Lithium battery negative plate, preparation method thereof and lithium battery

Abstract

The invention provides a lithium battery negative plate, a preparation method thereof and a lithium battery, and the lithium battery negative plate is provided with a first coating and a second coating which are sequentially laminated along a direction far away from a current collector, the negative electrode active material in the first coating comprises a first silicon-based material and a first carbon material; a negative electrode active material in the second coating comprises a second silicon-based material and a second carbon material; the silicon content in the first coating is higher than that in the second coating, and the multiplying power of the first carbon material in the first coating is smaller than that of the second carbon material in the second coating. Negative electrode active materials in the first coating and the second coating comprise silicon-based materials, so that the capacity of a battery prepared from the lithium battery negative electrode plate is favorably improved; the total proportion of the silicon content is reduced, and the discharge platform voltage of the battery is improved; and the carbon material in the negative active material in the second coating has higher multiplying power, so that the migration of lithium ions is accelerated, and the resistance is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a lithium battery negative electrode sheet, and in particular to a lithium battery negative electrode sheet and a preparation method thereof, and a lithium battery. Background Art

[0002] With the rapid advancement of modern technology and the continuous changes in people's lifestyles, the demand for portable electronic devices is growing. Various smart terminals such as mobile phones, laptops, and tablets have become widely popular. These devices require high-energy density, lightweight, compact, and stable batteries to provide power support. At the same time, people's expectations for the usage time of devices are getting higher and higher, and they hope to use them for a long time without frequent charging. In addition, as the consumer electronics market continues to expand and refine, new application scenarios and products continue to emerge, and the requirements for batteries are becoming more diverse. For example, wearable devices have stricter restrictions on battery size and weight, while some high-performance electronic devices have higher requirements for the battery's rapid charging and discharging capabilities.

[0003] Traditional battery technology has limitations in terms of energy and the environment, failing to fully meet the demands of green environmental protection and sustainable development. Lithium-ion batteries, with their relatively high energy density, low self-discharge rate, long cycle life, and environmental friendliness, have gradually become the mainstream choice in the consumer battery field, driving the development and transformation of the entire consumer electronics industry. Therefore, a preparation method to improve the performance of lithium-ion batteries is urgently needed.

[0004] Improving energy density, fast charging performance, safety performance and reducing costs are the goals of the lithium-ion battery industry. In the existing technology, the energy density of lithium batteries is improved by increasing the loading amount of active materials in the electrode. However, if the loading amount of active materials in the electrode is increased by increasing the thickness of the active material layer, it will lead to large battery polarization. In addition, the thicker battery electrode will also lead to an increase in the diffusion path of lithium ions and electrons, and the inhomogeneity of the internal and external polarization in the thickness direction of the electrode will be aggravated, thereby leading to an increase in resistance. If the loading amount of active materials in the electrode is increased through the active material layer, the porosity of the active material layer will be lower, the path of lithium ion movement in the thickness direction of the electrode will be longer, and the contact area between the active material and the electrolyte will be reduced, the electrolyte infiltration will face difficulties, the reaction site of the electrode will be reduced, and the internal resistance of the battery will increase, which will lead to a series of problems such as increased battery temperature, and poor rate performance and cycle performance.

[0005] CN116682939A discloses a negative electrode sheet and a lithium ion battery and a preparation method thereof, wherein the negative electrode sheet comprises a negative electrode current collector and an active material coating arranged on both sides of the negative electrode current collector; the active material coating comprises a first coating close to the negative electrode current collector and a second coating away from the negative electrode current collector; the negative electrode active material of the first coating comprises capacity-type graphite and silicon dioxide SiO x Negative electrode material; the negative electrode active material of the second coating includes rate-type graphite and silicon-carbon SiC negative electrode material.

[0006] CN115799446A discloses a lithium-ion battery negative electrode sheet, a preparation method and a lithium-ion battery thereof, comprising a current collector, a first coating and a second coating; the first coating is coated on the current collector, and the second coating is coated on the first coating; the first coating comprises graphite, and the second coating comprises a silicon negative electrode material.

[0007] The lithium battery negative electrode sheets disclosed in the prior art all have certain defects, and there is a problem that it is impossible to improve the battery's energy density while reducing the internal resistance during the battery cycle. Therefore, it is crucial to develop and design a new type of lithium battery negative electrode sheet, its preparation method, and lithium battery. Summary of the Invention

[0008] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a lithium battery negative electrode sheet, a preparation method thereof, and a lithium battery. The present invention limits the negative electrode active materials in the first coating and the second coating to include silicon-based materials, which helps to improve the capacity of the battery prepared with the lithium battery negative electrode sheet; in addition, the present invention reduces the total proportion of silicon content and improves the discharge platform voltage of the battery; in addition, the present invention limits the carbon material in the negative electrode active material in the second coating to have a higher rate, which accelerates the migration of lithium ions and reduces the resistance.

[0009] To achieve this object, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a lithium battery negative electrode plate, comprising a current collector and a first coating layer and a second coating layer stacked sequentially on at least one side surface of the current collector in a direction away from the current collector;

[0011] The negative electrode active material in the first coating layer includes a first silicon-based material and a first carbon material;

[0012] The negative electrode active material in the second coating layer includes a second silicon-based material and a second carbon material;

[0013] The silicon content in the first coating layer is higher than that in the second coating layer, and the ratio of the first carbon material in the first coating layer is lower than the ratio of the second carbon material in the second coating layer.

[0014] The present invention limits the negative electrode active materials in the first coating and the second coating to include silicon-based materials, which helps to improve the capacity of the battery prepared with the lithium battery negative electrode sheet; in addition, the present invention limits the negative electrode active material in the first coating to have a higher silicon content, and the negative electrode active material in the second coating to have a lower silicon content, so that the total silicon content ratio is reduced, and the discharge platform voltage of the battery prepared with the lithium battery negative electrode sheet is improved compared with the discharge platform voltage of the battery prepared with the single-layer coating lithium battery negative electrode sheet of the same proportion; in addition, the present invention limits the carbon material in the negative electrode active material in the second coating to have a larger rate, which accelerates the migration of lithium ions, compensates for the shortcoming of the increased migration distance of lithium ions in thick electrodes, and reduces resistance.

[0015] Preferably, the mass ratio of the first silicon-carbon material to the first carbon material in the negative electrode active material in the first coating layer is (10-20):(80-90), for example, it can be 10:90, 11:89, 12:88, 13:87, 14:86, 15:85, 16:84, 17:83, 18:82, 19:81 or 20:80, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0016] Preferably, the magnification of the first carbon material is 1C to 3C, for example, it can be 1C, 1.2C, 1.4C, 1.6C, 1.8C, 2C, 2.2C, 2.4C, 2.6C, 2.8C or 3C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0017] In the present invention, when the rate of the first carbon material is 1C~3C, the lithium battery has better performance. This is because the first carbon material at a rate of 1C~3C has a higher specific capacity and compaction, which can effectively increase the capacity of the battery cell and reduce the thickness of the electrode sheet, thereby increasing the energy density of the battery cell; within this rate range, the first carbon material maintains a continuous ion / electron transmission channel, ensuring the full release of the initial discharge capacity and the improvement of energy density; at the same time, it avoids the degradation of the electrode structure caused by high rate, reduces polarization and reduces internal resistance, thereby ensuring that the discharge platform voltage is stable and uniform, and comprehensively optimizing the performance of the lithium battery.

[0018] Preferably, the first silicon-based material includes any one of silicon-carbon material, silicon-oxygen material, pre-magnesium silicon-oxygen material or pre-lithium silicon-oxygen material, or a combination of at least two thereof. Typical but non-limiting combinations include a combination of silicon-carbon material and silicon-oxygen material, a combination of pre-magnesium silicon-oxygen material and pre-lithium silicon-oxygen material, or a combination of silicon-carbon material, silicon-oxygen material and pre-magnesium silicon-oxygen material.

[0019] Preferably, the first carbon material comprises any one or a combination of at least two of graphite, hard carbon or soft carbon. Typical but non-limiting combinations include a combination of graphite and hard carbon, a combination of graphite and soft carbon, or a combination of graphite, hard carbon and soft carbon.

[0020] Preferably, the mass ratio of the second silicon-carbon material to the second carbon material in the negative electrode active material in the second coating layer is (3-7):(93-97), for example, it can be 3:97, 3.5:96.5, 4:96, 4.5:95.5, 5:95, 5.5:94.5, 6:94, 6.5:93.5 or 7:93, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0021] In the present invention, when the mass ratio of the second silicon-based material to the second carbon material is (3-7): (93-97), the lithium battery has better performance. This is because the low content of the second silicon-based material ensures the basic capacity contribution and improves the initial discharge capacity; at the same time, the high proportion of the second carbon material can increase the compaction of the electrode, reduce the thickness of the electrode, and reduce the internal resistance of the electrode, as well as ensure efficient ion transmission during charging and discharging, so that the discharge platform voltage remains high and stable.

[0022] Preferably, the magnification of the second carbon material is 6C-10C, for example, it can be 6C, 6.5C, 7C, 7.5C, 8C, 8.5C, 9C, 9.5C or 10C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0023] In the present invention, when the rate of the second carbon material is 6C to 10C, the lithium battery has better performance. This is because the high-rate second carbon material has excellent lithium ion transmission ability, forming a gradient rate design with the carbon material of the first coating. The outer layer of high-rate second carbon material can efficiently transmit charges during the charging and discharging process, significantly reducing electrode polarization; at the same time, the excellent lithium ion transmission ability of the second carbon material itself alleviates the interface ion accumulation, and works together with the high proportion of carbon material to maintain a tight second coating structure and improve the overall performance of the lithium battery.

[0024] Preferably, the second silicon-based material includes any one of silicon-carbon material, silicon-oxygen material, pre-magnesium silicon-oxygen material or pre-lithium silicon-oxygen material, or a combination of at least two thereof. Typical but non-limiting combinations include a combination of silicon-carbon material and silicon-oxygen material, a combination of pre-magnesium silicon-oxygen material and pre-lithium silicon-oxygen material, or a combination of silicon-carbon material, silicon-oxygen material and pre-magnesium silicon-oxygen material.

[0025] Preferably, the second carbon material comprises any one or a combination of at least two of graphite, hard carbon or soft carbon. Typical but non-limiting combinations include a combination of graphite and hard carbon, a combination of graphite and soft carbon, or a combination of graphite, hard carbon and soft carbon.

[0026] Preferably, the first coating layer further includes a first conductive agent, a first suspending agent and a first binder.

[0027] Preferably, the mass ratio of the negative electrode active material, the first conductive agent, the first suspending agent and the first binder in the first coating layer is (95.8-97.6):(0.1-0.3):(0.2-0.5):(2-4).

[0028] The mass ratio of the negative electrode active material to the first conductive agent in the first coating layer of the present invention is (95.8-97.6):(0.1-0.3), for example, it can be 95.8:0.1, 95.8:0.2, 95.8:0.3, 97.6:0.1, 97.6:0.2 or 97.6:0.3, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0029] The mass ratio of the negative electrode active material to the first suspending agent in the first coating layer of the present invention is (95.8-97.6):(0.2-0.5), for example, it can be 95.8:0.2, 95.8:0.3, 95.8:0.4, 95.8:0.4, 97.6:0.2, 97.6:0.3, 97.6:0.4 or 97.6:0.4, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0030] The mass ratio of the negative electrode active material to the first binder in the first coating layer of the present invention is (95.8-97.6):(2-4), for example, it can be 95.8:2, 95.8:3, 95.8:4, 97.6:2, 97.6:3 or 97.6:4, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] Preferably, the first binder includes binder 1 and binder 2, and the mass ratio of binder 1 to binder 2 is (1.5~2.5):(0.5~1.5), for example, it can be 1.5:0.5, 1.5:0.7, 1.5:1.1, 1.5:1.3, 1.5:1.5, 2.5:0.5, 2.5:0.7, 2.5:1.1, 2.5:1.3 or 2.5:1.5, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0032] Preferably, the second coating layer further includes a second conductive agent, a second suspending agent and a second binder.

[0033] Preferably, the mass ratio of the negative electrode active material, the second conductive agent, the second suspending agent and the second binder in the second coating layer is (95.8-97.6):(0.1-0.3):(0.2-0.5):(2-4).

[0034] The mass ratio of the negative electrode active material to the second conductive agent in the second coating layer of the present invention is (95.8-97.6): (0.1-0.3), for example, it can be 95.8:0.1, 95.8:0.2, 95.8:0.3, 97.6:0.1, 97.6:0.2 or 97.6:0.3, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0035] The mass ratio of the negative electrode active material to the second suspending agent in the second coating layer of the present invention is (95.8-97.6): (0.2-0.5), for example, it can be 95.8:0.2, 95.8:0.3, 95.8:0.4, 95.8:0.4, 97.6:0.2, 97.6:0.3, 97.6:0.4 or 97.6:0.4, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0036] The mass ratio of the negative electrode active material to the second binder in the second coating layer of the present invention is (95.8-97.6):(2-4), for example, it can be 95.8:2, 95.8:3, 95.8:4, 97.6:2, 97.6:3 or 97.6:4, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0037] Preferably, the second binder includes binder three and binder three, and the mass ratio of binder three to binder four is (1.5~2.5):(0.5~1.5), for example, it can be 1.5:0.5, 1.5:0.7, 1.5:1.1, 1.5:1.3, 1.5:1.5, 2.5:0.5, 2.5:0.7, 2.5:1.1, 2.5:1.3 or 2.5:1.5, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0038] Preferably, the first conductive agent and the second conductive agent independently include conductive carbon black (SP) and / or carbon nanotubes (CNTs).

[0039] Preferably, the first suspending agent and the second suspending agent each independently comprise carboxymethyl cellulose (CMC).

[0040] Preferably, the binder 1, binder 2, binder 3 and binder 4 each independently include SBR binder and / or PAA binder.

[0041] Preferably, the thickness of the lithium battery negative electrode sheet is 90 to 200 μm, for example, it can be 90 μm, 110 μm, 130 μm, 150 μm, 170 μm, 190 μm or 200 μm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0042] In a second aspect, the present invention provides a method for preparing the negative electrode sheet of a lithium battery according to the first aspect, the method comprising:

[0043] A double-layer coating technology is used to coat the surface of the current collector with a first negative electrode slurry, and then coat the surface of the first negative electrode slurry with a second negative electrode slurry, and then dry to obtain a lithium battery negative electrode sheet.

[0044] Preferably, the viscosity of the first negative electrode slurry is 2000-3500 mPa·s, and the solid content is 40-55%.

[0045] The viscosity of the first negative electrode slurry in the present invention is 2000-3500 mPa·s, for example, 2000 mPa·s, 2500 mPa·s, 3000 mPa·s or 3500 mPa·s, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0046] The solid content of the first negative electrode slurry of the present invention is 40-55%, for example, it can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54% or 55%, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0047] Preferably, the viscosity of the second negative electrode slurry is 2000-3500 mPa·s, and the solid content is 40-55%.

[0048] The viscosity of the second negative electrode slurry of the present invention is 2000-3500 mPa·s, for example, 2000 mPa·s, 2500 mPa·s, 3000 mPa·s or 3500 mPa·s, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0049] The solid content of the second negative electrode slurry of the present invention is 40-55%, for example, it can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54% or 55%, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0050] The viscosity of the second negative electrode slurry of the present invention is 2000-5000 mPa·s, for example, it can be 2000 mPa·s, 2500 mPa·s, 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, 4500 mPa·s or 5000 mPa·s, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0051] The solid content of the second negative electrode slurry of the present invention is 40-55%, for example, it can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54% or 55%, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0052] Preferably, the method for preparing the first negative electrode slurry comprises:

[0053] The first silicon-carbon material and the first carbon material are dry-mixed, then mixed with the first binder and the first suspending agent colloid, and then mixed with the first conductive agent to obtain the first negative electrode slurry.

[0054] Preferably, the method for preparing the second negative electrode slurry comprises:

[0055] The second silicon-carbon material and the second carbon material are dry-mixed, then mixed with the second binder and the second suspending agent colloid, and then mixed with the second conductive agent to obtain the second negative electrode slurry.

[0056] Preferably, the viscosity of the second suspending agent colloid is 17000-23000 Pa·s, for example, it can be 17000 Pa·s, 18000 Pa·s, 19000 Pa·s, 20000 Pa·s, 21000 Pa·s, 22000 Pa·s, or 23000 Pa·s, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0057] Preferably, the viscosity of the first suspending agent colloid is 17000-23000 Pa·s, for example, it can be 17000 Pa·s, 18000 Pa·s, 19000 Pa·s, 20000 Pa·s, 21000 Pa·s, 22000 Pa·s or 23000 Pa·s, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0058] In a third aspect, the present invention provides a lithium battery, comprising the lithium battery negative electrode sheet described in the first aspect.

[0059] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] The present invention limits the negative electrode active materials in the first coating and the second coating to include silicon-based materials, which helps to improve the capacity of the battery prepared with the lithium battery negative electrode sheet; in addition, the present invention limits the negative electrode active material in the first coating to have a higher silicon content, and the negative electrode active material in the second coating to have a lower silicon content, so that the total silicon content ratio is reduced, and the discharge platform voltage of the battery prepared with the lithium battery negative electrode sheet is improved compared with the discharge platform voltage of the battery prepared with the single-layer coating lithium battery negative electrode sheet of the same proportion; in addition, the present invention limits the carbon material in the negative electrode active material in the second coating to have a larger rate, which accelerates the migration of lithium ions, compensates for the shortcoming of the increased migration distance of lithium ions in thick electrodes, and reduces resistance. DETAILED DESCRIPTION

[0062] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0063] Example 1

[0064] This embodiment provides a lithium battery negative electrode sheet, wherein the thickness of the lithium battery negative electrode sheet is 150 μm, and the lithium battery negative electrode sheet includes a current collector and a first coating layer and a second coating layer stacked in sequence on at least one side surface of the current collector in a direction away from the current collector;

[0065] The negative electrode active material in the first coating layer includes a first silicon-carbon material and a first graphite with a magnification of 2C in a mass ratio of 15:85;

[0066] The negative electrode active material in the second coating layer includes a second silicon-carbon material and a second graphite with a magnification of 8C in a mass ratio of 5:95;

[0067] The first coating layer further includes a first conductive carbon black, a first carboxymethyl cellulose and a first binder, wherein the mass ratio of the negative electrode active material, the first conductive carbon black, the first carboxymethyl cellulose and the first binder (composed of an SBR binder and a PAA binder in a mass ratio of 2:1) in the first coating layer is 96.6:0.2:0.3:3;

[0068] The second coating also includes a second conductive carbon black, a second carboxymethyl cellulose and a second binder. The mass ratio of the negative electrode active material, the second conductive carbon black, the second carboxymethyl cellulose and the second binder (composed of an SBR binder and a PAA binder in a mass ratio of 2:1) in the second coating is 96.6:0.2:0.3:3.

[0069] The preparation method of the lithium battery negative electrode sheet is as follows:

[0070] (1) dry-mixing a first silicon-carbon material with a first graphite, then remixing with a first binder and a first carboxymethyl cellulose glue having a viscosity of 20,000 Pa·s, and then mixing with a first conductive carbon black to obtain a first negative electrode slurry having a viscosity of 2,800 mPa·s and a solid content of 48%;

[0071] (2) dry-mixing the second silicon-carbon material with the second graphite, then remixing with the second binder and the second carboxymethyl cellulose glue having a viscosity of 20,000 Pa·s, and then mixing with the second conductive carbon black to obtain a second negative electrode slurry having a viscosity of 2,800 mPa·s and a solid content of 48%;

[0072] (3) Using double-layer coating technology, the first negative electrode slurry is coated on the surface of the current collector, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry. After drying, the lithium battery negative electrode sheet is obtained.

[0073] Example 2

[0074] This embodiment provides a lithium battery negative electrode sheet, which is the same as that in Example 1, except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 3C in the negative electrode active material of the first coating layer is 17:83, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 10C in the negative electrode active material of the second coating layer is 3:97.

[0075] Example 3

[0076] This embodiment provides a lithium battery negative electrode sheet, which is the same as that in Example 1, except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 1C in the negative electrode active material of the first coating layer is 13:87, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 7C in the negative electrode active material of the second coating layer is 7:93.

[0077] Example 4

[0078] This embodiment provides a lithium battery negative electrode sheet, wherein the thickness of the lithium battery negative electrode sheet is 90 μm, and the lithium battery negative electrode sheet includes a current collector and a first coating layer and a second coating layer stacked in sequence on at least one side surface of the current collector in a direction away from the current collector;

[0079] The negative electrode active material in the first coating layer includes a first silicon-carbon material and a first graphite with a magnification of 3C in a mass ratio of 20:80;

[0080] The negative electrode active material in the second coating layer includes a second silicon-carbon material and a second graphite with a magnification of 10C in a mass ratio of 3:97;

[0081] The silicon content in the first coating layer is higher than that in the second coating layer, and the ratio of the first graphite in the first coating layer is lower than the ratio of the second graphite in the second coating layer.

[0082] The first coating layer further includes first carbon nanotubes, first carboxymethyl cellulose, and a first binder. The mass ratio of the negative electrode active material, the first carbon nanotubes, the first carboxymethyl cellulose, and the first binder (composed of an SBR binder and a PAA binder in a mass ratio of 1.5:1.5) in the first coating layer is 95.8:0.3:0.5:4.

[0083] The second coating also includes second carbon nanotubes, second carboxymethyl cellulose and a second binder. The mass ratio of the negative electrode active material, the second carbon nanotubes, the second carboxymethyl cellulose and the second binder (composed of SBR binder and PAA binder with a mass ratio of 1.5:1.5) in the second coating is 95.8:0.3:0.5:4.

[0084] The preparation method of the lithium battery negative electrode sheet is as follows:

[0085] (1) dry-mixing a first silicon-carbon material with a first graphite, then remixing with a first binder and a first carboxymethyl cellulose glue having a viscosity of 23,000 Pa·s, and then mixing with a first carbon nanotube to obtain a first negative electrode slurry having a viscosity of 3,500 mPa·s and a solid content of 55%;

[0086] (2) dry-mixing the second silicon-carbon material with the second graphite, then remixing with the second binder and the second carboxymethyl cellulose glue having a viscosity of 23,000 Pa·s, and then mixing with the second carbon nanotubes to obtain a second negative electrode slurry having a viscosity of 3,500 mPa·s and a solid content of 55%;

[0087] (3) Using double-layer coating technology, the first negative electrode slurry is coated on the surface of the current collector, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry. After drying, the lithium battery negative electrode sheet is obtained.

[0088] Example 5

[0089] This embodiment provides a lithium battery negative electrode sheet, wherein the thickness of the lithium battery negative electrode sheet is 200 μm, and the lithium battery negative electrode sheet includes a current collector and a first coating layer and a second coating layer stacked in sequence on at least one side surface of the current collector in a direction away from the current collector;

[0090] The negative electrode active material in the first coating layer includes a first silicon-carbon material and a first graphite with a magnification of 1C in a mass ratio of 10:90;

[0091] The negative electrode active material in the second coating layer includes a second silicon-carbon material and a second graphite with a magnification of 6C in a mass ratio of 7:93;

[0092] The silicon content in the first coating layer is higher than that in the second coating layer, and the ratio of the first graphite in the first coating layer is lower than the ratio of the second graphite in the second coating layer.

[0093] The first coating layer further includes a first conductive carbon black, a first carboxymethyl cellulose and a first binder, wherein the mass ratio of the negative electrode active material, the first conductive carbon black, the first carboxymethyl cellulose and the first binder (composed of an SBR binder and a PAA binder in a mass ratio of 2.5:0.5) in the first coating layer is 97.6:0.1:0.2:2;

[0094] The second coating also includes a second conductive carbon black, a second carboxymethyl cellulose and a second binder. The mass ratio of the negative electrode active material, the second conductive carbon black, the second carboxymethyl cellulose and the second binder (composed of SBR binder and PAA binder with a mass ratio of 2.5:0.5) in the second coating is 97.6:0.1:0.2:2.

[0095] The preparation method of the lithium battery negative electrode sheet is as follows:

[0096] (1) dry-mixing a first silicon-carbon material with a first graphite, then remixing with a first binder and a first carboxymethyl cellulose glue having a viscosity of 17000 Pa·s, and then mixing with a first conductive carbon black to obtain a first negative electrode slurry having a viscosity of 2000 mPa·s and a solid content of 40%;

[0097] (2) dry-mixing the second silicon-carbon material with the second graphite, then remixing with the second binder and the second carboxymethyl cellulose glue having a viscosity of 17000 Pa·s, and then mixing with the second conductive carbon black to obtain a second negative electrode slurry having a viscosity of 2000 mPa·s and a solid content of 40%;

[0098] (3) Using double-layer coating technology, the first negative electrode slurry is coated on the surface of the current collector, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry. After drying, the lithium battery negative electrode sheet is obtained.

[0099] Example 6

[0100] This embodiment provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 2C in the negative electrode active material of the first coating layer is 15:85, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 8C in the negative electrode active material of the second coating layer is 1:99.

[0101] Example 7

[0102] This embodiment provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 0.5C in the negative electrode active material of the first coating is 15:85, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 8C in the negative electrode active material of the second coating is 5:95.

[0103] Example 8

[0104] This embodiment provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 5C in the negative electrode active material of the first coating is 15:85, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 8C in the negative electrode active material of the second coating is 5:95.

[0105] Example 9

[0106] This embodiment provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 2C in the negative electrode active material of the first coating is 15:85, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 4C in the negative electrode active material of the second coating is 5:95.

[0107] Example 10

[0108] This embodiment provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 2C in the negative electrode active material of the first coating is 15:85, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 12C in the negative electrode active material of the second coating is 5:95.

[0109] Comparative Example 1

[0110] This comparative example provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 2C in the negative electrode active material of the first coating is 5:95, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 8C in the negative electrode active material of the second coating is 5:95.

[0111] Comparative Example 2

[0112] This comparative example provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 8C in the negative electrode active material of the first coating is 5:95, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 2C in the negative electrode active material of the second coating is 15:85.

[0113] Comparative Example 3

[0114] This comparative example provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 2C in the negative electrode active material of the first coating is 15:85, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 2C in the negative electrode active material of the second coating is 5:95.

[0115] Comparative Example 4

[0116] This comparative example provides a lithium battery negative electrode sheet, which is the same as Example 1 except that the mass ratio of the first silicon-carbon material to the first graphite with a magnification of 8C in the negative electrode active material of the first coating is 15:85, and the mass ratio of the second silicon-carbon material to the second graphite with a magnification of 2C in the negative electrode active material of the second coating is 5:95.

[0117] A lithium battery was prepared using the lithium battery negative electrode sheet provided in the above embodiment and comparative example. The preparation method is as follows: using the lithium battery negative electrode sheet as the negative electrode, using the lithium cobalt oxide material positive electrode sheet as the positive electrode, and 1M LiPF6 (EC:DEC:DMC=1:1:1, volume ratio) as the electrolyte, and performing winding, liquid injection, packaging and vacuum sealing processes to obtain a lithium ion soft pack battery;

[0118] The prepared lithium battery was subjected to an energy density test. The test method is: energy density of lithium battery = battery discharge energy / battery weight. The discharge energy test method is: charge to 4.5V at a constant current and constant voltage of 0.2C, with a cutoff current of 0.02C, and then discharge to 2.5V at 0.2C for 3 cycles. The discharge energy density is shown in Table 1.

[0119] The prepared lithium battery was subjected to an initial discharge capacity test. The test method was as follows: constant current and constant voltage charging at 0.2C at 25±3°C to 4.5V, with a cutoff current of 0.02C, and then discharged at 0.2C to 2.5V. The initial discharge capacity of the lithium battery is shown in Table 1.

[0120] The discharge platform voltage of the prepared lithium battery was tested using the following method: constant current and constant voltage charging at 0.2C at 25±3°C to 4.5V, with a cutoff current of 0.02C, followed by discharge at 0.2C to 2.5V. The ratio of the energy during discharge to the capacity of the lithium battery during discharge is the discharge platform voltage, and the obtained discharge platform voltages are shown in Table 1.

[0121] The internal resistance of the prepared lithium battery was tested by measuring the AC impedance at AC 1 kHz and the shipping voltage. The internal resistance obtained by the test is shown in Table 1.

[0122] Table 1

[0123]

[0124]

[0125] From Table 1, we can get:

[0126] (1) The lithium-ion battery prepared using the lithium battery negative electrode sheets provided in Examples 1 to 5 has a higher energy density, a larger initial discharge capacity, a higher discharge platform voltage, and a lower resistance;

[0127] (2) By comparing Example 1 with Example 6, it can be seen that the mass ratio of the second silicon-based material to the second carbon material in the negative electrode active material of the second coating layer will affect the performance of the negative electrode sheet of the lithium battery, thereby affecting the performance of the lithium battery; when the mass ratio of the second silicon-based material to the second carbon material is (3-7): (93-97), the lithium battery has better performance. This is because the low content of the second silicon-based material ensures the basic capacity contribution and improves the first discharge capacity; at the same time, the high proportion of the second carbon material can increase the compaction of the electrode sheet, reduce the thickness of the electrode sheet, reduce the internal resistance of the electrode sheet, and ensure efficient ion transmission during charging and discharging, so that the discharge platform voltage remains high and stable;

[0128] (3) By comparing Example 1 with Examples 7 and 8, it can be seen that the rate of the first carbon material in the negative electrode active material of the first coating will affect the performance of the negative electrode sheet of the lithium battery, thereby affecting the performance of the lithium battery; when the rate of the first carbon material is 1C to 3C, the lithium battery has better performance. This is because the first carbon material at a rate of 1C to 3C has a higher specific capacity and compaction, which can effectively increase the capacity of the battery cell and reduce the thickness of the electrode sheet, thereby increasing the energy density of the battery cell; within this rate range, the first carbon material maintains a continuous ion / electron transmission channel, ensuring the full release of the first discharge capacity and the improvement of the energy density; at the same time, it avoids the degradation of the electrode structure caused by high rates, reduces polarization and reduces internal resistance, thereby ensuring that the discharge platform voltage is stable and uniform, and comprehensively optimizing the performance of the lithium battery;

[0129] (4) By comparing Example 1 with Examples 9 and 10, it can be seen that the rate of the second carbon material in the negative electrode active material of the second coating layer will affect the performance of the negative electrode sheet of the lithium battery, thereby affecting the performance of the lithium battery; when the rate of the second carbon material is 6C to 10C, the lithium battery has better performance. This is because the high-rate second carbon material has excellent lithium ion transmission ability, forming a gradient rate design with the carbon material of the first coating layer. The high-rate second carbon material in the outer layer can efficiently transmit charges during the charge and discharge process, significantly reducing electrode polarization; at the same time, the excellent lithium ion transmission ability of the second carbon material itself alleviates the interfacial ion accumulation, and works together with the high proportion of carbon material to maintain a tight second coating structure, thereby improving the overall performance of the lithium battery;

[0130] (5) By comparing Example 1 with Comparative Examples 1 to 4, it can be seen that the present invention contributes to improving the capacity of the battery prepared with the negative electrode sheet of the lithium battery by limiting the negative electrode active material in the first coating and the second coating to include silicon-based materials; in addition, the present invention reduces the total silicon content by limiting the negative electrode active material in the first coating to have a higher silicon content and the negative electrode active material in the second coating to have a lower silicon content, and improves the discharge platform voltage of the battery prepared with the negative electrode sheet of the lithium battery compared with the discharge platform voltage of the battery prepared with the single-layer coating negative electrode sheet of the same proportion; in addition, the present invention accelerates the migration of lithium ions by limiting the carbon material in the negative electrode active material in the second coating to have a larger rate, thereby compensating for the shortcoming of the increased migration distance of lithium ions in thick electrodes and reducing resistance.

[0131] 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 thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and disclosure scope of the present invention.

Claims

1. A lithium battery negative electrode sheet, characterized in that: The lithium battery negative electrode sheet includes a current collector and a first coating layer and a second coating layer stacked in sequence on at least one side surface of the current collector in a direction away from the current collector; The negative electrode active material in the first coating layer includes a first silicon-based material and a first carbon material; The negative electrode active material in the second coating layer includes a second silicon-based material and a second carbon material; The silicon content in the first coating layer is higher than that in the second coating layer, and the ratio of the first carbon material in the first coating layer is lower than the ratio of the second carbon material in the second coating layer.

2. The negative electrode sheet for lithium battery according to claim 1, characterized in that: The mass ratio of the first silicon-carbon material to the first carbon material in the negative electrode active material in the first coating layer is (10-20):(80-90); Preferably, the magnification of the first carbon material is 1C to 3C; Preferably, the first silicon-based material comprises any one or a combination of at least two of a silicon-carbon material, a silicon-oxygen material, a pre-magnesium silicon-oxygen material or a pre-lithium silicon-oxygen material; Preferably, the first carbon material includes any one of graphite, hard carbon or soft carbon, or a combination of at least two of them.

3. The lithium battery negative electrode sheet according to claim 1 or 2, characterized in that: The mass ratio of the second silicon-carbon material to the second carbon material in the negative electrode active material in the second coating layer is (3-7):(93-97); Preferably, the magnification of the second carbon material is 6C to 10C; Preferably, the second silicon-based material includes any one or a combination of at least two of a silicon-carbon material, a silicon-oxygen material, a pre-magnesium silicon-oxygen material or a pre-lithium silicon-oxygen material; Preferably, the second carbon material includes any one of graphite, hard carbon or soft carbon, or a combination of at least two of them.

4. The negative electrode sheet for lithium batteries according to any one of claims 1 to 3, characterized in that: The first coating layer also includes a first conductive agent, a first suspending agent and a first binder; Preferably, the mass ratio of the negative electrode active material, the first conductive agent, the first suspending agent and the first binder in the first coating layer is (95.8-97.6):(0.1-0.3):(0.2-0.5):(2-4); Preferably, the first binder includes a binder 1 and a binder 2, and the mass ratio of the binder 1 to the binder 2 is (1.5-2.5):(0.5-1.5); Preferably, the second coating layer further comprises a second conductive agent, a second suspending agent and a second binder; Preferably, the mass ratio of the negative electrode active material, the second conductive agent, the second suspending agent and the second binder in the second coating layer is (95.8-97.6): (0.1-0.3): (0.2-0.5): (2-4); Preferably, the second binder includes binder three and binder three, and the mass ratio of binder three to binder four is (1.5-2.5):(0.5-1.5).

5. The lithium battery negative electrode sheet according to any one of claims 1 to 4, characterized in that: The thickness of the lithium battery negative electrode sheet is 90 to 200 μm.

6. A method for preparing a lithium battery negative electrode sheet according to any one of claims 1 to 5, characterized in that: The preparation method comprises: A double-layer coating technology is used to coat the surface of the current collector with a first negative electrode slurry, and then coat the surface of the first negative electrode slurry with a second negative electrode slurry, and then dry to obtain a lithium battery negative electrode sheet.

7. The preparation method according to claim 6, characterized in that The viscosity of the first negative electrode slurry is 2000-5000 mPa·s and the solid content is 40-55%; Preferably, the viscosity of the second negative electrode slurry is 2000-5000 mPa·s, and the solid content is 40-55%.

8. The preparation method according to claim 6 or 7, characterized in that The method for preparing the first negative electrode slurry includes: The first silicon-carbon material and the first carbon material are dry-mixed, then mixed with the first binder and the first suspending agent colloid, and then mixed with the first conductive agent to obtain the first negative electrode slurry.

9. The preparation method according to any one of claims 6 to 8, characterized in that The method for preparing the second negative electrode slurry comprises: The second silicon-carbon material and the second carbon material are dry-mixed, then mixed with the second binder and the second suspending agent colloid, and then mixed with the second conductive agent to obtain the second negative electrode slurry.

10. A lithium battery, characterized in that: The lithium battery comprises the lithium battery negative electrode sheet according to any one of claims 1 to 5.

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