CN108428844A - A kind of new modified isolation film - Google Patents

Abstract

The present invention provides a novel modified isolation membrane, which includes an isolation membrane substrate, at least one surface of the isolation membrane substrate is provided with a coating, and HAP (hydroxyapatite) whiskers are arranged between the isolation membrane substrate and the coating. , the introduction of HAP whiskers provides a chain network structure for the coating, which effectively improves the mechanical strength and thermal stability of the separator, and the polyhydroxylation of HAP improves the compatibility and adhesion between coating particles At the same time, it also improves the wettability with the electrolyte, thereby effectively improving the lithium ion migration ability and the interface uniformity of the current in the lithium ion battery, preventing the occurrence of side reactions and the formation of lithium dendrites, and effectively improving the rate performance and cycle stability of the battery. .

Description

A New Modified Isolation Membrane

technical field

The invention belongs to the technical field of lithium ion batteries, and in particular relates to a novel modified isolation membrane.

Background technique

Separator is one of the four key materials of lithium-ion batteries. Its puncture resistance, heat resistance, electrolyte infiltration and liquid retention ability directly affect the performance of electrochemical devices. The existing problems such as poor compatibility between the separator and the electrolyte, easy thermal fusing, puncture by foreign objects, etc., greatly limit the application of electrochemical devices.

Modification is an effective method to improve the comprehensive performance of the separator and has been effectively applied in the industry, mainly including ceramic modification, polymer modification, and organic-inorganic composite modification. But there are also some problems: the adhesion between the ceramic particles and the matrix of the isolation film is weak, and it is easy to fall off during processing; the mechanical properties of the isolation film are only improved by ceramic particles by about 10%, and the room for improvement is limited; the polymer modified coating The mechanical strength is low, and it is easy to be squeezed and deformed; the organic/inorganic hybrid system is difficult to effectively exert the advantages of the material due to its weak compatibility.

Jiang Shan and others disclosed a preparation method of a lithium-ion secondary battery separator with a cross-linked structure composite layer (CN102888016B). In this invention, a strong oxidant solution is used to modify the surface of the polyolefin microporous base film to make it hydroxylated, and through chemical bonds The cross-linked layer linked to the surface of the separator has a long-lasting effect, achieving a long-lasting surface modification. However, the hydroxylation specific gravity of this preparation method is greatly affected by the grafting rate, and the hydroxylation specific gravity is limited. At the same time, excessive grafting will reduce the porosity of the separator and even cause the risk of pore blocking, which is not conducive to obtaining a high ion conductivity. and electrochemical performance. In addition, the preparation method has many steps and many and complicated factors affecting the preparation process, which is not conducive to industrial production. Liang Bo and others disclosed a solid polymer electrolyte porous membrane membrane liquid and its membrane-making method (CN 104538672B). This invention utilizes polyhydroxyl compounds to improve the mechanical properties and internal pore structure of polymer electrolyte membranes. The mass ratio to the polymer matrix is used to improve the mechanical properties of the polymer electrolyte and control the pore structure inside the membrane, so that the polymer electrolyte has a good liquid storage space and mechanical properties similar to the elliptical three-dimensional structure pores. However, the process is complicated and the cost is high, which is unfavorable for industrialized production.

Therefore, how to prepare high-efficiency functional isolation membranes at low cost has become an urgent problem to be solved.

Contents of the invention

In view of the problems existing in the background technology, the object of the present invention is to provide a novel modified isolation membrane, which can realize low-cost, high-efficiency improvement of the performance of the isolation membrane, and the coating included in the isolation membrane has high mechanical strength, heat resistance and stability It has strong electrolyte wettability and lithium ion migration ability.

In order to achieve the above object, the present invention adopts the following technical solutions:

A novel modified isolation membrane, comprising an isolation membrane substrate, at least one surface of the isolation membrane substrate is provided with a coating, and a HAP (hydroxyapatite) crystal is arranged between the isolation membrane substrate and the coating. beard.

Preferably, the separator substrate is polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyester film (PET), cellulose film, polyimide film (PI), poly Any of amide film (PA), spandex or aramid film.

Preferably, the coating is any one of a ceramic coating, a polymer coating, and a ceramic/polymer blend coating.

Preferably, the material used for the ceramic coating is any one of aluminum oxide, calcium oxide, zinc oxide, magnesium oxide, titanium dioxide, silicon dioxide, tin dioxide, zirconium dioxide, cerium oxide, and magnesium sulfate.

Preferably, the material used for the polymer coating is any one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyacrylonitrile, polyoxyethylene, polymethyl acrylate, polyethyl acrylate .

Preferably, the HAP whiskers are acicular fibers, and there are a large number of hydroxyl groups on the surface of the HAP whiskers.

Preferably, the HAP whiskers have a diameter of 0.1-3 μm and a length of 20-300 μm.

Preferably, the added amount of HAP whiskers is 0.1%-70%, preferably 5%-20%.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The introduction of HAP long whiskers in the modified isolation film provides a chain network structure, which can provide a space frame for ceramic coatings and polymer coatings, effectively improving the structural stability of the coating; at the same time, the network structure Increase the mechanical strength of the coating and effectively improve the puncture and short circuit resistance of the isolation film;

(2) The polyhydroxylated surface of HAP whiskers effectively improves the compatibility with other particles in the separator and coating, and effectively improves the adhesion of the interface;

(3) The polyhydroxylated surface and high modulus of HAP whiskers improve the wettability of the separator and electrolyte, thereby improving the lithium ion migration ability of the separator and ensuring the uniformity of the current density at the interface, preventing interface damage-repair The occurrence of such side reactions and the formation of lithium dendrites can effectively improve the rate performance and cycle stability of the battery.

Description of drawings

Fig. 1 is the morphology diagram of the HAP-modified polymer organic coating in Example 2 of the present invention.

Detailed ways

The content of the present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited only to the content described in the embodiments.

Comparative example 1

Add alumina and binder polyvinylidene fluoride (PVDF) with a mass ratio of 90:10 into solvent deionized water and mix uniformly to form a slurry and make the solid content of the slurry 40% (alumina and polyvinylidene fluoride The mass accounts for 40% of the total amount of alumina, polyvinylidene fluoride, and deionized water), and then the slurry is evenly coated on one side of the 9 μm thick isolation film substrate polyethylene by using the dimple coating method to obtain a wet Membrane, after the wet film is dried in an oven, a composite porous isolation membrane is obtained, wherein the thickness of the coating after drying is 3 μm.

Comparative example 2

Add polyacrylonitrile with a mass ratio of 96:4 and binder polymethyl acrylate into solvent deionized water and mix evenly to make a slurry and make the solid content of the slurry 50%, and then use the dimple coating method to coat the slurry The material was evenly coated on one side of the 20 μm-thick isolation film substrate polypropylene to obtain a wet film, and the wet film was dried in an oven to obtain a composite porous isolation film, wherein the thickness of the coating after drying was 4 μm.

Example 1

Alumina, HAP whiskers and binder polyvinylidene fluoride (PVDF) with a mass ratio of 87:5:8 were added to solvent deionized water and mixed evenly to form a slurry and the solid content of the slurry was 50%, wherein HAP The diameter of the whiskers is 0.1 μm and the length is 100 μm, and then the slurry is evenly coated on both sides of the 9 μm thick isolation film substrate polyethylene by screen printing to obtain a wet film. After the wet film is dried in an oven, a composite porous isolation film is obtained. , the thickness of the coatings coated on both sides of the isolation film base is 2 μm after drying.

Example 2

Add polyacrylonitrile, HAP whiskers and binder epoxy resin with a mass ratio of 73:7:20 into the solvent ethylene carbonate and mix uniformly to make a slurry and make the solid content of the slurry 55%, wherein the HAP crystal The diameter of the whiskers is 0.1 μm and the length is 100 μm. Then, the slurry is evenly coated on both sides of the 20 μm-thick isolation membrane substrate polypropylene by extrusion coating method to obtain a wet film. After the wet film is dried in an oven, a composite porous isolation membrane is obtained. Wherein, the thickness of the coating coated on both sides of the isolation film base is 3 μm after drying, and the coating morphology is shown in FIG. 1 .

Example 3

A mixture of calcium oxide, polymethyl acrylate, and HAP whiskers (wherein 10% is HAP whiskers with a diameter of 0.1 μm and a length of 150 μm, and the remaining 80% is calcium oxide and polyacrylic acid), oxidized The mass ratio of calcium to polymethyl acrylate is 3:1,) and 10% of the binder polyvinylidene fluoride is added to the solvent deionized water and mixed uniformly to make a slurry and make the solid content of the slurry 70%, and then use Transfer coating The slurry is uniformly coated on one side of the isolation membrane substrate polyvinylidene fluoride to obtain a wet film, and the wet film is dried in an oven to obtain a composite porous isolation membrane, wherein the thickness of the coating after drying is 6 μm.

Example 4

A mixture of magnesium sulfate, polyethylacrylate and HAP whiskers (wherein 20% is a HAP whisker with a diameter of 0.3 μm and a length of 270 μm, and the remaining 65% is magnesium sulfate, polyacrylic acid, and magnesium sulfate) with a mass percentage content of 85%. The mass ratio with polyethyl acrylate is 1:3,) and 15% of the binder epoxy resin is added to the solvent deionized water and mixed evenly to make a slurry and make the solid content of the slurry 70%, and then use transfer coating Spread the slurry evenly on one side of the polyamide base of the isolation membrane to obtain a wet film. After the wet film is dried in an oven, a composite porous isolation membrane is obtained, wherein the thickness of the coating after drying is 6 μm.

Example 5

A mixture of magnesium oxide, polyvinylidene fluoride, and HAP whiskers with a mass percentage content of 94% (wherein 40% is a HAP whisker with a diameter of 0.2 μm and a length of 240 μm, and the remaining 54% is magnesium oxide and polyvinylidene fluoride, The mass ratio of magnesium oxide to polyvinylidene fluoride is 5: 1,) and 6% of the binder polytetrafluoroethylene is added to the solvent deionized water and mixed uniformly to form a slurry and the solid content of the slurry is 20%, and then The slurry is uniformly coated on one side of the polyimide base of the isolation membrane by dip coating to obtain a wet film, and the wet film is dried in an oven to obtain a composite porous isolation membrane, wherein the thickness of the coating after drying is 5 μm.

Li-ion secondary battery preparation:

(1) Preparation of positive electrode sheet

The active material lithium cobalt oxide, the conductive agent conductive carbon, and the binder polyvinylidene fluoride (PVDF) are added to the solvent N-methylpyrrolidone (NMP) in a mass ratio of 96:2.0:2.0 and mixed uniformly to form a positive electrode slurry, and then Coated on the current collector aluminum foil, and after drying at 85°C, cold pressing, slicing, trimming, slitting, and welding of tabs are carried out to make a positive electrode sheet.

(2) Preparation of negative electrode sheet

Active material graphite, conductive agent conductive carbon, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are added to solvent deionized water in a mass ratio of 96.5:1.0:1.0:1.5 and mixed evenly to make negative electrode slurry, and then Coated on the copper foil of the current collector and dried at 85°C, cold-pressed, sliced, edge-trimmed, stripped, and lug-welded to make a negative electrode sheet.

(3) Preparation of electrolyte

LiPF ₆ was prepared with ethylene carbonate (EC) and diethyl carbonate (DEC) to form LiPF ₆ with a concentration of 1.0 mol/L (wherein the mass ratio of EC and DEC was 3:7) to obtain an electrolyte.

(4) Preparation of lithium-ion secondary battery

The positive electrode sheet, separator, and negative electrode sheet are wound into a battery cell, and then the battery cell is placed in an aluminum-plastic bag, and the above electrolyte is injected, and the lithium-ion secondary battery is made into a lithium-ion secondary battery through packaging, formation, and capacity.

Using the above preparation method for lithium ion secondary batteries, the separators in Comparative Examples 1-2 and Examples 1-5 were respectively selected to prepare corresponding lithium ion secondary batteries.

The performance test of the separator prepared by Comparative Example 1-2 and Example 1-5, the lithium-ion secondary battery prepared by the separator obtained by Comparative Example 1-2 and Example 1-5 adopts the following process:

(1) Test of the puncture resistance strength of the separator: a round nail with a diameter of 0.5 mm pierces the separator at a speed of 50 mm/min.

(2) Test of the thermal shrinkage rate of the isolation film: punch the isolation film into a square piece with a knife die, put the isolation film in a constant temperature oven at a specific temperature, take it out after a specific time, and measure the thermal shrinkage rate of the isolation film before and after heat treatment .

(3) Adhesion test of the isolation film: the isolation film was punched into strips with a knife die, and the adhesion of the coating was tested with a high-speed iron tensile meter 180.

(4) Low-temperature discharge rate test of the lithium-ion secondary battery: charge the lithium-ion secondary battery at 0°C at a rate of 0.5C, and discharge at a rate of 2C. The capacity retention rate was calculated as follows: capacity retention rate=(battery capacity after cycle/room temperature capacity of battery before cycle)×100%.

(5) Test of the cycle performance of the lithium-ion secondary battery at room temperature: the lithium-ion secondary battery was charged at a rate of 0.5C and discharged at a rate of 0.5C at room temperature for 500 cycles in sequence. The capacity retention rate is calculated as follows: capacity retention rate=(capacity of lithium ion secondary battery after 500 cycles/room temperature capacity of lithium ion secondary battery before cycle)×100%.

Table 1 gives the parameters of Comparative Examples 1-2 and Examples 1-5.

Table 2 shows the performance test results of the separators and lithium ion secondary batteries of Comparative Examples 1-2 and Examples 1-5.

As can be seen from Example 1 and Comparative Example 1 in Table 1 and Table 2, due to the addition of HAP whiskers in Example 1, although the adhesive content in Example 1 has only 8% (in Comparative Example 1 Adhesive content is 10%), but the bonding performance that the test draws is indeed higher than the performance in Comparative Example 1 more than one time; Simultaneously the puncture resistance strength, rate performance and cycle stability that the test of embodiment draws are all It is much higher than that of Comparative Example 1, indicating that the addition of HAP whiskers has obvious effects on improving the adhesion, puncture resistance strength, rate performance, and cycle stability of the separator.

As can be seen from the performance test results in Table 2, compared with Comparative Examples 1 and 2, the puncture resistance strength of the porous isolation membrane compounded with HAP whiskers in Examples 1 to 5 of the present invention is greatly improved, and the interface is sticky at the same time. Bonding and thermal stability are also improved. At the same time, the longer the aspect ratio of the whiskers and the larger the filling amount, the better the network effect, which is manifested in better puncture resistance strength, interface adhesion and thermal stability.

Compared with Comparative Examples 1 and 2, the lithium-ion batteries in Examples 1 to 5 have greatly improved the compatibility between coatings and the wettability of the electrolyte due to the use of highly hydroxylated HAP whiskers, thereby Improve the rate performance and cycle stability of lithium-ion batteries.

Table 1 The parameters of comparative examples 1-2 and embodiments 1-5

Table 2 Performance test results of separators and lithium-ion secondary batteries of Comparative Examples 1-2 and Examples 1-5

Claims (8)

1. A novel modified isolation membrane, comprising an isolation membrane substrate, characterized in that: at least one surface of the isolation membrane substrate is provided with a coating, and a HAP ( hydroxyapatite) whiskers. 2. A novel modified isolation membrane according to claim 1, characterized in that: the isolation membrane substrate is polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyester film (PET), cellulose film, polyimide film (PI), polyamide film (PA), spandex or aramid film. 3. A novel modified isolation membrane according to claim 1, characterized in that: said coating is any one of ceramic coating, polymer coating, and ceramic/polymer blend coating. 4. A novel modified isolation membrane according to claim 3, characterized in that: the material used for the ceramic coating is aluminum oxide, calcium oxide, zinc oxide, magnesium oxide, titanium dioxide, silicon dioxide, tin dioxide , Zirconium dioxide, Ceria, Magnesium sulfate in any one. 5. A novel modified isolation membrane according to claim 3, characterized in that: the material used in the polymer coating is polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyacrylonitrile, Any of polyoxyethylene, polymethyl acrylate, and polyethyl acrylate. 6. A novel modified isolation membrane according to claim 1, characterized in that: the HAP whiskers are needle-like fibers, and there are a large number of hydroxyl groups on the surface of the HAP whiskers. 7. A novel modified isolation membrane according to claim 1, characterized in that: said HAP whiskers have a diameter of 0.1-3 μm and a length of 20-300 μm. 8. A novel modified isolation membrane according to claim 1, characterized in that: the added amount of the HAP whiskers is 0.1%-70%, preferably 5%-20%.