A positive electrode sheet, its preparation method, and an electrochemical device thereof

Abstract

This invention provides a positive electrode sheet, its preparation method, and an electrochemical device. The positive electrode sheet includes a current collector and an active material layer on at least one surface of the current collector. The active material layer includes a positive electrode active material, a conductive agent, and a binder. The binder includes PVDF. The PVDF has a weight-average molecular weight to intrinsic viscosity cofactor of λ, where λ satisfies: λ = (Mw / 10 6 The formula is: Mw × (IV / 300), 0.7 ≤ λ ≤ 2.1; where Mw is the weight-average molecular weight of the PVDF, in g / mol; and IV is the intrinsic viscosity of the PVDF, in mL / g. The positive electrode sheet of this invention controls the molecular weight and intrinsic viscosity of PVDF to satisfy a specific relationship, which ensures high bonding strength of PVDF while forming a superior three-dimensional network structure in the positive electrode sheet, effectively suppressing positive electrode particle breakage and improving the high-temperature cycle performance of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and relates to a positive electrode sheet, a preparation method thereof and an electrochemical device. BACKGROUND

[0002] Ternary lithium batteries are widely used in the fields of electric vehicles and energy storage systems. However, under high-temperature cycle conditions, the ternary positive electrode material in the ternary lithium battery has the problem of particle breakage, which leads to rapid capacity decay of the ternary lithium battery. In the traditional positive electrode formula, the content of the binder such as PVDF (polyvinylidene fluoride) is usually 1.0-2.0 wt%, and in the prior art, the typical mass ratio of the positive electrode active material (such as a ternary material), the conductive agent (such as acetylene black, Super P, etc.) and the binder (such as PVDF) is about (96-98):(1-2):(1.0-2.0). Under this ratio, the main role of PVDF is defined as binding the active material and the conductive agent on the current collector, and the amount thereof is used as a prerequisite to ensure the basic bonding strength to avoid excessive use. Although the basic bonding performance can be ensured, the breakage of the positive electrode particles under high temperature cannot be effectively inhibited.

[0003] Although some researches have mentioned that increasing the content of the binder can improve the electrode performance, too high a content of the binder will lead to problems such as an increase in the sheet resistance and a decrease in the porosity, which will affect the battery performance. Therefore, the existing positive electrode sheet has the following problems:

[0004] (1) Particle breakage under high temperature: the existing low-PVDF-content formula cannot form a strong and tough binding network to buffer stress, which causes the contact between the active material and the conductive agent to fail, the internal resistance to increase sharply, and the capacity to decay rapidly.

[0005] (2) Insufficient stability of the electrode microstructure: due to the limited amount of the binder, the three-dimensional network structure formed is relatively fragile. In the long-term cycle process, the integrity of the electrode sheet structure will gradually be lost, which will exacerbate the isolation and shedding of the active material.

[0006] Based on the above research, it is necessary to provide a positive electrode sheet that can significantly improve the high-temperature performance of the battery without excessively sacrificing the conductivity and ion migration ability of the electrode. SUMMARY

[0007] The application aims to provide a positive electrode sheet, a preparation method thereof and an electrochemical device. The positive electrode sheet can form a more optimal three-dimensional network structure in the positive electrode sheet by controlling the molecular weight and the intrinsic viscosity of PVDF to satisfy a specific relationship, effectively inhibit the breakage of the positive electrode particles, and improve the high-temperature cycle performance of the battery while ensuring that PVDF has high bonding strength.

[0008] To achieve the application purpose, the following technical solutions are adopted in the application:

[0009] In a first aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising a current collector and an active material layer on at least one side surface of the current collector, the active material layer comprising a positive electrode active material, a conductive agent and a binder, the binder comprising PVDF;

[0010] The PVDF's weight-average molecular weight-intrinsic viscosity synergistic factor is λ, which satisfies:

[0011] λ=(Mw / 10 6 )×(IV / 300), 0.7≤λ≤2.1, for example, it can be 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 or 2.1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0012] Wherein, Mw is the weight-average molecular weight of the PVDF, in g / mol; and IV is the intrinsic viscosity of the PVDF, in mL / g.

[0013] The synergistic factor λ of the weight-average molecular weight-intrinsic viscosity of the PVDF described in this invention, within a specific range, enables the PVDF to construct a more resilient and buffered three-dimensional network bonding structure. This actively constrains and disperses the volume change stress caused by lithium-ion insertion / extraction during the charging and discharging process of the cathode particles, thereby physically slowing down the particle breakage process, reducing the generation and degree of microcracks in the particles, and significantly improving the high-temperature cycle life of the battery. If the synergistic factor is too small, the mechanical properties will decrease, leading to a reduction in the tensile strength and toughness of the material. If the synergistic factor is too large, the slurry viscosity will be too high, greatly increasing the processing difficulty.

[0014] The weight-average molecular weight (Mw) described in this invention is a statistical average molecular weight based on mass, calculated using the weight proportions of different molecular weight components as weights. Specifically, Mw reflects the overall mass distribution of the polymer molecular chain and is numerically equal to the sum of all molecular weights multiplied by their corresponding weight fractions, divided by the total weight.

[0015] An exemplary method for determining the weight-average molecular weight is gel permeation chromatography (GPC). First, the PVDF sample is dissolved in a solvent (such as N-methylpyrrolidone (NMP) or dimethylformamide (DMF)) to form a homogeneous solution. Then, the solution is injected into the chromatographic system, and components of different molecular weights are separated by the chromatographic column. The concentration of each component is analyzed using a detector (such as a differential refractive index detector or a multi-angle laser light scattering detector), and the molecular weight curve is compared with that of a standard sample to calculate Mw and the molecular weight distribution coefficient (PDI = Mw / Mn), with a temperature control accuracy of ±0.1℃.

[0016] The intrinsic viscosity (IV) described in this invention refers to the specific viscosity when the concentration of a polymer solution approaches zero. It represents the contribution of a single molecule to the viscosity of the solution and is a viscosity that reflects the properties of the polymer. Its value does not change with concentration.

[0017] For example, the intrinsic viscosity test method is the Ubbelohde viscometer method: Prepare a PVDF solution (commonly NMP or DMF), with the concentration controlled within the range of relative viscosity (the ratio of solution viscosity to pure solvent viscosity) of 1.2 to 2.0. Place the solution and pure solvent separately into an Ubbelohde viscometer, and under constant temperature conditions (25.0 ± 0.1℃), measure the time (t) for the solution to flow through the capillary and the time (t0) for the pure solvent to flow through. Calculate the IV value using the formula: IV = [ln(t / t0) / c], where c is the solution concentration (unit: g / dL), and ln represents the natural logarithm. During calculation, ensure the concentration approaches zero to meet the definition.

[0018] Preferably, the weight-average molecular weight of the PVDF is 900,000 g / mol to 1,600,000 g / mol, for example, it can be 900,000 g / mol, 1,000,000 g / mol, 1,100,000 g / mol, 1,200,000 g / mol, 1,300,000 g / mol, 1,400,000 g / mol, 1,500,000 g / mol, or 1,600,000 g / mol, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0019] If the weight-average molecular weight of the PVDF described in this invention is too small, it will not be sufficient to achieve a suitable electrode peeling force and a protective effect on the positive electrode particles; if the weight-average molecular weight of the PVDF is too large, the slurry viscosity will be too high, affecting the production yield.

[0020] Preferably, the intrinsic viscosity of the PVDF is 210 mL / g to 630 mL / g, for example, it can be 210 mL / g, 250 mL / g, 300 mL / g, 400 mL / g, 500 mL / g, 600 mL / g or 630 mL / g, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] Preferably, the surface resistivity and porosity of the positive electrode sheet satisfy the following:

[0022] Y = (23.8362 + 3.8947 × X) ± 3;

[0023] Wherein, X is the surface resistance of the positive electrode plate, in mΩ / mm. 2 Y represents the porosity of the positive electrode sheet.

[0024] Excessive concern in existing technologies regarding the side effects of increasing PVDF content leads to the belief that PVDF, being an insulator, will block electron conduction paths, resulting in a significant increase in electrode resistivity and affecting rate performance. Furthermore, it is believed that PVDF will clog electrode pores, reduce electrode porosity and electrolyte wettability, thereby increasing ion migration resistance.

[0025] In response, this invention breaks the conventional wisdom that high PVDF content inevitably leads to performance degradation. By adjusting the ratio of conductive agent to binder, the sheet resistance and porosity of the positive electrode sheet satisfy a specific relationship, ensuring efficient electrolyte wetting and rapid lithium-ion transport. Therefore, this invention successfully balances the problems of increased internal resistance and decreased wettability caused by binder, ensuring that the battery has excellent high-temperature cycle performance and low room-temperature impedance.

[0026] The Y=(23.8362+3.8947×X)±3 mentioned in this invention means that Y is in the range of (23.8362+3.8947×X)-3 to (23.8362+3.8947×X)+3.

[0027] Preferably, the surface resistivity of the positive electrode is 0.07 mΩ / mm. 2 ~2mΩ / mm 2 For example, it could be 0.07 mΩ / mm 2 0.1mΩ / mm 2 0.25mΩ / mm 2 0.5mΩ / mm 2 0.75mΩ / mm 2 1mΩ / mm 2 1.5mΩ / mm 2 1.75mΩ / mm 2 or 2mΩ / mm 2However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0028] Preferably, the porosity of the positive electrode sheet is 20% to 40%, for example, it can be 20%, 25%, 30%, 35% or 40%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Preferably, the PVDF content in the active material layer is 1.0wt% to 5.0wt%, for example, it can be 1.0wt%, 2.0wt%, 3.0wt%, 4.0wt% or 5.0wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] Preferably, the content of the conductive agent in the active material layer is 2.0wt% to 6.0wt%, for example, it can be 2.0wt%, 3.0wt%, 4.0wt%, 5.0wt% or 6.0wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Preferably, the conductive agent includes any one or a combination of at least two of Super P, carbon nanotubes, graphene, Ketjen black, acetylene black, carbon nanofibers, activated carbon, fullerene, or KS-6 (large particle graphite powder).

[0032] Preferably, the positive electrode active material includes LiNi. x Co y Mn z O2, where 0.3≤x≤0.8, for example, it can be 0.3, 0.5, 0.6, 0.7 or 0.8; 0.1≤y≤0.35, for example, it can be 0.1, 0.15, 0.2, 0.25, 0.3 or 0.35; 0.1≤z≤0.35, for example, it can be 0.1, 0.15, 0.2, 0.25, 0.3 or 0.35; and x+y+z=1.

[0033] For example, the positive electrode active material includes NCM333 (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), NCM532 (LiNi) 0.5 Co 0.3 Mn 0.2 O2), NCM622 (LiNi) 0.6 Co 0.2 Mn 0.2 O2) or NCM811 (LiNi 0.8 Co 0.1 Mn 0.1Any one or at least two of O2).

[0034] Preferably, the current collector comprises aluminum foil or carbon-coated aluminum foil.

[0035] In a second aspect, the present invention provides a method for preparing a positive electrode sheet as described in the first aspect, the method comprising the following steps:

[0036] A positive electrode active material, a conductive agent, a binder, and a solvent are mixed to obtain a positive electrode slurry. The positive electrode slurry is coated on at least one side of a current collector, and after drying and rolling, the positive electrode sheet is obtained.

[0037] Thirdly, the present invention provides an electrochemical device comprising a positive electrode as described in the second aspect.

[0038] Preferably, the electrochemical device includes a battery or a capacitor.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The synergistic factor λ of the weight-average molecular weight-intrinsic viscosity of the PVDF described in this invention, within a specific range, enables the PVDF to construct a more resilient and buffered three-dimensional network bonding structure. This actively constrains and disperses the volume change stress caused by lithium-ion insertion / extraction during the charging and discharging process of the cathode particles, thereby physically slowing down the particle breakage process, reducing the generation and degree of microcracks in the particles, and significantly improving the high-temperature cycle life of the battery. After 500 charge-discharge cycles at 45°C and a 5C rate, the capacity retention rate is significantly improved to over 95%, effectively solving the problem of rapid high-temperature degradation of cathode materials. Detailed Implementation

[0041] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0042] Example 1-1

[0043] This embodiment provides a positive electrode sheet, which includes a current collector (specifically an aluminum foil) and active material layers on both sides of the current collector. The active material layers include 96 wt% positive active material (specifically NCM622), 3 wt% conductive agent (specifically Super P), and 1.0 wt% binder, wherein the binder includes PVDF.

[0044] The PVDF has a weight-average molecular weight Mw of 1,000,000 g / mol and an intrinsic viscosity IV of 210 mL / g.

[0045] The PVDF's weight-average molecular weight-intrinsic viscosity synergistic factor is λ, where λ = (Mw / 10 6 )×(IV / 300)=0.7;

[0046] The surface resistance X of the positive electrode is 0.14 mΩ / mm. 2 The porosity Y is 25.1%, and the surface resistivity and porosity of the positive electrode sheet satisfy: Y = (23.8362 + 3.8947 × X) ± 3;

[0047] The method for preparing the positive electrode sheet includes the following steps:

[0048] The positive electrode active material, conductive agent, binder and N-methylpyrrolidone are mixed to obtain a positive electrode slurry. The positive electrode slurry is coated on both sides of the current collector. After drying and rolling, the positive electrode sheet is obtained.

[0049] Examples 1-2 to 1-9, Comparative Example 1 and Comparative Example 2

[0050] Examples 1-2 to 1-9, Comparative Examples 1 and 2 provide a positive electrode sheet, which is the same as that in Example 1-1 except that the weight-average molecular weight Mw, intrinsic viscosity IV and synergistic factor λ of the PVDF are different as shown in Table 1.

[0051] The positive electrode sheets obtained in Examples 1-1 to 1-9, Comparative Examples 1 and 2 above were used to prepare lithium-ion batteries. Specifically, the positive electrode sheet, negative electrode sheet, and separator were assembled together, and after electrolyte injection and capacity testing, a lithium-ion battery was obtained. The negative electrode sheet includes a copper foil and negative active material layers on both sides of the copper foil. The negative active material layers include a negative active material (specifically NCM622), a conductive agent (specifically Super P), and a binder (specifically PVDF) in a mass ratio of 96:2:2. The separator is a polyethylene separator. In the electrolyte, the solute is 1.0 mol / L lithium hexafluorophosphate, and the solvent is a mixed solvent of ethylene carbonate, diethyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1.

[0052] The high-temperature capacity performance of the lithium-ion batteries prepared in Examples 1-1 to 1-9, Comparative Examples 1 and 2 was tested. The test method was as follows: 0.33C constant capacity, battery cycling at 5C / 5C-45℃, voltage range of 3.0~4.2V, capacity retention rate = (discharge capacity of the nth cycle / discharge capacity of the first cycle) × 100%. The test results are shown in Table 1.

[0053] Table 1

[0054]

[0055] As can be seen from Table 1 above:

[0056] As can be seen from the comparison of Examples 1-1 to 1-9, when the synergistic factor of the weight-average molecular weight and intrinsic viscosity of the PVDF meets a specific numerical range, the weight-average molecular weight of the PVDF is further preferably between 900,000 and 1,600,000 g / mol. As can be seen from Examples 1-1, Comparative Example 1 and Comparative Example 2, when the synergistic factor of the weight-average molecular weight and intrinsic viscosity of the PVDF of the present invention is within a specific range, the PVDF can form a more robust and elastic three-dimensional network bonding structure, effectively avoiding the breakage of positive electrode particles and improving the high-temperature cycle performance of the battery.

[0057] Example 2-1

[0058] This embodiment provides a positive electrode sheet, which includes a current collector (specifically an aluminum foil) and active material layers on both sides of the current collector. The active material layers include 96 wt% positive active material (specifically NCM622), 3 wt% conductive agent (specifically Super P), and 1.0 wt% binder, wherein the binder includes PVDF.

[0059] The PVDF has a weight-average molecular weight Mw of 1,000,000 g / mol and an intrinsic viscosity IV of 250.

[0060] The PVDF's weight-average molecular weight-intrinsic viscosity synergistic factor is λ, where λ = (Mw / 10 6 )×(IV / 300)=0.83;

[0061] The surface resistance X of the positive electrode is 0.07 mΩ / mm. 2 The porosity Y is 24.1%, and the surface resistivity and porosity of the positive electrode sheet satisfy: Y = (23.8362 + 3.8947 × X) ± 3;

[0062] The method for preparing the positive electrode sheet includes the following steps:

[0063] The positive electrode active material, conductive agent, binder and N-methylpyrrolidone are mixed to obtain a positive electrode slurry. The positive electrode slurry is coated on both sides of the current collector. After drying and rolling, the positive electrode sheet is obtained.

[0064] Examples 2-2 to 2-9

[0065] Examples 2-2 to 2-9 provide a positive electrode sheet. Except for the changes in the content of each component, the surface resistance X, the porosity Y, and whether the surface resistance and porosity satisfy Y=(23.8362+3.8947×X)±3 as shown in Table 2, the positive electrode sheet is the same as that in Example 2-1.

[0066] The positive electrode sheets obtained in Examples 2-1 to 2-9 above are used to prepare lithium-ion batteries. Specifically, the positive electrode sheet, negative electrode sheet and separator are assembled together, and after liquid injection and formation and capacity testing, lithium-ion batteries are obtained. The negative electrode sheet, separator and electrolyte used are the same as those used in the lithium-ion battery prepared in Example 1-1.

[0067] The high-temperature capacity performance of the lithium-ion batteries prepared in Examples 2-1 to 2-9 was tested using the same method as that used in Example 1-1.

[0068] The DCR of the lithium-ion batteries prepared in Examples 2-1 to 2-9 at 25°C was tested. The test method was as follows: constant current and constant voltage charging to 50% SOC, followed by discharge DCR testing at 5C-10s. The test results are shown in Table 2.

[0069] Table 2

[0070]

[0071] As can be seen from Table 2 above:

[0072] As can be seen from Examples 2-1 to 2-9, the sheet resistance X and porosity Y of the present invention satisfy Y=(23.8362+3.8947×X)±3, which enables the battery to have both excellent high-temperature cycle performance and low room-temperature DCR. When the sheet resistance of the positive electrode is too high, that is, there is relatively less conductive agent and relatively more binder, the battery performance is poor. Even if the porosity is further increased, the electrical performance is still poor. When the sheet resistance of the positive electrode is too low, there is relatively more conductive agent, and the electrical performance of the battery is still poor.

[0073] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A positive electrode plate, characterized in that, The positive electrode sheet includes a current collector and an active material layer on at least one side of the current collector. The active material layer includes a positive electrode active material, a conductive agent, and a binder. The binder includes PVDF. The PVDF's weight-average molecular weight-intrinsic viscosity synergistic factor is λ, which satisfies: λ=(Mw / 10 6 )×(IV / 300),0.7≤λ≤2.1; Wherein, Mw is the weight-average molecular weight of the PVDF, in g / mol; and IV is the intrinsic viscosity of the PVDF, in mL / g.

2. The positive electrode sheet according to claim 1, characterized in that, The weight-average molecular weight of the PVDF is 900,000 g / mol to 1,600,000 g / mol; Preferably, the intrinsic viscosity of the PVDF is 210 mL / g-630 mL / g.

3. The positive electrode sheet according to claim 1, characterized in that, The surface resistivity and porosity of the positive electrode sheet satisfy the following: Y = (23.8362 + 3.8947 × X) ± 3; Wherein, X is the surface resistance of the positive electrode plate, in mΩ / mm. 2 Y represents the porosity of the positive electrode sheet.

4. The positive electrode sheet according to claim 3, characterized in that, The surface resistivity of the positive electrode is 0.07 mΩ / mm. 2 ~2mΩ / mm 2 ; Preferably, the porosity of the positive electrode sheet is 20% to 40%.

5. The positive electrode sheet according to claim 1 or 2, characterized in that, The PVDF content in the active material layer is 1.0 wt% to 5.0 wt%. Preferably, the content of the conductive agent in the active material layer is 2.0wt% to 6.0wt%.

6. The positive electrode sheet according to claim 1 or 2, characterized in that, The conductive agent includes any one or a combination of at least two of Super P, carbon nanotubes, graphene, Ketjen Black, acetylene black, carbon nanofibers, activated carbon, fullerene, or KS-6. Preferably, the positive electrode active material includes LiNi. x Co y Mn z O2, where 0.3≤x≤0.8, 0.1≤y≤0.35, 0.1≤z≤0.35, and x+y+z=1.

7. The positive electrode sheet according to claim 1 or 2, characterized in that, The current collector includes aluminum foil or carbon-coated aluminum foil.

8. A method for preparing a positive electrode sheet as described in any one of claims 1-7, characterized in that, The preparation method includes the following steps: A positive electrode active material, a conductive agent, a binder, and a solvent are mixed to obtain a positive electrode slurry. The positive electrode slurry is coated on at least one side of a current collector, and after drying and rolling, the positive electrode sheet is obtained.

9. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode as described in any one of claims 1-7.

10. The electrochemical device according to claim 9, characterized in that, The electrochemical device includes a battery or a capacitor.

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