Lithium ion battery diaphragm as well as preparation method and application thereof

Abstract

The invention discloses a lithium ion battery diaphragm and a preparation method and application thereof.The preparation method of the diaphragm comprises the following steps that polyolefin, ultra-high molecular weight polyethylene, inorganic nanoparticles, an antioxidant, a cross-linking agent and a diluent are mixed to obtain a mixture, the mixture is evenly mixed and heated to 120-200 DEG C, hot-pressing film forming is conducted after standing and defoaming are conducted, and the diaphragm is obtained. And sequentially carrying out cooling shaping, extraction and drying to obtain the lithium ion battery diaphragm. The ultra-high molecular weight polyethylene and the cross-linking agent are utilized to improve the strength of the diaphragm, and the inorganic nanoparticles improve the ionic conductivity and heat resistance of the diaphragm and inhibit lithium dendrites. When the prepared diaphragm is applied to the lithium ion battery, the cycle performance and the cycle stability of the battery can be obviously improved, and the problem of thermal runaway caused by local overheating is solved. Compared with a commercial PP diaphragm, the prepared diaphragm has the advantages that the ionic conductivity is improved by 45.25%, and the first-cycle reversible specific discharge capacity of an assembled battery is obviously improved.

Description

Technical Field

[0001] The present invention relates to the technical field of diaphragms, and particularly relates to a lithium-ion battery diaphragm and its preparation method and application. Background Art

[0002] Lithium-ion batteries, with core characteristics such as high energy density, no memory effect, long cycle life, and low self-discharge, greatly meet the requirements of portable devices such as laptop computers, digital cameras, and smartphones for thinness and long-term battery life. At the same time, lithium-ion batteries not only dominate the market in the consumer electronics field, but also dominate the market of new energy electric vehicles. Almost all the batteries of new energy electric vehicles on the market use lithium-ion batteries, providing important support for the global energy structure to transform towards low-carbon.

[0003] The diaphragm is a key component of lithium-ion batteries because it prevents physical contact between the positive and negative electrodes of the battery, and the performance of lithium-ion batteries is greatly affected by the material and structure of the diaphragm.

[0004] Although the reported lithium-ion battery diaphragm materials in the literature cover PVDF, PTFE, PAN, PMMA, PVDF-HFP, PP, PE and their composite systems, those actually used on a large scale are still dominated by polyolefins (such as PP, PE), which benefits from their low raw material cost and mature preparation process. However, polyolefin diaphragms have several inherent defects: insufficient electrolyte wettability leads to low ionic conductivity, and poor high-temperature resistance causes thermal shrinkage and the risk of battery short circuit. Summary of the Invention

[0005] In order to improve the cycle performance and cycle stability of lithium-ion batteries in the prior art and to improve the thermal runaway problem caused by local overheating, the present invention provides a lithium-ion battery diaphragm and its preparation method and application.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A preparation method of a lithium-ion battery diaphragm, comprising the following steps:

[0008] Mix polyolefin, ultra-high molecular weight polyethylene, inorganic nanoparticles, antioxidant, crosslinking agent and diluent to obtain a mixture. In the mixture, the content of each component is, by mass percentage: polyolefin 10% - 50%, ultra-high molecular weight polyethylene (UHMWPE) 1% - 10%, inorganic nanoparticles 0.1% - 10%, antioxidant 0.1% - 1%, crosslinking agent 0.1 - 5%, diluent 30% - 80%;

[0009] Mix the mixture evenly and heat it up to 120°C - 200°C. After standing for defoaming, perform hot pressing to form a film, and then shape it by quenching in a water bath. Then, extract with various extractants to reduce shrinkage and dry to obtain a lithium-ion battery separator. Among them: the standing defoaming time is between 10 and 120 minutes, the hot pressing temperature is between 120°C and 200°C, the pressure is 5 - 25 Mpa, the water bath quenching temperature is between 0°C and 25°C, and the extraction time is between 1 and 24 hours.

[0010] In a further embodiment, the polyolefin is high-density polyethylene (HDPE) or high-density polypropylene (HDPP); the molecular weight of the ultra-high molecular weight polyethylene is between 1 million and 5 million.

[0011] In a further embodiment, the inorganic nanoparticles are at least one of inert ceramic oxides (such as alumina, silica, zirconia, titanium dioxide, lead dioxide, etc.), acidic oxides (such as iron oxide, nickel oxide, copper oxide, zinc oxide, etc.), nitrides (such as boron nitride, silicon nitride, aluminum nitride, etc.), borides (such as titanium diboride, aluminum boride, etc.), sulfides (such as zinc sulfide, etc.), ferroelectric materials (such as barium titanate, etc.), phosphates (such as aluminum dihydrogen phosphate, etc.), halides (such as aluminum iodide, etc.), clays (such as montmorillonite, etc.), carbon nanotubes (CNT), and zeolites.

[0012] In a further embodiment, the antioxidant is at least one of antioxidant 1076, antioxidant 1010, and antioxidant 168.

[0013] In a further embodiment, the crosslinking agent is at least one of silane coupling agent A171, silane coupling agent A151, and silane coupling agent A172.

[0014] In a further embodiment, the diluent is at least one of dioctyl phthalate, liquid paraffin, dibutyl phthalate, soybean oil, palm oil, xylene, dicyclohexyl phthalate, dimethyl sulfoxide, and stearic acid.

[0015] In a further embodiment, the extractant includes one of dichloromethane, ethanol, water, n-hexane, acetone, toluene, and gasoline.

[0016] The lithium-ion battery separator prepared by the above preparation method has excellent performance. When the lithium-ion battery separator is applied in a lithium-ion battery, the performance of the lithium-ion battery can be significantly improved.

[0017] The present invention has the following beneficial effects:

[0018] In the preparation process of the lithium-ion battery separator of the present invention, ultra-high molecular weight polyethylene and a cross-linking agent are used to enhance the strength of the separator, and inorganic nanoparticles are used to improve the ionic conductivity and heat resistance of the separator and inhibit lithium dendrites. Compared with the commercial PP separator Celgard-2500, the ionic conductivity of the separator prepared by the present invention is increased by 45.25%. The first-week reversible discharge specific capacity of the assembled battery reaches 150.25 mAh / g, and after 100 cycles, the capacity retention rate is 98.59%. The discharge specific capacity is increased by 23.27% compared with the commercial separator Celgard-2500, and the capacity retention rate is increased by 4.93%. Description of the Drawings

[0019] Figure 1 Comparison of charge-discharge curves at 1C of the battery assembled with the separator prepared in Example 1 of the present invention and the commercial separator Celgard-2500;

[0020] Figure 2 Comparison of ionic conductivities of the separator prepared in Example 1 of the present invention and the commercial separator Celgard-2500. Detailed Description of the Invention

[0021] The following further illustrates the present invention in conjunction with embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the embodiments cited do not limit the present invention.

[0022] In addition, in the preparation process of the following embodiments, unless otherwise specified, they are all conventional means in the prior art in this field, so they will not be described in detail; all raw materials used in the present invention are commercially available products and can be obtained through commercial purchase.

[0023] Example 1

[0024] High-density polyethylene, ultra-high molecular weight polyethylene, nano-aluminum oxide, antioxidant 1010, silane coupling agent A151, and dioctyl phthalate were ultrasonically treated in a two-necked flask for 15 minutes according to the mass ratio of 26.5%, 1.5%, 1%, 0.5%, 0.5%, and 70%. Then, it was heated to 100 °C while stirring. When it reached 100 °C, stirring was maintained for 1 hour. Then, it was heated to 190 °C while stirring. When it reached 190 °C, stirring was maintained for 1 hour. After that, it was left standing for 1 hour. Subsequently, it was taken out and quenched and cooled in water at 10 °C. Then, the sample was taken out and hot-pressed in a vulcanizer. The hot-pressing temperature was 200 °C, the pressure was 15 Mpa, and the duration was 15 minutes. Subsequently, the sample was taken out and quenched and cooled in water at 0 °C. Then, it was heat-set at a high temperature, transferred to ethanol for extraction for 12 hours, then transferred to water for extraction for 12 hours, and then transferred to n-hexane for extraction for 6 hours. It was heat-set at a high temperature again and dried. The dried separator was cut into circular pieces to assemble the battery for testing.

[0025] The battery was assembled with the commercial separator Celgard-2500 as Comparative Example 1.

[0026] Figure 1 The charge-discharge curves of the battery assembled with the separator prepared in Example 1 of the present invention and the commercial separator Celgard-2500 at 1C are compared; from Figure 1 It can be seen that the initial reversible discharge specific capacity of the battery assembled with the separator prepared by the present invention is as high as about 160 mAh / g; after 100 cycles, the capacity retention rate is 98.59%, and the discharge specific capacity is increased by 23.27% compared with the commercial separator Celgard-2500.

[0027] Figure 2 The ionic conductivity of the separator prepared in Example 1 of the present invention and the commercial separator Celgard-2500 is compared. Compared with the commercial PP separator Celgard-2500, the ionic conductivity of the composite membrane is measured by AC impedance measurement of a stainless steel|separator|stainless steel blocking battery using a chemical workstation with a frequency range of 0.1 - 100 KHz. The thickness of the prepared thin film is about 80 μm, the thickness of the PP separator is about 25 μm, and the area is about 2.01 cm -2 for impedance measurement. Therefore, the ionic conductivity can be calculated by the following equation: (C)

[0028]

[0029] In this equation, σ is the ionic conductivity, and R b is the bulk resistance, and h and S are the thickness and area of the membrane, respectively.

[0030] After calculation, the ionic conductivity of Comparative Example 1 is 2.63×10 -4 S cm -1 and the ionic conductivity of Example 1 is 3.82×10 -4 S cm -1 The ionic conductivity of the separator prepared by the present invention is increased by 45.25%.

[0031] Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium ion battery separator, characterized in that: The following steps are involved: Polyolefin, ultra-high molecular weight polyethylene, inorganic nanoparticles, antioxidant, cross-linking agent and diluent are mixed to obtain a mixture, the mixture is mixed evenly and heated to 120°C-200°C, allowed to stand for degassing and then hot pressed into a film, and then cooled, shaped, extracted and dried in sequence to obtain a lithium-ion battery separator.

2. The method for preparing a lithium ion battery separator according to claim 1, characterized in that: In the mixture, the contents of the various components are as follows, by mass percentage: 10% to 50% polyolefin, 1% to 10% ultra-high molecular weight polyethylene, 0.1% to 10% inorganic nanoparticles, 0.1% to 1% antioxidant, 0.1% to 5% crosslinking agent, and 30% to 80% diluent.

3. The method for preparing a lithium ion battery separator according to claim 1, characterized in that: The temperature of the hot pressing is 120° C.-200° C., and the pressure is 5-25 MPa.

4. The method for preparing a lithium ion battery separator according to claim 1, characterized in that: The polyolefin is high-density polyethylene or high-density polypropylene; the molecular weight of the ultra-high molecular weight polyethylene is between 1 million and 5 million.

5. The method for preparing a lithium ion battery separator according to claim 1, characterized in that: The inorganic nanoparticles are at least one of inert ceramic oxides, acidic oxides, nitrides, borides, sulfides, ferroelectric materials, phosphates, halides, clays, carbon nanotubes, and zeolites.

6. The method for preparing a lithium ion battery separator according to claim 1, characterized in that: The antioxidant is at least one of antioxidant 1076, antioxidant 1010, and antioxidant 168.

7. The method for preparing a lithium ion battery separator according to claim 1, characterized in that: The cross-linking agent is at least one of silane coupling agent A171, silane coupling agent A151, and silane coupling agent A172.

8. The method for preparing a lithium ion battery separator according to claim 1, characterized in that: The diluent is at least one of dioctyl phthalate, liquid paraffin, dibutyl phthalate, soybean oil, palm oil, xylene, dicyclohexyl phthalate, dimethyl sulfoxide, and stearic acid.

9. A lithium ion battery separator, characterized in that: The lithium-ion battery separator is prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: The lithium ion battery comprises the lithium ion battery separator according to claim 9.

Images