A Flexible All-in-One Lithium-Sulfur Battery - ACS Publications

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A Flexible All-in-One Lithium-Sulfur Battery Minjie Yao, Rui Wang, Zifang Zhao, Yue Liu, Zhiqiang Niu, and Jun Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06936 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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A Flexible All-in-One Lithium-Sulfur Battery Minjie Yao, Rui Wang, Zifang Zhao, Yue Liu, Zhiqiang Niu,* and Jun Chen Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, 300071, P. R. China. *E-mail: [email protected]

KEYWORDS: carbon nanotubes, flexible, all-in-one, integrated, lithium-sulfur battery

ABSTRACT: The recent boom in flexible and wearable electronic devices has increased the demand for flexible energy storage devices. Flexible lithium-sulfur (Li-S) battery is considered to be a promising candidate due to its high energy density and low cost. Herein, a flexible Li-S battery was fabricated based on an all-in-one integrated configuration, where multi-walled carbon nanotubes/sulfur (MWCNTs/S) cathode, MWCNTs/manganese dioxide (MnO2) interlayer, polypropylene (PP) separator and Li anode were integrated together by combining blade coating with vacuum evaporation methods. Each component of the all-in-one structure can be seamlessly connected with the neighboring layers. Such an optimal interfacial connection can effectively enhance electron and/or load transfer capacity by avoiding the relative displacement or detachment between two neighboring components at bending strain. Therefore, the flexible all-in-one Li-S batteries display fast electrochemical kinetics and remain stable electrochemical performance under different bending states.

ACS Paragon Plus Environment

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The portable electronics are moving towards ultrathin, integrated, flexible or even wearable devices.1-8 In order to match these electronic devices for achieving completely self-powered flexible electronic system, flexible energy storage devices with simplified and integrated configurations have to be considered and designed.9-15 Among various energy storage devices, lithium-sulfur (Li-S) battery is a promising candidate due to its high theoretical energy density of 2567 W h kg−1 and low cost, as a result, flexible Li-S batteries are highly desired.16-21 The successful assembly of flexible Li-S batteries depends mainly on the preparation of flexible components and the design of innovative configuration.22 Recently, various freestanding carbon/sulfur composite films have been designed to directly serve as the cathodes of flexible Li-S batteries due to their excellent mechanical and electrochemical properties.23-28 However, most of these flexible Li-S batteries were still assembled by sandwiching separator between two isolated electrodes to achieve a conventional stacked configuration. When these flexible devices with conventional stacked configuration are bent into different levels, the displacing and detaching between their neighboring components induced by external deformation will seriously degrade the performance of the devices and even result in short circuit.29,30 Therefore, the flexible Li-S batteries with different electrode and configuration should be further developed. Recently, flexible energy storage devices with all-in-one configuration, where all components including electrodes, separator and even current collectors are integrated together, have been designed.31-37 The continuous seamless connection of the all-in-one devices could not only ensure efficient electron and/or load transfer capacity, but also avoid the relative displacement or detachment between the neighboring components under frequent mechanical deformation, leading to enhanced mechanical property and electrochemical stability.38 However, to date, the flexible Li-S batteries with all-in-one configuration have not been achieved since the utilization


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of isolated carbon/sulfur composite film cathodes and Li foil anode limits the consecutive and seamless integration between them. Therefore, a strategy has to be developed to design an all-in-one configuration of flexible Li-S batteries. Porous polymer separator is an essential component in conventional Li-S batteries. Owing to its natural flexibility, it can also act as a compliant substrate to support electrodes and buffer internal stress during bending.39-42 If the functional components of Li-S batteries could be integrated onto polymer separator to achieve an all-in-one configuration, highly flexible Li-S batteries with stable performance will be achieved. Inspired by this, we design a flexible integrated all-in-one structure, where multi-walled carbon nanotubes/sulfur (MWCNTs/S) cathode, MWCNTs/manganese dioxide (MnO2) interlayer and evaporated lithium anode are integrated onto polypropylene (PP) separator together by combining blade coating with vacuum evaporation methods. Such an all-in-one configuration could not only decrease the interfacial contact resistance, but also avoid relative displacement or detachment between the neighboring components under bending states to ensure effective electron and load-transfer capacity. Therefore, the resultant Li-S batteries show high flexibility and stable electrochemical performance under different mechanical deformation. It provides a strategy to assemble all components of a flexible energy storage device into an all-in-one integrated architecture with excellent structural stability. RESULTS AND DISCUSSION Figure 1a schematically shows the fabrication process of the all-in-one configuration by combining blade coating with vacuum evaporation methods. In a typically experiment, MWCNTs and MnO2 nanowires were first mixed to obtain the composite slurry. The slurry was directly poured and coated onto one side of the PP separator (step I of Figure 1a) to sever as polysulfide barrier, which is a common strategy to mitigate the dissolution and shuttle of


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polysulfide in the electrochemical process.43,44 Various MnO2 structures have been reported to trap lithium polysulfides, like hollow microspheres, nanosheets and so on.45-50 In our case, MnO2 nanowires and MWCNTs were simultaneously selected to provide polysulfide trappers and electron transfer pathway, respectively, since they are easy to interconnect with each other to form a stable skeleton structure. The MnO2 nanowires were prepared by a hydrothermal method51 and can be indexed as α-MnO2 with a diameter of 30-40 nm and length of 3-5 μm (Figure S1-S3, Supporting Information). Owing to the interconnection of MnO2 nanowires and MWCNTs, the coated slurry is easy to form a film and firmly adhere to the surface of the PP separator, named as MWCNTs/MnO2@PP. In order to achieve the seamless connection between interlayer and cathode, MWCNTs were also introduced into the cathode. Therefore, MWCNTs/S composite slurry was further prepared and coated onto the top of the MWCNTs/MnO2 coating layer by a similar process (step II of Figure 1a) to obtain the all-in-one integrated MWCNTs/S-MWCNTs/MnO2@PP structure. Benefitting from the high conductivity and intrinsic flexibility of MWCNTs, the MWCNTs/S cathode is free of metal current collector. To further achieve the integration with flexible Li anode, Li was directly deposited onto the other side of the separator in the MWCNTs/S-MWCNTs/MnO2@PP structure by vacuum evaporation method (step III of Figure 1a). Therefore, the all-in-one structure integrating flexible Li anode, separator, interlayer and cathode into one monolith (MWCNTs/S-MWCNTs/MnO2@PP@Li) was achieved. The cross-sectional scanning electron microscopy (SEM) image and corresponding elemental maps clearly show the integrated configuration of the all-in-one structure (Figure 1b-f). In such an all-in-one structure, each component can be firmly connected with its neighboring layers. It is noted that the thickness of PP separator is about 25 μm. The thin MWCNTs/MnO2 interlayer


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with a thickness of about 6 μm is closely adhered to one side of the PP separator and exhibits continuous porous network structure (Figure 2a), which is conducive to the infiltration of electrolyte and provides polysulfides anchored sites.52,53 The MWCNTs/S layer possesses a thickness of 19 μm and displays a similar network structure (Figure 2b). More importantly, the MWCNTs at the neighboring boundaries of the two coating layers could bridge the MWCNTs/MnO2 interlayer and MWCNTs/S cathode due to Van Der Waals force among MWCNTs and the function of binder. Although a two-step blade coating process was carried on, there was almost no permeation of coating materials for PP separator (Figure S4a and b, Supporting Information), effectively avoiding the short circuit in practical devices. Moreover, the loading of MWCNTs/MnO2 interlayer (0.20-0.48 mg cm-2) is much smaller than that of the MWCNTs/S layer (2.0 mg cm-2). Thus, the content of sulfur in the integrated structure of two coating layers is still around 60% (Figure S5, Supporting Information). In addition, the size of the integrated structure can be scaled up easily, since the area of the integrated coating layer mainly depends on the size of PP separator. In order to illustrate that, a large MWCNTs/S-MWCNTs/MnO2@PP structure with a size of 30 cm  7 cm (Figure 1g) was assembled

by

an

automatic

coating

machine.

Furthermore,

such

an

MWCNTs/S-MWCNTs/MnO2@PP structure exhibits excellent flexibility and its smooth structure could maintain well without obvious crack or exfoliation of coating materials after bending even rolling test (Figure 1h). Apart from MWCNTs/MnO2 interlayer and MWCNTs/S cathode, the Li layer with a thickness of about 10 μm can be further integrated onto the other side of PP separator by thermal evaporation method (Figure 1b and i). The Li layer is uniform and smooth except only a small amount of bulges on its surface (Figure 2c), which may result from the deposition of residual Li


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vapor during cooling process. The seamless connection between Li layer and PP separator indicates that deposited Li layer could adhere to the surface of the separator closely. More importantly, the deposition of Li has no impact on the porous structure of PP separator (Figure S4c, Supporting Information) to guarantee Li ion transport and avoid short circuit. Furthermore, the all-in-one structure still exhibits excellent flexibility (Figure 1j). The strain-stress curve shows that the MWCNTs/S-MWCNTs/MnO2@PP structure can withstand a high elastic strain of 10.6% with a Young’s modulus of 155 MPa (Figure 2d). In contrast, the stacked structure consisting of isolated PP separator, MWCNTs/MnO2 film and MWCNTs/S film (Figure S6, 7 and Table S1, Supporting Information), exhibits a characteristic behavior of multilayer structure with five regions, as shown in Figure 2d. Region I is the first linear region (ɛ ˂ 0.30%) and represents the elastic deformation of the stacked structure. With further tensile force, there is an obvious yield plateau from the strain of 0.30% to 0.36% (Region II), which can be ascribed to the fracture of the MWCNTs/S film with an inherent fracture strain of 0.31% (Figure S8a, Supporting Information). After that, the remaining stacked structure of MWCNTs/MnO2 film and PP separator needs a certain buffer force to return to the balanced state (Region III). Similarly, the other yield plateau appears at the strain of 0.83% to 0.92% (Region IV), corresponding to the fracture of MWCNTs/MnO2 film (inherent strain of 0.78%). Region V is the representative strain-stress curve of PP separator. In this region, the remaining PP separator reaches to its maximal elastic deformation at the strain of 11.68%, comparable with that of the pure PP separator (12.43%) (Figure S8b, Supporting Information). It is noted that the damage of any component will seriously degrade the performance of devices and even lead to their failure. Therefore,

compared

with

the

stacked

structure,

the

all-in-one

structure

of

MWCNTs/S-MWCNTs/MnO2@PP can withstand a larger strain, indicating its higher flexibility.


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Since the integrated configuration could efficiently enhance the load transfer capacity to avoid the relative detachment between the neighboring layers during deformation process.54 In addition, the Li layer anode with PP separator (Li@PP) also exhibits excellent mechanical property. Furthermore, the Li layer could still remain seamless connection with the PP without crack even after cyclic bending/unbending, further indicating the structural stability of the Li layer (Figure S9, and detailed information in Supporting Information). Figure 2e shows a mechanical model of bending multilayered film structure. The strain in multilayered film is mainly caused by external bending moment. In our case, it is noticed that the integrated coating layer itself is composed of MWCNTs/S layer and MWCNTs/MnO2 layer, named as MWCNTs/S-MWCNTs/MnO2 film (Figure S10, Supporting Information). The mechanical strength of MWCNTs/S and MWCNTs/MnO2 coating layers mainly depends on the interconnected MWCNTs network. Furthermore, the MWCNTs at the interface could bridge the MWCNTs/S coating layer and MWCNTs/MnO2 coating layer to achieve their seamless connection. Therefore, the integrated MWCNTs/S and MWCNTs/MnO2 coating layers could be regarded as one monolith, corresponding to the “film” in the model. The integrated coating layer in top surface is in tension and the PP separator in bottom surface is in compression.55 In our case, elastic moduli of integrated coating layer (Yf) is larger than that of PP separator substrate (Ys), as a result, the strain in the top surface (εtop) is given by: 𝜀𝑡𝑜𝑝 =

(

𝑑𝑓 + 𝑑𝑠 (1 + 2𝜂 + χ𝜂2) 2𝑅

)

(1 + 𝜂)(1 + χ𝜂)

(1)

Where R is curvature radius; df and ds are the thicknesses of the integrated coating layer and PP separator, respectively; η = df / ds and χ = Yf / Ys. In our case, df = 25 μm, ds = 25 μm, Yf = 358 MPa, Ys = 276 MPa, the calculated values of η and χ are 1.0 and 1.3, respectively. In previous reports, a substrate with a lower elastic modulus is beneficial for achieving a smaller curvature


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radius, since it can not only support coating layers but also buffer the internal stress during bending.43 As a result, the integrated structure of MWCNTs/S-MWCNTs/MnO2@PP is expected to achieve a large bending level. According to Equation (1), when the top surface, which suffers from the largest strain in the integrated structure, reaches its maximum (~10.6%), the corresponding curvature radius will be minimal (~220 μm), indicating the excellent mechanical strength of the MWCNTs/S-MWCNTs/MnO2@PP structure. The flexible energy storage devices often require that their electrodes possess stable conductivity

at

different

mechanical

deformation.

Therefore,

the

conductivity

of

MWCNTs/S-MWCNTs/MnO2@PP structure was measured at different bending levels. Impressively, the integrated structure shows nearly unchanged conductivity even when it was bent to nearly 180° (Figure 2f, Figure S11and Table S2 in Supporting Information). Furthermore, there is also almost no change in its conductivity after 10000 repeated bending cycles (Figure S12 and Table S2, Supporting Information). Such stable electrical conductivity under various bending levels and frequent bending cycles is attributed to the excellent flexibility and structural stability of the all-in-one configuration. As suggested in Figure S13, the integrated coating layer could still seamlessly contacts with the PP separator without exfoliation and remains its continuous network structure. Therefore, coupled with the deposition of Li, the as-prepared all-in-one MWCNTs/S-MWCNTs/MnO2@PP@Li structure is expected to be applied in both conventional and flexible Li-S batteries. The all-in-one structure was first assembled in the coin-type batteries, named as all-in-one Li-S batteries, to explore its basic electrochemical properties. Figure 3a shows the representative cyclic voltammetry (CV) curves of such all-in-one Li-S batteries in the initial 10 cycles. During the first cathodic scan, there are two reduction peaks at about 2.27 and 2.01 V, attributing to the


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reduction of the sulfur (S8) to long-chain lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) and further to short-chain lithium sulfides (Li2S2 or Li2S), respectively.56,57 In the following anodic scan, two oxidation peaks are observed at 2.33 and 2.39 V, corresponding to the reverse transformation from Li2S/Li2S2 to soluble polysulfides and solid sulfur, respectively. Furthermore, there is a small oxidation peak between 2.4-2.5 V, which is attributed to the electrochemical reaction of the surface-bound intermediates (S2O32−) and polysulfides to S8.58 In addition, the CV curve of the all-in-one device at the first cycle is different from that of subsequent cycles, as shown in Figure 3a. It is ascribed to the electrochemical activation process of the MWCNTs/S cathode during initial charge/discharge process.59 These are consistent with those of common Li-S batteries, indicating that the design of the all-in-one configuration is feasible. As a contrast, the stacked Li-S batteries composed of the isolated commercial Li foil (Figure S14), PP separator, MWCNTs/MnO2 film as well as MWCNTs/S film and the all-in-one Li-S batteries without MWCNTs/MnO2 interlayer were also fabricated. In the case of the stacked Li-S batteries, their dynamics is relatively sluggish. As a result, the corresponding oxidation peaks shift to higher potentials and the one at high potential overlaps with the small shoulder peak between 2.4 and 2.5 V (Figure S15). Therefore, there is no small shoulder peak in the CV curves of stacked Li-S battery. In addition, the CV curves of the stacked Li-S batteries and the all-in-one Li-S batteries without MWCNTs/MnO2 interlayer exhibit and/or obvious decreased intensity of catholic and anodic peaks during the initial 10 cycles (Figure S15, 16, Supporting Information). It further indicates

that

the

all-in-one

configuration

processes

enhanced

dynamics

and

the

MWCNTs/MnO2 layer can effectively prevents polysulfide intermediates from dissolving into the electrolyte.60,61


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The galvanostatic charge/discharge (GCD) profiles of the all-in-one Li-S battery show that it delivers a discharge specific capacity of 1202.1 mAh g-1 at 0.1 C in the first charge process (Figure 3b), with a Coulombic efficiency up to 96% (Figure S17, Supporting Information). In contrast, the stacked Li-S battery delivers a discharge specific capacity of 1170.9 mAh g−1, indicating the all-in-one configuration with an enhanced utilization efficiency of sulfur. Moreover, the absence of MWCNTs/MnO2 interlayer leads to a much lower discharge capacity of 760.3 mAh g−1 at 0.1 C, which further indicates the MWCNTs/MnO2 interlayer can effectively restrict the dissolution of polysulfides. In addition, with the cycling at 0.1, 0.2, 0.5, 1.0 and 2.0 C, the all-in-one Li-S battery delivered high reversible capacities of 1183.9, 1059.1, 945.7, 856.6 and 682.3 mAh g−1, respectively (Figure 3c). The potential difference (polarization, ΔE) between the charge plateaus and corresponding discharge plateaus at different rate represents the reaction kinetics of electrochemical process. Compared with the stacked Li-S battery and the all-in-one Li-S battery without interlayer, the all-in-one Li-S battery exhibits lower polarization at different current densities (Figure S18, 19 and Table S3, Supporting Information). It suggests that all-in-one Li-S battery possesses fast electrochemical kinetics since the seamless connection between neighboring components provide continuous electron/ion transfer pathway. As a result, the fresh all-in-one Li-S battery shows the smaller intrinsic resistance (Rs) and charge-transfer impedance (Rct) of 3.2 and 47.8 Ω, respectively (Figure 3d). And the Nyquist plots were fitted by an equivalent circuit model (Figure S20, Supporting Information). Impressively, the all-in-one Li-S battery also shows excellent long cycle stability with a low capacity decay rate of only 0.064% per cycle due to the integrated configuration coupling with the confinement effect of MWCNTs/MnO2 interlayer (Figure 3e), while the stacked Li-S battery and the all-in-one Li-S battery without interlayer exhibit faster capacity decay rates of 0.085% and 0.073%, respectively.


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In addition, it is noted that when the areal sulfur loadings increased to 3.0 and 4.5 mg cm-2, the corresponding integrated structures exhibit lower specific capacities and faster capacity decay in comparison with the case of 1.4 mg cm-2 (Figure S21, Supporting Information). It is ascribed to that the sluggish kinetic that originate from the thicker thickness of electrodes would induce larger polarization and lower utilization of sulfur.62 Apart from coin-typed Li-S batteries, the excellent mechanical flexibility and structural stability of the all-in-one structure endows it to be directly used in flexible Li-S batteries. As a proof of concept, the soft-packaged Li-S batteries were fabricated based on the MWCNTs/S-MWCNTs/MnO2@PP@Li structure, named as all-in-one soft-packaged Li-S batteries. Their GCD profiles (Figure 4a) exhibit two well-defined voltage plateaus at 2.31 and 2.11 V, assigned to the two-step conversion of sulfur to Li2Sn (4 ≤ n ≤ 8) and further to Li2S2 or Li2S, respectively. And it delivers a discharge capacity of 1134.4 mAh g−1 and a reversible charge capacity of 1114.7 mAh g−1 at 0.1 C, respectively, which is much higher than that of the previously reported Li ion batteries with all-in-one configuration and comparable to that of Li-S batteries with inflexible all-in-one configuration (Table S4, Supporting Information). Then, the discharge capacity stabilizes at 583 mAh g−1 after 100 cycles, suggesting faster capacity decay (Figure S22, Supporting Information). This is attributed to that the amount of as-obtained polysufides in the soft-packaged batteries is much larger than that in coin-type batteries, leading to more severe shuttle of polysufides. In addition, the enhanced current density on the surface of Li anode would induce a larger concentration gradient of Li ions. As a result, the high concentration of polysufides and large fluctuation of Li ions lead to the unstable solid electrolyte interface film. Therefore, the soft-packaged Li-S batteries present a faster capacity fading rate than coin-type Li-S batteries. 63,64


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In addition, the stable electrochemical performance in different mechanical deformation is also required for flexible energy storage devices. To investigate the electrochemical stability of the all-in-one soft-packaged Li-S battery under different bending states, it was bent to different levels and started then charged and discharged for 20 cycles at 1.0 C under each bending state (Figure 4b). The bending levels were controlled by adjusting the distance (L) between two ends of the battery (original length as L0). In the first 20 cycles, the flexible all-in-one Li-S battery was at the initial state and also exhibits faster capacity decay than that of coin-type Li-S battery.65,66 After that, the discharge capacity tends to be stable, even when the battery was bent at different levels. In contrast, the capacity of the stacked soft-packaged Li-S battery with conventional stacked configuration was continually degrading, not only in the first 20 cycles, but also in the subsequent cycles. Furthermore, compared with the all-in-one soft-packaged Li-S battery, the stacked soft-packaged battery showed a much larger capacity drop after it was bent to different levels. The stable electrochemical performance of all-in-one soft-packaged Li-S battery at bending states is ascribed to its structure. Since the excellent structural stability of the all-in-one configuration could effectively avoid the relative sliding and detaching between the neighboring components to ensure continuous electron/ion transfer pathway. On the contrary, the simple physical contact between the stacked components cannot resist their relative detachment, which would block the electron/ion transport and even cause the damage of components, leading to sharp capacity decay or failure of device. This can be further illustrated by the cross-sectional SEM images of above two configurations at bending states. When folded nearly to 180°, the all-in-one structure still remains intact integrated structure without relative displacement or detachment between two neighboring components (Figure 4c), while the stacked structure shows obvious gaps between the neighboring components (Figure 4d). Therefore, the all-in-one


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soft-packaged battery exhibits lower and nearly unchanged Rs (3.5 Ω) and Rct (71.7 Ω) under different bending states (Figure 4e). However, the stacked soft-packaged battery not only displays a larger Rs (5.1 Ω) and Rct (114.2 Ω) at flat state, but also exhibits the increased Rs and Rct of 6.2 Ω and 177.6 Ω under bending state, respectively. As a result, the all-in-one configuration is beneficial to the stable electrochemical performance of flexible Li-S batteries at bending states (Figure 4f). To more intuitively demonstrate the excellent flexibility of the all-in-one soft-packaged Li-S batteries, two all-in-one soft-packaged Li-S batteries were in series to light the “Lotus Light” with a white light-emitting diode (LED, 3.0 V) (Figure 4g). Impressively, when the batteries were rolled, twisted and even folded, the “Lotus Light” could be lit well and keep the similar brightness. The corresponding discharge-charge curves at 1.0 C are almost overlapped under different bending states, further indicating the electrochemical stability of the all-in-one soft-packaged devices (Figure S23, Supporting Information). After that, the soft-packaged batteries were recovered to the flat state and the “Lotus Light” still could be lightened up at a similar brightness. This further indicates that the flexible all-in-one Li-S battery is promising to serve as a practical flexible energy storage device. CONCLUSIONS In summary, a strategy combining slurry coating with vacuum evaporation methods was developed to achieve a flexible all-in-one integrated MWCNTs/S-MWCNTs/MnO2@PP@Li structure, which includes the cathode, interlayer, separator and anode of a Li-S battery. Its continuous seamless connection could effectively avoid the relative displacement or detachment between the neighboring components at different mechanical deformation to ensure the load and/or electron transfer capacity. Therefore, the all-in-one structure shows excellent flexibility


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and structural stability. More importantly, the as-prepared all-in-one soft-packaged Li-S battery exhibits fast electrochemical kinetics and stable electrochemical performance under different bending status, even in rolling, twisting or folding. EXPERIMENTAL SECTION Preparation of all-in-one structure: The 70 mg MnO2 nanowires, 20 mg MWCNTs and 10 mg polyvinylidene fluoride (PVDF) powder were dispersed in N-methyl-2-pyrrolidone (NMP) and stirred for 2 h to obtain the composite slurry. The slurry was poured and coated on one side of the PP separator (Celgard 2400) with a slot coating machine (MSK-AFA-HC100). The MWCNTs/MnO2 coating layer was formed on the PP separator after vacuum drying at 50°C for 12 h. Then, MWCNTs/S composite slurry composed of 70 mg commercial sulfur power with a particle size of 20-40 nm, 20 mg MWCNTs, 10 mg PVDF and NMP was directly poured and coated

onto

the

top

of

the

MWCNTs/MnO2@PP

structure.

The

MWCNTs/S-MWCNTs/MnO2@PP structure was obtained after vacuum drying at 50°C for 12 h. After that, Li layer was further deposited on the other side of PP separator in the MWCNTs/S-MWCNTs/MnO2@PP structure by a thermal evaporation system (ZJ-27/CF35, Reborn). The vacuum degree of evaporating chamber was less than 1× 10-4 Pa. The target temperature of heating source was controlled at about 600 °C by adjusting heating current. The deposition rate of lithium vapor was about 10 nm s-1. After that, the all-in-one structure was obtained and stored in the argon glove box. Fabrication of Li-S batteries: The Li-S batteries were assembled in an argon-filled glove box (Mikrouna Universal 2440/750). The large area all-in-one structure was easily cut into desired sizes to fabricate different types of Li-S batteries. The electrolyte was 1.0 mol L-1 lithium bis (triﬂuoromethanesulfonyl) imide (LiTFSI) in a mixed solution of 1,3-dioxolane (DOL) and


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1,2-dimethoxyethane (DME) (v/v = 1:1) with 1 wt% LiNO3 addition. The 2032 coin-type cells were assembled using the all-in-one wafers with a diameter of 16 mm. The soft-packaged Li-S batteries were assembled by sealing the all-in-one structure (5 cm × 4 cm) into an Al/plastic package with Al and Cu strip as cathode current collector and anode current collector, respectively. The conventional stacked Li-S batteries were assembled by sandwiching the PP separator and electrolyte between the isolated MWCNTs/S film, MWCNTs/MnO2 film and Li foil (about 10 μm). The corresponding electrolyte/sulfur (E/S) rate in the coin-type cells and soft-packaged Li-S batteries was about 30 and 150 μL mg-1, respectively. Characterizations and electrochemical measurements: The microstructures and morphologies of materials and integrated structure were characterized via field-emission SEM (JEOL JSM7500F). X-ray diffraction (XRD) patterns were collected to analyze the structure of the materials using a Rigaku MiniFlex600 instrument (Cu Kα radiation). And the Raman spectra were performed on the confocal Raman microscope (DXR, Thermo Fisher Scientific) with the operating wavelength of 633 nm. Thermogravimetric analysis (TGA) (Netzsch STA 449 F3 Jupiter analyzer) measurement was conducted to determine the mass fractions of sulfur and MnO2 in the integrated coating layer in an Ar flow and from room temperature to 900°C at a heating rate of 10°C min-1.The tensile tests were measured on a Dynamic Mechanical Analyzer (TA DMA Q800). The cyclic voltammetry curves were tested by the electrochemical workstation (CHI660E) at a scan rate of 0.1 mV s-1 in the window of 1.7-2.8 V. Galvanostatic charge/discharge tests of Li-S batteries were carried out by a battery test system (LAND, CT2001A) in the range of 1.7-2.8 V. The electrochemical impedance spectroscopy (EIS) of the devices was measured in the frequency range of 100 kHz to 10 mHz (Zahner-IM6ex).


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ASSOCIATED CONTENT The authors declare no competing financial interest.

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Figure S1-23 and Table S1-4 are included in Supporting Information. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by Ministry of Science and Technology of People’s Republic of China (2017YFA0206701), National Natural Science Foundation of China (21573116 and 51602218), Ministry of Education of China (B12015), Scientific Research Program of Tianjin Municipal Education Commission (2017KJ248), Natural Science Foundation of Tianjin Municipal Science and Technology Commission (18JCQNJC02400), and the Young Thousand Talents Program.


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Figure 1. (a) Schematic drawings illustrating the preparation of the all-in-one structure: (Ι) coating the MWCNTs/MnO2 slurry onto one side of PP separator; (II) coating the MWCNTs/S slurry onto the top of the MWCNTs/MnO2 coating layer; (III) depositing Li on the other side of the separator in the MWCNTs/S-MWCNTs/MnO2@PP structure. (b) Cross-sectional SEM image of the all-in-one structure and (c-f) corresponding elemental mappings of C, S and Mn. (g, h) Optical images of MWCNTs/S-MWCNTs/MnO2@PP structure. (i, j) Optical images of the all-in-one MWCNTs/S-MWCNTs/MnO2@PP@Li structure.


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Figure 2. SEM images of (a) MWCNTs/MnO2 coating layer, (b) MWCNTs/S coating layer and (c) Li layer. (d) Stress-strain curves of all-in-one structure of MWCNTs/S-MWCNTs/MnO2@PP and stacked structure of MWCNTs/S film, MWCNTs/MnO2 film and PP separator. (e) Mechanical model of the multilayer film structure bent into a cylindrical roll. (f) Normalized sheet resistance of all-in-one structure at different bending states, where R0 and L0 are the initial resistance and length of the structure, respectively; R and L are the resistance and the distance between two ends of the structure under different bending states, respectively. Insets: diagram of bending.


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Figure 3. (a) CV curves of the all-in-one Li-S battery between 1.7 and 2.8 V at a scan rate of 0.1 mV s-1. The electrochemical properties of the batteries with all-in-one, stacked and all-in-one without interlayer structures: (b) discharge-charge profiles at first cycle at 0.1 C, (c) rate performance, (d) EIS spectra, (e) long cycling performance at 1.0 C.


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Figure 4. (a) Discharge-charge profiles at first cycle of all-in-one soft-packaged Li-S battery at 0.1 C. (b) Cycling performance of flexible all-in-one and stacked soft-packaged Li-S batteries at 1C in flat and different bending states (with L/L0: 1, 0.8, 0.6, 0.4, 0.2, 1 and Scale bar: 2 cm). Cross-sectional SEM image of (c) all-in-one and (d) stacked structure under folding state. (e) EIS spectra of fresh all-in-one and stacked soft-packaged Li-S batteries at flat and bending states. (f) Comparison of the electrochemical performance between all-in-one and stacked batteries (C0.2nL is the second capacity at corresponding bending state, where n = 4, 3, 2, 1). (g) The optical images show that a “Lotus light” was lighten up by two all-in-one soft-packaged Li-S batteries in series under different bending states: initial flat, rolling, twisting and folding state, respectively.


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