Composite binder as well as pole piece and battery applying same

Abstract

The invention provides a composite binder, and a pole piece and a battery using the same, the composite binder comprises a first binder, the first binder is a block polymer, and the first binder is prepared through a block reaction of polyvinylidene fluoride and a conjugated polymer; the conjugated polymer comprises at least one of polyacetylene and polythiophene; wherein the mass content of polyvinylidene fluoride used for preparing the first binder is 50-90%. The conjugated polymer and PVDF are subjected to a block reaction to obtain the first binder, and the first binder has good binding performance, ionic conductivity and electronic conductivity.

Description

Technical Field

[0001] The present invention belongs to the field of batteries, and particularly relates to a composite binder, a pole piece using the same, and a battery. Background Art

[0002] As an important component in lithium-ion batteries, the binder accounts for about 1% - 10% in the pole piece, but it plays a crucial role. For example, its function is to bond between the electrode active material, the conductive agent, and the current collector, so that the electrode active materials have overall connectivity while maintaining a certain porosity, and at the same time firmly fix the electrode material on the current collector to prevent the active material particles from swelling and falling off during the charge and discharge processes of the battery, and reduce the impedance between the current collector and the electrode material. Since the binder is in a very special environment for a long time, it must be able to resist the influence of various external factors. In the electrode, the performance requirements that an excellent binder needs to meet include: (1) having sufficient bonding strength to bond and hold the active material, while enhancing the contact between the electrode active material and the conductive agent and bonding the active material to the current collector to better stabilize the structure of the pole piece; (2) having good electrochemical stability, the binder for the positive electrode material is not oxidized under high voltage conditions, and the binder for the negative electrode material is not reduced under low voltage conditions.

[0003] As a commonly used binder for the positive electrode, polyvinylidene fluoride (PVDF) can firmly bond the active material and the conductive agent on the current collector to prevent the shedding of the active material during the cycling process. In addition, PVDF can also absorb the electrolyte, increase the wettability of the electrode, and improve the ion transport rate. However, PVDF is an electronic insulator and can only provide bonding force and ionic conductivity. If the content of PVDF in the formula is too high, it will not only affect the capacity of the main material but also reduce the overall conductivity of the electrode and affect the high-rate discharge performance; if the content of PVDF is too low, it will lead to insufficient bonding force between the electrode material and the current collector, which may cause the peeling between the active material layer and the current collector during the charge and discharge process, thereby affecting the cycle stability and capacity retention rate of the battery. Summary of the Invention

[0004] In order to improve the conductive effect of the electrode, the present invention provides a composite binder, a pole piece using the same, and a battery.

[0005] According to one aspect of the present application, a composite binder is provided. The composite binder includes a first binder, the first binder is a block polymer, and the first binder is prepared by a block reaction of polyvinylidene fluoride and a conjugated polymer; the conjugated polymer includes at least one of polyacetylene and polythiophene; wherein, the mass content of polyvinylidene fluoride used in preparing the first binder is 50-90%. In the present invention, the first binder is obtained by a block reaction of a conjugated polymer and PVDF, and the first binder has good adhesion performance, ionic conductivity and electronic conductivity.

[0006] Preferably, the raw materials for preparing the first binder further include an initiator, and the initiator includes at least one of sodium dodecyl sulfate, cetyl alcohol, benzoyl peroxide, and N,N-dimethylaniline.

[0007] Preferably, the mass content of the initiator used in preparing the first binder is 0.1-2%.

[0008] Preferably, the conjugated polymer includes polyacetylene and polythiophene. Calculated by mass percentage, polyacetylene: polythiophene = 4-8: 2-6. Further, by combining multiple conjugated polymers, the electronic conductivity of the first binder can be further enhanced. This is because of the introduced conjugated π-electron groups, whose adjacent double bonds are alternately linked by single bonds. By controlling the arrangement of the molecular chains and the types of blocks, the movement of electrons on the molecular chains can be further regulated.

[0009] Preferably, the method for preparing the first binder includes the following steps: mixing polyvinylidene fluoride, conjugated polymer, initiator and solvent, and reacting at 30-45 °C for 60-360 minutes to obtain the first binder.

[0010] Preferably, the solvent includes N-methylpyrrolidone.

[0011] Preferably, the composite binder further includes a second binder, and the second binder is prepared by physically mixing raw materials; wherein, the raw materials for preparing the second binder include polyvinylidene fluoride and a conjugated polymer; in the second binder, the mass content of the conjugated polymer is not higher than 50%.

[0012] Preferably, calculated by mass ratio, in the composite binder, second binder: first binder

[0013] = 5-9: 1-5. Further, by combining the second binder, the compatibility, viscosity and adhesion performance of the first binder in the positive electrode slurry can be improved.

[0014] According to the second aspect of the present application, a pole piece is provided, and the pole piece includes the composite binder as described above.

[0015] Preferably, in the electrode, the mass content of the composite binder is 0.8% to 6%.

[0016] According to the third aspect of the present application, a battery is provided, and the battery includes the electrode as described above. Description of the Drawings

[0017] Figure 1 It is the reaction equation involved in the first binder provided in Embodiment 1 of the present application. Detailed Embodiments

[0018] In order to enable those skilled in the art to better understand the technical solutions in the present application, the technical solutions in the present application will be clearly and completely described below in conjunction with the embodiments of the present application and the accompanying drawings in the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0019] Embodiment 1

[0020] 1. Raw materials of the first binder

[0021] The raw materials for preparing the first binder are shown in Table 1.

[0022] Table 1. Raw materials for preparing the first binder

[0023] Component Material Mass percentage / PVDF 49％ Conjugated polymer Polyacetylene 49％ Initiator Sodium dodecyl sulfate 2％

[0024] 2. Method for preparing the first binder

[0025] According to the ratio in Table 1, PVDF, conjugated polymer, and initiator are mixed to obtain a slurry, and then the viscosity of the slurry is adjusted to 12000 mPa·s by adding a solvent (NMP), and the first binder is obtained after reacting at 35°C for 120 minutes.

[0026] 3. Application of the first binder

[0027] In this embodiment, the composite binder used is the first binder.

[0028] According to the mass ratio of positive electrode active material (ternary material LiNi 0.9 Mn 0.05 Co 0.05 O2): conductive agent (carbon black): composite binder = 96:2:2, the raw materials are weighed, and after adding NMP and dispersing and stirring for 120 minutes, a positive electrode slurry is obtained. The positive electrode slurry is coated on the surface of the aluminum foil, and after processes such as coating and cold pressing in sequence, a positive electrode sheet is prepared.

[0029] Weigh the raw materials according to the mass ratio of negative electrode active material (natural graphite): conductive agent (carbon black): binder (SBR) = 97:1:2, and add pure water for dispersion and stirring for 120 minutes to obtain the negative electrode slurry. Coat the negative electrode slurry on the surface of the copper foil, and then perform processes such as coating and cold pressing to prepare the negative electrode sheet. Subsequently, the negative electrode sheet, positive electrode sheet and separator are made into a 18650 lithium battery.

[0030] Example 2

[0031] In this example, the first binder, composite binder, positive electrode and battery are prepared with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder in this example, the mass content of polyacetylene used is 25% (achieved by increasing or decreasing the content of PVDF). Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode and battery in this example are strictly the same as those in Example 1.

[0032] Example 3

[0033] In this example, the first binder, composite binder, positive electrode and battery are prepared with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder in this example, the mass content of polyacetylene used is 10% (achieved by increasing or decreasing the content of PVDF). Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode and battery in this example are strictly the same as those in Example 1.

[0034] Example 4

[0035] In this example, the first binder, composite binder, positive electrode and battery are prepared with reference to the formula and method provided in Example 2. The difference from Example 2 is that when preparing the first binder in this example, polyacetylene is replaced by poly(thiophene) in equal mass portions. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode and battery in this example are strictly the same as those in Example 2.

[0036] Example 5

[0037] In this example, the first binder, composite binder, positive electrode and battery are prepared with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder in this example, cetyl alcohol is used to replace sodium dodecyl sulfate in equal mass portions. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode and battery in this example are strictly the same as those in Example 1.

[0038] Example 6

[0039] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder in this example, the dosage of the initiator is 1% (achieved by increasing or decreasing the content of PVDF). Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 1.

[0040] Example 7

[0041] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder in this example, the conjugated polymers used are polyacetylene and polythiophene (keeping the mass percentage of the conjugated polymers in the formula unchanged). Calculated by mass ratio, polyacetylene: polythiophene = 5:5. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 1.

[0042] Example 8

[0043] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder in this example, the conjugated polymers used are polyacetylene and polythiophene (keeping the mass percentage of the conjugated polymers in the formula unchanged). Calculated by mass ratio, polyacetylene: polythiophene = 8:2. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 1.

[0044] Example 9

[0045] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder in this example, the conjugated polymers used are polyacetylene and polythiophene (keeping the mass percentage of the conjugated polymers in the formula unchanged). Calculated by mass ratio, polyacetylene: polythiophene = 4:6. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 1.

[0046] Example 10

[0047] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the composite binder in this example, a second binder is further included. Calculated by mass ratio, the second binder: the first binder = 5:5. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 1. Specifically, the steps for preparing the second binder in this example are: mixing 50% by mass of PVDF and 50% by mass of polyacetylene, then adding an appropriate amount of NMP, and dispersing for 120 minutes with a dispersion disk to obtain the second binder.

[0048] Example 11

[0049] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 10. The difference from Example 10 is that when preparing the second binder in this example, the content of PVDF is 30%. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 10.

[0050] Example 12

[0051] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 10. The difference from Example 10 is that when preparing the composite binder in this example, a second binder is further included. Calculated by mass ratio, the second binder: the first binder = 9:1. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 10.

[0052] Example 13

[0053] This example prepares the first binder, composite binder, positive electrode, and battery with reference to the formula and method provided in Example 10. The difference from Example 10 is that when preparing the composite binder in this example, a second binder is further included. Calculated by mass ratio, the second binder: the first binder = 3:7. Except for the above differences, the operating steps for preparing the first binder, composite binder, positive electrode, and battery in this example are strictly the same as those in Example 10.

[0054] Comparative Example 1

[0055] This comparative example prepares the positive electrode and battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the positive electrode in this comparative example, PVDF with an equal mass fraction is used to replace the composite binder. Except for the above differences, the operating steps for preparing the positive electrode and battery in this comparative example are strictly the same as those in Example 1.

[0056] Comparative Example 2

[0057] This comparative example prepared the second binder, the positive electrode, and the battery with reference to the formula and method provided in Example 10. The difference from Example 10 is that when preparing the positive electrode, the composite binder was replaced with the second binder in equal parts by mass. Except for the above difference, the operating steps for preparing the second binder, the positive electrode, and the battery in this comparative example were strictly the same as those in Example 10.

[0058] Comparative Example 3

[0059] This comparative example prepared the first binder, the composite binder, the positive electrode, and the battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder, the content of PVDF used was 40% (achieved by increasing or decreasing the content of PVDF). Except for the above difference, the operating steps for preparing the first binder, the composite binder, the positive electrode, and the battery in this comparative example were strictly the same as those in Example 1.

[0060] Comparative Example 4

[0061] This comparative example prepared the first binder, the composite binder, the positive electrode, and the battery with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the first binder, the content of PVDF used was 95% (achieved by increasing or decreasing the content of the conjugated polymer). Except for the above difference, the operating steps for preparing the first binder, the composite binder, the positive electrode, and the battery in this comparative example were strictly the same as those in Example 1.

[0062] Test Example

[0063] 1. Test Objects

[0064] The electrode sheets and batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 4.

[0065] 2. Test Methods

[0066] (1) Electrode Sheet Peel Strength: A 3 cm × 15 cm tape was closely adhered to the electrode sheet, and the tape and the electrode sheet were stretched at a constant speed under a constant tensile force on a tensile testing machine, and the force required to peel the electrode sheet powder was recorded.

[0067] (2) Electrode Sheet Resistance: The electrode sheet was made into a symmetrical battery, and its liquid-phase resistance was measured.

[0068] (3) DCR: The voltage and current per second of the battery cell were recorded while it was placed on the charging cabinet. The battery cell was left standing for 4 h and then discharged at 1C for 10 s. The voltage and current at the last second of standing and 1C discharge were taken. Among them, DCR was calculated according to the following formula: DCR = (V2 - V1) / (I2 - I1).

[0069] (4) Discharge ratio: The battery cell is charged at a constant current and constant voltage of 0.5C to 4.2V in a discharge cabinet, and then discharged at a constant current of 0.2C and 1C to 2.5V respectively. Among them, the discharge ratio is calculated according to the following formula: Discharge ratio = 1C capacity / 0.2C capacity.

[0070] 3. Test results and analysis

[0071] It can be seen from Examples 1 to 4 that introducing different conjugated polymers into the first binder and adjusting their contents will affect the electronic conductivity and bonding performance of the first binder.

[0072] Furthermore, it can be seen from Examples 5 and 6 that in the copolymerization reaction, the initiator used will affect the polymerization effect, and thus affect the conductive performance and bonding performance of the first binder. And, by comparing the data of Comparative Examples 1 to 2, it can be known that compared with pure PVDF and simply mixing polyacetylene and PVDF as the positive electrode binder, by polymerizing polyacetylene and PVDF in the present invention, the electronic conductive effect of the first binder can be significantly enhanced. Combining Comparative Examples 3 to 4, it can be known that the content of the conjugated polymer in the first binder will also affect the electronic conductive performance.

[0073] Furthermore, it can be seen from Example 1 and Examples 7 to 9 that by combining polythiophene and polyacetylene and adjusting their ratios, the electronic conductive ability can be further enhanced. This may be because all the conjugated π-electron groups in polythiophene and polyacetylene are alternately linked by single bonds between adjacent double bonds. By controlling the mass ratio of the two, the arrangement of the block polymer molecular chain and the type of blocks can be regulated, and thus the movement of electrons on the molecular chain can be regulated.

[0074] In Example 10, by combining the second binder, not only can the bonding performance of the composite binder be enhanced, but also on the premise of maintaining a certain level of electronic conductive effect, the usage amount of the first binder can be reduced, thereby reducing the cost of the positive electrode binder. Furthermore, it can be seen from Example 11 that if the content of PVDF in the second binder is too low, its bonding performance will decline. Combining Examples 12 to 13, it can be known that the ratio of the first binder to the second binder will also affect the bonding performance and electronic conductive performance of the composite binder.

[0075] Table 2. Test results of this test example

[0076] Group Peeling force / N Pole piece resistance / Ω DCR / mΩ Discharge ratio / % Example 1 0.70 0.13 24.5 98.6 Example 2 0.52 0.07 16.4 102.6 Example 3 0.46 0.11 23.6 99.2 Example 4 0.46 0.08 16.9 102.1 Example 5 0.64 0.15 24.8 98.1 Example 6 0.62 0.18 25.2 97.6 Example 7 0.73 0.08 16.9 102.1 Example 8 0.75 0.06 16.1 103.3 Example 9 0.71 0.10 23.2 99.8 Example 10 0.78 0.14 24.7 98.3 Example 11 0.65 0.15 24.8 98.1 Example 12 0.74 0.14 24.7 98.3 Example 13 0.75 0.16 25.0 98.0 Comparative example 1 0.80 0.34 26.2 96.1 Comparative example 2 0.65 0.21 26.3 96.8 Comparative example 3 0.60 0.20 25.8 97.2 Comparative example 4 0.82 0.28 27.4 94.7

[0077] The above embodiments are only used to illustrate the technical solutions of the present application rather than to limit the protection scope of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A composite adhesive, characterized in that: The composite binder includes a first binder, the first binder is a block polymer, and the first binder is prepared by block reaction of polyvinylidene fluoride and a conjugated polymer; the conjugated polymer includes at least one of polyacetylene and polythiophene; Wherein, the mass content of the polyvinylidene fluoride used to prepare the first binder is 50-90%.

2. The composite adhesive according to claim 1, characterized in that: The raw materials for preparing the first binder also include an initiator, and the initiator includes at least one of sodium dodecyl sulfate, hexadecanol, dibenzoyl peroxide, and N,N-dimethylformamide.

3. The composite adhesive according to claim 1, characterized in that: The mass content of the initiator used to prepare the first adhesive is 0.1-2%.

4. The composite adhesive according to claim 1, characterized in that: The conjugated polymer includes the polyacetylene and the polythiophene. Calculated by mass ratio, the polyacetylene: the polythiophene = 4-8: 2-6.

5. The composite adhesive according to claim 1, characterized in that: The method for preparing the first binder comprises the following steps: mixing the polyvinylidene fluoride, the conjugated polymer, the initiator and a solvent, and reacting them at 30-45° C. to obtain the first binder.

6. The composite adhesive according to claim 1, characterized in that: The composite binder also includes a second binder, which is prepared by physically mixing raw materials; Wherein, the raw materials for preparing the second binder include the polyvinylidene fluoride and the conjugated polymer; in the second binder, the mass content of the conjugated polymer is not higher than 50%.

7. The composite adhesive according to claim 6, characterized in that: Calculated by mass ratio, in the composite binder, the second binder: the first binder = 5-9:1-5.

8. A pole piece, characterized in that: The pole piece comprises the composite adhesive as claimed in any one of claims 1 to 7.

9. The pole piece according to claim 8, characterized in that: In the pole piece, the mass content of the composite binder is 0.8% to 6%.

10. A battery, characterized in that: The battery comprises the electrode sheet as claimed in claim 8 or 9.

Images