Negative electrode sheet and battery

Abstract

This application relates to the field of battery technology, specifically to a negative electrode sheet and a battery. The negative electrode sheet includes: (a) a negative electrode current collector; (b) a first film layer disposed on at least one surface of the negative electrode current collector; the first film layer includes a first negative electrode active material, which comprises primary particulate graphite and a silicon-based material; (c) a second film layer disposed on the surface of the first film layer away from the negative electrode current collector; the second film layer includes a second negative electrode active material, which comprises secondary particulate graphite. The negative electrode sheet of this invention not only reduces structural damage caused by the volume expansion of silicon-based materials but also improves kinetic performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to negative electrode sheets and batteries. Background Technology

[0002] Lithium-ion batteries (LIBs), as the most widely used energy storage system, have been rapidly applied in various fields since their commercialization. In recent years, with the development of portable electronic devices and the increasing demand for electric vehicles, the energy density of current LIBs has gradually become insufficient to meet people's needs, making the development of high-energy-density LIBs an urgent task.

[0003] Silicon anode materials are widely recognized as next-generation lithium-ion battery anode materials due to their high specific capacity and low potential. However, silicon-based materials experience significant volume expansion during charging and discharging, which can easily damage the electrode structure. Furthermore, compared to traditional lithium-ion graphite anodes, silicon anodes exhibit poorer kinetic performance. These shortcomings limit the development and application of silicon anode materials. Therefore, researching and solving the expansion and kinetic problems of silicon-based anode materials and anode sheets is a core development direction for related industry and academia. Summary of the Invention

[0004] In view of this, the present invention provides a negative electrode and a battery. The negative electrode features a layered design of active material layers containing graphite and silicon-based materials. Primary granular graphite and silicon-based materials are placed in a first film layer near the negative electrode current collector, while secondary granular graphite is placed in a second film layer. This not only reduces structural damage caused by the volume expansion of the silicon-based material but also improves kinetic performance. Furthermore, by adjusting the areal capacity-related technical parameters of the first and second film layers, the areal capacity of the second film layer is made slightly larger than that of the first film layer. This reduces the overall polarization of the electrode, helps alleviate the over-intercalation problem of the active material, balances the lithium-ion distribution between the first and second film layers, and improves reversible capacity.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] In a first aspect, the present invention provides a negative electrode sheet, the negative electrode sheet comprising:

[0007] (a) Negative electrode current collector;

[0008] (b) A first film layer disposed on at least one side surface of the negative electrode current collector; the first film layer includes a first negative electrode active material, the first negative electrode active material including primary particulate graphite and silicon-based material;

[0009] (c) A second film layer, wherein the second film layer is disposed on the surface of the first film layer away from the negative electrode current collector; the second film layer includes a second negative electrode active material, wherein the second negative electrode active material includes secondary particulate graphite.

[0010] In an embodiment of the present invention, the negative electrode sheet satisfies the following relationship:

[0011] C2×K2×L2=C1×[K1×X+G×(1-X)]×L1×α

[0012] Wherein, C2 is the coating surface density of the second film layer.

[0013] K2 is the specific capacity of the secondary graphite particles in the second film layer.

[0014] L2 represents the mass percentage of the second negative electrode active material in the second membrane layer.

[0015] C1 is the coating density of the first film layer.

[0016] K1 is the specific capacity of the primary graphite particles in the first film layer.

[0017] X represents the mass percentage of primary graphite particles in the first film layer within the first anode active material, and G represents the specific capacity of the silicon-based material in the first film layer.

[0018] L1 represents the mass percentage of the first negative electrode active material in the first membrane layer.

[0019] α is a correction factor.

[0020] In this embodiment of the invention, the value of K2 ranges from 340 to 360 mAh / g.

[0021] In this embodiment of the invention, the value of L2 ranges from 95% to 97%.

[0022] In this embodiment of the invention, the value of K1 ranges from 340 to 360 mAh / g.

[0023] In this embodiment of the invention, the value of X ranges from 0.5 to 0.97.

[0024] In this embodiment of the invention, the value of G ranges from 1300 to 1700 mAh / g.

[0025] In this embodiment of the invention, the value of L1 ranges from 94.2% to 95.7%.

[0026] In this embodiment of the invention, the value of α ranges from 1.01 to 1.06.

[0027] Preferably, the primary granular graphite is carbon-coated primary granular graphite.

[0028] Preferably, the carbon-coated primary particulate graphite has a particle coating degree >50%, a carbon coating amount >2%, and a particle size D. 50 It is 6–10 μm.

[0029] In embodiments of the present invention, silicon-based materials include silicon-oxygen materials and / or silicon-carbon composite materials.

[0030] As a preferred option, the particle size D of the silicon-based material is... 50 It is 6–10 μm.

[0031] Preferably, silicon-based materials account for 3% to 50% of the total mass of the first negative electrode active material.

[0032] Preferably, the secondary particulate graphite is carbon-coated secondary particulate graphite.

[0033] Preferably, the carbon-coated secondary particulate graphite has a particle coating degree of >50% and a carbon coating amount of >2%.

[0034] In an embodiment of the present invention, the first film layer comprises the following components by mass percentage:

[0035] The active material of the first negative electrode is 94.2%–95.7%;

[0036] The first conductive agent is 1.5% to 1.7%;

[0037] The first adhesive is 2.8% to 5%.

[0038] In an embodiment of the present invention, the second film layer comprises the following components in the indicated mass percentages:

[0039] The second negative electrode active material is 95%–97%;

[0040] The second conductive agent is 1% to 1.5%;

[0041] The second adhesive is 2% to 3.5%.

[0042] Preferably, the first adhesive accounts for a larger mass percentage of the first film layer than the second adhesive accounts for a larger mass percentage of the second film layer.

[0043] Secondly, the present invention provides a battery comprising the aforementioned negative electrode.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] This invention employs a layered design for the active material layer of the negative electrode, which contains graphite and silicon-based materials. Primary granular graphite and silicon-based materials are placed in a first film layer near the negative electrode current collector, while secondary granular graphite is placed in a second film layer. Specifically, the primary granular graphite in the first film layer is blended with silicon-based materials to increase the specific capacity of the negative electrode, and the high structural strength of the primary granular graphite is used to reduce structural damage caused by the volume expansion of the silicon-based materials. The secondary granular graphite in the second film layer enhances kinetics. Based on this design, this invention can significantly reduce the film resistance, full-charge rebound rate, and DCR (direct current resistance) growth rate of the negative electrode, effectively improving the stability of the electrode structure during long-cycle operation and enhancing the cycle performance of the battery.

[0046] This invention adjusts the areal density of the first and second film layers, the mass ratio of the negative electrode active material, and other technical parameters related to areal capacity, so that the negative electrode sheet satisfies the following relationship: C2×K2×L2=C1×[K1×X+G×(1-X)]×L1×α. This makes the areal capacity of the second film layer slightly larger than that of the first film layer, reduces the overall polarization of the electrode sheet, helps to alleviate the problem of over-intercalation of active material, balances the lithium ion distribution between the first and second film layers, and improves the reversible capacity.

[0047] Therefore, this invention can effectively improve the fast-charging performance and cycle stability of silicon-graphite anodes while maintaining high energy density. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the negative electrode structure of the present invention.

[0049] Wherein, 1 represents the negative electrode current collector, 2 represents the first film layer, and 3 represents the second film layer. Detailed Implementation

[0050] This invention discloses a negative electrode sheet and a battery. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments. Those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0051] In the description of this invention, it should be noted that the terms "first," "second," etc., are used for descriptive purposes only and are not intended to indicate or imply relative importance.

[0052] In the description of this invention, the list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0053] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0054] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0055] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0056] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0057] Specifically, the present invention adopts the following technical solution:

[0058] In a first aspect, the present invention provides a negative electrode sheet, the negative electrode sheet comprising:

[0059] (a) Negative electrode current collector;

[0060] (b) A first film layer disposed on at least one side surface of the negative electrode current collector; the first film layer includes a first negative electrode active material, the first negative electrode active material including primary particulate graphite and silicon-based material;

[0061] (c) A second film layer, wherein the second film layer is disposed on the surface of the first film layer away from the negative electrode current collector; the second film layer includes a second negative electrode active material, wherein the second negative electrode active material includes secondary particulate graphite.

[0062] In this invention, on the one hand, the volume change of silicon during lithium insertion / extraction can compress graphite, causing graphite particle displacement and lamellar cracking. This can also lead to demolding and material shedding at the electrode layer. Since primary graphite particles have greater structural strength than secondary graphite particles, this invention uses a composite material of primary graphite particles and silicon-based materials as the first negative electrode active material in the first film layer. This can improve the electrode's specific capacity while reducing the structural damage to graphite and the electrode caused by the stress from the silicon-based material. On the other hand, the second film layer is closer to the electrolyte, preferentially undergoing lithium insertion, and has a higher lithium-ion concentration around the active material particles compared to the first film layer. Therefore, it places higher demands on the material's kinetics. Since the kinetic performance of secondary graphite particles is superior to that of primary graphite particles, this invention uses secondary graphite particles as the second negative electrode active material in the second film layer, which can improve the battery's kinetic performance.

[0063] In an embodiment of the present invention, the negative electrode sheet satisfies the following relationship:

[0064] C2×K2×L2=C1×[K1×X+G×(1-X)]×L1×α

[0065] Wherein, C2 is the coating surface density of the second film layer.

[0066] K2 is the specific capacity of the secondary graphite particles in the second film layer.

[0067] L2 represents the mass percentage of the second negative electrode active material in the second membrane layer.

[0068] C1 is the coating density of the first film layer.

[0069] K1 is the specific capacity of the primary graphite particles in the first film layer.

[0070] X represents the mass percentage of primary graphite particles in the first film layer within the first anode active material.

[0071] G represents the specific capacity of the silicon-based material in the first film layer.

[0072] L1 represents the mass percentage of the first negative electrode active material in the first membrane layer.

[0073] α is a correction factor.

[0074] Because the lithium insertion / extraction kinetics of the first film layer are slightly lower than those of the second film layer, and the active material in the second film layer preferentially undergoes lithium insertion / extraction, this difference can lead to over-intercalation of the active material in the second film layer and incomplete lithium insertion in the first film layer, resulting in reversible capacity decay. When the negative electrode sheet of the present invention satisfies the above relationship, it can ensure that the areal capacity of the second film layer is slightly greater than that of the first film layer, which helps to alleviate the problem of over-intercalation of the active material and balance the lithium ion distribution between the first and second film layers.

[0075] Preferably, the value of C2 is in the range of 50–100 g / m³. 2 For example, the value of C2 ranges from 50 g / m³. 2 55g / m 2 60g / m 2 65g / m 2 70g / m 2 72g / m 2 75g / m 2 80g / m 2 85g / m 2 90g / m 2 95g / m 2 100g / m 2 Any value in the range or any value within the range formed by any pair of the above values.

[0076] In a specific embodiment of the present invention, the value of C2 ranges from 58 to 85 g / m³. 2 .

[0077] In this embodiment of the invention, the value of K2 ranges from 340 to 360 mAh / g. For example, the value of K2 can be any one of 340 mAh / g, 342 mAh / g, 344 mAh / g, 346 mAh / g, 348 mAh / g, 350 mAh / g, 352 mAh / g, 354 mAh / g, 356 mAh / g, 358 mAh / g, or 360 mAh / g, or any value within the range formed by any two of the above values.

[0078] In this embodiment of the invention, the value of L2 ranges from 95% to 97%. For example, the value of L2 can be any value among 95%, 95.2%, 95.4%, 95.6%, 95.8%, 96%, 96.2%, 96.4%, 96.6%, 96.8%, and 97%, or any value within the range formed by any pair of the above values.

[0079] Preferably, the value of C1 is in the range of 30–60 g / m³. 2 For example, the value of C1 ranges from 30 g / m³. 2 35g / m 2 40g / m 2 43.5g / m 2 45g / m 2 45.5g / m 2 50g / m 2 55g / m 2 57g / m 2 60g / m 2Any value in the range or any value within the range formed by any pair of the above values.

[0080] In a specific embodiment of the present invention, the value of C1 ranges from 30 to 57 g / m³. 2 .

[0081] Preferably, the value of C2 / C1 is 1 to 3. For example, the value of C2 / C1 is any one of the following: 1.0, 1.04, 1.2, 1.4, 1.5, 1.52, 1.54, 1.56, 1.57, 1.58, 1.6, 1.62, 1.64, 1.66, 1.68, 1.7, 2.0, 2.2, 2.4, 2.6, 2.8, 2.83, 3.0, or any value within the range of any two of the above values.

[0082] In a specific embodiment of the present invention, the value of C2 / C1 is 1.04 to 2.83.

[0083] In this embodiment of the invention, the value of K1 ranges from 340 to 360 mAh / g. For example, the value of K1 can be any one of 340 mAh / g, 342 mAh / g, 344 mAh / g, 346 mAh / g, 348 mAh / g, 350 mAh / g, 352 mAh / g, 354 mAh / g, 356 mAh / g, 358 mAh / g, or 360 mAh / g, or any value within the range formed by any pair of the aforementioned values.

[0084] In this embodiment of the invention, the value of X ranges from 0.5 to 0.97. For example, the value of X is any value among 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, and 0.97, or any value within the range formed by any pair of the above values.

[0085] In this embodiment of the invention, the value of G ranges from 1300 to 1700 mAh / g. For example, the value of G can be any one of 1300 mAh / g, 1350 mAh / g, 1400 mAh / g, 1450 mAh / g, 1500 mAh / g, 1550 mAh / g, 1600 mAh / g, 1650 mAh / g, or any value within the range formed by any pair of the aforementioned values.

[0086] In this embodiment of the invention, the value of L1 ranges from 94.2% to 95.7%. For example, the value of L1 can be any value among 94.2%, 94.4%, 94.6%, 94.8%, 95%, 95.2%, 95.4%, 95.6%, and 95.7%, or any value within the range formed by any pair of the above values.

[0087] In this embodiment of the invention, the value of α ranges from 1.01 to 1.06. For example, α can be any value from 1.01, 1.02, 1.03, 1.04, 1.05, and 1.06, or any value within the range formed by any two of the above values. Since the lithium insertion / extraction kinetics of the first film layer are slightly lower than those of the second film layer, and the active material in the second film layer preferentially undergoes lithium insertion / extraction, this difference can lead to over-intercalation of the active material in the second film layer and incomplete lithium insertion in the first film layer, resulting in reversible capacity decay. α within the above range ensures that the areal capacity of the second film layer is slightly larger than that of the first film layer, helping to mitigate the over-intercalation problem of the active material and balance the lithium-ion distribution between the first and second film layers.

[0088] Preferably, the primary graphite particles are carbon-coated primary graphite particles. Carbon coating further enhances the kinetic properties of the primary graphite particles. Furthermore, when the secondary graphite particles in the second film are also carbon-coated, carbon coating of the primary graphite particles can reduce the kinetic difference between the first and second films, thus balancing the overall potential distribution of the electrode.

[0089] Preferably, the carbon-coated primary particulate graphite has a particle coating degree >50%, a carbon coating amount >2%, and a particle size D. 50 The particle size is 6–10 μm. For example, the carbon coating amount of the carbon-coated primary graphite particles is any value selected from 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, and 10%, or any value within the range of any pair of values ​​mentioned above. For example, the particle size D of the carbon-coated primary graphite particles is… 50 It is any value among 6μm, 7μm, 8μm, 9μm, and 10μm, or any value within the range of any pair of the above values.

[0090] In embodiments of the present invention, silicon-based materials include silicon-oxygen materials and / or silicon-carbon composite materials.

[0091] As a preferred option, the particle size D of the silicon-based material is... 50 The particle size is 6–10 μm. For example, the particle size D of silicon-based materials... 50 It is any value among 6μm, 7μm, 8μm, 9μm, and 10μm, or any value within the range of any pair of the above values.

[0092] Preferably, the silicon-based material accounts for 3% to 50% of the total mass of the first negative electrode active material. For example, the silicon-based material accounts for any value from 3%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, and 50% of the total mass of the first negative electrode active material, or any value within a range formed by any pair of the above values. When the proportion exceeds this range, the structural damage to the electrode layers caused by the excessive lithium intercalation expansion of the silicon-based material leads to a rapid decline in cell performance; when the proportion is less than this range, the energy density advantage brought by using silicon-based materials cannot be fully utilized.

[0093] Preferably, the secondary graphite particles are carbon-coated secondary graphite particles. After carbon coating, the kinetic properties of the secondary graphite particles can be further improved.

[0094] Preferably, the carbon-coated secondary particulate graphite has a particle coating degree >50% and a carbon coating amount >2%. For example, the carbon coating amount of the carbon-coated secondary particulate graphite is any value selected from 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any value within the range of any pair of the above values.

[0095] In an embodiment of the present invention, the first film layer comprises the following components by mass percentage:

[0096] The active material of the first negative electrode is 94.2%–95.7%;

[0097] The first conductive agent is 1.5% to 1.7%;

[0098] The first adhesive is 2.8% to 5%.

[0099] In an embodiment of the present invention, the second film layer comprises the following components in the indicated mass percentages:

[0100] The second negative electrode active material is 95%–97%;

[0101] The second conductive agent is 1% to 1.5%;

[0102] The second adhesive is 2% to 3.5%.

[0103] Preferably, the first binder accounts for a larger mass percentage of the first film layer than the second binder. Because the volume expansion of the silicon-based material particles in the first film layer is much greater than that of the primary graphite particles, the first film layer requires greater adhesion to maintain charge connectivity between the active material particles, thereby ensuring full utilization of the specific capacity while reducing electrical insulation and lithium plating.

[0104] In embodiments of the present invention, the first conductive agent and the second conductive agent independently include at least one of conductive carbon black (Super P), vapor-generated carbon fiber (VGCF), graphene, and carbon nanotubes (CNTs).

[0105] Preferably, the first conductive agent contains carbon nanotubes, and the mass percentage of carbon nanotubes in the first film layer is 0 to 0.08%. The linear structure of carbon nanotubes can significantly improve the long-range transport of electrons in the electrode and compensate for the polarization of the electrode layer caused by the poor conductivity of silicon-based materials.

[0106] Preferably, both the first adhesive and the second adhesive are water-based adhesives.

[0107] In embodiments of the present invention, the first adhesive and the second adhesive independently include at least one of styrene-butadiene rubber (SBR), nitrile rubber, butadiene rubber, modified styrene-butadiene rubber, sodium carboxymethyl cellulose (CMC-Na), polyacrylic acid (PAA), sodium polyacrylate (PAA-Na), waterborne polyacrylonitrile copolymer, and polyacrylate.

[0108] In this embodiment of the invention, the negative electrode current collector is one of homogeneous copper foil, porous copper foil, or copper foil with a carbon coating.

[0109] Secondly, the present invention provides a battery comprising the aforementioned negative electrode.

[0110] In embodiments of the present invention, the battery structure includes, but is not limited to, button cells, pouch cells, cylindrical cells, etc.

[0111] This application does not impose any particular restrictions on the positive electrode, separator, and electrolyte in the battery. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0112] The reagents, instruments, and materials used in this invention can all be obtained through commercial channels.

[0113] The present invention will be further illustrated below with reference to the embodiments:

[0114] Example 1:

[0115] 1. Preparation of the negative electrode:

[0116] a) Preparation of the first film layer

[0117] The first negative electrode active material (a mixture of carbon-coated primary granular artificial graphite and pre-lithium silicate material in a mass ratio of 8:2), conductive carbon black Super P (SP), single-walled carbon nanotubes (SWCNTs), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95.7%:1.42%:0.08%:1.9%:0.9%, deionized water was added, and the mixture was stirred to form a homogeneous slurry, thus obtaining the first film layer slurry. The carbon coating amount of the primary granular artificial graphite was 3%, and the particle coating degree was >50%; the primary granular artificial graphite and the pre-lithium silicate material... 50 All are 8μm.

[0118] The first film layer slurry was uniformly coated on both sides of the negative electrode current collector (8μm copper foil), and the coating surface density of the first film layer was 45g / m². 2 .

[0119] b) Preparation of the second film layer

[0120] The second negative electrode active material (carbon-coated secondary particulate artificial graphite with a carbon coating amount of 3% and a particle coating degree of >50%), SP, PAA, and SBR are mixed in a mass ratio of 97%:1%:1.3%:0.7%, deionized water is added, and the mixture is stirred into a uniform slurry to obtain the second film layer slurry.

[0121] The second film layer slurry was uniformly coated onto the first film layer, and the coating surface density of the second film layer was 70.46 g / m². 2 .

[0122] c) Preparation of negative electrode sheet

[0123] After drying and cold pressing, the negative electrode sheet is obtained, as shown in the schematic diagram below. Figure 1 As shown, the compacted density is 1.6 g / cm³. 3 .

[0124] 2. Preparation of the positive electrode:

[0125] The positive electrode active material (lithium nickel cobalt manganese oxide LiNi) 0.8 Co 0.1 Mn 0.1 SP, carbon nanotubes, and PVDF5130 binder were mixed in a ratio of 97:1.6:0.4:1. NMP was then added and stirred to form a uniform and stable positive electrode slurry. This slurry was then uniformly coated onto a positive electrode current collector (13μm aluminum foil) with a coating density of 195 g / m². 2 After drying and cold pressing, a positive electrode sheet is obtained with a compaction density of 3.4 g / cm³. 3 .

[0126] 3. Selection of the separator membrane:

[0127] Polypropylene membrane with a thickness of 12μm was selected as the base membrane for the separator.

[0128] 4. Cell assembly process:

[0129] Arrange the positive electrode, separator, negative electrode, and separator in sequence, and use winding as the assembly method.

[0130] 5. Preparation of electrolyte:

[0131] LiPF6 was dissolved in a solvent containing ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1, with a LiPF6 concentration of 1.2 mol / L.

[0132] 6. Battery manufacturing:

[0133] Electrolyte is injected into the dry battery cell and soaked for 24 hours. Then, it is formed at 45°C. The formation process is as follows: charge to 3.4V at 0.05C and then charge to 3.75V at 0.2C. The battery cell is manufactured after aging at room temperature for 24 hours.

[0134] Example 2:

[0135] The difference between this embodiment and Embodiment 1 is that the formulations of the first and second films of the negative electrode are different.

[0136] The formulation of the first membrane layer is as follows: the first negative electrode active material: SP:SWCNT:PAA:SBR = 94.2%: 1.64%: 0.06%: 2.8%: 1.3%;

[0137] The formulation of the second membrane layer is as follows: second negative electrode active material: SP:PAA:SBR = 96%:1%:2%:1%.

[0138] Example 3:

[0139] The difference between this embodiment and Embodiment 1 is that the coating surface density of the first film layer and the second film layer is different.

[0140] The surface density of the first film layer is 43.7 g / m². 2 ;

[0141] The surface density of the second film layer is 71.82 g / m². 2 .

[0142] Example 4:

[0143] The difference between this embodiment and Embodiment 1 is that the coating surface density of the first film layer and the second film layer are different, and the formulations of the first film layer and the second film layer of the negative electrode sheet are different.

[0144] The surface density of the first film layer is 43.5 g / m². 2 ;

[0145] The surface density of the second film layer is 70 g / m². 2 ;

[0146] The formulation of the first membrane layer is as follows: the first negative electrode active material: SP:SWCNT:PAA:SBR = 94.2%: 1.64%: 0.06%: 2.8%: 1.3%;

[0147] The formulation of the second membrane layer is as follows: the second negative electrode active material is SP:PAA:SBR = 95%:1.5%:2.3%:1.2%.

[0148] Example 5:

[0149] The difference between this embodiment and Embodiment 1 is that the coating surface density of the first film layer and the second film layer is different, and the formulation of the first film layer of the negative electrode sheet is different.

[0150] The surface density of the first film layer is 45.5 g / m². 2 ;

[0151] The surface density of the second film layer is 72 g / m². 2 ;

[0152] The formulation of the first membrane layer is as follows: the first negative electrode active material is SP:SWCNT:PAA:SBR = 95%:1.53%:0.07%:2.3%:1.1%.

[0153] Example 6:

[0154] The difference between this embodiment and Embodiment 1 is that the specific capacities of the graphite and silicon oxide materials used are different, and the mixing ratio of graphite and silicon oxide materials in the first film layer is different. In Embodiment 6, the specific capacity of the primary graphite particles is 340 mAh / g, the specific capacity of the secondary graphite particles is 360 mAh / g, and the specific capacity of the pre-lithiated silicon oxide material is 1300 mAh / g. The negative electrode active material in the first film layer is a mixture of carbon-coated primary granular artificial graphite and pre-lithiated silicon oxide material, with a mass ratio of 9.7:0.3. The formulations and processes of the first and second film layers are the same as in Embodiment 1.

[0155] The surface density of the first film layer is 56.5 g / m². 2 ;

[0156] The surface density of the second film layer is 58.5 g / m². 2 .

[0157] Example 7:

[0158] This embodiment differs from Embodiment 1 in that the specific capacity of the silicon-oxygen material used is different, and the mixing ratio of graphite and silicon-oxygen material in the first film layer is different. The specific capacity of the pre-lithiated silicon-oxygen material used in Embodiment 7 is 1700 mAh / g. In Embodiment 7, the negative electrode active material of the first film layer is a mixture of carbon-coated primary particulate artificial graphite and pre-lithiated silicon-oxygen material in a mass ratio of 5:5. The formulation of the first film layer is as follows: first negative electrode active material: SP:SWCNT:PAA:SBR = 94.2%: 1.53%: 0.07%: 2.8%: 1.4%.

[0159] The surface density of the first film layer coating is 30 g / m². 2 ;

[0160] The surface density of the second film layer is 85 g / m². 2 .

[0161] Comparative Example 1:

[0162] The difference between this comparative example and Example 1 is that the negative electrode includes a film layer.

[0163] The negative electrode active material (a mixture of carbon-coated primary granular artificial graphite, carbon-coated secondary granular artificial graphite, and pre-lithium silicon oxide material in a mass ratio of 4.5:4.5:1), SP, SWCNT, PAA, and SBR are mixed in a mass ratio of 95.7%:1.42%:0.08%:1.9%:0.9%, deionized water is added, and the mixture is stirred to form a uniform slurry, thus obtaining the film slurry.

[0164] The film slurry was uniformly coated onto the negative electrode current collector (8μm copper foil), and the coating surface density was 115.46 g / m². 2 .

[0165] Comparative Example 2:

[0166] The difference between this comparative example and Example 1 is that the negative electrode is the same as the two films of the negative electrode in Example 1, with the two films interchanged.

[0167] a) Preparation of the first film layer

[0168] The first negative electrode active material (carbon-coated secondary particulate artificial graphite with a carbon coating amount of 3% and a particle coating degree of >50%), SP, PAA, and SBR are mixed in a mass ratio of 97%:1%:1.3%:0.7%, deionized water is added and stirred into a uniform slurry to obtain the first membrane layer slurry.

[0169] The first film layer slurry was uniformly coated onto the negative electrode current collector (8μm copper foil), and the coating surface density of the first film layer was 70.46 g / m². 2 .

[0170] b) Preparation of the second film layer

[0171] The second negative electrode active material (a mixture of carbon-coated primary particulate artificial graphite and pre-lithium silicate material in a mass ratio of 8:2), conductive carbon black Super P (SP), single-walled carbon nanotubes (SWCNTs), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95.7%:1.42%:0.08%:1.9%:0.9%, deionized water was added, and the mixture was stirred to form a homogeneous slurry, thus obtaining the second film layer slurry. The carbon coating amount of the primary particulate artificial graphite was 3%, and the particle coating degree was >50%; the carbon coating amount of the primary particulate artificial graphite and the pre-lithium silicate material was... 50 All are 8μm.

[0172] The second film layer slurry is uniformly coated onto the first film layer, and the coating surface density of the second film layer is 45 g / m². 2 .

[0173] c) Preparation of negative electrode sheet

[0174] After drying and cold pressing, a negative electrode sheet is obtained with a compaction density of 1.6 g / cm³. 3 .

[0175] Comparative Example 3:

[0176] The difference between this comparative example and Example 1 is that the negative electrode is the same as the negative electrode in Example 1, with the two layers of graphite material interchanged.

[0177] a) Preparation of the first film layer

[0178] The first negative electrode active material (a mixture of carbon-coated secondary particulate artificial graphite and pre-lithium silicon oxide material in a mass ratio of 8:2), conductive carbon black Super P (SP), single-walled carbon nanotubes (SWCNT), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95.7%:1.42%:0.08%:1.9%:0.9%, deionized water was added and stirred into a uniform slurry to obtain the first film layer slurry.

[0179] The first film layer slurry was uniformly coated onto the negative electrode current collector (8μm copper foil), and the coating surface density of the first film layer was 45g / m². 2 .

[0180] b) Preparation of the second film layer

[0181] The second negative electrode active material (carbon-coated primary particulate artificial graphite), SP, PAA, and SBR are mixed in a mass ratio of 97%:1%:1.3%:0.7%, deionized water is added, and the mixture is stirred into a uniform slurry to obtain the second film layer slurry.

[0182] The second film layer slurry was uniformly coated onto the first film layer, and the coating surface density of the second film layer was 70.46 g / m². 2 .

[0183] c) Preparation of negative electrode sheet

[0184] After drying and cold pressing, a negative electrode sheet is obtained with a compaction density of 1.6 g / cm³. 3 .

[0185] Comparative Example 4:

[0186] The difference between this comparative example and Example 1 is that the coating surface density of the first and second films of the negative electrode is different.

[0187] The surface density of the first film layer is 43.2 g / m². 2 ;

[0188] The surface density of the second film layer is 72.33 g / m². 2 .

[0189] Comparative Example 5:

[0190] The difference between this comparative example and Example 1 is that the coating surface density of the first and second films of the negative electrode is different.

[0191] The surface density of the first film layer is 45.8 g / m². 2 ;

[0192] The surface density of the second film layer is 69.59 g / m². 2 .

[0193] Comparative Example 6:

[0194] The difference between this comparative example and Example 6 is that the negative electrode includes a film layer.

[0195] The negative electrode active material (a mixture of carbon-coated primary granular artificial graphite, carbon-coated secondary granular artificial graphite, and pre-lithium silicon oxide material in a mass ratio of 4.925:4.925:0.15), SP, SWCNT, PAA, and SBR are mixed in a mass ratio of 95.7%:1.42%:0.08%:1.9%:0.9%, deionized water is added, and the mixture is stirred to form a uniform slurry, thus obtaining the film slurry.

[0196] The film slurry was uniformly coated onto the negative electrode current collector (8μm copper foil), and the coating surface density of the film was 115g / m². 2 .

[0197] Comparative Example 7:

[0198] The difference between this comparative example and Example 1 is that the negative electrode includes a film layer.

[0199] The negative electrode active material (a mixture of carbon-coated primary granular artificial graphite, carbon-coated secondary granular artificial graphite, and pre-lithium silicon oxide material in a mass ratio of 3.75:3.75:2.5), SP, SWCNT, PAA, and SBR are mixed in a mass ratio of 94.2%:1.53%:0.07%:2.8%:1.4%, deionized water is added, and the mixture is stirred to form a uniform slurry, thus obtaining the film slurry.

[0200] The film slurry was uniformly coated onto the negative electrode current collector (8μm copper foil), and the coating surface density of the film was 90 g / m². 2 Its electrode surface capacity, i.e., Q1+Q2, is the same as that in Example 7.

[0201] Battery parameter measurement and performance testing

[0202] The parameters and performance of the batteries in the above embodiments and comparative examples were measured and tested. The specific methods are as follows:

[0203] 1. Surface volume measurement

[0204] The areal capacity Q1 of the first film layer and the areal capacity Q2 of the second film layer are calculated using formulas (I) and (II), respectively:

[0205] Q1=C1×[K1×X+G×(1-X)]×L1 Formula (I)

[0206] Q2=C2×K2×L2 Formula (II)

[0207] Wherein, C1 is the coating surface density of the first film layer, K1 is the specific capacity of the primary graphite particles in the first film layer, X is the mass percentage of the primary graphite particles in the first film layer in the first negative electrode active material, G is the specific capacity of the silicon-based material in the first film layer, and L1 is the mass percentage of the first negative electrode active material in the first film layer.

[0208] C2 is the coating surface density of the second film layer, K2 is the specific capacity of the secondary particle graphite in the second film layer, and L2 is the mass percentage of the second negative electrode active material in the second film layer.

[0209] Then, using formula (III), calculate the correction factor α:

[0210] Formula (III) for α = Q2 / Q1

[0211] 2. Membrane resistance test

[0212] Experimental equipment: BER1300 diaphragm resistor (IEST Yuaneng Technology), with an electrode diameter of 14mm.

[0213] The testing method is as follows:

[0214] 1) Do not place a negative electrode on the diaphragm resistance meter. Set the test pressure in the MRMS software to 25 MPa and hold for 25 seconds. Click "Start Test" and then click "Reset to Zero" after the result appears.

[0215] 2) Cut the rolled negative electrode sheet into rectangles approximately 5cm × 10cm and place them between the two electrodes of the film resistance meter. Set the test parameters on the MRMS software, with the pressure and holding time the same as in step 1), and start the test. The software automatically reads data such as the thickness, resistance, resistivity, and conductivity of the negative electrode sheet. Six locations are randomly selected for testing on each negative electrode sheet, and the average value is the film resistance value of the negative electrode sheet.

[0216] 3. Full charge rebound rate test

[0217] 1) Use a micrometer to measure the thickness of the negative electrode sheet after rolling and record it as C1;

[0218] 2) Charge the battery after capacity testing to 100% SOC using a 1C charging current according to the standard test procedure. Disassemble the fully charged battery and use a micrometer to measure the thickness of the fully charged negative electrode. Record the electrode thickness as C2.

[0219] 3) Full charge rebound rate = (C2-C1) / C1×100%.

[0220] 4. First-time efficiency test

[0221] 1) In a 25℃ constant temperature chamber, charge at a constant current of 1C to 4.25V, let stand for 30 minutes, and then charge at a constant current of 0.2C to 4.25V. Record the total charging capacity A.

[0222] 2) After standing for 30 minutes, discharge at a constant current of 1C to 2.5V and record the discharge capacity B;

[0223] 3) First-effect = B / A × 100%.

[0224] 5. Loop testing

[0225] 1) Let it stand in a constant temperature chamber at 25℃ for 30 minutes, then charge it to 4.25V with a constant current of 1C;

[0226] 2) After standing for 30 minutes, discharge at a constant current of 1C to 2.5V and record the initial discharge capacity C1;

[0227] 3) Cycle 1) to 2) until the battery discharge capacity is lower than 80% of the initial discharge capacity C1, then stop the test and record the discharge capacity Cn, where n is the number of cycles; the capacity retention rate after n cycles = discharge capacity Cn / initial discharge capacity × 100%.

[0228] 6. DCR growth rate test

[0229] 1) At 25℃, charge the battery to 4.25V using a constant current of 1C;

[0230] 2) After standing for 30 minutes, discharge at a constant current of 1C for 30 minutes (adjust to 50% SOC);

[0231] 3) Let stand for 60 minutes;

[0232] 4) Discharge at a constant current of 2C for 30s, record the open circuit voltages U1 and U2 and the discharge current I before and after discharge, and the battery DCR = (U1-U2) / I, so the battery DCR is D1;

[0233] The battery with SOC below 80% of the initial capacity in the 1C cycle test was tested again according to the process from 1) to 4), and the DCR after the cycle was D2.

[0234] DCR growth rate = (D2-D1) / D1.

[0235] The measurement and testing results are shown in the table below.

[0236] Table 1. Areal capacity of the first membrane layer

[0237]

[0238] Note: C1 is the coating surface density of the first film layer, K1 is the specific capacity of primary graphite particles in the first film layer, X is the mass percentage of graphite particles (in Examples 1-7 and Comparative Example 3, primary graphite particles are used; in Comparative Examples 1, 6, and 7, the sum of primary and secondary graphite particles are used) in the first negative electrode active material, G is the specific capacity of silicon-based material in the first film layer, L1 is the mass percentage of the first negative electrode active material in the first film layer, and Q1 is the areal capacity of the first film layer.

[0239] Table 2. Areal capacity of the second membrane layer

[0240]

[0241]

[0242] Note: C2 is the coating areal density of the second film layer, K2 is the specific capacity of the secondary particle graphite in the second film layer, L2 is the mass percentage of the second negative electrode active material in the second film layer, and Q2 is the areal capacity of the second film layer. The calculation formula corresponding to the K2 value in Comparative Example 2 in Table 2 is actually K2×X+G×(1-X).

[0243] Table 3

[0244] Example 1 23686 23921 1.01 Example 2 23315 23675 1.02 Example 3 23001 24383 1.06 Example 4 22537 23275 1.03 Example 5 23774 24444 1.03 Example 6 19941 20428 1.02 Example 7 28967 29401 1.02 Comparative Example 1 49723 / / Comparative Example 2 23921 21533 0.90 Comparative Example 3 23686 23921 1.01 Comparative Example 4 22738 24556 1.08 Comparative Example 5 24107 23626 0.98 Comparative Example 6 40088 / / Comparative Example 7 58286 / /

[0245] Table 4 Performance Test Results

[0246]

[0247]

[0248] Based on the membrane resistance data, Examples 1-5 are superior to Comparative Examples 1-5. Specifically, in Example 2, the increased content of the conductive agent resulted in a decrease in membrane resistance compared to Example 1. In Example 3, the areal capacitance of the second membrane layer was slightly increased compared to Example 1, leading to a slight decrease in membrane resistance, but the overall difference was not significant. Comparative Example 4 also showed improved membrane resistance due to the larger overall graphite content in its electrode compared to the other comparative examples.

[0249] From the data on the first efficiency of the cells, the overall difference between the examples and the comparative examples is small. This is mainly because the active materials are the same. The first efficiency of the comparative examples 1 to 5 is slightly lower, mainly due to the lag in the electrochemical reaction caused by polarization, which leads to incomplete delithiation of some silicon-based materials at low SOC.

[0250] From the perspective of full charge rebound, Examples 1-5 are significantly better than Comparative Examples 1-5. This is mainly due to the layered design and the high strength of the primary graphite particles, which reduces the damage to the electrode structure caused by silicon volume expansion.

[0251] Based on the cycling and DCR growth data, Examples 1-5 are significantly superior to Comparative Examples 1-5. Specifically, Comparative Examples 2-5 exhibit better cycling performance than Comparative Example 1, demonstrating that the layered structure design effectively improves the stability of the electrode structure during long-term cycling. Furthermore, the DCR growth of Examples 1-5 is superior to that of Comparative Examples 4-5, proving that under constrained conditions, the overall polarization of the electrode is further reduced.

[0252] Compared to Example 1, Comparative Example 1, due to the absence of a layered structure design, suffers from the volume expansion of silicon, which, without the constraint of the surface material area, disrupts the overall structural compactness of the electrode, exacerbating electrical insulation and demolding phenomena during cycling. Furthermore, due to the differences in specific capacity and lithium intercalation speed between graphite and silicon, a potential difference is generated inside the electrode, reducing fast charging and power performance.

[0253] Compared to Example 1, in Comparative Example 2, the positions of the first and second film layers are interchanged, with silicon located on the surface of the negative electrode. The electrode rebound caused by lithium intercalation expansion is greater than that of a single-layer electrode, and its full-charge rebound rate is the highest among all examples and comparative examples. This loose electrode structure will cause the electrical connection between the conductive agent and the active material to break, and accelerate the failure of the binder in the second film layer, resulting in cycle deterioration. Furthermore, the contact between silicon and the surface electrolyte will produce side reactions, further reducing the first efficiency and cycle performance.

[0254] Compared to Example 1, Comparative Example 3 swapped the positions of primary and secondary graphite particles. The primary particles were closer to the electrolyte in the second film layer. Since the fast charging capability of the primary particles was less than that of the secondary particles, their lithium intercalation rate was also lower when faced with a large number of lithium ions. As a result, the goal of reducing the overall potential difference of the electrode was not achieved, polarization occurred, which led to increased DCR growth and poor cycle performance.

[0255] Compared to Example 1, in Comparative Example 4, since α is greater than 1.06, the areal capacity of the upper and lower layers is significantly different. Because lithium ions have a long migration distance to travel from the second film layer to the first film layer, lithium ions preferentially insert into the second film layer. As a result, the first film layer will exhibit incomplete lithium insertion. During cycling, incomplete lithium insertion of silicon will cause loss of active lithium and deteriorate the cycling performance.

[0256] Compared to Example 1, in Comparative Example 5, since α is less than 1.01, lithium intercalation preferentially occurs in the second film layer near the electrolyte. The low capacity of the second film layer leads to graphite over-intercalation, which causes graphite cracking, increased polarization, and aggravated growth in film resistance and DCR, resulting in poor cycle performance.

[0257] Examples 6 and 7 are electrochemical systems with different energy densities than Example 1. Compared to Comparative Example 6, Example 6 shows improvements in kinetics and cycle life, and Example 7 shows improvements in performance compared to Comparative Example 7, demonstrating that the present invention is applicable to different graphite-doped silicon systems and effectively improves performance.

[0258] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A negative electrode sheet characterized by comprising: The negative electrode includes: (a) Negative electrode current collector; (b) A first film layer, wherein the first film layer is disposed on at least one side surface of the negative electrode current collector; the first film layer includes a first negative electrode active material, wherein the first negative electrode active material is primary particulate graphite and silicon-based material; (c) A second film layer, wherein the second film layer is disposed on the surface of the first film layer away from the negative electrode current collector; the second film layer includes a second negative electrode active material, wherein the second negative electrode active material is secondary particulate graphite; The negative electrode sheet satisfies the following relationship: C2×K2×L2=C1×[K1×X+G×(1-X)]×L1×α Wherein, C2 is the coating surface density of the second film layer. K2 is the specific capacity of the secondary graphite particles in the second film layer. L2 represents the mass percentage of the second negative electrode active material in the second membrane layer. C1 is the coating density of the first film layer. K1 is the specific capacity of the primary graphite particles in the first film layer. X represents the mass percentage of primary graphite particles in the first film layer within the first anode active material. G represents the specific capacity of the silicon-based material in the first film layer. L1 represents the mass percentage of the first negative electrode active material in the first membrane layer. α is a correction factor, and the value of α ranges from 1.01 to 1.06; The value of X ranges from 0.8 to 0.

97.

2. The negative electrode sheet according to claim 1, characterized in that, The value of K2 ranges from 340 to 360 mAh / g. The value of L2 ranges from 95% to 97%. The value of K1 ranges from 340 to 360 mAh / g. The value of G ranges from 1300 to 1700 mAh / g. The value of L1 ranges from 94.2% to 95.7%.

3. The negative electrode sheet according to claim 1, characterized by The primary particulate graphite is carbon-coated primary particulate graphite. The carbon-coated primary particle graphite has a particle coating degree of >50%, a carbon coating amount of >2%, and a particle size D 50 of 6-10 μm.

4. The negative electrode sheet according to claim 1, characterized by The silicon-based materials include silicon-oxygen materials and / or silicon-carbon composite materials; The particle size D of the silicon-based material is 6 to 10 μm. 50 is 6 to 10 μm.

5. The negative electrode sheet according to claim 1, characterized by The silicon-based material accounts for 3% to 50% of the total mass of the first negative electrode active material.

6. The negative electrode sheet according to claim 1, wherein The secondary particle graphite is carbon-coated secondary particle graphite. The carbon-coated secondary particulate graphite has a particle coating degree of >50% and a carbon coating amount of >2%.

7. The negative electrode sheet according to claim 1, wherein The first film layer comprises the following components in the indicated mass percentages: The active material of the first negative electrode is 94.2%–95.7%; The first conductive agent is 1.5% to 1.7%; First adhesive 2.8%–5%; The second film layer comprises the following components in the indicated mass percentages: The second negative electrode active material is 95%–97%; The second conductive agent is 1% to 1.5%; The second adhesive is 2% to 3.5%.

8. The negative electrode sheet according to claim 7, characterized by The first adhesive accounts for a greater proportion of the mass of the first film layer than the second adhesive accounts for the greater proportion of the mass of the second film layer.

9. A battery, characterized by The battery includes the negative electrode sheet according to any one of claims 1 to 8.

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