Positive plate and lithium ion battery

Abstract

The invention discloses a positive plate and a lithium ion battery. The positive plate comprises a positive current collector and a positive membrane, the preparation method comprises the following steps: dissolving a binder in N-methyl pyrrolidone during preparation of a positive electrode diaphragm, and stirring to obtain a first glue solution; adding carbon nanotubes into the first glue solution, and stirring to obtain a second glue solution; adding a conductive agent into the second glue solution, and stirring to obtain a third glue solution; adding part of the positive electrode active material into the third glue solution, and stirring to obtain a fourth glue solution; adding the rest of the positive electrode active material into the fourth glue solution, and stirring to obtain positive electrode slurry; and coating the positive electrode slurry on a positive electrode current collector to obtain the positive electrode plate. According to the invention, the process of adding the carbon nanotubes is preposed, and the lithium cobalt oxide is added twice, so that the dispersibility of the carbon nanotubes and the main material lithium cobalt oxide can be improved, the carbon nanotubes are attached to the surface of the lithium cobalt oxide in a net structure, the resistivity of the positive pole piece can be reduced, and the high-temperature long-cycle electrochemical performance of the battery can be remarkably improved; and the service life of the battery is prolonged.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and particularly to a positive electrode sheet and a lithium ion battery. Background Art

[0002] Lithium ion batteries have been widely used in fields such as portable electronic devices, electric vehicles, and energy storage systems due to their high energy density, long life, and low self-discharge rate. However, with the increase in the operating ambient temperature, especially in high-temperature environments, the performance of lithium ion batteries will significantly decline. This performance decline is mainly reflected in aspects such as increased battery impedance and accelerated capacity attenuation, seriously affecting the service life and application scope of the battery. Especially in applications that require long-term cycling, such as electric vehicles and large-scale energy storage systems, this problem is particularly prominent.

[0003] Research shows that the electrochemical impedance of the positive electrode film is one of the key factors affecting the high-temperature performance of lithium ion batteries. Under high-temperature conditions, the ion transport rate of the positive electrode material decreases, resulting in an increase in the internal resistance of the battery, which in turn causes a rapid attenuation of the battery capacity. In addition, high temperature will also accelerate the side reaction between the electrode material and the electrolyte, further deteriorating the battery performance. Therefore, for batteries that need to be cycled for a long time in high-temperature environments such as 35°C / 45°C, developing a positive electrode film with appropriate impedance characteristics has become an important way to improve the high-temperature cycling performance of lithium ion batteries. Summary of the Invention

[0004] Embodiments of the present invention provide a positive electrode sheet and a lithium ion battery to solve the problem of high electrochemical impedance of the positive electrode film.

[0005] Specifically, the present invention provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode film disposed on the positive electrode current collector. The areal density of the positive electrode film is 300~400 g / m 2 , and the tap density is 4.0~4.28 g / cm³; the positive electrode film includes a positive electrode active material, a binder, a conductive agent, and carbon nanotubes; the positive electrode active material is a lithium cobaltate material; by mass percentage, the mass ratio of the positive electrode active material is 94.5%~98.2%, the mass ratio of the binder is 1%~3%, the mass ratio of the conductive agent is 0.5%~2%, and the mass ratio of the carbon nanotubes is 0.3%~0.5%; The positive electrode sheet is prepared by the following steps: Dissolve the binder in N-methylpyrrolidone, and stir for a first time at a first rotation speed to obtain a first adhesive solution; Add the carbon nanotubes to the first adhesive solution, and stir for a second time at a second rotation speed to obtain a second adhesive solution; Add the conductive agent to the second adhesive solution, and stir for a third time at a third rotation speed to obtain a third adhesive solution; Add a part of the positive electrode active material into the third adhesive solution, and obtain a fourth adhesive solution after stirring at a fourth rotation speed for a fourth time; Add the remaining part of the positive electrode active material into the fourth adhesive solution, and obtain a positive electrode paste after stirring at a fifth rotation speed for a fifth time; Defoam the positive electrode paste, then adjust the viscosity and solid content of the paste, and then coat it on the positive electrode current collector, and obtain the positive electrode sheet after baking.

[0006] Optionally, the ratio of the mass of the positive electrode active material added into the third adhesive solution to the mass of all the positive electrode active materials is 40% - 60%.

[0007] Optionally, the first rotation speed is 400 - 600 rpm; and / or, the first time is 25 - 35 min.

[0008] Optionally, the second rotation speed is 1500 - 2000 rpm; and / or, the second time is 18 - 25 min.

[0009] Optionally, the third rotation speed is 1800 - 2400 rpm; and / or, the third time is 120 - 180 min.

[0010] Optionally, the fourth rotation speed is 1500 - 2000 rpm; and / or, the fourth time is 30 - 50 min.

[0011] Optionally, the fifth rotation speed is 1500 - 2000 rpm; and / or, the fifth time is 120 - 180 min.

[0012] Optionally, the mass ratio of the positive electrode active material is 97.4% - 98.2%, the mass ratio of the binder is 1.0% - 1.5%, the mass ratio of the conductive agent is 0.5% - 0.6%, and the mass ratio of the carbon nanotubes is 0.3% - 0.5%.

[0013] Optionally, the binder includes one or more of polyvinylidene fluoride and sodium carboxymethyl cellulose; and / or, the conductive agent includes one or more of acetylene black and carbon nanotubes.

[0014] The present invention also provides a lithium - ion battery, including the positive electrode sheet as described in any one of the above.

[0015] The beneficial effects of the present invention are as follows: In the positive electrode sheet and battery provided by the present invention, by advancing the addition process of carbon nanotubes and adding lithium cobaltate in two times, the dispersibility of carbon nanotubes and the main material lithium cobaltate can be improved, so that the carbon nanotubes are attached to the surface of lithium cobaltate in a "mesh structure", significantly reducing the resistivity of the positive electrode sheet, making the resistivity of the positive electrode sheet lower than 400 Ω·cm, and significantly improving the high-temperature long-cycle electrochemical performance of the battery, thereby extending the service life of the battery. Brief Description of the Drawings

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0017] Figure 1 are SEM diagrams of the positive electrode membranes in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention; Figure 2 are electrochemical performance diagrams of the lithium-ion batteries in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention; Figure 3 are internal resistance change rate diagrams of the lithium-ion batteries in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention. Detailed Description of the Embodiments

[0018] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0019] The embodiment of the present invention provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode membrane disposed on the positive electrode current collector. The areal density of the positive electrode membrane is 300~400 g / m 2 , and the tap density is 4.0~4.28 g / cm³; the positive electrode membrane includes a positive electrode active material, a binder, a conductive agent and carbon nanotubes; the positive electrode active material is a lithium cobaltate material; by mass percentage, the mass ratio of the positive electrode active material is 94.5%~98.2%, the mass ratio of the binder is 1%~3%, the mass ratio of the conductive agent is 0.5%~2%, and the mass ratio of the carbon nanotubes is 0.3%~0.5%; The positive electrode sheet is prepared by the following steps: Dissolve the binder in N-methylpyrrolidone, and stir for a first time at a first rotation speed to obtain a first glue solution; Add the carbon nanotubes into the first adhesive solution, and stir for a second time at a second rotation speed to obtain a second adhesive solution; Add the conductive agent into the second adhesive solution, and stir for a third time at a third rotation speed to obtain a third adhesive solution; Add a part of the positive electrode active material into the third adhesive solution, and stir for a fourth time at a fourth rotation speed to obtain a fourth adhesive solution; Add the remaining part of the positive electrode active material into the fourth adhesive solution, and stir for a fifth time at a fifth rotation speed to obtain a positive electrode paste; Defoam the positive electrode paste, then adjust the viscosity and solid content of the paste, and then coat it on the positive electrode current collector, and obtain the positive electrode sheet after baking.

[0020] When the positive electrode sheet of the present invention is prepared, by advancing the addition process of the carbon nanotubes and adding the lithium cobaltate in two times, the dispersibility of the carbon nanotubes and the main material lithium cobaltate can be improved, so that the carbon nanotubes are attached to the surface of the lithium cobaltate in a "network structure", significantly reducing the resistivity of the positive electrode sheet, making the resistivity of the positive electrode sheet lower than 400 Ω·cm, and significantly improving the high-temperature long-cycle electrochemical performance of the battery, thereby extending the service life of the battery.

[0021] In an embodiment of the present invention, the ratio of the mass of the positive electrode active material added into the third adhesive solution to the mass of all the positive electrode active materials is 40% - 60%.

[0022] Specifically, the first rotation speed is 400 - 600 rpm; and / or, the first time is 25 - 35 min. The second rotation speed is 1500 - 2000 rpm; and / or, the second time is 18 - 25 min. The third rotation speed is 1800 - 2400 rpm; and / or, the third time is 120 - 180 min. The fourth rotation speed is 1500 - 2000 rpm; and / or, the fourth time is 30 - 50 min. The fifth rotation speed is 1500 - 2000 rpm; and / or, the fifth time is 120 - 180 min. The mass ratio of the positive electrode active material is 97.4% - 98.2%, the mass ratio of the binder is 1.0% - 1.5%, the mass ratio of the conductive agent is 0.5% - 0.6%, and the mass ratio of the carbon nanotubes is 0.3% - 0.5%. The binder includes one or more of polyvinylidene fluoride and sodium carboxymethyl cellulose. The conductive agent includes one or more of acetylene black and carbon nanotubes.

[0023] The present invention also provides a lithium-ion battery, including the positive electrode sheet of any one of the above, so as to have all the effects of the positive electrode sheet.

[0024] The technical solutions of the present invention will be described below in conjunction with specific embodiments.

[0025] Example 1 Preparation of the positive electrode sheet: Taking 100 parts as a basis, prepare 97.9 parts of lithium cobaltate, 1.2 parts of polyvinylidene fluoride (PVDF, binder), 0.5 parts of conductive carbon black, and 0.4 parts of carbon nanotubes. Dissolve PVDF in N-methylpyrrolidone (NMP), stir at a speed of 500 rpm for 30 min to obtain the first adhesive solution. Add carbon nanotubes into the first adhesive solution, stir at a speed of 1800 rpm for 20 min to obtain the second adhesive solution. Add conductive carbon black into the second adhesive solution, stir at a speed of 2000 rpm for 140 min to obtain the third adhesive solution. Add 48.95 parts of lithium cobaltate into the third adhesive solution, stir at a speed of 1800 rpm for 40 min to obtain the fourth adhesive solution. Add 48.95 parts of lithium cobaltate into the fourth adhesive solution, stir at a speed of 1800 rpm for 150 min to obtain the positive electrode slurry. Slowly stir and degas the positive electrode slurry in vacuum for 30 min, adjust the viscosity and solid content, and then coat it on the positive electrode current collector aluminum foil, and obtain the positive electrode sheet after baking.

[0026] Preparation of the negative electrode sheet: Stir graphite, sodium carboxymethylcellulose, styrene-butadiene rubber (SBR) and N-methylpyrrolidone according to the mass ratio of 98.2:0.8:1.0:1.5, and disperse evenly to obtain the negative electrode slurry. Uniformly load the negative electrode slurry on the copper foil current collector through a coater, and obtain the negative electrode sheet after high-temperature baking and rolling.

[0027] Assembly of the lithium-ion battery: Adopt the winding type to wind the prepared positive electrode sheet, negative electrode sheet and upper and lower layer separators in sequence to obtain a wound core. Package the wound core with an aluminum-plastic film, inject the electrolyte, and assemble it into a lithium-ion battery through processes such as formation and grading.

[0028] Example 2 The difference between Example 2 and Example 1 is only that: the lithium cobaltate added into the third adhesive solution is 32.63 parts, and the lithium cobaltate added into the fourth adhesive solution is 65.27 parts.

[0029] Example 3 The difference between Example 3 and Example 1 is only that: the lithium cobaltate added into the third adhesive solution is 39.16 parts, and the lithium cobaltate added into the fourth adhesive solution is 58.74 parts.

[0030] Example 4 The difference between Example 4 and Example 1 is only that: the lithium cobaltate added into the third adhesive solution is 58.74 parts, and the lithium cobaltate added into the fourth adhesive solution is 39.16 parts.

[0031] Example 5 The difference between Example 5 and Example 1 is only that: the lithium cobaltate added into the third adhesive solution is 78.32 parts, and the lithium cobaltate added into the fourth adhesive solution is 19.58 parts.

[0032] Example 6 The difference between Example 6 and Example 1 is only that: the lithium cobaltate is 97.4 parts; the lithium cobaltate added into the third glue solution is 48.7 parts, and the lithium cobaltate added into the fourth glue solution is 48.7 parts.

[0033] Example 7 The difference between Example 7 and Example 1 is only that: the lithium cobaltate is 98.2 parts; the lithium cobaltate added into the third glue solution is 49.1 parts, and the lithium cobaltate added into the fourth glue solution is 49.1 parts.

[0034] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is only that: Dissolve PVDF in N-methylpyrrolidone (NMP), stir at a speed of 500 rpm for 30 min to obtain the first glue solution. Add conductive carbon black into the first glue solution, stir at a speed of 2000 rpm for 140 min to obtain the second glue solution. Add 97.9 parts of lithium cobaltate into the second glue solution, stir at a speed of 1800 rpm for 190 min to obtain the third glue solution. Add carbon nanotubes into the third glue solution, stir at a speed of 1800 rpm for 60 min to obtain the positive electrode paste.

[0035] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is only that: Dissolve PVDF in N-methylpyrrolidone (NMP), stir at a speed of 500 rpm for 30 min to obtain the first glue solution. Add conductive carbon black into the first glue solution, stir at a speed of 2000 rpm for 140 min to obtain the second glue solution. Add 48.95 parts of lithium cobaltate into the second glue solution, stir at a speed of 1800 rpm for 40 min to obtain the third glue solution. Add 48.95 parts of lithium cobaltate into the third glue solution, stir at a speed of 1800 rpm for 150 min to obtain the fourth glue solution. Add carbon nanotubes into the fourth glue solution, stir at a speed of 1800 rpm for 60 min to obtain the positive electrode paste.

[0036] Performance Test Positive electrode sheet resistance test: Take the compacted positive electrode sheet and place it on the IEST BER1300 device. After setting the parameters of the test sample, conduct the test and record the test data, as shown in Table 1.

[0037] High-temperature cycling test: For each example and comparative example, 3 lithium-ion batteries were selected as the batteries to be tested. The batteries to be tested were connected to a Neware charge-discharge instrument, and cyclic charge-discharge tests were carried out on the lithium-ion batteries in a high-temperature environment to verify their long-life cycling performance. The capacity retention rate and internal resistance change rate of the lithium-ion batteries were recorded at 100 cycles, 400 cycles, and 800 cycles, as shown in Table 1. Among them, the changes in the capacity retention rate and internal resistance change rate of the batteries to be tested in Example 1, Comparative Example 1, and Comparative Example 2 are shown in Figure 2 , Figure 3 .

[0038] Electron scanning microscope test: After coating and drying, the positive electrode film was cut into small squares and then pasted on the sample stage. For samples with poor conductivity / non-conductivity, coating treatment was required to prevent the charging effect. The sample stage was placed in a vacuum chamber, and the image was made clear by adjusting the focal length, brightness, etc. The SEM image of the sample was taken and saved under appropriate conditions, as shown in Figure 1 .

[0039] Table 1 As shown in Table 1 and referring to Figures 1 to 3 , through the improvement of the preparation method in Example 1, the resistivity of the prepared positive electrode sheet was 372.7 Ω·cm, and the capacity retention rate and internal resistance change rate were better.

[0040] Figure 1 In (a) is the SEM image of the positive electrode film in Example 1, (2) is the SEM image of the positive electrode film in Comparative Example 1, and (3) is the SEM image of the positive electrode film in Comparative Example 2. Comparing Example 1 with Comparative Example 1 and Comparative Example 2, since the lithium cobaltate was added in two batches and the carbon nanotubes were added before the lithium cobaltate, the added carbon nanotubes were continuously stirred in the subsequent processes, so the dispersion of the carbon nanotubes was good; adding the lithium cobaltate in two batches made it easy for the lithium cobaltate to be evenly mixed with the carbon nanotubes, and then the conductive agent and the carbon nanotubes were evenly dispersed on the surface of the lithium cobaltate. The evenly dispersed carbon nanotubes could form a continuous conductive network, reducing the resistance in the electron transmission path, thereby reducing the internal resistance and polarization of the battery, so the high-temperature long-cycle electrochemical performance of the battery could be significantly improved, thus extending the service life of the battery. In Comparative Example 1 and Comparative Example 2, the carbon nanotubes were added at the end, and the carbon nanotubes were not easily dispersed evenly and agglomerated.

[0041] Examples 2 to 5 were compared with Example 1, and the proportion of lithium cobaltate added to the third glue solution was changed. Among them, in Examples 2 and 3, the proportion of lithium cobaltate added to the third glue solution was reduced, so that too much lithium cobaltate was added finally, which affected the uniformity of lithium cobaltate dispersion, thus affecting the formation of the conductive network and the improvement of battery performance. In Examples 4 and 5, the proportion of lithium cobaltate added to the third glue solution was increased, resulting in uneven mixing of carbon nanotubes and lithium cobaltate, because a large amount of lithium cobaltate might limit the dispersion of carbon nanotubes. In addition, too much lithium cobaltate might cause too high local concentration in the early stage, resulting in uneven structure of the electrode material, and further affecting the performance and life of the battery.

[0042] Examples 6 and 7 were compared with Example 1, and the total amount of lithium cobaltate was changed, resulting in changes in both the capacity retention rate and the internal resistance change rate of the battery. However, compared with Comparative Example 1, the high-temperature long-cycle electrochemical performance of the battery could still be significantly improved, thus extending the service life of the battery.

[0043] The above-described embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The invention comprises a positive electrode current collector and a positive electrode film arranged on the positive electrode current collector, wherein the surface density of the positive electrode film is 300-400 g / m 2 , the compaction density is 4.0-4.28g / cm³; the positive electrode film comprises a positive electrode active material, a binder, a conductive agent and carbon nanotubes; the positive electrode active material is a lithium cobalt oxide material; in terms of mass percentage, the mass proportion of the positive electrode active material is 94.5%-98.2%, the mass proportion of the binder is 1%-3%, the mass proportion of the conductive agent is 0.5%-2%, and the mass proportion of the carbon nanotubes is 0.3%-0.5%; The positive electrode sheet is prepared by the following steps: Dissolving the binder in N-methylpyrrolidone, and stirring at a first speed for a first time to obtain a first glue solution; Adding the carbon nanotubes into the first glue solution, and stirring at a second speed for a second time to obtain a second glue solution; Adding the conductive agent into the second glue solution, and stirring at a third speed for a third time to obtain a third glue solution; adding a portion of the positive electrode active material into the third glue solution, and stirring at a fourth speed for a fourth time to obtain a fourth glue solution; Adding the remaining portion of the positive electrode active material into the fourth glue solution, and stirring at a fifth speed for a fifth time to obtain a positive electrode slurry; The positive electrode slurry is degassed, the viscosity and solid content of the slurry are adjusted, and then the slurry is coated on the positive electrode current collector, and the positive electrode sheet is obtained after baking.

2. The positive electrode sheet according to claim 1, characterized in that: The ratio of the mass of the positive electrode active material added to the third glue solution to the mass of all the positive electrode active materials is 40% to 60%.

3. The positive electrode sheet according to claim 1, characterized in that: The first rotation speed is 400-600 rpm; and / or the first time is 25-35 min.

4. The positive electrode sheet according to claim 1, characterized in that: The second rotation speed is 1500-2000 rpm; and / or the second time is 18-25 min.

5. The positive electrode sheet according to claim 1, characterized in that: The third rotation speed is 1800-2400 rpm; and / or the third time is 120-180 min.

6. The positive electrode sheet according to claim 1, characterized in that: The fourth rotation speed is 1500-2000 rpm; and / or the fourth time is 30-50 min.

7. The positive electrode sheet according to claim 1, characterized in that: The fifth rotation speed is 1500-2000 rpm; and / or the fifth time is 120-180 min.

8. The positive electrode sheet according to claim 1, characterized in that: The mass proportion of the positive electrode active material is 97.4% to 98.2%, the mass proportion of the binder is 1.0% to 1.5%, the mass proportion of the conductive agent is 0.5% to 0.6%, and the mass proportion of the carbon nanotubes is 0.3% to 0.5%.

9. The positive electrode sheet according to claim 1, characterized in that: The binder includes one or more of polyvinylidene fluoride and sodium carboxymethyl cellulose; and / or the conductive agent includes one or more of acetylene black and carbon nanotubes.

10. A lithium ion battery, characterized in that: A positive electrode sheet comprising any one of claims 1 to 9.

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