CN118588881A - A method for preparing a cathode sheet, a cathode sheet, an electrochemical device and an electronic device - Google Patents

Abstract

The present invention discloses a method for preparing a pole piece, a positive pole piece, an electrochemical device and an electronic device. The pole piece preparation method comprises the following steps: applying a first slurry on the surface of a positive current collector to form a first coating, and then applying a second slurry on the first coating to form a second coating; wherein the first slurry comprises a lithium iron phosphate material; the second slurry comprises a lithium supplement material and a lithium iron phosphate material; and the thickness of the second coating accounts for 25-50% of the total thickness of the first coating and the second coating. The pole piece preparation method of the present invention can better exert the performance of the lithium supplement, is not limited by the coating amount, and can be applied to high coating amount design.

Description

A method for preparing a cathode sheet, a cathode sheet, an electrochemical device and an electronic device

Technical Field

The invention relates to a method for preparing a pole piece, a positive pole piece, an electrochemical device and an electronic device.

Background Art

The cruising range is one of the important parameters of new energy vehicles. Improving the cruising range mainly depends on the improvement of the energy density of the battery cell. The increase in the gram capacity of the positive and negative electrode materials can fundamentally improve the energy density of the battery cell.

The widely studied lithium supplement Li ₅ FeO ₄ (LFO) can release 4 equivalent lithium ions, and has a capacity of up to 726mAh/g, with an initial efficiency of <10%. It is an ideal lithium supplement additive, and reacts at 3.4-4V and around 4V. However, the lithium supplement product LiFeO ₂ of LFO has a stable structure and is difficult to decompose, and has poor electronic conductivity, which leads to an additional increase in polarization, thus affecting the release of lithium ions in LFO. In addition, as the amount of positive electrode coating increases, the increase in polarization will also lead to insufficient reaction of LFO, thus affecting the lithium supplement effect, and then affecting the cycle performance of the battery.

Summary of the invention

The technical problem to be solved by the present invention is to overcome the defect that the lithium replenishment technology of the battery in the prior art brings additional polarization, resulting in poor lithium replenishment effect, and provide a pole piece preparation method, a positive pole piece, an electrochemical device and an electronic device.

The present invention solves the above technical problems through the following technical solutions:

In a first aspect, the present invention provides a method for preparing a pole piece, which comprises the following steps: applying a first slurry on the surface of a positive electrode current collector to form a first coating, and then applying a second slurry on the first coating to form a second coating; wherein the first slurry comprises a lithium iron phosphate material; the second slurry comprises a lithium supplement material and a lithium iron phosphate material; and the thickness of the second coating accounts for 25-50% of the total thickness of the first coating and the second coating.

In a second aspect, the present invention also provides a positive electrode sheet, which includes a positive electrode current collector, a first coating and a second coating, wherein the first coating is located between the positive electrode current collector and the second coating, and the first coating includes a lithium iron phosphate material; the second coating includes: a lithium supplement material and a lithium iron phosphate material; the thickness of the second coating accounts for 25-50% of the total thickness of the first coating and the second coating.

In a third aspect, the present invention further provides an electrochemical device, which comprises the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte as described above.

In a fourth aspect, the present invention further provides an electronic device comprising the electrochemical device as described above.

The positive and progressive effects of the present invention are:

The present invention mixes and coats the lithium replenisher material (such as LFO) and the lithium iron phosphate material (LFP) on the upper layer of the positive electrode sheet, and the thickness of the mixed layer of the upper lithium iron phosphate material (LFP) and the lithium replenisher material (such as LFO) accounts for 25%-50% of the total coating thickness, and the lithium replenisher material is only distributed in the range of 50-75% away from the current collector. Compared with the single-layer coating of the lithium iron phosphate material (LFP) and the lithium replenisher material (such as LFO), on the one hand, the lithium replenisher material (such as LFO) in the double-layer design can avoid the insufficient reaction of the lithium replenisher material (such as LFO) close to the current collector due to the increase in polarization during the first charging process, thereby better exerting the performance of the lithium replenisher. On the other hand, the lithium replenishment effect of the lithium replenisher material (such as LFO) is not limited by the coating amount, and can be applied to the high coating amount design.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of an electrode obtained by the electrode lithium replenishment method according to Example 1 of the present invention (A: second coating layer; B: first coating layer).

FIG2 is a graph showing the capacity retention rates at different rates measured at 25° C. when the electrodes of Comparative Examples 1, 2 and Example 1 are used in batteries.

DETAILED DESCRIPTION

The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The experimental methods in the following examples without specifying specific conditions are carried out according to conventional methods and conditions, or selected according to the product specifications.

In the method for preparing the pole piece described in the first aspect of the present invention:

In some preferred embodiments of the present invention, the thickness of the second coating layer accounts for 30-40% of the total thickness of the first coating layer and the second coating layer.

Preferably, the surface density of the first coating layer is 0.095-0.37 g/1540.25 mm ² , for example 0.3 g/1540.25 mm ² .

Preferably, the surface density of the second coating layer is 0.08-0.225 g/1540.25 mm ² , for example 0.2 g/1540.25 mm ² .

Preferably, the total surface density of the first coating layer and the second coating layer is 0.32-0.45 g/1540.25 mm ² , for example 0.3 g/1540.25 mm ² .

The surface density refers to (the mass of the electrode current collector after coating the slurry and drying - the mass of the electrode current collector before coating) / the coating area of the electrode current collector.

Preferably, the lithium supplement material is LFO, and the LFO refers to Li ₅ FeO ₄ .

The lithium iron phosphate material is abbreviated as LFP, and the LFP refers to lithium iron phosphate (LiFePO ₄ ).

When preparing the first slurry and the second slurry, preferably, the median particle size of the added lithium iron phosphate material is 0.8-1.1 μm, and the particle size range of the primary particles of the lithium iron phosphate material is 0.1-0.4 μm.

When preparing the second slurry, preferably, the median particle size of the added lithium supplement agent is 5-8 μm.

Preferably, in the first coating layer formed after the first slurry is dried, the mass content of the lithium iron phosphate material in the first coating layer is 92%-97%, for example, 95%.

Preferably, in the second coating layer formed after the second slurry is dried, the mass content of the lithium iron phosphate material in the second coating layer is 72%-95.5%, for example, 94%.

Preferably, in the second coating layer formed after the second slurry is dried, the mass content of the lithium supplement material in the second coating layer is 2%-20%.

Preferably, the first slurry does not contain a lithium supplement material, such as LFO.

Preferably, the first slurry and the second slurry respectively further include one or more of a positive electrode conductor, a positive electrode binder and a positive electrode dispersant.

Preferably, the positive electrode conductive agent is selected from one or more of SP, acetylene black, nano metal powder, carbon nanotube and graphene.

Preferably, the positive electrode binder is selected from one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, sodium carboxymethyl cellulose and styrene-butadiene rubber.

The positive electrode dispersant may be a conventional dispersant in the art.

The content of the positive electrode conductor is preferably 1%-3%, for example 2%, wherein % refers to the mass percentage of the first coating formed after the first slurry is dried or the second coating formed after the second slurry is dried.

The content of the positive electrode binder is preferably 1.5%-4%, for example 2%, wherein % refers to the mass percentage of the first coating layer formed after the first slurry is dried or the second coating layer formed after the second slurry is dried.

The content of the positive electrode dispersant is preferably 0.01-1%, where % refers to the mass percentage of the first coating layer formed after the first slurry is dried or the second coating layer formed after the second slurry is dried.

More preferably, the first slurry comprises: lithium iron phosphate: 92%-97%; positive electrode conductor: 1%-3%, positive electrode binder: 1.5%-4%; positive electrode dispersant: 0.01-1%, where % refers to the percentage of the mass of each component to the mass of the first coating formed after the first slurry is dried.

For example, the first slurry includes: lithium iron phosphate positive electrode main material: 95%; positive electrode conductor: 2%, positive electrode binder: 2%; positive electrode dispersant: 1%, % refers to the percentage of the mass of each component to the mass of the first coating formed after the first slurry is dried.

More preferably, the second slurry includes: lithium iron phosphate: 72%-95.5%; lithium supplement LFO: 2%-20%; positive electrode conductor: 1%-3%, positive electrode binder: 1.5%-4%; positive electrode dispersant: 0.01-1%, % refers to the percentage of the mass of each component to the mass of the second coating formed after the second slurry is dried.

For example, the second slurry includes: lithium iron phosphate positive electrode main material: 94%; lithium supplement LFO: 2%; positive electrode conductor: 1%, positive electrode binder: 2%; positive electrode dispersant: 1%, % refers to the percentage of the mass of each component to the mass of the second coating formed after the second slurry is dried.

In the present invention, the first slurry includes the components of the first coating layer and a solvent. The second slurry includes the components of the second coating layer and a solvent.

As mentioned above, the mass of the first coating formed after the first slurry is dried = the mass of the first slurry - the mass of the solvent. The mass of the second coating formed after the second slurry is dried = the mass of the second slurry - the mass of the solvent.

The solvent may be a conventional solvent in the art, such as N-methylpyrrolidone or deionized water.

Preferably, the solid content of the first slurry and the second slurry is independently 45%-70%. The solid content can be adjusted by adding the solvent to the slurry according to the conventional method in the art.

In the present invention, the first coating layer is formed by drying the first slurry, and the second coating layer is formed by drying the second slurry.

In the positive electrode sheet described in the second aspect of the present invention:

In the present invention, preferably, the surface density of the first coating layer is 0.095-0.37 g/1540.25 mm ² .

In the present invention, preferably, the surface density of the second coating layer is 0.08-0.225 g/1540.25 mm ² .

In the present invention, preferably, the total coating area density of the first coating layer and the second coating layer is 0.32-0.45 g/1540.25 mm ² .

In the present invention, the median particle sizes of the lithium supplement material and the lithium iron phosphate material are as described above.

In the present invention, the first coating layer is formed by drying the first slurry as described above, and the second coating layer is formed by drying the second slurry as described above.

In the electrochemical device described in the third aspect of the present invention:

In the present invention, the negative electrode sheet can be selected from one or a mixture of two of a graphite negative electrode, a silicon-carbon negative electrode, and a silicon-oxygen negative electrode.

In the present invention, the negative electrode sheet can be a conventional negative electrode in the art. Preferably, the negative electrode includes a negative electrode current collector and a negative electrode coating located on the negative electrode current collector. The components of the negative electrode coating include: negative electrode main material: 93%-98%; negative electrode conductive agent: 0.2%-1.5%, negative electrode binder: 1%-3%; negative electrode dispersant: 0.01-1.5%, and % refers to the percentage of the mass of each component to the mass of the negative electrode coating.

Preferably, the negative electrode conductive agent is selected from one or more of SP, acetylene black, nano silver powder, carbon nanotubes and graphene.

Preferably, the negative electrode binder is selected from one or more of vinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, SBR and styrene-butadiene rubber.

The surface density of the negative electrode coating on the negative electrode current collector is preferably 0.134-0.207 g/1540.25 mm ² .

The positive electrode sheet and the negative electrode sheet may also be roll-pressed according to the conventional art.

Preferably, after the rolling, the thickness of a single side of the positive electrode sheet is 79-131 μm, and the compaction density is 2.2-2.6 g/cm ³ , and the thickness of a single side of the negative electrode sheet is 48-93 μm, and the compaction density is 1.45-1.8 g/cm ³ .

Preferably, the electronic device is a lithium battery, and the N/P (ie, N/P=negative electrode capacity per unit area/positive electrode capacity per unit area) of the lithium battery is preferably 1.02-1.12.

On the basis of being in accordance with the common sense in the art, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present invention.

The preparation method of the slurry is as follows:

(1) Preparation of lower positive electrode slurry:

The formula of the lower positive electrode slurry is: lithium iron phosphate positive electrode main material: 95%; positive electrode conductive agent SP: 2%, positive electrode binder PVDF (polyvinylidene fluoride): 2% and positive electrode dispersant 1%, where % refers to the mass percentage of each component in the mass of the lower positive electrode slurry formula. When homogenizing, first mix the lower layer formula materials in a blender, then add N-methylpyrrolidone according to a solid content of 45%-70% and stir and mix to obtain the lower positive electrode slurry;

(2) The upper positive electrode slurry formula: lithium iron phosphate positive electrode main material: 94%; lithium supplement LFO: 2%; positive electrode conductive agent SP: 1%, positive electrode binder PVDF (polyvinylidene fluoride): 2% and positive electrode dispersant 1%, where % refers to the mass percentage of each component in the mass of the upper positive electrode slurry formula. When homogenizing, first mix the upper layer formula materials in a blender, then add N-methylpyrrolidone at a solid content of 45%-70% and stir and mix to obtain the upper positive electrode slurry.

Example 1

1. Method of positive electrode lithium supplement coating:

Double-layer coating: The lower positive electrode slurry (containing LFP) is uniformly coated on the positive electrode current collector at a surface density of 0.3g/ ^1540.25mm2 to obtain a first coating layer; the upper positive electrode slurry (containing a mixed layer of LFP and LFO) is coated on the upper layer of the first coating layer at a surface density of 0.2g/ ^1540.25mm2 to obtain a second coating layer. The total positive electrode coating surface density is 0.3g/ ^1540.25mm2 . The thickness of the second coating layer (i.e., the mixed layer containing LFP and LFO) accounts for 40% of the total coating thickness, so that LFO is only distributed within a range of 50-75% away from the current collector.

FIG1 is a schematic diagram of the structure of an electrode obtained by the electrode lithium replenishment method according to Example 1 of the present invention (A: second coating layer; B: first coating layer).

2. Preparation of battery cells:

The preparation of the battery cell is completed according to the following process.

Preparation of negative electrode sheet: negative electrode slurry: negative electrode main material: 95%; negative electrode conductive agent carbon nanotube: 1.5%, negative electrode binder styrene butadiene rubber: 2% and negative electrode dispersant 1.5%, fully stir in a mixer, then add N-methyl pyrrolidone or deionized water according to 45%-70% solid content and stir and mix to prepare negative electrode slurry. The negative electrode slurry is evenly coated on the negative electrode current collector at a surface density of 2g/ ^1540.25mm2 , and a negative electrode sheet of a specified size is prepared by a coating dryer.

Rolling process: The positive electrode sheet in Example 1 and the prepared negative electrode sheet were subjected to a rolling process. After rolling, the thickness of the positive electrode on one side was 131 μm, and the density was between 2.2 g/cm ^3. The thickness of the negative electrode on one side was 89 μm, and the density was between 1.45 g/cm ³ .

The subsequent processes are slitting, cutting, stacking or winding to orderly make the positive electrode, negative electrode and separator into battery cells.

3. Preparation of batteries:

The bare battery cell JR is put into the shell and fully baked to reduce the water content to below 450ppm. The processes of liquid injection, formation, sealing and inspection are completed to prepare square hard shell, soft pack and cylindrical batteries.

Comparative Example 1

LFP and LFO were mixed and coated in a single layer, with a coating surface density of 0.305 g/1540.25 mm ² , for comparison with other examples. Other conditions (such as the LFP and LFO mixed slurry formula, total coating thickness) were the same as in Example 1.

Comparative Example 2

LFP and LFO were mixed and coated in a single layer, with a coating surface density of 0.38 g/1540.25 mm ² , for comparison with other examples. Other conditions (such as the LFP and LFO mixed slurry formula, total coating thickness) were the same as in Example 1.

After testing, the capacity retention rates of the electrodes of Comparative Examples 1 and 2 and Example 1 at different rates at 25°C for batteries are shown in Figure 2. The test method is as follows: After charging to 3.65C at 0.3C constant current and constant voltage, the batteries are discharged to 2.5V at different rates of 0.2C/0.3C/0.5C/1C.

The comparison results show that compared with Comparative Example 1, since the embodiment uses a double-layer coating process, LFO and LFP are mixed and coated on the upper layer, the insufficient reaction of LFO near the current collector due to the increase in polarization is avoided, and the polarization is improved; compared with Comparative Example 2, the double-layer coating process of the embodiment is suitable for a high coating amount scheme, and the polarization improvement effect on the high coating amount is more obvious, and the polarization is improved. The battery prepared by the electrode of Example 1 has good cycle performance, energy density and high safety.

Although the specific embodiments of the present invention are described above, it should be understood by those skilled in the art that this is only for illustration and the protection scope of the present invention is defined by the appended claims. Those skilled in the art may make various changes or modifications to these embodiments without departing from the principles and essence of the present invention, but these changes and modifications all fall within the protection scope of the present invention.

Claims (10)

1. The preparation method of the pole piece is characterized by comprising the following steps: coating a first slurry on the surface of a positive electrode current collector to form a first coating, and then coating a second slurry on the first coating to form a second coating; wherein the first slurry comprises a lithium iron phosphate material; the second slurry comprises a lithium-replenishing agent material and a lithium iron phosphate material; the thickness of the second coating layer is 25-50% of the total thickness of the first coating layer and the second coating layer.

2. A method of manufacturing a pole piece as claimed in claim 1, wherein,

The surface density of the first coating is 0.095-0.37g/1540.25mm ²;

And/or the second coating has an areal density of 0.08-0.225g/1540.25mm ².

3. The method for preparing a pole piece according to claim 1, wherein the median particle diameter of the lithium iron phosphate material added in preparing the first slurry and the second slurry is 0.8 to 1.1 μm;

and/or, in preparing the second slurry, the median particle diameter of the lithium supplement material added is 5-8 μm.

4. The method for preparing a pole piece according to claim 1, wherein in a first coating layer formed after the first slurry is dried, the mass content of the lithium iron phosphate material is 92% -97% of the mass content of the first coating layer;

and/or, in the second coating formed after the second slurry is dried, the mass content of the lithium supplementing agent material in the second coating is 2-20%.

5. The method of making a pole piece of claim 1, wherein the first slurry is free of lithium-compensating materials.

6. The method of making a pole piece of claim 1, wherein the first slurry and the second slurry each independently have a solids content of 45% to 70%.

7. A positive electrode sheet comprising a positive electrode current collector, a first coating and a second coating, wherein the first coating is located between the positive electrode current collector and the second coating, and the first coating comprises a lithium iron phosphate material; the second coating comprises a lithium-replenishing agent material and a lithium iron phosphate material; the thickness of the second coating layer is 25-50% of the total thickness of the first coating layer and the second coating layer.

8. The positive electrode sheet of claim 7, wherein the first coating has an areal density of 0.095-0.37g/1540.25mm ²;

And/or the second coating has an areal density of 0.08-0.225g/1540.25mm ².

9. An electrochemical device comprising the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte according to claim 7 or 8.

10. An electronic device comprising the electrochemical apparatus of claim 9.