CN111900342A - Positive pole piece and preparation method and application thereof - Google Patents

Abstract

The present invention provides a positive electrode sheet and a preparation method and application thereof. The present invention is to coat the surface of the positive electrode current collector with a lithium iron phosphate positive electrode active material containing small particles as a first positive electrode active material layer by means of double-layer coating. , the use of the first positive electrode active material layer can effectively improve the needle penetration rate of the high-voltage system cell; the double-layer coating method of the present invention has a simple process, does not require secondary coating, and can save production time; the present invention Under the premise of greatly improving the safety of acupuncture, the energy density loss of the method is less than 3%.

Description

A kind of positive pole piece and its preparation method and use

technical field

The present invention relates to the technical field of lithium ion batteries, in particular to a positive electrode piece, a preparation method and application thereof, and the use of the positive electrode piece can improve the safety of a high voltage system of a lithium ion battery.

Background technique

Since its commercialization, lithium-ion batteries have been widely used as power sources for various mobile devices due to their high energy density, high power density, good cycle performance, no memory effect, and green environmental protection. With the large-scale application of lithium-ion batteries, the safety problem of batteries has become increasingly prominent.

At present, the positive electrode of a lithium ion battery includes an aluminum foil and a positive electrode active material layer, and the negative electrode includes a copper foil and a negative electrode active material layer. In the safety testing of lithium-ion batteries, acupuncture experiments are often used. During the acupuncture experiment, it is easy to trigger the internal short circuit phenomenon caused by the contact between the positive electrode aluminum foil and the negative electrode active material layer, and then cause thermal runaway, which means that the lithium ion battery in the acupuncture experiment has a potential safety hazard. It shows that the safety of the lithium ion battery in the prior art is low.

SUMMARY OF THE INVENTION

At present, 4.45V high-voltage battery cell acupuncture is one of the more difficult tests to pass, which seriously affects the safety of lithium-ion batteries. The main reason is that the most dangerous of the four internal short-circuit methods is easily triggered in the acupuncture test—the contact between the aluminum foil and the negative electrode active material (such as graphite), resulting in thermal runaway.

In order to solve the above problems, the present invention proposes the following technical solutions:

A positive pole piece, the positive pole piece comprises a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector, the positive electrode active material in the positive electrode active material layer is selected from olivine-type lithium iron phosphate, The ratio [111]/[311] of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane in the X-ray of the positive electrode active material is less than 9.

Preferably, the [111]/[311] is less than 6; further preferably, the [111]/[311] is less than 3; further preferably, the [111]/[311] is less than 1.

Further, the positive electrode active material layer is arranged into two layers, namely a first positive electrode active material layer and a second positive electrode active material layer; the first positive electrode active material layer is arranged on the surface of the positive electrode current collector, and the second positive electrode active material layer is arranged on the surface of the positive electrode current collector. The material layer is arranged on the surface of the first positive electrode active material layer; wherein, the first positive electrode active material in the first positive electrode active material layer is selected from lithium iron phosphate with an olivine type structure, and the first positive electrode active material is exposed to X-rays. The ratio [111]/[311] of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane is less than 9. Preferably, the [111]/[311] is less than 6; further preferably, the [111]/[311] is less than 3; further preferably, the [111]/[311] is less than 1.

Wherein, the median particle size D ₅₀ of the first positive electrode active material is 1-2 μm.

Wherein, the crystal planes of the olivine-type lithium iron phosphate in X-rays include 111 crystal planes and 311 crystal planes.

Wherein, the median particle size _D50 of the second positive electrode active material is 5-20 μm.

Wherein, the content of the first binder in the first positive electrode active material layer is greater than the content of the second binder in the second positive electrode active material layer.

Wherein, the thickness of the first positive electrode active material layer is 2-10 μm.

The present invention also provides a lithium ion battery, the lithium ion battery includes the above-mentioned positive pole piece.

Beneficial effects of the present invention:

The invention provides a positive electrode piece, a preparation method and application thereof, and the use of the positive electrode piece can improve the acupuncture pass rate of a high-voltage system cell under the premise of ensuring that the electrical performance of a lithium ion battery is not greatly lost. , improve the safety of lithium-ion battery high-voltage system. The study found that with the progress of the charge-discharge cycle, the lattice of the positive active material, especially the first positive active material, will expand and contract, and the diffraction intensity of the 111 crystal plane in the X-ray [111] represents the degree of expansion and contraction, and the contraction is obvious. It will cause the positive electrode active material layer, especially the first positive electrode active material layer, and the positive electrode current collector (such as aluminum foil) to be poorly bonded, which will significantly reduce the needle penetration rate of the battery after cycling. At the same time, the diffraction intensity of the 311 crystal plane [ 311] represents the order degree of the crystal structure of the positive electrode active material. The larger the peak intensity, the higher the order degree, which means the more stable the structure is and the less likely it is to collapse during the cycle. The ratio [111]/[311] of the diffraction intensity [111] to the diffraction intensity [311] of the 311 crystal plane is less than 9, especially when it is used as the first positive electrode active material in the first positive electrode active material layer. The acupuncture pass rate of the lithium ion battery is significantly improved, that is, the safety performance of the lithium ion battery is significantly improved.

Further, in the positive electrode sheet of the present invention, the first positive electrode active material layer is coated between the second positive electrode active material layer and the positive electrode current collector, and at the same time, the content of the binder in the first positive electrode active material layer is determined. is larger than the content of the binder in the second positive electrode active material layer, thereby further improving the bonding degree between the positive electrode current collector surface coating and the positive electrode current collector, that is, protecting the positive electrode current collector and making the positive electrode current collector surface coating During the acupuncture experiment, it can separate the positive electrode current collector and the negative electrode surface coating, which can effectively reduce the internal short circuit phenomenon caused by the contact between the positive electrode current collector and the negative electrode surface coating in the acupuncture experiment, and improve the safety of lithium-ion batteries. sex. In addition, the introduction of the first positive electrode active material layer can also ensure that the energy density loss of the lithium ion battery is less.

The present invention provides a positive electrode sheet and a preparation method and use thereof. Further, the present invention is to coat the surface of the positive electrode current collector with a lithium iron phosphate positive electrode active material containing small particles by means of double-layer coating as the first positive electrode Active material layer, the use of the first positive electrode active material layer can effectively improve the needle penetration rate of the high-voltage system cell; the double-layer coating method of the present invention has a simple process, does not require secondary coating, and can save production time ; On the premise that the method of the present invention greatly improves the safety of acupuncture, the loss of energy density is less than 3%.

Description of drawings

FIG. 1 is a schematic structural diagram of a positive electrode plate of the present invention.

Wherein, A is the positive electrode current collector; B is the first positive electrode active material layer; C is the second positive electrode active material layer.

Detailed ways

As mentioned above, the present invention proposes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the surface of the positive electrode current collector, and the positive electrode active material in the positive electrode active material layer is selected from Lithium iron phosphate with olivine structure, the ratio [111]/[311] of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane in the X-ray of the positive electrode active material is less than 9. Preferably, the [111]/[311] is less than 6; further preferably, the [111]/[311] is less than 3; further preferably, the [111]/[311] is less than 1.

Further, the positive electrode active material layer is arranged into two layers, namely a first positive electrode active material layer and a second positive electrode active material layer; the first positive electrode active material layer is arranged on the surface of the positive electrode current collector, and the second positive electrode active material layer is arranged on the surface of the positive electrode current collector. The material layer is arranged on the surface of the first positive electrode active material layer; wherein, the first positive electrode active material in the first positive electrode active material layer is selected from lithium iron phosphate with an olivine type structure, and the first positive electrode active material is exposed to X-rays. The ratio [111]/[311] of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane is less than 9. Preferably, the [111]/[311] is less than 6; further preferably, the [111]/[311] is less than 3; further preferably, the [111]/[311] is less than 1.

According to the present invention, the first positive electrode active material layer includes a first positive electrode active material, a first conductive agent and a first binder; the second positive electrode active material layer includes a second positive electrode active material, a second conductive material agent and second binder.

According to the present invention, the median particle diameter _D50 of the first positive electrode active material is 1-2 μm.

According to the present invention, the median particle diameter D ₅₀ of the second positive electrode active material is 5-20 μm.

According to the present invention, the first positive active material is lithium iron phosphate with a median particle size D ₅₀ of 1-2 μm; the second positive active material is lithium cobalt oxide with a median particle size D ₅₀ of 5-20 μm.

According to the present invention, the ratio [003]/[006] of the diffraction intensity [003] of the 003 crystal plane to the diffraction intensity [006] of the 006 crystal plane in the X-ray of the second positive electrode active material is 18-24, such as 20 -22, like 21.

According to the present invention, the first conductive agent and the second conductive agent are the same or different, and are independently selected from at least one of conductive carbon black, carbon nanotubes and graphene.

According to the present invention, the first adhesive and the second adhesive are the same or different, and are independently selected from polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyurethane, At least one of polyvinyl alcohol, polyvinylidene fluoride, and vinylidene fluoride-fluorinated olefin copolymer.

According to the present invention, in the first positive electrode active material layer, the mass percentage of each component is:

The first conductive agent is 2wt%-5wt%, the first binder is 20wt%-45wt%, and the first positive active material is 50wt%-78wt%.

According to the present invention, the thickness of the first positive electrode active material layer is 2-10 μm.

Exemplarily, the mass percentage of the first conductive agent is 2wt%, 3wt%, 4wt%, 5wt%, and the mass percentage of the first adhesive is 20wt%, 22wt%, 25wt% , 28wt%, 30wt%, 33wt%, 35wt%, 38wt%, 40wt%, 45wt%, the mass percentage of the first positive active material is 50wt%, 52wt%, 55wt%, 58wt%, 60wt%, 62wt%, 65wt%, 68wt%, 70wt%, 72wt%, 75wt%, 78wt%.

According to the present invention, in the second positive electrode active material layer, the mass percentage of each component is:

The second conductive agent is 1wt%-6wt%, the second binder is 1wt%-4wt%, and the second positive electrode active material is 90wt%-98wt%.

According to the present invention, the thickness of the second positive electrode active material layer is 35-60 μm.

Exemplarily, the mass percentage of the second conductive agent is 1.0wt%, 1.5wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, and the mass percentage of the second adhesive The content is 1wt%, 1.5wt%, 2wt%, 3wt%, 4wt%, and the mass percentage of the second positive active material is 98wt%, 97wt%, 96wt%, 94wt%, 92wt%, 90wt%.

According to the present invention, the positive electrode current collector is selected from aluminum foil.

The present invention also provides a lithium ion battery, the lithium ion battery includes the above-mentioned positive pole piece.

According to the present invention, the lithium ion battery further includes a negative electrode, an electrolyte and a separator.

The present invention also provides a method for preparing the above-mentioned positive electrode, the method comprising the following steps:

preparing a positive electrode slurry including a positive electrode active material, the ratio [111]/[311] of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane in X-rays of the positive electrode active material is less than 9;

The positive electrode slurry is coated on the positive electrode current collector by coating equipment, dried, cut, and sliced to prepare the positive electrode sheet.

According to the present invention, the method specifically comprises the following steps:

(1-1) Prepare the first positive electrode slurry including the first positive electrode active material, wherein the diffraction intensity of the first positive electrode active material in X-rays of the 111 crystal plane [111] The diffraction intensity of the 311 crystal plane [311 ] the ratio [111]/[311] is less than 9;

(1-2) preparing a second positive electrode slurry including a second positive electrode active material;

(1-3) Coating the first positive electrode slurry and the second positive electrode slurry on the positive electrode current collector by double-layer coating equipment, drying, cutting, and tableting, to prepare the positive electrode sheet.

According to the present invention, the method further specifically comprises the following steps:

(1-1) Mix the first positive electrode active material, the first conductive agent and the first binder, then add N-methylpyrrolidone and stir to prepare the first positive electrode slurry; wherein, the first positive electrode active material In X-rays, the ratio [111]/[311] of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane is less than 9;

(1-2) Mixing the second positive electrode active material, the second conductive agent and the second binder, then adding N-methylpyrrolidone and stirring to prepare the second positive electrode slurry;

(1-3) Coating the first positive electrode slurry and the second positive electrode slurry on the positive electrode current collector by double-layer coating equipment, drying, cutting, and filming, to prepare a positive electrode sheet.

The present invention also provides a method for preparing the above-mentioned lithium ion battery, the method comprising:

(1) according to the preparation method of above-mentioned positive pole piece, prepare positive pole piece;

(2) The lithium ion battery is prepared by combining the positive electrode and the negative electrode.

According to the present invention, the step (2) specifically includes:

(2-1) Mix the negative electrode active material, conductive agent, binder and thickener, add deionized water and stir to make negative electrode slurry; then coat the negative electrode slurry on the negative electrode current collector, dry, Slitting and filming to prepare negative pole pieces;

(2-2) The positive pole piece prepared in step (1) and the negative pole piece prepared in step (2-1) are made into a battery together with a separator and an aluminum plastic film, and then liquid injection, aging, chemical formation, preconditioning are carried out. The lithium ion battery is prepared through processes such as cycling.

The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following examples are only for illustrating and explaining the present invention, and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the intended protection scope of the present invention.

The experimental methods used in the following examples are conventional methods unless otherwise specified; the reagents, materials, etc. used in the following examples can be obtained from commercial sources unless otherwise specified.

In the description of the present invention, it should be noted that the terms "first", "second" and the like are only used for descriptive purposes, and do not indicate or imply relative importance.

Example 1

(1) Preparation of positive electrode sheet

(1-1) Preparation of the first positive electrode slurry

64wt% of the first positive electrode active material (median particle size _D50 of olivine-type lithium iron phosphate of 1-2 μm, and the first positive electrode active material in the first positive electrode active material layer is in the X-ray. The ratio [111]/[311] of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane was 0.82), 4 wt% of the first conductive agent (conductive carbon black) and 32 wt% of the first conductive agent (conductive carbon black) The binder (polyvinylidene fluoride) is mixed, and then N-methylpyrrolidone is added to stir and disperse to prepare the first positive electrode slurry.

(1-2) Preparation of the second positive electrode slurry

97.8 wt % of the second positive electrode active material (lithium cobalt oxide LiCoO ₂ with a median particle diameter D ₅₀ of 5-20 μm), and the second positive electrode active material in the second positive electrode active material layer is 003 crystallized in X-rays. Diffraction intensity of the plane [003] to the diffraction intensity of the 006 crystal plane [006] ratio [003]/[006] is 21), 1.1 wt % of the second conductive agent (conductive carbon black) and 1.1 wt % of the second conductive The binder (polyvinylidene fluoride) is mixed, and then N-methylpyrrolidone is added to stir and disperse to prepare the second positive electrode slurry.

(1-3) Then the first positive electrode slurry and the second positive electrode slurry are coated on the positive electrode current collector at one time (double-sided coating) by double-layer coating equipment, dried, cut, and tableted to prepare In the positive electrode plate, the thickness of the first positive electrode active material layer formed by the first positive electrode slurry is 8 μm; the thickness of the second positive electrode active material layer formed by the second positive electrode slurry is 45 μm.

(2) Preparation of negative electrode sheet

96.9wt% of negative active material (artificial graphite), 0.5wt% of conductive agent (conductive carbon black), 1.3wt% of binder (styrene-butadiene rubber (SBR)) and 1.3wt% of thickener (carboxyl Sodium methyl cellulose (CMC)), add deionized water and stir, and disperse to prepare negative electrode slurry. Then, the negative electrode slurry is coated on the negative electrode current collector (double-sided coating), dried, cut, and tableted to prepare a negative electrode pole piece.

(3) Preparation of batteries

The positive pole piece obtained in the first step and the negative pole piece obtained in the second step are made into a battery together with the diaphragm and aluminum-plastic film, and then processes such as liquid injection, aging, chemical formation, and sorting are carried out. Electrochemical performance and safety performance (mainly acupuncture abuse) were tested.

The temperature of the preparation environment of the pole piece should be kept at 20-30°C, and the humidity should be ≤40% RH.

The equipment used in the preparation of the pole pieces includes: mixer, coater, roller press, slitter, tablet machine, ultrasonic spot welding machine, top and side sealing machine, inkjet printer, film sticker, liquid injection machine , Forming cabinets, cold presses, sorting cabinets, vacuum ovens, etc.

Example 2

Others are the same as Example 1, the difference is only in the ratio of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane in the X-ray of the first positive electrode active material in the first positive electrode active material layer [ 111]/[311] is 2.67.

Example 3

Others are the same as Example 1, the difference is only in the ratio of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane in the X-ray of the first positive electrode active material in the first positive electrode active material layer [ 111]/[311] is 5.36.

Example 4

Others are the same as Example 1, the difference is only in the ratio of the diffraction intensity [111] of the 111 crystal plane to the diffraction intensity [311] of the 311 crystal plane in the X-ray of the first positive electrode active material in the first positive electrode active material layer [ 111]/[311] is 7.93.

Example 5

Others are the same as Example 1, except that the content of the first binder in the first positive electrode slurry is 5 wt %, the content of the first positive active material is 64 wt %, and the content of the first conductive agent is 31 wt %.

Example 6

Others are the same as Example 1, except that the content of the first binder in the first positive electrode slurry is 18 wt %, the content of the first positive active material is 64 wt %, and the content of the first conductive agent is 18 wt %.

Example 7

Others are the same as in Example 1, except that the thickness of the first active material layer is 15 μm.

Comparative Example 1

Others are the same as in Example 1, except that the first positive electrode active material layer is not provided.

Comparative Example 2

Others are the same as in Example 1, the difference is only in the ratio of the diffraction intensity [111] of the first positive electrode active material in the first positive electrode active material layer in the X-ray of the 111 crystal plane to the diffraction intensity of the 311 crystal plane [311] [111] /[311] is greater than 9.94.

Acupuncture experiments were carried out on the lithium ion batteries of the above-mentioned Examples 1-7 and Comparative Examples 1-2, and the test process was as follows:

The lithium-ion battery was cycled 100 times under the condition of 0.7C/1.0C, and then a high-temperature steel needle with a diameter of ф(4±0.5) mm (the cone angle of the needle tip was 45°C-60°C, and the surface of the needle was smooth and clean) No rust, no oxide layer and no oil pollution), at a speed of (30mm/s±5mm/s), penetrate from the direction perpendicular to the electrode plate of the cell, and the puncture position should be close to the geometric center of the punctured surface (the needle stays at in the cell). One hour after the occurrence of acupuncture, the passage of the acupuncture was observed. The test results are shown in Table 1.

From the results in Table 1, it can be seen that the pass rate of the acupuncture test of Comparative Example 1 is low, that is, the safety is low, and it cannot meet the safety performance requirements of lithium ion batteries. However, the pass rates of the acupuncture experiments of Examples 1-7 of the present invention are all higher than those of Comparative Example 1, indicating that adjusting the diffraction intensity of the 111 crystal plane of the first positive electrode active material in the first positive electrode active material layer in the X-ray [111] The ratio [111]/[311] of the diffraction intensity [311] of the 311 crystal plane can improve the safety performance of lithium-ion batteries.

Table 1

In addition, it can be seen from the results in Table 1 that adjusting the content of the binder in the first positive electrode active material layer and the second positive electrode active material layer can also effectively improve the safety of the lithium ion battery.

The foregoing summarizes the features of several embodiments, which may enable those of ordinary skill in the art to better understand various aspects of the present application. One of ordinary skill in the art can readily use this application as a basis for designing or modifying other compositions for carrying out the same purposes and/or achieving the same advantages of the embodiments disclosed herein. Those with ordinary knowledge in the technical field can also understand that these equivalent examples do not deviate from the spirit and scope of the application, and they can make various changes, substitutions and modifications to the application without departing from the spirit of the application. with category. Although the methods disclosed herein have been described with reference to specific operations performed in a specific order, it should be understood that these operations may be combined, subdivided, or reordered to form equivalent methods without departing from the teachings of the present application. Accordingly, unless specifically indicated herein, the order and grouping of operations are not limitations of the present application.

The embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (10)

1. A positive pole piece comprises a positive pole current collector and a positive pole active material layer arranged on the surface of the positive pole current collector, wherein a positive pole active material in the positive pole active material layer is selected from lithium iron phosphate with an olivine structure, and the ratio [111]/[311] of the diffraction intensity [111] of a 111 crystal face to the diffraction intensity [311] of a 311 crystal face of the positive pole active material in X-rays is less than 9.

2. The positive electrode sheet according to claim 1, wherein [111]/[311] is less than 6.

3. The positive electrode sheet according to claim 2, wherein [111]/[311] is less than 3.

4. The positive electrode sheet according to claim 1, wherein the positive electrode active material layer is provided in two layers, a first positive electrode active material layer and a second positive electrode active material layer; the first positive electrode active material layer is arranged on the surface of the positive electrode current collector, and the second positive electrode active material layer is arranged on the surface of the first positive electrode active material layer; wherein the first positive electrode active material in the first positive electrode active material layer is selected from lithium iron phosphate having an olivine structure, and a ratio [111]/[311] of a diffraction intensity [111] of a 111 crystal plane to a diffraction intensity [311] of a 311 crystal plane in X-rays of the first positive electrode active material is less than 9.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein the first positive electrode active material has a median particle diameter D₅₀Is 1-2 μm.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein the second positive electrode active material has a median particle diameter D₅₀Is 5-20 μm.

7. The positive electrode sheet according to any one of claims 1 to 6, wherein a content of the first binder in the first positive electrode active material layer is larger than a content of the second binder in the second positive electrode active material layer.

8. The positive electrode plate according to any one of claims 1 to 7, wherein the first positive electrode active material layer comprises the following components in percentage by mass:

2-5 wt% of a first conductive agent, 20-45 wt% of a first binder and 50-78 wt% of a first positive electrode active material;

in the second positive electrode active material layer, the mass percentage of each component is as follows:

1-6 wt% of second conductive agent, 1-4 wt% of second adhesive and 90-98 wt% of second positive active material.

9. The positive electrode sheet according to any one of claims 1 to 8, wherein the thickness of the first positive electrode active material layer is 2 to 10 μm.

10. A lithium ion battery comprising the positive electrode sheet of any one of claims 1 to 9.

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