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2 Journal of Membrane Science 461 (14) Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: Performance of through-hole anodic aluminum oxide membrane as a separator for lithium-ion battery Jingjuan Chen a,b, Suqing Wang a,n, Liangxin Ding a, Yanbin Jiang a, Haihui Wang a,n a School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 51641, Guangdong, China b School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 243, Anhui, China article info Article history: Received 2 January 14 Received in revised form 16 February 14 Accepted 3 March 14 Available online 12 March 14 Keywords: Lithium-ion battery AAO membrane Separator LiFePO 4 abstract A high porosity through-hole anodic aluminum oxide (AAO) membrane has been prepared by a two-step anodization procedure. The as-prepared AAO membrane is around μm thick with a uniform pore size of about 8 nm. Moreover, we also found that the AAO membrane not only has a good mechanical strength but also shows better performances on electrolyte infiltration and retention compared with the commercial polymer separator. The LiFePO 4 /graphite battery using the AAO separator exhibits better cycling stability, rate capability and low-temperature performance than those using the polymer separators. The excellent electrochemical performances can be attributed to good hydrophilic characteristic, high porosity and through-pore structure of the AAO separators used. Therefore, the AAO separator is very promising to be applied in lithium-ion batteries. & 14 Elsevier B.V. All rights reserved. 1. Introduction Lithium ion batteries (LIBs) are electrochemical energy storage devices with unique ability to deliver the stored chemical energy as electric energy efficiently without any kind of gaseous exhaust [1 5]. So far, LIBs have monopolized the portable electronic market (used in mobile phones and laptop computers), but their applications in high power equipments are mostly restricted by safety [6]. The separator plays very important roles and is crucial to the safety and durability of LIBs [7 1]. The most commercially used microporous polymer separators are made of polyethylene (PE) or/and polypropylene (PP) which have low melting points and poor mechanical strengths. Thus, the polymer separators could easily be punctured by lithium dendrite generated at high rate or long cycling test. Besides, the polymer separators usually show poor wettability with organic electrolyte due to their inherent hydrophobic property and low porosity (45%) [11 18]. The above-mentioned defects would induce safety issues and affect the overall electrochemical performance, which would severely hinder the high safety and high power applications of LIBs, such as HEVs and EVs [19 21]. In order to overcome these drawbacks, many strategies have been proposed, such as non-woven fabric mats [22], microporous membranes based on PVdF [8,23 24] and polymer incorporated with the ceramic fillers (such as SiO 2 and Al 2 O 3 ) [16,25 26]. But the presence of organic materials still affects the thermal stability of the separators at higher temperature (41 1C) and does not solve the safety problems [11]. In our previous work, inorganic membranes such as porous Al 2 O 3 membranes and porous SiO 2 membranes, prepared by a high-temperature sintering process, have been investigated as the separators for high safety LIBs [27,28]. However, the thickness of the Al 2 O 3 and the SiO 2 separators are as high as μm, limiting the volume energy density of the batteries. Furthermore, the curving and irregular pores in these separators prolong the diffusion path of lithium ions in the separators and reduce the rate capabilities of batteries. Therefore, a thinner inorganic separator with through-hole structure is an optimal choice to solve the above issues in LIBs. Herein, we proposed a through-hole porous anodic aluminum oxide (AAO) membrane as a separator for LIB. The prepared AAO membrane is around μm thick and its mechanical strength is sufficient for battery assembly. The through-hole AAO membrane also has a high porosity of 72% for adsorbing enough electrolytes to get high ionic conductivity. The electrochemical performances of the LiFePO 4 /graphite batteries using the AAO separator have been investigated in detail. For comparison, the commercial polymer separator (Celgard ) was tested under the same conditions. 2. Experimental section 2.1. Preparation of AAO membranes n Corresponding authors. Tel./fax: þ addresses: cesqwang@scut.edu.cn (S. Wang), hhwang@scut.edu.cn (H. Wang). AAO membranes used in this work were fabricated using a two-step anodization process [29,3]. High-purity aluminum foil /& 14 Elsevier B.V. All rights reserved.

3 J. Chen et al. / Journal of Membrane Science 461 (14) (99.999%,.3 mm thick) was cut into discs ( 16 mm) with tailing end of 5 5mm 2 and pressed with a tablet machine, followed by annealing at 5 1C for 5 h under Ar atmosphere. The as-prepared Al specimens were degreased in acetone and etched in.1 M NaOH aqueous solution to remove the surface oxide layer. The specimens were then electropolished in a mixture of perchloric acid and ethanol (HClO 4 :C 2 H 5 OH¼1:4, v/v) at a constant voltage of 15 V for 3 min to remove surface irregularities. The first-step anodization was carried out in a.3 M oxalic acid solution for 6 h, and the solution temperature was keep at 1C in an ice-water bath. Next, the AAO layer was removed by immersing the specimens in a mixture solution of 6 wt% H 3 PO 4 and 1.8 wt% H 2 CrO 4 at 8 1C for 6 h. Highly ordered porous AAO was prepared by second anodization of the textured Al surface, and the reaction time was extend to 24 h. Through-hole AAO membranes were obtained by voltage pulse detachment method in a mixture of perchloric acid and ethanol (HClO 4 :C 2 H 5 OH¼1:1, v/v) by applying voltage of 45 V. The constant voltage was applied by a DC regulated power supply (Digital, Manson SDP 23). At last, the AAO membranes was immersed in 5 wt% H 3 PO 4 for 3 min to enlarge the pores and then treated at 1C for 5 h to remove any impurities. The asprepared AAO membranes were ultrasonically cleaned with acetone and deionized water, and then vacuum dried at 8 1C for 12 h Characterization of the AAO membranes The morphology and microstructure of the AAO membrane were characterized by scanning electron microscopy (SEM, LEO 153 VP). The contact angles of the separators with organic electrolyte of 1 M LiPF 6 /ECþDEC (1:1, w/w) were tested by the contact angle tester (Dataphysics OCA Micro). The porosity of the AAO separator was calculated using the following equation [27]: Porosity ¼ 1 m 1% ð1þ ρv where m is the mass of the AAO separator, ρ is the density of Al 2 O 3 (3.98 g cm 3 ) and V is the volume of the AAO separator. Electrolyte infiltration and retention of the separators were evaluated as described in Ref. [27]: Electrolyte uptake ¼ W 1 W W 1% ð2þ Electrolyte retention ¼ W x W 1% ð3þ W 1 W where W is the weight of separator, W 1 is the initial weight of the separator after absorbing the electrolyte (1 M LiPF 6 /ECþDEC (1:1, w/w)) for 1 h, W x is the equilibrium weight of the electrolyteinfiltrated separator stored at 5 1C after x min. Two parallel measurements were carried out for both separators under the same conditions. The batteries for ionic conductivity tests were fabricated by sandwiching the electrolyte-infiltrated separators between two stainless steel (SS) discs, and the electrochemical impedance spectroscopy (EIS) was measured on an electrochemical workstation (Zahner IM6ex) with the potential perturbation of 1 mv over a frequency range from 1 MHz to.1 Hz. Each measurement was carried out after the electrolyte or the testing batteries were held at a certain temperature for 1 h to reach full thermal equilibration. The ionic conductivity (s) was calculated according to the following equation [1, 28]: s ¼ l ð4þ RS where l is the thickness of the separator, R is the bulk electrolyte resistance and S is the area of the separator Electrochemical measurements A graphite electrode consisting of 85 wt% graphite (Hitachi Powdered Metals Co. Ltd.), 5 wt% acetylene black and 1 wt% PVdF and a positive electrode consisting of 75 wt% LiFePO 4 (Tianjin STL Co. Ltd.), 15 wt% acetylene black and 1 wt% PVdF were made on copper foil and aluminum foil, respectively. The specific capacity of the LiFePO 4 /graphite battery was calculated based on the mass of LiFePO 4. The electrolyte was 1 M LiPF 6 /ECþDEC (1:1, w/w). CR32 coin-type cells were assembled in an argon-filled glove box (Mikrouna). The batteries were cycled over the voltage range of V using a NEWARE Battery Testing System. For the low temperature tests, all the batteries were cycled three times and charged to 4. V at the rate of.5 C at room temperature, then transferred into the low temperature chamber and held for 1 h for equilibrium, followed by being discharged to 2.4 V at.5 C. The impedances of the LiFePO 4 /graphite batteries were measured on the Zahner IM6ex electrochemical workstation. The frequency range was set from.1 Hz to 1 MHz with the potential amplitude of 5 mv. Before the impedance measurement, the batteries were cycled three times at.5 C and subsequently charged to 5% of overall state of charge (SoC¼5%). 3. Results and discussion Fig. 1 shows the SEM images of the as-prepared AAO membrane. As displayed in Fig. 1, the open-hole structure is successfully fabricated. Well-ordered nanopore arrays are observed in the cross-section view of the AAO membrane (Fig. 1(b)); the diameter of the pores is about 8 nm and the interpore distance is about 15 nm. The porosity of the AAO membrane is 72% calculated by Eq. (1), which is much higher than that of the commercial polymer separator ( 45%) [14 16]. Large pores and through-hole structure of the AAO separator can shorten the diffusion path of lithium ion and minimize the ionic resistance, resulting in the improvement of Fig. 1. SEM images of AAO membrane (a) top view, (b) cross-section view and (c) bottom view.

4 24 J. Chen et al. / Journal of Membrane Science 461 (14) the electrochemical performance of the battery [31]. The asprepared AAO membrane is stable under the pressure of MPa applied by the tablet machine, indicating the mechanical strength of the AAO membrane can meet the requirements of the separator used in large-scale battery stacks [32,33]. The wettability of the separator with liquid electrolyte is a very important parameter [8]. A separator with good wettability can retain the electrolyte effectively and facilitate electrolyte diffuse smoothly. The wettabilities of the polymer separator and the AAO separator are evaluated by contact angle measurements (Fig. 2). The contact angle of the polymer separator with electrolyte is 14.31, indicating its poor wetting property. The poor wettability of the separator leads to high cell resistance and then deteriorates the electrochemical performance of LIBs, which severely hampers the high power applications of LIBs [18,34]. For the AAO separator, the droplet of electrolyte is instantly infiltrated into the membrane and the contact angle is as small as 37.21, due to the capillary force of the pores in the AAO membrane and the close polarity between AAO membrane and electrolyte [,26]. Separator should absorb and hold enough amount of electrolyte to provide a good media for ion transportation during charge discharge process. Here, the electrolyte uptake properties of the polymer separator and the AAO separator have been investigated. The electrolyte uptakes of the polymer separator and the AAO separator are wt% and 1 wt% calculated by Eq. (2), respectively. This means that the polymer separator and the AAO separator are filled with the electrolyte after soaking in the electrolyte, taking into account the density of separators and electrolyte (e.g..9 g cm 3 for polypropylene in Celgard, 3.98 g cm 3 for Al 2 O 3 and 1.22 g cm 3 for electrolyte). Then the time dependence of the normalized electrolyte retention of separators at 5 1C was calculated by Eq. (3) (Fig. 3). The electrolyte retentions of both separators decrease at the beginning. After 15 min, the electrolyte retention of the polymer separator decrease to 43.7% while that of the AAO separator retains up to 79.5%. Moreover, no electrolyte loss was found in the AAO separator during the period from 15 min to min. The excellent electrolyte retention of the AAO separator is mainly attributed to the strong affinity of the hydrophilic Al 2 O 3 toward solvent molecules and the capillarity of the nanopores in AAO separator [26,35,36]. One of the important requirements of a separator for LIB is its ability to transport lithium ions. Fig. 4 shows the ionic conductivities of the liquid electrolyte and both separators infiltrated with electrolyte under different temperatures. The ionic conductivities of the electrolyte-infiltrated AAO separator are lower than those of the liquid electrolyte, but higher than those of the polymer separator infiltrated with electrolyte in the whole temperature range. At room temperature ( 1C), the AAO separator infiltrated with electrolyte has an ionic conductivity of 1.96 ms cm 1, which is higher than that of the polymer separator (.67 ms cm 1 ). The ionic conductivity of the AAO separator infiltrated with electrolyte reaches.64 ms cm 1 even at 1C, which means that the battery using the AAO separator can be used at low temperatures. The high ionic conductivity of the AAO separator infiltrated with electrolyte can be attributed to its high porosity [27]. Fig. 5 shows the electrochemical performances of LiFePO 4 / graphite batteries using different separators. In Fig. 5(a), the battery using the polymer separator has an initial discharge capacity of 88. mah g 1 with a coulombic efficiency of 66.3%. The battery using the AAO separator exhibits an initial discharge capacity of 12.9 mah g 1 with a coulombic efficiency of 72.6%. In Fig. 5(b), the battery using the AAO separator has higher discharge capacity than that using the polymer separator. The specific capacities of both batteries increase during the first few Normalized Electrolyte Retention (%) 1 8 AAO separator Polymer separator Time (min) Fig. 3. Time dependence of the electrolyte retention of the polymer separator and the AAO separator at 5 1C. Conductivity (ms cm -1 ) a c b a: liquid electrolyte by conductivity meter b: polymer separator + electrolyte by EIS c: AAO + electrolyte by EIS Temperature ( o C) Fig. 4. Dependence of ion conductivity on temperature for liquid electrolyte, the electrolyte-infiltrated the polymer separator and the AAO separator. Fig. 2. Contact angle images of (a) the polymer separator and (b) the AAO separator with the electrolyte of 1 M LiPF 6 /ECþDEC (1:1, w/w).

5 J. Chen et al. / Journal of Membrane Science 461 (14) Voltage (V) Polymer Separator AAO Separator C.5C 1C 2C Polymer Separator AAO Separator 5C 1C.2C Cycle Number Polymer Separator AAO Separator 8 1 Cycle Number Fig. 5. Initial voltage profiles (a) and cycling performance (b) of the LiFePO 4 / graphite batteries using the polymer separator and the AAO separator. cycles due to the infiltration of electrolyte, while the battery using the polymer separator increases more significantly, due to the poorer wettability of the polymer separator with organic electrolyte. The first cycle capacity loss is commonly attributed to the formation of the solid electrolyte interface (SEI) layer [37]. After two cycles, the coulombic efficiencies of both batteries maintain at over 9%. The other reason for the improved performance of the battery using the AAO separator is that the Al 2 O 3 can capture trace amounts of moisture and acidic impurity in the electrolyte to reduce the side reactions [27,38]. The rate capabilities and low-temperature performances of LiFePO 4 /graphite batteries are also investigated (Fig. 6). The discharge capacities drop with the current rate increasing for both batteries. The battery using the AAO separator always exhibits higher discharge capacity than that using the polymer separator at the same current rate. At.2 C, the discharge capacities of batteries using the polymer separator and the AAO separator are 14.7 and mah g 1, respectively. When the current rate increases to 1C, the discharge capacities of batteries using the polymer separator and the AAO separator are 54.5 and 62.3 mah g 1, respectively. Fig. 7 shows the discharge performances of the batteries using different separators at low temperatures. For the battery using the polymer separator, the discharge capacities are only 77.1 mah g 1 at 1C and 23.7 mah g 1 at 1C. For the battery using the AAO separator, the discharge capacities increase to 86.2 mah g 1 at 1C and 35.7 mah g 1 at 1C. In addition, the discharge voltage plateaus drop for both batteries at low 1 8 Coulombic Efficiency (%) Fig. 6. Rate capabilities of the LiFePO 4 /graphite batteries using the polymer separator and the AAO separator. Voltage (V) b d a: Polymer Separator o C b: Polymer Separator - o C c: AAO Separator o C d: AAO Separator - o C Fig. 7. Discharge curves of the LiFePO 4 /graphite batteries using the polymer separator and the AAO separator at low temperatures. temperature, resulting from the increase of the lithium ion transfer resistance. The discharge voltage plateaus of the battery using the AAO separator are significantly higher than those using the polymer separator, attributing to the unique structure of the AAO separator. The through-hole structure of the AAO could minimize the ionic resistance of the battery [31]. As seen above, the rate performance and low-temperature performance of the LiFePO 4 /graphite battery are significantly improved by replacing the polymer separator with the AAO separator. In order to further understand the influence of the separator on the electrochemical performance, the impedance of the batteries is investigated by EIS. Fig. 8 shows the Nyquist plot of the batteries using the polymer separator and the AAO separator at room temperature. The impedance spectra are fitted using an equivalent circuit (insert of Fig. 8). According to previous literatures [39,], the ohmic resistance R e represents the resistance of the electrolyte; the (CPE 1 ) (R f ) parallel elements are related to solid electrolyte interface (SEI) film, in which R f is SEI film resistance and CPE 1 is a capacitive element usually related to the electrode roughness and thickness. The (CPE 2 ) (R ct ) parallel elements are used to simulate the electronic and ionic conductive resistance between electrode and electrolyte. R ct is the charge transfer resistance, which is related to the combined transfer ability of electrons and ions. Warburg impedance Z w is related to the solid-state diffusion a c

6 26 J. Chen et al. / Journal of Membrane Science 461 (14) Z"(Ω) of Li-ions in the LiFePO 4 particles, corresponding to the slopping line at the low frequency. The fitting impedance results of the equivalent circuit are listed in Table 1. All the fitting values of the battery using the AAO separator are smaller than those using the polymer separator, attributing to the higher ionic conductivity and excellent wettability with electrolyte by using the AAO separator, which is in agreement with the results of electrochemical performances. 4. Conclusions The through-hole AAO membrane has been prepared by a two step anodization process. The as-prepared AAO membrane is μm thick with a high porosity of 72% and good mechanical strength. It also exhibits excellent wettability with organic electrolyte due to the capillary force of the pores in AAO membrane and the close polarity of Al 2 O 3 with electrolyte. The electrolyte infiltrated AAO membrane exhibits excellent ionic conductivity and high electrolyte retention performance. Furthermore, the LiFeO 4 /graphite batteries using the AAO separators exhibit higher rate capabilities and better low temperature performances than that using the polymer separator. The above-mentioned results make the AAO membrane a promising separator for use in high safety and high power LIBs. Acknowledgment Z'(Ω) Fig. 8. Nyquist plots of the LiFePO 4 /graphite batteries using the polymer separator and the AAO separator, the inset is equivalent circuit used for fitting the experimental EIS data. Table 1 Fitted impedance parameters of the LiFePO 4 /graphite batteries using the polymer separator and the AAO separator. 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