Thermally stable non-aqueous ceramic-coated separators with enhanced nail penetration performance

Journal of Power Sources 427 (2019) 271–282

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Thermally stable non-aqueous ceramic-coated separators with enhanced nail penetration performance

T

Bokyung Junga,1,∗, Byungmin Leeb,1, Yong-Cheol Jeongc, Joowook Leea,b, Seung Rim Yanga, Hana Kimb, Myungkook Parkb a

Energy Laboratory, SAIT, Samsung Electronics, 130 Samsung-ro, Suwon-si, Gyeonggi-do, 16678, Republic of Korea Platform Materials Team 1, Samsung SDI, 130 Samsung-ro, Suwon-si, Gyeonggi-do, 16678, Republic of Korea c Micro/Nano-Scale Manufacturing R&D Department, KITECH, Ansan, 426-910, Republic of Korea b

HIGHLIGHTS

GRAPHICAL ABSTRACT

non-aqueous Al O and Mg(OH) • The CCSs are investigated. ceramic-polyurethane/ • Crosslinked PVdF composite gives thermal stabi2

3

2

lity at 200 °C.

E and H of ceramic coating layers • The are determined by nanoindentation. but tough Mg(OH) CCS im• Flexible proves safety performance in nail per

2

netration.

Al O and Mg(OH) CCSs improve • The high-temperature electrochemical 2

3

2

performance.

ARTICLE INFO

ABSTRACT

Keywords: Ceramic-coated separator Nanoindentation Battery safety test Nail penetration Lithium-ion battery

Two types of non-aqueous ceramic-coated separators, based on Al2O3 and Mg(OH)2, with extremely high thermal stability are prepared at a pilot scale. To investigate the factors determining the electrochemical and safety performance of the battery, the structure and properties of Al2O3 and Mg(OH)2 separators are characterized at the entire separator and coating-layer level using nanoindentation. Both separators exhibit almost 0% thermal shrinkage, even at 200 °C, and good electrolyte wettability. The mechanical properties of the coating layers, represented as indentation modulus and hardness, reveal that the Mg(OH)2 separator is more flexible than Al2O3 separator, which is brittle, and exhibits a trend of hardness strengthening with increasing penetration depth. These mechanical properties of the coating layer of separators dominantly affect nail penetration more than electrochemical performance. Electrochemical performance is affected more by the uniformity and electrochemical stability of the ceramic/binder coating layer. Indeed, Al2O3 and Mg(OH)2 cells show similar value of capacity retention of 78.6 and 77.8% after 1100 cycles, and internal resistance of 128.5 and 132.0%, respectively. Pouch cells equipped with Mg(OH)2 separators do not show any events, evaluated as L3, while Al2O3 separator-incorporated cells are evaluated as L6 during the nail penetration test.

Corresponding author. E-mail address: [email protected] (B. Jung). 1 These authors contributed equally to this work. ∗

https://doi.org/10.1016/j.jpowsour.2019.04.046 Received 17 December 2018; Received in revised form 12 March 2019; Accepted 10 April 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.

Journal of Power Sources 427 (2019) 271–282

B. Jung, et al.

1. Introduction

ceramic to binder and binder to co-binder. Then, fundamental properties of the separators were investigated at the total separator level, and micro-scale mechanical properties of the ceramic coating layer were also studied by nanoindentation. Additionally, electrochemical performance was evaluated with cylindrical cells of 18650-model with a reduced internal cell volume and capacity, thus called a mini-18650. Finally, a nail penetration test was performed as a battery safety test. Understanding the relationship between structure, properties, and performance of the separators will give insight towards designing them for enhanced electrochemical performance and adequate safety.

Lithium-ion batteries (LIBs) are routinely utilized in day-to-day applications such as mobile devices, power tools, and even electric vehicles (EVs). To account for the extended application of LIBs for EVs, they have been developed with increased energy and power densities, which is necessary for longer driving ranges with one charge. With increased cell capacity and energy density, battery safety has become one of the most important aspects in the development of LIBs [1,2]. Safety should be guaranteed at the cell, module, pack, and ultimately system levels. Failure at even one of these levels can trigger severe higher-level events [1]. Therefore, many efforts have been made to develop effective safety devices for each level of LIBs [3–5]. Recently, the role of the separator has gained attention with respect to safety at the cell level [6–9]. Commercial separators, typically polyethylene (PE) and polypropylene (PP), have been developed with sufficient mechanical and electrochemical stability and high porosity. However, these polyolefintype separators suffer from a poor electrolyte affinity due to their hydrophobicity and low thermal structure integrity due to their low melting point, which leads to an internal short and finally a thermal runaway [10]. There have been attempts to address these problems by filling the polyolefin with thermally stable ceramic particles such as fumed silica (SiO2) or alumina (Al2O3) [10] or by combining it with mechanically strong ultrahigh molecular weight polyethylene [11]. Alternatively, polyolefin has been substituted with various polymers with high thermal stability such as polyimide [12], polyacrylonitrile [13], and cellulose nanofiber [14] by electrospinning, wet-laid, or paper-making processes. Although these non-woven fabric separators have high porosity and enhanced thermal shrinkage, it is hard to control their thickness and porosity due to the limited manufacturing methods mentioned above [15]. Alternatively, a porous organic polymer-coated separator has been made by non-solvent induced phase separation to create a thermally stable porous coating layer using an aramid polymer with a high thermal stability [16] and a high-boiling point solvent. However, it is still difficult to avoid cost increases in its manufacturing process due to the solvent removal step [17]. Different from these approaches, ceramic-coated separators (CCSs) are the most attractive candidate for the development of safety-enhanced separators. By coating a ceramic slurry composed of ceramic particles and binder onto a polyolefin base film, the thermal shrinkage of polyolefin can be highly suppressed [18–21]. Additionally, the hydrophilic surface property of ceramic particles and the high porosity of CCSs can provide electrolyte wettability and efficient lithium ion transport, respectively, which improves the electrochemical performance [18,21]. Furthermore, not only the coating process is viable for mass production, but it is also the most effective way to make separators at low costs. Within this context, we previously developed various non-aqueous CCSs with excellent thermal stabilities of less than 2% of dimensional shrinkage in extremely harsh conditions, i.e. at 200 °C [22,23]. This was achieved by using a thermally crosslinkable binder/polyvinylidene fluoride (PVdF) co-binder and ceramic particles. In particular, the poly (urethane) (PU) binder, with its multifunctional acrylate groups, long flexible chain, and good miscibility with the PVdF co-binder, ensures that ceramic particles are firmly interconnected with one another and with binders, even at high temperatures [22]. In the present study, we extend our work to explore what kind of CCS structures and properties can give an extremely high thermal stability and ultimately contribute to better electrochemical performance and battery safety. To design a safety-enhanced separator with a high thermal stability, one needs to establish the relationship between the structure and properties of the ceramic-coating layer and their effect on the separator performance. By doing so, CCSs with high thermal stability were prepared by using different ceramic particles, including Al2O3 and Mg(OH)2, at the same compositions, namely the ratio of

2. Experimental 2.1. Materials An aliphatic polyurethane acrylate oligomer (SC2152, functionality = 15, molecular weight = 20,000 g mol−1, Miwon Specialty Chemical Co., Ltd., Republic of Korea), poly (vinylidene fluoride) (PVdF, KF9300, Kureha, Japan), and benzoyl peroxide (BPO, 75%, Aldrich, MO, USA) were used as received. Dimethylacetamide (DMAc) and acetone (Samchun Pure Chemicals, Republic of Korea) were purchased and used without further purification. Al2O3 (AES11, Sumitomo Chemicals, Japan) and Mg(OH)2 (AM103S, Akchemtech Co., Ltd, Republic of Korea) were used as received without purification. A commercially available PE separator (G14AB1, 14 μm, porosity = 42%, Toray, Japan) was used as received. A cathode comprising a mixture of Li(Ni, Co, Al)O2 (NCA) and Li(Ni, Co, Mn)O2 (NCM) (NCA/NCM/ carbon/PVdF = 96/1.8/2.2 by weight, loading level = 40.73 mg cm−2) and an anode of graphite (graphite/styrene-butadiene rubber (SBR)/ carboxymethylcellulose (CMC) = 97.5/1.5/1.0 by weight, loading level = 20.50 mg cm−2) were kindly provided by Samsung SDI and used for the electrochemical performance test. Additionally, for the nail penetration test, a cathode of NCA (NCA/carbon/PVdF = 95/2/3 by weight, loading level = 38.53 mg cm−2) and an anode of graphite (graphite/SBR/CMC = 98/1.2/0.8 by weight, loading level = 22.76 mg cm−2) were used, which were also provided by Samsung SDI. A liquid electrolyte, 1 M LiBF4 in propyl carbonate (PC) (PANAX ETEC Co. Republic of Korea) was used for the measurements of shutdown and meltdown of the separators. For electrochemical and nail penetration tests, a liquid electrolyte of 1.15 M LiPF6 in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate with the additives of 1% lithium difluorophosphate (MP1) and 1.5% vinyl chloride (VC) (EC/EMC/DMC 2/4/4 by volume, PANAX ETEC Co. Republic of Korea) was used. 2.2. Preparation of Al2O3- and Mg(OH)2-coated separators Each ceramic slurry, Al2O3 and Mg(OH)2 (580.6 g, 25 wt% in acetone), was homogeneously mixed with a PVdF solution (34.6 g, 7 wt % in DMAc/acetone), and a PU binder solution (8.1 g, 30 wt% in acetone) with benzoyl peroxide (BPO) (10 wt% relative to PU) was added to prepare a coating solution of a 15 wt% solid content with additional acetone. The coating was performed with a pilot-scale direct metering (DM) coater (DM coating head with 500 mm width, Kobayashi Engineering Works., Ltd. Japan). The pristine PE separator was coated with the prepared coating solution and dried to remove excess organic solvent. The separator was thermally cured in a convection oven at 85 °C for 24 h. 2.3. Characterization of basic separator properties The coating thicknesses of the CCSs were averaged thickness values measured along the transverse direction of ten sheets of the separator with a portable micrometer (QuantuMike, Mitutoyo, Japan). The loading level of the ceramic layer was determined by Eq. (1), where WCCS and WPE indicate the weight of the ceramic-coated separator and 272

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reached at a speed of 10 μN s−1. A typical loading-unloading plot during the nanoindentation test and surface changes are shown in Fig. 1. Hardness (H) (Eq. (4)) and reduced elastic modulus (Er) (Eq. (5)) were evaluated by the Oliver-and-Pharr model from measured loadingunloading versus displacement data as follows, where Ac, hc, S, and β are the projected contact area, contact depth, stiffness, and geometrical constant of the used tip, respectively.

the PE base film, respectively, and the unit area of the separator is denoted as A. The specimen was prepared by punching the 10 separator sheets with a size of 50 mm × 50 mm for each sheet.

Loading level [g/m2] = (WCCS

WPE ) / A

(1)

Gurley numbers were measured by the JIS P8117 method using a Digital Oken Type Air-Permeability tester (EGO-1-55-1MR, Asahi Seiko Co., Ltd. Japan). The surface morphologies of the base film and CCSs were investigated by field emission scanning electron microscopy (UHR FE-SEM, SU-9000, Hitachi, Japan). The surface morphologies of the separators were observed as-prepared, while the cross-sectional morphologies were observed by cutting the sample with a cross-section polisher (IBe19520CCP, JEOL, Japan). Electrolyte wettability was measured by Eq. (2), where W1 and W2 indicate the mass of the separator before and after soaking in the electrolyte for 24 h, respectively.

Electrolyte uptake [wt%] = (W2

H=

Er =

EIT =

Thermal stability was investigated by measuring the dimensional changes of a cross marked on the separators before and after thermal treatment in a convection oven at 200 °C for 10 min. In Eq. (3), L1 and L2 indicate the length of the cross marked on the separator before and after heat treatment, respectively.

Thermal shrinkgage ratio [%] = (L1

L 2 ) / L1

(4)

S Ac

(5)

2

With the reduced elastic modulus in Eq. (5), indentation modulus (EIT) can be finally determined by

(2)

W1)

Pmax Pmax = Ac 2.88hc

(1 1 Er

vs2) (1

vi2) Ei

(6)

where υs is the poissonʹs ratio of the sample, Ei and υi are the elastic modulus and the poissonʹs ratio of the indenter, respectively. In Eq. (6), the indentation modulus was calculated by assuming the υs as 0.5 for ceramic-binder composite and with the known value of Ei and υi as 1141 GPa and 0.7, respectively for the Berkovich-type diamond tip. Each separator was measured at 16 grids over an area of 5 μm × 5 μm. Maximum and minimum hardness and indentation modulus results were excluded because they could be caused by slippage of the AFM tip on the ceramic particles.

(3)

The shut-down and melt-down behaviors of the separators were evaluated by measuring their impedance change, which was done by soaking in an electrolyte (1 M LiBF4 in PC) overnight while sandwiched between nickel foils and pressed between customized hot plates (Ilshin Autoclave Co. LTD., Republic of Korea) at 5 MPa. The impedance at 1 kHz was measured by LCR meter (IM 1533, Hioki, Japan) at a heating rate of 3 °C min−1 from room temperature up to 200 °C. The adhesion strength of the ceramic-coating layers was estimated by the 180° peel test between the ceramic-coating layer and the 3 M adhesive tape (Scotch Magic tape, 18 mm. 3 M) attached to its surface using a Universal Testing Machine (Instron 3343, Instron, USA).

2.7. Nail penetration test Pouch-type cells were prepared to have a theoretical capacity of approximately 1320 mAh and a 1.04 capacity ratio of negative electrode to positive electrode. The electrodes, comprising of NCA and graphite, were prepared with sizes of 34 mm × 69 mm (W × H) for the cathode and 35 mm × 70 mm for the anode, and the separator was folded alternatively and sandwiched between the cathode and anode assembly by z-stacking method. The ceramic-coated layer faced the cathode in the electrode assembly to prevent transitional-metal dissolution from the cathode. The total numbers of different layers in the zstacked assembly were 7 layers of cathode, 8 layers of anode, and 18 layers of separator containing an excess layer of the separator to cover the assembly. The z-stacking of cells was carried out in a dry room under controlled humidity. The electrolyte was injected and the cells were vacuum-sealed by sequential steps of differing pressure and vacuum times. The electrolyte comprised of 1.15 M LiPF6 in EC/EMC/ DMC with 1% MP1 and 1.5% VC (2/4/4 by volume) and was used without any purification. For a comparison with the control group, pouch cells with a PE base film were also prepared. To ensure electrolyte uptake into the electrode/separator assembly, all cells were kept for 12 h at 25 °C before pre-charging. The cells were degassed after precharging to 20% SOC. Subsequently, formation of the cell was carried out as per following procedures. In constant current (CC) and constant voltage (CV) mode, cells were first charged at a rate of 0.1C to the cutoff voltage of 4.25 V with a current cut-off at 0.05C. Subsequently, cells were discharged at 0.1C in CC mode to the cut-off voltage of 2.8 V. This cycle was repeated, followed by charging at CC/CV mode to 100% SOC with a cut-off voltage of 4.3 V. After formation, cells were degassed again by vacuum-sealing and end-sealed again to have a final cell dimension of 48 mm × 90 mm × 2 mm (W × H × D). The open-circuit voltage (OCV) and alternating current internal resistance (AC-iR) were checked before the nail penetration test using a resistance meter (BT3554, Hioki, Japan). The thermocouple was attached to the left side of the cell body. During the test, all cells were restrained between steel jigs, with holes in the middle and were equipped with a spacer. The number of spacers was adjusted to keep the same thickness as that of

2.4. Electrochemical properties of the separator Ionic conductivity (σ) was calculated by measuring the bulk resistance (R) of CR2032-type coin cells, which were filled with electrolyte (1.15 M LiPF6 EC/EMC/DMC = 2/4/4 by volume with additive of 1% MP1 and 1.5% VC) and the separators were sandwiched between two stainless-steel electrodes, by electrochemical impedance spectroscopy (1400A Cell Test System, Solartron Analytical, USA) in the frequency range of 10−2 to 106 Hz with an amplitude of 10 mV. The values were calculated according to the relationship σ = l/RS, where l is the thickness of the separator, S is the contact area between the separator and the stainless-steel blocking electrode, and R is the bulk resistance. 2.5. Bulk mechanical properties Bulk mechanical properties, including tensile strength and elongation along the machine direction (MD) and transverse direction (TD), were measured on the separator specimens with a size of 10 mm × 100 mm using a Universal Testing Machine (Instron 3343, Instron, USA). The puncture strength was measured on the separators before and after heat treatment at 200 °C for 10 min in the convection oven by the JIS 1019 method using a Handy Compression Testing machine (KES-G5, Kato Tech Co., Ltd. Japan). 2.6. Nanoindentation The mechanical properties of the CCSs were investigated using the nanoindentation method as illustrated in Fig. 1. A Berkovich-type diamond tip with a 79° effective cone angle was loaded normal to the separator surface until the predetermined maximum load (Pmax) was 273

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Fig. 1. Illustration of (a) typical loading-unloading vs. displacement curve and (b) surface change during a nanoindentation test.

the tested cell. Then, the entire area of the jigs adjacent to the cathode and anode tap of the pouch cell was insulated using a polyimide tape to prevent a short circuit. The test was carried out by moving the sample mount stage toward the vertically fixed nail on the chamber ceiling. A steel nail with a length of 75 mm and a diameter of 2.5 mm penetrates the center of the cell at a rate of 50 mm s−1. During nail penetration, the temperature and OCVs of the pouch cells were recorded.

3. Results and discussion 3.1. Morphologies and coating uniformity of separators Two different CCSs are prepared with ceramic particles of Al2O3 and Mg(OH)2 to investigate how the overall properties and structure of CCSs affect electrochemical performance and battery safety tests. Al2O3 is utilized most extensively as a ceramic coating material with a high thermal stability for CCSs, while Mg(OH)2 has been recently studied for the development of safety-enhanced CCSs [24]. Mg(OH)2 is known for its unique flame-retardant effect, which arises from the release of water molecules upon decomposition at a high temperature (approximately 330 °C) [25]. Additionally, the two materials are different with respect to hardness and particle geometry; for example, Al2O3 has a spherical shape with a Mohs hardness of 9, and Mg(OH)2 is a platelet-shaped particle with an aspect ratio of 3.0 and a Mohs hardness of 2.5 [26]. Given their similar thermal structural integrities, we hypothesize that the separator coated with ceramic particles of different particle shapes and hardness would have coating layers with different structures and properties and thus behave differently in electrochemical and battery safety tests. To avoid discrepancies induced by different coating thicknesses and uniformities, we tightly control coating parameters such as wire bar size, speed ratio of line to bar, and so on, in the pilotscale DM coater so that all the separators have an almost identical ceramic-coating layer thickness, irrespective of ceramic species. Thus, the coating thicknesses of both Al2O3 and Mg(OH)2 separators are quite similar to each other, 18.8 ± 1.3 and 18.5 ± 1.9 μm, respectively, which is summarized in Table 1 and shown in Fig. 2. The loading level (the weight of the coating layer per unit area) is calculated by the weight difference between PE and CCSs per unit area. Both the loading level and the density of the coating layer of the Al2O3 separator are much higher than of the Mg(OH)2 separator, but the volumetric amounts of their coating layers are not largely different upon taking into account the different densities of the ceramic particles. For example, the true densities of Al2O3 and Mg(OH)2 are 3.89 g cm−3 and 2.34 g cm−3, respectively. Indeed, the ratio of the loading level of Al2O3 to Mg(OH)2 corresponds approximately to their true density ratio. This means that both separators have a similar coating layer volume on the

2.8. Electrochemical performance The cycle life of LIBs with Al2O3- and Mg(OH)2-coated separators were tested using so-called mini-18650 cells, which are cylindrical-type 18650 cells with a hollowed-out cylindrical PVdF insert to reduce the effective internal cell volume and total capacity of the cell. The cathodes used in this study comprised a mixture of NCA/NCM active materials with a carbon black conducting material and a PVdF binder, and graphite with a SBR/CMC binder was used for the anodes. The cathodes and anodes measured 54 mm × 130 mm and 58 mm × 160 mm, respectively. The separators were cut into 60 mm × 1000 mm strips, and three mini-18650 cells were assembled for each separator. The cycle performance of the cells was tested in a temperature-controlled chamber set to 45 °C under CC/CV mode, charging at 1 C to 4.25 V with a 0.05C current cut-off, and in CC mode discharging at 1 C to 2.8 V. At the end of 100, 200, 300, 500, and 1,100 cycles, the conditions of the cells were evaluated by measuring the standard capacity, direct current internal resistance (DC-iR), and AC impedance at room temperature. To obtain the standard capacity value after cycling, the cells were first discharged at CC mode at a rate of 0.2C, and then charged by CC/CV mode at a rate of 0.5C–4.25 V with a 0.05C cut-off. Before measuring the DC-iR, the cells were successively charged by CC mode at a rate of 0.2C to SOC 50% and then were discharged at a rate of 2 C for 10 s. The DC-iR of the cells was calculated from the IeV curves obtained during discharge. The AC impedance of the cells was also measured at SOC 50% in the frequency range of 10−1 to 104 Hz with a 10 mV voltage amplitude.

Table 1 Basic properties of Al2O3- and Mg(OH)2-coated separators. Separator

Thickness [μm]

Loading [g m−2]

Densitya [g cm−3]

Gurley number [s/ 100 cc air]

Peel strength (MD/TD) [gf mm−1]

Electrolyte uptake [%]

Ionic conductivity [mS cm−1]

PE Al2O3 Mg(OH)2

14.1 18.8 18.5

0 8.37 4.99

N.A. 1.86 1.22

184 207 234

N.A. 7/5 4/4

78.9 97.2 103.8

1.106 0.760 0.734

a

Density of coating layer is calculated by dividing the loading value by thickness of coating layer. 274

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Fig. 2. SEM images of (a, b) base film PE separator, (c, d) Al2O3-, and (e, f) Mg(OH)2-coated separator, with the top view in the left column and the cross-sectional view in the right column. Each inset scale bar indicates 3 μm, except for the scale bar in magnified images, which denotes 1 μm.

base film. Coating uniformity of CCSs is an important factor in determining electrochemical performance. A non-uniform ceramic-coating layer causes an inhomogeneous current flow to occur during charging and discharging, which ultimately leads to a severe deterioration of the life cycle of the LIBs. To investigate the areal coating uniformity, the surface morphologies of Mg(OH)2- and Al2O3-coated separators are observed at the top and cross-sectional views with SEM. As shown in the top-view SEM images in Fig. 2(a–c), all the separators have uniform ceramic layer coverage on the microporous PE base film without any significant vacancies. A dense and uniform coating is also critical for obtaining a separator with an extremely high thermal stability. This uniform coating ability can be attributed to the excellent film-forming properties of the polyurethane acrylate/PVdF binder, as well as the stable coating formulae during the pilot-scale coating process [22]. The cross-sectional images in Fig. 2(d–f) show that all the separators have a similar range of coating thicknesses as previously measured. The ceramic particles are shown to be closely packed in the coating layer and to have an approximately equal thickness and uniform surface. Also, the Mg(OH)2-coated separator shows that the Mg(OH)2 particles keep their platelet shape intact in the coating layer. Further, the Gurley number in each separator is quite reasonably increased, not exceeding 250 s 100 mL−1 after coating the ceramic layer on PE, which has 184 s 100 mL−1. This reflects that the ceramic coating layer does not block the pores of the PE base film and is effectively coated to allow lithium ions to permeate through the separator in the cells. The adhesion of the coating layer upon base film is 7/5 and 4/4 gf mm−1 along the MD/TD for the Al2O3- and Mg(OH)2-coated

separators, respectively. An adequate adhesion force is important for the assembly of the electrode and separator; otherwise, the ceramiccoating layer could fall off, leading to non-uniform current flow through the separators in the cells [27]. 3.2. Electrolyte wettability Electrolyte wettability of the separator is investigated by measuring the amount of electrolyte soaked into the separator after incubation for 24 h. As summarized in Table 1, electrolytes in the Mg(OH)2- and Al2O3-coated separators are very well-soaked, while the PE base film shows a relatively low electrolyte uptake of less than 80%. However, the ionic conductivity of the CCSs is lower than that of the PE base film due to their increased thickness of up to 32% compared to PE, which is fairly reasonable and still falls into the range for CCSs usually found in the literature. Even with the increased thickness of the CCSs, a good ionic conductivity is contributed to the present PU/PVdF binder, due to its electrolyte-like chemical groups, including the polyol of PU and PVdF itself. 3.3. Thermal shrinkage and shut-down properties Generally, a polyolefin-based separator such as PE shows anisotropic shrinkage upon heating due to its microporous structure and internal stresses induced by the stretching steps in the manufacturing process of polyolefin-based films [15]. To investigate the suppression of thermal shrinkage of PE by coating with Al2O3 and Mg(OH)2, all the separators are marked with a cross and treated with heat at 200 °C for 275

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Fig. 3. Thermal shrinkage of (a) base film PE separator, Al2O3- and Mg(OH)2-coated separators before and (b) after heat treatment at 200 °C for 10 min and their SEM images of surface morphologies (c) at the top view and (d) cross-sectional view. The scale bar indicates 30 μm for PE and 3 μm for Al2O3 and Mg(OH)2 separators. The scale bar for inset images denotes 1 μm.

10 min. To exclude the dimensional shrinkage of separators from thinning edge during cutting, the dimensional change is calculated by measuring the change in length of the cross before and after heat treatment. The Al2O3- and Mg(OH)2-coated separators show almost no dimensional change along the MD/TD, as demonstrated in Fig. 3(a and b), while PE shrinks significantly up to 75/76% (MD/TD). Apparently, the enhanced thermal stability of these CCSs arises from a combined effect of the ceramic particles and the binders. Ceramic particles typically have a low thermal expansion coefficient compared to a polymer such as PE; the thermal expansion coefficient is a parameter showing the extent of dimensional change upon heating at a constant pressure. For example, ceramic particles such as Al2O3 have a low thermal expansion coefficient of 8.1 × 10−6 m m−1 K−1, which is a much lower when it is compared with 1.1–1.8 × 10−4 m m−1 K−1 of high-density polyethylene [28,29]. In addition, the thermally crosslinkable PU binder and PVdF co-binder system can form a very uniform, firm, and thermally stable matrix between ceramic particles after thermal curing by the highly functionalized acrylate group of PU and the good miscibility of PU with the PVdF binder. Therefore, a ceramic layer keeps the overall shape of the CCSs, even upon heating to 200 °C, while the PE below melts down and flows into the pores of the ceramic-coating layer, forming a composite layer of ceramic particles and PE. These phenomena are verified by comparing the surface and cross-sectional SEM images of heat-treated separators in Fig. 3(c and d). As expected, the overall thickness of the PE separator increases to approximately 180 μm upon heating, which is vigorously induced by two-dimensional shrinkage due to the intrinsically low thermal stability of PE and its anisotropic structure. The surface morphology of PE also shows that a lamellar phase is randomly formed over a wide area by melting-recrystallization of PE chains during the heat treatment. As PE melts down and flows into the interstitial volume between ceramic particles, the overall thickness of the CCSs is decreased by up to 46% and 58% for the Al2O3 and Mg(OH)2 separators, respectively. It is difficult to see the interstitial volume between ceramic particles in the coating layer in the cross-sectional images, as a dense film filled that volume instead. The

only difference between the surface morphologies of the Al2O3 and Mg (OH)2 separators is that Mg(OH)2 particles are more embedded by the PE melt than the Al2O3 particles, as shown in Fig. 3(d). Thermal stability of the separators in a dry condition, where no electrolytes are included and no mechanical stress is exerted on the separator, cannot be criteria in determining whether the separators are thermally stable or not in real cells. It can be assumed that the PE would melt down in a non-uniform manner in the presence of liquid electrolyte due to the randomly increased flow of PE in the cells. Therefore, this causes the thickness of the ceramic composite layer to vary over a wide area range of the separator, leading to an internal short in various thermally abusive conditions. Additionally, it has been reported that the mechanical properties of the separator are definitely alleviated in the presence of electrolyte [30,31]. In a more practical test of the thermal stability of the separators, the shut-down behavior of the electrolyte-soaked separators sandwiched between conductive metal foil is evaluated. To test the thermal stability of separators upon heating up to 200 °C, the electrolyte is used, which comprises a LiBF4 salt with a high melting point (almost 300 °C) and an organic solvent such as PC with a high boiling point of 240 °C. Therefore, the separator is continually heated in an electrolyte-soaked state throughout the test without any chemical reactions from electrolyte components. As shown in Fig. 4, as the PE separator is heated, the micropores in the base film start to close at approximately 130 °C, which is clearly observed by an abrupt increase in impedance. However, the PE separator melts down at approximately 145 °C because its thermal stability is not enough to endure the thermomechanical stress as the results of thermal stability test in dry. In contrast to PE, the pores of CCSs start to close at a slightly higher temperature than bare PE (135 °C), which is correlated to the crosslinked interface between substrate PE and the ceramic coating of the CCSs. The crossliked interface will be explained more in detail later. In addition, the impedance value of CCSs remains consistent or even increases upon heating above 145 °C, the melt-down temperature of the base film. Upon heating, the CCSs form the ceramic composite layer of PE and the ceramic-coating 276

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one sheet of the heat-treated separator is too delicate to be handled, all the separators are prepared by folding them into multiple layers, followed by thermal treatment at 200 °C for 10 min. After thermal treatment, the Al2O3 separator only shows 60 gf of puncture strength, while the Mg(OH)2 separator exhibits 527 gf, a much higher value compared to the Al2O3 separator. This is also checked by visualization of the heattreated separators as shown in Fig. S1. The visual images demonstrate that the stretched Al2O3 separator is broken due to brittleness, although it still retains its overall shape even after thermal treatment at 200 °C. In contrast, the Mg(OH)2-coated separator forms a more flexible film that can be stretched over the observed area. Although the toughness of thermally treated CCSs cannot be precisely measured either in tensile or compression mode due to the difference in surface area along the separators, the Mg(OH)2 separator seems to be much tougher than the Al2O3 separator, as the Mg(OH)2 separator is somewhat elongated but not broken, while the Al2O3 separator is more brittle.

Fig. 4. Shut-down and melt-down behavior of separators.

layer and can keep their dimensional stability and structural integrity. This indicates that the CCSs can endure thermal and mechanical stress even in the presence of an electrolyte.

3.5. Nanoindentation Except for the puncture strength of the heat-treated separators, Al2O3- and Mg(OH)2-coated separators do not show much difference in common bulk mechanical properties. This might be because common analysis tools for separators are unable to appropriately characterize the inhomogeneous structure of the ceramic-coating layer. Within this context, nanoindentation is performed to characterize the mechanical properties of the ceramic-coating layer on the submicron scale, excluding the effect of the base film of PE. The mechanical properties are measured on both pristine and heat-treated separators to determine any change in mechanical properties according to the species of ceramic particles of the CCSs after thermal treatment. In this study, we have utilized an automated protocol to subsequently obtain loading-unloading curves under the given maximum loading force of 100 μN in order not to exceed 1 μm of maximum displacement of CCSs. Therefore, the present study focuses on the characterization of the ceramic-coating layer of the CCSs. From 16 center points on a 4 × 4 grid over a 5 μm × 5 μm area on the separator, any unreasonable patterns in the loading-unloading curve, usually observed due to particle slippage, are removed from the calculation of mechanical properties. The surface morphology of each separator and the corresponding loading-unloading curves are shown in Fig. 5. As the indenter tip approaches to the surface of the ceramic coating layer, until the loading force reaches the predetermined maximum, the displacement varies according to the type of separator. Then, the indenter tip retracts from the surface, and the indented surface is recovered to a certain level. The plots of loading-unloading vs. displacement for each separator, which are typical for viscoelastic materials, are shown in Fig. 5. This indicates that hysteresis, occurs as it does in viscoelastic polymers [34,35]. We conjecture that this behavior can be simultaneously attributed to the ceramic particles and binder network in the ceramic-coating layer. From the sequential loading-unloading curves of each separator, two representative material parameters, indentation modulus and hardness, are calculated as explained in Fig. 1. The indentation modulus (EIT) is an intrinsic property of materials, which can be defined as resistance to reversible deformation. However, hardness (H) is an extrinsic property of the material, which is a resistance to irreversible deformation [36]. The EIT and H values are plotted against the loading force in Fig. 6 for each separator.

3.4. Bulk mechanical properties First, the mechanical properties of the separators are characterized by tensile strength and elongation in tensile mode, which is necessary for resisting the forces present during the winding process in battery assembly [15]. As expected, the tensile strength of Al2O3- and Mg (OH)2-coated separators are similar to each other but lower than PE by 30% for Al2O3 and 24% for Mg(OH)2, as presented in Table 2. This is due to the crosslinked interface between PE and the ceramic-coating layer of the CCSs. When the ceramic slurry is coated on the PE base film, the PU/PVdF binder dissolved in acetone is incorporated into the surface of PE through micropores of 50–60 nm in size. This is facilitated by the combined effect of wetting of PE and capillary infiltration of acetone into porous structure of PE. After the thermal curing process, a thin, densely crosslinked interface is formed, and this can decrease the tensile strength and elongation of PE. The high tensile strength value of PE is mostly due to the intertwined network structure of PE chains formed by the manufacturing process. Indeed, this phenomenon has been reported in the literature for both aqueous and non-aqueous CCSs [32,33]. In the case of aqueous CCSs, a wetting agent is used such as poly (vinyl alcohol) or the sodium salt form of carboxymethyl cellulose (CMC), combined with other aqueous binders, for the enhanced wetting of PE. Therefore, it can be speculated that the wetting agent also fills the micropores of PE during the aqueous coating process of CCSs. The reduced toughness of the PE base film at the interface leads to a decrease in break elongation value of the present CCSs for the same reason as tensile strength. It is noted that the Mg(OH)2 separator is slightly more flexible than Al2O3. However, all the CCSs have improved puncture strengths of approximately 517 gf, which is almost 20% higher than the approximately 430 gf of PE. The puncture strength should be over at least 300 gf mil−1, a minimum required value in the industry [15]. This implies that Al2O3- and Mg(OH)2-coated separators can resist the penetration of active electrode materials encountered in the battery assembly process. The puncture strengths of pristine Al2O3- and Mg(OH)2-coated separators are the same, but the thermal puncture strengths, measured on the heat-treated separators in this study, vary greatly. Due to the fact that Table 2 Mechanical properties of Mg(OH)2- and Al2O3-coated separators. Separator

Thermal shrinkage (MD/TD) [%]

Tensile strength (MD/TD) [kgf cm−2]

Elongation (MD/TD) [%]

Puncture strength [gf]

Thermal puncture strength [gf]

PE Al2O3 Mg(OH)2

77/76 4/4 2/2

1175/1208 824/924 891/999

132/136 110/103 124/124

427.94 517.02 517.44

N.A. 60 527

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Fig. 5. AFM images of surface morphologies of PE (a–b), Al2O3 (c–d), and Mg(OH)2 separators before (left column) and after thermal treatment (right column) at 200 °C for 10 min and their corresponding loading-unloading curves during nanoindentation tests.

Notably, the Mg(OH)2 separator is more flexible than the Al2O3 separator in the absence of heat treatment as shown in Fig. 6. The indentation modulus of the Mg(OH)2 separator is not as high as that obtained for Al2O3 and is even similar to PE as shown in Fig. 6(a). This might be due to Mg(OH)2 having an intrinsically lower Mohs hardness than Al2O3. However, the hardness measured by the resistance of the ceramic coating layer to plastic deformation (and not the hardness of the ceramic particle itself) is quite high, as high as the Al2O3 separator, and of course much higher than PE. Additionally, the hardness of the Mg(OH)2 separator starts to increase from 70 μN of loading force, at which point the separator is indented over 400 nm in depth, which corresponds to a few of layers of Mg(OH)2 particles. Due to the unique particle shape of Mg(OH)2, which is a thin plate-like structure with a relatively high aspect ratio of 3, the Mg(OH)2 coating layers exhibit an interlayered structure, as shown in the magnified SEM images in Fig. 2(f). Therefore, the hardness seems to be strengthened with an increase in penetration depth, like the strain-hardening phenomenon observed in polymers. Much more force is needed to deform materials that exhibit strain hardening at the same strain after initial stress. Generally, materials with greater strain hardening are tougher and exhibit ductile rather than brittle failure [37]. As the indenter tip passes through a few layers of Mg(OH)2, which are chemically connected by the crosslinked PU/PVdF binder, less plastic deformation occurs. In contrast, the hardness of Al2O3 shows an almost consistent value of 383 MPa with increasing penetration depth, which is similar to the value obtained for the Mg(OH)2 separator. The penetration depth-dependence of the hardness of the Mg(OH)2 separator is much more apparent in the thermally treated separators, where the hardness of the

Mg(OH)2 separator is 502 MPa in contrast with 374 MPa for the Al2O3 separator, as shown in Fig. 6(d). In the case of the heat-treated separators, the significantly higher hardness of the Mg(OH)2 separator is due to not only the structure of the ceramic particles but also to the combined effect of the ceramic and PE layers. Considering that hardness is related more to fracture behavior of the material than the elastic region of displacement, this trend is also similarly observed in the measurement of thermal puncture strength of the separators, as shown previously in Table 2 and Fig. S1. This implies that the Mg(OH)2 separator has a more durable coating layer than the Al2O3 separator when the separator is exposed to heat in the cells under the penetration load as summarized in Table 3. Different from the behavior of hardness, indentation moduli of Al2O3 and Mg(OH)2 separators are compromised upon thermal heating of CCSs because the PE melts and fills the pores of the ceramic-coating layer, as observed in the SEM images in Fig. 3. This is also indirectly shown in the surface morphologies during the nanoindentation test as shown in Fig. 4(b) and (c). Both separators exhibit a lamellar phase of PE melt between ceramic particle domains in the AFM images, which is more apparently observed in the Mg(OH)2-coated separator, as was also shown in the SEM images in Fig. 3. This means that the composite between the Mg(OH)2 coating layer and PE may be strong enough to endure penetration. However, the mechanical properties of thermally treated PE are much lower than those obtained for pristine PE. Because such a high elastic modulus and hardness are contributed to the interconnected structure of PE, it is reasonable to conclude that both mechanical properties are alleviated by the morphological change of PE, as shown in the AFM images in Fig. 4(b). 278

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Fig. 6. Average indentation modulus (EIT) (left) and hardness (H) vs. load plots (right) for PE, Al2O3- and Mg(OH)2-coated separators (a) before and (b) after heat treatment at 200 °C for 10 min.

A level of safety is assigned to each result according to EUCAR safety standards, which define the level of danger for automotive applications and are widely used [43]. Before the nail penetration tests, the cells are preconditioned to maintain a certain level of OCV (above 4.26 V) and AC-iR. By doing so, the cells with an OCV less than 4.2 V and ambiguously higher or lower AC-iR values than the rest of the cells are excluded from the tests. The conditions of the pouch cells used for the nail penetration tests are summarized in Table 4. The schematic experimental setup for the nail penetration test and plots of OCV and temperature profiles of the cells during the tests are shown in Fig. 7. All the cells are restrained between jigs, preventing movement or twisting by the applied nail during the test. The cells assembled with PE and Al2O3-coated separators show a rupture, flame, and fire but no explosion, which is assigned as level 6 (L6) by EUCAR standards. They exhibit an abrupt voltage drop to 0 V directly after being penetrated and a significant increase in temperature up to approximately 215 °C and 250 °C, respectively. However, it should be noted that the actual internal temperature of the cell could be much higher than detected because the thermocouple is attached to the side of the pouch cells, as shown in Fig. 7 [44]. This is to exclude the fact that cells can be ruptured by the thermocouple itself, as much more force is exerted on the parts of the cells attached to the thermocouple.

Table 3 Mechanical properties of Mg(OH)2- and Al2O3-coated separators characterized by nanoindentation method. Separator

PE Al2O3 Mg(OH)2

Before heat treatment

After heat treatment

Indentation modulus [MPa]

Hardness [MPa]

Indentation modulus [MPa]

Hardness [MPa]

453 ± 34 553 ± 36 479 ± 38

304 ± 23 379 ± 23 374 ± 76

252 ± 13 387 ± 15 386 ± 23

126 ± 11 374 ± 20 502 ± 42

3.6. Nail penetration The nail penetration test is an important test for simulating internal shorts in cells that can occur from manufacturing defects such as a metal particle wound in the jelly roll, a wrinkle in the separator, or a mismatch in the winding [38]. Therefore, nail penetration is used to induce an internal short in cells by mechanical abuse, and the cells should pass this test for application in EVs. In this study, nail penetrations tests are performed to explore how the properties and structures of the separators affect the nail penetration of the cells. Generally, mechanical or thermal abuse tolerance of the cells differs according to cell size, capacity, cell design, state of charge (SOC), and types of active material in the cell [39,40]. Considering these factors, we establish a test protocol for nail penetration to exhibit on-off type behavior in the nail penetration test by using a certain capacity level not exceeding 1500 mAh and at SOC 100%. An NCA/graphite electrode pair is chosen, as it is characterized by a lower onset temperature and higher peak selfheating rate than NCM/graphite, which was analyzed using an accelerated rate calorimeter (ARC) [41,42]. This implies that the NCA/ graphite electrode has lower thermal stability, which would cause the cells to experience much harsher conditions during nail penetration. For repeatability of the test, three cells for each separator are assembled, including Al2O3- and Mg(OH)2-coated separators and PE only.

Table 4 Pouch cell conditions and EUCAR levels for nail penetration. Separator

Capacity [mAh]

SOC [%]

AC-iR [mΩ]

OCV [V]

EUCAR level

PE

1302 1302 1308 1312 1308 1308 1307 1306 1309

100 100 100 100 100 100 100 100 100

18.3 18.0 18.1 18.6 16.8 19.2 17.2 17.2 18.2

4.26 4.26 4.26 4.26 4.26 4.26 4.26 4.26 4.26

L6 L6 L6 L6 L6 L6 L3 L3 L3

Al2O3 Mg(OH)2

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Fig. 7. Schematic illustration of (a) nail penetration test and pouch cell, and voltage (blue)-temperature (red) profiles of cells assembled with (b) PE, (c) Al2O3-, and (d) Mg(OH)2-coated separators during nail penetration. The inset arrows indicate the instant of penetration. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 8. Electrochemical performances of mini-18650 cells equipped with PE, Al2O3, and Mg(OH)2 separators: (a) cycle life at 45 °C and (b) DC-iR growth during the cycling test after the predetermined cycle ended.

The OCV and temperature profiles of cells with PE and Al2O3-coated separators are consistent with results found for thermally abused cells in the literature [44]. In contrast, cells assembled with the Mg(OH)2coated separator show no venting, fire, flame, or rupturing, which is assigned as L3. Additionally, they exhibit only a moderate voltage drop to 4.17 V directly after nail penetration and then recover to 4.18 V within a few seconds, followed by a very slow decrease in voltage to approximately 4 V. At the same time, the temperature of the cells very slowly increases to 43 °C and then remains steady. When a nail is forced to penetrate through a cell, various internal shorts can be created. The possible internal short circuits are classified as type I (AleCu short), type II (CuCathode active materials), type III (Al-Anode active materials), and type IV (Cathode-Anode short) [44]. The mode of internal short circuits could be quite random in the cells, even if the cells are made in the same conditions, which means it is very difficult to discern which internal short triggers the explosion of cells in nail penetration tests. However, it is worth noting that, regardless of which mode causes the internal short circuit, the separator should still have a sufficient thermal structural integrity in the cells. In this respect, the safety performance of the Mg(OH)2 separator is greater than that of the Al2O3 separator, even though their dimensional thermal stabilities are similar. Whether a separator is damaged by mechanical abuse or by induced thermal causes after nail penetration, it should keep its structural integrity to

prevent electrode materials from self-heating by increasing short resistance in the cells. In this sense, the Mg(OH)2 separator seems to effectively prevent an internal short by the strengthening of its hardness according to the penetration depth. However, further study is needed to comprehensively understand the mechanical properties of the coating layer to validate the relationship between its properties and the safety performance of the separators. 3.7. Electrochemical performance To obtain discernable results for electrochemical performance according to the types of separator, the life cycle of mini-18650 cells is tested at 45 °C rather than 25 °C. It has been reported that elevated temperatures accelerate the degradation of battery materials, including electrodes and electrolytes [45]. Therefore, under the given electrodes and electrolytes, separators would experience an electrochemically harsh condition at the elevated temperature. Cells made with both Al2O3 and Mg(OH)2 CCSs show better performances than the cell made with the bare PE separator, as shown in Fig. 8. After 1100 cycles, the Al2O3- and Mg(OH)2-coated separator cells show capacity retention rates of 78.6 and 77.8%, respectively; in contrast, the bare PE separator cells show capacity retention rates of 70.3%, as seen in the inset image of Fig. 8 (a). Generally, there should not be a significant capacity loss of more than 1–2% between the types of pilot-scale separators; otherwise, 280

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the adapted binders of the ceramic coating layer would have severely low electrochemical stability, or the porosity and tortuosity of the PE base film would deteriorate during cycling of the battery. Indeed, PE exhibits a similar trend of capacity fade to the Al2O3- and Mg(OH)2coated separators for the initial 100–200 cycles. However, a slightly steeper capacity fade starts from 200 cycles and it becomes much more severe after 500 cycles. The thermally crosslinkable binder system has been reported to be electrochemically stable up to 6.2 V according to linear sweep voltammetry results of the previous study [22]. Therefore, the binder is electrochemically stable in the voltage range between 2.8 and 4.25 V during repeated charge and discharge. The capacity retention rate difference between Al2O3 and Mg(OH)2 remains consistent at approximately 1% until 300 cycles, and after 300 cycles the difference gap become smaller. It seems that the Mg(OH)2 separator is electrochemically stabilized later than the Al2O3 separator. The lifetime of both CCSs retains similar levels, less than 1% at 1100 cycles, indicating that both ceramic-coating layers are electrochemically stable enough to exhibit a good long-term cycle performance. The DC-iR value, which represents the internal resistance of cells during discharging, is also measured at the start and end of the predetermined cycles (i.e. 100, 200 …). For a given electrode and electrolyte, the DC-iR values increase in the order of PE, Mg(OH)2, and Al2O3 separators. However, it is noteworthy that the overall DC-iR growth of the Al2O3 and Mg(OH)2 separators is still lower than that of the commercially available CCS (see supporting information Fig. S2). By 500 cycles, the DC-iR growth of PE remains almost at the same level as Al2O3 and Mg(OH)2 separators but it rapidly increases up to 56.7% after 1100 cycles as shown in Fig. 8(b). Meanwhile, both ceramic-coated separators show the DC-iR growth of 28.5% and 32.0% compared to the initial state of cells. These long-term cycling test of cells over 1000 cycles clearly shows that both Al2O3 and Mg(OH)2 separators are more excellent than PE separator in electrochemical performance, which is due to good electrochemical stability of ceramic coating layers as elucidated in post mortem study with x-ray photoelectron spectroscopy (XPS) (Fig. S4 in supporting information).

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4. Conclusions In this study, the CCSs of different ceramic particles exhibiting extremely high thermal stabilities were investigated at the entire separator and coating-layer levels. The morphology and structure of the ceramic-coating layer are main factors in determining the mechanical properties of the CCS coating layer and play a key role in battery safety performance rather than in electrochemical performance. Especially, the nail penetration test results are apparently dictated as pass or failure mode according to the mechanical properties of the ceramiccoating layers of different CCSs. A flexible but sufficiently tough Mg (OH)2 separator, rather than a brittle Al2O3 separator, improves the nail penetration performance of pouch cells. In contrast, the coating uniformity and electrochemical stability of the separator are more dominant factors for an excellent electrochemical performance. Contrasting with PE, which has a 70.3% capacity retention, both Al2O3 and Mg (OH)2 separators exhibit a good cycle lifetime over 77% of capacity retention after 1100 cycles. Additionally, the internal resistance of the cells during discharge was under 135% compared to initial state after 1100 cycles, which is contrast to 157% of PE. The ceramic/binder composites and thermally stable binder are critical components for providing the separator with an enhanced safety performance. Moreover, the comprehensive characterization of the structure and properties of ceramic coating layers give insight toward appropriately designing and manufacturing the separators. Acknowledgements This work was supported by funds from Samsung Electronics Co. Ltd (RRA0114ZZ-HHRM). We thank Kang-Han Kim for the support and analysis of mechanical properties of the separators using 281

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Glossary AC-iR: alternating current internal resistance BPO: benzoyl peroxide CC: constant current CCS: ceramic-coated separator CMC: carboxymethylcellulose CV: constant voltage DC-iR: direct current internal resistance DM: direct metering DMAc: dimethylacetamide EMC: ethyl methyl carbonate LIB: lithium-ion battery MD: machine direction MP1: lithium difluorophosphate NCA: Li(Ni, Co, Al)O2 NCM: Li(Ni, Co, Mn)O2 NIPS: non-solvent induced phase separation PU: poly (urethane) PVdF: poly (vinylidene fluoride) SBR: styrene-butadiene rubber TD: transverse direction VC: vinyl chloride XPS: x-ray photoelectron spectroscopy

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