or/and chemical absorption. The electrochemical performance for Li-S batteries has been well enhanced by using the above materials, nevertheless the complicated preparation method and the decrease of practical energy density would make the Li-S batteries difficult to industrialize and commercialize. Recently, introducing interlayers between the separator and cathode or improving separator with functionalized materials has been widely exploited to prolong the cyclability of Li-S batteries[16]. The interlayer is not only as barrier to trap the LiPSs by physical or chemical effect, but also as upper current collector to reutilize the sulfur species[17-19]. Up to now, many carbonaceous materials (graphene[20, 21],

Journal Pre-proof carbon nanotubes[22, 23] and carbon nanofibers[24]) have been used to functionalize the separator in Li-S batteries. Because most of these carbon hosts are non-polar and only have weak physical interactions with LiPSs, the LiPSs may eventually diffuse into the anode with result of a relatively short cycling life[25]. Therefore, some polar compounds (metal oxides[26, 27], MOFs[28, 29], and metal sulfides[30, 31]) which could bond with LiPSs were introduced subsequently. However, most of these materials are so insulative as to retard the electron transportation, thus decreasing the utilization of active sulfur and rate capability. As a result, to develop a highly

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conductive polar material as interlayer becomes a growing trend.

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Since it is first used as cathode in Li-S batteries by Manthiram’s group[32], titanium nitride (TiN) with advantages of excellent electrical conductivity (4.0 × 106 S

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m-1) and high chemical stability has drawn numerous attentions. Owing to the strong

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chemical adsorption of TiN to LiPSs with the formation of Ti-S and S-N bonds during

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the electrochemical process, a series of TiN composites, such as mesoporous-TiN microspheres[33], TiN-hollow nanospheres[34], hierarchically porous TiN[35],

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TiN@CNT[36], TiN/CNF[37] and TiN composite with graphene[38] or rGO[39], have been applied in Li-S batteries and gained remarkable achievements. Even so,

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there still remains challenge to obtain excellent electrochemical performance and cycling life in the condition of high sulfur loading and high current density for Li-S batteries systems.

Herein, based on our previous work[40], a novel sandwich-like membrane of BNC-TiN-BNC, where the top and bottom substrates consisting of BNCNF membrane and the commercial TiN nanoparticles is used as packing layers, is prepared by an environment-friendly and simple electrospinning and spraying method. Compared with only BNCNF membrane, on the one hand, the packing layer with TiN nanoparticles can fill the voids between the nanofibers and further retard the migration of LiPSs by physical barriers. And on the other hand, the high electronic conductivity and polarity of TiN nanoparticles can facilitate the utilization of sulfur and the chemical absorption to LiPSs. Meanwhile, the Li-S batteries with sandwich-like

BNC-TiN-BNC

composite

membrane

demonstrate

excellent

Journal Pre-proof electrochemical performance and rate capability with high sulfur loading. With the sulfur loading of 2.5 mg cm-2, the cell with BNC-TiN-BNC interlayer delivers a high initial discharge capacity of 642.3 mA h g-1 at high current density of 2.0 C, and the capacity remains 594 mA h g-1 with only the decay rate of 0.025% over 300 cycles. 2. Experimental 2.1 Materials The used materials include commercial titanium nitride nanoparticles (TiN) (~20

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nm particle size, 99.9% metals basis, Macklin), polyacrylonitrile (PAN) (average

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Mw=150,000, Aldrich), boron oxide (B2O3, AR, mesh≤200, Aladdin), N, N-dimethylformamide (DMF) (anhydrous, 99.8%, Alfa), sublimed sulfur powder

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(99.9%, Aladdin) and commercial porous carbon (CMK-3) (Nan Jing XFNANO

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Corporation Limited). All these materials are directly applied without further

2.2 Materials synthesis

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treatment.

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The membrane of BNC-TiN-BNC was prepared by a combination of electrospinning and spraying method. In a typical synthesis, 1.2 g PAN and 0.3 g

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B2O3 were dispersed in 20 mL DMF followed by continuously stirring to form the transparent solution A, and meanwhile, 0.35 g TiN and 0.35 g citric acid were dispersed in 10 mL ethyl alcohol to form the transparent solution B. And then, 5 mL solution A was added in a 10 mL needle tube for electrospinning to obtain the BNCNF precursor nanofibers membrane, during which the applied voltage, needle to collector distance and feeding rate were 12 kV, 18 cm and 0.4 mL h-1, respectively. After this, 10 mL solution B was sprayed uniformly on the surface of the obtained BNCNF precursor nanofibers membrane to form the packing layers. Subsequently, 5 mL solution A was electro-spun on the above dense layer, thus the BNC-TiN-BNC precursor membrane like “sandwich” structure was prepared. Thereafter, the composite NFs membrane was stabilized in a tube furnace at 280 °C for 2 h in air, followed by the further nitriding in ammonia (NH3) ambiance at 1000 °C for 2 h (heating rate, 5° min-1) to obtain the BNC-TiN-BNC nanocomposite membrane. The

Journal Pre-proof areal weight of the BNC-TiN-BNC membrane as interlayer in Li-S batteries is about 1.3 mg cm-2 and an average thickness of 28 µm. Similarly, the BNCNF membrane was prepared according to our previous report[40] with the same experimental conditions. Additionally, the CMK-3-S cathode composites were fabricated with the sulfur ratio of 60 wt% by the melt-sulfur diffusion method, and subsequently, the obtained product was further heated at 155 °C with the protection of Argon atmosphere to get

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the CMK-3-S composite powder. 2.3 Material characterization

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The structure of the materials was measured by X-ray diffraction (XRD)

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(Model-Rigaku-D-Max-2500). The morphology and elemental analysis of the materials were characterized by transmission electron microscopy (TEM) (Model:

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JEM-2010) equipped with energy dispersive X-ray (EDX) and scanning electron

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microscope (SEM) (Model: HITACHI-S-4700) equipped with energy dispersive spectrometer (EDS). The BET method was employed to analyse the specific surface

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area of samples. The Raman (Renishaw RM-1000) and FT-IR (Nicolet 380) were also applied to confirm the structure of the materials, respectively. The test of electrolyte

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infiltration was measured with a contact angle meter by the sessile drop method (Cam-plus Micro, Tantec Inc., USA). X-ray photoelectron spectroscopy (XPS) (Model: XPS, Thermo K-Alpha) was used to knowledge the bonding energy. The absorption of LiPSs was performed by UV-vis (UV-3600). The sulfur content of CMK-3-S was tested by the thermogravimetric analysis (TGA) in nitrogen flow. 2.4 Electrochemical characterization The cathode film of CMK-3-S was fabricated by mixing homogeneously the CMK-3-S powder, acetylene black (AB) and polyvinylidene fluoride (PVDF) at the weight ratio of 8:1:1 in an N-methylpyrrolidone (NMP) solution, followed by stirring overnight to form the slurry. And then, the slurry was evenly pasted onto Al foil and dried at 60 °C for 12 h in a vacuum furnace. The areal sulfur loading on the cathode was around 2.5 mg cm-2. The anode was a Li metal wafer and the electrolyte consisted

Journal Pre-proof of 1 M LiTFSI salt in a solvent mixture of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1 v/v) with 2% LiNO3 additive. And then, all Li-S batteries were assembled into 2032 type coin cells with 0.2 mL electrolyte solution in a glove box. The charge-discharge tests were conducted with a Neware battery test system in the 1.7-2.8 V (vs. Li/Li+) range and the capacities were calculated based on the mass of sulfur. Cyclic voltammogram (CV) plots and Electrochemical impedance spectra (EIS) were measured with a CHI-760E electrochemical workstation. And

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Electrochemical impedance spectra (EIS) were performed over the frequency range of

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10-6 to 102 KHz, the CV plots were performed in the 1.5-3.0 V (vs. Li/Li+) range. In order to demonstrate the strong absorption of BNC-TiN-BNC to LiPSs, a 5

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mM Li2S6 solution was prepared by mixing the stoichiometric amounts of lithium

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sulfide and sulfur powders in a solvent mixture of DME and DOL (1:1 v/v), followed

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by stirring overnight at 60 °C in the Argon-filled glovebox. And then, the BNC-TiN-BNC membrane was ground uniformly. Subsequently, 10 mg of

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BNC-TiN-BNC powder was dispersed into 4 mL of Li2S6 solution with a transparent glass vial. Similarly, blank Li2S6 and BNCNF powder as the contrast samples were

glovebox.

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also prepared via the above method. All samples were performed in the Argon-filled

3. Results and discussion

The characterizations of morphologies and structures for the BNC-TiN-BNC membrane with sandwich structure are shown in Fig.1. The BNC-TiN-BNC membrane as shown in Fig. 1b, consists with two layers of BNCNF membrane as substrates and an intermediate layer of TiN nanoparticles as physical fillers. The whole thickness of BNC-TiN-BNC membrane is around 28 µm, where the thicknesses of the BNCNF monolayer substrate and the filling layer with TiN nanoparticles are around 7 and 14 µm, respectively. And by observation of the top surface morphology of BNC-TiN-BNC membrane as shown in Fig. 1a, the substrate membrane also maintains a fibrous structure with the average diameter of around 200 nm. Compared

Journal Pre-proof with the monolayer BNCNF membrane where voids exist among nanofibers in our previous report[40], the BNC-TiN-BNC membrane exhibits a like cross-linked structure among nanofibers, which is based on the “sandwich” structure with a filled layer of TiN nanoparticles and the conglutination effect of citric acid. By the observation of the filled layer as shown in Fig. S1a, the commercial TiN nanoparticles are proven to be uniformly distributed throughout the surface of BNCNF membrane skeleton and mainly fill the voids among the nanofibers to form a physical barrier, which is beneficial to retard the migration of LiPSs. In addition, the BNC-TiN-BNC

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membrane also demonstrates excellent flexibility and good mechanical strength as

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shown in Fig. S1b, which not only contributes to remit immediately the volume expansion of the sulfur cathode but also benefits to make the membrane

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commercializable.

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The crystal structures of BNC-TiN-BNC and BNCNF membrane are

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characterized by XRD, and the obtained results are displayed in Fig. 1c. Compared with the XRD pattern of BNCNF membrane, besides the characteristic peak at 25.5°

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which comes from the amorphous BNCNF substrate, the BNC-TiN-BNC membrane exhibits five characteristic peaks at 36.8°, 42.6°, 61.9°, 74.2° and 77.9°, which are

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perfectly assigned to the (111), (200), (220), (311) and (222) reflections of crystalline TiN (JCPDS No. 38-1420), respectively. To further clarify the morphology and crystal nature of BNC-TiN-BNC composite, their TEM and HRTEM images are analyzed as shown in Fig. S2. Fig. S2a shows that the nanoparticles are distributed densely throughout the nanofibers and the average particle size is about 20 nm, which is consistent with the particle size of pure TiN nanoparticles. And meanwhile, the HRTEM (as shown in Fig. S2b), coming from the amplification of the yellow zone in the Fig. S2a, shows an interlayer spacing of 0.21 nm which corresponds well to the (200) plane of TiN[37, 38]. Hence, the above results indicate that, even during the calcination process, the commercial TiN nanoparticles in the BNC-TiN-BNC membrane still maintain their original potentials, which are conducive to adsorb the LiPSs with the polar effect of the Ti-S and S-N bonds. Moreover, the structure of BNC-TiN-BNC composite is well characterized by

Journal Pre-proof FTIR as shown in Fig. 1d. Except two typical absorption peaks for BNCNF substrates at 805 cm-1 and 1391 cm-1[41-43], there exist no absorption peaks. Raman spectroscopies are applied to study the structure of BNC-TiN-BNC and BNCNF composites (Fig. 1e). Three carbon characteristic peaks at about 1344 cm-1, 1584 cm-1 and 2705 cm-1 are named for the D, G and 2D bands, respectively. The D-band corresponds to the defects and disordered degree of carbon matrix, and the G-band arises from the E2g phonon of sp2 C atoms[44, 45]. The ratio of IG/ID of BNC-TiN-BNC composite is about 1.04, which is lower than that of BNCNF

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composite (1.24). This value indicates that the packing layer consisting of TiN

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nanoparticles may help promote the increment of aromatic structures in the carbon matrix so as to further accelerate the electron and ion transportation within the

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BNC-TiN-BNC composite[45]. In addition, the N2 adsorption-desorption isotherms

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are also studied as shown in Fig. 1f. The BET specific surface area of BNC-TiN-BNC composite is only 114.3 m2 g-1, which is lower than that of BNCNF composite (224.1

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m2 g-1). And the pore size distribution as shown in Fig. S3 for the above two samples

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also demonstrate that, the BNCNF composites constructs with the micro-meso pores, but the BNC-TiN-BNC composites is mainly made up of mesopores. The above

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results represent the TiN nanoparticles have filled the voids among the nanofibers, consistent with the observation of the SEM images as shown in Fig. S1a. To evaluate the effect of BNC-TiN-BNC membrane with sandwich-like structure on the improvement of electrochemical performance for Li-S batteries, the electrochemical measurements are carried out. Taking the practical application in the future into consideration, we design all batteries with the areal sulfur loading of about 2.5 mg cm-2 and the TGA result of the CMK-3-S cathode is shown in Fig. S4. The rate performances of the Li-S batteries with BNC-TiN-BNC composite interlayer, BNCNF interlayer and without interlayer (bare CMK-3-S) are measured and the results are shown in Fig. 2a. The initial discharge capacity of the cell with BNC-TiN-BNC membrane is 1189.1 mA h g-1 at 0.2 C, and with the increment of the current densities from 0.5 to 2.0 C, the final reversible discharge capacities are stabilized at 1076.6 (at 0.2 C), 885.7 (at 0.5 C), 715.1 (at 1.0 C) and 589.0 (at 2.0 C) mA h g-1, respectively,

Journal Pre-proof which are higher than those of the cells with BNCNF interlayer and bare CMK-3-S, especially at the high current density of 2.0 C. Moreover, when the rate is set back to 0.2 C, a high capacity of 990.8 mA h g-1 can be recovered, which is equal to 83.32% of the initial discharge capacity. These above results indicate that the Li-S battery with BNC-TiN-BNC composite possesses excellent electrochemical stability and reversible discharge capacities. To further analyze the reason, the galvanostatic charge/discharge tests are performed with the current density from 0.2 to 2.0 C and the obtained results are

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shown in Fig. S5a-c. For all batteries, there exist two distinct discharge plateaus.

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However, the plateaus of the battery with BNC-TiN-BNC composite interlayer are mainly longer and smoother than those of the batteries with BNCNF interlayer and

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bare CMK-3-S at the same current density. More importantly, compared with the

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other two samples, the battery with the BNC-TiN-BNC composite membrane presents

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the lowest voltage hysteresis (ΔE), indicating that the BNC-TiN-BNC membrane with sandwich structure is beneficial to suppress immensely the migration of LiPSs to thus

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decrease the polarization effects and accelerate the electrochemical reaction kinetics for Li-S battery. The CV test of the Li-S battery with the BNC-TiN-BNC composite

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interlayer as shown in Fig. S5d is also performed at a scan rate of 0.1 mV s-1. There exist three reduction peaks at 2.24, 1.93 and 1.67 V in the first cathodic scan, which correspond to the following: (Ⅰ) the breakage of the S8 ring to long chain Li2Sx (4
Journal Pre-proof analyze the compatibility between the membrane and electrolyte. From the Fig. S6c, the contact angle between the BNC-TiN-BNC membrane and electrolyte is 23.6°, which is less than that of BNCNF membrane (29.0°) and pure PP separator (34.1°). The result demonstrates that, owing to the polar effect of TiN nanoparticles and BNCNF membrane with large specific surface areas as substrates, the obtained BNC-TiN-BNC membrane possesses better compatibility with the polar electrolyte and has ability to absorb much more electrolyte solutions, which contribute to accelerate the Li ion and electron transfer, thus leading to the improvement of

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electrochemical performance.

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Attributed to the above advantages of BNC-TiN-BNC composite membrane, the long-term cycling performances of the cells with different interlayers have been

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compared, and the results are delivered in Fig. 2b-d. From the Fig. 2b, the cell with

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BNC-TiN-BNC composite interlayer exhibits more higher initial discharge capacity of 1015.6 mA h g-1 than the cell with BNCNF interlayer (1003.5 mA h g-1) and the

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bare CMK-3-S (714.7 mA h g-1) at current density of 0.5 C, and meanwhile, even after 250 cycles, the cell with BNC-TiN-BNC composite membrane still maintains

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627.4 mA h g-1 with the capacity decay rate of only 0.153% per cycle. In contrast, the

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discharge capacities of the cell with BNCNF interlayer and bare CMK-3-S drop fast to 673.1 and 356.1 mA h g-1 over 100 cycles with the capacity decay rate of 0.329% and 0.502% per cycle, respectively. Furthermore, with the increment of the current density to 1.0 C, the cell with BNC-TiN-BNC composite interlayer exhibits outstanding initial discharge capacity of 823.8 mA h g-1 and remains 665.8 mA h g-1 over 250 cycles with the capacity decay rate of 0.077% (Fig. 2c), which is much lower than the cell with BNCNF interlayer (0.162%) over 155 cycles and bare CMK-3-S (0.182%) over 136 cycles, respectively. Most importantly, even at high current density of 2.0 C, the cell with BNC-TiN-BNC composite membrane still presents prominent initial discharge capacity of 642.3 mA h g-1 and excellent cycling stability with the capacity retention of 92.5 % over 300 cycles (Fig. 2d). Compared with other reported modified separators and functionalized interlayer (Table S1), the BNC-TiN-BNC membrane demonstrates that it can effectively retard the migration of

Journal Pre-proof LiPSs with its special sandwich structure and facilitate the reutilization of active sulfur as upper current collector, and further to achieve relatively high electrochemical performance. Meanwhile, the electrical resistivity of BNC-TiN-BNC and BNCNF membrane are tested with four-probe tester respectively. The electrical resistivity of BNC-TiN-BNC composite membrane is 17.6 Ω∙cm, which is slightly larger than that of BNCNF membrane (6.7 Ω∙cm) and is still small. The reason maybe that, the TiN nanoparticles possess high electrical conductivity, but the contact between the TiN

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nanoparticles and the BNCNF membrane is in a point-to-point fashion to make the

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electron conduction scattered. Hence, the electrical conductivity of BNC-TiN-BNC composite membrane is a bit smaller than BNCNF membrane.

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Furthermore, to confirm the effect of BNC-TiN-BNC composite membrane, the

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electrochemical impedance spectra (EIS) is carried out for the first cycle, and the

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results are shown in Fig. 3. At the same time, linear diagrams of charge transfer resistance (Rct) are drawn as shown in Fig. S7 to present the variational trend vividly

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during the discharge/charge process. Combing with the results as displayed in Fig. 3 and Fig. S7, under the condition of open-circuit potential (OCP), the initial Rct value

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of the cell with BNC-TiN-BNC composite interlayer is lower than those of the cell with BNCNF interlayer and bare CMK-3-S, indicating that the filling layer consisting of TiN nanoparticles benefits to promote the interfacial charge transfer. Moreover, during the processes from discharge at 2.2 V (D-2.2 V) to charge at 2.2 V (C-2.2V), both of the cells with BNC-TiN-BNC composite or BNCNF membrane exhibit much flatter tendency than the cell without any interlayer for the charge transfer resistance, and there are not any increments for the Rct values. Even under the condition of D-1.7 V, since the soluble LiPSs have been transformed to the insulating Li2S with the result of the increment for the Rct values, the impedance of the cell with BNC-TiN-BNC composite interlayer is still lower than others, showing that the BNC-TiN-BNC composite can anchor the soluble LiPSs to suppress their migration to the Li anode. Interestingly, the Rct values of all the batteries decrease remarkably over one cycle. However, the Rct value of the cell with BNC-TiN-BNC composite interlayer is still

Journal Pre-proof lower than the other samples, further suggesting that the BNC-TiN-BNC composite as upper current collector has ability to adsorb much more LiPSs and promote the reutilization of the active sulfur. According to all the results discussed above, the reasons why the BNC-TiN-BNC composite can improve the electrochemical performance could be explained with the following factors. First of all, the high conductivity of TiN nanoparticles benefits to accelerate the electron transfer and decrease the interfacial impedance. Secondly, both of the polar effect of TiN nanoparticles and the large specific surface area of BNCNF membrane as substrates

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contribute to facilitate the infiltration of the electrolyte. Thirdly, the BNC-TiN-BNC

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composite as interlayer can anchor the LiPSs effectively with the chemical adsorption and the physical barriers so as to further suppress the shuttle effect.

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In order to further analyze how the BNC-TiN-BNC composite membrane effects

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on anchoring the LiPSs, the BNC-TiN-BNC composite membrane is measured by

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SEM with the corresponding EDS and TEM with the corresponding EDX line scan before and after cycling. For the BNC-TiN-BNC composite membrane before cycling,

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as shown in Fig. 4a and Fig. S8a, there exist significant Ti signals in both of the cross-section and voids of the nanofibers, respectively. The results are consistent with

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the TEM images (Fig. S2) and XRD analysis (Fig. 1c), suggesting again that the TiN nanoparticles can fill into the nanofibers effectively with the spraying method. And meanwhile, there also exist strong B and N signals in the cross-section of the nanofibers, indicating that the TiN nanoparticles can composite well with BNCNF nanofibers. Furthermore, the BNC-TiN-BNC composite membrane after cycling has also been analyzed, and the results are displayed in Fig. 4b and Fig. S8b. From the SEM image as shown in Fig. S8b, compared with the original nanofibers, the surfaces of the nanofibers over cycling have become rough and the diameters of the nanofibers over cycling have increased to around 300 nm from the original diameter of 200 nm (Fig. S8a), and also, there exist strong S signal in the void and cross-section of the BNCNF nanofibers with the observation of the element component by EDS (the right of the Fig. S8b) and the element distribution by EDX line scan (the right of the Fig. 4b). More importantly, the intensity of the S signal is between those of the B and Ti

Journal Pre-proof signals, delivering that the adsorption of the BNC-TiN-BNC composite on the soluble LiPSs comes from both of the TiN nanoparticles and BNCNF substrates. These above results demonstrate that the BNC-TiN-BNC composite membrane has ability to intercept and further immobilize the soluble LiPSs with its sandwich structure, in order that it can effectively suppress the shuttle effect and then promote the reutilization of the captured LiPSs. XPS is performed to further analyze the interaction between the BNC-TiN-BNC composite membrane and the soluble LiPSs. To eliminate the influence of electrolyte,

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before the XPS test, the BNC-TiN-BNC composite membrane after cycle has been

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washed three times with the electrolyte solution of diethyl carbonate (DEC). As shown in Fig. 5a, there exist the signals of B, C, N, Ti and O for the BNC-TiN-BNC

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composite membrane before and after cycle. And meanwhile, the signal of O element

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might be attributed to the formation of an oxide or oxynitride passivation layer[33,

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47]. As expected, the appearance of the S and Li signals for the BNC-TiN-BNC membrane after cycle indicates that the BNC-TiN-BNC membrane has effect on

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anchoring the soluble LiPSs. With the analysis of the Ti 2p spectra as shown in (Fig. 5b), there exist three peaks before cycle which correspond to the Ti-O bond (463.98

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and 458.31 eV), Ti-N-O bond (462.57, 456.86 and 456 eV) and Ti-N bond (461.27 and 455.29 eV)[48, 49]. And meanwhile, an additional peak at about 456.65 eV appears after the lithiation, which is attributed to the formation of the S-Ti-N bond[47]. Besides, the peaks of the Ti-N bond after cycle have shifted to a lower binding energy (460.79 and 454.89 eV), suggesting the increase of electron density at the center of Ti atom[47]. The spectra of N 1s exhibit four predominant peaks before and after cycle as shown in Fig. 5c, which are attributed to the N=B/N-B bond (399.6 eV), pyridinic N (398.2 eV), Ti-N-O (397.5 eV) and Ti-N (396.47 eV), respectively[32, 48, 49]. The N=B/N-B bond and pyridinic N come from the BNCNF substrates, which have effect on improving the electron conductivity and contributing to the adsorption of the LiPSs according to our previous report[40]. Moreover, a new peak at about 400.90 eV also appears after cycle which corresponds to the formation of the S-N-Ti bond[47]. In addition, the B 1s spectra for the BNC-TiN-BNC

Journal Pre-proof composite membrane also exhibit two predominant peaks before and after cycle as shown in Fig. 5d, which is consistent with our previous report. And meanwhile, the new peak on behalf of the S loss energy appearing at about 186.5 eV demonstrates again that the BNCNF substrates with numerous of the Lewis acid-base sites can trap the LiPSs effectively[40]. More importantly, Fig. S9 exhibits the spectrum of S 2p of the BNC-TiN-BNC membrane after cycle, and the peaks at 166.97 and 169.10 eV are assigned to sulphate species which arose from the oxidation of the LiPSs in the air[50]. Furthermore, there exist two peaks at 163.64 (S 2p1/2) and 162.15 eV (S 2p3/2), which

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represent a +0.54 and +0.55 eV shift compared to the binding energy of the soluble

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LiPSs, indicating again the strong interaction between the LiPSs and BNC-TiN-BNC composite membrane[47, 51, 52]. In summary, the reason why the BNC-TiN-BNC

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composite membrane can capture the LiPSs with chemical adsorption effectively

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mainly depends on the following two factors: (Ⅰ) the polar nature of the TiN

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nanoparticles contributes to bond with the LiPSs by the formation of the S-N-Ti and S-Ti-N bonds; (Ⅱ) the BNCNF substrates possess abundant Lewis acid sites (the B

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atoms) and Lewis base sites (the N atoms), and the formation of N=B/N-B bond has also improved the polarization of the BNCNF substrates, thus promoting the chemical

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absorption with the LiPSs.

To further prove these points of view, the self-discharge tests of the cells with different interlayers at 1.0 C are carried out, and the results are shown in Fig. 6a. Firstly, these above cells are discharged and charged for one cycle at 1.0 C, and then they will be under static state for 24 h at room temperature, finally all cells are discharged to 1.7 V and the obtained discharge capacities of the cells before and after rest will be studied. Compared to the cells with BNCNF interlayer and bare CMK-3-S, the cell with BNC-TiN-BNC interlayer exhibits higher initial discharge voltage and more stable discharge platform. The reason is that the high conductivity of TiN nanoparticles as filling layer makes the sulfur in the cathode be used more efficiently and the sandwich structure of BNC-TiN-BNC interlayer further decreases the accumulation of LiPSs in the electrolyte to improve the reutilization of the active sulfur. As a result, the capacities of the cells with BNC-TiN-BNC interlayer before

Journal Pre-proof and after rest are 1329.6 and 1154.6 mA h g-1, respectively, which are far more than those of the cells with BNCNF interlayer (849.9 and 822.9 mA h g-1) and bare CMK-3-S (318.7 and 312.8 mA h g-1). And meanwhile, to more intuitively evaluate the fast adsorption ability of the BNC-TiN-BNC membrane for the LiPSs, both of BNCNF and BNC-TiN-BNC membrane are measured as shown in Fig. 6b. All of the original solution colors in three bottles are brown yellow, which are consistent with the color of Li2S6 electrolyte solution. And after 3 h, the solution color of BNC-TiN-BNC powder turns much lighter than that of BNCNF powder. Finally, after

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6 h, the solution color of BNC-TiN-BNC powder turns transparent. Furthermore, all

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of the upper solutions from the adsorption test for 6 h are measured by UV test, and the results are shown in Fig. 6c. The characteristic peak of Li2S6 disappears in the

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BNC-TiN-BNC containing solution, suggesting that the BNC-TiN-BNC composite

4. Conclusion

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has much stronger adsorptivity with LiPSs than the BNCNF membrane.

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In summary, a novel flexible sandwich-structured BNC-TiN-BNC composite

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membrane has been designed and applied as interlayer for Li-S battery to exhibit outstanding electrochemical performance with a sulfur loading of 2.5 mg cm-2. With the BNC-TiN-BNC composite membrane as interlayer, the cell has displayed a high initial discharge capacity of 823.8 mA h g-1 and remains 665.8 mA h g-1 over 250 cycles at 1.0 C with the capacity decay rate of 0.077%. And even at a high current density of 2.0 C, the cell still presents prominent initial discharge capacity of 642.3 mA h g-1 and excellent cycling stability with the capacity retention of 92.5 % over 300 cycles. The reasons why the BNC-TiN-BNC composite membrane can improve the electrochemical performance are attributed to the following factors: firstly, the BNC-TiN-BNC composite membrane has strong chemical adsorbability to LiPSs by

Journal Pre-proof

the combination of the N=B/N-B structure and S-Ti-N or S-N-Ti bonds; secondly, the sandwich structure of the BNC-TiN-BNC composite membrane can provide a more effective physical barriers to suppress the migration of LiPSs; thirdly, the nature of high conductivity and polarity for the TiN nanoparticles as filling layer contribute to accelerate the Li ion and electron transfer. Therefore, the unique sandwich-structured

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design provides a great potential for the practical applications of the Li-S battery with

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a high sulfur loading.

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Acknowledgements

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This work was financially supported by the National Natural Science Foundation of

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China (21773290), the Natural Science Foundation of Guangdong Province, China

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(2018A030313182), the Opening Project of State Key Laboratory of Advanced Chemical Power Sources, and Shenzhen Bureau of Science, Technology and

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Innovation Commission (JCYJ20170811154527927).

References

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Journal Pre-proof cathode for high performance lithium/dissolved polysulfide batteries. J. Power Sources 321 (2016)

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87-93.

Journal Pre-proof Fig. 1. SEM images for the BNC-TiN-BNC membrane of (a) top surface and (b) cross-section, Characterization of BNCNF and BNC-TiN-BNC samples: (c) XRD patterns, (d) FTIR spectra, (e) Raman spectra and (f) N2 adsorption-desorption isotherms. Fig. 2. (a) Rate performance of CMK-3-S coupled with BNC-TiN-BNC interlayer, BNCNF interlayer and without any interlayer, (b-d) Cycling performance of CMK-3-S with different interlayers at series current densities of (b) 0.5 C, (c) 1.0 C and (d) 2.0 C. Fig. 3. The AC impedance spectra of the cells with BNC-TiN-BNC composite interlayer, BNCNF interlayer and without interlayer for the first cycle. The discharge/charge process is performed at

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the current density of 1.0 C.

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Fig. 4. (a) EDX line scan with drift corrected spectrum profile scanning (left) and the

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corresponding element distributions on the cross-section of BNC-TiN-BNC composite (right) before cycling, (b) EDX line scan with drift corrected spectrum profile scanning (left) and the

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corresponding element distributions on the cross-section of BNC-TiN-BNC composite (right) after

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cycling.

Fig. 5. (a) XPS survey scan spectra for the BNC-TiN-BNC composite membrane before and after

2p, (c) N 1s, (d) B 1s.

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cycle, and XPS spectra of the BNC-TiN-BNC composite membrane before and after cycle: (b) Ti

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Fig. 6. (a) Self-discharge tests of the cells with different interlayers at 1.0 C, (b) Optical photo of the blank Li2S6 solution (left), BNCNF-containing (middle) and BNC-TiN-BNC-containing (right) for 0 h, 3h and 6 h, respectively, (c) UV spectra of the upper solution from the absorption tests for 6 h.

Journal Pre-proof

Declaration of Competing Interest Statement Herein, we submit the manuscript entitled “A sandwich-structured TiN/BN-C composite interlayer with enhanced performance for Li-S batteries” to “Journal of Electroanalytical Chemistry” for possible publication. We declare on behalf of all authors that the work described was original research that has not been published previously and is not under consideration for publication elsewhere. The publication of the manuscript is approved by all authors and tacitly or explicitly by the

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responsible authorities where the work was carried out. If accepted, it will not be

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published elsewhere in the same form, in English or in any other language, without

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the written consent of the Publisher.

Journal Pre-proof Author Statement Author Contribution: Jinghui Zhu: Main experimenter, writing-original draft and writing-review & editing; Caiming Jiao: Experimental participant; Tuo Kang: Experimental participant; Liubiao Zhong: Writing-review & editing; Sanfei Zhao: Project administration;

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Yejun Qiu: Project administration, writing-review & editing.

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Graphical abstract

Highlight:

A novel BNC-TiN-BNC membrane with sandwich-like structure is prepared through electrospinning and spraying method.

TiN nanoparticles can fill in voids among nanofibers and promote the electron

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

transfer.

chemical absorption for LiPSs.

Li-S batteries with BNC-TiN-BNC membrane exhibit outstanding cycling

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stability with high sulfur loading.

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

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The sandwich-like structure can improve largely the physical restriction and

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Figure 1

Figure 2

Figure 3

Figure 4

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Figure 6