A lithium ion battery negative electrode sheet

Abstract

The application provides a lithium ion battery negative pole piece, which comprises graphite, hard carbon and a copper foil, the graphite is covered on the surface of the copper foil, and the hard carbon is covered on the upper layer of the graphite. The negative pole piece can realize fast charging and guarantee high energy density.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more specifically, to a negative electrode sheet for lithium-ion batteries. Background Technology

[0002] Currently, the main anode materials for lithium-ion batteries are graphite, hard carbon, soft carbon, and silicon-based materials, each with different characteristics. Graphite, for example, is inexpensive, stable, and has a low voltage plateau, making it the most widely used anode material. However, its expansion, cycling, and kinetics are relatively average. Hard carbon, on the other hand, is an amorphous carbon material with large interlayer spacing, good kinetics, low lithium intercalation strain, and high capacity, but it suffers from low initial efficiency, low compaction, easy water absorption, and significant voltage hysteresis. Some reports have suggested mixing graphite and hard carbon to leverage their respective properties, but this simple mechanical mixing method cannot fully utilize the advantages of both materials.

[0003] The development of new energy vehicles has progressed to the point where they now account for 45% of total vehicle sales. However, the charging speed of new energy vehicles has always been a major obstacle to their further development. Lithium-ion batteries have evolved from initial 1C fast charging to the current 2C fast charging, and even up to 6C fast charging. Fast charging remains a technological challenge that requires rapid breakthroughs. Summary of the Invention

[0004] The inventors discovered that the current industry's microscopic limitations on fast charging performance can be summarized into the following aspects: (1) migration and diffusion of lithium ions in electrode materials; (2) transport of lithium ions in electrolytes; (3) transfer of lithium ions at the phase interface, including desolvation of electrolyte molecules, charge transfer at the interface, and ion migration. For anode materials, the design ideas for achieving fast charging are as follows: (1) improve the ionic conductivity and electronic conductivity of the material to accelerate lithium ion and charge transfer; (2) improve the wetting ability of the electrolyte on the surface of the anode material to reduce the interface transfer resistance; (3) suppress the structural transformation and volume expansion of the material under high current conditions to improve cycle life; (4) prevent lithium deposition on the anode surface to ensure safety performance. This invention is a design invention for the anode to enable lithium-ion batteries to achieve higher charging rates, thereby solving the current problem of slow charging. The innovation of this invention is to combine the high capacity and long cycle performance of graphite with the high rate advantage of hard carbon to achieve fast charging technology. Hard carbon is coated on the upper layer and graphite is coated on the lower layer using a double-layer coating technology. Hard carbon and graphite in a certain ratio can achieve fast charging while ensuring high energy density.

[0005] In a first aspect, the present invention provides a negative electrode sheet for a lithium-ion battery. According to an embodiment of the invention, the negative electrode sheet comprises graphite, hard carbon, and copper foil, wherein the graphite covers the surface of the copper foil, and the hard carbon covers the upper layer of the graphite. The negative electrode sheet according to the present invention utilizes the high rate-capacity performance of hard carbon to more quickly receive lithium ions from the electrolyte in the upper layer, and then transfer them layer by layer in the active material to the lower layer of graphite. This enables higher-rate charging of lithium-ion batteries, solving the current problem of slow charging.

[0006] According to embodiments of the present invention, the lithium-ion battery negative electrode may further include at least one of the following additional technical features:

[0007] According to an embodiment of the present invention, the mass ratio of hard carbon to graphite is 1:(1-19), for example 1:19, 1:18, 1:17, 1:16, 1:15, 1:14, 1:13, 1:12, 1:11, 1:10, 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, or 1:1. This ratio allows for both fast charging and high energy density.

[0008] According to an embodiment of the present invention, the mass ratio of the hard carbon to the graphite is 1:(1-3). This ratio enables fast charging while ensuring high energy density, and the effect is superior.

[0009] According to an embodiment of the present invention, the thickness of the copper foil is 8 micrometers.

[0010] In another aspect, the present invention also provides a method for preparing a negative electrode sheet for a lithium-ion battery. According to an embodiment of the present invention, the method includes:

[0011] 1) Graphite powder, conductive carbon, carbon nanotubes, sodium carboxymethyl cellulose, styrene-butadiene rubber and water are mixed to obtain the first slurry;

[0012] 2) Hard carbon powder, conductive carbon, carbon nanotubes, sodium carboxymethyl cellulose, styrene-butadiene rubber and water are mixed to obtain a second slurry;

[0013] 3) The first slurry is applied to one side of the copper foil, and the second slurry is applied to the surface of the first slurry. Then, a drying process is performed to obtain a copper foil with a one-sided coating.

[0014] 4) The first slurry is coated onto the other side of the copper foil with one-sided coating, and the second slurry is coated onto the surface of the first slurry. After drying, a lithium-ion battery negative electrode sheet is obtained. This method is simple to operate. The negative electrode sheet prepared by this method utilizes the high rate capability of hard carbon to more quickly receive lithium ions from the electrolyte in the upper layer, which are then transferred layer by layer to the graphite in the lower layer within the active material. This enables higher rate charging of lithium-ion batteries, solving the current problem of slow charging.

[0015] According to embodiments of the present invention, the above method may further include at least one of the following additional technical features:

[0016] According to an embodiment of the present invention, the graphite powder is artificial graphite powder.

[0017] According to an embodiment of the present invention, the mass ratio of the artificial graphite powder, conductive carbon, carbon nanotubes, sodium carboxymethyl cellulose and styrene-butadiene rubber is 93:2:2:3.

[0018] According to an embodiment of the present invention, the solid content of the first slurry is 48%.

[0019] According to an embodiment of the present invention, the mass ratio of the hard carbon powder, conductive carbon, carbon nanotubes, sodium carboxymethyl cellulose, styrene-butadiene rubber and water is 93:2:2:3.

[0020] According to an embodiment of the present invention, the solid content of the second slurry is 48%.

[0021] According to an embodiment of the present invention, the first slurry is coated on one side surface of the copper foil, and the second slurry is coated on the surface of the first slurry by means of a double-layer coating device.

[0022] According to an embodiment of the present invention, the mass ratio of the second slurry to the first slurry is 1:(1-19), for example 1:19, 1:18, 1:17, 1:16, 1:15, 1:14, 1:13, 1:12, 1:11, 1:10, 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, or 1:1. This ratio allows for both fast charging and high energy density.

[0023] According to an embodiment of the present invention, the mass ratio of the second slurry to the first slurry is 1:(1-3). This ratio enables fast charging while ensuring high energy density, and the effect is superior.

[0024] According to an embodiment of the present invention, the thickness of the copper foil is 8 micrometers.

[0025] According to an embodiment of the present invention, the density of the coating on the copper foil having a coating on one side is 8.5 mg / cm³. 2 .

[0026] In another aspect, the present invention also provides a lithium battery semi-finished cell. According to an embodiment of the present invention, the lithium battery includes the lithium-ion battery negative electrode sheet described above or the lithium-ion battery negative electrode sheet obtained according to the method described above.

[0027] In another aspect, the present invention also provides a lithium battery. According to an embodiment of the present invention, the lithium battery comprises the aforementioned lithium battery semi-finished cell.

[0028] According to embodiments of the present invention, the present invention provides at least one of the following technical effects produced by a negative electrode sheet:

[0029] 1. The high rate capability of hard carbon allows it to receive lithium ions from the electrolyte more quickly in the upper layer, and then transfer them layer by layer to the graphite in the lower layer through the active material.

[0030] 2. At high rates, lithium ions travel faster in the electrolyte than in the active material. If the negative electrode active material cannot receive lithium ions from the electrolyte in time, they will accumulate on the surface, leading to lithium plating and posing a safety risk. High-porosity hard carbon is used to first receive a large number of lithium ions as a buffer, after which the lithium ions are transported within the active material. Attached Figure Description

[0031] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0032] Figure 1 These are electron microscope images according to Embodiment 1 of the present invention;

[0033] Figure 2 This is a schematic diagram of the negative electrode sheet according to an embodiment of the present invention. Detailed Implementation

[0034] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0035] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0036] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0037] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0038] Example 1

[0039] Positive electrode active material powder, conductive carbon, carbon nanotubes, and PVDF were mixed in a mass ratio of 94:2:1:3. NMP was then added in a high-speed mixer and uniformly mixed to form a slurry with a solid content of 45-80%. This slurry was coated onto one side of a 12-micron thick aluminum foil using a transfer coating machine and dried, maintaining a coating weight of 17.5 mg / cm² per unit area after drying. The same process was then repeated on the other side of the aluminum foil to obtain a semi-finished positive electrode sheet. The positive electrode sheet was then rolled to a compaction density of 3.0 g / cm³ to obtain a rolled positive electrode sheet.

[0040] Artificial graphite powder, conductive carbon, carbon nanotubes, CMC, and SBR were mixed in a mass ratio of 93:2:2:3. Deionized water was then added in a high-speed mixer and the mixture was homogeneously mixed to form a slurry with a solid content of 48%. Hard carbon powder, conductive carbon, carbon nanotubes, CMC, and SBR were also mixed in a mass ratio of 93:2:2:3. Deionized water was then added in a high-speed mixer and the mixture was homogeneously mixed to form a slurry with a solid content of 48%. Both slurries were then coated onto one side of an 8-micrometer thick copper foil at a weight ratio of 5:95, with hard carbon on top and graphite on the bottom. The coating was dried, maintaining a dry coating weight of 8.5 mg / cm². The same process was then repeated on the other side of the copper foil, and the coating was dried to obtain a semi-finished negative electrode sheet. Electron micrographs are shown below. Figure 1 .

[0041] The exposed metal foil portion of the aforementioned electrode sheets is processed and welded into tabs, then wound with a separator to form a core. The core is then wrapped in aluminum-plastic film to create a semi-finished cell, which is then injected with electrolyte. After formation and capacity testing, the finished lithium-ion battery is obtained.

[0042] Example 2

[0043] Positive electrode active material powder, conductive carbon, carbon nanotubes, and PVDF were mixed in a mass ratio of 94:2:1:3. NMP was then added in a high-speed mixer and uniformly mixed to form a slurry with a solid content of 45-80%. This slurry was coated onto one side of a 12-micron thick aluminum foil using a transfer coating machine and dried, maintaining a coating weight of 17.5 mg / cm² per unit area after drying. The same process was then repeated on the other side of the aluminum foil to obtain a semi-finished positive electrode sheet. The positive electrode sheet was then rolled to a compaction density of 3.0 g / cm³ to obtain a rolled positive electrode sheet.

[0044] Artificial graphite powder, conductive carbon, carbon nanotubes, CMC, and SBR were mixed in a mass ratio of 93:2:2:3. Deionized water was then added in a high-speed mixer and the mixture was homogeneously mixed to form a slurry with a solid content of 48%. Hard carbon powder, conductive carbon, carbon nanotubes, CMC, and SBR were also mixed in a mass ratio of 93:2:2:3. Deionized water was then added in a high-speed mixer and the mixture was homogeneously mixed to form a slurry with a solid content of 48%. Both slurries were then coated onto one side of an 8-micron thick copper foil using a double-layer coating apparatus, with hard carbon on top and graphite on the bottom, at a weight ratio of 10:90. The coating was dried, maintaining a dry coating weight of 8.5 mg / cm². The same process was then repeated on the other side of the copper foil, and the coating was dried to obtain a semi-finished negative electrode sheet.

[0045] The exposed metal foil portion of the aforementioned electrode sheets is processed and welded into tabs, then wound with a separator to form a core. The core is then wrapped in aluminum-plastic film to create a semi-finished cell, which is then injected with electrolyte. After formation and capacity testing, the finished lithium-ion battery is obtained.

[0046] Example 3

[0047] Positive electrode active material powder, conductive carbon, carbon nanotubes, and PVDF were mixed in a mass ratio of 94:2:1:3. NMP was then added in a high-speed mixer and uniformly mixed to form a slurry with a solid content of 45-80%. This slurry was coated onto one side of a 12-micron thick aluminum foil using a transfer coating machine and dried, maintaining a coating weight of 17.5 mg / cm² per unit area after drying. The same process was then repeated on the other side of the aluminum foil to obtain a semi-finished positive electrode sheet. The positive electrode sheet was then rolled to a compaction density of 3.0 g / cm³ to obtain a rolled positive electrode sheet.

[0048] Artificial graphite powder, conductive carbon, carbon nanotubes, CMC, and SBR were mixed in a mass ratio of 93:2:2:3. Deionized water was then added in a high-speed mixer and the mixture was homogeneously mixed to form a slurry with a solid content of 48%. Hard carbon powder, conductive carbon, carbon nanotubes, CMC, and SBR were also mixed in a mass ratio of 93:2:2:3. Deionized water was then added in a high-speed mixer and the mixture was homogeneously mixed to form a slurry with a solid content of 48%. Both slurries were then coated onto one side of an 8-micron thick copper foil using a double-layer coating apparatus, with hard carbon on top and graphite on the bottom, at a weight ratio of 15:85. The coating was dried, maintaining a dry coating weight of 8.5 mg / cm². The same process was then repeated on the other side of the copper foil, and the coating was dried to obtain a semi-finished negative electrode sheet.

[0049] The exposed metal foil portion of the aforementioned electrode sheets is processed and welded into tabs, then wound with a separator to form a core. The core is then wrapped in aluminum-plastic film to create a semi-finished cell, which is then injected with electrolyte. After formation and capacity testing, the finished lithium-ion battery is obtained.

[0050] The above process was repeated in Examples 4-10 and Comparative Examples 1-2. The results are shown in Table 1 below.

[0051] Table 1

[0052]

[0053]

[0054] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0055] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a negative electrode sheet for a lithium-ion battery, characterized in that, include: 1) Graphite powder, conductive carbon, carbon nanotubes, sodium carboxymethyl cellulose, styrene-butadiene rubber and water are mixed to obtain the first slurry; 2) Hard carbon powder, conductive carbon, carbon nanotubes, sodium carboxymethyl cellulose, styrene-butadiene rubber and water are mixed to obtain a second slurry; 3) The first slurry is coated on one side of the copper foil, and the second slurry is coated on the surface of the first slurry. Then, a drying process is performed to obtain a copper foil with a one-sided coating. 4) The first slurry is coated on the other side of the copper foil with one side coating, the second slurry is coated on the surface of the first slurry, and then dried to obtain the negative electrode sheet of the lithium-ion battery. The mass ratio of the hard carbon powder to the graphite powder is 1:(1-3).

2. The method according to claim 1, characterized in that, The graphite powder is artificial graphite powder. The mass ratio of the artificial graphite powder, conductive carbon, carbon nanotubes, sodium carboxymethyl cellulose, and styrene-butadiene rubber is 93:2:2:

3. The solid content of the first slurry is 48%.

3. The method according to claim 1, characterized in that, The solid content of the second slurry is 48%.

4. The method according to claim 1, characterized in that, The first slurry is coated onto one side of the copper foil, and the second slurry is coated onto the surface of the first slurry using a double-layer coating equipment. The mass ratio of the second slurry to the first slurry is 1:(1-3).

5. The method according to claim 1, characterized in that, The thickness of the copper foil is 8 micrometers; The density of the coating on the copper foil with one side coating is 8.5 mg / cm³. 2 .

6. A lithium battery semi-finished cell, characterized in that, The lithium battery semi-finished cell includes a lithium-ion battery negative electrode sheet obtained by the method according to any one of claims 1-5.

7. A lithium battery, characterized in that, The lithium battery includes the lithium battery semi-finished cell as described in claim 6.

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