Hybrid silica membranes with a polymer nanofiber skeleton and their application as lithium-ion battery separators

Accepted Manuscript Hybrid silica membranes with a polymer nanofiber skeleton and their application as lithium-ion battery separators Wenzheng Zheng, Yun Zhu, Bing Na, Ruihua Lv, Hesheng Liu, Weiping Li, Haiying Zhou PII:

S0266-3538(16)31545-7

DOI:

10.1016/j.compscitech.2017.03.022

Reference:

CSTE 6707

To appear in:

Composites Science and Technology

Received Date: 24 October 2016 Revised Date:

12 March 2017

Accepted Date: 17 March 2017

Please cite this article as: Zheng W, Zhu Y, Na B, Lv R, Liu H, Li W, Zhou H, Hybrid silica membranes with a polymer nanofiber skeleton and their application as lithium-ion battery separators, Composites Science and Technology (2017), doi: 10.1016/j.compscitech.2017.03.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Hybrid silica membranes with a polymer nanofiber skeleton and their application as lithium-ion battery separators

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Wenzheng Zheng#, Yun Zhu#, Bing Na*, Ruihua Lv, Hesheng Liu, Weiping Li, Haiying Zhou Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, School of

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Chemistry, Biology and Materials Science, East China University of Technology, Nanchang, 330013,

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People’s Republic of China

Abstract: Inorganic nanoparticles are frequently adopted to modify polymer porous membranes, aiming to fabricate high performance lithium-ion battery separators. However, the loading of inorganic nanoparticles is usually low, which limits their effects on the performance of modified polymer porous

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membranes. This study reports novel hybrid membranes with highly loaded silica nanoparticles of 67.5 wt% as a matrix. Good strength and flexibility of hybrid silica membranes are ensured by

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interpenetrated polymer nanofibers as a mechanical skeleton. The hybrid silica membranes have excellent dimensional stability at 200

, and high liquid electrolyte uptake and resultant ionic

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conductivity. The coin cells with hybrid silica membranes as separators show superior discharge capacity and cyclic performance even at a high temperature of 120

.

Keywords: hybrid composites; nanoparticles; electro-spinning

*

Correspondence author. Fax: +86 791 83897982. E-mail address: [email protected], [email protected]

# contributed equally 1

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1. Introduction Lithium-ion batteries consist of cathodes, anodes and sandwiched separators with soaking of liquid electrolytes [1, 2]. At least two demands are required for high performance separators. First, they should

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have high electrolyte uptake and affinity towards liquid electrolytes, enabling fast ion transportation and high ionic conductivity [3-7]. Second, superior dimensional stability at elevated temperatures is

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necessary especially in the harsh conditions [3-5]. It can prevent short-circuit between cathodes and anodes, and thus reduce the risk of fire-catching and even explosion [8-11]. The popular

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polyolefin-based separators have low liquid electrolyte affinity and inferior dimensional stability [12-14]. Therefore, extensive studies have been carrying out to fabricate high performance separators. Adding inorganic particles is promising to improve liquid electrolyte affinity and dimensional stability of polymer separators [15-19]. It can be realized by doping inorganic particles in the solutions

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for fabrication of polymer separators. By this approach inorganic particles are mostly dispersed and isolated in the polymer matrix. Thus, effect of inorganic particles on the performance of polymer

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separators is less outstanding. On the other hand, inorganic particles are directly incorporated into polymer separators by surface coating to produce multilayered composite membranes; and polymer

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separators serve as supporting layers for inorganic particles. It is widely adopted for post-modification of polyolefin-based separators and nonwoven fabrics [20-22]. The modified separators exhibit remarkable improvement in the liquid electrolyte affinity and dimensional stability, thus responsible for enhanced electrochemical performance of lithium-ion batteries. Obviously, it is inorganic particles that contribute to superior performance of modified separators. In the reported studies inorganic particles only account for a small portion of modified separators

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because of their low loadings [19-23]. Thus, one may wonder that membranes consisting of inorganic particles could be extremely intriguing as high performance separators of lithium-ion batteries. However, it is not easy to fabricate such a kind of separators with enough mechanical integrity solely from

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inorganic particles.

Fibrous membranes fabricated by electrospinning are widely used as separators due to their high

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porosity [4, 5, 24, 25]. Blending of inorganic particles in the solutions for electrospinning is an approach to modify fibrous membranes, aiming to improve separator performance. However,

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incorporation of inorganic particles in the solutions could disturb electrospinning process and their loading is usually low [26]. On the other hand, surface coating or doping of electrospun membranes by inorganic particles is alternative for modification of separators [18, 19, 27]. Unfortunately, inorganic particles are not penetrated throughout electrospun membranes and their loading is not high.

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In this contribution, silica nanoparticles with 67.5 wt% loading, interpenetrated with electrospun membranes, was achieved with aid of vacuum filtration. The electrospun membranes act as a skeleton to

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ensure good mechanical strength and flexibility. The hybrid silica membranes have excellent dimensional stability at 200

and high ionic conductivity after soaking a liquid electrolyte. Meanwhile,

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superior discharge capacity and cyclic performance are exhibited by coin cells with hybrid silica membranes as separators even at elevated temperatures. 2. Experimental Section 2.1. Materials The PVDF, supplied by Solvay, USA, had a melt flow index of 6 g/10 min (230 ℃, 5 kg). The PAN, with a number average molecular weight of about 50 kg/mol, was purchased from Yuyao Suhe Plastics

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Co. Ltd, China. Hydrophilic silica nanoparticles were obtained from Evonik Industries, Germany. They had an average particle size of 12 nm and a specific area of 200 m2/g, respectively. The liquid electrolyte was a 1M LiPF6 solution with ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl

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carbonate (EMC) (1:1:1 by volume) as solvents. Other reagents and chemicals were also commercially available.

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2.2. Preparation of nanofibrous membranes

PVDF and PAN with a weight ratio of 1:1 were co-dissolved in N, N-dimethylformamide at 80 ℃

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to produce homogeneous solutions with a concentration of 0.3 g/mL (based on the total amount of PVDF and PAN). The above solution was loaded in a syringe equipped with a metal needle, and pushed out at a constant flow rate. Under an applied voltage of 15 kV the solution was transformed into nanofibers after solvent evaporation. The nanofibers were collected by an aluminum foil that was 15 cm

electrospinning.

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away from the needle. As a result, nanofibrous membranes were obtained through so-called solution

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2.3. Fabrication of hybrid silica membranes

The resultant nanofibrous membranes were thoroughly penetrated by a silica nanoparticle

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suspension under vacuum at 0.09 MPa to produce hybrid silica membranes. The suspension was obtained by dispersion of silica nanoparticles in acetone with a concentration of 0.5 % (w/v) under intensive ultrasonication. A certain amount of PVDF was added in the suspension to improve adhesion among silica nanoparticles; and the ratio of PVDF to silica nanoparticles was 1/10 (by weight). Afterwards, the hybrid silica membranes were vacuum dried at 60 solvents.

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overnight to remove residual

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2.4. Characterizations Morphological observation was carried out by a Nova NanoSEM 450 scanning electron microscope (SEM). Cross-section of samples was obtained by cryo-fracture in liquid nitrogen; and before

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measurements a thin gold layer was sputtered. Mechanical properties were measured by a universal testing machine with a cross-head rate of 5 mm/min at room temperature. The membranes were punched

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into dog-bone specimens with a gauge length of 6 mm and a width of 4 mm. Dimensional stability was checked by placing membranes in an oven at 200

for 0.5 h. Electrolyte uptake was determined from

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the weight difference before and after soaking in the electrolyte for 4 h. Electrolyte wettability of membranes was evaluated by placing a small drop of the electrolyte on them. Ionic conductivity was performed on the membranes with the electrolyte sandwiched between two stainless steel plates with aid of a Princeton PARSTAT 2273 electrochemical workstation. Impedance spectra were collected over the Electrochemical stability was determined by liner sweep

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frequency range from 1 Hz to 1 MHz.

voltammetry measurements with a scanning rate of 5 mV/s on the electrochemical workstation at room

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temperature. The electrolyte soaked membranes were sandwiched between a stainless steel plate and a lithium plate. The electrochemical performance of coin cells was measured by a Land CT2001A cell

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testing system. The coin cells were charged up to 4.2 V and then discharged to 2.5 V at various current densities from 0.2 C to 3C. The coin cells consisted of a cathode with a LiFePO4 mixture, an anode with a lithium metal and an electrolyte soaked membrane between them. The LiFePO4 mixture was prepared by mixing LiFePO4 (80 wt %), Super-P (10 wt %) and a cell-grade PVDF (10 wt %) in N-methyl-2-pyrrolidone as a solvent. The mixture was cast onto aluminum foils and then cut into circles after vacuum drying. Of note, all samples for above electrochemical measurements were assembled in

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an argon-filled glove box to avoid possible decomposition of the electrolyte. 3. Results and Discussion To produce hybrid membrane with highly loaded silica nanoparticles, in this case vacuum filtration

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of a silica suspension by nanofibrous membranes produced from solution electrospinning was adopted. The nanofibers act as a mechanical skeleton to ensure good strength and flexibility of hybrid membranes

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with silica nanoparticles as a matrix. Figure 1 gives the schematic representation of fabrication of hybrid silica membranes with a nanofiber skeleton.

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Nanofibrous membranes consisting of numerous nanofibers with diameters of about 600 nm were produced from PVDF/PAN blend solution by solution electrospinning (Figure 2a-b). Loose stacking of the nanofibers results in abundant interconnected micropores among them, which provides an opportunity to accommodate silica nanoparticles. With aid of vacuum filtration, the micropores were

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filled by silica nanoparticles; thus hybrid silica membranes with the nanofibers as a mechanical skeleton were obtained. As demonstrated by SEM micrographs in Figure 2c-f, silica nanoparticles are thoroughly

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penetrated the whole cross-section of resultant hybrid membranes to form a continuous matrix. This morphology is totally different from the reported composite membranes by surface coating where

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inorganic particles prevailed adjacent the surface of polymer microporous membranes [19]. Correspondingly, the loading of silica nanoparticles in the resultant hybrid membranes reaches 67.5 wt%, determined from the weight difference before and after immersion in a 4 % (v/v) HF aqueous solution at room temperature to completely remove silica nanoparticles. It is much higher than that of hybrid membranes produced by simultaneous electrospraying and elecrospinning [28]. On the other hand, the packing of silica nanoparticles is not dense, yielding profuse nanopores among them. It could ensure ion

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transportation through hybrid silica membranes while as lithium-ion battery separators. Good mechanical integrity is exhibited by hybrid silica membranes, albeit of high loading of silica nanoparticles. The hybrid silica membranes can be bended or twisted, during which little detaching of

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silica nanoparticles occurs (Figure 3a). It is advantageous over multilayered composite membranes where inorganic particles are not tightly bonded with the supported microporous membranes. Moreover,

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hybrid silica membranes can be stretched to a strain of more than 100% with a fracture strength over 10 MPa (Figure 3b). It can meet the requirements for assembly of the hybrid silica membranes as

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lithium-ion battery separators. Of course, the interpenetrated nanofibers are responsible for reinforcement of the hybrid silica membranes, while the mechanical properties of nanofibrous membranes themselves are taken into account.

Figure 4 depicts optical photographs of hybrid silica membranes before and after heat treatment at for 0.5 h. For comparison, those of nanofibrous membranes are also included as references.

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200

Superior dimensional stability is exhibited by hybrid silica membranes, as compared to that of

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nanofibrous membranes. The interpenetrated silica nanoparticles, as a rigid skeleton, prevent thermal shrinkage of hybrid membranes. Note that dimensional stability of hybrid silica membranes is much

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superior to that of composite membranes where inorganic particles are not interpenetrated. Superior dimensional stability of hybrid silica membranes could ensure high safety of lithium-ion batteries in the harsh conditions.

The hybrid silica membranes have good affinity towards the liquid electrolyte, as do the nanofibrous membranes. As demonstrated by optical photographs as insets in Figure 5, the liquid electrolyte is completely spread on both hybrid silica membranes and nanofibrous membranes. It should

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be correlated with high polarity of PVDF/PAN nanofibers; and the presence of silica nanoparticles seems to have little effect on the wettabliity of the liquid electrolyte. On the other hand, liquid electrolyte uptake of hybrid silica membranes is higher than that of nanofibrous membranes (Figure 5).

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The nanopores among silica nanoparticles with high surface area should be responsible for it [29]. Ionic conductivity is very critical for application of membranes as lithium-ion battery separators.

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Figure 6a shows temperature dependent electrochemical impendence spectra of hybrid silica membranes as examples. The Nyquist plots at the intercept with real axis in the high-frequency region are decreased

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with temperatures. It corresponds to the reduced resistance with temperature increment, as a result of enhanced ion transportation in the liquid electrolyte. Ionic conductivity was deduced from resistance; and corresponding results are given in Figure 6b. At each temperature ionic conductivity of hybrid silica membranes soaked with the liquid electrolyte are higher than that of nanofibrous counterparts. For

nanofibrous membranes at 20

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instance, ionic conductivity of 1.68 and 1.50 mS/cm is exhibited by hybrid silica membranes and , respectively. The improved ionic conductivity in the hybrid silica

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membranes arises from enhanced liquid electrolyte uptake. On the other hand, temperature dependent ionic conductivity can be described by Arrhenius formula (the inset in Figure 6b). The activation energy,

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represented by the slope of fitted lines, is nearly same for hybrid silica membranes and nanofibrous membranes. It makes sense that the improved ionic conductivity at elevated temperatures is mainly contributed by the activated ion transportation in the liquid electrolyte rather than by the separators. Electrochemical window of hybrid silica membranes were evaluated; and the corresponding results are given in Figure 7. The electrochemical stability of hybrid silica membranes can persist up to about 5 V, above which steep increase in the current occurs. It is comparable to that of nanofibrous membranes.

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The electrochemical stability can ensure no decomposition of hybrid silica membranes during operation of lithium-ion batteries within a voltage range between 2.5 and 4.2 V. Electrochemical performance of coin cells with hybrid silica membranes as separators were

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performed at room temperature. The nanofibrous membranes were adopted as reference separators. Figure 8a-b compares the 1st and 100th charge and discharge curves at 1 C-rate. Good reproducibility

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with respect to charge and discharge is observed, irrespective of the type of separators. As further demonstrated in Figure 8c, there is little decay in the discharge capacity for both kinds of coin cells upon

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charge-discharge over 100 cycles at 1 C-rate. It corresponds to excellent cyclic performance of assembled coin cells with the electrochemically stable separators. On the other hand, discharge capacity shows some difference among coin cells with two types of separators. Higher discharge capacity is observed for coin cells with hybrid silica membranes as separators at each cycle. It is about 5 mAh/g

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higher than that of coin cells with nanofibrous membranes as separators. Meanwhile, superior C-rate performance is also achieved for coin cells with hybrid silica membranes as separators (Figure 8d). At

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each C-rate higher discharge capacity is observed for coin cells with hybrid silica membranes as separators. The improved electrochemical performance in the coin cells with hybrid silica membranes as

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separators is basically correlated with their high liquid electrolyte uptake and ionic conductivity [18, 30]. Excellent dimensional stability could ensure operation of coin cells with hybrid silica membranes as separators at elevated temperatures. To illustrate this, electrochemical measurements on coin cells were carried out at 60 and 120

, respectively. Commercially available polyolefin-based Celgard 2400

membranes with low dimensional stability were used as references. Coin cells with three kinds of separators can be reversibly charged and discharged at 60

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where little change in the dimensions is

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expected for separators. As shown in Figure 9a as an example, good reproducibility in the charge and discharge is exhibited by coin cells with hybrid silica membranes over 80 cycles at 1 C-rate. Figure 9b collects cyclic discharge capacity for three kinds of separators at 60

. Coin cells with hybrid silica

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membranes as separators show a discharge capacity of about 149 mAh/g during entire cyclic measurements. As a comparison, coin cells with Celgard 2400 membranes has a low discharge capacity, discharge

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and the value is decayed with cycle numbers to some extent. On the other hand, at 120

capacity of coin cells with hybrid silica membranes is about 152 mAh/g, and the discharge capacity

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remains nearly intact during cycles (Figure 9c-d). Note that improved discharge capacity arises from enhanced ionic conductivity at elevated temperatures. In contrast, coin cells with Celgard 2400 membranes can not be stably charged up to 4.2 V in the first cycle(Figure 9c). As a result, discharge becomes impossible and the discharge capacity is zero. It could be caused by thermal shrinkage of

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micropores in the Celgard 2400 membranes with low dimensional stability at 120

. The above results

clearly demonstrate that hybrid silica membranes can ensure excellent performance of assembled coin

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cells at elevated temperatures due to their superior dimensional stability. In combination with high discharge capacity, hybrid silica membranes could be suitable as high performance separators of

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lithium-ion batteries. 4. Conclusion

Hybrid membranes with silica nanoparticles as a matrix for lithium-ion separators have been successfully fabricated. The interpenetrated polymer nanofibers act as a skeleton to ensure good mechanical strength and flexibility. The silica nanoparticles in the hybrid membranes contribute to excellent dimensional stability and high liquid electrolyte uptake. As a result, coin cells with hybrid

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membranes as separators show superior discharge capacity and cyclic performance even at elevated temperatures. The hybrid silica membranes have great potentials as high performance separators of lithium-ion batteries. In addition, our approach can be extended to fabricate other hybrid membranes by

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choosing various types of inorganic nanoparticles and polymer nanofibers.

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Acknowledgements

This work is financially supported by the project of Jiangxi Province Advantageous Science and

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Technology Innovation Team (20153BCB24001) and the National Natural Science Foundation of China (No. 51163001).

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Figure 1 Schematic diagram of fabrication of hybrid silica membranes with a polymer nanofiber

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skeleton.

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Figure 2 SEM micrographs of (a, b) nanofibrous membranes and (c-f) hybrid silica membranes at low and high magnifications, respectively. (a-d) surface, (e, f) cross-section.

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(b) mechanical response of hybrid silica membranes and nanofibrous membranes under stretching.

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Figure 4 Optical photographs of (a, c) nanofibrous membranes and (b, d) hybrid silica membranes (a, b)

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Figure 5 Electrolyte uptake of nanofibrous membranes and hybrid silica membranes. Insets are optical

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Figure 6 (a) electrochemical impendence spectra of hybrid silica membranes soaked with the liquid electrolyte with respect to temperatures, (b) temperature dependent ionic conductivity for hybrid silica membranes and nanofibrous membranes. In (b) Arrhenius plots of ionic conductivity is included as an inset.

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160

Figure 8 Electrochemical performance of lithium-ion batteries with hybrid silica membranes and

AC C

nanofibrous membranes as separators at room temperature: (a) 1st and 100th charge and discharge curves at 1 C-rate for (a) hybrid silica membranes and (b) nanofibrous membranes, (c) cyclic performance at 1 C-rate, (d) C-rate performance.

23

Voltage (V)

4.0 3.6 3.2 2.8

1st 80th

2.4 0

20

40

60

80

160

b

140

RI PT

a

120

100

hybrid silica membrane nanofibrous membrane Celgard 2400 membrane

80 0

100 120 140 160

20

Capacity (mAh/g)

3.6 3.2

hybrid silica membrane Celgard 2400 membrane

2.4 0

40

80

TE D

Voltage (V)

4.0

2.8

120

180

d

M AN U

c

40

160

60

80

Cycle numbers

Discharge capacity (mAh/g)

4.4

SC

4.4

Discharge capacity (mAh/g)

ACCEPTED MANUSCRIPT

150 120

90 60 30

hybrid silica membrane nanofibrious membrane Celgard 2400 membrane

0

200

Capacity (mAh/g)

0

5

10

15

20

25

30

Cycle numbers

EP

Figure 9 Electrochemical performance of lithium-ion batteries with 1 C-rate at elevated temperatures: (a) 1st and 80th charge and discharge curves for hybrid silica membranes at 60

, (c) 1st charge and discharge curves for hybrid silica membranes and Celgard 2400

AC C

capacity at 60

, (b) cyclic discharge

membranes at 120

, (d) cyclic discharge capacity at 120

24

.