CN112467107A - High-safety positive plate and lithium ion battery thereof - Google Patents

Abstract

The invention discloses a high-safety positive plate and a lithium ion battery thereof. The active material layer in the pole piece is designed to be double-layer, wherein the first active layer close to the current collector is made of a mixed active material of lithium iron phosphate and lithium phosphate, the theoretical mass capacity of the lithium phosphate is 694mAh/g, and the lithium iron phosphate can be used as a lithium supplement material to be added into lithium iron phosphate, so that the disadvantage of small mass capacity (140mAh/g) of the lithium iron phosphate can be overcome. Meanwhile, lithium phosphate also has good thermal stability, and the lithium iron phosphate and the lithium phosphate are compounded and then have a synergistic effect, so that the needling safety performance of the prepared battery cell can be obviously improved under the condition of keeping the energy density.

Description

High-safety positive plate and lithium ion battery thereof

Technical Field

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a high-safety positive plate and a lithium ion battery thereof.

Background

Lithium ion batteries are widely used in portable electronic products, energy storage devices and new energy vehicles due to their advantages of high energy density, long cycle life, no memory effect, environmental friendliness, etc. With the coming of the 5G era of mobile phones and the development of new energy automobiles with high endurance mileage, the demand on the energy density of lithium ion batteries is higher and higher. However, the higher the energy density of the lithium ion battery is, the worse the safety of the lithium ion battery is, and the spontaneous combustion and explosion accidents of many electric vehicles and hybrid electric vehicles are caused by the safety problem of the power battery, so that the further development of the lithium ion battery in the field of new energy is severely restricted.

In order to improve the technical defects, the safety performance of the battery cell is usually verified by testing means such as overcharge, furnace temperature, needling, external short circuit and extrusion. The needling is a safety test which simulates the safety of the battery core when an internal short circuit occurs and is also a recognized and most difficult test to pass. The needling performance of the battery cell can be improved by coating two or more than two positive active materials (wherein one layer of the positive active material close to the current collector is lithium iron phosphate) on the positive electrode by a multilayer coating method. However, the lithium iron phosphate has low mass capacity, and thus, the capacity or energy density of the cell is significantly reduced. Therefore, it is urgently required to develop a positive electrode active material and a lithium ion battery thereof, which can improve the cell needling performance without reducing the energy density thereof.

Disclosure of Invention

In order to improve the technical problem, the invention provides a high-safety positive plate and a lithium ion battery thereof.

The invention realizes the technical effects through the following technical scheme:

a positive plate comprises a current collector, a first active layer and a second active layer, wherein the first active layer is arranged between the current collector and the second active layer, and is formed on at least one surface of the current collector; the active material of the first active layer includes lithium iron phosphate and lithium phosphate.

According to an embodiment of the present invention, the weight percentage of the lithium iron phosphate in the active material of the first active layer may be 0.01 to 99.99%; preferably 70-99%; exemplary are 0.01%, 70%, 90%, 95%, 99%, 99.99%.

According to an embodiment of the present invention, the particle size D50 of the lithium iron phosphate in the active material of the first active layer is 0.05 to 2.00 μm, preferably 0.1 to 0.3 μm.

According to the embodiment of the invention, the lithium iron phosphate in the active material of the first active layer has a sheet-like morphology.

According to an embodiment of the present invention, the weight percentage of the lithium phosphate in the active material of the first active layer may be 0.01 to 99.99%; preferably 1-30%; exemplary are 0.01%, 1%, 5%, 10%, 30%, 99.99%.

According to an embodiment of the present invention, the particle diameter D50 of the lithium phosphate in the active material of the first active layer is 0.01 to 2.00 μm, preferably 0.05 to 0.2 μm.

According to an embodiment of the present invention, the lithium phosphate in the active material of the first active layer has a spherical morphology.

According to an embodiment of the present invention, the first active layer further contains a conductive agent and/or a binder.

According to the embodiment of the present invention, the first active layer is coated to a thickness of 1 to 20 μm; exemplary are 1 μm, 6 μm, 10 μm, 20 μm.

According to an embodiment of the present invention, the first active layer is formed on both surfaces of the current collector.

According to an embodiment of the invention, the thickness of the second active layer is higher than the thickness of the first active layer.

Preferably, the second active layer is coated to a thickness of 20 μm or more; exemplary are 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm.

According to an embodiment of the present invention, the second active layer is made of at least one second positive electrode active material of lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, a lithium rich manganese based material, lithium iron phosphate; preferably, the lithium-rich manganese-based material is lithium manganate.

According to an embodiment of the present invention, the second active layer further contains a conductive agent and/or a binder.

According to the embodiment of the invention, the mixing mass ratio of the active substance, the adhesive and the conductive agent of the first active layer is (40-98): (1-50): 1-10).

According to the embodiment of the invention, the mixing mass ratio of the second positive electrode active material, the adhesive and the conductive agent is (70-98): (1-10): (1-20).

According to an embodiment of the present invention, the conductive agent is at least one of acetylene black, conductive carbon spheres, conductive graphite, carbon nanotubes, conductive carbon fibers, graphene, and reduced graphene oxide.

According to an embodiment of the invention, the binder is selected from at least one of polyvinylidene fluoride (PVDF), a copolymer of vinylidene fluoride-hexafluoropropylene, a polyamide, polyacrylonitrile, a polyacrylate, a polyacrylic acid, a polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, a polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene and styrene-butadiene rubber.

The invention also provides a lithium ion battery, which comprises the positive plate.

According to an embodiment of the present invention, the lithium ion battery further contains an electrolyte, a separator, and a negative electrode sheet.

Preferably, the active material of the negative electrode sheet includes at least one selected from artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, hard carbon, silicon carbon compounds, and lithium titanate.

The invention also provides a preparation method of the lithium ion battery, which comprises the following steps: and assembling the positive plate, the electrolyte, the negative plate and the shell to prepare the lithium ion battery.

According to an embodiment of the present invention, the method for preparing a lithium ion battery comprises the steps of:

s1, preparation of first active layer slurry:

mixing lithium iron phosphate with lithium phosphate to obtain an active material of a first active layer; then mixing the active substance, the adhesive and the conductive agent of the first active layer, and then adding a solvent for dispersion to obtain first active layer slurry;

s2, preparation of second active layer slurry:

mixing a second positive electrode active material, an adhesive and a conductive agent, and then adding a solvent for dispersion to obtain second active layer slurry;

s3, preparing the positive plate:

coating the first active layer slurry on at least one surface of the current collector, then coating the second active layer slurry on the first active layer, drying, rolling and die-cutting to obtain a positive plate;

s4, preparation of the negative plate:

mixing the active material, the adhesive, the thickening agent and the conductive agent of the negative plate, then adding the solvent for dispersion to obtain negative slurry, then coating the negative slurry on a negative current collector, drying, rolling and die cutting to obtain the negative plate;

s5, preparing a battery cell:

and assembling the positive plate, the negative plate, the electrolyte and the diaphragm into the lithium ion battery.

According to an embodiment of the present invention, the thickener is one or two of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

According to an embodiment of the invention, the electrolyte is a common commercial electrolyte.

The invention has the beneficial effects that:

the theoretical mass capacity of the lithium phosphate used by the invention is up to 694mAh/g, and the lithium phosphate can be used as a lithium supplement material to be added into lithium iron phosphate, so that the disadvantage of small mass capacity (140mAh/g) of the lithium iron phosphate can be compensated. The lithium phosphate and the lithium iron phosphate have small particles (the lithium phosphate D50 is 0.05-0.2 mu m, the lithium iron phosphate D50 is 0.1-0.3 mu m), the lithium iron phosphate is flaky and can be tightly distributed on the current collector, and the lithium phosphate is spherical and is filled in gaps of the flaky lithium iron phosphate, so that the positive current collector can be protected from less contact with a negative active material during needling, internal short circuit current is reduced, and the needling throughput rate is improved (the internal short circuit generated by the positive current collector and the negative electrode is a key factor for causing thermal runaway during needling). Meanwhile, lithium phosphate also has good thermal stability, and the lithium iron phosphate and the lithium phosphate are compounded and then have a synergistic effect, so that the safety performance of the prepared battery cell can be obviously improved under the condition of keeping the energy density.

Drawings

FIG. 1 is a schematic structural diagram of a positive plate according to the present invention;

in the figure: 1. a current collector; 2. a first active layer; 3: a second active layer.

Detailed Description

The technical solution of the present invention will be further described in detail with reference to specific embodiments. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.

Unless otherwise indicated, the raw materials and reagents used in the following examples are all commercially available products or can be prepared by known methods.

Example 1

Referring to fig. 1, a positive electrode sheet includes a current collector 1, a first active layer 2, and a second active layer 3, wherein the first active layer 2 is disposed between the current collector 1 and the second active layer 3, the first active layer 2 is formed on two surfaces of the current collector 1, and the second active layer 3 is thicker than the first active layer 2.

A preparation method of a lithium ion battery comprises the following steps:

s1, preparation of first active layer slurry:

lithium iron phosphate (LFP) and Lithium Phosphate (LPO) are mixed according to the weight percentage of 80%: 20% of active material mixed to form a first active layer; then, the active material, the binder (PVDF) and the conductive carbon black of the first active layer were mixed in a mass ratio of 96 wt%: 2 wt%: 2 wt% of the active layer slurry is uniformly mixed and then dispersed in N-methyl pyrrolidone to obtain uniform first active layer slurry; wherein the particle size D50 of lithium phosphate is 0.1 μm, and the particle size D50 of lithium iron phosphate is 0.2 μm;

s2, preparation of second active layer slurry:

and mixing a second positive electrode active material nickel cobalt lithium manganate (NCM), a binder (PVDF) and conductive carbon black according to the mass ratio of 97 wt%: 2 wt%: 1 wt% of the active layer slurry is uniformly mixed and then dispersed in deionized water to obtain uniform second active layer slurry;

s3 preparation of positive plate

Sequentially coating the prepared first active layer slurry on two surfaces of an aluminum foil to form a first active layer, and drying at 85 ℃; then sequentially coating the second active layer slurry on the two first active layers to form a second active layer, and drying at 85 ℃; after cold pressing, cutting into pieces and die cutting, drying for 8 hours at 85 ℃ under vacuum condition to obtain a positive pole piece P1;

wherein the thickness of the first active layer is 10 μm, and the thickness of the second active layer is 50 μm;

s4 preparation of negative plate

The graphite, the styrene butadiene rubber as a binder, the carboxymethyl cellulose sodium as a thickener and the conductive carbon black as a conductive agent are mixed according to the mass ratio of 95 wt%: 2 wt%: 1.5 wt%: 1.5 wt% of the mixture is uniformly mixed, then the mixture is dispersed in deionized water to obtain negative electrode slurry, the negative electrode slurry is uniformly coated on two surfaces of a copper foil, and the two surfaces are dried for 6 hours at 110 ℃ and compacted by a roller press to obtain a negative electrode sheet N1;

s5, preparing the lithium ion battery:

and assembling the prepared positive plate P1, the negative plate N1 and a diaphragm to prepare a stacked core, packaging the stacked core by adopting an aluminum-plastic film, baking the stacked core for 48 hours in a vacuum state to remove moisture, injecting an electrolyte purchased from New Zezhou corporation (the solvent of the electrolyte is EC: PC: DMC: EMC ═ 2:1:4:3 (volume ratio) and the solute of lithium hexafluorophosphate is 1.0M), and forming and sorting the battery to obtain the square soft package lithium ion battery, wherein the mark is C1.

Example 2

Example 2 differs from example 1 in that: the thickness of the first active layer was 6 μm, and a lithium ion battery C2 was prepared.

Example 3

Example 3 differs from example 1 in that: the thickness of the first active layer was 15 μm, and a lithium ion battery C3 was prepared.

Example 4

Example 4 differs from example 1 in that: the thickness of the first active layer was 20 μm, and a lithium ion battery C4 was prepared.

Example 5

Example 5 differs from example 1 in that: the mass content of lithium phosphate in the active material of the first active layer was 5%, and a lithium ion battery C5 was prepared.

Example 6

Example 6 differs from example 1 in that: the mass content of lithium phosphate in the active material of the first active layer was 10%, and a lithium ion battery C6 was prepared.

Example 7

Example 7 differs from example 1 in that: the mass content of lithium phosphate in the active material of the first active layer was 30%, and a lithium ion battery C7 was prepared.

Example 8

Example 8 differs from example 1 in that: the first active layer slurry and the second active layer slurry were coated on only one surface of the aluminum foil, and a lithium ion battery C8 was prepared.

Example 9

Example 11 differs from example 1 in that: the second positive electrode active material is lithium cobaltate, and the lithium ion battery C9 is prepared.

Example 10

Example 10 differs from example 1 in that: and the second positive electrode active material is lithium manganate, and the lithium ion battery C10 is prepared.

Example 11

Example 11 differs from example 1 in that: the particle diameter D50 of lithium phosphate in the active material of the first active layer was 0.05 μm, and the particle diameter D50 of lithium iron phosphate was 0.1 μm.

Example 12

Example 12 differs from example 1 in that: the particle diameter D50 of lithium phosphate in the active material of the first active layer was 0.2 μm, and the particle diameter D50 of lithium iron phosphate was 0.3 μm.

Example 13

Example 13 differs from example 1 in that: the lithium phosphate and the lithium iron phosphate in the active material of the first active layer are both spherical.

Example 14

Example 14 differs from example 1 in that: the lithium phosphate and the lithium iron phosphate in the active material of the first active layer are both in a sheet form.

Example 15

Example 15 differs from example 1 in that: both lithium phosphate and lithium iron phosphate in the active material of the first active layer are amorphous.

Comparative example 1

The comparative example provides a method of making a lithium ion battery, comprising the steps of:

(1) preparation of positive plate

The preparation method comprises the following steps of (1) mixing nickel cobalt lithium manganate (NCM), a binder (PVDF) and conductive carbon black according to the mass ratio of 97 wt%: 2 wt%: 1 wt% of the active layer slurry is uniformly mixed and then dispersed in N-methyl pyrrolidone to obtain uniform first active layer slurry; sequentially coating the prepared first active layer slurry on two surfaces of an aluminum foil to form a single active layer, and drying at 85 ℃ to prepare a positive pole piece;

wherein the thickness of the active layer is 60 μm;

(2) preparation of negative plate

The graphite, the styrene butadiene rubber as a binder, the carboxymethyl cellulose sodium as a thickener and the conductive carbon black as a conductive agent are mixed according to the mass ratio of 95 wt%: 2 wt%: 1.5 wt%: 1.5 wt% of the mixture is uniformly mixed, then the mixture is dispersed in deionized water to obtain negative electrode slurry, the negative electrode slurry is uniformly coated on two surfaces of a copper foil, and the two surfaces are dried for 6 hours at 90-130 ℃ and compacted by a roller press to obtain a negative electrode sheet;

(3) preparing a lithium ion battery:

and assembling the prepared positive plate, the prepared negative plate and the diaphragm to obtain a laminated core, packaging by adopting an aluminum-plastic film, baking for 48 hours in a vacuum state to remove moisture, injecting a commercially available electrolyte, and forming and sorting the battery to obtain the square soft package lithium ion battery, wherein the D1 is recorded.

Comparative example 2

Comparative example 2 differs from example 1 in that: the active material of the first active layer was only lithium iron phosphate (no lithium phosphate was added), and a lithium ion battery D2 was prepared.

Comparative example 3

Comparative example 3 differs from comparative example 1 in that: the positive electrode slurry is prepared from 97 wt% of lithium cobaltate, a binder (PVDF) and conductive carbon black according to the mass ratio: 2 wt%: 1 wt% of the active layer slurry is uniformly mixed and then dispersed in deionized water to obtain a uniform single active layer slurry coating;

wherein the thickness of the active layer is 60 μm;

the rest was the same as in comparative example 1, and thus a lithium ion battery D3 was produced.

Comparative example 4

Comparative example 4 differs from comparative example 3 in that: the active material of the first active layer is lithium manganate, and the lithium ion battery D4 is prepared.

Test examples

In order to examine the safety performance of the lithium ion batteries prepared in the embodiments 1 to 12 and the comparative examples 1 to 4 of the invention, the safety performance of the battery cell is verified by a needling test method.

The test method of the acupuncture comprises the following steps:

1. and (3) placing the lithium ion battery in a constant temperature box at 25 ℃, and standing for 30 minutes to keep the temperature of the lithium ion battery constant. The lithium ion battery reaching the constant temperature was charged at a constant current of 1C to an upper limit voltage (4.3V), and then charged at a constant voltage of 4.3V to a current of 0.05C. And transferring the fully charged lithium ion battery to a nail penetration testing machine, keeping the testing environment temperature at 25 +/-2 ℃, using a steel nail with the diameter of 5mm to uniformly penetrate through the center of the lithium ion battery at the speed of 25mm/s, keeping for 1 hour, and recording that the lithium ion battery is not fired, not exploded, not smoked and passes. And testing 10 lithium ion batteries each time, wherein the number of the lithium ion batteries passing the needling test is used as an index for evaluating the safety performance of the lithium ion batteries.

2. The energy density test method comprises the following steps:

and (3) placing the lithium ion battery in a constant temperature box at 25 ℃, and standing for 30 minutes to keep the temperature of the lithium ion battery constant. And charging the lithium ion battery reaching the constant temperature to the voltage of 4.3V at a constant current of 1C, then charging the lithium ion battery at a constant voltage of 4.3V to the current of 0.05C, discharging the lithium ion battery at 1C to the voltage of 3.0V, and recording the discharge energy.

Energy density is the discharge energy/(mass of lithium ion battery).

The results of the needling performance tests of comparative examples D1-D4 and examples C1-C10 are shown in Table 1 below.

As can be seen from the results in table 1, the positive electrode first coating layer can improve the needle safety, and the addition of lithium phosphate to the first coating layer can increase the energy density.

Specifically, as can be seen from examples 1 to 4, the thicker the first active coating layer of the positive electrode, the better the needle-punching safety of the battery.

As can be seen from examples 5 to 7, the higher the content of lithium phosphate in the positive electrode first coating layer, the higher the energy density of the battery.

As can be seen from example 8, coating the positive electrode first active layer on only one surface of the aluminum foil can also improve the safety of the battery.

It can be seen from examples 9 to 10 that the lithium iron phosphate and lithium phosphate mixed material as the first positive electrode active coating layer is also suitable for different second positive electrode active material systems (lithium cobaltate and lithium manganate).

As can be seen from examples 11 to 12, the particle sizes of lithium phosphate and lithium iron phosphate in the first coating layer of the positive electrode were too large, and the battery could have a reduced safety against needle punching due to small particle size and a better safety against needle punching.

As can be seen from examples 13 to 15, the morphology of lithium phosphate and lithium iron phosphate in the first coating layer of the positive electrode has an influence on the needle safety, wherein the needle safety is the best when spherical lithium phosphate is matched with sheet-shaped lithium phosphate.

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

Claims (10)

1. A positive plate is characterized by comprising a current collector, a first active layer and a second active layer, wherein the first active layer is arranged between the current collector and the second active layer, and is formed on at least one surface of the current collector; the active material of the first active layer includes lithium iron phosphate and lithium phosphate.

2. The positive electrode sheet according to claim 1, wherein the weight percentage of the lithium iron phosphate in the active material of the first active layer may be 0.01 to 99.99%; preferably 70 to 99%.

And/or the particle size D50 of the lithium iron phosphate in the active material of the first active layer is 0.05-2.00 μm, preferably 0.1-0.3 μm.

And/or the lithium iron phosphate in the active material of the first active layer is in a flaky shape.

3. The positive electrode sheet according to claim 1 or 2, wherein the weight percentage of lithium phosphate in the active material of the first active layer may be 0.01 to 99.99%; preferably 1 to 30%.

And/or the particle diameter D50 of lithium phosphate in the active material of the first active layer is 0.01-2.00 μm, preferably 0.05-0.2 μm.

And/or the lithium phosphate in the active material of the first active layer is in a spherical shape.

And/or, the first active layer further contains a conductive agent and/or a binder.

4. The positive electrode sheet according to any one of claims 1 to 3, wherein the first active layer is coated to a thickness of 1 to 20 μm.

And/or, the first active layer is formed on both surfaces of the current collector.

And/or the thickness of the second active layer is higher than that of the first active layer. Preferably, the second active layer is coated to a thickness of 20 μm or more.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein the second active layer is made of at least one second positive electrode active material selected from the group consisting of lithium cobaltate, lithium nickel cobalt manganese, lithium nickel cobalt aluminate, a lithium rich manganese based material, and lithium iron phosphate; preferably, the lithium-rich manganese-based material is lithium manganate.

And/or, the second active layer further contains a conductive agent and/or a binder.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein the first active layer has a mixing mass ratio of the active material, the binder and the conductive agent of (40 to 98): 1 to 50: (1 to 10).

7. The positive electrode sheet according to any one of claims 1 to 6, wherein the second positive electrode active material, the binder and the conductive agent are mixed at a mass ratio of (70-98): 1-10: (1-20).

8. The positive electrode sheet according to any one of claims 1 to 7, wherein the conductive agent is at least one of acetylene black, conductive carbon spheres, conductive graphite, carbon nanotubes, conductive carbon fibers, graphene, and reduced graphene oxide.

9. The positive electrode sheet according to any one of claims 1 to 8, wherein the binder is at least one selected from the group consisting of polyvinylidene fluoride (PVDF), a copolymer of vinylidene fluoride and hexafluoropropylene, a polyamide, a polyacrylonitrile, a polyacrylate, a polyacrylic acid, a polyacrylate, a sodium carboxymethyl cellulose, a polyvinylpyrrolidone, a polyvinyl ether, a polymethyl methacrylate, a polytetrafluoroethylene, a polyhexafluoropropylene, and a styrene-butadiene rubber.

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

Preferably, the lithium ion battery further comprises an electrolyte, a diaphragm and a negative plate.

Preferably, the active material of the negative electrode sheet includes at least one selected from artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, hard carbon, silicon carbon compounds, and lithium titanate.

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