Ultra-thin and Strong Electrospun Porous Fiber Separator - ACS

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Ultra-thin and Strong Electrospun Porous Fiber Separator Jiaolong Pan, Ze Zhang, Hai Zhang, Peipei Zhu, Junchao Wei, Jianxin Cai, Ji Yu, Nikhil A. Koratkar, and Zhenyu Yang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00855 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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ACS Applied Energy Materials

Ultra-thin and Strong Electrospun Porous Fiber Separator Jiao-Long Pana,#, Ze Zhanga,#, Hai Zhanga, Pei-Pei Zhua, Jun-Chao Weia, Jian-Xin Caib, Ji Yua, Nikhil Koratkarc,d*, and Zhen-Yu Yanga* a

School of Chemistry, Nanchang University, Nanchang, Jiangxi, 330031, P. R. China.

b

School of Resources and Environmental Science, Nanchang University, Nanchang, Jiangxi, 330031, P. R. China.

c

Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110, 8th Street, Troy, NY 12180, USA

d

Department of Materials Science and Engineering. Rensselaer Polytechnic Institute, 110, 8th Street, Troy, NY 12180, USA

#

J. L. Pan and Z. Zhang contributed equally to this work

*E-mail: [email protected] or [email protected]

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Abstract An ideal separator of lithium-ion battery (LIBs) should have a zero ionic resistance. Low ionic resistance (high ionic conductivity) will greatly help to realize very fast ion diffusion and super-high rate capability of LIBs. The most effective technique to achieve low ionic resistance of separator is to reduce its thickness or increase its porosity. Paradoxically, the low thickness and high porosity will inevitably decrease the mechanical strength of separators. Inspired by the hierarchical structures of abalone shell, we demonstrate in this work an ultra-thin silica-anchored layered (PVdF/PE/PVdF) porous fiber separator prepared via electrospinning. The separator displays both ultra-thin thickness (~20 µm thick) and high mechanical strength of ~11.2 MPa, as well as high porosity, which results in high electrolyte uptake (~380%) and ionic conductivity (~2.5 mS cm–1). When such thin separator was deployed in a LiFePO4/Li cell, and the cell can deliver an initial discharge capacity of 134.3 mAh g– 1

at a high rate of 10 C and maintain a capacity of 129.2 mAh g–1 after 300

charge-discharge cycles, showing excellent high-rate performance. More interestingly, this study demonstrates a pathway for the development of ultra-thin and high-mechanical-strength electrospun separators for high-rate Li-ion batteries.

Keywords: Abalone shells, Electrospinning; Ultra-thin; Strong; Fiber separator; Low ionic resistance; High rate Li-ion batteries

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1. Introduction The separator, as an electronically isolating layer between the cathode and anode to avoid an internal short circuit, becomes more and more important in the development of high power lithium-ion batteries (LIBs). An ideal separator should have a zero ionic resistance, which will help to realize very fast ion diffusion and super-high rate capability of LIBs. It is very difficult to perform a zero ionic resistance in practice, but a low ionic resistance (high ionic conductivity) can be achieved by a low thickness and high porosity of a separator. Paradoxically, the low thickness and high porosity will inevitably decrease the mechanical strength of the separators, resulting in safety hazard of LIBs. Therefore, a critical challenge in separator fabrication is to reduce its thickness and increase its porosity, while simultaneously maintaining its mechanical strength and thermal stability.1-3 Currently,

microporous

polyethylene

(PE),

polypropylene

(PP)

or

their

combinations are widely used as commercial separators for LIBs because of their low thickness, high electrical resistance, and chemical stability. Nevertheless, both PE and PP show low melting point, and easily shrink or even melt at elevated temperatures, which represents a significant safety hazard, especially in case of thermal runaway situations4-6. In addition, commercial polyolefin separators possess low porosity, poor wettability, and low electrolyte uptake.7,8 To overcome the aforementioned drawbacks of commercial separators, considerable efforts have been made to develop superior separators by ceramic-coatings.9-12 Specifically, the ceramic-coated modification effectively promotes the mechanical strength, thermal stability and electrolyte uptake 3



of the separators. However, the surface-coated ceramic materials tend to easily detach from the separator, resulting in deterioration in the performance as well as pore blockage. Recently, electrospinning technology is favorable to obtain nanofibers for energy storage.13-15 The fibrous electrospun-membranes have become an alternative option for separators due to their unique one-dimensional structure and high porosity,16-20 which ensures high electrolyte uptake and high ionic conductivity. More interestingly, the thickness of the electrospun-separator can be tailored ranging from tens of microns to hundreds of microns by adjusting the volume of the spinning solution. However, the biggest challenge has been met to prepare the separators with high mechanical strength by electrospinning techniques. In the past three decades, rigid biological materials, such as shells and bones, have been attractive as models for synthetic structural composites due to their unusual combinations of mechanical behaviors.21 Attempts at bio-mimetic structure of a nacreous layer of a mollusk shell have also shown reasonable success.22-24 There are three important features of mollusk shell that distinguished it from the others as shown in hierarchical structure of abalone shells25,26 (Figure S1): (i) non-mineralized “soft” structures that are composed of fibrous constituents, such as collagen, chitin, and other biopolymers; (ii) mineralized “hard” structure that consists of assembled composites of minerals, such as calcium carbonate and silica; (iii) the closely packed layered architecture. Interestingly, the building blocks of the rigid biological materials are primarily minerals and biopolymers, mostly in combination, and the intricate and 4


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ingenious hierarchical structures are responsible for the outstanding performance of each material. Inspired by the hierarchical structures of abalone shell, we have used poly(vinylidene fluoride) (PVdF) or PE (“soft” structure) and SiO2 nanoparticles (“hard” structure) in combination as the elemental building blocks to fabricate the ultra-thin SiO2-anchored layered (PVdF/PE/PVdF) porous fiber separators by simple electrospinning method. As expected, such electrospun ultra-thin layered separators display several advantages: (1) ultra-thin thickness of ~20 µm, less than that of any electrospun separators (~30 µm to ~300 µm) and the commercial layered separator (~25 µm); (2) high mechanical strength of ~11.2 MPa, ~3.5 times that of pure PVdF separator and ~1.3 times that of commercial PP separator; (3) highly porous structure, helping to display high electrolyte uptake capability (~380%) and high ionic conductivity (~2.5 mS cm–1); (4) unique layered (PVdF/PE/PVdF) structure, helping to reinforce the separator and arrest thermal runaway by cutting off the Li+ diffusion channel. As a result, an Li/LiFePO4 cell with as-prepared layered PVdF/PE/PVdF separator delivers a high initial discharge capacity of 134.3 mA h g–1 at 10 C rate, and the discharge capacity is maintained at 129.2 mA h g–1 after 300 charge/discharge cycles, indicating outstanding rate capability and cycle stability. 2. Experimental Section 2.1 Materials Commercial polyethylene (PE) and polypropylene (PP) separators (thickness, ~25 µm; porosity, 40%) were purchased from SK Energy Company (Seoul, Korea). 5



Poly(vinylidene fluoride) (PVdF, Mw= ~300000), polyethylene (PE, Mw= ~600000) were obtained from Shanghai Ofluorine Chemical Technology. SiO2 nanoparticles (NPs, particle diameter, ~100 nm; surface area, ~340 m2 g–1) was supplied from Evonic industries. The liquid electrolyte containing LiPF6 (1 mol L–1) in a mixture of ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/dimethyl carbonate (DMC) (1:1:1, by volume) was purchased from Guotai Huarong Company (Zhangjiagang, China). N,N-dimethylformamide (DMF), Ammonium bicarbonate (NH4HCO3) and n-butanol were purchased from Sigma Aldrich. LiFePO4 was obtained from Hydro-Qubec. All these reagents were used without further purification. 2.2 Preparation of SiO2-anchored PVdF/PE/PVdF layered separator The SiO2-anchored PVdF/PE/PVdF layered ultra-thin separator was prepared by electrospinning of a mixture of ammonium bicarbonate and SiO2 nanoparticles in a polymer solution. Firstly, a pretreatment of SiO2 nanoparticles is necessary to ensure their homogeneous dispersion in the polymer solution (see in Figure S2). In detail, SiO2 nanoparticles were washed several times with absolute ethyl alcohol, and dried in a vacuum oven at 80 oC for 2 h to completely remove the absorbed water. Then, the treated SiO2 nanoparticles were added into the polymer solution for the electrospinning process. Further, certain amount of sodium dodecyl benzene sulfonate surfactant (~1 wt.%) was added into the solution to ensure the homogeneous dispersion of SiO2 NPs. Typically, PVdF or PE were completely dissolved in DMF to obtain a 13 wt.% solution, respectively. Then, different amounts of NH4HCO3 (0, 5, 10, 15 and 20 wt.%) and treated SiO2 NPs (0, 5, 10, 15 and 20 wt.%) were added into 6


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the two polymer solutions successively, and stirred for 10 h. To further ensure the As for the electrospinning process, the layered polymer fibrous composite separator was obtained by successively spinning PVdF, PE and PVdF solution including NH4HCO3 and SiO2 NPs with a 20-gauge needle at a flow rate of ~0.8 ml h–1 for 3 h, respectively. The traveled distance of syringe needle was set as ~15 cm to control the thickness of the separators. The electrospinning distance, the applied voltage, and relative humidity were fixed at 30 cm, 21 kV, and 30%, respectively. Finally, the collected separator was dried at 80 oC for 24 h to remove residual solvent and achieve complete decomposition of NH4HCO3. The pure PVdF and PVdF/PE/PVdF separator was also obtained via the same preparation process without the addition of NH4HCO3 and SiO2 NPs. 2.3 Physical characterization The surface morphologies of the obtained separators were characterized by scanning electron microscope (SEM, FEI Quanta 200F) and transmission electron microscope (TEM, FEI Tecnai G2 F20). Mechanical properties were obtained using a universal testing machine (UTM4000, SUNS, Shenzhen). The porosities of the obtained separators were determined by using n-butanol uptake tests. Thermal shrinkage tests at various temperatures were carried out in an incubator (DHG 9011A, Jinghong Shanghai) for 1 h to evaluate the dimensional stability of the separators. The porosity (P) was calculated by using equation (1), where M1 and M2 are the weight of the separator before and after impregnation in n-butanol, respectively; ρBuOH is the density of n-butanol; r and d represents the radius and the thickness of the 7



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separators.

=

( )

∗ ∗

∗ 100%

(1)

Liquid electrolyte uptakes were measured by soaking the separators in the liquid electrolyte of 1 M LiPF6 in EC/EMC/DMC (1:1:1 in volume) at room temperature. The electrolyte uptake (EU) was calculated using equation (2), where W1 and W2 are the weights of the separators before and after immersion in the above electrolyte.

=

( )

∗ 100%

(2)

2.4 Electrochemical performance evaluation The ionic conductivities of liquid electrolyte-soaked separators were measured by electrochemical impedance spectroscopy (EIS) (Potentiostat/Galvanostat/ZRA, GAMRY). The impedance measurements were performed on liquid electrolytesoaked separators sandwiched between two stainless steel plates in the frequency range of 10 mHz to 100 kHz. The ionic conductivity (σ) was calculated using equation (3), where d is the separator thickness, S is the cross-sectional area, and Rb is the bulk resistance obtained from the Nyquist plots.

σ =

∗

(3)

The LiFePO4 cathode was prepared by blending LiFePO4 powder (80 wt.%), carbon black conductor (10 wt.%) and PVdF binder (10 wt.%). The charge/discharge tests of Li/LiFePO4 cells containing liquid electrolyte-soaked separators (the obtained PVdF/PE/PVdF separator and commercial PE separator) were carried out by using 2025-type cells with potential range of 4.2-2.5 V to evaluate the cycling performance. Different C-rates (0.5 C, 1 C, 3 C, 5 C, 10 C; 1 C = 170 mA g–1) were applied to 8


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evaluate the rate performance of the cell.

3. Results and Discussion As illustrated in Figure 1a, PVdF and PE solutions including NH4HCO3 and SiO2 NPs are electrospun successively to obtain the SiO2 anchored layered PVdF/PE/PVdF porous fiber separator. In biomimicry, three different contributions have been identified and are believed to operate synergistically to achieve high mechanical properties of the ultra-thin separator: (i) the “bricks and mortars” reinforcement effect of the SiO2 NPs in the polymer matrix; (ii) the mineral bridge (interfaces of layers) strongly attached together, and the asperities (SiO2) on the surface of layer could produce frictional resistance and strain hardening; (iii) energy is required for viscoelastic deformation (stretching and shearing) of the polymer matrix. These features will greatly help to perform the ultra-thin SiO2 anchored PVdF/PE/PVdF separator, while simultaneously reinforce its mechanical strength. Figure 1b shows the typical layered structure of the PVdF/PE/PVdF separator, in which SiO2 NPs are anchored in PVdF or PE fiber matrix and interlayers. A cross-section view of the obtained SiO2-anchored PVdF/PE/PVdF sandwiched separator is shown in Figure 1c and Figure S3. The thickness is calculated as ~20.1 µm, which to our knowledge is the smallest thickness for the electrospun separators reported and also smaller than that of commercial layered separators (25 µm). Furthermore, the obtained ultra-thin separator exhibits good flexibility as shown in Figure 1c inset.

9



Figure 1. Schematic diagrams of SiO2-anchored layered thin separator. (a) Fabrication process and (b) Layered structure, and (c) SEM image of the cross-section of thin separator. (Inset shows the digital photo of flexible thin separator)

Based on reducing the thickness of separator, more porosity has been created simultaneously to further lower ion resistance and increase ion conductivity of separator by NH4HCO3, a “sacrificial” pore-creating agent, due to the abundant small-molecule gases produced from its thermal decomposition at moderate temperature. Firstly, the influence of the added amount of NH4HCO3 on the porous structure of the separator is investigated. The SEM and TEM images of as-prepared separator modified with various NH4HCO3 contents (0, 5, 10, 15 and 20 wt.%) are presented in Figure 2. The pristine PVdF electrospun fibers show smooth surfaces and an average diameter of ~120 nm (Figure 2a and b). When NH4HCO3 is added, the fibers show increasingly rough surfaces. Even with NH4HCO3 content of 10 wt.% (Figure 2e and 2f), the continuous and porous fiber network is preserved. It should be noted that the polymer network collapses when the NH4HCO3 content is increased to 15 wt.% and above. As seen in Figure 2g-j, such high concentration of NH4HCO3 leads to a discontinuous structure of polymer fibers, due to the rapid evolution and escape of numerous gases. Besides, the influence of NH4HCO3 on PE fibers is also investigated 10


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and similar results were observed as can be seen in Figure S4. Nevertheless, with a reasonable amount (10 wt.%) of NH4HCO3, the polymer fibers with a continuous and porous structure can be obtained. Under such conditions, the polymer fibers are interwoven to form an integral and mechanically stable networked membrane structure.

Figure 2. SEM and TEM images of PVdF fibers of PVdF/PE/PVdF separator with various NH4HCO3 contents: (a, b) 0 wt.%; (c, d) 5 wt.%; (e, f) 10 wt.%; (g, h) 15 wt.%; (i, j) 20 wt.%.

To further enhance the mechanical properties of the separators, optional ratio of SiO2 NPs (“hard” structure) have been introduced subsequently into the polymer solutions (“soft” structure) with the optimized NH4HCO3 content of 10 wt.%. The low-resolution SEM images (Figure S5) of the separators show that the separators exhibit an interwoven structure of one-dimensional fibers. With increasing content of SiO2 NPs, the SiO2 particles can be observed on the exposed surfaces of polymer fibers. Figure 3 shows high-resolution SEM images of the obtained layered ultra-thin separator with different mass ratio of SiO2 NPs (5, 10, 15 and 20 wt.%). As seen from Figure 3a-d, with the increasing amount of SiO2 NPs, some SiO2 NPs might be partially exposed on the surface of PVdF fibers, thus improve the fiber surface roughness. In detail, as seen in Figure 3a, SiO2 NPs are homogeneously wrapped within the interior of the PVdF fibers for a NP loading of 5 wt.%, and the fibers 11



present a smooth surface. With increasing amount of added SiO2 NPs, the surface roughness of the electrospun fibers increases. However, even when the added amount of SiO2 NPs is raised to 15 wt.% (Figure 3c), only minimal SiO2 NPs can be found anchored on the fiber surfaces, indicating good dispersion of the NPs within the PVdF fibers. TEM images in Figure S6 provide a clear observation of PVdF fibers decorated with SiO2 NPs, which present as “dark dots” in the fibers based on the mass-thickness contrast.27 The electrospun fibers shows some bumps on the edges, which result from the partially exposed SiO2 NPs. The corresponding EDS results further suggest the coexistence of C, F, Si and O elements. Such superior dispersibility of the SiO2 NPs can be attributed to the adhesion effect between SiO2 and the polymer chains28,29, as well as the highly porous structure of PVdF fibers due to the utilization of NH4HCO3. Meanwhile, the exposed SiO2 NPs also promote electrolyte uptake of the separator due to interaction with ionic species in the electrolyte via Lewis acid/base interactions30,31 which increases the electrolyte wettability. However, we find that numerous SiO2 NPs get exposed on the fiber surfaces when 20 wt.% of SiO2 NPs are added, and the polymer fibers become messy and discontinuous (Figure 3d). Therefore, the optional amount of SiO2 NPs added into the polymer solutions is 15 wt.%. It should be noted that the addition of SiO2 NPs do not negatively influence the flexibility of the obtained PVdF/PE/PVdF porous separator as indicated in the inset of Figure 1c.

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e

12 with 15 wt% SiO2

PVdF/PE/PVdF

10

with 10 wt% SiO2

8

Stress (MPa)

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with 5 wt% SiO2

6 4 pure PVdF

pure PVdF/PE/PVdF

2 0 0

10

20

30

40

50

60

Strain (%)

Figure 3. SEM images of ultrathin PVdF/PE/PVdF separators anchoring with various amount of SiO2 NPs (10 wt.% NH4HCO3 ): (a) 5 wt. %, (b) 10 wt. %, (c) 15 wt. % and (d) 20 wt.%; (e) Stress-strain curves of pure PVdF, pure sandwiched PVdF/PE/PVdF and the PVdF/PE/PVdF separators with various amount of SiO2 NPs.

The high mechanical strength of separator with ultra-thin thickness is considered to be a vital attribute for the LIB safety, especially due to high stacking pressure in prismatic cells and tight winding of the electrodes in cylindrical cells. The typical stress strain curves for the tensile strength of the separators are presented in Figure 3e. As seen, the tensile strength for as-prepared PVdF/PE/PVdF separators are dramatically improved, once SiO2 NPs are introduced into the polymer separators. In detail, the tensile strength values of pure PVdF separator and pure sandwiched PVdF/PE/PVdF separator (~20 µm) are ~4.3 and ~5.7 MPa, while the SiO2-anchored PVdF/PE/PVdF separators offer tensile strength values of ~6.0, ~9.3 and ~11.2 MPa for different addition amount of SiO2 NPs and 10% NH4HCO3. Particularly, the PVdF/PE/PVdF separator with ~15 wt.% SiO2 still exhibits higher mechanical strength than that (~8.9 MPa) of the commercial PP separator (Figure S7). These results indicate that the PVdF/PE/PVdF porous fiber separator anchored by the incorporation of SiO2 NPs can be endowed with superior mechanical strength. The high mechanical strength can be ascribed to the reinforcement effect of the SiO2 NPs 13



in the polymer matrix, in which the polymers as non-mineralized “soft” structures could be strongly attached to the tiles with “hard” structure (SiO2 NPs) together, thus the “brick and cement” structures produce high mechanical strength of fibrous separator. More interestingly, the PVdF or PE layers with the exposed SiO2 NPs (the asperities) can be closely packed to greatly enhance frictional resistance and strain hardening, resulting in more mechanical strength of the electrospun SiO2-anchored PVdF/PE/PVdF layered separator. Apart from high mechanical strength, high thermal stability is also critical for separators used in LIBs. The morphological changes of commercial PE separator, PP separator and the PVdF/PE/PVdF separator (anchored with 15 wt.% SiO2) are investigated by transient temperature gradient in air for 1 h (the initial diameters of all the three separators are 18 mm). As seen in Figure 4a, the SiO2-anchored PVdF/PE/PVdF porous layered separator only suffers slight shrinkage after exposure to various temperatures and appears relatively undamaged. However, both PE separator and PP separator show severe shrinkage and burned surface at 170 oC, indicating that the superior thermal stability of SiO2-decorated PVdF/PE/PVdF separator. The calculated area shrinkages of the three separators are displayed in Figure 4b. The SiO2-decorated PVdF/PE/PVdF separator suffers almost no shrinkage at 110 oC and 140 oC, and the shrinkage at 170 oC is calculated as only ~3%, much lower than that of the PP separator (~18%) and PE separator (~30%), respectively. The large shrinkage of PP and PE separators can lead to the anode and cathode touching each other, resulting in electrical shorting and fire hazard. Further, we find 14


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that when compared to the PVdF/PE/PVdF separator with 5 wt.% or 10 wt.% SiO2, the PVdF/PE/PVdF separator with ~15 wt.% SiO2 is more stable, while indicates that the more the SiO2 NPs concentration in the polymer separator, the better is its dimensional and thermal stability at elevated temperatures.

Figure 4. Thermal properties of the as-obtained separators: (a) the visual shape change and (b) the calculated shrinkage of commercial PE, PP separator, and the PVdF/PE/PVdF separator anchored with different content of SiO2 at various temperatures; (c-f) SEM images of PVdF/PE/PVdF separator decorated with 15 wt.% SiO2 after heat treatment under various temperatures.

Figure 4c-f show the SEM images of the PVdF/PE/PVdF porous separator anchored with 15 wt.% SiO2 after thermal treatment at different temperatures. There is no significant change in the fibrous morphology of the fibers at 110 oC (Figure 4c), while the polymer fibers begin to distort and twist at 140°C (Figure 4d), and eventually melt to form a dense and smooth surface at 200°C (Figure 4f). For our sandwich layered structure comprised of the PE membrane clamped by two PVdF membranes, the PE membrane will melt before the PVdF and form a dense layer to close the channel for Li+ transport, while the PVdF membranes still retain their structural integrity preventing short-circuits of the anode and cathode. In other words, 15



the sandwiched structure provides a safety cutoff in case of a thermal runaway situation by cutting off the diffusion path of Li+ ions, therefore providing a safer battery separator. 400

100

(a)

400

200 60 10 wt.%

5 wt.%

15 wt.%

150 100

PVdF/PE/PVdF with SiO2

40

Electrolyte uptake (%)

250

Electrolyte uptake (%)

300

80

Porosity (%)

(b)

350

300

200 PVdF/PE/PVdF with 15 wt.% SiO2 PVdF/PE/PVdF with 10 wt.% SiO2 PVdF/PE/PVdF with 5 wt.% SiO2 PP

100

50 PP

0

0

0

30

60

90

120

150

180

210

240

Time (min)

20

3.0

(d)

200

(c)

2.5

150

6 4

100

2 0

0

0

2

4

6

8

10

PVdF/PE/PVdF with 15 wt% SiO2 PVdF/PE/PVdF with 10 wt% SiO2 PVdF/PE/PVdF with 5 wt% SiO2 PP

50

2.2

-1

Ionic conductivity (mS cm )

2.5 8

-Z``(Ω)

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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2.0

2.0 1.5 1.0 0.67 0.5 0.0

0

50

100

150

5 wt.%

200

PP

Z` (Ω )

10 wt.%

15 wt.%

PVdF/PE/PVdF with SiO2

Figure 5. The Electrolyte uptake and Ionic conductivity of SiO2-anchored PVdF/PE/PVdF porous separator and the commercial PP separator. (a) the data of porosity and electrolyte uptake, (b) the electrolyte uptake curves, (c) EIS profile, (d) the ionic conductivities at room temperature.

The porosity and electrolyte uptake of the separator play important roles in improving the electrochemical performance of LIBs. It is found that the addition of SiO2 NPs will reduce the porosity to some degree, but the SiO2-anchored PVdF/PE/PVdF porous separator (15 wt.% SiO2) still shows a high porosity value of ~58%, far more than that of 36% of commercial PP separator (Figure 5a). Whereas SiO2 NPs can improve the surface wettability and promote the electrolyte uptake, the SiO2-anchored PVdF/PE/PVdF porous separators offer electrolyte uptakes in the 16


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range of 280 to 380% accordingly as shown in Figure 5a-b, much higher than that of the commercial PP separator (~200%). The low porosity of the PP separator is the primary reason for its relatively low electrolyte uptake. It is known that the electrolyte uptake of the separators are greatly affected by their thickness. Impressively, the ultra-thin SiO2-anchored PVdF/PE/PVdF porous separator (~20 µm) provides larger electrolyte uptake capability than many polymer separator (~50 µm) and commercial PP separator (25 µm), which can be expected to perform high ionic conductivity. To confirm enhanced Li+ diffusion within the ultra-thin SiO2-anchored PVdF/PE/PVdF porous separator, the separators are fully soaked in the liquid electrolyte, and then placed between two stainless steel electrodes for electrochemical impedance spectroscopy (EIS) measurements. The obtained Nyquist plots are linear (Figure 5c), indicating that the conductivity mainly comes from ion conduction.32,33 The fitted bulk resistances (Rb) of the soaked ultra-thin SiO2-anchored PVdF/PE/PVdF porous separator are much lower than that of soaked PP separator, resulting in much higher ionic conductivities. The ionic conductivities at room temperature of the separators are calculated and presented in Figure 5d. Typically, the soaked ultra-thin SiO2-anchored PVdF/PE/PVdF porous separators show ionic conductivities of ~2.0, ~2.2, and ~2.5 mS cm–1 for different SiO2 NP content of 5 wt.%, 10 wt.% and 15 wt.%, respectively. The conductivities are nearly 3-4 times higher than that of soaked PP separator (~0.67 mS cm–1) since the ultra-thin SiO2-anchored PVdF/PE/PVdF porous separator exhibit much higher electrolyte uptake. Typical electrospun-separators display a thickness of > 60 µm, which is much 17



higher than that (25 µm) of commercial PP separators. However, the PVdF/PE/PVdF separator developed in this study shows a thickness of only ~20 µm, which is much lower than that of the reported electrospun-separators. As compared to similar works on the layered separators34-41 summarized in Figure 6, our PVdF/PE/PVdF separator shows the lowest thickness, the high ionic conductivity and the largest electrolyte uptake reported to date. Such desirable features can be ascribed to its highly porous microstructure and the incorporation of SiO2 NPs in the polymer membrane. These results demonstrate the potential of the ultra-thin SiO2-anchored PVdF/PE/PVdF porous separator in LIBs.

Figure 6. Comparison of the ultra-thin SiO2-anchored PVdF/PE/PVdF porous separator with other similar separators 34-41 in terms of their thickness, electrolyte uptake and ionic conductivity.

Further, we test the voltage stability of the separators by assembling coin cells with the separators sandwiched between metallic Li anode and stainless steel electrode. Figure 7 shows the linear sweep voltammograms of the separators at a scan rate of 1 mV s–1. There are no obvious current peak for both PP and PVdF/PE/PVdF 18


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separator from the open circuit voltage to ~ 5 V, which is believed to be suitable for LIBs system. Notably, the current of the PP separator increases at the potential of ~5.05 V, while the increasing current of PVdF/PE/PVdF separator occurs at the potential of ~5.40 V. These results clearly bring out the superior stability of the PVdF/PE/PVdF separator and its applicability for LIBs. 0.5 0.4 0.3

Current (mA)

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


PP PVdF/PE/PVdF with 15 wt% SiO2

0.2 0.1 0.0 -0.1 -0.2 0

1

2

3

4

5

6

+

Potential (V, vs. Li/Li ) Figure 7. The linear sweep voltammograms of commercial PP and the PVdF/PE/PVdF separator at a scan rate of 1 mV s–1.

The higher electrolyte uptake and ionic conductivities of the ultra-thin SiO2-anchored

PVdF/PE/PVdF

porous

separator

suggest

their

superior

electrochemical performance in LIBs. To verify this, 2025-type coin cells are assembled with various separators in a glove box with LiFePO4 as cathode and metallic Li as anode. As seen in Figure 8a, the cell with the PVdF/PE/PVdF separator shows the typical and stable charge and discharge potential plateaus of LiFePO4 at various C-rates from 0.5 C to 10 C (1 C = 170 mA g–1). The shrinkage of both charge and discharge potential plateaus with increasing C-rate are far more moderate when 19



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compared to that of the cell with a commercial PP separator (Figure 8b), indicating low polarization. Even at 10 C rate, the gap between charge and discharge potential plateaus is calculated as ~0.40 V for the cell with the SiO2-decorated PVdF/PE/PVdF separator, much lower than that (~0.82 V) of the cell with a commercial PP separator (Figure

8b).

These

results

vividly

demonstrate

that

the

SiO2-decorated

PVdF/PE/PVdF separator facilitates the electrochemical redox kinetics of the LiFePO4 cathode due to its high electrolyte uptake and ionic conductivity. Specifically, the cell with the PVdF/PE/PVdF separator (anchored with 15 wt.% SiO2) exhibits discharge capacities of 164.1, 156.2, 150.9, 144.3, and 134.8 mAh g–1 (Figure 8c) at C-rates of 0.5, 1, 3, 5, and 10 C, respectively. In addition, the capacity recovers to 158.5 mAh g–1 at 0.5 C rate after 25 cycles at various charge/discharge rates, higher than that (148.2 mA h g–1) of the cell with a PP separator. Furthermore, as shown in Figure 8d, the long-term cycle performance at 10 C of the cell with the PVdF/PE/PVdF separator (anchored by 15 wt.% SiO2) is obviously superior to that of the cell with PP separator. After 300 cycles, the discharge capacity remains 129.2 mAh g–1 for the cell with the SiO2-anchored PVdF/PE/PVdF porous separator, implying a low capacity decay (fade) rate of ~0.01% per cycle. However, the capacity of the cell with PP separator rapidly decreases to 113.6 mAh g–1 at the 300th cycle, and the capacity decay rate is calculated as ~0.05%. It has been reported that SiO2 NPs show excellent compatibility with Li anode by effectively stabilizing the Li electrodeposition, and thus inhibit the formation and growth of lithium dendrites.42-44 In the SiO2 NPs-decorated sandwiched separator, the well-anchored 20


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SiO2 NPs on the polymer fibers facilitate the good compatibility of the separator and Li anode, which can help improve the electrochemical performance of the battery. Therefore, the presence of SiO2 NPs in the PVdF/PE/PVdF separator not only helps to improve the physical and chemical properties of the PVdF/PE/PVdF separator, thereby improving the safety of LIBs, but also contributes to the enhanced rate capability and cycle stability.

(b)

4.5

PP separator

PVdF/PE/PVdF with 15 wt% SiO2

4.0

+

+

Potential (V, vs. Li/Li )

4.0

Potential (V, vs. Li/Li )

4.5

3.5

3.0 0.5 C 1C 3C 5C 10 C

2.5

3.5

3.0 0.5 C 1C 3C 5C 10 C

2.5

2.0

2.0 0

20

40

60

80

100

120

140

160

0

20

40

-1

100

120

140

160

150

(d)

170

0.5 C

160 1C 150

3C 5C

140

10 C

130

PVdF/PE/PVdF with 15 wt.% SiO2 PVdF/PE/PVdF with 10 wt.% SiO2 PVdF/PE/PVdF with 5 wt.% SiO2 PP

120 110

100 140

0.5 C

Discharge capacity (mAh g -1)

-1

80

Specific capacity (mA h g )

(c) 180

100

60

-1

Specific capacity (mA h g )

80 130 60

120 110

40 100 20 90

PVdF/PE/PVdF with 15 wt.% SiO2 PP 0

80

0

5

10

15

20

25

30

Coulombic efficiency (%)

(a)

Discharge capacity (mAh g )

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


0

30

60

90

120

150

180

210

240

270

300

Cycle number

Cycle number

Figure 8. Charge/discharge curves at different C-rates of the cell with (a) the as-prepared ultra-thin layered separator and (b) PP separator, (c) the rate capability and (d) cycle performance at 10 C rate of the cell with the separators containing different SiO2 content.

The soaked SiO2-anchored PVdF/PE/PVdF porous separators are also characterized by EIS measurement to verify their interfacial resistances. The Nyquist plots before and after cycling (Figure 9a) contain a depressed semicircle in high 21



frequency region and a sloped line in the low frequency region. Although the fitted charge-transfer resistances of both cells with PP and SiO2-anchored PVdF/PE/PVdF porous separator display slight increase after 300 cycles at 10 C, the cell cycled with SiO2-anchored PVdF/PE/PVdF porous separator shows much lower charge-transfer resistances. The lower resistances will promote faster electrochemical redox kinetics, resulting in improved rate capability and long-term cycle stability of the electrodes. Furthermore, the SiO2-anchored PVdF/PE/PVdF porous separator can still maintain the fibrous micro-morphology after repeated charge/discharge cycles (Figure 9b). Most importantly, SiO2 NPs are still anchored to the polymer fibers, and elemental Si shows homogeneous distribution (Figure 9e). This demonstrates that SiO2 NPs are effectively anchored with the polymer matrix by “brick and cement” pattern and do not get detached in spite of repeated charge-discharge cycling. The cross-section of SEM image in Figure 9f suggests that the thickness of the PVdF/PE/PVdF separator is ~40 µm after cycling. The increased thickness can be attributed to the common swelling property of polymer membranes upon soaking in the electrolyte. Notably, the nearly doubling of thickness confirms the excellent electrolyte uptake of the PVdF/PE/PVdF separator (see in Figure 5b). In addition, it is important that the separators maintain their mechanical properties to endure the long-term cycling. As shown in Figure 9g, both PP and the PVdF/PE/PVdF separators suffer from the decrease of mechanical strength as compared with their own pristine values, but the situation is more acceptable for our sandwiched PVdF/PE/PVdF separator. Specifically, in the sandwiched PVdF/PE/PVdF separator, the undetached SiO2 NPs 22


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are beneficial for maintaining a mechanical strength of ~9.8 MPa after 300 cycles. The value is much higher than that (5.8 MPa) of PP separator after 300 cycles. These results demonstrate the superiority of our ultra-thin PVdF/PE/PVdF separator when compared with the commercial PP separator. The high maintained mechanical strength of the separator during cycling ensures the safety of LIBs system.

g 10 8

Stress (MPa)

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


6 After 300 cycles

4

PP PVdF/PE/PVdF

2 0 0

5

10

15

20

25

30

35

40

Strain (%)

Figure 9. (a) EIS profiles of the cell with PP and the PVdF/PE/PVdF separator anchored by 15 wt.% SiO2 before cycling and after 300 cycles at 10 C rate; (b) SEM image of the PVdF/PE/PVdF separator decorated with ~15 wt.% SiO2 after 300 cycles and the corresponding element maps of (c) C, (d) F, and (e) Si; (f) cross-section SEM image of the PVdF/PE/PVdF separator decorated with ~15 wt.% SiO2 after 300 cycles; (g) mechanical strength of the separators after 300 cycles of charge/discharge.

3. Conclusion 23



1) Inspired by the hierarchical structures of abalone shell, an ultra-thin SiO2-anchored PVdF/PE/PVdF porous layered fibrous separator (~20 µm in thickness) has been fabricated via the electrospinning method. The reinforcement effect of “soft” structures (PVDF or PE), “hard” structure (SiO2 NPs) and layered heterogeneous interfaces (exposed asperity SiO2 NPs) produces high mechanical strength of this ultra-thin separator, although large porosity is obtained by a low-cost pore-creating agent (NH4HCO3). 2) The obtained electrospun ultra-thin separator shows high mechanical strength of ~11.2 MPa, high electrolyte uptake of ~380%, high ionic conductivity of ~2.5 mS cm-1, and promising thermal stability with an extremely low area shrinkage of ~3% at 170 oC, because of high melting temperature of PVdF, good electrochemical stability and high affinity to Li+ ions of SiO2 NPs. 3) The employed LiFePO4 cathode with such ultra-thin separator can deliver outstanding high-rate performance with an initial discharge capacity of 134.3 mA h g– 1

at 10 C, and remains 129.2 mAh g–1 after 300 cycles. Such an enhancement in

electrochemical performance can be attributed to the ultra-thin and porous separator structure and the “brick and cement” reinforcement effect between polymers and SiO2 NPs. In addition, the SiO2 NPs are tightly anchored on the fibers and remain intact during the repeated charge/discharge cycles. Therefore, our study offers a new approach for ultra-thin and strong separator materials for high-performance LIBs.

Acknowledgements 24


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Financial supports from the National Natural Science Foundation of China (No. 21263016, 21363015, 51662029) and the Jiangxi Province Research Program of Science and Technology (No. 2011BBE50023) are gratefully acknowledged. NK acknowledges support from the USA National Science Foundation (Awards 1435783, 1510828, 1608171).

ASSOCIATED CONTENT Supporting Information available: Illustration of the structure of tough biological materials, photograph of PVdF/DMF electrospinning solution, SEM and TEM images of PVdF fibers, stress-strain curve of PP separator.

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The table of contents entry and TOC figure

An ultra-thin and strong SiO2-anchored layered fiber separator is fabricated via electrospinning method inspired by the hierarchical structure of abalone shell for high-rate lithium-ion batteries.

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Ultra-thin and Strong Electrospun Porous Fiber Separator - ACS

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