Journal of Membrane Science 509 (2016) 19–26

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Journal of Membrane Science

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

A thin inorganic composite separator for lithium-ion batteries

Yaocheng Zhang a, Zhonghui Wang a, Hongfa Xiang a,n, Pengcheng Shi a, Haihui Wang b,c,nn

a School of Materials Science and Engineering, Hefei University of Technology, Anhui Hefei 230009, PR Chinab School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, PR Chinac School of Chemical Engineering, The University of Adelaide, Adelaide SA 5005, Australia

a r t i c l e i n f o

Article history:Received 22 January 2016Received in revised form19 February 2016Accepted 21 February 2016Available online 24 February 2016

Keywords:Lithium-ion batteryInorganic composite separatorAluminum oxide

x.doi.org/10.1016/j.memsci.2016.02.04788/& 2016 Elsevier B.V. All rights reserved.

esponding author at: School of Materials Sciity of Technology, Anhui Hefei 230009, PR Chresponding author at: School of Chemical Ene, Adelaide SA 5005, Australia.ail addresses: hfxiang@hfut.edu.cn (H. Xiang),@scut.edu.cn (H. Wang).

a b s t r a c t

A thin inorganic composite membrane composed of 94 wt% Al2O3 and 6 wt% styrene-butadiene rubber(SBR) polymer binder is prepared via an aqueous solution casting process. 1 wt% polyethylene glycol(PEG) is introduced into the casting suspension for the preparation of a 37 mm-thick inorganic compositeseparator. PEG plays a key role to enhance the stability of the casting suspension to separate the thinmembrane from the substrate, and to increase the porosity of the membrane. The as-prepared Al2O3/SBRseparator shows a superior thermal stability under 130 °C with no any shrinkage, higher electrolyteuptake/retention and ionic conductivity than the common polyethylene (PE) separator. In LiNi1/3Co1/3Mn1/3O2|graphite cells, the inorganic composite separator exhibits excellent cycling stability and goodrate performance.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

After successful applications in mobile electronic devices, li-thium-ion batteries (LIBs) have been regarded as promising powersources for electric vehicles (EVs) and large-scale energy storagesystems (ESSs) in recent years because of their advantages onenergy density, power density and cycle lifetime [1–4]. In a typicalLIB, an electrolyte-permeable porous separator is necessarily usedto keep active electrodes (cathode and anode) apart but to permitthe free flow of lithium ions through the liquid electrolyte filling intheir open porous structure [5]. Even though the separator doesnot directly participate in the electrochemical reactions in a bat-tery, it does significantly affect the cell performance of the batteryas well as safety characteristics. Microporous membranes madefrom polyethylene (PE) and polypropylene (PP) matrices havewidely used as separators for commercial lithium-ion batteriesdue to their good chemical stability and mechanical strength atroom temperature. However, these organic separators usually havelow melting points (135 °C for PE and 165 °C for PP) so that theycould undergo obvious dimensional shrinkage even at the lowertemperatures than their melting points, which may result in thedirect contact of the anode and the cathode, i.e., short circuit. Thisinternal short circuit may trigger thermal runaway of LIBs and

ence and Engineering, Hefeiina.gineering, The University of

even a fire or explosion. Additionally, poor wettability of thepolyolefin separators to nonaqueous electrolytes limits their high-power applications for EVs and ESSs [6,7].

Inorganic ceramic materials have been investigated as fillersinto the polymer matrix to obtain the composite polymer se-parator with an enhanced thermal stability and electrolyte wett-ability [8–11]. In our previous papers [12,13], we prepared purelyinorganic Al2O3 and SiO2 porous membranes, which was used asseparators in LiFePO4|graphite or LiMn2O4|Li cells and showedhigher discharge capacity and rate capability, and better low-temperature performance than those using the commercial poly-mer separators due to their high porosity and wettability. In orderto combine the advantages on the flexibility of composite polymerseparators and the thermal stability of purely inorganic separators,inorganic composite separators containing inorganic ceramicpowders as the main component are attractive [13–15]. The in-organic composite separators are composed of a high content ofceramic powders and a small amount of polymer binders or ma-trix. The former provides the separator with excellent thermalstability and electrolyte wettability, while the latter is designed forthe flexibility of the separator. Recently, ceramic layers includingSiO2 [16], Al2O3 [17,18], TiO2 [19,20] were coated on commercialpolyolefin separators in order to minimize the thermal shrinkageof polyolefin separators. This type of composite separators in-herited the shutdown function of polyolefin separators, but alsotheir high cost. Recently, Jung et al. [21] developed a ceramic se-parator composed of Li7La3Zr2O12 and 10–20% polymer binder ongraphite negative electrode. Zhang et al. [14] blended CaCO3powers with Teflon emulsion (61.5% solid content in water) andhot-rolled the mixture into a self-standing membrane. However,

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Fig. 1. Photographs of the membranes containing different contents of SBR. (a) 6%, (b) 20% and (c) 30%.

Y. Zhang et al. / Journal of Membrane Science 509 (2016) 19–2620

that inorganic composite separator had the thickness of 175–190 mm, which was much thicker than commercial separators (25–40 mm). So far, it still keeps challenges to prepare a thin and self-standing inorganic composite separator. In order to achieve aninorganic composite separator with high porosity, the content ofthe polymer binder should be a low level, e.g. o10%. Afterwards,the main difficulty is the poor mechanical strength of a thincomposite separator with a small content of binder.

In this study, a thin and self-standing inorganic compositemembrane composed of 94% Al2O3 particles and 6% styrene-bu-tadiene rubber (SBR) binder was prepared by a solution castingmethod. In order to obtain a 37 mm-thick inorganic compositeseparator with the comparative thickness of commercial poly-olefin separators, polyethylene glycol (PEG) was used as a multi-functional agent in the membrane-making process. Firstly, it washelpful to evenly mix Al2O3 particles with the SBR binder. Sec-ondly, it acted as a pore-forming agent for increasing the porosityof the separator. Finally, it was the key to separate the thinmembrane from a glass substrate. During the membrane-makingprocess, deionized water was used as the only solvent/ dispersant,which made the preparation process environmental friendliness.Besides no thermal shrinkage, the thin inorganic composite se-parator exhibited attractive cell performance compared to thecommercial PE separator in LiNi1/3Co1/3Mn1/3O2|graphite cells. Inthe LiNi1/3Co1/3Mn1/3O2|graphite cells, the Al2O3/SBR separatorexhibits excellent compatibility with both the electrodes, longcycling stability and superior rate capability.

2. Experimental

2.1. Preparation of the composite separators

The inorganic composite separators were prepared by a solu-tion casting method. Neutral activated alumina (Al2O3) (300 mesh,Sinopharm Chemical Reagent Co., Ltd.) and Al2O3 ultrathin parti-cles (100–300 nm, Xianzheng Co., Ltd.) were mixed firstly by themass ratio of 1:3. Then the mixed Al2O3 particles and styrene-butadiene rubber (SBR, Jingbang Power Source Co., Ltd.) withpolyethylene glycol (PEG, M.W.¼2000, Sinopharm Chemical Re-agent Co., Ltd.) were mixed thoroughly in deionized water to forma viscous slurry at room temperature. The mass of PEG is 1 wt%based on the total mass of Al2O3 and SBR. Then the slurry was cast

on a clean glass substrate and spread by blade to obtain a mem-brane with a controllable thickness. After drying at 55 °C for 6 h,the glass substrate with the membrane was immersed in deio-nized water for 1 h to remove PEG. At the same time, the wetmembrane was spontaneously separated from the substrate. Afterdrying at 55 °C for 24 h, the as-prepared membrane was cut intodiscs (Φ16 mm) for testing. For comparison, the no-PEG-assistedmembranes with different ratios of Al2O3 particles and SBR binderwere prepared in a similar process with the addition and removalof PEG.

2.2. Physical and electrochemical measurements of the separators

The morphologies of the composite separators were in-vestigated by a field emission scanning electron microscopy(FESEM, Hitachi S-4800). The thermal shrinkage of separators wasobserved after they were placed in an oven and heated at 130 °Cfor 40 min. The thermal stability of the separators was evaluatedby differential scanning calorimetry (DSC, TA DSC 2920) in thetemperature range of 50–200 °C at a heating rate of 10 °C min�1

and thermogravimetric (TG) analysis (TGA 6000, Seiko Instru-ments) in N2 atmosphere from 100 to 800 °C at a heating rate of20 °C min�1. The contact angles between separators and theelectrolyte of 1 M LiPF6/ethylene carbonate (EC)þdimethyl car-bonate (DMC) (1:1, w/w) were tested by the contact angle tester(DSA10-Mk2, KRUSS Gmbh Germany). As described in our pre-vious paper [12], the electrolyte uptake (U) and retention ratio (R)were calculated according to the following equations.

=−

×( )

UW W

WElectrolyte uptake 100%

11 0

0

=−−

×( )

RW WW W

Electrolyte retention 100%2

x 0

1 0

where W0 is the net weight of pristine separator, W1 and Wx arethe initial and equilibrium weights of the wet separator after ab-sorbing the electrolyte solution and standing at 50 °C for 30 min atleast, respectively. For each testing point, three parallel measure-ments were carried out under the same conditions. For compar-ison, the electrolyte retention (R) of some separator was normal-ized based on the 1st point of each measurement. All the

Fig. 2. Photographs and SEM images of the membrane cast on the glass substrate. (a) photograph of the membrane without PEG, (b) photograph of the membrane with PEG,(c) top surface SEM images of membrane with PEG, (d) cross section SEM images of membrane with PEG, (e) bottom surface SEM images of the membrane with PEG,(f) bottom surface SEM images of the membrane without PEG.

Y. Zhang et al. / Journal of Membrane Science 509 (2016) 19–26 21

measurements were carried out in an Ar-filled glove box(MBraun).

2.3. Electrochemical performance measurements

The ionic conductivities of the separators immersed with theelectrolyte solution of 1 M LiPF6/ECþDMC (1:1, w/w) were mea-sured by an electrochemical impedance spectroscopy (EIS) methodusing a CHI660e electrochemical work station (Shanghai Chen-hua). The impedance measurements were performed on theelectrolyte-immersed separators sandwiched between two

stainless steel electrodes over a frequency range of 100 kHz to 1 Hzwith AC amplitude of 10 mV. The ionic conductivity (s) was cal-culated by the following equation:

σ =× ( )d

R AIonic conductivity

3b

where d and A are the thickness and the effective area of the se-parator, respectively, and Rb is the bulk impedance of the wetseparator obtained at the high frequency intercept of the Nyquistplot on the real axis.

Fig. 3. Photographs of composite membranes with 1% PEG and different contents of SBR. (a) 4%, (b) 5% and (c) 6%.

Fig. 4. Schematic illustration of the membrane making process. (a) Without PEG and (b) with PEG.

Y. Zhang et al. / Journal of Membrane Science 509 (2016) 19–2622

Cell performance was evaluated in CR2032-type coin cells witha LiNi1/3Co1/3Mn1/3O2 positive electrode and a graphite negativeelectrode. To make a positive electrode laminate, 80 wt% LiNi1/3Co1/3Mn1/3O2 (Umicore, Belgium), 6 wt% conductive carbon ad-ditive (KS-6, TIMCAL), 6 wt% conductive carbon additive (Super-P,TIMCAL) and 8 wt% polyvinylidene fluoride (PVDF, aladdin) weredispersed in 1-methyl-2-pyrrolidinone (NMP, Sinopharm Chen-mical Reagent Co., Ltd.) to form a slurry, which was then spreadonto an aluminum foil current collector. To make a negativeelectrode laminate, 90 wt% commercial graphite (FT-1), 5 wt%conductive carbon additive (SFG, TIMCAL), 2 wt% carboxymethylcellulose (CMC, aladdin) and 3 wt% SBR were dispersed in deio-nized water, which was then spread onto a copper foil currentcollector. After vacuum drying at 80 °C for 10 h, both the positiveand negative laminates were punched into discs (Φ14 mm) forassembling the cells. The mass loadings of the positive and ne-gative electrodes were controlled at �4.0 mg cm�2 and�1.9 mg cm�2, respectively. The specific capacity of the LiNi1/3Co1/3Mn1/3O2|graphite full-cell was calculated on the mass of theLiNi1/3Co1/3Mn1/3O2 cathode. The electrolyte used was 1 M LiPF6/ECþDMC (1:1, w/w). The LiNi1/3Co1/3Mn1/3O2|graphite cells wereassembled in the glove box.

The cell performance tests on the LiNi1/3Co1/3Mn1/3O2|graphite

cells were performed between 2.5 and 4.3 V on an Arbin BT2000battery cycler. All the cells were initially cycled twice at 0.1 C(1 C¼150 mA g�1) for activation. The cycling performance wasmeasured at 0.5 C and rate capabilities was obtained at differentcurrent densities ranging from 0.5 to 8 C for charge and discharge.

3. Results and discussion

Zhang et al. [14] prepared the inorganic composite separatorcontaining 92% CaCO3 and 8% PTFE by a facile solution castingmethod, but its big thickness of 175–190 mm was not welcome inviews of volumetric energy density and cell resistance. However,there are many challenges for the thin inorganic composite se-parator. Firstly, a big challenge is originated from the preparationof thin inorganic composite membranes by solution casting, e.g.,the comparable thickness of commercial polyolefin separators,o40 mm. As shown in Fig. 1, it is quite difficult to prepare anunbroken membrane using a low content of polymer binder (6%SBR in Fig. 1a).

A high content of polymer binder, exceeding 20% (Fig. 1b), even30% SBR is necessary to get an unbroken membrane, but theporosity of the membrane will dramatically decrease with

Fig. 5. Thermal stability of the inorganic composite separator and the PE separator. Photographs (a) before and (b) after being held at 130 °C for 40 min, (c) DSC curves,(d) TG curves.

Y. Zhang et al. / Journal of Membrane Science 509 (2016) 19–26 23

increasing the polymer composition (Fig. 1c). The porosity istightly related to the mass ratio of Al2O3 and SBR. The higher massratio of Al2O3 and SBR (the lower content of SBR), the higherporosity of the received membrane. In order to prepare a flexible,thin and highly porous membrane, two issues need be solved:(1) a low content of polymer binder is used for high porosity of thecomposite membrane, but the binder can strongly anchor the tinyinorganic powers to build up a flexible membrane. One of the keyissues is the good dispersibility of the binder in the inorganicpowers suspension; (2) it is a big challenge to separate a thinmembrane from a substrate after the solution casting followed bydrying. It seems to be impossible to peel off a dry membrane withless than 40 mm-thickness and a low content of polymer binderstrongly adhered to the glass substrate.

In order to weaken the connection of the membrane and thesubstrate, we introduce 1 wt% PEG into the suspension for solutioncasting. After immersing the dry membrane adhered to the sub-strate into water, PEG will be dissolved into water and the mem-brane be separated spontaneously from the substrate. As shown inFig. 2a, the membrane prepared without PEG can be hardly se-parated from the glass substrate and broke into pieces since thecontent of polymer binder is low and the membrane is too thin.The membrane prepared with PEG can be easily peeled off and theas-prepared membrane is flexible (Fig. 2b). The top surface mor-phology of the membrane with PEG was characterized by SEM asshown in Fig. 2c. It is clearly shown that Al2O3 particles arehomogeneously distributed and connected by SBR binders. Theclear and homogeneous pores with the size of several hundrednanometers are formed after removing distilled water and PEG.The pore size of the inorganic composite membrane will be mainly

related to the size of Al2O3 particles. In our study, we use themixture of neutral activated alumina (Al2O3) (300 mesh) and Al2O3ultrathin particles (100–300 nm) by the mass ratio of 1:3. The bigAl2O3 particles are beneficial to the mechanical strength of themembrane, and the ultrathin particles provide the abundant mi-croporous structure. The porosity of the Al2O3/SBR separator isalso estimated to be around 68% based on the electrolyte uptake,the electrolyte density and the geometrical volume of the se-parator, which is much higher than that of the commercial poly-mer separators (�40%) [13]. From the cross-section SEM image(Fig. 2d), the thickness of the Al2O3/SBR membrane is about 37 mm.It is also possible to obtain much thinner membrane by increasingthe content of SBR or using a nonwoven matrix. The bottom sur-face (Fig. 2e) of the Al2O3/SBR membrane is also highly porous,quite similar as its top surface, suggesting that the membraneprepared with PEG is symmetrical. However, the bottom surface(Fig. 2f) of the membrane prepared without PEG is dense, whichclearly suggests the high content of the polymer binder in the sideclose to the glass substrate. The inhomogeneous dispersing of thepolymer binder directly results to the difficulties for the prepara-tion of the membrane and the poor mechanical properties of themembrane as well as the limitation for the ionic transportationbetween-in. In addition, it is also found that 6% SBR is the optimalcontent for the preparation of inorganic composite separators with1% PEG. As shown in Fig. 3, when the SBR content is lower than 6%(4% or 5%) it is difficult to form a thin and flexible membrane. Thatis, Al2O3 ultrathin particles (100–300 nm) with high surface arearequire a content of SBR binder no less than 6%. Therefore, theinorganic composite separator containing 6% SBR and with theassistance of 1% PEG is indicated as the Al2O3/SBR separator below.

Fig. 6. Wettability of the separators toward the electrolyte. (a) Contact angle for the PE separator, (b) contact angle for the Al2O3/SBR separator, (c) the time dependencenormalized electrolyte retention and (d) the impedance spectra of the separators.

Y. Zhang et al. / Journal of Membrane Science 509 (2016) 19–2624

Fig. 4 shows schematic illustration of the membrane prepara-tion process with and without PEG. Without PEG, the SBR in thewet membrane cast on the glass substrate is more or less apt toprecipitate to the bottom of the membrane during the dryingprocess. As a result, the dense bottom part of the compositemembrane was tightly adhered to the glass substrate so that themembrane cannot be separated from the substrate. Since thecontent of SBR is low, the Al2O3 particles on the top of the mem-brane lead to the poor mechanical strength and the membranewas very brittle. PEG has three roles during the membrane pre-paration process. Firstly, it acts as a dispersing agent, which en-hances the stability of the casting suspension during the dryprocess. PEG helps the polymer binder uniformly sticking theAl2O3 particles like a surfactant, which is quite important to obtainthe relatively symmetrical porous membrane. Secondly, PEG as asoluble polymer can help weaken the connection between theinorganic composite membrane and the glass substrate in thewater so that the thin membrane could be separated sponta-neously from the substrate. Last but not the least, PEG is a goodpore-forming agent after its removal from the membrane in thewater.

The thermal shrinkage of separators is closely related to thesafety of lithium-ion batteries, since a severe shrinkage caused bythe heat generation during cycling especially at high current rateswould result in the short circuit of batteries [22,23]. The thermalshrinkages of separators were observed by measuring the di-mensional change (area-based) after they were subjected to heattreatment at 130 °C for 40 min (Fig. 5a and b). Since PE has amelting point of about 135 °C and the corresponding microporous

separator is prepared through stretching processes, the PE se-parator easily lose dimensional stability above 100 °C [24,25]. Asshown in Fig. 5b, the PE separator has a big shrinkage of 84%.However, the Al2O3/SBR separator has almost no shrinkage. Thegood thermal stability of the Al2O3/SBR separator can be attributedto the thermally stable Al2O3 particles and high heat resistanceSBR as binder. It must be pointed out that compared with otherbinders such as PVDF and polymethyl methacrylate (PMMA), theelastomeric SBR possesses higher flexibility, stronger binding forceand higher heat resistance [17,26]. Fig. 5c and d shows the DSC andTG curves of the PE and the Al2O3/SBR separators, respectively. InFig. 5c, the PE separator shows an endothermic peak at 147.5 °C inthe DSC curve, corresponding to the melting point of PE. Mean-while, the Al2O3/SBR membrane shows no endothermic peak atthe same temperature range, which indicates that the Al2O3/SBRseparator is thermally stable between 50 and 200 °C. From Fig. 5d,it can be seen that up to 600 °C the PE separator experiences aweight loss of over 90%, but the inorganic composite separatoronly has a small weight loss of about 5% due to its low content ofpolymer binder. The low weight loss of the inorganic compositeseparator demonstrates its qualification to separate the cathodeand the anode at high temperatures.

The wettability of separators was evaluated by electrolytecontact angle test, and the results are shown in Fig. 6a and b. TheAl2O3/SBR separator was quickly wetted by electrolyte with con-tact angle of 0°, while the PE separator was hardly wetted byelectrolyte (with contact angle of 44.8°) because the nonpolar PEseparator has intrinsically poor wettability with the commercialnonaqueous electrolytes [13,27]. The superior electrolyte

0 50 100 150 200

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Volta

ge (V

)

Specific Capacity (mAh g )

(a)

Al O /SBRPE

0 20 40 60 80 1000

20

40

60

80

100

120

140

160

180

Cap

acity

(m

Ah

g)

Cycle number

PE Al O /SBR

(b)

0 20 40 60 80 100 120 140 160 1802.0

2.4

2.8

3.2

3.6

4.0

4.4

Volta

ge (V

)

Specific Capacity (mAh g )

PE

(c)

8C 4C 2C 1C 0.5C

0 20 40 60 80 100 120 140 1602.0

2.4

2.8

3.2

3.6

4.0

4.4

Volta

ge (V

)

Specific Capacity (mAh g )

(d)

Al O /SBR

8C 4C 2C 1C 0.5C

Fig. 7. Cell performance of the LiNi1/3Co1/3Mn1/3O2|graphite cells using the separators. (a) Initial voltage profiles at 0.1 C, (b) cycling performance at 0.5 C, rate capability of(c) the PE separator and (d) the Al2O3/SBR separator.

Y. Zhang et al. / Journal of Membrane Science 509 (2016) 19–26 25

wettability of the Al2O3/SBR separator is attributed to the hydro-philic nature of the oxide particles and the capillary force of thepores in the Al2O3/SBR separator. Moreover, the electrolyte uptakeof the Al2O3/SBR separator (0.8159 g cm�3) was larger than that ofthe PE separator (0.6768 g cm�3), due to the good electrolytewettability of inorganic particles and high porosity of the Al2O3/SBR separator. The time dependence of the normalized electrolyteretention of the separators at 50 °C is shown in Fig. 6c. After 3 hthe electrolyte stored in the Al2O3/SBR separator is 40%, but only23% in the PE separator. The result of electrolyte uptake is wellaccordance with the electrolyte contact angle testing. The Al2O3/SBR separator presents superior electrolyte wettability than the PEseparator, owing to the nano-porous structure and high hydro-philicity of Al2O3 particles toward nonaqueous electrolytes. It isknown that the wettability of separator greatly affects cell per-formance [28]. Considering the excellent electrolyte wettability,the Al2O3/SBR separator is expected to provide higher ion con-ductivity. Fig. 6d shows the Nyquist curves of the liquid electro-lyte-soaked separators. The straight line inclined towards the Z′axis represents the electrode/electrolyte double layer capacitancebehavior. Thus, the bulk resistance Rb of the separators can beacquired from the high-frequency intercept of the Nyquist plot onthe Z′ axis. It is seen that the bulk resistance of the Al2O3/SBRseparator (3.45Ω) is slight lower than the PE separator (3.86Ω).According to the Eq. (3) in Section 2, the ionic conductivity of theAl2O3/SBR separator is calculated to be 0.93 mS cm�1, which sur-passes that of the PE separator (0.40 mS cm�1). The higher ionicconductivity of the Al2O3/SBR separator is due to its better elec-trolyte wettability and higher porosity than the PE separator [29].These results suggest that the ionic conductivity of the Al2O3/SBR

separator can well meet the conductivity requirement of lithium-ion battery applications.

The cell performance of the inorganic composite separator isevaluated in the LiNi1/3Co1/3Mn1/3O2|graphite cells by comparingwith the PE separator. Fig. 7a shows the initial charge–dischargevoltage profiles of the LiNi1/3Co1/3Mn1/3O2|graphite cells using thePE separator and the Al2O3/SBR separator. The first discharge ca-pacity of the cell using the PE separator at 0.1 C is 161 mAh g�1,which is slightly higher than the cell using the Al2O3/SBR separator(156 mAh g�1). However, the cell using the Al2O3/SBR separatorexhibits the high Coulombic efficiency of 88%, compared that usingthe PE separator of only 81%. The improvement on Coulombic ef-ficiency is because of the reduced side reactions during the acti-vation process of the battery, since Al2O3 can capture the traceamounts of moisture and acidic impurity in the electrolyte [30].Fig. 7b shows the cycling performance of the cells with the PEseparator and the Al2O3/SBR separator. At 0.5 C, the cycling per-formance of both cells is very stable. But the discharge capacity ofthe cell using the Al2O3/SBR separator remains 90% of its initialcapacity after 100 cycles, while the cell using the PE separatorremains 87%. It can be attributed to the good wettability of Al2O3particles which offered expedite channels for Liþ ions to passthrough. The excellent cycling stability of the cell using the Al2O3/SBR separator strongly supports the good compatibility betweenthis inorganic composite separator with both the cathode and theanode.

Rate performance of the LiNi1/3Co1/3Mn1/3O2|graphite cells withthe PE separator and the Al2O3/SBR separator were also evaluatedat current rates of 0.5 C, 1 C, 2 C, 4 C and 8 C (Fig. 7c and d). At0.5 C, the discharge capacities of the cells using the polymer

Y. Zhang et al. / Journal of Membrane Science 509 (2016) 19–2626

separator and the Al2O3/SBR separator are 160 and 156 mAh g�1,respectively. At 1 C, the cell using the Al2O3/SBR separator deliversthe comparative capacity to that using the PE separator. However,the cell with the Al2O3/SBR separator shows better rate capabilitythan that with the PE separator at high current rates (2 C, 4 C and8 C). When the current rate increases to 8 C, the discharge capa-cities of the cells using the Al2O3/SBR separator and the PE se-parator are 128 and 105 mAh g�1, respectively. The reason for theimprovement on rate capability can be also attributed to thehigher uptake and the better wettability of the Al2O3/SBR se-parator toward the electrolyte as well as its high porosity. Theseresults on the cell performance suggest that this inorganic com-posite separator could be a promising choice in power batteries forEVs.

4. Conclusion

A 37 mm-thick inorganic composite membrane, consisting of94% Al2O3 and 6% SBR, has been prepared via a facile solutioncasting with 1% PEG. Since a very small amount of binder is nee-ded and the only solvent is deionized water during the mem-brane-making process, the Al2O3/SBR membrane is attractive inviews of environmental friendliness and cost. The effects of PEGduring making the thin membrane are from three ways: to en-hances the stability of the casting suspension as a dispersingagent, to separate the thin membrane from the substrate as a so-luble polymer, and to increase the porosity of the membrane as apore-forming agent. As the separator of LIBs, the Al2O3/SBRmembrane shows superior thermal stability, enhanced wettabilitytoward nonaqueous electrolyte, higher porosity and ionic con-ductivity compared with the commonly used PE separator. In theLiNi1/3Co1/3Mn1/3O2|graphite cells, the Al2O3/SBR separator ex-hibits excellent compatibility with both the electrodes, long cy-cling stability and superior rate capability. Nevertheless, the me-chanical strength of the inorganic composite membrane is notgood enough, still further work needs to be done for this inorganiccomposite separator with high strength. In conclusion, this workprovides a facile PEG-assisting way to prepare thin inorganiccomposite separators, which would be attractive for high perfor-mance lithium ion batteries with high safety characteristics.

Acknowledgments

This study was supported by the National Natural ScienceFoundation of China (Grant no. 51372060) and the Australian Re-search Council (ARC) through the Future Fellow Program(FT140100757).

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A thin inorganic composite separator for lithium-ion batteriesIntroductionExperimentalPreparation of the composite separatorsPhysical and electrochemical measurements of the separatorsElectrochemical performance measurements

Results and discussionConclusionAcknowledgmentsReferences