Synthesis of an Al2O3-coated polyimide nanofiber mat and its electrochemical characteristics as a separator for lithium ion batteries

Journal of Power Sources 248 (2014) 1211e1217

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Synthesis of an Al2O3-coated polyimide nanofiber mat and its electrochemical characteristics as a separator for lithium ion batteries Juneun Lee a, Cho-Long Lee a, Kyusung Park b, Il-Doo Kim a, * a b

Department of Materials Science & Engineering, Korea Advanced Institute of Science & Technology, 335 Science Road, Daejeon 305-701, Republic of Korea Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, United States

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Polyimide (PI) nanofibers were synthesized by electrospinning a poly(amic acid) solution followed by an imidization process.
Thin Al2O3 over-layers were coated on both sides of a PI separator via a dip-coating process.
Al2O3-coated PI separator exhibited superior capacity, cyclability and rate capabilities.
Al2O3-coated PI separator is a promising separator candidate for nextgeneration lithium ion batteries.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 August 2013 Received in revised form 5 October 2013 Accepted 14 October 2013 Available online 23 October 2013

Polyimide (PI) nanofibers with an average diameter of 300 nm, possessing superior electrolyte wettability and thermal stability, are synthesized by electrospinning a poly(amic acid) (PAA) solution followed by an imidization process. The large pore volume of the PI nanofiber mat can facilitate faster Liþ-ion transport and greater rate capability, but it also cause an irreversible increase in cell impedance during long term cycling. To overcome these problems, thin Al2O3 over-layers are coated on both sides of a PI separator via a dip-coating process. The Al2O3-coated PI separators exhibit enhanced capacity, cyclability (95.53% retention after 200 cycles at 1 C), and rate capabilities (78.91% at 10 C) compared to the bare PI separator (68.65% at 10 C) and a commercial polypropylene separator (18.25% at 10 C) with a limited increase of cell impedance. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Lithium ion battery Nonwoven separator Electrospinning Polyimide Composite Dip-coating

1. Introduction Lithium ion batteries with high energy densities and stable cycle performance have been used for portable electronic devices such as laptops, smart phones and tablet PCs [1e3]. As environmentally friendly applications and high energy demands have come to the

* Corresponding author. E-mail address: [email protected] (I.-D. Kim). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.10.056

fore recently, research on plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), electric vehicles (EVs) and smart grids have been actively conducted [2,4,5]. Accordingly, there are new and increasing requirements of battery components including the cathode, anode, electrolyte and separator [2,6,7]. In particular, the battery separator plays an important role in achieving high power density as well as safe operation of batteries. The separator prevents physical contact between the cathode and anode and also retains liquid electrolyte, allowing lithium ions to move freely inside of a cell to match the external electrical current [8e11].

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Commercially available lithium ion battery separators are made of polyolefins such as polyethylene (PE) and polypropylene (PP). They have advantages such as outstanding mechanical strength and good electrochemical stability at an acceptable cost, but some intrinsic disadvantages impede their application to nextgeneration battery systems. Poor thermal stability [12,13] could cause direct contact between the cathode and anode, which would induce extensive current flow, Joule heating, and possible safety hazards such as fire/explosion. In addition, inferior wettability [8e 10,14] to polar liquid electrolytes and low porosity (less than 40%) [8,10] limit electrochemical performances of lithium ion batteries. Nonwoven separators, which provide high porosity and price competitiveness, are promising alternatives to commercial separators. A paper making method [15], solution extrusion method [16], melt blowing method [17] and electrospinning [18e22] have been used to prepare nonwoven separators. Among these methods, electrospinning has drawn considerable attention due to its capacity to produce a fully interconnected pore structure with a large surface area and high porosity [18e22]. The large pore size, however, could lead to the self-discharge and uneven current distribution, which can cause local lithium dendrite growth [23,24]. Caution must also be exercised if the anode is composed of alloytype materials (e.g. Si and Sn), which undergo significant volume changes during charge/discharge. The dimensional change of an electrode could induce direct contact between the anode and cathode through the large pores, which is not acceptable in battery technology. These considerations led us to further modify the surface of the PI separator, i.e. the electrode/separator surface, to reduce excess electrolyte exposure and to enhance mechanical strength. In this study, an Al2O3-nanoparticle-coated electrospun nonwoven polyimide (Al2O3-PI) separator was developed for the first time by an electrospinning method and a subsequent dipcoating process. Polyimide (PI) is a well-known engineering polymer that is used for its mechanical strength, chemical stability and thermal stability up to 500  C [25e27]. Furthermore, the polar groups in the polyimide are expected to improve the electrolyte wettability [28,29]. Here, Al2O3 particles were coated on an electrospun nonwoven PI layer to enhance electrochemical performance and long term stability. Intrinsic thermal and dimensional stabilities of ceramic particles reduce the thermal shrinkage of a separator, and the hydrophilicity of the ceramic particles can also improve the wetting behaviors. As a reference, a commercial PP separator (Celgard 2400) was compared to demonstrate the advantage of the PI separator and the use of the Al2O3 overlayer. 2. Experimental 2.1. Synthesis Pyromellitic dianhydride (PMDA) was purchased from AlfaAesar. 4,40 -Oxydianiline, Al2O3 nanopowders (13 nm) and N,Ndimethylfromamide (DMF) were purchased from SigmaeAldrich. Poly(vinylidene fluoride-hexafluoro propylene) (PVdF-HFP, Kynar 2801) was obtained from Arkema. All chemical reagents were used without further purification. For the electrospinning step, a poly(amic acid) (PAA) solution was prepared by dissolving 2 g of PMDA and 1.84 g of ODA in 16 g of DMF. This solution was mechanically stirred for 12 h. Electrospinning was conducted by extruding the PAA solution through a stainless steel needle (25 gauge, inner diameter ¼ 250 mm). High voltage (15 kV) was applied between the needle and a grounded stainless steel roll collector located 15 cm from the needle. The solution was supplied from a syringe pump at a constant feed rate (0.2 ml h1). The thickness of the PAA nanofiber mat was optimized by adjusting the electrospinning time. The collected PAA nanofiber

mats were dried under a vacuum for 24 h and calcined to achieve polyimide (PI) nanofiber mats. These heat treatments were performed by holding at 100, 200, and 300  C for 2 h, respectively, and the heating rate between each step was 5  C min1 [30]. In order to make a dip-coating solution, Al2O3 nanopowders were dispersed in acetone via sonication for 3 h. PVdF-HFP was then added to the Al2O3 solution with a weight ratio of 4/1, Al2O3/ PVdF-HFP (w/w). Furthermore, a ballmilling step was employed to obtain a homogeneous solution. The Al2O3 solution was coated on the PI nanofiber mats by a dip-coating process.

2.2. Characterization The morphology of the fibers and the surface structure of the separators were observed by a field emission scanning electron microscope (FE-SEM, Nova 230, FEI). The thickness of each separator was measured using a micrometer (MDC-25PJ, Mitutoyo). The synthesis of the PI membrane was confirmed by Fourier transform infrared spectroscopy (FT-IR, IF66V/S & HYPERION 3000, Bruker) with the attenuated total reflectance (ATR) method. The thermal stability of the separators was analyzed by using a thermogravimetry analyzer (TGA, TG 209 F3, NETZSCH) and differential scanning calorimetry (DSC, DSC 204 F1, NETZSCH) at a constant heating rate of 5  C min1 in air. Thermal shrinkage was checked by heating the separators in an oven at various temperatures from 150 to 200  C for 30 min. The electrolyte wettability was observed with an electrolyte droplet on a separator and confirmed by a contact angle analyzer (Phoenix300, SEO). The electrolyte uptake was determined by using Eq. (1) with the weights of separators before and after soaking them in a liquid electrolyte for 2 h in an argon filled glove box.

Electrolyte uptakeð%Þ ¼




Wf  Wi

. Wi  100

(1)

where Wi and Wf are the weights of the separator before and after soaking in the electrolyte. The porosity was calculated using Eq. (2).

Porosityð%Þ ¼




Wwet  Wdry

.


rb  Vdry



(2)

where Wdry and Wwet are the weights of the separator before and after the soaking in n-butanol, rb is the density of n-butanol, and Vdry is the apparent volume of the separator. The size of each separator for characterization was 3.24 cm2. The electrochemical stability of a coin cell was determined by the linear sweep voltammetry (LSV) method with a potentiostat (WPG100e, Wonatech). A liquid electrolyte-soaked separator was sandwiched between lithium metal as a counter electrode and stainless steel as a working electrode. Experiments were carried out at a scan rate of 1.0 mV s1 in a voltage range of 3.5e6.0 V vs. Liþ/Li. The ionic conductivity and cell resistance were evaluated via electrochemical impedance spectroscopy (EIS, SI1260, Solatron). All experiments were conducted at an amplitude of 5 mV with a frequency range of 1 Hze100 kHz. The difference of the cell resistance was investigated after cycling at 0.5 C for 10 and 200 cycles. Also, the ionic conductivity was calculated using Eq. (3).

s ¼ d=Rb S

(3)

where d is the thickness of the separators, Rb is the bulk resistance and S is the area of the separators. These values were measured using a symmetric cell comprised of two stainless steel (SS) electrodes and a liquid electrolyte-soaked separator. Coin cells (2032 type coin cell) were assembled in an argon filled glove box. A blended cathode (Li(Ni0.5Co0.2Mn0.3)O2/LiMn2O4/

J. Lee et al. / Journal of Power Sources 248 (2014) 1211e1217

Carbon Black/PVdF ¼ 55.2/36.8/4/4 w/w/w/w) and graphite anode (Graphite/PVdF ¼ 95/5 w/w) were employed with a liquid electrolyte of 1 M LiPF6 in EC/DEC (1/1 v/v). The separator (3.24 cm2) was wetted with the liquid electrolyte and located between the cathode and anode. The following experiments were performed in a voltage range of 2.8 Ve4.2 V vs. graphite using a battery cycle tester (Series 4000, Maccor). The rate capability was evaluated by discharging at various current densities (0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C) after charging at a constant current of 0.2 C and constant voltage (cut-off at 4.2 V if the current fell below 0.01 C) condition. The cycle performance was investigated at rates of 0.5 C and 1 C for 200 cycles. Specific capacities were calculated based on the weight of active materials in the cathode. 3. Results and discussion Fig. 1a shows the FT-IR spectrum of the as-synthesized PI separator, which confirms the formation of the PI phase. The absorbance peaks at 1780, 1725, 1380 and 725 cm1 indicate C]O symmetric stretching, C]O asymmetric stretching, CeN stretch and C]O bonding, respectively [25,26]. Disappearance of the 2900e3200 cm1 COOH peak indicates that most of the PAA precursor has been transformed into PI phase after the imidization process. The morphology of the PI separator is shown in Fig. 1bec. The diameter of the PI nanofibers is approximately 300 nm and the fibers are randomly aligned. The commercial PP separator (Celgard 2400) reportedly has ca. 50 nm-sized pores as a result of employing a dry-stretching process [31]. On the other hand, the electrospun nonwoven PI separator presents micro-sized pores with a highly porous structure. The micro-sized pores facilitate lithium ion transport, and are thus desirable to develop high power lithium ion batteries, but they also could have greater likelihood of inducing electrolyte decomposition. In contrast, the PI separator sandwiched by Al2O3 overlayers (hereafter, Al2O3-PI separator) displays a different surface morphology (Fig. 1d). The micro-sized pores in the PI separator are covered homogeneously with Al2O3 nanoparticles. Contrary to the PI separator, the outer Al2O3 layer of the Al2O3-PI separator offers nano-sized pores in a range of a few nm to 50 nm. The limited thickness of the Al2O3 layer (<3 mm) should mitigate Liþ-ion transport hindrance through the nanopores. Moreover, it also contributes to the electrolyte wettability [23,32e37]. The surface hydrophilicity of Al2O3 [38] and the small amount of PVdF-

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Table 1 Physical and electrochemical properties of the PP, PI and Al2O3-PI separators.

PP PI Al2O3-PI

Thickness (mm)

Porosity (%)

Electrolyte uptake (%)

Ionic conductivity (mS cm1)

25 20 27

36.31 91.42 70.38

65.61 1190.33 440.21

0.1898 0.3063 0.3638

HFP [39] would play important roles in realizing facile ionic transport as well as fast wetting behavior. In parallel, the inner PI layer provides mechanical support and large spaces to retain the electrolyte. This is confirmed by a cross-section image showing the micro-porous PI layer sandwiched between the nano-porous Al2O3 layers (Fig. 1e). The porous structures of the separators are examined in terms of the porosity and the electrolyte uptake (Table 1). The porosity of the Al2O3-PI separator is 70.38 %, which is much higher than that of the PP separator (36.31 %), although it is lower than that of the PI separator (91.42 %). While the outer nano-porous Al2O3 layer lowers the total porosity, the inner porous PI layer still provides large pore volumes. The electrolyte uptake also shows that the porous structure of the Al2O3-PI separator is fairly large. The electrolyte uptake of the Al2O3-PI separator (440.21%) is much higher than that of the PP separator (65.61%) and lower than that of the PI separator (1190.33%). Although the Al2O3 coating reduces porosity and electrolyte uptake, the Al2O3-PI separator presents considerably high performance measures such that it is desirable alternative to conventional lithium ion battery separators. The wetting behaviors of the separators are further investigated by a liquid electrolyte absorption test (Fig. 2a). The PP separator is hardly wetted by the liquid electrolyte owing to its hydrophobicity and low surface energy [8e10,14]. Poor electrolyte wettability might cause unwet pores which would deteriorate the long-term cyclability of a lithium ion battery. On the other hand, the PI separator and the Al2O3-PI separator absorb the liquid electrolyte quickly (within 1 min). For the PI separator, the high electron affinity functional groups of PI would contribute to fast electrolyte absorption [28,29]. In addition to PI, the Al2O3-PI separator has extra Al2O3 layers and PVdF-HFP as a binder. The hydrophilic Al2O3 [38] and PVdF-HFP [39] would further facilitate fast electrolyte absorption. To evaluate the wetting properties of the separators quantitatively, the wetting angles have been measured and the

Fig. 1. (a) FT-IR spectrum of the PI separator; (bec) FE-SEM images of the PI separator, (dee) surface and cross sectional images of the Al2O3-PI separator.

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Fig. 2. (a) Photographs showing liquid electrolyte wetting behavior of the PP, PI, and Al2O3-PI separators. (b) Photographs of static liquid electrolyte contact angles of the separators. 1 M LiPF6 in EC/DEC (1/1 v/v) was used as a liquid electrolyte.

results are shown in Fig. 2b. The angles of droplets on the PP and PI separators are 27.86 and 22.09 , respectively. The Al2O3-PI separator exhibits the smallest value, 18.41, owing to the reasons explained above. With excellent electrolyte wettability, the Al2O3PI separator is expected to deliver better rate capability and cyclability. The thermal shrinkage is evaluated by placing the separators in an oven at various temperatures in a range from 150  C to 200  C for 30 min (Fig. 3a). The PP separator shows significant shrinkage at above 150  C. This vulnerable thermal property of PP suggests that

the PP separator may induce physical contact between the cathode and anode at high temperature. On the other hand, the PI separator and the Al2O3-PI separator don’t shrink up to 200  C due to the thermal stability of PI [25,28]. In this study, it is difficult to verify how the Al2O3 coating layer contributes to the thermal property of the Al2O3-PI separator owing to the thermally stable PI. It is expected to promote high temperature stability of the Al2O3-PI separator [32,40e45]. These thermal characteristics of PP and PI are confirmed by DSC (Fig. 3b). PP displays an endothermic peak at 160  C. This peak corresponds to the melting point of PP. In contrast,

Fig. 3. (a) Photographs showing thermal shrinkage of the PP, PI and Al2O3-PI separators as a function of exposure temperature. (b) DSC curves and (c) TGA curves of the PP and PI separators.

J. Lee et al. / Journal of Power Sources 248 (2014) 1211e1217

(a)10

(b)0.15 Current (mA)

PP PI Al2O3-PI

8 6

-Z'' (Ohm)

1215

4

PP PI Al2O3-PI

0.10

0.05

2 0

0.00 0

2

4

6

8

10

3.5

4.0

4.5

Z' (Ohm)

5.0

5.5

6.0

Voltage (V)

Fig. 4. (a) Nyquist plots of the cells (stainless steel/separator/stainless steel) for the liquid electrolyte-soaked PP, PI and Al2O3-PI separators. (b) Linear sweep voltammograms of the separators at a voltage scan rate of 1.0 mV s1.

retard the decomposition voltage of the carbonate electrolyte (4.5 V vs. Liþ/Li) to more than 5 V. These results imply that the Al2O3-PI separator is compatible with the carbonate electrolyte and can be applicable to a lithium ion battery. The rate capabilities of coin full cells (Li(Ni0.5Co0.2Mn0.3)O2/ LiMn2O4 cathode and graphite anode) employing the separators are shown in Fig. 5. In the case of the PP separator, higher ohmic polarization is observed as the discharge current density increases. The smallest loss in discharge capacities at each current density is observed in the Al2O3-PI separator. Fig. 5d displays a summary of the rate capabilities for the PP, PI, and Al2O3-PI separators. The Al2O3-PI separator presents the largest discharge capacities over various discharge current densities from 0.2 C to 10 C. In addition, the capacity retention ratios of the Al2O3-PI separator are higher than those of other separators. For 1 C, the capacity retention ratio of the Al2O3-PI separator (97.59% of discharge capacity at 0.2 C) is slightly higher than those of the other separators (PI: 95.54%, PP: 94.45%). The difference in the discharge capacities becomes larger

PI exhibits no peaks because it does not melt but rather it decomposes above 500  C. It can be seen that PI experiences thermal degradation above 500  C in the TGA curve (Fig. 3c) while PP begins to lose weight at about 250  C. Accordingly, the impressive thermal properties and the dimensional stability at high temperature originate from the intrinsic thermal stability of PI. The bulk resistance of a cell is measured from electrochemical impedance spectra to calculate the ionic conductivity of the separators (Fig. 4a). The PP separator exhibits the smallest ionic conductivity (0.19 mS cm1) and the largest bulk resistance (4.07 U). The bulk resistance of the PI separator (2.02 U) is slightly lower than that of the Al2O3-PI separator (2.29 U). Nevertheless, the ionic conductivity of the Al2O3-PI separator (0.36 mS cm1) surpasses that of the PI separator (0.31 mS cm1) on account of the thicker Al2O3-PI separator and the small difference in the bulk resistances. The electrochemical stability of the cells is tested by a linear sweep voltammetry experiment (Fig. 4b). No anodic currents are observed below 5.0 V vs. Liþ/Li for all separators. All separators

0.2C 0.5C 1C 2C 5C 10C

4.2

Voltage (V)

4.0 3.8 3.6 3.4

(b)4.4 4.0 3.8 3.6 3.4

3.2

3.2

3.0

3.0

2.8

0

20

40

60

80

100

120

0.2C 0.5C 1C 2C 5C 10C

4.2

Voltage (V)

(a) 4.4

2.8

140

0

-1

Voltage (V)

4.0 3.8 3.6 3.4 3.2 3.0 2.8

0

20

40

60

80

100

120 -1

Discharge Capacity (mAh g )

140

(d)160

0.2C 0.5C

140 -1

0.2C 0.5C 1C 2C 5C 10C

4.2

40

60

80

100

120

140

Discharge Capacity (mAh g )

Capacity (mAh g )

(c)4.4

20

-1

Discharge Capacity (mAh g )

1C

2C

5C

10C 0.2C

120 100 80 60

PP PI Al2O3-PI

40 20 0

0

5

10

15

20

25

30

35

Cycle

Fig. 5. Discharge voltage profiles of coin full cells with the (a) PP, (b) PI and (c) Al2O3-PI separators as a function of C-rate. (d) Rate capability of cells with the PP, PI and Al2O3-PI separators at constant current charge/discharge rates (0.2 C/0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C; 5 cycles at each C-rate).

100

Capacity (mAh/g)

140 120

80

100 60

80 60

40

PP PI Al2O3-PI

40 20 0

0

40

20

80

120

160

0 200

(b)160

100

140

Capacity (mAh/g)

(a) 160

120

80

100 60

80 60

40

PP PI Al2O3-PI

40 20 0

0

40

80

20

120

160

Coulombic efficiency (%)

J. Lee et al. / Journal of Power Sources 248 (2014) 1211e1217

Coulombic efficiency (%)

1216

0 200

Cycle

Cycle

Fig. 6. Cycle performance of cells with the PP, PI, and Al2O3-PI separators at constant current charge/discharge conditions: (a) 0.5 C/0.5 C (b) 1.0 C/1.0 C.

at higher current densities. At higher current density of 10 C, the Al2O3-PI separator holds 73.91% of the discharge capacity at 0.2 C. On the other hand, the PP and PI separators present values of 18.25% and 68.65%, respectively. The larger differences in discharge capacities for the separators could be attributed to the electrolyte

(a)

80 70

-Z'' (Ohm)

60 50 40 30 20 10 0

(b)

80

0

10

20

30

40

50

60

70

80

0

10

20

30

40

50

60

70

80

70

-Z'' (Ohm)

60 50 40 30 20 10 0 80

(c)

70

-Z'' (Ohm)

60 50 40 30 20

4. Conclusions

10 0

wettability and the corresponding difference in the Liþ-ion transport. The Al2O3 coating layer offers nano-porous structures consisting of hydrophilic Al2O3 nanoparticles and PVdF-HFP binder. The favorable structure of the Al2O3-PI separator would facilitate ionic diffusion and electrolyte retention for superior rate capability. Cycle performance is evaluated at constant current charge/ discharge (0.5 C/0.5 C, 1 C/1 C) conditions (Fig. 6). A Li(Ni0.5Co0.2Mn0.3)O2/LiMn2O4 blended cathode matched with a graphite anode shows an impressive cycle life at 0.5 C cycling. The higher discharge capacities of the PI and Al2O3-PI separator coin cells relative to those of the PP separator cell are also reproduced in this test. At higher current density of 1 C for 200 cycles, the capacity retention ratios of the PP, PI and Al2O3-PI separator cells are 86.87%, 94.94%, and 95.56%, respectively. In the case of the PP separator coin cell, a continuous capacity drop is observed for the initial 40 cycles, which is mainly responsible for the inferior cyclability data. To further understand the outstanding electrochemical performance of the Al2O3-PI separator, electrochemical impedance spectra are compared between the 10th and the 200th cycles (Fig. 7). The cells are fully discharged (2.8 V) before measurement after constant current charge/discharge (0.5 C/0.5 C) cycles. The electrode dimension, mass loading, and electrolyte are the same and only the separators are varied among the cells. The PP separator cell shows the highest initial impedance, 39.3 U, while the PI separator cell provides the lowest initial impedance, 18.5 U. After 200 cycles of charge and discharge, however, the impedance values are increased by 24.1 U and 32.6 U, respectively. Therefore, in the case of the PI separator, the initial advantage in the cell impedance is not fully maintained during long term cycling. This seemingly could be attributed to greater electrolyte decomposition at the electrode/electrolyte interface owing to larger amount of electrolyte in the separator. After coating the PI separator with Al2O3, however, the cell impedance is increased only by 19 U during 200 cycles, and hence the final impedance is 45.8 U, which is a much smaller value compared to PP (63.4 U) and PI (51.1 U) separators. The Al2O3 nanoparticle layer in the Al2O3-PI separator may play a key role in alleviating the increase of cell impedance. It can reduce the total amount of electrolyte in contact with electrodes while the core PI separator can facilitate Liþ-ion transport through the large pore structure.

0

10

20

30

40

50

60

70

80

Z' (Ohm) Fig. 7. Nyquist plots of coin full cells with the (a) PP, (b) PI and (c) Al2O3-PI separators: green square and brown circle plots indicate the impedance spectra after 10 and 200 cycles, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

A nonwoven polyimide separator sandwiched between thin Al2O3 overlayers is synthesized by an electrospinning route followed by dip-coating of Al2O3 nanopowders. The Al2O3-PI separator shows a distinctive structure comprised of an inner PI nanofiber support layer and an outer Al2O3-coating layer with nano-sized pores, resulting in impressive thermal stability, superior electrolyte wettability and high porosity. The Al2O3-coated PI separator shows ideal electrochemical performance including

J. Lee et al. / Journal of Power Sources 248 (2014) 1211e1217

outstanding rate capability and high capacity owing to the PI inner membrane as well as a limited cell impedance increase during long term cycling due to the Al2O3 surface layer. The Al2O3-PI separators are thus a promising separator candidate for next-generation lithium ion batteries. Acknowledgments This work was supported by the Center for Integrated Smart Sensors funded by the Ministry of Education, Science and Technology as a Global Frontier Project (CISS-2012M3A6A6054188). I.D. Kim acknowledges support by the Engineering Research Center (ERC-N01120073) program from the Korean National Research Foundation. References [1] J. Hassoun, B. Scrosati, Adv. Mater. 22 (2010) 5198e5201. [2] J. Hassoun, S. Panero, P. Reale, B. Scrosati, Adv. Mater. 21 (2009) 4807e4810. [3] Y.S. Hu, R. Demir-Cakan, M.M. Titirici, J.O. Muller, R. Schlogl, M. Antonietti, J. Maier, Angew. Chem. Int. Ed. 47 (2008) 1645e1649. [4] M. Rosa Palacin, Chem. Soc. Rev. 38 (2009) 2565e2575. [5] H. Li, Z. Wang, L. Chen, X. Huang, Adv. Mater. 21 (2009) 4593e4607. [6] G. Wang, H. Liu, J. Liu, S. Qiao, G.M. Lu, P. Munroe, H. Ahn, Adv. Mater. 22 (2010) 4944e4948. [7] H.-W. Lee, P. Muralidharan, R. Ruffo, C.M. Mari, Y. Cui, D.K. Kim, Nano Lett. 10 (2010) 3852e3856. [8] P. Arora, Z.M. Zhang, Chem. Rev. 104 (2004) 4419e4462. [9] G. Venugopal, J. Moore, J. Howard, S. Pendalwar, J. Power Sources 77 (1999) 34e41. [10] S.S. Zhang, J. Power Sources 164 (2007) 351e364. [11] X. Huang, J. Solid State Electrochem. 15 (2011) 649e662. [12] I. Uchida, H. Ishikawa, M. Mohamedi, M. Umeda, J. Power Sources 119 (2003) 821e825. [13] M.S. Wu, P.C.J. Chiang, J.C. Lin, Y.S. Jan, Electrochim. Acta 49 (2004) 1803e 1812. [14] Y.M. Lee, J.W. Kim, N.S. Choi, J.A. Lee, W.H. Seol, J.K. Park, J. Power Sources 139 (2005) 235e241. [15] A. Mathur, U.S Patent 6,517,676, 2003. [16] S.J. Law, H. Street, G.J. Askew, U.S Patent 6,358,461, 2002. [17] Y. Sudou, H. Suzuki, S. Nagami, K. Ikuta, T. Yamamoto, S. Okijima, S. Suzuki, H. Ueshima, U.S Patent Appl. 20060073389, 2006.

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