1.2V NiMH battery.Special research on nickel-metal hydride battery separators

Special study on nickel-metal hydride battery separators (top-secret documents) provided by electronic enthusiasts. Special study on nickel-hydrogen battery separators (top-secret documents). Type 1 of polyolefin non-woven fabrics. Traditional wet-laid fiber composition: PP, PP/PE fiber size : 15-20 micron Features: Special study on the use of polyethylene and polynickel metal hydride battery separators (top secret document)

Types of polyolefin nonwoven fabrics

1. Traditional wet-laid fiber composition: pp, pp/pE Fiber size: 15-20 microns Features: Using a mixture of polyethylene and polypropylene with high uniformity, it was developed from the production of wet-laid papermaking to meet the needs of The combination of pp and pE, so fibers with relatively larger diameters should be used.

2. Spliced fiber method Fiber composition: pp, pE+pEVOH Fiber size: 2-8 microns, 15-20 microns Features: It is also prepared by wet method, but the difference is that water flow is used to splice and entangle the fibers. Because of the problem of pinhole formation during the entanglement process, the minimum quantitative value of the product is limited to approximately 55g/m2. When the diameter of the spliced fibers is 2-8 microns, fibers with larger diameters are required to be cross-linked to enhance the strength of the product. If the assembly is not perfect, a uniform product will not be obtained. The raw materials for making such separators contain both polyethylene fibers and polypropylene fibers, and in some cases ethylene-vinyl alcohol copolymer fibers.

3. Dry fiber composition: pure pp fiber size: 8-12 microns Features:

4. Melt-blown fiber composition: pure pp fiber size: 1-5 microns. Features: It is made of polypropylene using the melt-blown method. It usually does not contain additives, so it will not reduce battery performance.

Surface treatment of polyolefin non-woven fabrics Polyolefin non-woven fabric materials are generally hydrophobic and require surface treatment to make them hygroscopic before being used as alkaline battery separators. The most commonly used methods are:

1. Treatment with surfactant is to make it hydrophilic by coating anionic or nonionic surfactant. The main disadvantage of this method is that the surfactant will quickly escape from the separator, causing contamination of the battery's electrolyte and electrodes. As a result, the separator will become hydrophobic, and the introduction of impurities will cause a decrease in battery performance.

2. Corona discharge is a method of subjecting the separator to high-voltage discharge to oxidize the polymer surface to make it hydrophilic. This treatment method cannot be used for too long, and the water absorption of the separator will be lost over time.

3. Fluorine gas treatment is to treat the diaphragm with fluorine gas diluted in an inactive gas such as nitrogen or argon, and another reactive gas such as sulfur dioxide is also added to the mixed gas. The separator treated with this method has a certain degree of hydrophilicity, and this method has the least impact on the tensile strength of the separator.

4. Adding hydrophilic fibers. By adding a relatively small portion of hydrophilic fibers, the polyolefin non-woven separator can be water-soaked. A commonly used fiber is ethylene vinyl alcohol copolymer (pEVOH).

5. Sulfonation treatment: This process includes hydrophilic treatment of the surface of non-woven fabrics in sulfuric acid or similar chemical solvents. This process brings sulfuric acid into contact with the fiber surface, but this also causes the degradation of the polyolefin due to chemical oxidation reactions. The result of the treatment will be a reduction in the tensile properties of the non-woven fabric. Because of degradation caused by oxidative reactions, only polypropylene fibers covered with polyethylene can be used, which limits fiber diameter to 15 microns or larger. Fine pure polypropylene fibers cannot be processed by this method.

6. Graft copolymerization of vinyl groups is the most commonly used method in the above process. All types of non-woven membranes are treated to provide effective and long-lasting water soakability. Grafting acrylic onto vinyl is usually the most effective method using radiation techniques. The radiation grafting process can produce uniform and excellent hydrophilic separators, and at the same time, it can improve the mechanical properties of non-woven separators. Another advantage of this process is that it adds ion-exchange properties to the separator, thereby improving the battery's performance, such as battery retention time and cycling after charging.

Diaphragm performance

1. Mechanical properties: Tensile strength is a very important parameter, which changes with the structure of the separator, the type of fiber, the diameter of the fiber and the basis weight of the separator. Surface treatment also affects the mechanical properties of the separator.

①Melt-blown method-grafting quantitative (g/m2): 50 Tensile strength MD (N/m): 1200 Elongation (%): 20

② Dry method - Grafting quantitative (g/m2): 58 Tensile strength MD (N/m): 3500 Elongation (%): 20

③Wet method - Grafting quantitative (g/m2): 58 Tensile strength MD (N/m): 3500 Elongation (%): 20

④Wet method - sulfonation quantitative (g/m2): 62 Tensile strength MD (N/m): 2100 Elongation (%): 11

⑤ Spliced fiber method - Fluorine treatment quantitative (g/m2): 60 Tensile strength MD (N/m): 3700 Elongation (%): 25

⑥Quantitative amount of hydrophilic fiber added in the spliced fiber method (g/m2): 57 Tensile strength MD (N/m): 2000 Elongation (%): 20

Separators with tensile strength above 3000N/m can be used in roll batteries without problems. If the tensile strength is less than 2000N/m, care must be taken when winding the battery to avoid damage to the separator and short circuit of the battery. Separators produced by the melt-blown method can only be used for square batteries because the tension on the separators during the battery production process is not serious. A particular benefit of using radiation grafting is the increased tensile strength of the separator. ②Chemical stability In order to extend the service life of the battery and ensure reliable operation, the separator must have high chemical stability inside the battery. The figure below shows the relationship between the time and tensile strength of three types of grafted separators immersed in 30% KOH solution at a temperature of 70°C. For comparison this figure lists the effects of soaking a typical nylon diaphragm. These results represent the degradation properties of nylon while clearly demonstrating the long-term stability of polyolefin materials. In addition, the table below shows that the tensile properties of polyolefin nonwoven separators are not affected even after hundreds of charge and discharge cycles. ③The rapid water-soakability of the water-soakable separator in the electrolyte can ensure rapid and effective battery production. The table below gives typical imbibition rates for optimal liquid removal from the separator of a NiMH battery. These data clearly show that all grafted nonwoven membranes have good water-soakability and wetting speed. The non-woven separator made of fine fibers and small pores by melt-blown method has the highest wetting speed. The separator is only suitable for use in prismatic cells after radiation grafting, which is being developed for use in electric vehicles. To ensure stable battery performance during cycling, the water-soakability of the separator must not be affected during the battery's operating life. However, corona discharge and fluorination treatment on the surface of the non-woven separator will degrade its water immersion performance over time. However, radiation grafting is a permanent treatment for polyolefin materials, and they can always remain water-soaked. The figure below shows the wetting speed as a function of time for two grafted polypropylene non-woven separators and a fluorine-treated non-woven separator in KOH at 70°C. The results showed that the grafted nonwoven membrane was the best at maintaining water immersion.

④ Absorption and retention of electrolyte The ability of the separator to absorb and retain electrolyte is an important parameter for battery performance. For ideal battery performance, the separator must have a high and uniform absorption rate of electrolyte. Furthermore, to ensure long cycle life, the separator must be able to retain sufficient electrolyte to prevent the separator from drying out during cycling. The table below gives quantitative data on the absorption and retention of electrolyte by a separator of approximately 60 g/m2. The electrolyte retention capacity is measured by the amount of electrolyte squeezed out after the separator is pressurized through pressure. In order to maximize the cycle life of the battery, the electrolyte should be retained as much as possible. 1. Electrolytic melt-blown method - absorption and retention of grafted electrolyte: 300 (g/m2) Electrolyte retention: 19.7 (g/m2) Electrolyte retention rate (%): 6.6

2. Dry method-grafting electrolyte absorption and retention: 190 (g/m2) Electrolyte retention: 14.8 (g/m2) Electrolyte retention rate (%): 7.8

3. Wet-grafting electrolyte absorption and retention: 160 (g/m2) Electrolyte retention: 9.3 (g/m2) Electrolyte retention rate (%): 5.8

4. Absorption and retention of wet-sulfonated electrolyte: 140 (g/m2) Electrolyte retention: 8.8 (g/m2) Electrolyte retention rate (%): 5.7

5. Spliced fiber method - absorption and retention of grafted electrolyte: 180 (g/m2) Electrolyte retention: 13.7 (g/m2) Electrolyte retention rate (%): 7.6

6. Spliced fiber method-fluorine treatment electrolyte absorption and retention: 230 (g/m2) electrolyte retention: 12.6 (g/m2) electrolyte retention rate (%): 5.5

7. The absorption and retention of electrolyte by splicing fiber method plus hydrophilic fiber: 210 (g/m2) Electrolyte retention: 14.3 (g/m2) Electrolyte retention rate (%): 6.8 The main impact on the electrolyte retention ability It is the diameter of the fiber, and the diameter of the fiber is also a factor that controls the pore size of the separator. For square batteries, non-woven separators made by melt-blown method are the best choice, while for sealed roll batteries, dry-laid non-woven separators are the best choice.

⑤Impurity separation/self-discharge performance Ni-MH batteries have a large self-discharge. However, the self-discharge rate of nickel-metal hydride batteries using grafted and sulfonated polyolefin non-woven separators can reach the level of nickel-cadmium batteries. One reason for this improved performance is that the polyolefin non-woven separator has strong chemical stability, which can prevent battery contamination, and impurities are the cause of accelerated self-discharge. For example, nylon separators can accelerate battery self-discharge due to the introduction of nitrogen in impurities. In addition, a very important feature of the radiation-grafted polyolefin non-woven separator is that it can effectively reduce the impurity content that causes accelerated self-discharge. There is evidence that the ion exchange properties of this type of membrane can absorb and block impurities such as metal ions and ammonia. These contaminations will cause corrosion of the electrodes in the battery, just like there are impurities inside the electrodes. For example, nitrate impurities may reduce in the positive electrode and become ammonia in the negative electrode, thereby accelerating self-discharge. This is called a nitrogen conduction reaction. The ammonia absorption capacity of the grafted separator: 1, the wet method-grafted ammonia absorption capacity (NH3mmol/g): 3.0

2. Spliced fiber method-grafted ammonia absorption capacity (NH3mmol/g): 3.6

3. Dry method - grafted ammonia absorption capacity (NH3mmol/g): 4.0

4. Melt-blown method-grafted ammonia absorption capacity (NH3mmol/g): 5.2

5. Melt-blown method - non-grafted ammonia absorption capacity (NH3mmol/g): 0

Charge retention capacity of nickel-metal hydride batteries after 7 days of storage at 70°C: 1, grafted pp charge rate (%): 80

2. Fluorine treatment charging rate (%): 54

3. Coronadischargepp charging rate (%): 50