Contents lists available at ScienceDirect

Journal of Membrane Science

journal homepage: www.elsevier.com/locate/memsci

A nano-silica modified polyimide nanofiber separator with enhancedthermal and wetting properties for high safety lithium-ion batteries

Ying Wang, Suqing Wang⁎, Junqi Fang, Liang-Xin Ding, Haihui Wang⁎

School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China

A R T I C L E I N F O

Keywords:Lithium-ion batteriesElectrolyte wettabilitySafetySeparatorLiMn2O4

A B S T R A C T

The commercial polyolefin separators still possess two well-known drawbacks: poor wettability and thermalstability. Herein, a thin and lightweight silica filled in polyimide (PI) nanofibers membrane is prepared byelectrospinning. Without any binders, the nano-silica particles are firmly embedded in the PI nanofibers withhigh structure stability. The PI-SiO2 membrane with high porosity (90%) presents enhanced conductivity due tothe excellent electrolyte wettability and large electrolyte uptake (about 2400%). In addition, the PI-SiO2

membrane displays good mechanical flexibility and enhanced thermal stability up to 250 °C, which significantlyimprove the safety of lithium-ion batteries when used as a separator. The LiMn2O4/Li cell with the PI-SiO2

separator exhibits highly improved rate capability and cycling stability at different temperatures (25 °C and55 °C), which make PI-SiO2 membrane as a promising secure separator candidate for high-performance andsafety lithium-ion batteries.

1. Introduction

Due to high energy density, long cycle life, and environmentalfriendliness [1–3], lithium-ion batteries (LIBs) not only successfullyapplied to cell phones, laptops and digital cameras but also garneredthe most promising candidates for electric vehicles and energy storagesystems (ESSs) [4–6]. In order to satisfy expanded applications ofbatteries, reliable electrochemical property and reinforced safetytolerance are essentially required. Separator plays the vital role ofsafety by preventing physical contact of the cathode and anodeelectrodes as well as influencing the transfer of lithium ions insidethe cell [7]. Current commercial separators for LIBs are porouspolyolefin membranes, which made from polyethylene (PE) or/andpolypropylene (PP). Those separators are reliable for most portableelectronic applications due to their good chemical stability withelectrolyte and mechanical strength [8,9]. However, polyolefin separa-tors have two intrinsic limitations. One is the low porosity whichinduces the poor wettability with organic electrolyte and generates highionic conduction resistance in the interfacial layer of separator andelectrodes [9,10]. The other is the unsatisfactory thermal stability dueto the low melting point of polyolefin, which leads to an internal shortcircuit between electrodes and even causes a fire or explosion atelevated temperatures [11].

Recently, numerous approaches have been adopted to overcome theaforementioned problems. For example, previous studies have demon-

strated that grafting polymers (poly(ethylene oxide) and poly(methyl-methacrylate)) on the polyolefin separators could generate a stablecoating layer and effectively enhanced the wettability due to the highpolarity and affinity between electrolyte and polymer [12,13]. How-ever, this method reduces porosity of the separator and induces poorelectrolyte uptake. The emergence of non-woven membranes fabricatedby electrospinning can highly increase the porosity of the separator andenhance the electrolyte uptake [14–16], but the poor thermal stabilitystill limits their applications at high temperature. Generally, inorganicceramic particles which possess the merits of thermal stability andhydrophilicity have been introduced into polymer separators [17]. Forinstance, Kim et al. developed a ceramic-coated polyethylene (PE)separator using dip-coating method, which achieved good thermalstability (160 °C) and wettability for liquid electrolyte [18]. Yanget al. synthesized a SiO2/Al2O3-coated electrospun polyimide fibrousseparator with high porosity and electrolyte uptake, exhibited goodelectrochemical performance [19]. Unfortunately, the introduction ofinorganic coating layer inevitably increased the thickness and weight ofthe separator [17]. More seriously, the binders used to form the ceramiccoating layer showed a high shrinkage at elevated temperature,resulting in particles shedding from separators and influencing thefilm-forming properties [20].

Herein, we developed a novel flexible thin PI-SiO2 compositemembrane integrating with the comprehensive merits of high electro-lyte affinity, excellent dimensional and thermal stability. Because of the

http://dx.doi.org/10.1016/j.memsci.2017.05.023Received 15 January 2017; Received in revised form 4 May 2017; Accepted 7 May 2017

⁎ Corresponding authors.E-mail addresses: cesqwang@scut.edu.cn (S. Wang), hhwang@scut.edu.cn (H. Wang).

Journal of Membrane Science 537 (2017) 248–254

Available online 08 May 20170376-7388/ © 2017 Elsevier B.V. All rights reserved.

MARK

slight amounts of SiO2 nanoparticles tightly embedded in the polyimidenanofibers, the membrane shows a negligible thickness and weightincrease, and simultaneously maintains an outstanding structuralstability. Notably, the PI-SiO2 membrane with high polarity andporosity shows significantly enhanced electrolyte wettability, and theneffectively facilitates the transport of lithium ions. More importantly,due to the excellent mechanical and heat tolerance of SiO2 and PI[21–23], the PI-SiO2 hybrid membrane displays excellent dimensionalflexibility and good thermal stability. The LiMn2O4/Li cell using PI-SiO2

separator exhibits excellent electrochemical properties with better ratecapability (80 mA h g−1 at 5 C) and more stable cycling performance,surpassing that of using commercial PP separator.

2. Experimental section

2.1. Preparation of membranes

SiO2 with particle size of 15 nm was purchased from Alfa Aesar,4,4′-oxydianiline (ODA), pyromelliticdianhydride (PMDA), and di-methylformamide (DMF) were supplied by Sinopharm ChemicalReagent Co., Ltd, China. ODA (7.924 g, 0.039 mol) was first dissolvedin 95 ml DMF. After ODA completely dissolved, PMDA (8.805 g,0.04 mol) was divided into four equal parts and added into the systemstep by step with intense mechanical stirring at 0 °C for 3 h. Then SiO2

nanoparticles (0.836 g) was dispersed in DMF (5 ml) and added into thesystem to obtain pristine polyamic acid (PAA) solution containing 5 wt% SiO2. Electrospinning process was conducted at 15 kV using a syringewith a stainless steel needle with a diameter of 0.23 mm, the workingdistance was set as 15 cm. The as-spun PAA-SiO2 membrane wasimidized at high temperature to obtain the PI-SiO2 membrane usingthe following program: heating up to 100, 200, and 300 °C at a rate of2 °C min−1 and isotherming at each temperature for 2 h in vacuum. Forcomparison, the PI membrane was fabricated without adding of SiO2

nanoparticles.

2.2. Material characterizations

The surface and cross-sectional morphologies were characterizedusing scanning electron microscope (SEM, Hitachi SU8000) andtransmission electron microscopy (TEM, JEOL 2100F). Elementalanalysis was performed on an Elementar Vario EL cube.Thermogravimetric (TG) analysis was scanned from 25 to 1000 °C ata heating rate of 10 °C min−1 under N2 atmosphere using a NETZSCHSTA44C. Differential scanning calorimetry (DSC) measurement wascarried out from 30 to 500 °C at a heating rate of 10 °C min−1 under N2

atmosphere (NETZSCH DSC 214). The air permeability of separator wascharacterized by a Gurley-type densometer (4110N, Gurley).Mechanical properties of membranes (membrane size: 20 mm×40 mm)were tested using Instron-5565 at a stretching speed of 1 mm min−1.The wetting property of the separator was tested using a contact angletester (Dataphysics OCA40 Micro). Electrolyte immersion-height testwas evaluated by measuring the absorbed electrolyte height of separa-tor at room temperature for 1 h. The porosity of the membrane wastested by a liquid absorption and calculated according to the equation:

Porosity W WρV

= −w d

(1)

Where Wd is the weight of dry separator, Ww is the weight of theseparator absorbed hexadecane for 1 h, while ρ and V represent thedensity of the hexadecane and the volume of the separator, respectively[24].

The electrolyte uptake and retention of the separator were calcu-lated by weighting the separator. W0 is the pristine weight of separator.After soaking in the electrolyte for 1 h, the separator was taken out andweighed (W1) again. Last, the separator was stored at 50 °C andmeasuring the equilibrium weights (Wx) for different periods of time.

The electrolyte retention and electrolyte uptake of the separator werecalculated as follows [25]:

Electrolyte retention W WW W

= −−

× 100%x 0

1 0 (2)

Electrolyte uptake W WW

= − × 100%1 0

0 (3)

To measure the ionic conductivity, the separator with electrolytewas tested by sandwiching between two stainless steel (SS) electrodes.The ionic conductivity was calculated as follows:

σ dR S

=⋅b (4)

Where d is the thickness of the separator, S is the area of SS electrodes,the bulk resistance (Rb) of the separator was obtained from electro-chemical impedance spectroscopy (EIS) by Zahner IM6ex electroche-mical workstation with the frequency range from100 kHz to 1 Hz [25].

2.3. Electrochemical characterizations

In order to check the electrochemical stability of membranes, thelinear sweep voltammetry (LSV) was conducted. The membrane wassandwiched between lithium metal counter electrode and stainless steelworking electrode at a scan rate of 1 mV s−1 from the open circuitpotential (positive scan and negative scan, respectively). TheCR2025coin cell was assembled in an argon-filled glove box. Thecathode was prepared by coating a slurry consisting of 80 wt%LiMn2O4 (Huizhou Desay Battery Technology Co., Ltd.), 10 wt% superP and 10 wt% PVDF in N-methyl-2-pyrrolidone (NMP, SinopharmChemical Reagent Co., Ltd, China) on the aluminum foil and vacuumdried at 80 °C for 8 h. The electrolyte was 1 M LiPF6/ethylene carbonate(EC) + dimethyl carbonate (DEC) (1:1, v/v). The discharge and chargemeasurements were conducted on a NEWARE battery tester (Shenzhen,China) in a voltage range of 3.5–4.3 V (vs. Li+/Li).

3. Results and discussion

Fig. 1 shows the morphologies of the PI and PI-SiO2 membranes. Alarge area membrane of more than 30 cm×11 cm can be easilyfabricated, which is beneficial to practical applications (Fig. S1). Thecross-sectional SEM images (Fig. 1a and d) show that the averagethickness of PI and PI-SiO2 membranes is about 20 µm. The thicknesscan be easily adjusted by electrospinning time. Both PI and PI-SiO2

membranes feature interconnected nanofibrous structure and thediameter of the nanofibers is about 250 nm (Fig. 1b and e). The high-magnification SEM image and TEM image of the PI-SiO2 nanofiber(inset of Fig. 1e and f) demonstrates that the SiO2 nanoparticles almosthomogeneously scatter in the PI nanofibers. Furthermore, elementalmapping results of the PI-SiO2 membrane (Fig. S2) present a uniformdistribution of elements. To verify the lightweight of PI-SiO2 mem-brane, the mass densities of the membranes are tested and shown inTable S1. The PI-SiO2 membrane exhibits much lower mass density thanPP membrane.

The porosity of the membrane was tested by liquid absorptionmethod [24]. The porosities of PI (91%) and PI-SiO2 membranes (90%)are much higher than that of PP membrane (45%), owing to the well-developed pores formed by interconnected nanofibrous structure. Theair permeability reflects the mobility and transportation ability of thelithium ions in the membranes, which can be represented by the Gurleyvalue [9]. It should be noted that the Gurley values of PI and PI-SiO2

membranes (5 s per 100 cm3) are much lower than PP membrane (15 sper 100 cm3) under pressure of 0.02 MPa, verifying the multi-porousstructure of the membranes and a fast transportation of lithium ions inthe PI and PI-SiO2 membranes.

Contact angle measurement with liquid organic electrolyte was

Y. Wang et al. Journal of Membrane Science 537 (2017) 248–254

249

conducted to characterize the electrolyte wettability of membranes(Fig. 2a-c). After 2 s, PI and PI-SiO2 membranes quickly absorbed theelectrolyte with contact angle of 10° and 0°, respectively, while the PPmembrane was hardly wetted by electrolyte with contact angle of 44°.The electrolyte wettability was also measured by testing the electrolyteimmersion-height (Fig. 2d). Obviously, the immersion-height of PI-SiO2

membrane (10.2 cm) is greater than that of PI (7.5 cm) and PP (1.7 cm).The smaller contact angle and higher immersion-height confirm thatthe adding of SiO2 nanoparticles is beneficial to improve the wettingproperty of the membrane [21,26].

Moreover, the quantitative testing of electrolyte uptake was also

investigated. As shown in Fig. 3a, the electrolyte uptake process of allmembranes was stabilized within 5 mins. After 40 mins, the electrolyteuptakes reached 169%, 2156% and 2400% for PP (16 µm), PI (20 µm)and PI-SiO2 (20 µm) membranes, respectively. Meanwhile, the electro-lyte uptakes of membranes with different thicknesses were alsoconducted (Table S2). It can be find that the electrolyte uptake declinedwith the membrane thickness increased. By comparing the results, theelectrolyte uptake of thick PI and PI-SiO2 membranes (60 µm and100 µm) are also much higher than previous reports [27,28]. Thesuperior electrolyte uptakes of PI and PI-SiO2 membranes is due to thehigh porosity (90%) and polarity of the non-woven mat, leading to goodcompatibility with liquid electrolytes [19,26–28]. The electrolyteretention of the membranes at 50 °C is shown in Fig. 3b. It is clearlyto find that all electrolyte uptakes of membranes reduced greatly at thebeginning. After 150 min, the electrolyte retention stabilized at 0%,70% and 73% for PP, PI and PI-SiO2 membranes, respectively.Obviously, the PI-SiO2 membrane shows superior wettability, electro-lyte uptake and electrolyte retention compared with PI and PPmembranes. This extraordinary liquid electrolyte wettability of PI-SiO2 membrane is attributed to the high electrolyte affinity of SiO2

particles, the well interconnected porous structure and the similarpolarity between the membranes and electrolyte [21].

The ionic conductivity of the membrane with electrolyte wasdetermined by EIS with stainless steel (SS) as blocking electrodes. Itcan be observed that the bulk resistances (Rb) of the PP, PI and PI-SiO2

membranes are 3 Ω, 0.8 Ω and 0.6 Ω from Fig. 3c and d. The calculatedionic conductivity is: PI-SiO2 membrane (2.27 mS cm−1)> PI mem-brane (1.71 mS cm−1)> PP membrane (0.35 mS cm−1). With higherporosity and the electrolyte uptake, the PI-SiO2 membrane could fixmore quantity of electrolyte, which facilitates the fluent flow of Li ionsthrough the channels in the PI-SiO2 membrane. The physical andelectrochemical parameters of various membranes are summarized inTable 1.

The most important role of a separator is to prevent physical contactof the positive and negative electrodes while permitting free ion flow[7]. The dimensional tolerance of PP, PI and PI-SiO2 membranes uponmechanical deformation and thermal shrinkage were tested. Afterbending/twisting/wrinkling, the PI and PI-SiO2 membranes can fully

Fig. 1. (a) Cross-sectional and (b) surface SEM image of PI (the inset is corresponding high-magnification image); and (c) TEM image of PI nanofibers; (d) cross-sectional and (e) surfaceSEM image of PI-SiO2 (the inset is corresponding high-magnification image); (f) TEM image of PI-SiO2 nanofibers.

Fig. 2. Contact angle images of (a) PP membrane (b) PI membrane and (c) PI-SiO2

membrane; (d) the electrolyte immersion-heights of the PP, PI and PI-SiO2 membranes(after 1 h soaking in EC: DEC =1:1 v/v containing 1 M LiPF6).

Y. Wang et al. Journal of Membrane Science 537 (2017) 248–254

250

recover to its initial state without any wrinkles, indicating excellentmechanical flexibility, while PP membrane shows many wrinkles afterfolding into small size (Fig. 4a). Fig. S3 shows the stress-strain curves ofPP, PI and PI-SiO2 membranes before and after absorbing the electro-lyte. The maximum stress of PP membrane is about 22.3 MPa. The PI-SiO2 membrane showed the maximum stress up to 4.66 MPa and7.75 MPa before and after absorbing the electrolyte, respectively,which are higher than that of PI membrane (3.77 MPa and6.01 MPa), indicating that the adding of SiO2 nanoparticles helps toenhance tensile strength of the membrane. Meanwhile, the goodcompatibility of the PI and PI-SiO2 membranes with electrolyte leadsto the rise of the maximum stress after absorbing the electrolyte[27,28]. In addition, the PI and PI-SiO2 membranes could maintaintheir original dimensions without any thermal shrinkage up to 250 °C,while PP membrane suffers severe thermal shrinkage and deformationdue to its low melting point (Fig. 4b). As for the quantitative testing, thethermogravimetric analysis was also conducted. It can be seen thatthere are no any clear and sharp weight loss until 500 °C of PI and PI-SiO2 membranes, while PP membrane loses weight sharply at 250 °C(Fig. S4). The 5% difference between the PI and PI-SiO2 membranesafter testing represents the content of SiO2. The thermal stability of themembranes were also evaluated by DSC measurements (Fig. 4c). A bigendothermic peak at 142 °C is found in the DSC curve of PP membrane.

The PI and the PI-SiO2 membranes show no significant peaks during30 °C to 500 °C, indicating excellent thermal stability.

The LSV measurement was conducted to test the electrochemicalstability of PI and PI-SiO2 separators (Fig. 5). It can be observed that PI-SiO2 is stable between 0V and 5.0V without obvious oxidation/reduction peak. The electrochemical stability is greatly enhanced byadding SiO2 nanoparticles. The electrochemical performance wasevaluated using LiMn2O4/Li coin cells (Fig. 6). The cells using PP, PIand PI-SiO2 separators deliver about 99, 103 and 108 mA h g−1 at acurrent rate of 0.2C, respectively. With current density increased, thecell with PI-SiO2 separator exhibits higher capacity retention than thatusing PP and PI separators. The discharge capacities of cells using PI-SiO2 separator are 105, 102, 98, 80 and 55 mA h g−1 at 0.5, 1, 2, 5 and10C (1C =148 mA g−1), respectively (Fig. 6a). The superior ratecapability is attributed to the improved wettability property whichfacilitates the transport of lithium ions between the separator and theelectrode. The discharge voltage plateaus of the cell using the PI-SiO2

separator are higher than that of using PP, PI separators, revealinglower electrochemical polarization (Fig. 6b). The cycling performancesat a rate of 5C were also studied (Fig. 6c), the cell using PI-SiO2

separator exhibits an impressive initial value of 80 mA h g−1 andstabilizes at 77 mA h g−1 after 100 cycles (corresponds to a capacityretention of 96.3%). As further verify the effect of SiO2 nanoparticles,

Fig. 3. (a) Electrolyte uptakes of PP, PI and PI-SiO2 membranes; (b) electrolyte retentions of PP, PI and PI-SiO2 membranes at 50 °C; (c) the EIS spectra of SS/separator/SS cells using PP,PI and PI-SiO2 membranes; (d) the enlarged green square area in Fig. 3c. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle).

Table 1Physical and chemical properties of the PP, PI and PI-SiO2 membranes.

Membrane Thickness (μm) Porosity (%) Gurley value (sec. 100 ml−1) Contact angle (°) Electrolyte uptake (%) Ionic conductivity (mS cm−1) Density (g cm−3)

PP 16 45 15 44 169 0.35 0.56PI 20 91 5 10 2156 1.71 0.15PI-SiO2 20 90 5 0 2400 2.27 0.16

Y. Wang et al. Journal of Membrane Science 537 (2017) 248–254

251

the high temperature (55 °C) cycling performance was also tested(Fig. 6d). Obviously, the LiMn2O4/Li cell using PP separator showedapparent capacity degradation in 80 cycles. From 80th to 84th cycles,the capacity sharp declined to 58.3 mAh g−1 and then to 0, whereas thecell using PI-SiO2 separator exhibits much better cycling performance(almost 100% capacity retention after 80 cycles) than that of PI and PPseparators. The improved capacity retention is ascribed to the adding ofthe SiO2 nanoparticles, which can capture the trace moisture and HF inthe electrolyte and suppress the dissolution of Mn ion from the activeLiMn2O4 at elevated temperature [29–33].

The electrochemical impedance spectroscopy (EIS) measurementswere conducted to evaluate the conductivity of the cells using differentseparators. As shown in Fig. S5, the cell with PI-SiO2 separator showsthe lowest resistance. The reduced resistance compared with the cellwith PI separator can be attributed to the introduction of SiO2

nanoparticles which absorbed the trace impurities (H2O and HF) and

stabilized the electrolyte [34].The morphology of PI-SiO2 separator after cycling was tested to

check the affinity between SiO2 nanoparticles and polymer nanofiber(Fig. 7). From the photograph of the tested Pi-SiO2 separator (inset inFig. 7a), it is clear that the dimension and surface of PI-SiO2 separatorare well maintained without any damage. The SEM and TEM images ofthe tested PI-SiO2 separator (Fig. 7a and b) show that the morphologyof the nanofibers still maintains interconnected fibrous structure. Thehigh magnification TEM image of the single nanofiber (Fig. 7c) revealsthat the SiO2 nanoparticles still tightly embed in the nanofiber withoutshedding, which further verify the excellent structure stability of PI-SiO2. Accordingly, the superior affinity between SiO2 and the nanofiberhelp to achieve in a splendid battery performance.

Fig. 4. Dimensional stability test: (a) wrinkle test; (b) thermal shrinkage after exposure at 250 °C for 1 h; (c) DSC curves of PP, PI and PI-SiO2 membranes.

Y. Wang et al. Journal of Membrane Science 537 (2017) 248–254

252

4. Conclusions

In summary, a thin and flexible PI-SiO2 composite membrane wassuccessfully synthesized and used as a high safety lithium-ion separator.Due to the three-dimensional (3D) fibrous network structure as well astightly cohesion between SiO2 particles and PI nanofibers, the PI-SiO2

membrane shows high porosity, excellent electrolyte wettability,extraordinary ionic conductivity and outstanding stability (chemicalstability, good mechanical flexibility, and excellent thermal stability).In addition, LiMn2O4/Li cells using the PI-SiO2 separator exhibitssuperior rate capability (80 mA h g−1 at 5C) and excellent cycle lifeeven at high temperature (99.1% capacity retention after 100 cycles at

55 °C). Considering the easy fabrication of PI-SiO2 separator, therationally designed PI-SiO2 separator will be very promising forindustrialization.

Acknowledgements

The authors greatly acknowledge the financial support by theNational Key Research Program of China (No. 2016YFA0202601),National Natural Science Foundation of China (Nos. 51621001,21576100, 21536005), and Pearl River S & T Nova Program ofGuangzhou (201610010062).

Fig. 5. Linear sweep voltammograms of PI and PI-SiO2 separators: (a) negative scan (−0.4–3.0 V); (b) positive scan (2.5–5.5 V).

Fig. 6. Electrochemical behaviors of LiMn2O4/Li cells: (a) rate capability with PP, PI and PI-SiO2 separators (0.2–10C); (b) discharge curves using different separators; (c) cyclicperformance at 5C at room temperature; (d) cycling performance at 55 °C.

Y. Wang et al. Journal of Membrane Science 537 (2017) 248–254

253

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.memsci.2017.05.023.

References

[1] J.-M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithiumbatteries, Nature 414 (2001) 359–367.

[2] Y. Zhu, F. Wang, L. Liu, S. Xiao, Z. Chang, Y. Wu, Composite of a nonwoven fabricwith poly (vinylidene fluoride) as a gel membrane of high safety for lithium ionbattery, Energy Environ. Sci. 6 (2013) 618–624.

[3] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am.Chem. Soc. 135 (2013) 1167–1176.

[4] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: a batteryof choices, Science 334 (2011) 928–935.

[5] T.H. Kim, J.S. Park, S.K. Chang, S. Choi, J.H. Ryu, H.K. Song, The current move oflithium ion batteries towards the next phase, Adv. Energy Mater. 2 (2012) 860–872.

[6] A. Zhamu, G. Chen, C. Liu, D. Neff, Q. Fang, Z. Yu, W. Xiong, Y. Wang, X. Wang,B.Z. Jang, Reviving rechargeable lithium metal batteries: enabling next-generationhigh-energy and high-power cells, Energy Environ. Sci. 5 (2012) 5701–5707.

[7] P. Arora, Z. Zhang, Battery separators, Chem. Rev. 104 (2004) 4419–4462.[8] M.H. Ryou, Y.M. Lee, J.K. Park, J.W. Choi, Mussel-Inspired Polydopamine-Treated

Polyethylene Separators for High-Power Li-Ion Batteries, Adv. Mater. 23 (2011)3066–3070.

[9] S.S. Zhang, A review on the separators of liquid electrolyte Li-ion batteries, J. PowerSources 164 (2007) 351–364.

[10] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, X. Zhang, A review of recent developmentsin membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci.7 (2014) 3857–3886.

[11] X. Zhu, X. Jiang, X. Ai, H. Yang, Y. Cao, TiO2 ceramic-grafted polyethyleneseparators for enhanced thermostability and electrochemical performance oflithium-ion batteries, J. Membr. Sci. 504 (2016) 97–103.

[12] C. Man, P. Jiang, K.-w. Wong, Y. Zhao, C. Tang, M. Fan, W.-m. Lau, J. Mei, S. Li,H. Liu, Enhanced wetting properties of a polypropylene separator for a lithium-ionbattery by hyperthermal hydrogen induced cross-linking of poly (ethylene oxide), J.Mater. Chem. A 2 (2014), pp. 11980–11986.

[13] C. Yang, Z. Li, W. Li, H. Liu, Q. Xiao, G. Lei, Y. Ding, Batwing-like polymermembrane consisting of PMMA-grafted electrospun PVdF-SiO2 nanocompositefibers for lithium-ion batteries, J. Membr. Sci. 495 (2015) 341–350.

[14] P. Raghavan, J.-W. Choi, J.-H. Ahn, G. Cheruvally, G.S. Chauhan, H.-J. Ahn, C. Nah,Novel electrospun poly (vinylidene fluoride-co-hexafluoropropylene)-in situ SiO2

composite membrane-based polymer electrolyte for lithium batteries, J. PowerSources 184 (2008) 437–443.

[15] D. Bansal, B. Meyer, M. Salomon, Gelled membranes for Li and Li-ion batteriesprepared by electrospinning, J. Power Sources 178 (2008) 848–851.

[16] Y. Ding, P. Zhang, Z. Long, Y. Jiang, F. Xu, W. Di, The ionic conductivity andmechanical property of electrospun P(VdF-HFP)/PMMA membranes for lithium ionbatteries, J. Membr. Sci. 329 (2009) 56–59.

[17] Q. Cheng, W. He, X. Zhang, M. Li, X. Song, Recent advances in composite

membranes modified with inorganic nanoparticles for high-performance lithiumion batteries, RSC Adv. 6 (2016) 10250–10265.

[18] W.-K. Shin, D.-W. Kim, High performance ceramic-coated separators prepared withlithium ion-containing SiO2 particles for lithium-ion batteries, J. Power Sources 226(2013) 54–60.

[19] X. Liang, Y. Yang, X. Jin, Z. Huang, F. Kang, The high performances of SiO2/Al2O3-coated electrospun polyimide fibrous separator for lithium-ion battery, J. Membr.Sci. 493 (2015) 1–7.

[20] F. Croce, M.L. Focarete, J. Hassoun, I. Meschini, B. Scrosati, A safe, high-rate andhigh-energy polymer lithium-ion battery based on gelled membranes prepared byelectrospinning, Energy Environ. Sci. 4 (2011) 921–927.

[21] D. Lin, D. Zhuo, Y. Liu, Y. Cui, All-Integrated Bifunctional Separator for Li DendriteDetection via Novel Solution Synthesis of a Thermostable Polyimide Separator, J.Am. Chem. Soc. 138 (2016) 11044–11050.

[22] W. Jiang, Z. Liu, Q. Kong, J. Yao, C. Zhang, P. Han, G. Cui, A high temperatureoperating nanofibrous polyimide separator in Li-ion battery, Solid State Ion. 232(2013) 44–48.

[23] X. Huang, Separator technologies for lithium-ion batteries, J. Solid StateElectrochem. 15 (2011) 649–662.

[24] J. Dai, C. Shi, C. Li, X. Shen, L. Peng, D. Wu, D. Sun, P. Zhang, J. Zhao, A rationaldesign of separator with substantially enhanced thermal features for lithium-ionbatteries by the polydopamine-ceramic composite modification of polyolefinmembranes, Energy Environ. Sci. 9 (2016) 3252–3261.

[25] Y. Zhang, Z. Wang, H. Xiang, P. Shi, H. Wang, A thin inorganic composite separatorfor lithium-ion batteries, J. Membr. Sci. 509 (2016) 19–26.

[26] J. Shayapat, O.H. Chung, J.S. Park, Electrospun polyimide-composite separator forlithium-ion batteries, Electrochim. Acta 170 (2015) 110–121.

[27] Q. Wang, W.-L. Song, L. Wang, Y. Song, Q. Shi, L.-Z. Fan, Electrospun polyimide-based fiber membranes as polymer electrolytes for lithium-ion batteries,Electrochim. Acta 132 (2014) 538–544.

[28] Q. Wang, Z. Jian, W.-L. Song, S. Zhang, L.-Z. Fan, Facile fabrication of safe androbust polyimide fibrous membrane based on triethylene glycol diacetate-2-propenoic acid butyl ester gel electrolytes for lithium-ion batteries, Electrochim.Acta 149 (2014) 176–185.

[29] T. Yim, H.-J. Ha, M.-S. Park, K.J. Kim, J.-S. Yu, Y.-J. Kim, A facile method forconstruction of a functionalized multi-layered separator to enhance cycle perfor-mance of lithium manganese oxide, RSC Adv. 3 (2013) 25657–25661.

[30] W. Cho, S.-M. Kim, J.H. Song, T. Yim, S.-G. Woo, K.-W. Lee, J.-S. Kim, Y.-J. Kim,Improved electrochemical and thermal properties of nickel rich LiNi0.6Co0.2Mn0.2O2

cathode materials by SiO2 coating, J. Power Sources 282 (2015) 45–50.[31] J.-H. Yoo, W.-K. Shin, S.M. Koo, D.-W. Kim, Lithium-ion polymer cells assembled

with a reactive composite separator containing vinyl-functionalized SiO2 particles,J. Power Sources 295 (2015) 149–155.

[32] W.-K. Kim, D.-W. Han, W.-H. Ryu, S.-J. Lim, H.-S. Kwon, Al2O3 coating on LiMn2O4

by electrostatic attraction forces and its effects on the high temperature cyclicperformance, Electrochim. Acta 71 (2012) 17–21.

[33] Z. Liu, H. Wang, L. Fang, J.Y. Lee, L.M. Gan, Improving the high-temperatureperformance of LiMn2O4 spinel by micro-emulsion coating of LiCoO2, J. PowerSources 104 (2002) 101–107.

[34] M. Yanilmaz, Y. Lu, J. Zhu, X. Zhang, Silica/polyacrylonitrile hybrid nanofibermembrane separators via sol-gel and electrospinning techniques for lithium-ionbatteries, J. Power Sources 313 (2016) 205–212.

Fig. 7. Characterizations of PI-SiO2 separator after 100 cycles: (a) SEM image (inset is the photograph of the separator); (b) TEM image; (c) high-magnification TEM image.

Y. Wang et al. Journal of Membrane Science 537 (2017) 248–254

254