CN102522517A - Cellulose/inorganic particle composite diaphragm for lithium secondary battery and preparation method thereof - Google Patents

Abstract

The invention provides a cellulose/inorganic particle composite separator for lithium secondary batteries and a manufacturing method thereof. The cellulose separator is made by beating cellulose raw materials such as wood pulp, hemp pulp or cotton pulp into cellulose with a microfibrous structure, and mixing it with inorganic particles to form a cellulose/inorganic particle composite wet state membrane material, and then passing through a solvent Replace and dry to form a diaphragm, and the diaphragm made of the cellulose/inorganic particle compound can be used as a non-aqueous lithium ion battery diaphragm material. The diaphragm prepared by using the present invention has good heat resistance, chemical solvent resistance and good mechanical properties, and can meet the requirements of the diaphragm for non-aqueous electrolyte energy storage devices. In addition, the use of renewable cellulose as raw material has low cost, The advantages of simple process and environmental protection.

Description

Cellulose/inorganic particle composite separator for lithium secondary battery and manufacturing method thereof

technical field

The present invention relates to a separator for electrochemical devices and its manufacturing method, more particularly, the present invention relates to a separator made of composites of cellulose and inorganic microparticles, which has high heat resistance and chemical stability, and its application .

Background technique

In recent years, electrochemical devices for energy storage have received extensive attention. Rechargeable secondary batteries have been widely used in digital products such as mobile phones, video cameras, cameras, and notebook computers, and have begun to be used in fields such as electric vehicles. The requirements for high power and high capacity of lithium-ion secondary batteries are increasing day by day. At some point, lithium-ion batteries are prone to safety hazards such as smoke, fire, explosion, and even personal injury, making high-capacity and power lithium-ion batteries It has not been widely used yet, so improving the safety performance of lithium-ion batteries is the key to the development of lithium-ion secondary batteries. Lithium-ion batteries are generally obtained by laminating and winding strip-shaped positive electrodes, negative electrodes, and separators, and are helically wound bodies that ensure a large effective electrode area. The separator basically prevents the short circuit of the two poles, and uses its microporous structure to allow ions to pass through to carry out the battery reaction, but under some conditions such as misoperation, an abnormal current is generated and the internal temperature rises, causing the resin separator to thermally deform and block the microstructure. The hole stops the battery reaction.

The separator is an important part of the secondary battery, and its performance determines the interface structure, internal resistance, safety, etc. of the battery, and directly affects the capacity, cycle performance and other characteristics of the battery. A separator with excellent performance plays an important role in improving the overall performance of the battery. However, the preparation process of the separator for the secondary battery in the past is complicated, the pores of the separator are uneven and the porosity is low, which affects the quality of the secondary battery. Now commonly used battery separators such as polyethylene (PE) and polypropylene (PP) have a melting temperature lower than 160°C (for example, the self-closing temperature of PE diaphragm is 135-140°C, and the self-closing temperature of PP diaphragm is about 160°C), In some cases, such as the external temperature is too high, the discharge current is too large, or the thermal inertia of the electrolyte is heated, the temperature of the battery may continue to rise even if the current is interrupted, so the separator may be completely destroyed. cause the battery to short circuit, causing the battery to explode or catch fire. In addition, the tensile strength of uniaxially stretched PE separators and PP separators in the transverse direction is much worse than that in the longitudinal direction, and there is a danger of membrane rupture in the case of battery stacks or accidental impacts. There are many factors for the increase of internal heat and temperature of high-capacity and high-power batteries, so it is particularly important to improve the high-temperature resistance of batteries.

Chinese Patent Publication No. CN 101752539A discloses a polyimide for lithium-ion secondary battery and a lithium-ion battery. According to the principle of pore formation, a kind of pore-forming substance is incompatible with polyamic acid, and can be mixed with polyamic acid through use. The pore-forming substance reacts but does not react with the polyamic acid to remove the pore-forming substance. The other is incompatible with the polyamic acid. By using a substance that can dissolve the pore-forming substance but not dissolve the polyamic acid, the pore-forming The substance dissolves, thereby forming micropores. Although the polyimide material used in this patent has good heat resistance, its high cost is difficult for industrial application.

Chinese Patent Publication No. CN 101779311A discloses a polyolefin microporous film base material for non-aqueous secondary battery diaphragm and its preparation method and application. material and containing inorganic additive components, and then prepared non-aqueous secondary battery separator material by coagulation, water washing and drying. The polyolefin substrate used in this patent has poor heat resistance, and the diaphragm preparation process is relatively complicated.

Cellulose is an abundant natural polymer material in nature, which has good heat resistance and chemical solvent resistance. At present, the papermaking process first mixes plant fibers with chemical substances, cooks, beats, disperses the cellulose in water, and adds A large amount of papermaking aids, and then the pulp is separated from the wet paper through the screen, and then the wet paper is dried, and the cellulose is tightly bound together by hydrogen bonds, van der Waals forces, etc. to form paper. Paper produced by the papermaking process is widely used and is also used as a separator in alkaline batteries or electrolytic capacitors. At present, the airtightness, strength, and electrochemical properties of cellulose paper prepared by conventional papermaking processes cannot meet the separator materials for secondary batteries. In the invention, cellulose with a microfibrous structure prepared by highly beating and dispersing is combined with inorganic particles, and wet paper is made through papermaking and rolling, and the moisture in the wet paper is replaced by an organic solvent, and then the diaphragm is prepared by drying. The membrane prepared by the method has uniform pore structure distribution, is convenient to prepare, is suitable for mass production, and has high heat resistance, and is especially suitable for lithium-ion battery membranes.

Contents of the invention

At present, the separators used in tertiary and secondary batteries are mainly polyolefin porous membrane materials, which have poor heat resistance and mechanical stability. In the case of thermal inertia, even if the current is interrupted, the temperature of the battery may continue to rise, so the separator may be completely destroyed and the battery may be short-circuited, causing the battery to explode or catch fire.

The purpose of the present invention is to provide a cellulose/inorganic microparticle composite membrane with good heat resistance, chemical stability and mechanical properties.

Another object of the present invention is to provide a method and application for preparing the above-mentioned composite diaphragm.

In order to solve the above-mentioned technical problems, the present invention is achieved through the following technical solutions: a cellulose/inorganic particle composite diaphragm, including cellulose and inorganic particles, and also includes a wet strength agent. Based on the total weight of the composite diaphragm, the composition is 75%-99% of cellulose, 0.5%-25% of inorganic particles, and 0.5%-5% of wet strength agent.

The cellulose has a microfibril structure, the diameter of the cellulose is 0.05 to 5.0 microns, and the diameter of the microfibrils on the fiber surface is 50 to 500 nanometers; the cellulose is obtained by making wood pulp, cotton pulp, hemp At least one of pulp, bamboo pulp and their composites is used as a raw material to prepare cellulose slurry with a microfibrous structure.

The oxidation potential of the inorganic particles is +4.5v or above, including silicon dioxide, aluminum oxide, titanium oxide, calcium oxide, zirconium oxide, tin oxide, magnesium oxide, silicon carbide, calcium carbonate, diatomaceous earth and their composites At least one inorganic filler in the material, the diameter of the inorganic particles is 0.1-2 microns.

The wet strength agent includes at least one of urea-formaldehyde resin, melamine-formaldehyde resin, polyethyleneimine resin and their composites.

The preparation method of the cellulose/inorganic particle composite membrane for lithium secondary battery of the present invention comprises the following steps:

a) Grinding and dissolving the cellulose raw material into a cellulose slurry in a pulper, the concentration of the slurry is 1% to 30%, the beating temperature is 20 to 50, and the standard freeness CFS value of the slurry is less than 600;

b) making a suspension of inorganic particles, the mass percentage concentration of the inorganic particles is 10% to 30%; vigorously stirring and mixing the prepared inorganic particle suspension with the cellulose slurry;

c) Making paper from the cellulose slurry containing inorganic particles into a film by using a papermaking process, and rolling and pressing to form a wet composite film, and the water content of the composite film is 10% to 40% by mass;

d) Using an organic solvent to replace the moisture in the wet paper;

e) drying the composite film after organic solvent replacement at room temperature to 100°C.

Cellulose/inorganic particle composite wet paper is made into wet paper through papermaking, beating, netting, and pressing; organic solvent replacement refers to the replacement of residual organic matter in wet paper with a solvent that is compatible with water and has a surface tension lower than that of water. Moisture, while retaining the voids in the wet paper, resulting in a porous paper with inorganic particles.

Application of the composite diaphragm in preparing diaphragm materials for energy storage devices, the energy storage devices being selected from at least one of non-aqueous electrolyte lithium ion batteries and capacitors.

Compared with the prior art, the beneficial effect of the present invention is that the prepared cellulose composite membrane has high heat resistance, chemical stability and good physical and mechanical properties, and the raw material used is biorenewable cellulose raw material, which has It has the characteristics of low cost, simple process and environmentally friendly materials.

The following are the various embodiments of the non-aqueous battery assembled after being immersed in the electrolyte by using the composite separator as the separator between the positive and negative electrodes of the battery. In addition, the comparative example of the non-aqueous battery assembled by the current method is also listed. Describes the test methods for separators and non-aqueous batteries.

Slurry freeness CFS value: dilute the slurry after cellulose beating to a mass percent concentration of 0.3%, measure the water volume of 1000 ml of slurry passing through the Canadian standard freeness meter at a temperature of 20.0, and calculate it as the CFS value.

Cellulose morphology and membrane pore size: use scanning electron microscopy to observe the size of the cellulose microfibril structure and the pore size and distribution on the surface and cross-section of the composite membrane.

Thermal stability: The size change of the composite separator was measured after the separator was treated in an oven at different temperatures for 30 minutes.

Air permeability: Gurley 4110N air permeability meter (USA) is used to measure the air permeability of the composite diaphragm sample, in seconds.

Membrane thickness: Use a micrometer (accuracy: 0.01 mm) to test the thickness of the composite diaphragm, randomly take 5 points on the sample, and take the average value.

Porosity: Using the following test method, soak the composite diaphragm in n-butanol for 2 hours, and then calculate the porosity according to the formula.

Tensile strength: The tensile strength and elongation of the composite diaphragm are tested by the plastic tensile test method of GB1040-79.

1) Preparation of positive electrode

First, mix 5.75 grams of positive electrode active material LiCoO ₂ and 0.31 grams of conductive agent acetylene black, and then add 6.39 grams of 5% polyvinylidene fluoride (PVDF) solution (solvent is N-methyl-2-pyrrolidone) , stirred to form a uniform positive electrode slurry.

The slurry was uniformly coated on an aluminum foil, then dried at 120°C, rolled, and punched to obtain a circular positive electrode sheet with a radius of 12 mm and a thickness of 80 microns, which contained 17.6 mg of active ingredient LiCO ₂ .

2) Preparation of negative electrode

4.74 grams of negative active material natural graphite and 0.10 gram of conductive agent acetylene black are mixed evenly, then add 2.55 grams of polyvinylidene fluoride (PVDF) solution (solvent is N-methylpyrrolidone) with a mass fraction of 10%, and stir to form a uniform negative electrode slurry.

The negative electrode slurry is evenly coated on the copper foil, then dried at 120°C, rolled, and punched to obtain a circular negative electrode sheet with a radius of 14 mm and a thickness of 70 microns, which contains 11.9 mg of active ingredient Natural graphite.

3) prepare battery with composite diaphragm of the present invention

The positive electrode obtained above, the negative electrode and the composite diaphragm are stacked in sequence and packed into a button cell, and mixed solvent (ethylene carbonate: methyl ethyl carbonate (EC/EMC) volume ratio is 1: 1) About 150 mg of electrolyte solution containing 1 mole of lithium hexafluorophosphate (LiPF ₆ ) was injected into the above battery, aged according to conventional methods, and the aluminum shell of the battery was sealed to obtain a lithium ion secondary battery.

4) Battery high temperature performance test

The test method is as follows: charge the battery at 1C to a 100% state of charge, place it in an oven, and raise the temperature of the oven from room temperature to 200°C at a rate of 5°C/min. If the battery voltage drops greater than 0.2 volts, it is considered a short circuit.

5) Battery performance test

The test method is as follows: at 25±-5°C, the battery is charged and discharged 250 times, and the remaining power is recorded. The higher the remaining power, the longer the battery life.

Detailed ways

The present invention will be further described below in conjunction with specific examples, but the present invention is not limited to the following examples.

Example 1

Add 1 liter of distilled water to 20 grams of hardwood pulp (needle kraft pulp), soak and grind at a high speed, beating to make a hardwood plant fiber slurry with a concentration of 2wt%, the CFS value is 600, and the fiber diameter is 0.8 micron. 1 gram of silicon dioxide with a particle size of 50nm is dispersed in 100 ml of water, 0.2 gram of urea-formaldehyde resin is added, and fully mixed with cellulose slurry, paper is made by a semi-automatic paper machine, and the wet paper is removed by squeezing with a press roller excess moisture. Put the rolled wet paper into ethanol, replace the moisture in the wet paper twice with ethanol, and dry at 35°C to make a cellulose composite separator with a thickness of 35 microns.

Example 2

Add 1 liter of distilled water to 20 grams of hemp pulp, after soaking, through high-speed grinding and beating, make a hardwood plant fiber slurry with a concentration of 2wt%, the CFS value is 500, and the fiber diameter is 0.8 micron. Silica is dispersed in 100 ml of water, 0.2 g of urea-formaldehyde resin is added, and fully mixed with cellulose slurry, paper is made by a semi-automatic paper machine, and the wet paper sheet is squeezed with a press roller to remove excess water. Put the rolled wet paper into acetone, replace the moisture in the wet paper twice with acetone, and dry at 35° C. to prepare a cellulose composite separator with a thickness of 35 μm.

Example 3

Add 1 liter of distilled water to 20 grams of hardwood pulp (needle kraft pulp), soak and grind at a high speed, beating to make a hardwood plant fiber slurry with a concentration of 2wt%, the CFS value is 300, and the fiber diameter is 0.6 micron. 1 gram of aluminum oxide with a particle size of 500nm is dispersed in 100 milliliters of water, 0.2 grams of melamine formaldehyde resin is added, and fully mixed with cellulose slurry, paper is made by a semi-automatic paper machine, and the wet paper is rolled with a press roll Press to remove excess water. Put the rolled wet paper into ethanol, replace the moisture in the wet paper twice with ethanol, and dry at 35°C to prepare a cellulose composite separator with a thickness of 30 microns.

Example 4

Add 1 liter of distilled water to 18 grams of cotton pulp pulp, after soaking, high-speed grinding and beating to make a hardwood plant fiber slurry with a concentration of 1.8wt%, the CFS value is 300, and the fiber diameter is 0.5 microns. Disperse 50nm aluminum oxide in 100ml of water, add 0.1g of melamine formaldehyde resin, fully mix the above-mentioned substance with cellulose slurry, make paper through a semi-automatic paper machine, and use a press roller to squeeze the wet paper sheet to remove excess of moisture. Put the rolled wet paper into ethanol, replace the moisture in the wet paper twice with ethanol, and dry at 55°C to prepare a cellulose composite separator with a thickness of 28 microns.

Example 5

Add 1 liter of distilled water to 20 grams of hardwood pulp (acicular kraft pulp), soak and grind at a high speed, beating to make a hardwood plant fiber slurry with a concentration of 2wt%, the CFS value is 200, and the fiber diameter is 0.2 micron. 1 gram of diatomite with a particle size of 400nm is dispersed in 100ml of water, a wet strength agent for papermaking is added, and fully mixed with the cellulose slurry, paper is made by a semi-automatic paper machine, and the wet paper sheet is squeezed with a press roller Remove excess moisture. Put the rolled wet paper into acetone, replace the moisture in the wet paper twice with acetone, and dry at 35° C. to prepare a cellulose composite separator with a thickness of 35 μm.

Comparative example 1

The commercial polyolefin separator Celgard 2400 was used as a comparison to further clarify the advantages of the cellulose nanofiber separator described in the present invention.

The properties of the membranes prepared in Examples 1-5 were tested, and the results are listed in Table 1. As can be seen from the results in Table 1, the composite diaphragm prepared by the present invention has high temperature resistance, gas permeability and mechanical strength, and meets the requirements of lithium-ion battery diaphragms. From the test results of Examples 1-5 and Comparative Example 1 It can be seen that the temperature resistance and transverse tensile strength of commercial polyolefin separators are poor.

Table 1:

*The longitudinal and transverse strengths of the composite diaphragms prepared in the patent examples of the present invention are consistent.

The composite separator prepared in Examples 1-5 and the commercial separator in Comparative Example 1 were used to make batteries, and the high temperature resistance and life of the batteries were tested. The results obtained are listed in Table 2.

Table 2

Claims (11)

1. A cellulose/inorganic particle composite separator for a lithium secondary battery, comprising cellulose and inorganic particles, characterized in that it also includes a wet strength agent, and the composite separator is based on the total weight and consists of 75% to 99% of cellulose %, inorganic particles 0.5% to 25%, wet strength agent 0.5% to 5%. 2. The composite diaphragm according to claim 1, characterized in that the cellulose has a microfibril structure, the diameter of the cellulose is 0.05-5.0 microns, and the diameter of the microfibrils on the fiber surface is 50-500 nanometers. 3. The composite diaphragm according to claim 1, characterized in that the oxidation potential of the inorganic particles is +4.5v or above, and the diameter of the inorganic particles is 0.01-2 microns. 4. The composite diaphragm according to claim 1 or claim 2, characterized in that, the cellulose is made of at least one of wood pulp, cotton pulp, hemp pulp, bamboo pulp and their composites through a papermaking beating process. It is used as a raw material to make a cellulose slurry with a microfibril structure. 5. The composite diaphragm according to claim 1, wherein the inorganic particles include silicon dioxide, aluminum oxide, titanium oxide, zirconium oxide, tin oxide, magnesium oxide, silicon carbide, calcium carbonate, wollastonite and at least one inorganic filler in its composite. 6 . The composite diaphragm according to claim 1 , wherein the wet strength agent comprises at least one of urea-formaldehyde resin, melamine-formaldehyde resin, polyethyleneimine resin and their composites. 7. A method for preparing composite diaphragm as claimed in claim 1, is characterized in that, comprises the following steps: a) Grinding and dissolving the cellulose raw material into a cellulose slurry in a pulper, the concentration of the slurry is 1% to 30%, the beating temperature is 20 to 50°C, and the standard freeness CFS value of the slurry is less than 600; b) making a suspension of inorganic particles, the mass percentage concentration of the inorganic particles is 10% to 30%, and vigorously stirring and mixing the prepared inorganic particle suspension with the cellulose slurry; c) Making paper from the cellulose slurry containing inorganic particles into a film by using a papermaking process, and rolling and pressing to form a wet composite film, and the water content of the composite film is 10% to 40% by mass; d) using an organic solvent to replace the moisture in the wet membrane material; e) Drying the composite film after replacing the organic solvent at room temperature to 100°C. 8. The preparation method according to claim 7, characterized in that, the standard freeness CFS value is measured by a Canadian standard freeness tester, and the mass percent concentration of the cellulose slurry is determined to be 0.3%. 9. preparation method according to claim 7, it is characterized in that, described organic solvent replacement refers to by being compatible with water and surface tension is less than the water remaining moisture in the solvent replacement wet state film of water surface tension, simultaneously The voids in the wet membrane are preserved, resulting in a porous membrane with inorganic particles. 10. according to claim 7 or claim 9 described composite membrane preparation methods, the solvent that adopts in wherein said solvent replacement process comprises methyl alcohol, ethanol, n-propanol, acetone, toluene, hexanaphthene and compound thereof at least one solvent. 11. The application of the composite diaphragm according to any one of claims 1 to 7 in non-aqueous electrolyte lithium ion batteries.