Effective Cathode Design of Three-Layered Configuration for High

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The Effective Cathode Design of Three-Layered Configuration for High-Energy Li-S Batteries Si-Yu Liu, Chao-Ying Fan, Yan-hong Shi, Han-Chi Wang, Xing-Long Wu, and Jingping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14118 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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ACS Applied Materials & Interfaces

The Effective Cathode Design of Three-Layered Configuration for High-Energy Li-S Batteries Si-Yu Liu, Chao-Ying Fan, Yan-Hong Shi, Han-Chi Wang, Xing-Long Wu*, Jing-Ping Zhang* Faculty of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Jilin, Changchun 130024, China.

Email addresses: [email protected]; [email protected]

1 ACS Paragon Plus Environment

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

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ABSTRACT: Three-layered cathode structure was designed to minimize the shuttle effect of PSs and improve active material utilization. The three-layered configuration was fabricated by direct dropping pure sulfur composite slurry into the multifunctional dual barrier layers consisting of self-standing TiO2/C interlayer and very thin acetylene black layer (0.35 mg cm-2). In consequence, the decent discharge capacity of 963 mAh g−1 was acquired after 100 cycles at 0.1 C. With cycling at 0.1, 0.2, 0.5, 1 and 2 C, the cells displayed excellent reversible capacities of 1203, 1145, 1035, 934 and 820 mAh g−1, respectively. Furthermore, the cells still delivered satisfactory discharge capacity of 799 mAh g−1 after 300 cycles at 0.5 C. The light mass of threelayered configuration guarantees the energy density being effectively improved considering the overall mass of cathode. The energy density (603 Wh kg−1 after 100 cycles) was in a high level compared to reported ones. Therefore, it is believed that the synergistic design for three-layered cathode structure, which combines the mass-produced layer-by-layer structure, provides a novel protocol to the practical application of Lithium–sulfur batteries. KEYWORDS: Lithium–sulfur batteries, Three-layered structure, TiO2 interlayer, lightweight current collector, High energy density

1. INTRODUCTION

Batteries with high energy densities are essential to satisfy the continuously urgent demand of electrical vehicles, electronic devices and energy storage systems.1,2 However, current lithiumion batteries based on intercalated cathode materials (e.g., LiCoO2 and LiFePO4) cannot meet this demand.3-6 Lithium-sulfur (Li-S) batteries have attracted intensive interest with their excellent theoretical energy density of 2567 Wh kg−1.3,7 Furthermore, the low-price and nonpoisonous of sulfur is effectively improved environment protection and energy economy.4


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Nevertheless, the commerciality of Li-S battery is still hampered by several tough issues: (1) sulfur species is electrical insulation, resulting in the large polarization; (2) sulfur cathode suffers a large volume expansion during lithiation; (3) the formation of insoluble insulation layer on the electrode surface attributed to the shuttle effect of dissolved lithium polysulfides (PSs), which seriously impedes the full active material utilization.4,6,8-10 In order to solve these matters, the great mass of efforts have been centered on suppressing the shuttle effect of PSs.1,11-13 In this regard, the insertion of interlayer between active materials and separator has been certified to be an effective method to intercept the migrating PSs and reutilize the absorbed active materials, leading to an effective restraints of shuttle effect.2,14,15 In the initial stage, the barrier layer was pure carbon material.14-20 However, the interaction between hydrophobic and non-polar carbon materials and hydrophilic and polar polysulfide species is weak, resulting in their easy detachment from carbon materials.2,5 Metal oxides (MO), with polar and hydrophilic features, have strong chemical interaction with PSs, which is beneficial to restrain the shuttle effect.1,13 Therefore, to integrate the MO into carbon material holds great enhanced the electrochemical properties of Li-S batteries. In particular, TiO2 is a promising carrier for sulfur due to its superior binding energy with PSs.17,21-23 For example, Huang et. al. have reported an integrated, selective graphene/TiO2 film layer structure to mitigate the PSs diffusion.15 Although substantial progress has been achieved, the introduction of excess barrier layer will unavoidably increase the weight of cell and compromise the overall energy density. This issue is usually ignored in those publications where the capacities of batteries were calculated only based on the sulfur content. Therefore, the energy density and specific capacity may be uncompetitive and undesirable based on the whole weight of the cathode.



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Herein, the multifunctional dual barrier layers composed of the TiO2/C interlayer adhered on thin acetylene black (AB) coating layer on the separator were employed to promise the impenetrable immobilization for PSs. For avoiding the extra loading of Al current collector, the sulfur and carbon (CB-S) composite slurry was direct dipped onto the multifunctional dual barrier layers. The self-standing TiO2/C film is comprised of interconnected one dimensional nanofibers with TiO2 nanocrystals embedded into N-doping porous carbon. As schematically illustrated in Figure 1, the three-layered configuration with the Al foil current collector free can effectively restrain the transformation of PSs to anode and increase the utilization of sulfur, while the conventional cell still suffer from the severe shuttle loss. As a result, the battery exhibited superior reversible capacity of 963 mAh g-1 after 100 cycles at 0.1 C. Even after 300 cycles, a reversible capacity of 799 mAh g-1 was still maintained at 0.5 C. With cycling at 0.1, 0.2, 0.5, 1 and 2 C, the cells displayed great rate capacities of 1203, 1145, 1035, 934 and 820 mAh g−1. Moreover, the lightweight three-layered structure cathode also serves as current collector without Al foil. The lightweight and higher conducting of TiO2/C film and AB layer guarantee the higher energy density of the batteries. The excellent energy density of 603 Wh kg−1 was acquired at 0.1 C after 100 cycles (based on TiO2/C interlayer, AB layer and CB-S), which is in a high level compared to reported ones. This simple, low-cost and scalable fabrication of the three-layered structure provides an effective path for the practical utilization of Li-S batteries.

2. EXPERIMENTAL SECTION 2.1 Preparation of the electrode material Flexible TiO2/C film fabrication: TiO2/C film was prepared by the following steps. Firstly, the solution of tetrabutyl titanate (1.5 g, Ti(OC4H9)4, 97%, Aldrich), ethanol (4 mL)and acetic acid (3 mL) were stirred for 10 min. Then the polyvinylpyrrolidone (PVP, 0.4 g, Mw ≈ 360000,


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three-layered cathode Li 2 S8

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Li ano de

Figure 1. Graphical demonstration of the design for AB-TL@S cathode region and the function mechanism for excellent performance Li−S batteries.

Aladdin) was added to the solution with stirring for 30 min. Then, the freshly electrospining solution was electrospun with the operating parameters: a voltage of 15 kV and a flow rate of 1.0 mL h-1. Aluminum foil was 20 cm away from the needle point for collecting the nanofibers. The obtained nanofibers films were stabilized at 60 °C for 2 h under an air atmosphere. After that carbonized at 600 °C for 4 h in N2 flow. The fabrication of TiO2/C interlayer and AB layer adhered on the separator (AB-TL): Typically, AB and polyvinylidene fluoride (PVDF) were dispersed in N-methyl-2-pyrolidone (NMP) to form the slurry. The slurry was subsequently spread onto the glass fibre separator and the weight of AB coating can be controlled (about 0.7 mg). Then, the flexible TiO2/C film was immediately placed on the AB layer. The AB-TL was formed after drying at 60 ºC for 12 h. The fabrication of sulfur coated AB-TL (AB-TL@S): CB-S composites were prepared by the S (60 wt %) and carbon powder (30 wt %) dissolved in carbon disulfide, then heated at 155



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ºC in an autoclave for 12 h. A sulfur slurry (90 wt % CB-S, 10 wt % PVDF, NMP as solvent) was dropped onto AB-TL. The obtained integrated electrode was dried at 60 ºC for 12 h. The fabrication of unmodified electrode and single TiO2/C interlayer electrode: The sulfur slurry was directly spread onto an Al sheet and the single TiO2/C interlayer, respectively. Then the products was dried at 60 ºC for 12 h.

2.2 Material characterizations Powder X-ray diffraction (XRD, Rigaku P/max 2200VPC) with Cu Kα (λ = 1.5406 Å) radiation was used to analyze the crystal phase of the TiO2/C interlayer, CB-S and sulfur. The carbon content of the TiO2/C interlayer and the sulfur content of the cathode determined by thermogravimetric analysis (TGA, Pyris Diamond, PerkinElmer) from 25 °C to 800 °C in an air atmosphere. The structural feature of the TiO2/C interlayer was analyzed by Raman scattering spectra (JY HR-800, HORIBA JOBINYVON) with a laser beam wavelength (λ = 633nm). The microtopography and elemental distribution of the TiO2/C interlayer were explored by scanning electron microscopy (SEM, XL 30 ESEM-FEG, FEI Co.) and transmission electron microscope (TEM, JEM-2010F). In order to prove the micro-mesopores of the TiO2/C, N2 adsorptiondesorption testing was carried out. The powder of TiO2/C layer before and after cycles were analyzed by XPS (ESCALAB 250, Thermo) with Al Kα radiation and energy step size of 1 eV. 2.3 Electrochemical Measurement. The three-layered cathode structure as cathode and Li metal as anode to assemble CR 2032 coin cells in an Ar-filled glovebox. Lithium bistriﬂuoromethanesulfonimide lithium salt(LiTFSI, 1 mol L-1) in a solvent of dioxolane(DOL) and 1,2-Ethanediol dimethyl ether (DME) (VDOL: VDOL= 1:1) with 5 wt % LiNO3 was used as electrolyte, which be added in coin cell was


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controlled in 40 µL. Specific capacity of the cell was based on active materials. The energy density was based on the overall mass of cathode (including TiO2/C interlayer, AB layer and CBS). Galvanostatic charge-discharge tests were tested in the voltage of 1.7-2.8 V vs. Li+/Li at room temperature by using a battery-testing apparatus (LANDCT2001A). Cyclic voltammetry (CV) tests was proceeded by using electrochemical workstation (CHI750E) in a voltage of 1.72.8 V vs. Li+/Li at sweep rates of 0.01 mV s-1. The electrochemical impedance spectroscopy (EIS) was used for measuring the cells at different cycles with a 5 mV response excursion between the frequency ranges from 105 Hz to 10 mHz.

3. RESULTS AND DISCUSSION The three-layered configuration, composed of the CB-S composites slurry dipped into the TiO2/C interlayer coupled with very thin AB layer, was based on the separator using the layerby-layer covering method, as schematically exhibited in Figure 2a. First, the AB layer was spread onto the commercial separator with blade method to serve as both blinder and the further interception layer, then, the TiO2/C interlayer was immediately placed on the AB layer to act as simultaneously the current collector and the first barrier for lithium PSs. Soon after, the CB-S cathode slurry dissolved into NMP was quickly dripped into the TiO2/C interlayer. The coherent steps enables the compact contact of every layer for prosperous electron and ion transfer. In addition, the dual-protection of AB layer and TiO2/C interlayer provides the trustworthy immobilization for PSs on the cathode region. Moreover, the construction free of weighty Al foil minimizes the cell mass towards high energy density. The 3D nanostructure of TiO2/C largely increase the contact area with sulfur. As displayed in Figure 2b, the three-layered structure of the electrode is observed through the cross-sectional SEM images. The thin coating layer close to the separator is AB with thickness of 6 µm. Thus thin coating leads to the little impact for the overall



energy density of Li-S batteries. The TiO2/C approximately 80 µm thick is attached on the AB layer, to which the active materials layer (17 µm) is strongly adhered. The corresponding crosssectional mapping analysis (Figure 3) displays the distribution of C, O, Ti and S, which clearly shows the border edge of AB layer and TiO2/C interlayer. Moreover, from the overlapped mapping of Ti and S, besides on the surface of TiO2/C interlayer, many active materials slurry is uniformly penetrated into the TiO2/C interlayer. The front SEM image of the electrode is displayed in Figure 2c. The sulfur is evenly dispersed on the composite. Furthermore, the

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Li+

(b)

(c) TiO2 layer

S/C AB layer

separator 200 µm

2 µm

Figure 2. (a) The schematic diagram of the cell’s assembly process. (b) The Cross-sectional SEM images of electrode structure. (c) The SEM images of morphology of electrode.


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(a)

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O

C

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Figure 3. (a) The Cross-sectional SEM images of electrode structure, (b-e) the elemental mapping images of C, O, Ti, S and (f) the overlapping images of Ti and S.

corresponding mapping analysis (Figures S2a-c) also reveals that the element S and C are uniformly distributed. The crystalline phase of the pure S and CB-S products are explored by XRD, as displayed in Figure S2e. The diffraction image of the CB-S exhibits obvious characteristic peaks of Fddd orthorhombic S.2 The TGA curve demonstrated the sulfur content is 59 wt% in the electrode (Figure S2d). The TiO2/C interlayer was synthesized by the scalable electrospinning and subsequent onestep carbonization, as presented in Figure S1. Metal oxide of TiO2 has been confirmed as the effective chemical adsorbent of PSs because of the formation of S-Ti-O bonding.2,22 The particular structure of the TiO2/C interlayer is summarized in Figure S3. As shown in Figure S3a, the slight diffraction peaks are found in the XRD pattern, signifying the weak TiO2 crystallinity because of its ultrafine size. The peak at 25.6° is due to disordered carbon, which is further explored by Raman spectrum. As displayed in Figure S3b, the peaks at 1328 cm−1 and 1578 cm−1



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are ascribed to disordered and defective part of carbon (sp3-coordinated) and ordered graphitic crystallites of carbon (sp2-coordinated) respectively.24,25 The high ID/IG ratio of 1.14 illustrated the TiO2/C nanofibers contain a mass of disordered and defective sites. In addition, two pronounced peaks at 517 cm−1 and 639 cm−1, which are assigned to the A1g and E1g vibration modes of anatase TiO2, are displayed in spectrum.2,26 The content of carbon in the TiO2/C is 29 wt % by TGA test, as displayed in Figure S3c. Furthermore, XPS analysis is performed for exploring the chemical and electronic environment. The Peaks of N 1s, C 1s, O 1s, and Ti 2p in the XPS spectra are displayed in Figure S3d. The doublet peaks of Ti 2p with the binding energies at approximately 463.84 eV and 458.10 eV are consistent with Ti 2p1/2 and Ti 2p3/2 of Ti4+ as presented in Figure S3e.27,28 From the survey spectrum, the N signal is detected with the content of ~1.98%, suggesting the successful N doping of carbon. When intensively fitting the N 1s spectrum in Figure S3f, three predominant peaks at 400.76.3, 399.69 and 397.78 eV are discovered and identified to graphitic, pyrrolic and pyridinic nitrogen species, respectively.2,29 The N heteroatoms can strongly fix the PSs by the N-Li bonding.20,30 The morphologies of the TiO2/C are studied by SEM. As shown in Figure S4a, a large amount of one dimensional (1D) nanofibers are interconnected into three-dimensional (3D) network, which promises the integration and mechanical strength of self-standing film. The inserted image in Figure S4a iconically exhibits the excellent membrane flexibility. Clearly, the self-standing TiO2/C interlayer can be curved with well flexibility. The high-resolution SEM image (Figure S4b) displays the surface of TiO2/C nanofibers is smooth and the diameter (about 200 nm) of it is uniform. This unique and robust structure can well cushion severe volume change, which is beneficial for the excellent cycle stability. The high-resolution TEM image (Figure S4c) demonstrates that the nanofibers are made up of well grown granular nanocrystals


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with diameter of around 8 nm embedded into porous carbon. The ultrafine TiO2 nanocrystal provides more surface active sites for PSs adsorption and deposition. The crystal lattices with spacing of 0.356 nm is correspond to the (101) plane of the anatase TiO2. Furthermore, the corresponding mapping analysis in Figures S5b-e reveals that the elements of C, N, O, and Ti are uniformly distributed in the whole nanofibers. For purpose of exploring the porous structure of the TiO2/C interlayer, N2 adsorption-desorption isotherm test was performed as displayed in Figure S4d. Pore size distributions of the TiO2/C interlayer demonstrated the presence of welldeveloped pore size with an average diameter of 1-3 nm. The BET total pore volume and surface area of TiO2/C interlayer are 0.251 cm3 g-1 and 145.9 m2 g-1, respectively. This micromesoporous TiO2/C material with high surface area is in favor of enhancing the quick transport of electrolytes and facilitating diffusion of ion during the cycle process, herein improving the rate capability of the TiO2/C material. To explore the electrochemical process of the three-layered configuration, a series of electrochemical tests are launched. The CV curves at a sweep rate of 0.01 mV s-1 between the voltage 1.7 and 2.8 V is illustrated in Figure 4a. On the cathodic segment, two clear cathodic peaks at around 2.34 and 2.02 V are observed. The peak at 2.34 V related to S8 translated into long-chain PSs (Li2Sn, 4 < n < 8).7,31 The peak at 2.02 V is belonged to the further reduction of long-chain PSs to Li2S2 and Li2S. 31 The two adjacent peaks observed at 2.32 and 2.43 V during the anodic segment corresponds to the reverse reactions from Li2S/Li2S2 to Li2S8, and from Li2S8 to elemental S8.32,33 The intensity and position of peaks have little change during the following cycles, indicating the suppressed polarization and high electrochemical reversibility. By contrast, the peak intensity of the unmodified cell is smaller than that of AB-TL@S cell and gradually weaken during cycling in Figure S6a, Moreover, the polarization of the unmodified cell is more



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unmodified cell

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Figure 4. (a) Cyclic voltammetry profile in a potential range of 1.7–2.8 V vs. Li+/Li at a scan rate of 0.01mV s-1 and (b) the charge and discharge profile. (c) The cycle performance at 0.1 C and (d) the rate capacities at different current densities for AB-TL@S cell and unmodified cell. (e) The long-term cycle performance at 0.5 C for AB-TL@S cell. (f) The ratio of low discharge capacity (QL) to upper discharge capacity (QH; QL/QH) in the charge−discharge curve for ABTL@S cell and unmodified cell.

and more severe with cycling. As a result, the capacity and cycle stability of the unmodified cell


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is inferior to those of AB-TL@S cell during cycling. The charge-discharge profiles of AB-TL@S cell and unmodified cell in 1st, 2nd, 5th, 10th, 50th and 100th cycles are also displayed in Figures 4b and S6b, respectively. The electrochemical profiles of AB-TL@S cell curves are consistent with CV results and show the typical two-plateau behavior of a Li–S system even after 100 cycles, indicating the uncrossed electron and ion transfer with cycling. However, the charge/discharge plateaus of the unmodified cell sustains unstable with cycling. The ratio of low discharge capacity (QL) to upper discharge capacity (QH; QL/QH) in the charge−discharge curve reflects the synergistic effect of the leakage of PSs and the restriction of lithiation.34 As shown in Figure 4f, the higher ratio of 1.79 for AB-TL@S cell illustrates limited shuttle effect and full electrochemical conversion. As anticipated, the electrode with well-designed three-layered configuration displays superior cycle capacity and stability. The sulfur loading is about 1.1 mg cm−2. As displayed in Figure 4c, the initial discharge capacity of the cell is 1,707 mAh g−1 at 0.1 C (1 C =1675 mA g−1) and it still delivers a reversible capacity of 963 mAh g−1 after 100 cycles. The electrochemical property of three-layered without active materials is presented in Figure S7, which illustrated the “extra” capacity part compared to the theoretical capacity attribute to TiO2/C interlayer/AB layer in the first few cycles. As presented in Figure S8, the single TiO2/C interlayer cell delivers a good first cyclic capacity of 1235.3 mA h g-1 at 0.5 C, however it suffer quick capacity decay. This results may be imputed to the poor contact between TiO2/C interlayer and separator. On the contrary, the first cyclic capacity of the unmodified cell is only 985 mAh g−1. The AB-TL@S cell is cycled at different current densities to estimate the rate property, as displayed in Figure 4d. With cycling at 0.1, 0.2, 0.5, 1 and 2 C, the AB-TL@S cell shows high reversible capacities of


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this work Ref.26 Ref.22 Ref.12 Ref.23 Ref.19 Ref.27 Ref.24 Ref.14

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Figure 5. (a) The energy densities at 0.1 C for AB-TL@S cell. (b) The energy density and power density (based on the theoretical time) of the AB-TL@S cell compared to the reported literature.

1203, 1145, 1035, 934 and 820 mAh g−1, respectively, much better than those of unmodified cell. When the current density is switched back to 0.1 C, the discharge capacity is recovered to 1095 mAh g−1, illustrated the stability of the cathodic structure. These results indicate excellent cycle and rate performance due to the strong adhesion of these three covering layer, providing perfect conductive network for electrochemical conversion and effective chemical adsorption sites for the PSs. For evaluating the potential of the well-designed three-layered structure in high-power energy storage application, the long cycling performance of AB-TL@S cell at a high rate of 0.5 C has also been illustrated in Figure 4e. Even after 300 cycles, the AB-TL@S cell still holds a superior discharge capacity of 799 mAh g-1. Batteries with the light weight of the TiO2/C interlayer (1.3 mg cm−2) and AB layer (0.35 mg cm−2) as well as free of the weighty Al foil (6 mg cm−2) have much higher gravimetric energy densities. The initial discharge energy density of the AB-TL@S cell is 970 Wh kg−1 at 0.1 C and it still delivers a reversible energy density of 603 Wh kg−1 after 100 cycles in Figure 5a. As presented in Figure 5b, at a rate of 2 C, the energy density of the AB-TL@S cell is as high as 603 Wh kg−1 and the power density is 1202 W kg−1 (based on the theoretical time). Furthermore, the power density and energy density of the AB-


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TL@S cell are in a high level compared to reported ones, indicating the three-layered structure is promising for the practical utilization of Li-S batteries. The energy density based on whole weight of cathode region of the AB-TL@S cell is displayed in Figure S11 and the specific capacity based on cathode weight of the AB-TL@S cell is presented in Figure S12. Moreover, the material is cheap and the preparation methods is mass-producible and simple, further promoting the commodification of the Li-S batteries. EIS is further conducted after different discharge/charge cycles to excavate the electrochemical kinetics of two electrodes. As presented in Figures S6c and d, the intercept and semicircle in high-frequency region corresponded with the cell components resistance and charge transfer (Rct), respectively.30 The semicircle in middle-frequency region is ascribed to the mass transportation resistance attributed to the deposition of insoluble Li2S2 and Li2S.29,35,36 Obviously, the Rct of AB-TL@S cell is far less than that of unmodified cell before cycle because of well-established 3D conductive network derived from TiO2/C film and AB layer. The Rct immensely decrease after 10 cycles due to the full immersed of electrolyte in the electrode and the rearranged active materials. After 10 cycles, Rct has little charge, demonstrating fast charge transfer and effective entrapment of PSs even after the long cycling.37 In contrast, the unmodified cell delivers much larger resistance at every cycle. These results explicitly confirm that the threelayered cathode mitigates the dissolution of PSs as well as the deposition of nonconductive Li2S2 and Li2S. The XPS analysis of cycled cell is conducted to certify the chemical interaction between TiO2 and PSs. The cycled layer and cathode are disassembled in the glovebox and washed using the DOL solvent. The high-resolution XPS spectrum of the TiO2/C film after 100 cycles are shown in Figure S9. In the S 2p XPS spectrum, plentiful sulfur bonds can be noticed. The S 2p3/2 peaks of 168 eV, 166 eV and 163.8 eV in Figure S9a are corresponded to -NSO2CF3



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from the residual electrolyte, Li2SO3 from the decomposition of electrolyte and S-S bond from sulfur specie, respectively.26 The peak at 161 eV can be attributed to the Ti-S binding.38-41 Figure S9b shows two obvious characteristic peaks of TiO2 at 458.1 and 463.8 eV and a small peak at 467.2 eV revealing the Ti-S bonding between TiO2/C and PSs.26,39 To better confirm the design advantage of AB-TL@S cathode, we further investigated the (c)

(b)

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Figure 6. (a) The Cross-sectional SEM images of electrode structure after 100 cycles and (b-e) the elemental mapping images of C, O, Ti, S.

(c)

(b)

(a)

Ti

2 µm

(e)

(d) 2.5 µm

O

S

Figure 7. (a-b) the SEM of the reverse side of AB-TL@S cathode after 100 cycles and (c-e) the elemental mapping images of Ti, O, S.


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SEM images after 100 cycles. As presented in Figure 6a, the three-layered structure of the electrode after 100 cycles is observed through the cross-sectional SEM images. The corresponding cross-sectional mapping analysis (Figures 6b-e) shows that the distribution of C, O, Ti and S. Clearly, the active sulfur is well entrapped into the three-layered structure, preventing the migration of PSs towards the lithium anode. The SEM and mapping of the surface of AB-TL@S cathode in Figure S10 show the elemental sulfur and carbon are uniformly distributed on surface of AB-TL@S cathode, guaranteeing the effective electron and ion transfer. The reverse side of TiO2/C interlayer is illustrated in Figure 7. The SEM image in Figure 7a compared with the image before the cycle (Figure S4a) demonstrates the interconnected nanofibers architecture of TiO2/C film is well maintained without structural collapse. The diameter (about 320 nm) of the 1D nanofibers in Figure 7a compared with those in Figure S4b (about 200 nm) is clearly increased, it shows that there is PSs deposited in TiO2/C nanofibers. There are no nonconductive agglomerates deposited into the pore or surface of the TiO2/C nanofibers, guaranteeing the unimpeded electron and ion transfer path and good electrolyte infiltration for electrochemical conversion. As displayed by the sulfur mapping in Figures 7c-e, sulfur is uniformly deposited onto the TiO2/C nanofibers and there are no conglobate S8 particles on the surface or the pores because of the abundant chemical adsorption sites for PSs on the TiO2/C nanofibers. It explains that the TiO2/C interlayer can effectively entrap the PSs to serve as the barrier layer. 4. CONCLUSION In summary, the well-design of lightweight three-layered structure combined the selfstanding TiO2/C interlayer with the thin AB layer for improving active materials utilization and decrease the loading of inactive materials, hence synergistically acquire high energy density of



Page 18 of 25

Li−S batteries. The three-layered configuration was constructed by dipping the CB-S composites slurry into the multifunctional dual barrier layers. The first multifunctional layer composing of one-dimensional TiO2/C nanofibers act as the conductive network and the first barrier layer, which effectively prevent PSs and provide abundant nucleation sites for uniform deposition of sulfur species through enriched chemical adsorption sites of TiO2. The second multifunctional AB layer serves as both binder and the further interception layer. As a result, the cell with a welldesigned cathode region exhibited superior capacity and cycle stability. The decent reversible capacity of 963 mAh g−1 was acquired after 100 cycles at 0.1 C. Especially, the cells still maintain high discharge capacity of 799 mAh g−1 at 0.5 C after 300 cycles. With cycling at 0.1, 0.2, 0.5, 1 and 2 C, the cells show excellent rate capacities of 1203, 1145, 1035, 934 and 820 mAh g−1, respectively. Due to the light weight of the TiO2/C interlayer and AB layer as well as free of the weighty Al foil, Li-S batteries have much higher gravimetric energy densities (603 Wh kg−1 after 100 cycles), which is superior to reported ones. Therefore, the elaborate design of the lightweight three-layered configuration cathode would achieve high-energy-density Li−S batteries for practical application. ASSOCIATED CONTENT Supporting Information. The preparation of TiO2/C interlayer and the morphology characterization of TiO2/C film, the SEM, TEM, mapping and N2 adsorption-desorption isotherms images of the calcined TiO2/C film, the XRD of CB-S cathode, the SEM of CB-S composites on three layered structure and corresponding elemental mapping, cyclic voltammetry profile and the charge and discharge curve of unmodified cell, the EIS results of S@TiO2/C electrode and unmodified cell, the XPS survey spectrum of cycled TiO2/C film, the SEM image


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of the AB-TL@S morphology features at 0.1 C after 100 cycles and corresponding elemental mapping. AUTHOR INFORMATION Corresponding Author *Email addresses: [email protected] (J.-P.Z.); [email protected] (X.-L.W.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The Education Department of Jilin Province (111099108), Financial support from the National Natural Science Foundation of China (21573036, and 51602048), and the Fundamental Research Funds for the Central Universities (2412017FZ013). REFERENCES (1) Yang, Z. Z.; Wang, H. Y.; Lu, L.; Wang, C.; Zhong, X. B.; Wang, J. G.; Jiang, Q. C. Hierarchical TiO2 spheres as highly efficient polysulfide host for lithium-sulfur batteries. Sci. Rep. 2016, 6, 22990. (2) Yu, M.; Ma, J.; Song, H.; Wang, A.; Tian, F.; Wang, Y.; Qiu, H.; Wang, R. Atomic layer deposited TiO2 on a nitrogen-doped graphene/sulfur electrode for high performance lithiumsulfur batteries. Energy Environ. Sci. 2016, 9, 1495-1503. (3) Xu, C.; Xu, B.; Gu, Y.; Xiong, Z.; Sun, J.; Zhao, X. S. Graphene-based electrodes for electrochemical energy storage. Energy Environ. Sci. 2013, 6, 1388-1414. (4) Yin, Y. X.; Xin, S.; Guo, Y. G.; Wan, L. J. electrochemistry, materials, and prospects. Angew. Chem. Int. Ed 2013, 52, 13186-13200.



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Table of contents (TOC) three-layered cathode Al foil

AB layer

Separ ator

Li ano de

VS.

1200

Energy density (Wh kg -1)

absorb LiPSs

TiO2 layer

900

Sepa

rator

600

AB-TL@S cell unmodified cell

300 0

Li ano

0

20

40 60 Cycle

80

Polysulfide shuttle

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


de

100


Effective Cathode Design of Three-Layered Configuration for High

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