Document KRdV1yabeVp2nyoZ8LB0eO0Eo

Suitability Verification of Various Materials for Gasket in Lithium-ion Battery Contents Executive summary........................................................................................................................2 Introduction ...................................................................................................................................3 1. Basic properties comparison of materials for different gasket materials...................................3 1-1 The most suitable material for gaskets from characteristics point of view.................................3 1-2 Compression creep properties required for gaskets.................................................................4 1-3 Comparison of moisture permeability of gasket materials........................................................5 1-4 Comparison of electrolyte resistance for gasket materials.......................................................6 1-5 Conclusion...............................................................................................................................7 2. Evaluation under real gasketing conditions (in-situ) ................................................................8 2-1 Measurement of electrolyte vapor permeability through compressed state gaskets.................8 2-2 Evaluation of water vapor permeability under compressed state..............................................9 2-3 Conclusion.............................................................................................................................11 1 Executive summary This executive summary presents a comprehensive analysis of gasket materials for lithium-ion batteries (LiB) with a focus on their fundamental properties and practical suitability. The investigation primarily centers on the comparison between Ethylene Propylene Diene Monomer (EPDM), Polypropylene (PP), Polybutylene Terephthalate (PBT), and Perfluoroalkoxy (PFA) as potential gasket materials. The initial section scrutinizes the basic properties of these materials in the context of gasket applications. While EPDM exhibits commendable sealing properties vis--vis PFA, its suboptimal water vapor permeability and electrolyte resistance render it unsuitable for ensuring long-term product reliability and safety in LiB applications. Both PP and PBT were found to fall short of PFA across all essential characteristics, further underscoring the latter's dominance in this arena. In the subsequent phase, the practical implications of these findings were evaluated through an investigation into the permeability of electrolyte and water vapor. EPDM's pronounced permeability was identified as a significant drawback, suggesting its inadequacy as a viable gasket material for LiB batteries due to the potential degradation of battery performance. Conversely, PFA emerged as the unequivocal frontrunner among resin materials, distinguished by its superior impermeability in comparison to PP and PBT. The inherent attributes of PFA not only bolster battery safety and operational longevity but also position it as an indispensable component for ensuring sustained battery integrity. In conclusion, this study provides a comprehensive overview of gasket material properties and their real-world implications for LiB applications. PFA stands out as the preeminent choice, poised to deliver enduring safety, reliability, and performance, while alternative materials prove inadequate in replicating its unique virtues. The findings presented herein serve as a valuable guide for future material selection and engineering endeavors aimed at enhancing the efficiency and durability of LiB systems. 2 Introduction Lithium-ion batteries (LiB) must be sealed with gaskets to prevent water condensation due to intrusion of water vapor from the outside and to prevent leakage of electrolytes and gases from the inside. These roles are important in maintaining the battery performance and prolonging the life of LiB. The gaskets also prevent short circuits and leaks of potentially hazardous gases, thereby contributing to the security of battery safety. Therefore, materials for the gaskets are required to have properties such as excellent sealing properties, electrolyte resistance, electrical insulation properties, heat resistance, low moisture permeability. 1. Basic properties comparison of materials for different gasket materials 1-1 The most suitable material for gaskets from performance properties point of view Basic characteristics required for gasket materials include heat resistance, chemical resistance (electrolyte), electrical properties, flame retardancy, low water absorption, etc. Table 1 lists the properties of materials commonly used in LiB gaskets. It is known that temperatures near gaskets become high during laser welding in the LiB manufacturing process and fast charging. Therefore, heat resistance is required in the gasket material. Maintaining the dimensional stability of the gasket and preventing thermal degradation are significant. Properties related to heat resistance such as melting point and continuous operating temperatures are important indicators. The LiB gasket is assembled by a process called "crimping" in which a gasket is inserted between a metal lid, can and electrode terminal, finally compressed. Therefore, the mechanical properties of the material affect the ease of crimping and the sealing property after crimping. Also, since the gaskets are in contact with the electrolyte, it is important for gaskets that the electrolyte does not cause swelling, deterioration, extraction, etc. in order to keep the sealing performance for a long period of time. In addition, in the case of a short circuit between electrodes, casings and lids can cause ignition, there is a risk of spread of fire due to electrolyte. Therefore, insulation property and flame retardancy of the material are also important. Low water absorption rate is also an important factor because water permeation decreases LiB performance. Table 1, comparison of various properties, shows that PFA is the only material that combines high levels of properties such as heat resistance, chemical resistance, insulation, flame retardancy, and low water absorption, suggesting that it is extremely difficult for materials other than PFA to substitute for PFA. In addition, in terms of elastic modulus that indicates the deformability of the material, PFA has a lower value than PP and PBT, and shows excellent crimping/processability. 3 Table 1: Comparison of basic material properties used in gaskets Physical Mechanical Thermal Electrical Flame retardancy Properties Specific gravity Melting point Tensile strength Tensile elongation Tensile modulus Heat distortion temperature Continuous use temperature Coefficient of linear expansion Volume resistivity Surface arc resistance Comparative tracking Index Flammability Limiting oxygen index Acid Chemical resistance Alkaline Other Organic solvent Water absorption Test method ASTM D792 ASTM D4591 ASTM D638 ASTM D638 ASTM D638 1,82 Mpa UL 746B ASTM D696 ASTM D257 ASTM D495 JIS C 2134 UL 94 ASTM D286 Unit Mpa % Mpa 10-5/K cm s V % ASTM D543 - PFA 2.14 305 25-35 300 - 400 300 - 500 50 260 12 1018 >300 >600 V-0 >95 Excellent ASTM D543 - Excellent ASTM D543 ASTM D570 % Excellent <0.01 PP 0.91 165 30-35 200 - 700 1500 - 1800 55 - 65 60 - 100 5.8 - 10.2 1018 135 - 165 >600 HB 18 Good Excellent not suitable for use 0.01 - 0.03 PBT 1.31 228 55 50 - 300 2000 - 3000 50 - 80 130 6.0 - 9.5 1016 75 - 190 250 HB 20 Excellent not suitable for use Good 0.08 - 0.09 EPDM 0.86 - 0.87 ~20 ~800 1.1 - 1.3 120 1013 n/a not suitable for use Excellent not suitable for use <5 1-2 Compression creep properties required for gaskets The most important role of gaskets for LiB is to insulate the interior from the exterior of the cell and vice versa. Therefore, it is important to prevent the intrusion of water from the outside, as well as the leakage of the internal electrolyte and gaseous electrolyte. There are two types of gasket permeation: One is permeation at the gasket-lid/electrode interface and the other is permeation inside the gasket material. The permeability inside the gasket material depends on permeability of the material itself. On the other hand, the permeation at the interface is affected by the surface pressure generated at the interface of the lid/electrode when compressed. In other words, the creep characteristics after compression are important. To compare the creep properties of the materials, a Compression Set test was performed in accordance with ASTM D395. The test conditions were at a compression ratio of 25%, a test temperature of 65C and retention time of 72hr. The test was conducted with a cylindrical specimen of 13 mm6 mm thick. The test materials used were NEOFLON PFA AP-201 (Daikin Industries, Ltd.) for PFA, NOVATEC PP BC03C (Japan Polypropylene Corporation) for PP, DURANEX PBT 2002 (Polyplastics Co., Ltd.) for PBT, and JSR EP EP65(JSR Corporation) for EPDM. Recovery ratio was used as an index of evaluation. (Recovery Ratio=the thickness recovered after release the thickness during compression). Since the recovery ratio indicates how much the material recovers from the compressed state, it is a suitable index for sealing performance. A schematic diagram of the test method is shown in Fig. 1. The test results are shown in Fig. 2. PFA has about five times higher recovery ratio than PP and PBT and shows 4 suitable compression creep characteristics for gaskets. Hence, it is considered that the permeability at the interface is reduced. On the other hand, EPDM, which is a rubber material, shows a better recovery rate than PFA and the permeability at the interface could be also reduced. Figure 1: Schematic representation of the Compression Set test Figure 2: Recovery rate of various materials 1-3 Comparison of moisture permeability of gasket materials The permeation between the gasket and the lid/electrode interface is described in the previous chapter. In this section, the permeation inside of the gasket material is shown. Measurements were performed by the cup method (Fig. 3 and Fig. 4) to compare the water permeability. The test method was as follows: Water was put into a SUS container, a 0.23 mm thick sheet of the test specimen was sandwiched by a SUS lid with an opening of 12.56cm2, and the cup was placed in an electric oven at 80C. The weight of the cup was measured at regular intervals, and the weight change from the initial weight was defined as the amount of moisture permeated. PFA, PBT, PP, and EPDM were used as materials, as of comparability to the previous chapter. The measurement results are depicted in Fig. 4. It shows that the amount of water vapor permeation is low in the order of PFA, PP, PBT, and EPDM. The water vapor permeation amount of PFA is about half of that of PP and about 1/3 of that of PBT. The amount of permeation for 5 EPDM was overwhelmingly larger than that of the other resin materials. Figure 3: Cup test Figure 4: Water Vapor Permeability of Gasket Materials 1-4 Comparison of electrolyte resistance for gasket materials Since the gasket is placed in contact with the liquid electrolyte, it is important not to be affected by the electrolyte. Swelling (swell) properties were evaluated to verify the electrolyte resistance. The swelling property of the electrolyte also impacts the penetration of the electrolyte inside the material. The test method can be described as following: A sheet of test specimens (76.2 mm25.4 mm3.2 mm) was placed in a glass beaker, and a solution of EC/DEC=3:7 was used as electrolyte. The entire test specimen was immersed, the beaker was closed and put in an electric oven at 60C. The test specimens were taken out and weighed at regular intervals. The weight change from the initial weight was noted, as an indication of the swelling property. The measurement results are shown in Fig. 5. It shows the weight change increases in the order 6 of PFA < PP < PBT < EPDM. Figure 5: Electrolyte immersion test results 1-5 Conclusion Although EPDM has excellent mechanical sealing properties compared to PFA, it cannot be used as an alternative material for Lithium Ion battery sealings, from long-term product reliability, especially emphasizing safety, due to its higher water vapor permeability and electrolyte resistance. PP and PBT, cannot substitute PFA as they are inferior to PFA in all properties. 7 2. Evaluation under real gasketing conditions (in-situ) In the first chapter, we compared the characteristics of different materials for gasketing applications. In this second chapter, we further assess these materials under actual use conditions, namely these materials are used as gaskets. Since the actual gasket is used in a compressed state, achieved by "crimping", the permeability of electrolyte vapor (2-1) and water vapor (2-2) was investigated. Fig. 6 shows a schematic diagram of the jig and gasket used for the measurement. 2-1 Measurement of electrolyte vapor permeability through compressed state gaskets An O-ring-shaped gasket was sandwiched between a lid and a container simulating the shape of an electrode, and compression was performed. The compression ratio was adjusted at 12.5% using a spacer. Prior to compression, the container was filled with the electrolyte and placed in a constant temperature bath. The weight change was measured after a predetermined period of time to compare the amount of permeated electrolyte. The measurement results at room temperature and 60C are shown in Fig. 7. At 60C, the permeation amount was increasing in the order of PFA < PBT < PP< EPDM. The amount of permeation through EPDM is much larger than that of the other materials. The permeation amount of through PP is larger than that of the PFA resin material at 60C, however it is lower than that through EPDM. This demonstrates the superiority of PFA for permeation suppression of electrolyte vapor. Using these permeation results, the hypothetical calculation under the actual use of the battery was performed. Assuming that in automotive application defining room temperature as nondriving time that lasts for 22 hours/day and 60C as driving time for 2 hours/day. It was assumed that 15 years of vehicle use, it would be approximately 120,000 hours at room temperature and 8 approximately 11,000 hours at 60C. Table 2 shows the calculated amount of electrolyte permeation for each material. Compared to PFA, the calculated total permeability for other materials showed larger values. The value for PBT was calculated to be about 1.5 times larger, for PP about 2 times larger, and for EPDM about 40 times larger. Since the electrolyte permeation affects the degradation of the battery performance, it is considered that the degradation using non-PFA-made gaskets will be accelerated at this magnification. Taken this into consideration, it is fair to mention that non-PFAS candidate materials cannot replace PFA material from the viewpoint of reliability in the long-term. Figure 7: Electrolyte permeability evaluation as gasket (left: room temperature, right: 60C) Table 2: Permeation volume assuming 15years of use in automotive application PFA PBT Permeation volume (g) 0.325 0.46 PP EPDM 0.66 13.08 2-2 Evaluation of water vapor permeability under compressed state The measurement was carried out in accordance with the method described in 2-1, replacing electrolyte with water. The test was carried out at room temperature and 80C (to reach suitable water vapor pressure). The measurement results are shown in Fig. 8. The permeation amount increases in the order of PFA < PBT < PP < EPDM. (Note: low permeation is better!) EPDM significantly permeates water vapor already at room temperature, while PP, PBT and PFA are not. At 80C this becomes much more pronounced. PP also at elevated temperature permeates significant amounts. These results are used to calculate effects under the actual use of the battery. Assuming in 9 automotive application defining room temperature as non-driving time that lasts for 22 hours/day and 80C as driving time 2 hours/day. Assuming 15 years of use, it would be about 120,000 hours at room temperature and about 11,000 hours at 80C. The assumed amount of water vapor leakage for each material is shown in Table 3. Compared to PFA, the calculated permeability for other materials showed larger values. PBT and PP are about 1.2 times larger, EPDM is 20 times larger, and water vapor permeates easily. Since the electrolyte permeation leads the degradation of the battery performance, it is considered that the degradation of PFA-made gaskets will be accelerated at this magnification. In other words, if the larger permeation affects the performance of the battery, the degradation of non PFAS candidates will be more serious than PFA made gasket. Given this in mind, it is fair to mention that non-PFAS candidate materials cannot replace PFA material from the viewpoint of reliability in the long-term. Figure 8: Evaluation of water vapor permeability as gasket (left: room temperature, right: 80C) Table 3 Estimated leakage 15years of use in automotive application PFA PBT PP EPDM Permeation volume (g) 0.39 0.51 0.56 8.4 10 2-3 Conclusion In summary, the investigation encompassing the permeability of electrolyte and water vapor in the context of gasket materials unveiled a significant difference in performance between EPDM and resin-based counterparts. Notably, EPDM exhibited an exceptionally heightened degree of permeability, suggesting its unsuitability as a gasket material for LiB batteries due to the potential for substantial degradation in battery performance. Within the spectrum of resin materials, this study conclusively affirmed PFA as the standout candidate, displaying superior impermeability compared to PP and PBT. PFA's intrinsic attributes lend robust support to the protraction of battery safety and operational lifespan, positioning it as an irreplaceable constituent. The findings underscore PFA's preeminence in sustaining battery integrity and anticipate challenges in seeking alternative materials capable of matching its distinctive virtues. 11