Negative plate and secondary battery

Abstract

The invention relates to the technical field of batteries, and discloses a negative electrode plate and a secondary battery, the negative electrode plate comprises a negative electrode current collector and a negative electrode active layer, the negative electrode active layer comprises a first coating and a second coating which are laminated, and the first coating is located between the negative electrode current collector and the second coating; the first coating comprises a first active material and a first binder, and the first binder comprises a spherical binder; the second coating comprises a second active substance and a second binder, the second active substance comprises a silicon-based material, and the second binder comprises at least one functional group of cyano and ester groups. The bonding effect between the whole negative electrode active layer and the current collector can be ensured through the spherical binder in the bottom coating, and the silicon-based material and the binder containing cyano and / or ester functional groups in the surface coating are matched, so that the cycle stability and rate capability of the battery can be improved while the energy density of the battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a negative electrode sheet and a secondary battery. BACKGROUND

[0002] With the increasing market demand, developing lithium ion secondary batteries with high energy density and high fast-charging efficiency has become a future development trend. The actual specific capacity of traditional graphite materials has approached its theoretical specific capacity (372 mAh / g), and it is difficult to improve further. The theoretical specific capacity of silicon-based materials (4200 mAh / g) is much higher than that of graphite materials, and it is considered to be the most potential negative active material for the next generation of high energy density lithium ion secondary batteries. However, during the process of lithium alloying / lithium dealloying, silicon-based materials usually undergo a volume change of 120%-300%, which easily leads to material pulverization, causing the loss of adhesion between silicon particles and between silicon particles and the current collector, aggravating the side reaction between the material and the electrolyte. At the same time, the destruction of the electrode structure can also block or distort the electrolyte infiltration channel, hindering the uniform and deep infiltration of the electrolyte into the entire electrode sheet, thereby deteriorating the cycle and rate performance of the battery. SUMMARY

[0003] Therefore, the technical problem to be solved by the present application is to overcome the contradiction between high energy density and long cycle life and large rate performance of the existing lithium ion secondary batteries, and to provide a negative electrode sheet capable of improving the cycle and rate performance of the battery while improving the energy density of the battery and a secondary battery comprising the negative electrode sheet.

[0004] According to an embodiment of the present application, in a first aspect, the present application provides a negative electrode sheet, comprising a negative current collector and a negative active layer arranged on at least one side surface of the negative current collector; wherein: The negative active layer comprises a first coating layer and a second coating layer arranged in layers, and the first coating layer is located between the negative current collector and the second coating layer. The first coating layer comprises a first active material and a first binder, and the first binder comprises a spherical binder. The second coating layer comprises a second active material and a second binder, the second active material comprises a silicon-based material, and the second binder comprises at least one functional group selected from a cyano group and an ester group.

[0005] In an optional embodiment, the second binder comprises a first non-spherical binder, and the first non-spherical binder comprises at least one functional group selected from a cyano group and an ester group.

[0006] In an alternative embodiment, the negative active layer comprises sodium element, the mass content of sodium element in the first coating layer is x, based on the mass of the first coating layer; the mass content of sodium element in the second coating layer is y, based on the mass of the second coating layer, satisfying: x≥y.

[0007] In an alternative embodiment, it satisfies: 0.1 wt%≤x≤5 wt%.

[0008] Further, in an alternative embodiment, it satisfies: 0.1 wt%≤x≤1.5 wt%.

[0009] In an alternative embodiment, it satisfies: 0 wt%≤y≤0.1 wt%.

[0010] In an alternative embodiment, the sodium element in the first coating layer is derived from a first sodium-containing binder, the first sodium-containing binder comprises sodium carboxymethyl cellulose, or comprises sodium carboxymethyl cellulose and a sodium-acrylic-containing polymer.

[0011] In an alternative embodiment, the sodium element in the second coating layer is derived from a second sodium-containing binder, the second sodium-containing binder comprises a sodium-acrylic-containing polymer.

[0012] In an alternative embodiment, the first active substance comprises a silicon-based material; the mass content of silicon element in the first coating layer is t, based on the mass of the first coating layer; the mass content of silicon element in the second coating layer is s, based on the mass of the second coating layer, satisfying: s≥t.

[0013] In an alternative embodiment, the first active substance comprises a silicon-based material; the mass content of silicon element in the first coating layer is t, based on the mass of the first coating layer; the mass content of silicon element in the second coating layer is s, based on the mass of the second coating layer, satisfying: s>t.

[0014] Optionally, it satisfies: 0.8 wt%≤s≤40 wt%.

[0015] Optionally, it satisfies: 0 wt%≤t≤10 wt%.

[0016] In an alternative embodiment, the first active substance comprises a silicon-based material, the silicon-based material in the first coating layer and / or the silicon-based material in the second coating layer satisfies at least one of the following conditions: (a) comprises at least one of silicon-oxygen, silicon-carbon, silicon element; (b) comprises at least one of spherical silicon-based material, block silicon-based material.

[0017] In an alternative embodiment, the average diameter of the spherical binder is 50 nm to 500 nm.

[0018] In an alternative embodiment, the spherical binder includes at least one of styrene butadiene rubber, nitrile butadiene rubber, and butadiene rubber.

[0019] In an alternative embodiment, the mass content of the spherical binder is C, based on the mass of the first coating, satisfying 0.3 wt% ≤ C ≤ 4 wt%.

[0020] In an alternative embodiment, the first binder further includes a polymer of any one of acrylic acid, a first acrylic salt, an acrylate, an acrylamide, and acrylonitrile, or a copolymer of at least two thereof, the first acrylic salt being at least one of lithium acrylate, potassium acrylate, calcium acrylate, and magnesium acrylate.

[0021] In an alternative embodiment, the mass content of the first binder is D, based on the mass of the first coating, satisfying 0.9 wt% ≤ D ≤ 7 wt%.

[0022] In an alternative embodiment, the first non-spherical binder includes a polymer of at least one of acrylonitrile, an acrylate, and methyl methacrylate, or a copolymer of at least one of acrylonitrile, an acrylate, and methyl methacrylate with at least one of acrylic acid, a second acrylic salt, and an acrylamide, the second acrylic salt including at least one of sodium acrylate, lithium acrylate, potassium acrylate, calcium acrylate, and magnesium acrylate.

[0023] Further, in an alternative embodiment, the first non-spherical binder includes a multi -component copolymer including acrylonitrile, an acrylate, and a second acrylic salt.

[0024] In an alternative embodiment, the mass content of the first non-spherical binder is E, based on the mass of the second coating, satisfying 0.3 wt% ≤ E ≤ 7 wt%.

[0025] In an alternative embodiment, the second binder further includes a second non-spherical binder, the second non-spherical binder including at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, and polyhexafluoropropylene.

[0026] In an alternative embodiment, the mass content of the second binder is F, based on the mass of the second coating, satisfying 0.5 wt% ≤ F ≤ 7.5 wt%.

[0027] In an optional embodiment, in the same cross section, the thickness of the first coating layer is A, and the thickness of the second coating layer is B, satisfying: 0.11≤A / B≤9.

[0028] Further, in an optional embodiment, it satisfies: 0.4≤A / B≤3.

[0029] In an optional embodiment, it satisfies: 0.1≤A / (A+B)≤0.9.

[0030] Further, in an optional embodiment, it satisfies: 0.3≤A / (A+B)≤0.5.

[0031] In an optional embodiment, it satisfies: 0.1≤B / (A+B)≤0.9.

[0032] Further, in an optional embodiment, it satisfies: 0.5≤B / (A+B)≤0.7.

[0033] According to an embodiment of the present application, in a second aspect, the present application provides a secondary battery, comprising a positive electrode sheet and the negative electrode sheet of the first aspect.

[0034] Further, in an optional embodiment, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer disposed on at least one side surface of the positive electrode current collector, the positive electrode active layer comprising a positive electrode active material and a third binder; the third binder comprises at least one of a polymer containing a cyano group and an ester group, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, and polyhexafluoropropylene.

[0035] Further, in an optional embodiment, the polymer containing a cyano group and an ester group is polymerized from at least a first monomer, a second monomer, and a third monomer; the first monomer comprises at least one of acrylonitrile and methacrylonitrile, the second monomer comprises at least one of acrylate, methacrylate, and vinyl acetate, and the third monomer comprises at least one of an acidic group-containing monomer, an alkaline group-containing monomer, and a halogen-containing monomer.

[0036] Further, in an optional embodiment, the mass content of the first monomer is 15 wt%-84 wt%, the mass content of the second monomer is 15 wt%-84 wt%, and the mass content of the third monomer is 1 wt%-10 wt%, based on the total mass of all monomers forming the polymer containing a cyano group and an ester group.

[0037] In an optional embodiment, the positive electrode active material comprises at least one of lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate.

[0038] Compared with the prior art, the technical scheme of the present application has the following advantages: 1. In the negative electrode sheet of the present application, the negative electrode active layer adopts a double-coating structure, the first coating is located between the negative electrode current collector and the second coating, and the first coating can increase the adhesion between the negative electrode active layer and the current collector by using spherical binders, thereby producing a good bonding interface, ensuring electron transmission, and reducing battery impedance. The second coating contains silicon-based materials, which can help to take advantage of the high specific capacity of silicon-based materials, improve the overall energy density of the battery, and help to build a more stable solid electrolyte interface (SEI) film, optimize the interface stability, and inhibit the growth of lithium dendrites. In particular, under fast charging conditions, it can promote the rapid transmission of lithium ions, reduce "dead lithium", and prolong the cycle life of the battery. On the third aspect, due to the surface properties of silicon, the adsorption energy of lithium ions on the surface of the silicon-based negative electrode is higher, and the migration energy is lower, which is conducive to the rapid insertion and extraction of lithium ions, and improves the rate performance of the battery. At the same time, the second coating also includes a binder containing cyano and / or ester functional groups. The cyano group has high polarity and strong electron affinity, can form hydrogen bonds or partial covalent bonds with the hydroxyl group or oxide layer (SiO2) on the surface of the silicon-based particles, and can also be combined with the surface of carbon materials through π-π conjugation, effectively preventing the silicon-based particles from falling off from the carbon skeleton or current collector during the expansion process, alleviating the negative electrode pulverization, reducing the side reaction with the electrolyte, and ensuring the smoothness of the electrolyte penetration channel; the ester group has good affinity with the electrolyte, which can improve the wettability of the electrolyte to the negative electrode sheet, promote the transmission of lithium ions in the negative electrode sheet, reduce the diffusion resistance of lithium ions, improve the kinetic performance, reduce the lithium precipitation phenomenon on the surface of the negative electrode sheet, and the good electrolyte wettability also helps to form a uniform and stable SEI film on the surface of the negative electrode, reduce the interface resistance, reduce the occurrence of side reactions, avoid the continuous consumption of active lithium, and improve the cycle stability of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0039] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the description of the specific embodiments or the prior art. Obviously, the drawings described below are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creating any inventive labor.

[0040] Figure 1 is the cross-sectional scanning electron microscope image (SEM image) of the first coating of the negative electrode sheet provided by Example 1-1 of the present application.

[0041] Figure 2 is the cross-sectional scanning electron microscope image (SEM image) of the second coating of the negative electrode sheet provided by Example 1-1 of the present application.

[0042] Figure 3is a structural schematic diagram of the negative electrode sheet provided in Embodiment 1-1 of the present application. In the figure, 1, negative electrode current collector; 2-1, first coating layer; 2-2, second coating layer. DETAILED DESCRIPTION

[0043] The following embodiments are provided in order to better further understand the present application, and are not limited to the best embodiments, and do not limit the content and protection scope of the present application, and any person under the enlightenment of the present application or the combination of the present application with other prior art features, any product same or similar to the present application falls within the protection scope of the present application.

[0044] It should be noted in the description of the present application that the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance. In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as there is no conflict between them.

[0045] In order to solve the problem that the lithium ion secondary battery in the related art cannot simultaneously consider high energy density, long cycle life and large rate performance, the present application provides the following solutions.

[0046] In a first aspect, the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector; wherein: The negative electrode active layer comprises a first coating layer and a second coating layer arranged in layers, and the first coating layer is located between the negative electrode current collector and the second coating layer; The first coating layer comprises a first active material and a first binder, and the first binder comprises a spherical binder; The second coating layer comprises a second active material and a second binder, the second active material comprises a silicon-based material, and the second binder comprises at least one functional group of a cyano group and an ester group.

[0047] The negative electrode active layer in the negative electrode sheet of the present application adopts a double-coating layer structure. The first coating layer is located between the negative electrode current collector and the second coating layer. For the convenience of description, the first coating layer can also be referred to as the bottom coating layer, and the second coating layer can be referred to as the surface coating layer. The bottom coating layer can increase the adhesion between the negative electrode active layer and the current collector by using a spherical binder, produce a good bonding interface, ensure electron transmission, and reduce battery impedance. The surface coating layer contains silicon-based materials, which can help to take advantage of the high specific capacity of silicon-based materials, improve the overall energy density of the battery, and help to build a more stable solid electrolyte interface (SEI) film, optimize the interface stability and inhibit the growth of lithium dendrites. In particular, it can promote the rapid transmission of lithium ions, reduce "dead lithium", and prolong the cycle life of the battery. Thirdly, due to the surface properties of silicon, the adsorption energy of lithium ions on the surface of the silicon-based negative electrode is higher, and the migration energy is lower, which is beneficial to the rapid insertion and extraction of lithium ions, and improves the rate performance of the battery.

[0048] At the same time, the surface coating layer also includes a binder containing cyano and / or ester functional groups. The cyano group has high polarity and strong electron affinity, which can form hydrogen bonds or partial covalent bonds with the hydroxyl group or oxide layer (SiO2) on the surface of the silicon-based particles, and can also be combined with the surface of the carbon material through π-π conjugation, effectively preventing the silicon-based particles from falling off the carbon skeleton during the expansion process, alleviating the negative electrode pulverization, reducing the side reaction with the electrolyte, and ensuring the smoothness of the electrolyte penetration channel. The ester group has good affinity with the electrolyte, which can improve the wettability of the electrolyte to the negative electrode sheet, promote the transmission of lithium ions in the negative electrode sheet, reduce the diffusion resistance of lithium ions, improve the kinetic performance, reduce the lithium precipitation phenomenon on the surface of the negative electrode sheet, and good electrolyte wettability also helps to form a uniform and stable SEI film on the negative electrode surface, reduce the interface resistance, reduce the occurrence of side reactions, avoid the continuous consumption of active lithium, and improve the cycle stability of the battery.

[0049] Therefore, by using the spherical binder in the bottom coating layer, the adhesion between the entire negative electrode active layer and the current collector can be ensured, and by using the silicon-based material and the binder containing cyano and / or ester functional groups in the surface coating layer, the energy density of the battery can be improved while the cycle stability and rate performance of the battery are improved. In contrast, if only the silicon-based material is added to the bottom coating layer without setting the silicon-based material in the surface coating layer, although it can guide the lithium deposition to occur inside the negative electrode active layer, the huge volume change of the silicon-based particles during the cycle process is more likely to cause the negative electrode material to pulverize, and also affects the rapid transmission of lithium ions, which is not conducive to the overall electrical performance of the battery. If the binder without cyano and / or ester functional groups is used in the surface coating layer, it is difficult to prevent the silicon-based particles from falling off and improve the wettability of the electrolyte.

[0050] It should be noted that the term "spherical binder" refers to the morphology of the binder in a scanning electron microscope (SEM) image as an ellipse, sphere or sphere-like shape, such as Figure 1 As an example, the spherical binder in the first coating layer may, for example, be at least one of styrene butadiene rubber, nitrile butadiene rubber, and butadiene rubber.

[0051] In some embodiments, the second binder comprises a first non-spherical binder, and the first non-spherical binder comprises at least one functional group of a cyano group or an ester group. In this way, the non-spherical binder can exist in the form of a line, film or fiber on the surface of the silicon-based material, which is beneficial to alleviate or even inhibit the volume change of the silicon-based particles during the circulation process. The cyano group or ester group functional group can further improve the binding effect of the second binder, more beneficially prevent the shedding of the silicon-based particles, and more helpfully improve the wettability of the electrolyte to the negative electrode sheet.

[0052] It should be noted that the term "non-spherical binder" refers to the morphology of the binder in a scanning electron microscope (SEM) image as a line, film or fiber on the surface of the silicon-based material, which can be referred to as Figure 2 As an example, the first non-spherical binder in the second coating layer may, for example, be a polymer of at least one of acrylonitrile, acrylate and methyl methacrylate, or a copolymer of at least one of acrylonitrile, acrylate and methyl methacrylate and at least one of acrylic acid, an acrylic acid salt and an acrylamide.

[0053] In some embodiments, the negative electrode active layer further comprises sodium elements, and the mass content of the sodium elements in the first coating layer is x based on the mass of the first coating layer; the mass content of the sodium elements in the second coating layer is y based on the mass of the second coating layer, and x≥y is satisfied.

[0054] It should be noted that the sodium elements are derived from a sodium-containing binder, and the sodium-containing binder may, for example, be at least one of sodium carboxymethyl cellulose and a sodium-acrylic acid-containing polymer. The term "sodium-acrylic acid-containing polymer" refers to a type of high molecular compound formed by polymerization of sodium-acrylic acid monomers themselves or copolymerization of sodium-acrylic acid monomers and other monomers (such as acrylamide).

[0055] The addition of the sodium-containing binder in the negative active layer can not only ensure excellent adhesion between the negative electrode materials (such as active material particles, conductive agent and binder) and between the negative active layer and the current collector, and stabilize the structure of the whole negative electrode sheet; more importantly, the sodium-containing binder can generate carboxylate after ionization, and the carboxylate can form electrostatic repulsion between each other, which can prevent the agglomeration of negative electrode material particles, increase the dispersion and uniformity of the slurry, and thus ensure the effective exertion of the battery capacity. However, the content of sodium element in the negative active layer is not the more the better, because on the one hand, the sodium ion will exchange with the lithium ion in the electrolyte or the electrode material during the charging and discharging process of the battery, interfere with the normal deintercalation process of the lithium ion, and reduce the migration efficiency of the lithium ion; on the other hand, the sodium ion may participate in the formation process of the SEI film, but due to its different chemical properties from the lithium ion, it may cause the instability of the SEI film structure; and thirdly, the sodium-containing binder will be coated on the surface of the active material particles and form a film, which affects the deintercalation speed of the negative electrode and increases the risk of lithium precipitation. Therefore, by controlling the content of sodium element in the bottom coating to be greater than or equal to the content of sodium element in the surface coating, the adhesion between the negative electrode materials and between the negative active layer and the current collector and the dispersion and uniformity of the slurry can be further improved, while avoiding the interference of the high content of sodium element in the surface coating on the deintercalation behavior of the lithium ion, further improving the migration rate of the lithium ion and reducing the risk of lithium precipitation, and reducing the influence of the participation of excessive sodium ion in the surface coating on the stability of the SEI film structure during the formation of the SEI film, so that the negative electrode structure can be stabilized and uniform, the surface lithium precipitation of the negative electrode can be prevented, and the kinetic performance and rate performance of the battery can be further improved.

[0056] The present application research finds that when the mass content x of sodium element in the first coating is less than the mass content y of sodium element in the second coating, not only the adhesion between the negative active layer and the current collector will decrease, the dispersion and stability of the material in the bottom coating will not be obviously improved, which is not conducive to the exertion of the battery performance, but also the excessive sodium element in the surface coating will affect the migration rate of the lithium ion in the surface coating, which will increase the risk of lithium precipitation on the surface of the negative electrode.

[0057] Specifically, in some embodiments, the mass content x of sodium element in the first coating layer satisfies: 0.1 wt%≤x≤5 wt%. In this way, the adhesion between the negative active layer and the current collector can be guaranteed to be more excellent, and the dispersibility and stability of the materials in the primer coating layer are better, while the migration rate of lithium ions in the primer coating layer is not affected, thereby ensuring the battery capacity to be exerted and the battery to have more excellent rate and cycle performance. When the mass content x of sodium element in the first coating layer is less than 0.1 wt%, the adhesion between the negative active layer and the current collector cannot be effectively improved, the dispersibility and stability of the materials in the primer coating layer are poor, which is not conducive to the effective exertion of the battery capacity and affects the electrochemical performance of the battery. When the mass content x of sodium element in the first coating layer is greater than 5 wt%, the presence of excessive sodium element will interfere with the normal deintercalation process of lithium ions in the electrode material, reduce the migration rate of lithium ions in the primer coating layer, affect the speed of lithium deintercalation of the negative electrode, and easily induce lithium precipitation in the negative electrode sheet.

[0058] It should be noted that the mass content x of sodium element in the first coating layer can be obtained by inductively coupled plasma (ICP) test. For example, the mass content x of sodium element in the first coating layer can be 0.1 wt%, 0.3 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt% or a value within the range formed by any two of the above values.

[0059] Further, in some embodiments, the mass content x of sodium element in the first coating layer satisfies: 0.1 wt%≤x≤1.5 wt%.

[0060] In some embodiments, the mass content y of sodium element in the second coating layer satisfies: 0 wt%≤y≤0.1 wt%. In this way, the adhesion, good dispersibility and uniform stability between the various substances in the second coating layer can be guaranteed to further improve the overall structural stability of the negative electrode sheet, and at the same time, the influence of excessive sodium element in the top coating layer on the migration rate of lithium ions, the formation of SEI film and lithium precipitation is avoided, thereby further improving the kinetic performance of the battery and improving the rate performance of the battery.

[0061] The present application research found that when the mass content y of sodium element in the second coating layer is greater than 0.1 wt%, the excessive sodium element in the top coating layer will reduce the migration rate of lithium ions, cause the uniformity and stability of the formed SEI film to decrease, and induce lithium precipitation.

[0062] It is to be noted that the mass content y of the sodium element in the second coating layer is obtained by inductively coupled plasma (ICP) test. Exemplarily, the mass content y of the sodium element in the second coating layer may be, for example, 0 wt%, 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.1 wt% or a value within a range between any two of the above values.

[0063] In some embodiments, the sodium element in the first coating layer is derived from a first sodium-containing binder, which includes sodium carboxymethyl cellulose, or includes sodium carboxymethyl cellulose and a sodium-acrylic-acid-containing polymer.

[0064] In some embodiments, the sodium element in the second coating layer is derived from a second sodium-containing binder, which includes a sodium-acrylic-acid-containing polymer.

[0065] The present application provides sodium elements in the negative active layer by sodium carboxymethyl cellulose and a sodium-acrylic-acid-containing polymer. Both of the above sodium-containing binders have -COONa, which is a strong-polarity ionizable group and generates carboxylate (-COO - ) and sodium ion (Na + ) after ionization. The electrostatic repulsion between carboxylates can prevent the agglomeration of the negative active layer and increase the dispersibility of the substances in the negative active layer. Meanwhile, the sodium-containing binders can also ensure excellent adhesion between the negative active material particles, between the negative active material particles and the conductive agent, and between the negative active layer and the current collector, thereby ensuring the uniformity and stability of the negative electrode sheet, effectively exerting the capacity of the battery, and further improving the cycle performance and rate performance of the battery.

[0066] In some embodiments, the first active material comprises a silicon-based material; the mass content of silicon in the first coating layer is t, based on the mass of the first coating layer; the mass content of silicon in the second coating layer is s, based on the mass of the second coating layer; and s ≥ t. The application introduces a silicon-based material into the first active material of the first coating layer, which can further improve the high specific capacity advantage and further improve the overall energy density of the battery. However, the application research finds that too much silicon content in the negative active layer can cause the failure of the bonding network and the conductive network in the negative active layer after volume expansion during battery cycling, and thus cause the negative active layer to have a "powder falling and peeling" phenomenon. In particular, too much silicon content in the first coating layer between the second coating layer and the current collector can cause more stress concentration after volume expansion, and thus more easily cause the above-mentioned "powder falling and peeling" phenomenon. To this end, the application controls the silicon content in the first coating layer to be equal to or lower than the silicon content in the second coating layer, so as to achieve a balance between high energy density and high cycle stability, thereby improving the overall electrochemical performance of the battery.

[0067] Further, in some embodiments, the first active material comprises a silicon-based material; the mass content of silicon in the first coating layer is t, based on the mass of the first coating layer; the mass content of silicon in the second coating layer is s, based on the mass of the second coating layer; and s > t. In the negative active layer provided by the application, the first coating layer is arranged between the current collector and the second coating layer. When the silicon-based material in the first coating layer expands in volume during battery cycling, the stress of the negative electrode sheet is more concentrated, and the failure of the bonding network and the conductive network in the first coating layer is more likely to occur, thereby causing the negative active layer to have a more serious "powder falling and peeling" phenomenon. To this end, the application further controls the silicon content in the first coating layer to be lower than the silicon content in the second coating layer. While ensuring that the battery has a higher overall energy density, the relatively low silicon content in the first coating layer can further avoid the phenomenon that too much silicon-based material causes more stress concentration due to volume expansion during battery cycling, thereby ensuring that the first coating layer has more excellent structural stability. In addition, the second coating layer introduces a specific binder containing an ester group or a cyano group, which can more easily absorb electrolyte, thereby softening the surface coating layer of the negative electrode sheet, and thus relieving the stress caused by the volume change of the relatively high silicon-based material during battery cycling. Therefore, even if the silicon-based material in the second coating layer is relatively high, the negative electrode sheet can still have excellent structural stability during cycling.

[0068] It should be noted that the mass content of silicon in the first coating layer and the second coating layer is obtained by stripping the first coating layer and the second coating layer of the electrode sheet, and then testing the silicon content in the first coating layer and the second coating layer by inductively coupled plasma (ICP).

[0069] In some embodiments, the following is satisfied: 0.8 wt%≤s≤40 wt%.

[0070] Exemplarily, the mass content of silicon element in the second coating layer can be, for example, 0.8 wt%, 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, or a value within a range between any two of the above values.

[0071] In some embodiments, the following is satisfied: 0 wt%≤t≤10 wt%.

[0072] Exemplarily, the mass content of silicon element in the first coating layer can be, for example, 0 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, or a value within a range between any two of the above values.

[0073] In some embodiments, the first active material comprises a silicon-based material, and the silicon-based material in the first coating layer and / or the silicon-based material in the second coating layer satisfies at least one of the following conditions: (a) comprises at least one of silicon oxide, silicon carbide, and silicon element; (b) comprises at least one of spherical silicon-based material and block silicon-based material.

[0074] In some embodiments, the average diameter of the spherical binder is 50 nm-500 nm. In this way, it can be ensured that the electrolyte still has good wetting effect when penetrating into the first coating layer, further improving the liquid retention performance of the first coating layer in the negative electrode sheet, while ensuring that the negative active layer and the current collector, the negative active material particles, and the conductive agent have sufficient adhesion, further improving the structural stability of the negative electrode sheet.

[0075] The present application research found that when the average diameter of the spherical binder in the first binder is less than 50 nm, the spherical binder in the first coating is prone to agglomeration, poor dispersion uniformity, which is not conducive to improving the adhesion between the negative active layer and the current collector, the coating is prone to powder and peeling off, and the channels formed between the active material particles are also blocked, affecting the infiltration of the electrolyte and the transmission of ions / electrons, thereby causing the cycle stability and rate performance of the battery to decrease, and the spherical binder with too low average diameter also increases the processing difficulty of the negative electrode sheet and makes coating difficult. On the contrary, if the average diameter of the spherical binder in the first binder is greater than 500 nm, the actual contact area between the spherical binder and the first negative active material will decrease, weakening the adhesion of the negative electrode sheet as a whole, failing to effectively ensure the dispersion uniformity of the negative active layer, causing stress concentration phenomenon due to the expansion and contraction of the negative active material, and affecting the cycle stability and rate performance of the battery.

[0076] It should be noted that the average diameter of the spherical binder is prepared by cutting the cross-section of the electrode sheet with argon ion gas or using a knife cutting method, testing the cross-section of the electrode sheet using a scanning electron microscope, and testing the length of the functional calibration of the diameter range of the spherical binder. For example, the average diameter of the spherical binder can be 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, or a value within the range of any two of the above values.

[0077] In some embodiments, the spherical binder includes at least one of styrene-butadiene rubber, nitrile-butadiene rubber, and butadiene rubber.

[0078] In some embodiments, the mass content of the spherical binder is C based on the mass of the first coating, satisfying: 0.3 wt%≤C≤4 wt%. In this way, the adhesion between the negative active layer and the current collector can be further increased, resulting in good interfacial bonding effect between the two, while ensuring good electron transmission at the interface between the first coating and the current collector, reducing the impedance of the battery, and further improving the electrochemical performance of the battery.

[0079] The present application research found that if the mass content C of the spherical binder in the first binder is less than 0.3 wt%, the adhesion between the negative active layer and the current collector cannot be effectively improved, causing the negative active layer to be prone to peeling off and affecting the cycle stability of the battery; if the mass content C of the spherical binder in the first binder is greater than 4 wt%, too much spherical binder will block the pores between the negative active materials, affecting the penetration of the electrolyte and the transmission of electrons, causing the rate performance of the battery to decrease.

[0080] Exemplarily, the mass content C of the spherical binder in the first binder can be, for example, 0.3 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, 3.5 wt%, 4.0 wt%, or a value within a range between any two of the above values.

[0081] In some embodiments, the first binder further comprises a polymer of any one of acrylic acid, a first acrylic acid salt, an acrylic acid ester, an acrylamide, an acrylonitrile, or a copolymer of at least two of them, the first acrylic acid salt being at least one of lithium acrylate, potassium acrylate, calcium acrylate, magnesium acrylate. The introduction of the acrylic polymer binder into the first coating layer in the present application can construct a polymer network structure with good self-adaptability, high interfacial activity, and good processability, so as to more effectively alleviate the volume change of the negative active material during the cycle process, and the acrylic polymer can be compounded with the spherical binder and the sodium-containing binder to improve the dispersibility and adhesion between the negative active materials and between the negative active material and the conductive agent in the first coating layer, and to improve the adhesion between the negative active layer and the current collector, further ensuring good interfacial bonding force, and more helping to improve the cycle stability and rate performance of the battery.

[0082] In some embodiments, the mass content of the first binder is D, based on the mass of the first coating layer, satisfying: 0.9 wt%≤D≤7 wt%. In this way, the first coating layer and the current collector can have excellent adhesion, produce a good bonding interface, ensure good electron transmission at the interface, and reduce the battery impedance; secondly, the active material particles in the first coating layer and between the active material particles and the conductive agent can have good dispersibility, uniform stability, and excellent adhesion; thirdly, the volume change of the negative active material in the first coating layer during the battery cycle process can be effectively inhibited, further improving the stability of the negative electrode structure; and fourthly, an appropriate amount of the first binder can also be conducive to improving ion transmission and ensuring a high energy density of the battery.

[0083] Exemplarily, the mass content D of the first binder can be, for example, 0.9 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, or a value within a range between any two of the above values.

[0084] In some embodiments, the first non-spherical binder comprises a polymer of at least one of acrylonitrile, acrylate, and methyl methacrylate, or the first non-spherical binder comprises a copolymer of at least one of acrylonitrile, acrylate, and methyl methacrylate with at least one of acrylic acid, a second acrylic acid salt, and acrylamide, the second acrylic acid salt comprising at least one of sodium acrylate, lithium acrylate, potassium acrylate, calcium acrylate, and magnesium acrylate.

[0085] Further, in some embodiments, the first non-spherical binder comprises a multi- copolymer containing acrylonitrile, acrylate, and a second acrylic acid salt. The present application further preferably uses a multi-copolymer containing acrylonitrile, acrylate, and a second acrylic acid salt as the first non-spherical binder, which can further improve the cycle stability and rate performance of the battery. Among them, the multi-copolymer non-spherical binder can effectively inhibit the volume change of the silicon-based material particles during the cycle process of the battery; the cyano group in acrylonitrile has high polarity and strong electron affinity, which can effectively prevent the silicon-based particles from falling off during the expansion process, alleviate the negative electrode pulverization, reduce the side reaction with the electrolyte, ensure the smoothness of the electrolyte infiltration channel; the ester group in acrylate has good affinity with the electrolyte, which can improve the wettability of the electrolyte and promote the transmission of lithium ions in the negative electrode sheet, improve the kinetic performance, reduce the lithium precipitation phenomenon on the surface of the negative electrode sheet, and help to form a uniform and stable SEI film on the surface of the negative electrode, thereby improving the cycle stability of the battery; the introduction of the acrylic acid salt can ensure sufficient adhesion between the second coating and the first coating, as well as between the various substances inside the second coating, and improve the dispersibility and uniformity and stability between the various substances in the second coating.

[0086] It can be understood that, in the second binder, when the second acrylic acid salt is sodium acrylate, the multi-copolymer containing acrylonitrile, acrylate, and sodium acrylate formed can be used as the first non-spherical binder, or can be used as a sodium-containing binder providing sodium elements.

[0087] In some embodiments, the mass content of the first non-spherical binder is E based on the mass of the second coating, and satisfies: 0.3 wt%≤E≤7 wt%. In this way, the volume change of the silicon-based material in the second coating during the cycle process of the battery can be further inhibited, the falling off of the silicon particles can be prevented, the negative electrode pulverization can be alleviated, the side reaction between the negative electrode sheet and the electrolyte can be reduced, the wettability of the electrolyte can be improved and the transmission of lithium ions in the negative electrode sheet can be promoted, the formation of a stable and uniform SEI film can be promoted, sufficient adhesion between the second coating and the first coating, as well as between the various substances inside the second coating can be ensured, thereby further improving the structural stability of the battery and improving its kinetic performance, reducing the lithium precipitation phenomenon, and further more favorably improving the cycle performance and rate performance of the battery.

[0088] The application researches and finds that if the mass content of the first non-spherical binder in the second coating is E lower than 0.3 wt%, the volume change of the silicon-based material cannot be effectively alleviated, the silicon particles are prone to fall off in the battery cycle process, the negative plate is prone to pulverization, secondly, the wettability of the electrolyte, the transport of lithium ions and the formation of the SEI film are affected, and thirdly, the low content of the first non-spherical binder in the second coating also leads to insufficient adhesion between the second coating and the first coating and between various substances in the second coating, thereby leading to a decrease in the structural stability of the negative plate and the cycle performance and rate performance of the battery cannot be effectively improved. On the contrary, if the mass content of the first non-spherical binder in the second coating is E higher than 7 wt%, the electrolyte infiltration channel and the lithium ion transport channel in the second coating are blocked, thereby affecting the wettability of the electrolyte to the negative plate and the normal deintercalation of lithium ions, and also affecting the formation of the SEI film, and the lithium precipitation phenomenon occurs, thereby leading to a decrease in the cycle performance and rate performance of the battery.

[0089] Exemplarily, the mass content E of the first non-spherical binder may be 0.3 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt% or a value within a range consisting of any two of the above values.

[0090] In some embodiments, the acrylate in the application includes at least one of methyl acrylate, ethyl acrylate, n-butyl acrylate.

[0091] In some embodiments, the second binder further includes a second non-spherical binder, and the second non-spherical binder includes at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, polyhexafluoropropylene. In this way, the application further introduces the second non-spherical binder to be matched with the first non-spherical binder containing a cyano and / or ester functional group, and the electrochemical stability and adjustable mechanical properties of the second non-spherical binder can more effectively alleviate the volume change of the silicon-based material in the battery cycle process, further improve the stability of the electrode structure, avoid electrode pulverization, and more effectively improve the wettability of the electrolyte and the transport efficiency of lithium ions, thereby improving the cycle stability and rate performance of the battery.

[0092] In some embodiments, the mass content of the second binder is F, based on the mass of the second coating layer, satisfying: 0.5 wt%≤F≤7.5 wt%. In this way, the volume change of the silicon-based material during battery cycling can be further alleviated, the shedding of silicon particles can be more effectively prevented, the negative electrode pulverization can be further alleviated, the side reaction with the electrolyte can be reduced, at the same time, the permeation channel of the electrolyte is ensured to be unobstructed, on the other hand, it can also be more conducive to improving the wettability of the electrolyte to the negative electrode sheet, further promoting the transport of lithium ions in the negative electrode sheet, more effectively improving the kinetic performance, further reducing the lithium precipitation phenomenon on the surface of the negative electrode sheet, and thirdly, a suitable amount of the second binder can also help to form a stable and uniform SEI film on the surface of the negative electrode, thereby more favorably improving the cycle performance and rate performance of the battery.

[0093] The present application research found that if the mass content F of the second binder in the second coating layer is less than 0.5 wt%, the volume change of the silicon-based material during battery cycling cannot be effectively inhibited, and even the material in the negative active layer is pulverized, and a side reaction with the electrolyte occurs, thereby adversely affecting the cycle stability and rate performance of the battery; if the mass content F of the second binder in the second coating layer is greater than 7.5 wt%, the wettability of the electrolyte, the transport of lithium ions and the formation of SEI film are affected, thereby affecting the cycle stability and rate performance of the battery, and an excessive amount of the second binder will reduce the mass content of the negative active material, affecting the energy density of the battery.

[0094] For example, the mass content F of the second binder can be 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 7.5 wt% or a value within the range of any two of the above values.

[0095] Further, in some embodiments, the thickness of the first coating layer is A and the thickness of the second coating layer is B in the same cross section, satisfying: 0.11≤A / B≤9. In this way, the first coating layer and the second coating layer with a specific thickness ratio can further ensure that the negative active layer containing the silicon-based material has a high overall energy density, while further improving the overall stability of the negative electrode sheet structure and improving the kinetic performance of the battery, so that the battery can have high energy density, excellent cycle performance and rate performance at the same time.

[0096] This application research found that if the ratio of the thickness A of the first coating to the thickness B of the second coating is less than 0.11, it means that the thickness of the first coating is too small relative to the thickness of the second coating. This leads to poor adhesion between the negative electrode and the current collector, and cannot effectively alleviate the stress concentration caused by the volume change of the silicon-based material during battery cycling, thus resulting in poor structural stability of the negative electrode and consequently poor cycle stability of the battery. Conversely, if the ratio of the thickness A of the first coating to the thickness B of the second coating is greater than 9, it indicates that the thickness of the second coating is too small relative to the thickness of the first coating. This leads to a decrease in the overall energy density of the battery, and affects the wetting of the negative electrode by the electrolyte, the transport of lithium ions, and the formation of the SEI film, thus leading to a decrease in the kinetic performance of the battery and consequently a decrease in the overall electrochemical performance of the battery.

[0097] It is understood that the thickness A of the first coating and the thickness B of the second coating refer to the thickness of the first coating and the second coating in the negative electrode active layer on either side of the current collector.

[0098] It should be noted that the thickness A of the first coating is obtained by cutting the cross-section of the negative electrode sheet with argon-ion gas and measuring the coating thickness using a scanning electron microscope; the thickness B of the second coating is obtained by cutting the cross-section of the negative electrode sheet with argon-ion gas and measuring the coating thickness using a scanning electron microscope. For example, the ratio of the thickness A of the first coating to the thickness B of the second coating can be, for example, 0.11, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, or a value within the range of any two of the above values.

[0099] Furthermore, in an optional implementation, the following condition is satisfied: 0.4 ≤ A / B ≤ 3. This allows for a further attainment of a balance between high energy density, excellent rate performance, and cycle stability, thereby further improving the overall electrochemical performance of the battery.

[0100] Furthermore, in some embodiments, the ratio of the thickness A of the first coating to the total thickness (A+B) of the first coating and the second coating satisfies: 0.1≤A / (A+B)≤0.9.

[0101] For example, A / (A+B) can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or a value within the range of any two of the above values.

[0102] Furthermore, in some embodiments, the ratio of the thickness A of the first coating to the total thickness (A+B) of the first coating and the second coating satisfies: 0.3≤A / (A+B)≤0.5.

[0103] Furthermore, in some embodiments, the ratio of the thickness B of the second coating to the total thickness (A+B) of the first coating and the second coating satisfies: 0.1≤B / (A+B)≤0.9.

[0104] For example, B / (A+B) can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or a value within the range of any two of the above values.

[0105] Furthermore, in some embodiments, the ratio of the thickness B of the second coating to the total thickness (A+B) of the first coating and the second coating satisfies: 0.5≤B / (A+B)≤0.7.

[0106] Optionally, in some embodiments, within the same cross section, the thickness of the first coating is A and the thickness of the second coating is B, satisfying: 6 μm ≤ A ≤ 70 μm, 6 μm ≤ B ≤ 70 μm, and 25 μm ≤ A + B ≤ 75 μm.

[0107] For example, the thickness A of the first coating is 6 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm or a value within the range of any two of the above values.

[0108] For example, the thickness B of the second coating is 6 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm or a value within the range of any two of the above values.

[0109] For example, the sum of the thickness A of the first coating and the thickness B of the second coating can be, for example, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm or a value within the range of any two of the above values.

[0110] Secondly, this application provides a secondary battery, including a positive electrode and the negative electrode as described in the first aspect. The secondary battery of this application, employing a specific negative electrode, can simultaneously achieve high energy density, as well as excellent cycle performance and rate performance.

[0111] Furthermore, in some embodiments, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector, the positive active layer including a positive active material and a third binder; the third binder includes at least one of a polymer containing cyano and ester groups, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, and polyhexafluoropropylene.

[0112] Furthermore, in some embodiments, the cyano- and ester-containing polymer is polymerized from at least a first monomer, a second monomer, and a third monomer; the first monomer includes at least one of acrylonitrile and methacrylonitrile, the second monomer includes at least one of acrylate, methacrylate, and vinyl acetate, and the third monomer includes at least one of a monomer containing an acidic group, a monomer containing a basic group, and a halogen-containing monomer. Thus, the cyano group in the first monomer has high polarity and strong electron affinity, which can ensure the adhesion between the various substances in the positive electrode active layer, prevent the pulverization and shedding of the positive electrode active material, and reduce the side reactions between the positive electrode sheet and the electrolyte. The ester group in the second monomer has good affinity with the electrolyte, which can improve the wettability of the electrolyte to the positive electrode sheet, promote the transport of lithium ions inside the positive electrode sheet, further improve the kinetic performance of the battery, and help form a stable and uniform SEI film on the surface of the positive electrode sheet, thereby improving the cycle stability of the battery. The third monomer can increase the dispersion between the various substances in the positive electrode active material, ensure the uniformity and stability of the positive electrode sheet, and can be compounded with the first and second monomers to optimize the performance of polymers containing cyano and ester groups, improve the interfacial effect of the polymer as a binder, and ensure the chemical stability of the binder, thereby improving the cycle stability and rate performance of the battery.

[0113] Furthermore, in some embodiments, based on the total mass of all monomers forming the cyano and ester-containing polymer, the mass content of the first monomer is 15 wt%-84 wt%, the mass content of the second monomer is 15 wt%-84 wt%, and the mass content of the third monomer is 1 wt%-10 wt%.

[0114] For example, the mass content of the first monomer may be 15 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 84 wt%, or a value within the range of any two of the above values; the mass content of the second monomer may be 15 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 84 wt%, or a value within the range of any two of the above values; the mass content of the third monomer may be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, or a value within the range of any two of the above values.

[0115] In some embodiments, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate.

[0116] The present application will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products.

[0117] Example 1-1 This embodiment provides a method for preparing a secondary battery, including the following steps: (1) Preparation of positive electrode plate Lithium cobalt oxide (LiCoO2), a cyano- and ester-containing polymer binder, and conductive carbon black (Super P) were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 98:1:1 and stirred evenly to form a positive electrode slurry. This slurry was then uniformly coated onto both sides of the positive electrode current collector aluminum foil. After drying and rolling, the positive electrode sheet was formed. The cyano- and ester-containing polymer was polymerized from a first monomer, a second monomer, and a third monomer. The first monomer was acrylonitrile, the second monomer was ethyl acrylate, and the third monomer was sodium acrylate. Based on the total mass of the first, second, and third monomers, the mass content of the first monomer was 65 wt%, the mass content of the second monomer was 30 wt%, and the mass content of the third monomer was 5 wt%.

[0118] (2) Preparation of negative electrode First coating: The first negative electrode active material, conductive carbon black (Super P), single-walled carbon nanotubes (CNTs), styrene-butadiene rubber (SBR, average diameter 150 nm), sodium acrylate-methyl acrylate copolymer, and sodium carboxymethyl cellulose (CMC-Na) were added to deionized water in a mass ratio of 95:0.7:1:1.2:0.8:1.3 and stirred evenly to prepare the first negative electrode slurry. The prepared first negative electrode slurry was coated on both sides of a 6 μm carbon-coated copper foil current collector and dried to prepare the first coating. The first negative electrode active material is composed of artificial graphite material and bulk silicon carbon material in a mass ratio of 9:1.

[0119] In this first coating, based on the total mass of the first coating, the spherical binder (styrene-butadiene rubber) has a mass content (C) of 1.2 wt%; the sodium-containing binders are sodium acrylate-methyl acrylate copolymer (sodium-containing binder II) and sodium carboxymethyl cellulose (CMC-Na, sodium-containing binder I), and based on the total mass of the first coating, the sodium content (x) in the first coating is 0.12 wt%, and the mass ratio of sodium acrylate to methyl acrylate in the sodium acrylate-methyl acrylate copolymer is 8.2:91.8; the first binder in the first coating is styrene-butadiene rubber, sodium acrylate-methyl acrylate copolymer, and sodium carboxymethyl cellulose (CMC-Na), and based on the total mass of the first coating, the first binder has a mass content (D) of 3.3 wt%, the artificial graphite material has a mass content of 85.5 wt%, the bulk silicon carbide material has a mass content of 9.5 wt%, and the silicon content (t) in the first coating is 1.9 wt%. The cross-sectional SEM image of the first coating is shown below. Figure 1 As shown, Figure 1 The spherical adhesive can be clearly seen in the box.

[0120] The second coating: A second negative electrode slurry is prepared by dissolving the second negative electrode active material, conductive carbon black (Super P), single-walled carbon nanotubes (CNTs), acrylonitrile-ethyl acrylate-sodium acrylate copolymer, and polyvinylidene fluoride in a mass ratio of 93.2:0.7:1:3.3:1.8 in N-methylpyrrolidone (NMP) and mixing them evenly. The second negative electrode active material is composed of artificial graphite and bulk silicon carbon in a mass ratio of 7:3. The prepared second negative electrode slurry is coated onto the surface of the first coating on the side away from the current collector. After drying and rolling, a second coating is formed on the surface of the first coating away from the current collector, thus obtaining the negative electrode sheet.

[0121] The second binder in the second coating consists of a first non-spherical binder (acrylonitrile-ethyl acrylate-sodium acrylate copolymer) and a second non-spherical binder (polyvinylidene fluoride). Based on the total mass of the second coating, the mass content E of the first non-spherical binder (acrylonitrile-ethyl acrylate-sodium acrylate copolymer) is 3.3 wt%, the mass content of the second non-spherical binder (polyvinylidene fluoride) is 1.8 wt%, and the sodium content y in the second coating is 0.01 wt%. Based on the total mass of the second coating, the mass content F of the second binder is 5.1 wt%, the mass content of the second negative electrode active material is 93.2 wt%, specifically, the mass content of artificial graphite material is 65.24 wt%, and the mass content of bulk silicon-carbon material is 27.96 wt%. Based on the total mass of the second coating, the mass content s of silicon in the second coating is 8.2 wt%. wt%; In the acrylonitrile-ethyl acrylate-sodium acrylate multi-component copolymer, the mass ratio of acrylonitrile, ethyl acrylate, and sodium acrylate is 65:33.65:1.35. The cross-sectional SEM image of the second coating is shown below. Figure 2 As shown, Figure 2 The adhesive, which has a film-like morphology, can be clearly seen in the box, and is therefore a non-spherical adhesive.

[0122] The schematic diagram of the negative electrode sheet prepared above is shown in Figure 1. Figure 3 As shown, the negative electrode sheet includes a negative electrode current collector 1 and negative electrode active layers disposed on both sides of the negative electrode current collector, with the negative electrode active layers on both sides being identical. The negative electrode active layer on one side includes a first coating layer 2-1 and a second coating layer 2-2 stacked together, with the first coating layer 2-1 located between the second coating layer 2-2 and the negative electrode current collector 1. In the single-sided negative electrode active layer, the thickness A of the first coating layer 2-1 is 12 μm, the thickness B of the second coating layer 2-2 is 18 μm, the ratio of the thickness A of the first coating layer 2-1 to the thickness B of the second coating layer 2-2 is 0.67, A / (A+B) is 0.4, and B / (A+B) is 0.6.

[0123] (3) Electrolyte preparation A 1 mol / L lithium hexafluorophosphate (LiPF6) solution was prepared using a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a mass ratio of 3:4:3 as organic solvents. The additives included fluoroethylene carbonate (FEC), vinylene carbonate, and succinate. Based on the mass of the electrolyte, the content of fluoroethylene carbonate (FEC) was 3 wt%, the content of vinylene carbonate was 1 wt%, and the content of succinate was 2.5%.

[0124] (4) Lithium-ion battery assembly The prepared lithium-ion negative electrode sheet, polypropylene porous separator and positive electrode sheet are stacked in sequence, and the battery core is made by winding process. Then, the core is made by packaging, liquid injection, formation and sorting processes to make a soft-pack lithium-ion battery with conventional winding structure.

[0125] The preparation methods and parameter settings of the remaining embodiments and comparative examples are basically the same as those of Examples 1-1. The differences are shown in Tables 1, 2 and 3.

[0126] In Tables 1-3, A and B represent: the thickness of the first coating (A) and the thickness of the second coating (B) within the same cross-section, respectively; C represents: the mass content of spherical adhesive in the first adhesive, based on the mass of the first coating; D represents: the mass content of the first adhesive, based on the mass of the first coating; E represents: the mass content of the first non-spherical adhesive, based on the mass of the second coating; F represents: the mass content of the second adhesive, based on the mass of the second coating; I represents: the mass content of the second non-spherical adhesive, based on the mass of the second coating; x represents: the mass content of sodium in the first coating, based on the mass of the first coating; y represents: the mass content of sodium in the second coating, based on the mass of the second coating; t represents... The mass content of silicon in the first coating is based on the mass of the first coating; s represents the mass content of silicon in the second coating is based on the mass of the second coating; SBR represents styrene-butadiene rubber; CMC-Na represents sodium carboxymethyl cellulose; P(NaAA-co-MAA) represents sodium acrylate-methyl acrylate copolymer; PAANa represents sodium polyacrylate; P(AN-co-EA-co-NaAA) represents acrylonitrile-ethyl acrylate-sodium acrylate copolymer; P(EA-co-NaAA) represents ethyl acrylate-sodium acrylate copolymer; P(AN-co-NaAA) represents acrylonitrile-sodium acrylate copolymer; PVDF represents polyvinylidene fluoride; P(AN-co-EA) represents acrylonitrile-ethyl acrylate copolymer.

[0127] Table 1

[0128] Table 2

[0129] Table 3

[0130] Besides the differences in content between Tables 1-3 and Examples 1-1, the following differences also exist: In Examples 1-3, the first non-spherical adhesive used was an ethyl acrylate-sodium acrylate copolymer (P(EA-co-NaAA)), with a mass ratio of ethyl acrylate to sodium acrylate of 98.65:1.35.

[0131] In Examples 1-3, the first non-spherical adhesive used was an acrylonitrile-sodium acrylate copolymer (P(AN-co-NaAA)), with a mass ratio of acrylonitrile to sodium acrylate of 98.65:1.35.

[0132] In Examples 2-5, the first non-spherical adhesive is an acrylonitrile-ethyl acrylate copolymer (P(AN-co-EA)), with a mass ratio of acrylonitrile to acrylate of 2:1.

[0133] In Examples 2-6 to 2-9, the first non-spherical adhesive is an acrylonitrile-acrylate-sodium acrylate copolymer, with a mass ratio of acrylonitrile, acrylate, and sodium acrylate of 60:31.3:8.7.

[0134] In Example 9-1, the silicon-based material used in the first coating and the second coating is spherical silicon-carbon material.

[0135] In Example 9-2, the silicon-based material used for the first coating is spherical silicon-carbon material, and the second coating uses spherical elemental silicon.

[0136] Test example: The lithium-ion secondary batteries provided in the above embodiments and comparative examples were tested as follows: (1) Energy density: The volume of the lithium-ion secondary battery provided in the above embodiments and comparative examples after the second sealing process is recorded as the cell volume, and the energy of the sorting process after being fully charged to 4.45 V and then discharged to 3 V after being left to stand for 5 min is recorded as the discharge energy; wherein, the ratio of discharge energy to cell volume is the energy density of the cell, and the unit is Wh / L.

[0137] (2) Capacity retention test: At 25°C, the lithium-ion secondary batteries provided in the above examples and comparative examples were charged at a constant current of 1C to 4.45 V, then charged at a constant voltage to the cutoff current of 0.05 C, left to stand for 5 min, and then discharged at a constant current of 1C to 3.0 V. This was the first cycle, and the above process was repeated for 300 cycles. The capacity retention rate (%) of the battery after 300 cycles = discharge capacity after 300 cycles / discharge capacity after the first cycle × 100%.

[0138] (3) Rate performance: Under conditions of 25±2℃, the following were performed sequentially: (a) The lithium-ion secondary batteries provided in the above examples and comparative examples were left to stand for 10 min; (b) The batteries were discharged at a constant current of 0.2 C to 3.0 V, then charged at a constant current of 0.7 C to 4.45 V, then charged at a constant voltage to the cutoff current of 0.05 C, left to stand for 10 min, then discharged at 0.2 C to 3.0 V, left to stand for 10 min, and the discharge specific capacity C0 at this rate was recorded; (c) The batteries were charged at a constant current of 0.7 C to 4.45 V, then charged at a constant voltage to the cutoff current of 0.05 C, left to stand for 10 min, then discharged at 0.5 C to 3.0 V, left to stand for 10 min; (d) The batteries were charged at a constant current of 0.7 C to 4.45 V, then charged at a constant voltage to the cutoff current of 0.05 C, left to stand for 10 min, then discharged at 1 C to 3.0 V, left to stand for 10 min; (e) The batteries were discharged at a constant current of 0.7 C to 3 ... (c) Charge at a constant current to 4.45 V, then charge at a constant voltage to the cutoff current of 0.05 C, let stand for 10 min, then discharge at 2 C to 3.0 V, and let stand for 10 min; (f) Charge at a constant current of 0.7 C to 4.45 V, then charge at a constant voltage to the cutoff current of 0.05 C, let stand for 10 min, then discharge at 3 C to 3.0 V, and let stand for 10 min. Record the discharge specific capacity C1 at this rate; (g) Calculate the capacity retention rate at 3 C as the rate performance using the formula: Rate performance (%) = C1 / C0 × 100%.

[0139] The test results are shown in Table 4.

[0140] Table 4

[0141] As can be seen from Tables 1-4, this application can ensure the bonding effect between the entire negative electrode active layer and the current collector by using the spherical binder in the bottom coating layer. Combined with the silicon-based material and the binder containing cyano and / or ester functional groups in the top coating layer, it can improve the battery's energy density while enhancing the battery's cycle stability and rate performance.

[0142] Compared with Examples 1-1, Comparative Example 1 omitted the spherical binder in the first coating of the negative electrode sheet. The resulting lithium-ion secondary battery showed a significant decrease in energy density, cycle capacity retention after 300 cycles, and rate performance. This means that the absence of the spherical binder in the first coating leads to poor adhesion between the negative electrode active layer and the current collector, resulting in a poor bonding interface, affecting electron transport, increasing battery impedance, and thus hindering the full utilization of the capacity of the negative electrode active material. The structural stability and kinetic performance of the negative electrode sheet deteriorate, leading to a deterioration in the overall energy density, cycle stability, and rate performance of the battery.

[0143] Compared with Examples 1-1, Comparative Example 2 omitted the first non-spherical binder in the second coating in the negative electrode sheet. The resulting lithium-ion secondary battery showed a significant decrease in energy density, cycle capacity retention after 300 cycles, and rate performance. This means that if the second coating uses a binder without cyano and / or ester functional groups, it is difficult to prevent silicon particles from falling off and improve electrolyte wettability, thus leading to a deterioration in the electrochemical performance of the battery.

[0144] Compared with Examples 1-1, Comparative Example 3 kept the total thickness of the negative electrode active layer unchanged, but used only a coating with the same composition as the second coating in Example 1-1 as the negative electrode active layer. Comparative Example 4 kept the total thickness of the negative electrode active layer unchanged, but used only a coating with the same composition as the first coating in Example 1-1 as the negative electrode active layer. The energy density, cycle capacity retention rate and rate performance of the resulting lithium-ion secondary battery all decreased. This fully demonstrates that if any layer of the negative electrode active layer is missing, it will affect the bonding effect inside the negative electrode sheet and affect the wettability of the electrolyte to the negative electrode sheet, thus making it impossible to achieve the advantage of the battery simultaneously taking into account high energy density, excellent cycle and rate performance.

[0145] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector; characterized in that, The negative electrode active layer includes a first coating and a second coating stacked together, wherein the first coating is located between the negative electrode current collector and the second coating. The first coating comprises a first active substance and a first binder, wherein the first binder comprises a spherical binder; The second coating comprises a second active material and a second binder. The second active material comprises a silicon-based material, and the second binder comprises at least one functional group selected from cyano and ester groups.

2. The negative electrode sheet according to claim 1, characterized in that, The second adhesive includes a first non-spherical adhesive, which includes at least one functional group selected from cyano and ester groups.

3. The negative electrode sheet according to claim 2, characterized in that, The negative electrode active layer includes sodium; based on the mass of the first coating, the mass content of sodium in the first coating is x; based on the mass of the second coating, the mass content of sodium in the second coating is y, satisfying: x≥y.

4. The negative electrode sheet according to claim 3, characterized in that, It satisfies: 0.1 wt% ≤ x ≤ 5 wt%, preferably 0.1 wt% ≤ x ≤ 1.5 wt%; And / or, 0 wt% ≤ y ≤ 0.1 wt%; And / or, the sodium element in the first coating is derived from a first sodium-containing binder, which includes sodium carboxymethyl cellulose, or a polymer containing sodium carboxymethyl cellulose and sodium acrylate; And / or, the sodium element in the second coating is derived from a second sodium-containing binder, which comprises a polymer containing sodium acrylate.

5. The negative electrode sheet according to claim 1, characterized in that, Within the same cross-section, the thickness of the first coating is A, and the thickness of the second coating is B, satisfying at least one of the following conditions: (1) 0.11≤A / B≤9, preferably 0.4≤A / B≤3; (2) 0.1≤A / (A+B)≤0.9, preferably 0.3≤A / (A+B)≤0.5; (3) 0.1≤B / (A+B)≤0.9, preferably 0.5≤B / (A+B)≤0.

7.

6. The negative electrode sheet according to claim 1, characterized in that, The first active material includes silicon-based materials, and the mass content of silicon in the first coating is t, based on the mass of the first coating. Based on the mass of the second coating, the mass content of silicon in the second coating is s; Satisfy: s≥t; Preferably, s>t; And / or, the first active material comprises a silicon-based material, and the silicon-based material in the first coating and / or the silicon-based material in the second coating satisfies at least one of the following conditions: (a) Includes at least one of silicon-oxygen, silicon-carbon, and elemental silicon; (b) Includes at least one of spherical silicon-based materials and bulk silicon-based materials.

7. The negative electrode sheet according to any one of claims 1-6, characterized in that, The average diameter of the spherical adhesive is 50 nm-500 nm; And / or, the spherical adhesive includes at least one of styrene-butadiene rubber, nitrile rubber, and butadiene rubber; And / or, based on the mass of the first coating, the mass content of the spherical adhesive is C, satisfying: 0.3wt% ≤ C ≤ 4 wt%; And / or, the first adhesive further includes a polymer or copolymer of any one of acrylic acid, a first acrylate, acrylate, acrylamide, and acrylonitrile, wherein the first acrylate is at least one of lithium acrylate, potassium acrylate, calcium acrylate, and magnesium acrylate. And / or, based on the mass of the first coating, the mass content of the first adhesive is D, which satisfies: 0.9wt%≤D≤7wt%.

8. The negative electrode sheet according to claim 2, characterized in that, The first non-spherical adhesive comprises a polymer of at least one of acrylonitrile, acrylate, and methyl methacrylate; or, the first non-spherical adhesive comprises a copolymer of at least one of acrylonitrile, acrylate, and methyl methacrylate with at least one of acrylic acid, a second acrylate, and acrylamide, wherein the second acrylate comprises at least one of sodium acrylate, lithium acrylate, potassium acrylate, calcium acrylate, and magnesium acrylate; preferably, the first non-spherical adhesive comprises a multi-component copolymer containing acrylonitrile, acrylate, and a second acrylate. And / or, based on the mass of the second coating, the mass content of the first non-spherical adhesive is E, satisfying: 0.3 wt% ≤ E ≤ 7 wt%; And / or, the second adhesive further includes a second non-spherical adhesive, the second non-spherical adhesive comprising at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, and polyhexafluoropropylene; And / or, based on the mass of the second coating, the mass content of the second adhesive is F, satisfying: 0.5wt%≤F≤7.5wt%.

9. A secondary battery, characterized in that, Includes a positive electrode and a negative electrode as described in any one of claims 1-8; Preferably, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector, the positive active layer including a positive active material and a third binder; The third adhesive includes at least one of the following: a polymer containing cyano and ester groups, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, and polyhexafluoropropylene; And / or, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate.

10. The secondary battery according to claim 9, characterized in that, The polymer containing cyano and ester groups is polymerized from at least a first monomer, a second monomer, and a third monomer; the first monomer includes at least one of acrylonitrile and methacrylonitrile, the second monomer includes at least one of acrylate, methacrylate, and vinyl acetate, and the third monomer includes at least one of a monomer containing an acidic group, a monomer containing a basic group, and a halogen-containing monomer. Preferably, based on the total mass of all monomers forming the cyano and ester-containing polymer, the mass content of the first monomer is 15 wt%-84 wt%, the mass content of the second monomer is 15 wt%-84 wt%, and the mass content of the third monomer is 1 wt%-10 wt%.

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