CN115939383B

CN115939383B - Positive electrode slurry, positive electrode plate and battery

Info

Publication number: CN115939383B
Application number: CN202111114025.0A
Authority: CN
Inventors: 许占; 袁晓涛; 陈培兴; 李风帆; 何科峰
Original assignee: BYD Co Ltd
Current assignee: BYD Co Ltd
Priority date: 2021-09-23
Filing date: 2021-09-23
Publication date: 2024-10-11
Anticipated expiration: 2041-09-23
Also published as: CN115939383A

Abstract

The application provides positive electrode slurry, which comprises a positive electrode active material, a binder and a conductive agent, wherein the binder is one or more selected from polyvinylidene fluoride, polytetrafluoroethylene and polyvinyl alcohol, and the conductive agent is one or more selected from tubular, granular and sheet conductive agents; the mass of the binder satisfies ：2.2k≤m≤2.6k,k＝[100B+m₁×B₁×d₁/(d₁+d₂)+m₂×B₂+m₃×B₃]/1781;B representing the specific surface area of the positive electrode active material, m ₁、B₁、d₁、d₂ representing the mass, specific surface area, outer diameter, inner diameter of the tubular conductive agent, m ₂、B₂ representing the mass and specific surface area of the granular conductive agent, and m ₃、B₃ representing the mass and specific surface area of the sheet conductive agent, based on 100g of the positive electrode active material. The positive plate prepared from the positive plate slurry has good structural stability and high energy density of the battery. The application also provides a positive plate and a battery.

Description

Positive electrode slurry, positive electrode plate and battery

Technical Field

The application relates to the technical field of batteries, in particular to positive electrode slurry, a positive electrode plate and a battery.

Background

Lithium ion batteries are widely used in the fields of portable electronic devices and electric automobiles due to their excellent safety and electrochemical properties. The positive electrode active material of the battery is critical to the performance of the battery, and the binder is used as an important component of the positive electrode of the battery, so that the binding and mixing uniformity of the positive electrode active material on the positive electrode current collector can be ensured. However, the addition amount of the binder in the current battery anode system lacks the rule of the system, so that too much binder addition can cause too low mixing gram capacity of the pole piece, reduce the energy density of the battery, seriously affect the stability of the pole piece structure and are not beneficial to the performance of the battery.

Disclosure of Invention

In view of the above, the application provides a rule for using the binder in the battery anode system to construct an effective anode binding network, thereby being beneficial to exerting high energy density of the battery while ensuring that the anode plate has good structural stability.

Specifically, in a first aspect, the present application provides a positive electrode slurry, the positive electrode slurry comprising a positive electrode active material, a binder, a conductive agent and a solvent, wherein the binder is selected from one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl alcohol (PVA), and the conductive agent is selected from one or more of a tubular conductive agent, a granular conductive agent, and a sheet conductive agent;

based on 100g of the positive electrode active material, the mass m of the binder satisfies the following relationship:

M is more than or equal to 2.2k and less than or equal to 2.6k; wherein the method comprises the steps of ,k＝[100B+m₁×B₁×d₁/(d₁+d₂)+m₂×B₂+m₃×B₃]/1781

Wherein B represents the specific surface area of the positive electrode active material, m ₁、B₁、d₁、d₂ represents the mass, specific surface area, outer diameter, and inner diameter of the tubular conductive agent, m ₂、B₂ represents the mass and specific surface area of the granular conductive agent, m ₃、B₃ represents the mass and specific surface area of the sheet conductive agent, and both B, B ₁、B₂ and B ₃ are in g units of m ²/g,m、m₁、m₂、m₃.

In the above relation, 100B represents the surface area of the positive electrode active material, m ₁×B₁×d₁/(d₁+d₂) represents the surface area of the outer surface of the tubular conductive agent, the binder forms a binding network covering the outer surface of the tubular conductive agent, m ₂×B₂、m₃×B₃ represents the particulate conductive agent, the surface area ,[100B+m₁×B₁×d₁/(d₁+d₂)+m₂×B₂+m₃×B₃] of the sheet-like conductive agent represents the total surface area of the positive electrode active material and the conductive agent, respectively, 1781 is an empirical value obtained by the present inventors through a series of experiments and experiments, which represents the total surface area of the positive electrode active material and the conductive agent corresponding to the optimum amount of the binder obtained based on 100g of the positive electrode active material.

In the above relation, when the amount of m is less than or equal to 2.2k and less than or equal to 2.6k based on 100g of positive electrode active material, the binder can fully cover the surfaces of the positive electrode active material and the conductive agent, fully bond the positive electrode active material and the conductive agent to form an effective bonding network, and maintain the structural stability of the positive electrode material layer formed by the positive electrode slurry, but since the binder is not an electrode active material, the higher the amount of m is, the better the higher the amount of m is, the higher the stripping force of the positive electrode material layer is, but when the amount of m is too high, the mass ratio of the positive electrode active material in the positive electrode slurry is reduced, so that the mixing gram capacity of the positive electrode sheet is reduced, and the energy density of the battery is reduced. Therefore, by controlling the use of the binder to meet the use rules defined by the relational expression, the positive plate containing the positive electrode material layer formed by the positive electrode slurry can have good structural stability, and meanwhile, the mixing gram capacity of the positive plate can not be obviously reduced, and the energy density of the battery can not be obviously reduced.

In an embodiment of the present application, the positive electrode active material may be selected from one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium cobalt oxide, lithium manganate, lithium nickel manganate, nickel Cobalt Manganese (NCM), nickel Cobalt Aluminum (NCA), and the like. Optionally, the B is in the range of 14-19.5m ²/g.

Alternatively, the specific surface area B ₁ of the tubular conductive agent is in the range of 350-740m ²/g.

Alternatively, the specific surface area B ₂ of the particulate conductive agent is in the range of 80-210m ²/g.

Alternatively, the specific surface area B ₃ of the sheet-like conductive agent is in the range of 20-90m ²/g.

In an embodiment of the present application, the tubular conductive agent may be selected from one or more of carbon nanotubes, carbon fibers, and the like. Because of the small diameter of the carbon fiber, its outer diameter d1 and inner diameter d2 can be approximately equal, i.e., d ₁/(d₁+d₂ above) ζ1.

In an embodiment of the present application, the granular conductive agent may be selected from one or more of carbon black, furnace black, graphite powder, etc., but is not limited thereto. Wherein the carbon black is selected from one or more of acetylene black, ketjen black, and provider P conductive carbon black. The graphite may be selected from KS-6, KS-15 large particle graphite powder, and S-O ultra-fine graphite powder.

In an embodiment of the present application, the sheet-shaped conductive agent may be selected from one or more of graphene, flake graphite, and the like, but is not limited thereto.

In some embodiments, the tubular conductive agent is selected from carbon nanotubes. The particulate conductive agent is selected from carbon black. The sheet-like conductive agent is selected from graphene.

In some embodiments of the application, the conductive agent contains one or more of carbon black, graphene, and carbon nanotubes. In some embodiments, the conductive agent is a mixture of carbon nanotubes and carbon black. Alternatively, the mass ratio of carbon nanotubes to carbon black is 1:1. In other embodiments, the conductive agent is a mixture of carbon nanotubes, carbon black, and graphene. The three-dimensional conductive agent can make the conductivity of the positive electrode material layer formed by the positive electrode slurry more excellent. Alternatively, the mass ratio of carbon nanotubes, carbon black, and graphene may be (0.01-10): 1: (0.01-10).

In a second aspect, the present application provides a positive electrode sheet including a positive electrode current collector and a positive electrode material layer disposed on the positive electrode current collector, the positive electrode material layer including a positive electrode active material, a binder and a conductive agent, the binder being selected from one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), the conductive agent being selected from one or more of a tubular conductive agent, a granular conductive agent, a sheet-like conductive agent;

wherein, based on 100g of positive electrode active material, the mass m of the binder satisfies the relation:

2.2k≤m≤2.6k;k＝[100B+m₁×B₁×d₁/(d₁+d₂)+m₂×B₂+m₃×B₃]/1781.

The parameters in the relation are as described in the first aspect of the present application, and are not described here again. The positive electrode material layer may be formed by coating and drying the positive electrode slurry according to the first aspect of the present application.

In an embodiment of the present application, the mass of the conductive agent is generally 0.4% to 2.5% of the mass of the positive electrode active material. Optionally, the conductive agent is typically present in the positive electrode material layer at a mass ratio of 0.4% to 2.4%, and in some embodiments, at a mass ratio of 0.7% to 1.2%.

According to the positive plate provided by the second aspect of the application, the usage amount of the binder in the positive plate material layer is regulated to meet the relation, so that the binder can cover the surfaces of other components in the positive plate material layer to form an effective binding network, the positive plate material layer is stably attached to the positive plate current collector, the positive plate is favorable for maintaining higher structural stability in the battery cycle process, the mixing gram capacity of the positive plate is obviously reduced due to the fact that the usage amount of the binder is not too high, and the exertion of the energy density of the battery is influenced. Therefore, the positive plate can lay a foundation for preparing batteries with high energy density and good cycle performance.

In a third aspect, the present application provides a battery comprising the positive electrode sheet according to the second aspect of the present application. The battery generally includes the positive electrode sheet, the negative electrode sheet, and a separator and an electrolyte between the positive and negative electrode sheets.

The amount of the binder in the positive plate of the battery is proper, so that the battery can achieve good cycle performance and high energy density.

Detailed Description

The embodiments of the present application will be further described below by way of a plurality of examples.

Examples

A method of preparing a positive electrode sheet comprising: mixing an anode active material lithium iron phosphate with a conductive agent (specifically one or more of carbon nanotubes, carbon black and graphene), a binder PVDF and a solvent to obtain an anode slurry; the positive electrode slurry is coated on a positive electrode current collector aluminum foil, and the positive electrode plate is obtained through baking and rolling. Wherein, when PVDF is added, the proper adding amount range of PVDF is calculated according to the relation of the application.

A method of making a battery comprising: providing a negative electrode sheet: mixing a negative electrode graphite material with carbon black, a binder (specifically, a mixture of CMC (sodium carboxymethylcellulose) and Styrene Butadiene Rubber (SBR)) and a solvent to obtain a negative electrode slurry; and coating the negative electrode slurry on a negative electrode current collector copper foil, and baking and rolling to obtain a negative electrode plate.

And assembling the positive electrode plate and the negative electrode plate into a full battery, wherein the design capacity of the full battery is 1.82Ah.

In example 1, the specific surface area b=15.5m ²/g of lithium iron phosphate, the specific surface area B ₁ of the carbon nanotube is 510m ²/g, the ratio d ₁/(d₁+d₂ of the outer diameter to the sum of the inner diameter and the outer diameter is 0.3, the specific surface area B ₂ of the carbon black is 122m ²/g, the mass m ₁ =0.3 g of the carbon nanotube based on 100g of lithium iron phosphate, the addition amount m ₂ =0.7 g of the carbon black, and the calculated addition mass m of PVDF is 2.08-2.45g. In example 1 of the present application, m=2.2 g was controlled.

Example 2 differs from example 1 in that: in the preparation of the positive electrode sheet, PVDF was added with a mass m=2.08 g.

Example 3 differs from example 1 in that: in the preparation of the positive electrode sheet, PVDF was added with a mass m=2.45 g.

Example 4

Example 4 differs from example 1 in that: and only graphene is used as a conductive agent of the positive plate, wherein the specific surface area B ₃ of the graphene is 56m ²/g, and the mass m ₃ =1 g of the graphene is based on 100g of lithium iron phosphate.

The added mass m of PVDF in the positive plate calculated according to the above relation of the present application ranges from 2.07 to 2.34g, with m=2.25 g in example 4 of the present application.

Example 5

Example 5 differs from example 1 in that: only carbon black is used as the conductive agent of the positive plate.

The added amount of carbon black m ₂ =1 g based on 100g of lithium iron phosphate, and the calculated added mass m of PVDF ranges from 2.06 to 2.44g, in example 5 of the present application, m=2.2 g.

Example 6

Example 6 differs from example 1 in that: graphene is also used as a conductive agent of the positive plate. That is, the conductive agent of the positive electrode sheet of example 6 is a mixture of carbon black, carbon nanotubes and graphene; the amounts and parameters of carbon black and carbon nanotubes were the same as in example 1, and those of graphene were the same as in example 4.

The added mass m of PVDF in the positive plate calculated according to the above relation of the present application ranges from 2.14 to 2.54g, with m=2.44 g in example 6 of the present application.

Example 7

In the preparation process of the positive electrode sheet of example 7, the specific surface area b=16.5m ²/g of lithium iron phosphate, the specific surface area B ₁ of the carbon nanotube was 580m ²/g, the ratio d ₁/(d₁+d₂ of the outer diameter to the sum of the inner and outer diameters) =0.32, the specific surface area B ₂ of the carbon black was 112m ²/g, the mass m ₁ =0.5 g of the carbon nanotube based on 100g of lithium iron phosphate, the addition amount m ₂ =0.5 g of the carbon black was calculated to obtain an addition mass m of PVDF in the range of 2.22 to 2.63g, and m=2.5 g was controlled in example 7 of the present application.

In addition, in order to highlight the advantageous effects of the present application, the following comparative examples 1 to 2 are also provided.

Wherein comparative example 1 differs from example 1 in that: in the preparation of the positive plate, PVDF is added with the mass of 1.8g.

Comparative example 2 is different from example 1 in that: in the preparation of the positive plate, the added mass of PVDF is 3g.

In order to support the beneficial effects of the present application, the peel force of the positive plates in each example and comparative example was tested, and the results are summarized in table 1 below; the samples and comparative cells were also tested for gram capacity of compound discharged at 0.33C and capacity retention at 45℃ for 500 weeks at 0.5C/0.5C cycles, and the results are summarized in table 2 below.

The testing method of the positive electrode mixing gram capacity comprises the following steps: the battery is charged and discharged in a voltage range of 2.0-3.8V, wherein the battery is charged to 3.8V at a constant current and a constant voltage of 1/3C during charging, then is charged at a constant voltage, the cut-off current is 0.05C, and is discharged to 2.0V at a constant current of 1/3C during discharging. And carrying out charge-discharge circulation for 3 times, taking the discharge capacity after the third circulation, and dividing the discharge capacity by the mass of the positive electrode material layer on the pole piece to obtain the positive electrode gram capacity of the battery.

The method for testing the cycle performance comprises the following steps: and (3) carrying out 0.5C/0.5C cycle test on the full battery at 45 ℃ with the voltage range of 2.0-3.8V, wherein during charging, 1C is firstly charged to 3.8V by constant current and then charged by constant voltage, the cut-off current is 0.05C, and during discharging, 1C is discharged to 2V. After repeating the above charge and discharge cycle for 500 weeks, the ratio of the discharge capacity after 500 weeks of the battery cycle to the 1 st discharge capacity was taken as the capacity retention rate of 500 weeks of the battery cycle.

Table 1 results of the positive electrode sheet peel force test of each of examples and comparative examples

	Stripping force (N)
		Example 1	0.2502
Example 2	0.2063
		Example 3	0.3046
Example 4	0.2089
		Example 5	0.2611
Example 6	0.2632
		Example 7	0.3409
Comparative example 1	0.1533
		Comparative example 2	0.4503

Table 2 results of performance test of the batteries of each example and comparative example

It can be seen from tables 1 and 2 that the positive electrode sheet had a peel force of 0.2N or more and a good structural stability when the binder addition amount of examples 1 to 3 and comparative example 2 was ∈ 2.08g while maintaining the composition of the positive electrode material layer. However, the binder of comparative example 2 was used in an excessive amount, which resulted in a significant increase in the stripping force of the electrode sheet, but also resulted in a decrease in the mass ratio of the positive electrode active material, resulting in a significant decrease in the mixing gram capacity of the positive electrode sheet in table 2, and eventually in a significant decrease in the battery energy density. Under the same conditions, the adhesive of the comparative example 1 is added in an amount of only 1.8g, and the amount is small, so that an effective bonding network is not formed, and the stripping force of the positive plate is low and the structural stability is poor. In addition, the positive plates in examples 4-7 of the application have larger stripping force, and the battery has excellent mixing gram capacity and cycle performance. The results show that the usage control strategy of the binder provided by the application can be beneficial to considering the higher stability of the positive plate and the higher gram capacity of the battery.

The above examples merely represent a few exemplary embodiments of the present application, which are described in more detail and are not to be construed as limiting the scope of the application. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the application, which are all within the scope of the application. Accordingly, the scope of protection of the present application is to be determined by the appended claims.

Claims

1. The positive electrode slurry is characterized by comprising a positive electrode active material, a binder, a conductive agent and a solvent, wherein the binder is one or more selected from polyvinylidene fluoride, polytetrafluoroethylene and polyvinyl alcohol, and the conductive agent is one or more selected from tubular conductive agents, granular conductive agents and sheet-shaped conductive agents;

2.2k≤m≤2.6k,k＝[100B+m₁×B₁×d₁/(d₁+d₂)+m₂×B₂+m₃×B₃]/1781

2. The positive electrode slurry of claim 1, wherein the tubular conductive agent is selected from carbon nanotubes.

3. The positive electrode slurry according to claim 1, wherein the particulate conductive agent is selected from one or more of carbon black, furnace black, and graphite powder.

4. The positive electrode slurry according to claim 1, wherein the sheet-like conductive agent is selected from one or more of graphene and flake graphite.

5. The positive electrode slurry of claim 1, wherein the conductive agent comprises one or more of carbon black, graphene, and carbon nanotubes.

6. The positive electrode slurry of claim 1, wherein the positive electrode active material is selected from one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium cobalt oxide, lithium manganate, lithium nickel manganate, nickel cobalt manganese, nickel cobalt aluminum.

7. The positive electrode slurry of claim 1, wherein B is in the range of 14-19.5m ²/g.

8. The positive electrode plate is characterized by comprising a positive electrode current collector and a positive electrode material layer arranged on the positive electrode current collector, wherein the positive electrode material layer comprises a positive electrode active material, a binder and a conductive agent, the binder is one or more selected from polyvinylidene fluoride, polytetrafluoroethylene and polyvinyl alcohol, and the conductive agent is one or more selected from tubular conductive agents, granular conductive agents and sheet conductive agents;

2.2k≤m≤2.6k;k＝[100B+m₁×B₁×d₁/(d₁+d₂)+m₂×B₂+m₃×B₃]/1781

9. The positive electrode sheet according to claim 8, wherein the mass of the conductive agent is 0.4% to 2.5% of the mass of the positive electrode active material.

10. A battery comprising the positive electrode sheet according to claim 8 or 9, or a positive electrode sheet made by the positive electrode slurry according to any one of claims 1 to 7.