A water-based Al2O3 ceramic coating for polyethylene-based microporous separators for lithium-ion batteries

Journal of Power Sources 315 (2016) 161e168

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A water-based Al2O3 ceramic coating for polyethylene-based microporous separators for lithium-ion batteries Hyunkyu Jeon 1, Daeyong Yeon 1, Taejoo Lee, Joonam Park, Myung-Hyun Ryou**, Yong Min Lee* Department of Chemical and Biological Engineering, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon 34158, Republic of Korea

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


A water-based Al2O3 ceramic coating for polyolefin separators are developed.
The anionic surfactant, i.e., DLSS improves the ceramic coating efficiency.
DLSS containing Al2O3 coating solution maintains a uniform solution over time.
Al2O3 coated separators improve cycle performance and rate capabilities.
Al2O3 coated separators improve dimensional stability at high temperature storage.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 October 2015 Received in revised form 18 February 2016 Accepted 9 March 2016

To develop an environmentally friendly and cost-effective water-based inorganic coating process for hydrophobic, polyolefin-based microporous separators, the effect of surfactants in an aqueous inorganic coating solution comprising alumina (Al2O3) on polyethylene (PE)-based microporous separators is investigated. By using a selected surfactant, i.e., disodium laureth sulfosuccinate (DLSS), the aqueous Al2O3 coating solution maintained a dispersed state over time and facilitated the formation of a uniform Al2O3 coating layer on PE separator surfaces. Due to the hydrophilic nature of the Al2O3 coating layers, the as-prepared, ceramic-coated PE separators had better wetting properties, greater electrolyte uptake, and larger ionic conductivities compared to those of the bare PE separators. Furthermore, half cells (LiMn2O4/Li metal) containing Al2O3-coated PE separators showed improved capacity retention over several cycles (93.6% retention after 400 cycles for Al2O3 coated PE separators, compared to 89.2% for bare PE separators operated at C/2) and rate capability compared to those containing bare PE separators. Moreover, because the Al2O3-coated layers are more thermally stable, the coated separators had improved dimensional stability at high temperatures (140  C). © 2016 Elsevier B.V. All rights reserved.

Keywords: Aqueous Ceramic coating Aluminum hydroxide Polyolefin-based microporous separators Lithium-ion batteries

1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M.-H. Ryou), [email protected]. kr (Y.M. Lee). 1 Both authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2016.03.037 0378-7753/© 2016 Elsevier B.V. All rights reserved.

In the early 1990s, SONY developed and released lithium-ion batteries (LIBs). These are the dominant type of power source used in mobile electronic devices including laptop computers, cell phones, and digital cameras [1e4]. Recently, considering global

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environmental problems closely associated with fossil fuel depletion and the effects of greenhouse gases, the demand for large-scale battery systems such as electric vehicles (EVs) and energy storage systems (ESSs) has received increased interest [5e7]. For successful implementation of these upcoming electric devices, they must satisfy several customer needs; firstly, they must have long operation times and be reasonably priced, and secondly, they must be safe for consumer use [8,9]. To improve battery energy density, new types of battery systems based on new active materials and chemistry, beyond those of the “intercalation”-chemistrybased commercialized LIBs, have been studied [3,6,7,10e12]. Also, although energy density must be increased, safety concerns remain paramount. High energy materials may threaten consumer safety if the batteries are placed under abnormal conditions, leading to fires, explosions, and smoke [13,14]. The separator plays an important role in enhancing the safety of LIBs by creating a physical separation between the positive and negative electrodes, yet allowing the movement of lithium ions through the liquid electrolyte via the porous separator structure during cycling [4,15]. Generally, microporous polyolefins, including polypropylene (PP), polyethylene (PE), or laminates of PP and PE, are used in commercial LIBs [4,16,17]. Under abnormal conditions, i.e., when the temperature of the inner side of the LIB polymer matrix reaches a critical value, the separators shrink, causing internal shorting. To improve the separator thermal stability, polymer layers (polyvinylidene fluoride, poly(acrylonitrile), poly(methyl methacrylate), or their copolymers) or ceramic coating layers containing various types of hydrophilic inorganic powders (SiO2, Al2O3, TiO2, MgO, Mg(OH)2, etc.) bonded using a small amount of a polymeric binder have been developed [17e20]. In contrast, because of the inherent hydrophobicity of polyolefin separators, non-polar organic solvents must be used to securely attach the separator and coating layers. Generally, organic solvents are expensive and toxic; thus, they must be removed during drying, increasing the final separator price. Herein, we describe the development of a water-based ceramic coating method using a surfactant and water-soluble polymeric binders. This process is economic and environmentally friendly compared to the existing commercialized organic-solvent-based ceramic coating processes. Amphiphilic surfactants, containing both hydrophobic and hydrophilic groups within a surfactant molecule, enable this process. Generally, surfactants are amphiphilic in character because they contain both a “water-loving” hydrophilic head, consisting of a relatively small ionic or polar group, attached to a non-polar “water-hating” hydrophobic tail, consisting of a long hydrocarbon chain [21,22]. Depending on the head group, amphiphiles can be ionic (cationic, anionic), zwitterionic, or nonionic. Because of the ease and low cost of manufacture, anionic surfactants are used in greater volumes than the other surfactant class. In particular, sulfosuccinate-type anionic surfactants are of practical importance as mild, high-foaming surfactants used in personal care products and in wool, fur, and leather treatment [22e24]. We have chosen disodium laureth sulfosuccinate (DLSS) for our system, and its chemical structure is shown in Fig. 1. We optimized the waterbased ceramic coating process using DLSS and investigated the physical properties of Al2O3 ceramic-coated separators. We also investigated the electrochemical properties of cells containing the separators. 2. Experimental 2.1. Materials Sodium carboxymethyl cellulose (CMC, WS-C, Dai-ichi Kogyo Seiyaku Co., LTD.), poly(vinylidene fluoride-co-

hexafluoropropylene) (PVdF-HFP, Kynar Flex®2801, Arkema Inc.), and aluminum oxide (Al2O3, AES-11, Sumitomo Chemical Co.) were used as received without further purification. Disodium laureth sulfosuccinate solution (28 wt% ASCO® DLSS, AK Chemtech Co., LTD.) and deionized (DI) water from a Milli-Q system (Millipore Co., USA, >18.2 MU cm1) were used. N-methyl-2-pyrrolidone (NMP) and acetone (purity > 99.9%, water content < 0.005%) were purchased from Aldrich. Lithium manganese oxide (LiMn2O4, Iljin Materials Co., Korea), artificial graphite (SCMG-AR, Showa Denko, Japan), polyvinylidene fluoride (PVdF, Solef-6020, Solvay Chemicals, Belgium), conductive additive (Super-P, Timcal, Switzerland), and Li metal foil (thickness ¼ 400 mm, Honjo Metal, Japan) were used. A mixture of 1.15 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/ethyl methyl carbonate (EC/EMC ¼ 3/7 v/v) was purchased from Panax Etec (Korea) and used without further purification. Microporous polyethylene separator (PE, ND420, Asahi Kasei E-materials, Japan, thickness ¼ 20 mm, porosity ¼ 41%) was used.

2.2. Preparation of Al2O3 ceramic-coated PE separators The coating solution was prepared by mixing Al2O3, DLSS, and CMC at a constant ratio in DI water (Al2O3/CMC/DLSS/DI water ¼ 38.9/1/0.1/60 by weight) for 24 h at room temperature (25  C), followed by ball milling for 2 h at 380 rpm. PE separators were coated by simple bar coating process, followed by drying in a fume hood for 10 min at 70  C. The coated separators were further dried in the vacuum oven for 24 h at 60  C to completely remove solvent prior to use. To investigate the effect of DLSS on the aqueous system, control coating solutions were prepared following the same procedure and at the same constituent ratio: 1) Al2O3/CMC/ water ¼ 38.9/1/60 by weight for an aqueous system without DLSS and 2) Al2O3/PVdF-HFP/NMP ¼ 38.9/1/60 by weight for a nonaqueous system without DLSS.

2.3. Characterization of Al2O3 ceramic-coated PE separators The surface morphologies of the bare and the Al2O3 ceramiccoated PE separators were investigated by field-emission scanning electron microscopy (FE-SEM, S4800, Hitachi, Japan). The air permeability (Gurley number) of separators was determined using a densometer (4110N, Thwing-Albert, USA). The electrolyte wettability of the separators was determined by comparing the electrolyte wetting area of bare and coated separators, which had the same size (radius ¼ 1.8 cm), at room temperature. The electrolyte uptake amount of the separators (original size: 3  3 cm) was determined by Eq. (1), where W1 and W2 indicate the weights of the separators before and after absorbing the liquid electrolyte, respectively:

Uptake amount ðwt:%Þ ¼ ðW2  W1 Þ=W1  100

(1)

The weight change was measured after soaking the separators in electrolyte for 12 h, followed by removal of excess electrolyte from the surfaces. The thermal stability of the separators (original size: 3  3 cm) was investigated by measuring their dimensional changes using Eq. (2) after heat exposure in an oven at 140  C for 30 min, where A1 and A2 indicate the area of the samples before and after oven storage, respectively:

Thermal shrinkage ratio ð%Þ ¼ ðA1  A2 Þ=A1  100

(2)

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Fig. 1. (a) Molecular structure of the surfactant: disodium laureth sulfosuccinate (DLSS). Schematic figures for the Al2O3 coating process on PE separators: (b) Digital camera images and (c) schematic figures showing the interfaces between the Al2O3-coating solution and PE separators for the Al2O3 coating solution with and without DLSS, respectively. Contact angle images of the PE separators and the droplet of Al2O3 coating solution (d) with and (e) without DLSS, respectively.

2.4. Electrode preparation The positive electrodes were prepared by casting an NMP-based electrode slurry (LiMn2O4/Super-P/PVdF ¼ 90/5/5 by weight) on aluminum foil (15 mm, Sam-A Aluminum) using a doctor blade, followed by drying in an oven at 130  C for 1 h. The negative electrodes were prepared by casting an NMP-based electrode slurry (graphite/Super-P/PVdF ¼ 93/2/5 by weight) on a copper foil (10 mm, Nippon Foil, Japan) using a doctor blade, followed by drying in an oven at 70  C for 2 h. Both positive and negative electrodes were roll-pressed with a gap-control-type roll pressing machine (CLP-2025, CIS, Korea) to control the thickness, density, and loading amount of the electrodes (thickness ¼ 44 mm,

density ¼ 1.76 g cm3, loading level ¼ 7.74 mg cm2 for positive electrodes, and thickness ¼ 43 mm, density ¼ 1.35 g cm3, loading level ¼ 5.8 mg cm2 for negative electrodes, respectively). 2.5. Electrochemical measurements The ionic conductivities (s) of the separators impregnated with liquid electrolyte were measured by sandwiching the liquid electrolyte-soaked separators between two stainless steel electrodes (diameter ¼ 1.6 cm, area ¼ 2.01 cm2). The ionic conductivities were calculated according to the relationship s ¼ l/RS, where l is the thickness of the separators, S is the contact area between the separator and the stainless steel blocking electrodes, and R is the

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bulk resistance measured by AC complex impedance analyses (VSP, Bio-Logic, USA) [25,26]. Thickness and bulk resistances of separators are listed in Table 1. The electrochemical stability of DLSS was investigated using linear sweep voltammetry (LSV). For the reduction potential measurement, the potentials were swept from the open-circuit voltage (OCV) to 0 V vs. Li/Liþ at a rate of 0.01 mV s1 using the graphite/PE separator/Li metal cells containing liquid electrolyte (1.15 M LiPF6 in EC/EMC ¼ 3/7 v/v) with and without 0.1 wt% DLSS. For the oxidation potential measurement, the potentials were swept from 0 to 7 V vs. Li/Liþ at a rate of 1.0 mV s1 using the stainless steel/PE separator/Li metal cells containing liquid electrolyte (1.15 M LiPF6 in EC/EMC ¼ 3/7 v/v) with and without 0.1 wt% DLSS. To evaluate cell performance, the 2032 coin-type half cells (LiMn2O4/Li metal) were assembled in a glove box filled with argon. After cell assembly, the cells were aged for 12 h before cycling. Using a charge/discharge cycle tester (PNE Solution, Korea), cells were cycled between 3.0 and 4.4 V vs. Li/Liþ in constant current (CC) mode for both charge and discharge processes at C/10 rate (0.087 mA cm2) at room temperature. The cells were also stabilized for the three subsequent cycles between 3.0 and 4.4 V vs. Li/Liþ at a constant current/constant voltage (CC/CV) mode for charging. CC mode was used for the discharge process at a C/5 rate (0.173 mA cm2) at room temperature. The 2032 coin-type full cells (LiMn2O4/graphite), assembled following the same procedure of half cells described above, were cycled between 3.0 and 4.3 V vs. Li/Liþ in constant current (CC) mode for both charge and discharge processes at C/10 rate (0.087 mA cm2) at room temperature. The cells were also stabilized for the three subsequent cycles between 3.0 and 4.3 V vs. Li/Liþ at a constant current/ constant voltage (CC/CV) mode for charging. CC mode was used for the discharge process at a C/5 rate (0.173 mA cm2) at room temperature. The rate capabilities of the half cells and full cells was evaluated by cycling at a variety of discharging current densities from C/ 2 to 20 C (C/2, 1, 3, 5, 7, 10, 15, 20, and C/2 in a CC/CV mode for charging and CC mode for discharging) at room temperature. Furthermore, half cells were cycled at C/2 (a CC/CV mode for charging and a CC mode for discharging, between 3.0 and 4.4 V vs. Li/Liþ) for 400 cycles at room temperature, while full cells were cycled at C/2 for charging (CC/CV mode) and 1 C for discharging (CC mode) between 3.0 and 4.3 V vs. Li/Liþ for 150 cycles at room temperature.

2.6. Battery safety measurements The battery safety was evaluated by monitoring open-circuit voltage (OCV) changes of fully charged full cells (LiMn2O4/ graphite) during heat exposure as a function of time. To prepare fully charged full cells, full cells were prepared and stabilized by following the previous preparation procedure mentioned above. As-prepared full cells were fully charged to 4.3 V at C/2 (a CC/CV mode for charging and CC mode for discharging) at 25  C, and then exposed to 140  C as monitoring OCV changes as a function of time [27].

3. Results and discussion The coating ability of the DLSS surfactant on commerciallyavailable hydrophobic polyolefin microporous separators was investigated. Microporous PE separators were coated with the Al2O3 coating solutions prepared both with and without (control) DLSS surfactant using a bar-coating technique, as shown in Fig. 1b and c. After bar-coating, the coating quality of each sample was investigated. As revealed in digital camera images, the Al2O3 coating solution without DLSS resulted in a very poor quality surface coating, having large numbers of longitudinal cracks parallel to the bar-coating direction (Fig. 1b). In contrast, the DLSS-containing Al2O3 coating solution uniformly covered the PE separator surfaces without defects (Fig. 1c). The poor quality surface formed from the coating solution without DLSS could be attributed to the poor affinity of water for the hydrophobic PE surfaces. There are strong molecular interactions between polar water molecules, resulting in the formation of liquid droplets to reduce surface tension, as shown in Fig. 1b-1. In contrast, DLSS decreases the surface tension of the liquid droplets because the non-polar tail groups project outwards onto the PE surfaces, and the polar anionic head groups remain in contact with water, as depicted in Fig. 1c-1. To investigate the affinity of the Al2O3 coating solution for the PE separator surface quantitatively, contact angle measurements were made. As demonstrated in Fig. 1d and e, DLSS-containing Al2O3 coating solutions (63.11 ± 0.43) had a lower contact angles with the PE surfaces in comparison to those without DLSS surfactants (199.27 ± 0.19). Therefore, we concluded that the DLSS surfactant molecules increase the affinity between polar water and the hydrophobic PE surface by decreasing surface tension of the aqueous solution, thus, producing a better quality Al2O3 ceramic coating. The surface morphology of the Al2O3 ceramic-coated PE separators was investigated in detail by SEM, and it was verified that Al2O3 ceramic particles cover the microporous PE separators uniformly, as shown in Fig. 2. Concerning upscaling of the inorganic composite coating process, another important advantage of using a DLSS-surfactantcontaining water-based Al2O3 coating solution is that this system maintains dispersion quality over time. Generally, maintaining a uniform dispersion of inorganic coating solutions is difficult. Without sonication or agitation, over time, inorganic particles agglomerate and settle due to van der Waals attractive forces and gravity and this accelerates as particle weight increases. As a result, as shown in Fig. 3a, the non-aqueous Al2O3 coating solution (NMP) had poor dispersion behavior, possibly due to the poor affinity between the non-polar solvent and the hydrophilic Al2O3 particles. Using water as a solvent diminishes this problem slightly; however, aqueous Al2O3 coating solutions without DLSS also phase separated on storage. Use of DLSS allows formation of a uniform Al2O3 aqueous dispersion. The surface of Al2O3 is positively charged (14.8 ± 2 mV), consequently, the anionic surfactants surround the Al2O3 particles, forming bilayers on the surfaces [28,29]. This generates steric repulsion between coated particles originating from the negative surface charge, thus, maintaining a well-dispersed Al2O3 coating solution as described in Fig. 3b.

Table 1 Physical properties of bare PE and Al2O3-coated PE separators.

PE separator Al2O3 coated PE separator

Thickness (mm)

Coating layer density (g cm3)

Gurley number (s 100 mL1)

Uptake amount

Bulk resistance (ohm)

Ionic conductivity (mS cm1)

20 26

e 1.13

280 290

77 100

1.492 1.529

0.667 0.846

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Fig. 3. Digital camera images of various types of Al2O3 coating solution based on (a) NMP solvent, water without DLSS, and water with DLSS after 30 days storage at 25  C. (b) A schematic figure showing the repulsion forces ascribed to DLSS in a uniform Al2O3 coating solution.

Fig. 2. SEM images of (a) bare PE separators and (b, c) Al2O3 coated PE separators with different magnifications.

The physical properties of Al2O3 ceramic-coated separators and bare PE separators such as coating thickness, Gurley number, amount of liquid electrolyte uptake, bulk resistance and ionic conductivity were investigated, and these data are summarized in Table 1. The thickness of the Al2O3 ceramic composite coating layer was controlled to be 6 ± 0.63 mm over all experiments, and the density was 1.13 g cm3. Generally, separator permeability is characterized by air permeability measurements, represented by the Gurley number. This is calculated by measuring the required time for a quantity of air to pass through a specific area of separator at a set pressure [17,27,30]. Thus, the Gurley number contains information about pore structure, tortuosity, porosity, and the thickness of the separators. After coating with Al2O3, the Gurley number increased slightly from 280 to 290 s 100 mL1, which implies that the coating layer does not significantly impede air permeability and is ascribed to the loosely packed structure of the coating layers, as seen in SEM images shown in Fig. 2b and c. Due to the hydrophilic moieties in Al2O3 and polymeric binders, the surfaces of the Al2O3 ceramic-coated separators became more hydrophilic compared to those of the bare PE separators, leading to better wetting by the polar liquid electrolyte. As shown in Fig. 4, a polar liquid electrolyte droplet immediately spreads and is absorbed over the Al2O3 ceramic-coated separator surface. In contrast,

the initial droplet shape is maintained on the bare PE separators for some time. The improved wettability of the Al2O3 ceramic-coated separators is reflected in both their uptake of liquid electrolyte and ionic conductivities. Al2O3 ceramic-coated separators absorbed larger amounts of liquid electrolyte (100%) compared to bare PE (77%), consequently, a higher ionic conductivities (for example, 0.846 and 0.667 mS cm2 for Al2O3 ceramic-coated separators and bare PE separators, respectively) were observed for the coated separators. To monitor the electrochemical stability of the residual DLSS under the cell operating condition, LSV was conducted for the

Fig. 4. Digital camera images of bare PE separators and Al2O3 coated PE separators on which a droplet of liquid electrolyte [1.15 M LiPF6 with EC/EMC ¼ 3/7 v/v] was placed.

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electrolytes containing 0.1 wt% DLSS. As shown in Fig. 5, the electrolytes containing 0.1 wt% DLSS had similar reduction and oxidation behavior to those of bare electrolytes. If negative electrodes are exposed to low potentials below 1 V vs. Li/Liþ, the negative electrodes contacting with the electrolyte is covered with a passivation film (the “solid electrolyte interphase”, SEI), which is formed by the reaction of the negative electrode surface with the electrolyte during charge, preventing further electrolyte decomposition [2,31]. In this regard, the reduction peak at ca. 0.6 V vs. Li/Liþ shown in Fig. 5a seems to be attributed to the electrochemical decomposition of electrolytes associated with forming SEI, which has been proved in several previous studies [2,31e33]. Similarly, there is no distinct oxidation peak during the anodic potential sweep, as can be seen in Fig. 5b, implying that DLSS is electrochemically stable up to 4.4 V vs. Li/Liþ. Considering these results, we could anticipate that DLSS is electrochemically stable in the LIB cell at a normal operating condition. The effect of Al2O3 ceramic-coated PE separators on LIBs was evaluated by assembling CR2032 coin-type half cells (LiMn2O4/Li metal). As shown in Fig. 6a and b, both cases (Al2O3 ceramic-coated PE separators and bare PE separators) had similar voltage profiles during precycling in the voltage range of 3.0e4.4 V (C/5, 0.173 mA cm2). Al2O3 ceramic-coated PE separators had revealed discharge capacities of around 113.5 mAh g1 and coulombic efficiencies of 99.1%; in contrast, those for bare PE separators were 113.3 mAh g1 and 98.7%, respectively. After precycling, when the cells were discharged to 3.0 V, AC impedance measurements were carried out. As shown in Fig. 6c,

Fig. 6. Voltage profiles of the cells containing (a) bare PE and (b) Al2O3 coated PE separators during precycling [C/10 between 3.0 and 4.4 V vs. Li/Liþ]. (c) Nyquist plots of the cells containing bare PE and Al2O3 coated PE separators after precycling.

Fig. 5. (a) Reduction potential (from OCV to 0 V vs. Li/Liþ at a scan rate ¼ 0.05 mV s1) and (b) the oxidation potential (from 0 to 7 V vs. Li/Liþ at a scan rate ¼ 1.0 mV s1) measurements for electrolyte (1.15 M LiPF6 in EC/EMC ¼ 3/7 v/v) with and without DLSS (0.1 wt%) using linear sweep voltammetry at 25  C.

cells containing Al2O3 ceramic-coated PE separators had lower total cell resistances (Rcell) compared to those containing bare PE separators. Generally, impedance spectra of the Li-ion cells consist of two partially overlapped semicircles and a straight slopping line at low frequency end [4,34]. The semicircle in the higher frequency range (the left-hand side) corresponds to the resistance (RSEI) due to

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lithium ion migration through the SEI, and the semicircle in the medium to low frequency range (the right-hand side) corresponds to the charge transfer resistance (Rct) between the electrodes and electrolytes [4]. On the other hand, the bulk resistance (Rb), indicated by the x-axis intercepts close to the origin, is related to the ionic conductivity through the separators. Rcell of the Li-ion cell is mainly contributed by the Rb, RSEI, and Rct, but not a simply summation of these three individual values [34]. Considering the fact that the liquid electrolyte is a lithium ion carrier in a battery system, it is speculated that the increased electrolyte uptake or the diminished electrolyte leakage due to the improved wetting ability of the Al2O3-coated PE separators facilitate not only the lithium ion migration through the electrode SEI layer, but also the charge transfer between the electrode and electrolyte, resulting smaller RSEI, and Rct values, respectively [35e37]. Considering these in mind, it is reasonable to expect that the smaller Rcell would bring the improvement of cycle performance and rate capability of the cells. The cycling capacity performance and rate capability of half cells (LiMn2O4/Li metal) were evaluated. Cells containing Al2O3 ceramiccoated PE separators had better cycling performance compared to those of bare PE separators. As shown in Fig. 7a, after 400 cycles (CC/CV mode for charging at C/2 (0.433 mA cm2), CC mode for discharging at C/2, respectively), the Al2O3 ceramic-coated PE separators retained 93.6% (103.02 mAh g1) of their initial discharge capacity; in contrast, the bare PE separators only retained 89.2% (98.16 mAh g1). Furthermore, Al2O3 ceramic-coated PE separators yielded a better rate capability compared bare PE separators. As shown in Fig. 7b, at the 60th cycle after discharging at 10 C (8.66 mA cm2), the cells containing Al2O3 ceramic-coated PE

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separators maintained 64% of their initial discharge; in contrast, the bare PE separators maintained 60%. Along with the reduced interfacial resistance of cells containing Al2O3 ceramic-coated PE separators shown in Fig. 6c, the ability of the Al2O3 ceramic coating layers to retain more electrolyte in the separator compared to the hydrophobic bare PE separators prevents both lack and leakage of electrolyte during repeated cycling. The effect of the Al2O3 ceramic-coated PE separators was also evaluated on the full cells (LiMn2O4/graphite). As shown in Fig. 7c and d, the cells employing Al2O3 ceramic-coated PE separators revealed almost similar cycle retention ability up to 150 cycles, while revealed a better rate capability compared to those of bare PE separators. Generally, microporous PE separators shrink as they melt or soften because of internal stress [18,37]. To investigate the thermal properties of the separators, each separator was cut into a square sample (3  3 cm) and dimensional changes were monitored during heating at 140  C. As demonstrated in Fig. 8a and b, the existence of thermally stable inorganic particle layers suppressed dimensional changes in the Al2O3 ceramic-coated separators, which retained their original dimensions. In contrast, the bare PE separators shrank to 41% of their original dimensions. In this regard, the effect of the Al2O3 ceramic coating layers on the safety of the LIBs were evaluated by monitoring open-circuit voltage (OCV) changes during heat exposure. Fully charged full cells (LiMn2O4/graphite) were exposed at 140  C and monitored the OCV as a function of time. As shown in Fig. 8c, the cells employing Al2O3 ceramic-coated PE separators well retained initial OCV over 120 min, while OCV of the bare PE separator containing unit cells was suddenly dropped to 0 V after 5 min. From the result, we could

Fig. 7. Comparison of (a) the cycle performance and (b) rate capability of the half cells (LiMn2O4/Li metal) containing bare PE and Al2O3 coated PE separators at 25  C, and (c, d) for the full cells (LiMn2O4/graphite), respectively.

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on hydrophobic PE separators was developed. The Al2O3-coated PE separators had improved wettability and higher liquid electrolyte uptake, factors that are closely related to the improved cycling performance and retained rate capabilities of the cells. Furthermore, the process is not only environmental friendly and costeffective, but also improve the electrochemical properties and safety of cells. Therefore, we believe that this DLSS-containing aqueous inorganic coating process is a milestone in the development of the next-generation of inorganic surface-coated polyolefin separators for LIBs.

Acknowledgement We acknowledge financial support from the Ministry of Education, Science and Technology (MEST) and National Research Foundation (NRF) of Korea through the Human Resource Training Project for Regional Innovation (2014066977) and IT R&D program of MOTIE/KEIT (10046314).

References

Fig. 8. Dimensional changes of bare PE separators and Al2O3 coated PE separators at (a) room temperature (25  C) and (b) after high temperature exposure (140  C for 30 min). (c) Open circuit voltage (OCV) changes of the fully charged full cells (LiMn2O4/ graphite) containing bare PE and Al2O3 coated PE separators during heat exposure at 140  C.

infer that the improved thermal stability of Al2O3 ceramic-coated PE separators ultimately helps enhance the safety of LIBs. 4. Conclusion Using DLSS enabled development of a water-based Al2O3 inorganic coating process, thus far, never achieved in this research field. Using this method, a well-dispersed Al2O3 inorganic coating solution that facilitates the formation of uniform Al2O3 coating layers

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