Silicon-based negative electrode sheet and preparation method thereof

Abstract

This invention discloses a silicon-based negative electrode sheet and its preparation method. The silicon-based negative electrode sheet includes a current collector, on the same surface of which a first coating and a second coating are disposed. The first coating is located on both sides of the second coating. The first coating is applied using a first negative electrode slurry, and the second coating is applied using a second negative electrode slurry. The first negative electrode slurry includes polyacrylic acid, and the second negative electrode slurry includes an alkaline salt of polyacrylic acid. The silicon-based negative electrode sheet provided by this invention exhibits low contamination, low or no edge effect, and small volume change during use.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries, and particularly relates to a silicon-based negative electrode sheet and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics and power applications due to their high energy density, fast charging capability, and high safety. However, with technological advancements, the demands for fast charging performance and battery capacity are increasing. In fast-charging batteries, edge effects and interface issues on the electrode sheets cause lithium-ion insertion / extraction at the edges of the cell to occur faster than in the main body (the central region). This leads to lithium plating at the edges of the electrode sheets, resulting in decreased battery capacity and even short circuits and fires. Furthermore, for silicon-based lithium-ion batteries, besides the capacity reduction caused by lithium plating, the silicon-based anode, while possessing the highest theoretical specific capacity, undergoes significant volume expansion during charging and discharging. This causes repeated rupture and formation of the SEI film on the electrode surface. Consequently, the silicon anode experiences rapid capacity decay and further lithium plating during lithium insertion / extraction cycles, leading to the peeling of the silicon-based material coating and significantly limiting the further development of silicon-based materials in the battery industry.

[0003] Chinese patent CN117199291A discloses a negative electrode active slurry, a battery negative electrode sheet, and a lithium-ion secondary battery. The raw material components of the negative electrode active slurry include a silicon-based negative electrode material and an inorganic lithium compound. The inorganic lithium compound reacts with active lithium to form a protective layer. In this raw material composition, the silicon-based negative electrode material, as the active material in the battery, undergoes an electrochemical reaction during charging and discharging, serving as the energy source of the battery. Adding a very small amount of inorganic lithium compound additive to the negative electrode slurry allows it to react during charging and discharging to generate a robust protective layer, reducing lithium dendrite growth and thus inhibiting lithium plating on the negative electrode surface, thereby extending the cycle life of the silicon-based negative electrode. However, the added inorganic lithium compound is toxic, and because it easily dissolves in the electrolyte, leakage can easily lead to pollution problems. Summary of the Invention

[0004] One of the objectives of this invention is to provide a silicon-based anode sheet that produces less pollution and has a lower edge lithium plating effect during use.

[0005] To achieve the above objectives, the present invention provides a silicon-based negative electrode sheet, the silicon-based negative electrode sheet including a current collector, a first coating and a second coating being provided on the same surface of the current collector along the Y direction, the first coating being located on both sides of the second coating, the first coating being coated with a first negative electrode slurry, the second coating being coated with a second negative electrode slurry, the first negative electrode slurry including polyacrylic acid, and the second negative electrode slurry including an alkaline salt of polyacrylic acid.

[0006] The silicon-based negative electrode provided by this invention utilizes the basic hydroxyl groups in the alkali salt of polyacrylic acid to neutralize the aggregated carboxyl groups in the polyacrylic acid. These carboxyl functional groups can form hydrogen bonds or covalent bonds with groups on the silicon surface, enhancing the adhesion between silicon particles and between silicon particles and the current collector. The highly flexible main chain allows it to withstand the stress caused by volume changes during lithium insertion / extraction in the silicon-based material, alleviating volume changes and reducing the likelihood of active material shedding and electrode cracking, thus ensuring the stability and integrity of the silicon electrode during cycling. Furthermore, the bonds formed between the carboxyl groups of polyacrylic acid and the silicon-based material, due to the direct hydrogen or covalent bond interaction between the carboxyl functional groups and silicon surface groups, produce a better effect than those formed with polyacrylic acid alone. This results in a smaller volume change in the second coating, and the lithium-ion transport efficiency of the first coating is lower than that of the second coating, meaning the lithium-ion insertion rate of the first coating is lower than that of the second coating. Therefore, the edge lithium plating effect of the first coating is suppressed. Because the alkali salts of polyacrylic acid are low-toxicity or non-toxic substances, and because polyacrylic acid and its alkali salts are high-molecular-weight substances, they are not easily dissolved in the electrolyte during use and exist in the form of colloids or gels, resulting in less pollution during use.

[0007] Furthermore, the lithium deposition coefficients k at the beginning and end of the silicon-based negative electrode satisfy Equation I, which is as follows:

[0008]

[0009] r1 is the ratio of the thickness d1 of the first coating to the thickness d2 of the second coating; r2 is the ratio of the width w1 of the first coating to the width w2 of the second coating; W1 is the addition ratio of binder in the first negative electrode slurry; W2 is the addition ratio of binder in the second negative electrode slurry; when k > 0, the electrode will not exhibit head and tail lithium plating; when k ≤ 0, there is a risk of head and tail lithium plating; and the smaller the value of k, the greater the risk of abnormal head and tail lithium plating.

[0010] When the head and tail lithium plating coefficients are within the range disclosed in the technical solution of this invention, the edge effect completely disappears. Furthermore, the inventors were pleasantly surprised to find that the silicon-based negative electrode does not exhibit rapid expansion in actual use.

[0011] The lithium deposition coefficient k of the silicon-based negative electrode in this application is greater than 0.

[0012] Furthermore, the range of r1 values ​​is 0.55 to 1.67; the range of r2 values ​​is 0.11 to 3.

[0013] When the values ​​of r1 and r2 are within the above range, it helps to ensure the uniformity and stability of the electrode material. Since the first coating and the second coating have different lithium ion insertion rates, when the difference in width and thickness between the first coating and the second coating is too large, there will be squeezing or gaps at the connection between the first coating and the second coating, which will easily lead to lithium plating.

[0014] Furthermore, by adjusting the thickness and width of the first and second coatings, the electrode structure can be adjusted, thereby improving the stability of the coating structure to meet the electrode performance design requirements.

[0015] When the value of k satisfies the relationship and range disclosed in the technical solution of this invention, if the values ​​of r1, r2, W1, and W2 are further optimized according to the preferred range disclosed in this invention, the inventors have found that the prepared silicon-based negative electrode not only completely eliminates the edge effect, but also achieves various electrode structures with different parameter combinations, thereby realizing different electrochemical performances. In actual use, the silicon-based negative electrode exhibits no rapid expansion, and its cold-pressing peel strength is further increased. The stability and integrity of the silicon electrode during cycling are further enhanced, resulting in lower resistance and higher rate capability.

[0016] Furthermore, the thickness d1 of the first coating is 70μm to 110μm, and the thickness d2 of the second coating is 60μm to 100μm.

[0017] The thickness of the first coating and the thickness of the second coating can be selected within the preferred range disclosed in the technical solution of the present invention. Those skilled in the art can select a suitable thickness according to actual needs. For example, the thickness of the first coating can be 100 μm, and correspondingly, the thickness of the second coating can be 100 μm.

[0018] Furthermore, the width w1 of the first coating is 30mm to 225mm, and the width w2 of the second coating is 75mm to 270mm.

[0019] The widths of the first coating and the second coating can be selected within the preferred range disclosed in the technical solution of the present invention. Those skilled in the art can select appropriate widths according to actual needs. For example, the width of the first coating can be 30 mm, and correspondingly, the width of the second coating can be 270 mm.

[0020] Furthermore, the addition ratio of the binder in the first negative electrode slurry W1 is 5% to 15%, and the addition ratio of the binder in the second negative electrode slurry W2 is 5% to 15%.

[0021] The addition ratio of the binder can be selected within the preferred range disclosed in the present invention. Those skilled in the art can select a suitable addition ratio according to actual needs. For example, the addition ratio of the binder in the first negative electrode slurry is 10%, and correspondingly, the addition ratio of the binder in the second negative electrode slurry can be 10%.

[0022] Furthermore, the polyacrylic acid has a molecular weight of 10,000 to 20,000, a molecular weight distribution of 1.5 to 6, and is polymerized by homopolymerization; the alkali salt of the polyacrylic acid has a molecular weight of 11,000 to 21,000, a molecular weight distribution of 1.5 to 5, and is polymerized by homopolymerization.

[0023] The molecular weight, molecular weight distribution, and polymerization method of the polyacrylic acid, as well as the molecular weight, molecular weight distribution, and polymerization method of the alkali salt of the polyacrylic acid, can be selected within the preferred range disclosed in the technical solution of this invention. Those skilled in the art can select appropriate molecular weight, molecular weight distribution, and polymerization method according to actual needs. For example, the molecular weight of the polyacrylic acid can be 10,000, the molecular weight distribution can be 1.5, and the polymerization method can be homopolymerization; the molecular weight M2 of the alkali salt of the polyacrylic acid can be 11,000, the molecular weight distribution can be 1.5, and the polymerization method can be homopolymerization.

[0024] Furthermore, the alkali salt of polyacrylic acid includes one or more of the sodium salt, potassium salt, and lithium salt of polyacrylic acid.

[0025] The alkali salt of the polyacrylic acid can be selected from the preferred range disclosed in the technical solution of this invention. Those skilled in the art can select the type of alkali salt of polyacrylic acid according to actual needs. The metal ion radii of the sodium salt, potassium salt, and lithium salt of polyacrylic acid are arranged from largest to smallest as potassium, sodium, and lithium. They form ionic bonds similar to hydrogen bonds with the carboxyl functional groups in the main body of the alkali salt of polyacrylic acid and the silicon in the silicon-based material. The smaller the metal ion radius, the greater the ionic bond strength and the stronger the adhesion.

[0026] Furthermore, the first negative electrode slurry comprises silicon-based active material, graphite, conductive agent, and polyacrylic acid; the second negative electrode slurry comprises nano-silicon, graphite, conductive agent, and an alkaline salt of polyacrylic acid.

[0027] In the technical solution disclosed in this invention, silicon-based active material is mainly used to form silicon matrix. The smaller the silicon-based active material particles, the better the battery performance. In order to avoid agglomeration caused by excessively small silicon-based active material particles, graphite is used to support the silicon-based active material particles. The conductive agent is used to make the silicon-based negative electrode sheet conductive. Polyacrylic acid and the alkaline salt of polyacrylic acid are used to bond silicon-based active material and graphite and other materials, so as to increase the cold-press peel strength on the current collector.

[0028] Furthermore, in the first negative electrode slurry, the weight ratio of silicon-based active material: graphite: conductive agent: polyacrylic acid is 25-35:45-55:5-15:5-15; and in the second negative electrode slurry, the weight ratio of nano-silicon:graphite: conductive agent: polyacrylic acid alkali salt is 25-35:45-55:5-15:5-15.

[0029] The weight ratio of silicon-based active material:graphite:conductive agent:polyacrylic acid in the first negative electrode slurry can be selected within the preferred range disclosed in the technical solution of this invention. Those skilled in the art can select a suitable weight ratio according to actual needs, such as 30:50:10:10. The weight ratio of silicon-based active material:graphite:conductive agent:alkaline salt of polyacrylic acid in the second negative electrode slurry can be selected within the preferred range disclosed in the technical solution of this invention. Those skilled in the art can select a suitable weight ratio according to actual needs, such as 30:50:10:10.

[0030] The silicon-based active material includes at least one of silicon-carbon, silicon-oxygen, and nano-silicon.

[0031] Furthermore, the graphite is natural graphite, and the conductive agent is superconducting carbon.

[0032] The above-mentioned materials are preferred materials in the technical solution of this invention.

[0033] The second objective of this invention is to provide a method for preparing the aforementioned silicon-based negative electrode sheet, comprising the following steps: preparing a first negative electrode slurry; preparing a second negative electrode slurry; coating the first negative electrode slurry onto the current collector in the area where the first coating is located, coating the second negative electrode slurry onto the current collector in the area where the second coating is located; and processing the coated current collector to obtain the silicon-based negative electrode sheet.

[0034] The above preparation method is simple, convenient and quick to operate, uses inexpensive raw materials, and has a high cost-performance ratio.

[0035] Furthermore, the preparation method further includes the following steps:

[0036] The preparation steps of the first negative electrode slurry are as follows: nano-silicon, natural graphite, superconducting carbon and polyacrylic acid are added and mixed evenly in a weight ratio of 25-35:45-55:5-15:5-15;

[0037] The preparation steps of the second negative electrode slurry are as follows: nano-silicon, natural graphite, superconducting carbon and alkali salt of polyacrylic acid are added and mixed evenly in a weight ratio of 25-35:45-55:5-15:5-15;

[0038] The steps for processing the coated current collector are as follows: baking the coated current collector, cold pressing and slitting the current collector during the baking process, drying it under vacuum, and then welding the tabs to obtain the silicon-based negative electrode sheet.

[0039] The preparation methods of the first negative electrode slurry and the second negative electrode slurry are simple and the raw materials are inexpensive. The coating process is simple and mature, and the cost is low. Those skilled in the art can refer to the existing technology and select process conditions according to the actual situation, such as baking temperature, continuing drying temperature and time.

[0040] Furthermore, the alkali salt of polyacrylic acid is prepared by the following method: an 80% neutral alkaline hydroxide solution is added dropwise to a polyacrylic acid solution to obtain an alkali salt solution of polyacrylic acid. The polyacrylic acid of this application is a polymer, and controlling its distribution in the electrode helps to control the expansion behavior of the electrode. When the carboxyl groups of polyacrylic acid dissociate, its molecular chains will partially stretch due to the influence of charge, making the structure of the electrode material more open and easier to diffuse. In order to make the carboxyl groups on the chain dissociate and stretch, an alkaline substance is added to control the pH value of the acrylate binder solution. The pH of the alkali salt solution of polyacrylic acid is 6-7.

[0041] Polyacrylic acid typically exhibits good stability under neutral to alkaline conditions. When the pH of the alkaline salt solution of polyacrylic acid is 6 to 7, it can effectively inhibit the edge deposition of lithium on the electrode surface, thereby extending the cycle life of the battery.

[0042] The preparation method of the alkali salt of polyacrylic acid is simple, convenient, mature, and low in cost.

[0043] Furthermore, the alkaline hydroxide includes one or more of sodium hydroxide, potassium hydroxide, and lithium hydroxide.

[0044] The alkaline hydroxide can be selected within the preferred range disclosed in this invention, and those skilled in the art can select a suitable type of alkaline hydroxide according to actual needs.

[0045] Beneficial effects

[0046] Compared with the prior art, the present invention has at least the following beneficial effects:

[0047] The silicon-based negative electrode provided by this invention utilizes the basic hydroxyl groups in the alkali salt of polyacrylic acid to neutralize the aggregated carboxyl groups in the polyacrylic acid. These carboxyl functional groups can form hydrogen bonds or covalent bonds with groups on the silicon surface, enhancing the adhesion between silicon particles and between silicon particles and the current collector. The highly flexible main chain allows it to withstand the stress caused by volume changes during lithium insertion / extraction in the silicon-based material, alleviating volume changes and reducing the likelihood of active material shedding and electrode cracking, thus ensuring the stability and integrity of the silicon electrode during cycling. Furthermore, the bonds formed between the carboxyl groups of polyacrylic acid and the silicon-based material, due to the direct hydrogen or covalent bond interaction between the carboxyl functional groups and silicon surface groups, produce a better effect than those formed with polyacrylic acid alone. This results in a smaller volume change in the second coating, and the lithium-ion transport efficiency of the first coating is lower than that of the second coating, meaning the lithium-ion insertion rate of the first coating is lower than that of the second coating. Therefore, the edge lithium plating effect of the first coating is suppressed. Because the alkali salts of polyacrylic acid are low-toxicity or non-toxic substances, and because polyacrylic acid and its alkali salts are high-molecular-weight substances, they are not easily dissolved in the electrolyte during use and exist in the form of colloids or gels, resulting in less pollution during use. Attached Figure Description

[0048] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0049] Figure 1 This is a schematic diagram of the structure of a negative electrode sheet according to an embodiment of the present invention.

[0050] Wherein: 1-current collector; 2-first coating; 3-second coating; YY direction. Detailed Implementation

[0051] The present invention will be further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0052] Negative electrode sheet

[0053] This invention provides a silicon-based negative electrode sheet, such as... Figure 1 As shown, the silicon-based negative electrode includes a current collector 1. A first coating 2 and a second coating 3 are provided on the same surface of the current collector 1 along the Y direction. The first coating 2 is located on both sides of the second coating 3. The first coating 2 is coated with a first negative electrode slurry, and the second coating 3 is coated with a second negative electrode slurry. The first negative electrode slurry includes polyacrylic acid, and the second negative electrode slurry includes an alkaline salt of polyacrylic acid.

[0054] Example 1

[0055] A method for preparing the lithium-ion battery negative electrode sheet includes the following steps:

[0056] Preparation of silicon-based negative electrode sheet

[0057] Preparation of the first negative electrode slurry: Nano-silicon, natural graphite, superconducting carbon (Super-P, conductive agent, the same below), and polyacrylic acid (binder, the same below) are added and mixed evenly in a weight ratio of 30:50:10:10 to obtain the first negative electrode slurry;

[0058] Preparation of the second negative electrode slurry: Nano-silicon, natural graphite, superconducting carbon, and sodium salt of polyacrylic acid are added in a weight ratio of 30:50:10:10 to prepare the second negative electrode slurry;

[0059] Coating of negative electrode paste: The thickness of the first coating is controlled to be 100 μm and the coating width is 30 mm. The thickness of the second coating is controlled to be 100 μm and the coating width is 270 mm. The first coating and the second coating are located on the same surface of the copper current collector, and the first coating is located on both sides of the second coating to obtain an electrode sheet.

[0060] The coated current collector is then cold-pressed and slit during the drying process at 85°C. After slitting, it is dried at 110°C for 10 hours under vacuum conditions, and then the tabs are welded to form a lithium-ion battery negative electrode sheet.

[0061] The preparation of a lithium-ion battery based on the above-mentioned negative electrode sheet includes the following steps:

[0062] Preparation of the positive electrode sheet:

[0063] Lithium cobalt oxide (LiCoO2), superconducting carbon, and binder polyvinylidene fluoride (PVDF) were mixed and stirred evenly at a weight ratio of 97.6:1.1:1.3 and then coated onto an aluminum current collector. The mixture was then trimmed, cut into sheets, and slit. After slitting, the sheets were dried at 80°C under vacuum for 10 hours, and then the tabs were welded to form the positive electrode sheet.

[0064] Electrolyte preparation: In an argon-filled glove box, with a moisture content <5 ppm and an oxygen content <5 ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (DEC), and propyl propionate (PP) are mixed in a mass ratio of 1:1:1:2 to obtain an organic solvent. The organic solvent is then mixed with lithium hexafluorophosphate (according to existing techniques) such that the lithium salt constitutes 14.5% of the electrolyte by weight, thus obtaining the electrolyte.

[0065] Battery manufacturing:

[0066] An 8μm oil-based separator is selected, and the above-mentioned positive electrode, separator, and silicon-based negative electrode are wound into a battery cell. The separator is located between the positive electrode and the silicon-based negative electrode. The positive electrode is led out by spot welding with aluminum tabs, and the silicon-based negative electrode is led out by spot welding with nickel tabs. Then, the battery cell is placed in an aluminum-plastic packaging bag, the electrolyte is injected, and after processes such as encapsulation, formation, and capacity testing, a lithium-ion battery is manufactured.

[0067] Example 2

[0068] Unlike Example 1, the width of the first coating is 150 mm, and the width of the second coating is 150 mm.

[0069] Example 3

[0070] Unlike Example 2, the addition ratio of polyacrylic acid is 15%, and the addition ratio of alkali salt of polyacrylic acid is 5%.

[0071] Example 4

[0072] Unlike Example 1, the thickness of the first coating is 100 μm and the thickness of the second coating is 60 μm.

[0073] Example 5

[0074] Unlike Example 4, the addition ratio of polyacrylic acid is 15%.

[0075] Example 6

[0076] Unlike Example 1, the width of the first coating is 225 mm, and the width of the second coating is 75 mm, with a thickness of 60 μm.

[0077] Example 7

[0078] Unlike Example 6, the addition ratio of polyacrylic acid is 15%, and the addition ratio of the alkali salt of polyacrylic acid is 5%.

[0079] Example 8

[0080] Unlike Example 6, the thickness of the first coating is 60 μm and the thickness of the second coating is 100 μm.

[0081] Example 9

[0082] Unlike Example 8, the addition ratio of polyacrylic acid is 5%, and the addition ratio of the alkali salt of polyacrylic acid is 5%.

[0083] Comparative Example 1

[0084] Unlike Example 1, the first negative electrode slurry was prepared using the same proportion of carboxymethyl cellulose (CMC) binder as in Example 1 instead of the polyacrylic acid. The first negative electrode slurry was coated on the entire current collector, while the second negative electrode slurry was not coated.

[0085] Comparative Example 2

[0086] Unlike Example 1, the first negative electrode slurry is coated on the entire current collector, while the second negative electrode slurry is not coated.

[0087] Comparative Example 3

[0088] Unlike Example 1, the second negative electrode slurry is coated on the entire current collector, while the first negative electrode slurry is not coated.

[0089] Comparative Example 4

[0090] Unlike Example 1, the thickness of the first coating is 40 μm.

[0091] Comparative Example 5

[0092] Unlike Example 1, the width of the first coating is 10 mm, and the width of the second coating is 290 mm.

[0093] Comparative Example 6

[0094] Unlike Example 1, the width of the first coating is 260 mm, and the width of the second coating is 40 mm.

[0095] Comparative Example 7

[0096] Unlike Example 1, the thickness of the second coating is 40 μm.

[0097] Comparative Example 8

[0098] Unlike Example 1, the binder used in the first negative electrode slurry is CMC.

[0099] Comparative Example 9

[0100] Unlike Example 1, the pH of the alkali salt solution of acrylic acid is 8.0.

[0101] Comparative Example 10

[0102] Unlike Example 1, the thickness of the first coating is 80 μm and the width of the first coating is 50 mm. The thickness of the second coating is 135 mm and the width of the second coating is 225 mm. The addition ratio of polyacrylic acid is 15%, and the addition ratio of the alkali salt of polyacrylic acid is 15%.

[0103] Table 1 shows the indicators and parameters of the examples and comparative examples, and Table 2 shows the performance test results of the examples and comparative examples.

[0104] Table 1. Indicators and parameters of the examples and comparative examples

[0105]

[0106]

[0107]

[0108] Table 2 Performance test results of the examples and comparative examples

[0109]

[0110] Examples 1 to 6 illustrate that when the indicators of each example meet the protection scope of the technical solution disclosed in this invention, the silicon-based negative electrode does not deposit lithium, the cold-pressing peel strength is greater than 9 and the coating is intact, achieving the expected technical effect of this invention. By adjusting the thickness and width of the first and second coatings, the electrode structure can be adjusted, thereby improving the stability of the coating structure to meet the electrode performance design requirements.

[0111] Examples 2 and 3 illustrate that increasing the proportion of polyacrylic acid binder can improve the cycle performance of the electrode and increase the battery capacity; decreasing the proportion of alkali salt in polyacrylic acid can further reduce the risk of lithium plating.

[0112] Examples 1, 2, 4, 6, and 8 illustrate that by adjusting the thickness and width of the main body and the head and tail regions of the electrode, different electrochemical performances can be obtained. This demonstrates that electrode performance can be designed through the design of the electrode structure. Example 2 exhibits the best overall performance, possessing a high rate capability and good cycle performance and specific capacity. Example 4 has poor cycle performance and capacity performance, but possesses the best rate capability. Example 6 exhibits the best cycle performance and the highest specific capacity.

[0113] Example 1 and Comparative Example 1 illustrate that using carboxymethyl cellulose adhesive and applying it indiscriminately leads to significant lithium plating due to edge effects. This is because carboxymethyl cellulose adhesive is not a polymer and has poor adhesion. It cannot utilize the high flexibility of the polymer backbone to withstand the stress caused by volume changes during the lithium insertion / extraction process of silicon-based materials. Furthermore, single-area coating cannot utilize the different interaction strengths of different adhesives with lithium ions to reduce the lithium ion transport in the edge area and increase the lithium ion transport in the middle area. In this case, Comparative Example 1 shows significant lithium plating, and its cold-press peel strength is significantly lower than that of Example 1, ultimately leading to coating detachment after 500 cycles at 25°C. The comparative example cannot achieve the technical effect of the present invention.

[0114] Example 1 and Comparative Examples 2 to 3 illustrate that using a single-area coating, it is impossible to utilize the different adhesives and lithium ions with varying strengths to reduce lithium ion transport in the edge region and increase lithium ion transport in the middle region. Therefore, regardless of whether polyacrylic acid or an alkaline salt of polyacrylic acid is applied, significant lithium plating due to edge effects will occur. Furthermore, the cold-press peel strength of Comparative Examples 2 to 3 is greater than that of Comparative Example 1. This is because the bonding ability of polyacrylic acid and the alkaline salt of polyacrylic acid is stronger than that of carboxymethyl cellulose used in Comparative Example 1. However, Example 1 does not exhibit lithium plating, while Comparative Examples 2 to 3 show significant lithium plating. Comparative Examples 2 and 3 cannot achieve the technical effects of this invention. The low lithium ion insertion rate of the first coating in Comparative Example 2 results in lithium ions not being effectively inserted into the electrode during high-rate charging and discharging, leading to lithium plating. In contrast, the second coating in Comparative Example 3 is prone to lithium plating at the edges.

[0115] Example 1 and Comparative Example 4 illustrate that when the coating thickness is lower than the range disclosed in the technical solution of this invention, the thinning of the first coating will affect the lithium ion insertion and extraction, resulting in lithium plating due to edge effects.

[0116] Example 1 and Comparative Example 5 illustrate that if the coating width ratio r2 is lower than the range disclosed in the technical solution of this invention, the amount of lithium ions transported will be higher than the capacity of the first coating due to the first coating being too narrow, resulting in lithium plating due to edge effect.

[0117] Example 1 and Comparative Example 6 illustrate that if the coating width ratio r2 is higher than the range disclosed in the technical solution of this invention, the amount of lithium ions transported in the second coating will be less than that in the first coating due to the excessive width of the first coating, which in turn leads to lithium plating due to edge effect.

[0118] Example 1 and Comparative Example 7 illustrate that if the coating thickness is lower than the range disclosed in the technical solution of this invention, the capacity will be reduced due to the second coating being too thin, and the encapsulation effect of the polyacrylic acid alkali salt binder on the active material will be affected. Ultimately, the coating will fall off after 500 cycles at 25°C, and the technical effect of this solution cannot be achieved.

[0119] Example 1 and Comparative Example 8 illustrate that using CMC as a binder for the head and tail regions of the slurry results in poor encapsulation of the silicon anode, limited mitigation of the volume expansion effect of the silicon anode, and poor improvement of the edge lithium plating effect. Lithium plating occurs in high-rate testing, and the coating peels off after 500 cycles at 25°C, failing to achieve the technical effect of this solution.

[0120] Example 1 and Comparative Example 9 illustrate that when using PAH alkaline salt with a higher degree of neutralization as the binder for the main region, the number of carboxyl functional groups in its molecule is sharply reduced, and its coating effect on the silicon anode begins to decline. The effect on the volume expansion effect of the silicon anode is limited, and the coating peels off after 500 cycles at 25°C, failing to achieve the technical effect of this solution.

[0121] Comparative Examples 1, 3, 5, 7, 8, and 9 show that the influencing factors for coating peeling are the amount of PAA added and the distribution area of ​​PAA. Beyond the specified limits, the designed double-coating structure loses its original effect of limiting the lithium-ion transport rate in the head and tail regions, and the volume expansion of the silicon anode during cycling causes coating peeling. The test methods used in the above embodiments and comparative examples are as follows:

[0122] Performance testing:

[0123] The electrodes and lithium-ion batteries of Examples 1-8 and Comparative Examples 1-5 were subjected to the following tests:

[0124] (1) Cold pressing peel strength of negative electrode sheet (180° peel test)

[0125] At room temperature (25℃±2℃), the negative electrode slurry is coated onto the surface of the current collector, dried, and cold-pressed to form an electrode sheet. The prepared electrode sheet is cut into test samples of 20*100mm size. To accurately reflect the bonding force of the electrode sheet inside the battery, the prepared sample is immersed in electrolyte for 4 hours at 60℃. After removal, the surface is wiped with lint-free paper, and the side to be tested is adhered with double-sided tape and pressed with a pressure roller to ensure complete adhesion to the electrode sheet. The other side of the double-sided tape of the sample is then adhered to the stainless steel surface. One end of the sample is bent in the opposite direction at a bending angle of 180°. The test is conducted using a high-speed rail tensile testing machine. One end of the stainless steel is fixed to the lower clamp of the tensile testing machine, and the bent end of the sample is fixed to the upper clamp. The angle of the sample is adjusted to ensure that the upper and lower ends are in a vertical position. Then, the sample is stretched at a speed of 50mm / min until the sample is completely peeled off from the substrate. The displacement and force during the process are recorded. The force at which the forces are balanced is generally considered to be the peeling force of the electrode sheet.

[0126] (2) Standard Discharge Test

[0127] At room temperature (25℃±2℃), discharge the battery at a given current of 0.2C to the termination voltage of 3.0V and let it rest for 5 minutes; charge the battery at a charging current of 0.5C to 4.45V. When the battery voltage reaches 4.45V, switch to constant voltage charging at 4.45V with a cutoff current of 0.02C; let it rest for 30 minutes; then discharge the battery at a given current of 0.02C to the termination voltage of 3.0V; let it rest for 5 minutes; repeat the charge and discharge cycle twice to obtain the actual discharge capacity of the battery.

[0128] (3) High-rate lithium plating test

[0129] At room temperature (25℃±2℃), allow the battery to stand for 5 minutes, then discharge at a constant current of 0.2C, with a cutoff voltage of 3.0V; allow it to stand for 5 minutes, then charge it at a constant current and constant voltage of 6.5C to 4.5V, with a cutoff rate of 0.02C; allow it to stand for 5 minutes, and repeat this charge-discharge cycle 50 times. After completion, test the battery's internal resistance and thickness, and disassemble the cell to check for lithium plating at the interface. (Thickness change rate less than 10%)

[0130] (4) Lithium-ion battery cycle performance test

[0131] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 30 minutes to allow it to reach a constant temperature. The battery was then charged at a constant current of 1C to a voltage of 4.45V, followed by charging at a constant voltage of 4.45V to a current of 0.025C, and then discharging at a constant current of 0.5C to a voltage of 3.0V. This constitutes one charge-discharge cycle. This charging and discharging process was repeated, and the capacity retention rate of the lithium-ion battery after 500 cycles was calculated. After 500 cycles, the interface was disassembled to observe the coating on the negative electrode surface.

[0132] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A silicon-based negative electrode sheet, characterized in that, The silicon-based negative electrode includes a current collector. A first coating and a second coating are provided on the same surface of the current collector along the Y direction. The Y direction is a direction parallel to the surface of the current collector. The first coating is located on both sides of the second coating. The first coating is coated with a first negative electrode slurry, and the second coating is coated with a second negative electrode slurry. The first negative electrode slurry includes polyacrylic acid, and the second negative electrode slurry includes an alkaline salt of polyacrylic acid.

2. The silicon-based negative electrode sheet according to claim 1, characterized in that, The lithium deposition coefficients k at the beginning and end of the silicon-based negative electrode sheet satisfy relation I, where relation I is: (I) r1 is the ratio of the thickness d1 of the first coating to the thickness d2 of the second coating; r2 is the ratio of the width w1 of the first coating to the width w2 of the second coating; W1 is the addition ratio of binder in the first negative electrode slurry; and W2 is the addition ratio of binder in the second negative electrode slurry. The lithium deposition coefficient k of the silicon-based negative electrode sheet is greater than 0.

3. The silicon-based negative electrode sheet according to claim 2, characterized in that, The range of r1 values ​​is 0.55 to 1.67; the range of r2 values ​​is 0.11 to 3.

4. The silicon-based negative electrode sheet according to claim 3, characterized in that, The thickness d1 of the first coating is 60μm~110μm, and the thickness d2 of the second coating is 60μm~270μm.

5. The silicon-based negative electrode sheet according to claim 3, characterized in that, The width w1 of the first coating is 30mm to 225mm, and the width w2 of the second coating is 75mm to 270mm.

6. The silicon-based negative electrode sheet according to claim 3, characterized in that, The polyacrylic acid has a molecular weight of 10,000 to 20,000 and a molecular weight distribution of 1.5 to 6. The alkali salt of the polyacrylic acid has a molecular weight of 10,000 to 21,000 and a molecular weight distribution of 1.5 to 5.

7. The silicon-based negative electrode sheet according to claim 1, characterized in that, The alkali salt of polyacrylic acid includes one or more of the sodium salt, potassium salt, and lithium salt of polyacrylic acid.

8. The silicon-based negative electrode sheet according to claim 1, characterized in that, The first negative electrode slurry comprises silicon-based active material, graphite, conductive agent and polyacrylic acid; the second negative electrode slurry comprises nano-silicon, graphite, conductive agent and alkaline salt of polyacrylic acid.

9. The silicon-based negative electrode sheet according to claim 8, characterized in that, The weight ratio of silicon-based active material: graphite: conductive agent: polyacrylic acid in the first negative electrode slurry is 25~35:45~55:5~15:5~15; the weight ratio of nano-silicon:graphite: conductive agent:alkaline salt of polyacrylic acid in the second negative electrode slurry is 25~35:45~55:5~15:5~15.

10. The silicon-based negative electrode sheet according to claim 8, characterized in that, The silicon-based active material includes at least one of silicon-carbon, silicon-oxygen, and nano-silicon.

11. A method for preparing a silicon-based negative electrode sheet according to any one of claims 1 to 10, characterized in that, The process includes the following steps: preparing the first negative electrode slurry; preparing the second negative electrode slurry; coating the first negative electrode slurry onto the current collector in the area where the first coating is located, coating the second negative electrode slurry onto the current collector in the area where the second coating is located; and processing the coated current collector to obtain the silicon-based negative electrode sheet.

12. The method for preparing a silicon-based negative electrode sheet according to claim 11, characterized in that, The pH of the alkaline salt solution of the polyacrylic acid is 6-7.

13. The method for preparing a silicon-based negative electrode sheet according to claim 12, characterized in that, The alkali salt solution of the polyacrylic acid contains one or more of sodium hydroxide, potassium hydroxide, and lithium hydroxide.

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