Comparative Study of the Adhesion Properties of Ceramic Composite Separators Using a Surface and Interfacial Cutting Analysis System for Lithium-Ion Batteries.

Because of the constantly increasing demand for highly safe lithium-ion batteries (LIBs), interest in the development of ceramic composite separators (CCSs) is growing rapidly. Here, an in-depth study of the adhesion properties of the Al2O3 ceramic composite coating layer of CCSs is conducted using a peel test and a surface and interfacial cutting analysis system (SAICAS). Contrary to the 90 and 180° peel tests, which resulted in different adhesion strengths even for the same CCS sample, the SAICAS is able to measure the adhesion properties uniformly as a function of depth from the surface of the coating layer. The adhesion strengths measured at the midlayer (F mid) and interface (F inter, interlayer between the separator and the ceramic coating layer) are compared for various types of CCS samples with different amounts of polymeric binder, and it is found that F inter is higher than F mid for all CCSs. Compared with F mid, F inter is significantly affected by storage in the liquid electrolyte (under wet condition).

Introduction

Lithium-ion batteries (LIBs) have been utilized as promising power sources for powering mobile electric devices for several decades.[1−3] Recently, the increasing demand for large-scale battery applications, such as electric vehicles (EVs) and energy-storage systems (ESSs), has increased the demand for the development of next-generation LIBs with high energy densities, high rate capabilities, long life cycles, low cost, and high safety. Safety must be the primary concern because LIBs that are not safe can harm consumers’ health and influence the very survival of the companies involved.[4,5] In general, commercialized LIBs contain large amounts of combustible organic solvents in the electrolytes and can cause thermal runaway when LIBs are exposed to detrimental conditions (e.g., high-temperature exposure, overcharge condition, high-temperature operation, and short circuit).[6,7]

Separators prevent electrical short circuits by keeping the positive and negative electrodes apart and complete the circuit as the ions pass through the microporous structures in the electrochemical cell.[8] In general, commercial separators are based on microporous polyolefin membranes made of polyethylene (PE), polypropylene, and their combination because of their superior properties, such as electrochemical stability, ease of processing, low cost, and high mechanical strength.[8−10] However, the hydrophobic surface character of polyolefin-based separators has hampered the battery performance because of their poor compatibility with the conventional polar solvents used in the liquid electrolyte.[9−11] Furthermore, the stretching process, which is indispensable for the formation of micropores in polyolefin-based separators, is the main origin of the dimensional shrinkage of these separators upon exposure to high temperature. This dimensional shrinkage of separators causes an internal short circuit between electrodes, resulting in serious safety issues.[12,13] Many battery manufacturers and researchers have considered ceramic composite separators (CCSs) to be a good option for overcoming the top-priority problems of polyolefin-based separators, such as the poor wetting ability to liquid electrolytes and their dimensional stability.[14−18]

In general, CCSs include composite coating layers, which consist of nanosized hydrophilic ceramic particles entangled with a small amount of polymeric binders. During the manufacturing process of LIBs, which is accompanied by high mechanical stress, some of the ceramic particles can detach from the separator surface, act as defects that diminish the electrochemical performance, and even cause the thermal runaway of LIBs ascribed to the nonuniform impedance.[19,20] To control the quality of CCSs, particularly in composite coating layers, a measurement and analysis method for evaluating the adhesion properties of composite coating layers is required.

In this study, we investigated the adhesion properties of the ceramic composite coating layers of CCSs using the surface and interfacial cutting analysis system (SAICAS). In contrast to the conventional peel test adhesion evaluation technique, the SAICAS can adjust the blade depth from the coating surface and can thus evaluate the adhesion properties of the coating layer in the region of interest.[16,21−23] First, we compared the adhesion properties measured using a peel test and a SAICAS technique. Then, we evaluated the effects of the amount of polymeric binders and the liquid electrolyte swelling on the adhesion properties of the ceramic composite coating layers of CCSs.

Results and Discussion

We prepared various types of CCSs with different amounts of polymeric binders, as listed in Table . For the facile preparation of Al2O3-based ceramic composite coating layer on the hydrophobic PE separators, we utilized the surfactant-assisted coating technique, as reported in our previous studies.[16,17] We denoted each CCS 0.5, 1.0, and 2.0 wt %, representing the amount of polymeric binder in the ceramic coating layer. The physical properties of each CCS, including the coating thickness, Gurley number, bulk resistance, ionic conductance, and ionic conductivity, are listed in Table (Figure S2 and Table S3). Regardless of the amount of polymeric binder, the thickness of the ceramic coating layer was uniformly controlled to be 6 μm, and the Gurley number was slightly increased for each case. The 0.5 wt % revealed the highest ionic conductivity that was ascribed to the increased wettability to the polar liquid electrolyte because of the existence of the hydrophilic Al2O3.[17] As the amount of polymeric binder increased, the ionic conductivity decreased slightly because the excess amount of polymeric binders impeded the porous structures of the composite. It is noteworthy that each CCS nonetheless displayed a higher ionic conductivity value than that of bare PE.

Table 1

Composition of the CCSs as a Function of the Polymeric Binder Materials

	solvent	solid contents
binder contents (binder/solution)	water	Al2O3	binder (CMC)	surfactant	Al2O3/binder ratio	binder/Al2O3 ratio
0.5 wt %	60	39.4	0.5	0.1	78.8	0.013
1.0 wt %	60	38.9	1	0.1	38.9	0.025
2.0 wt %	60	37.9	2	0.1	19.0	0.053

Table 2

Physical Properties of the CCSs

system	thickness (μm)	Gurley number (s 100 mL–1)	bulk resistance (ohm)	ionic conductance (S)	ionic conductivity (mS cm–1)
bare	18	171.9	1.188	0.842	0.595
0.5 wt %	24	178.2	1.149	0.870	0.821
1.0 wt %	24	182.5	1.207	0.828	0.781
2.0 wt %	24	183.3	1.335	0.749	0.706

Before the SAICAS study, we evaluated the adhesion properties of the ceramic composite coating layers of CCSs by the peel test. As demonstrated in Figure a,b, we utilized two different types of peeling techniques to pull the tapes adhered on the top of the coating layers at different angles of 90° (90° peel test, Figure a)[24] and 180° (180° peel test, Figure b)[25,26] from the substrate. Both these tests have been randomly used without deep consideration because, in principle, the adhesion properties measured using both techniques should be the same. Surprisingly, we found that the techniques resulted in a variation of adhesion values for each ceramic composite coating layer of CCSs containing different amounts of polymeric binders: for 0.5 and 1.0 wt %, the 90° peel test showed higher adhesion strengths than those found by the 180° peel test, whereas a smaller value was obtained for 2.0 wt % by the 90° peel test than by the 180° peel test (Figure c). Although the exact origin for these phenomena is currently not clearly understood, these cases show that the peel test technique is not appropriate for the precise evaluation of the adhesion property of the ceramic composite coating layers.

Figure 1

Schematics of the (a) 90° and (b) 180° peel tests. (c) Comparison of the adhesion strength of the CCSs measured by the 90 and 180° peel tests.

We measured the adhesion properties of the ceramic composite coating layers for CCSs with different amounts of polymeric binder. We used CCSs with the ceramic composite coating layers of 6 μm listed in Table and measured the midlayer (Fmid, 3 μm from the surface) and interfacial (Finter, 6 μm from the surface: interlayer between the separator and the ceramic coating layer) adhesion strengths of the ceramic composite coating layer by adjusting the blade depth from the coating layer surface.

To verify the validity of the SAICAS technique for separators, the CCS surface was monitored by scanning electron microscopy (SEM) after measuring Fmid and Finter. After the measurement of Finter, the porous separator substrate was exposed to the surface while the ceramic coating layer was uniformly covering the entire area of the CCS (Figure ). These results prove that the SAICAS blade is precisely regulated and that the SAICAS results for Fmid and Finter are reliable.

Figure 2

Schematics of the SAICAS measurements for (a) Finter and (b) Fmid. SEM images of the CCSs after measuring (c, e) Finter and (d, f) Fmid using a SAICAS.

For both cases, Fmid and Finter, CCSs with the higher amount of polymeric binder showed higher adhesion strength values (Figure ). For convenience, the adhesion strength of each case is summarized in Table . These results are reasonable because a higher amount of polymeric binder can entangle ceramic particles more firmly. It is remarkable that the Finter is higher than the Fmid for each case, which is in good agreement with the SAICAS results recently reported in our previous study of electrodes.[21] Thus, the higher Finter can be ascribed to the higher pressure of the heavier ceramic coating layers on the separator relative to the midlayer position. In contrast, heterosubstrates can affect the detachment property between them and Finter because Finter arises from the inorganic composite coating layer and the polymeric separator, whereas Fmid arises from the same substrate.[21] To eliminate this interfacial inhomogeneity issue, we prepared a 2 wt % CCS with a 10 μm coating thickness and investigated the Fmid at 3, 6, and 9 μm using SAICAS, where the blade does not meet the polymeric separator surface during the measurement. The Fmid of each case still showed an increasing trend with increasing depth from the surface (Figure ).

Figure 3

Adhesion strengths ((a) Finter and (b) Fmid) of the CCSs as a function of the amount of polymeric binder (0.1, 0.5, and 2.0 wt % CCSs) measured using a SAICAS.

Table 3

Adhesion Properties of the CCSs Measured Using SAICAS for Finter and Fmid

	SAICAS
system	Fmid (kN m–1)	Finter (kN m–1)
0.5 wt %	0.0640 ± 0.0044	0.1415 ± 0.0027
1.0 wt %	0.1045 ± 0.0056	0.1879 ± 0.0150
2.0 wt %	0.1601 ± 0.0122	0.2360 ± 0.0082

Figure 4

Fmid of the CCSs as a function of the blade depth measured using a SAICAS.

The effect of the storage of liquid electrolyte on the adhesion properties of the ceramic coating layers of CCSs was investigated. For this purpose, 2 wt % CCS with a 6 μm thick coating, as shown in Table , was immersed in the liquid electrolyte and stored at 25 and 60 °C for 12 h, and Fmid and Finter of each storage case were measured. For both temperatures, Fmid and Finter decreased after storage (Figure ). Finter after storage at 25 °C (F25-inter) and 60 °C (F60-inter) showed almost identical values (F25-inter ≈ F60-inter), whereas Fmid after storage at 25 °C (F25-mid) was higher than that after storage at 60 °C (F60-mid) (F25-mid > F60-mid). Again, as discussed above, we believe that these discrepancies can be ascribed to the heterogeneous nature of CCSs:[21]Finter is the adhesion strength between the heterosubstrates, whereas Fmid arises from the homosubstrates. From these results, it is inferred that the substrate affinity between the ceramic composite layers and the polymeric separators is more sensitive to the external environment (wet condition) than is the inner adhesion of the ceramic composite layers.

Figure 5

(a) Finter and (b) Fmid of CCSs measured using a SAICAS after storage of liquid electrolyte at 25 and 60 °C for 12 h.

We compared the adhesion properties of the ceramic composite coating layers measured by the 90° peel test and SAICAS. As discussed above (Figure and Table ), the SAICAS results revealed that the adhesion strength near the interfaces between the ceramic composite layer and polymer separators is higher than that for the midlayer of the ceramic composite layer (Finter > Fmid). In this case, the midlayer of the ceramic composite layer would be cracked and a portion of this layer should be pulled off to the tape surface (Figure a), resulting in the tape becoming nearly semitransparent because the opacity of the tape is proportional to the transferred amount of the ceramic composite layer. On the contrary, if Finter is smaller than Fmid (Finter < Fmid), the entire ceramic composite layer would be pulled off with the tape surface (Figure b), resulting in the tape becoming opaque white.

Figure 6

Schematics describing the detachment feature of the ceramic composite coating layer from the CCSs after the 90° peel test when (a) Finter > Fmid and (b) Finter < Fmid. (c) Digital camera images of the tapes and CCSs after the 90° peel test.

On the basis of the SAICAS results (Figure ), we expected the color of the tapes for all CCSs (0.5, 1.0, and 2.0 wt %) to remain semitransparent because Finter was larger than Fmid for every case (Finter > Fmid). However, after the 90° peel test, 0.5 and 1.0 wt % CCSs turned opaque white, whereas the tape for 2.0 wt % CCS remained semitransparent (Figure c). We believe that these conflicting results were due to the inherent morphologically inhomogeneous nature of the composite materials. For instance, the composite materials used in our study included nanometer-sized Al2O3 ceramic particles and polymeric binders. On the basis of Griffith’s crack theory,[27] the material fracture is originated due to the presence of microscopic flaws in the bulk material. If structural defects, that is, microscopic flaws, are present in the material, the fracture stress increases and the material can be easily fractured and/or torn apart in spite of its inherent mechanical strength. Again, together with the adhesion-strength discrepancies discussed for the 90 and 180° peel tests (Figure ), the comparison between the 90° peel test and SAICAS highlighted the incoherent characteristic of the peel test. SEM images for CCSs (0.5, 1.0, and 2.0 wt %) after the 90° peel test indicate that our presumption based on the change of tape color was reasonable; the polymer separator was exposed after the 90° peel test for 0.5 and 1.0 wt % CCSs, whereas the ceramic coating layers are partially ripped off the polymer separator for 2.0 wt % CCS (Figure ).

Figure 7

(a) Schematic of the observed area, and SEM images of the same after the 90° peel test for (b) 0.5, (c) 1.0, and (d) 2.0 wt % CCSs.

Regardless of the existence of microscopic flaws, SAICAS is less affected by the morphological nature of the substrate because it calculates the adhesion strength on the basis of the force applied by the limited length of the blade. Consequently, SAICAS can measure the adhesion strength more accurately than the peel test.

Conclusions

The adhesion properties of the ceramic coating layer of CCSs were measured using the peel test and SAICAS. We observed that the peel test and SAICAS reveal inexplicable differences in the adhesion properties of CCSs. These are closely related to the microscopic flaws existing in the bulk material that are closely related to the morphological feature of the CCSs. Because of the inherent inhomogeneity of CCSs consisting of ceramic particles and polymeric binders, the microscopic flaws appear to be inevitable, thus making the peel test inappropriate for evaluating the adhesion properties of CCSs. In contrast to the peel test, SAICAS is carried out on the basis of the length of the blade cutting through the ceramic coating layer and is thus less influenced by the morphological features of the CCSs. SAICAS could measure the adhesion strength as a function of the blade depth relative to the surface coating. By varying the blade depth relative to the coating layer surface, the adhesion strengths at the midlayer of the ceramic composite coating layers (Fmid) and the interlayer between the separator and the ceramic coating layer (Finter) were evaluated. For every CCS, Finter > Fmid. Finter, which is sensitive to the wet environment, decreased faster than Fmid when the CCSs were immersed in a liquid electrolyte.

Experimental Section

Ceramic-Composite Separator Preparation

To investigate the effect of binder contents on the ceramic layer adhesion, ceramic-composite separators were prepared by water-based slurry with different binder contents. Aluminum oxide (Al2O3, AES-11, Sumitomo Chemical Co.), disodium laureth sulfosuccinate solution (28 wt % ASCO DLSS, AK Chemtech Co., Ltd.), and sodium carboxymethyl cellulose (CMC, WS-C, Dai-ichi Kogyo Seiyaku Co., Ltd.) were used as the ceramic particle, surfactant, and binder polymer, respectively. The binder amount was increased from 0.5 to 2.0 wt % by decreasing the Al2O3 content while maintaining the surfactant content at 0.1 wt % and water content at 60 wt %. Single side of the PE separator was coated using a simple bar-coating process. The one-side-coated separators were dried in a fume hood for 10 min (70 °C), followed by further drying in a vacuum oven (24 h, 60 °C) for the complete removal of any remaining solvent before use. However, the thickness of the ceramic layer was maintained at 6 μm to ensure a similar internal structure. To compare the adhesion properties at each vertical position, only one control coating slurry (2.0 wt %) was used with the same procedure. To investigate the adhesion properties at three different positions (3, 6, and 9 μm), a 10 μm thick ceramic layer was formed on the PE separator.

Air Permeabilities of Separators

Gurley numbers of the separators were measured to determine the air permeabilities using a densometer (4110N, Thwing-Albert).[16]

Ionic Conductivities of Separators

To measure the ionic conductivities (σ) of the separators, the liquid electrolyte (a mixture of 1.15 M LiPF6 in ethylene carbonate/ethyl methyl carbonate (3:7 by vol., Panax Etec, South Korea))-soaked separators were sandwiched between two stainless steel electrodes (radius = 0.8 cm; area = 2.01 cm2), and the bulk resistance was measured by alternating current complex impedance analyses (VSP, Bio-Logic). The ionic conductivities were calculated according to the relationship σ = l/RS, where l is the separator thickness, S is the contact area between the separator and the stainless steel blocking electrodes, and R is the measured bulk resistance.[19]

Peel Test

The adhesion strength of the ceramic layer was measured using a peel tester (Versatile Peel Analyzer, Kyowa, Japan). For the peel test, 19 mm wide and 50 mm long sample pieces of 3M adhesive tape were attached to the ceramic-coated separator and the peel strength was measured. The tape was detached by peeling at an angle of 90° and a constant displacement rate of 30 mm min–1; the applied load was continuously measured, and force/displacement plots were obtained. To guarantee the reproducibility of the test, we conducted at least three measurements for each sample and averaged the adhesion strength value.

SAICAS Measurements

The adhesion strength between the PE separator and the ceramic layer and that at the mid-depth of the ceramic layer were measured using a SAICAS (Daipla Wintes Co., Ltd, Japan) with a diamond blade (shear angle = 45°; clearance angle = 10°; rake angle = 20°; and width of blade = 1 mm). To measure Fmid and Finter, the vertical force (0.5 N) was applied to the blade until it reached the region of interest. Then, the vertical force was removed. During the test, the blade moved horizontally at 2.0 μm s–1.

Morphological Analysis of Ceramic-Composite Separator

After the peel test and SAICAS measurements, the surface morphology of the ceramic-composite separator was characterized by field emission scanning electron microscopy (S4800, Hitachi, Japan).