Negative pole piece, electrochemical device and preparation method of negative pole piece

Abstract

The invention provides a negative pole piece, an electrochemical device and a preparation method of the negative pole piece, and belongs to the technical field of secondary batteries, and the negative pole piece comprises a negative current collector, a negative active material layer and a polymer film. Wherein the negative electrode active material layer is arranged on the negative electrode current collector; the polymer film is arranged on the negative electrode active material layer, the polymer film comprises a polymer PIMs, and the average pore size of the polymer film is 0.7-1 nm. According to the negative pole piece, the polymer film arranged on the surface is utilized to prevent transition metal ions dissolved out from a positive pole from diffusing into a negative pole active material layer interface while ensuring smooth passing of lithium ions, so that a metal insulating layer is prevented from being deposited on the negative pole interface, and the internal resistance of the battery is further inhibited from being increased; and the cycle life of the battery under the conditions of high temperature and high voltage is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a negative electrode sheet, an electrochemical device, and a method for preparing the negative electrode sheet. Background Art

[0002] As a high-efficiency energy storage device, lithium-ion batteries have been widely used in portable electronic digital products and electric vehicles, and play an increasingly important role in today's society.

[0003] Currently, the cathode materials used in lithium-ion batteries often contain transition metal elements. While these transition metal elements can provide lithium-ion batteries with higher capacity and charge-discharge voltages, they are also prone to gradually dissolving into the electrolyte in high-temperature, high-voltage environments and during the charge-discharge process. Dissolved transition metal ions diffuse and deposit onto the surface of the negative electrode under the influence of concentration gradients and potential differences. This increases the interfacial impedance of the negative electrode, worsens battery polarization, and damages the solid electrolyte interphase (SEI) film on the surface of the negative electrode. The damaged SEI film requires continuous consumption of electrolyte to repair, resulting in the continuous consumption of active lithium in the battery, which in turn leads to a serious reduction in the battery's cycle life under high-voltage and high-temperature conditions.

[0004] Therefore, it is necessary to design a negative electrode sheet, an electrochemical device, and a method for preparing the negative electrode sheet to improve the above problems. Summary of the Invention

[0005] The present invention provides a negative electrode plate, an electrochemical device and a method for preparing the negative electrode plate, which are used to improve the technical problem that the SEI film on the surface of the negative electrode plate is easily destroyed by diffused and deposited transition metal ions, resulting in increased battery internal resistance and decreased cycle life.

[0006] In a first aspect, the present invention provides a negative electrode plate, which includes a negative electrode current collector, a negative electrode active material layer and a polymer film.

[0007] The negative electrode active material layer is arranged on the negative electrode current collector; the polymer film is arranged on the negative electrode active material layer, the polymer film includes polymer PIMs, and the average pore size of the polymer film is 0.7-1 nm.

[0008] In one example of the present invention, the negative electrode active material layer includes a negative electrode material, the negative electrode material includes a silicon-based material, and the Young's modulus of the polymer film is 15-20 GPa.

[0009] In one example of the present invention, the thickness of the polymer film is 0.5-4 μm.

[0010] In one example of the present invention, the mass content of silicon in the negative electrode material is 13-100 wt %; the thickness of the polymer film and the mass content of silicon in the negative electrode material satisfy the relationship of formula (1): -2≤25x-y≤0(1) The thickness of the polymer film is x μm, and the mass content of silicon in the negative electrode material is y wt%.

[0011] In one example of the present invention, the porosity of the polymer is 50%-60%.

[0012] In one example of the present invention, the polymer PIMs includes a polymer of a first monomer and a second monomer, the first monomer includes a diamine monomer, and the second monomer includes at least one of a dianhydride monomer and a bismaleimide monomer.

[0013] In an example of the present invention, the silicon-based material includes at least one of silicon, a silicon-carbon composite, and silicon monoxide.

[0014] In a second aspect, the present invention further provides a method for preparing a negative electrode sheet, the method comprising: The polymer PIMs are dissolved in a first solvent to prepare a glue solution; the glue solution is applied to the surface of a bare negative electrode plate and dried to form a polymer layer to prepare a negative electrode plate; the bare negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector.

[0015] In one embodiment of the present invention, the preparation process of polymer PIMs includes: placing a first monomer and a second monomer into a second solvent for reaction to obtain a prepolymer solution; wherein the molar ratio of the first monomer to the second monomer is 1:(1-1.05), the first monomer comprises a diamine monomer, the second monomer comprises at least one of a dianhydride monomer and a bismaleimide monomer, and the solid content of the prepolymer solution is 18-20 wt %; The prepolymer solution is subjected to atomization and imidization or curing treatment in sequence to obtain polymer PIMs.

[0016] In one embodiment of the present invention, the steps of atomizing and imidizing or curing the prepolymer solution to obtain polymer PIMs include: The prepolymer solution is spray-dried to obtain prepolymer droplets; wherein the inlet temperature of the spray drying is 160-180°C, the outlet temperature of the spray drying is 80-90°C, and the atomization pressure of the spray drying is 0.3-0.5 MPa; the prepolymer droplets are imidized or cured to obtain polymer PIMs powder; wherein the heating temperature of the imidization or curing treatment is 250-300°C.

[0017] In a third aspect, the present invention further provides an electrochemical device, comprising a positive electrode sheet, a separator, an electrolyte, and a negative electrode sheet according to any of the above examples, or a negative electrode sheet prepared by any of the above example preparation methods.

[0018] The negative electrode plate provided by the present invention has a polymer film composed of PIMs constructed on the surface of the negative electrode active material layer. The polymer film has a microporous structure with an average pore size of 0.7-1 nm. This ensures the smooth passage of lithium ions while blocking the diffusion of transition metal ions dissolved from the positive electrode into the interface of the negative electrode active material layer. This prevents the formation of a metal insulating layer by deposition at the negative electrode interface, thereby suppressing the increase in the battery's internal resistance and improving the battery's cycle life under high temperature and high voltage conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings are incorporated into and constitute a part of the specification, illustrate embodiments consistent with the present application, and together with the specification, are used to explain the principles of the present application. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, other embodiments can be obtained based on these drawings without inventive efforts.

[0020] In the attached figure: Figure 1 Schematic diagram of the structure of the negative electrode sheet in one embodiment of the present invention; Figure 2 Schematic diagram of a process for preparing a negative electrode sheet according to an embodiment of the present invention; Figure 3 Schematic diagram of the process for preparing polymer PIMs according to one embodiment of the present invention.

[0021] The reference numerals are as follows: 10. Negative electrode current collector; 20. Negative electrode active material layer; 30. Polymer film. DETAILED DESCRIPTION

[0022] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following examples and the features in the examples can be combined with each other unless they conflict. It should also be understood that the terms used in the examples of the present invention are for the purpose of describing specific embodiments, not for the purpose of limiting the scope of protection of the present invention. The test methods for which specific conditions are not specified in the following examples are generally carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0023] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" and "one" cited in this specification are only for the convenience of description and are not used to limit the scope of implementation of the present invention. Changes or adjustments to their relative relationships should be regarded as the scope of implementation of the present invention without substantially changing the technical content.

[0024] As a highly efficient energy storage device, the energy density and rate performance of lithium-ion batteries are primarily determined by the positive electrode material. Currently, most of the positive electrode materials used in lithium-ion batteries contain transition metal elements with variable charge and discharge characteristics. While these transition metal elements can provide lithium-ion batteries with higher capacity and charge and discharge voltages, they are also prone to gradually dissolving into the electrolyte in high-temperature, high-voltage environments and during the charge and discharge process. Transition metal ions dissolved in the electrolyte will deposit on the surface of the negative electrode under the influence of concentration gradients and potential differences, increasing the interfacial impedance of the negative electrode and damaging the SEI film on the surface of the negative electrode. This causes the damaged SEI film to continuously consume the active lithium in the electrolyte for repair, leading to a serious reduction in the battery's cycle life under high-voltage and high-temperature conditions.

[0025] The reason is that the SEI film on the surface of the existing negative electrode contains a large amount of organic and inorganic lithium salts. The lithium ions in these lithium salts undergo ion exchange reaction with the transition metal ions in the electrolyte, resulting in the SEI film not only being unable to block the entry of transition metal ions at the negative electrode interface, but also aggravating the deposition of transition metals at the negative electrode interface, thereby worsening the negative electrode interface impedance and continuously damaging the SEI film, causing the battery cycle life to decline.

[0026] In order to solve the above problems, the present invention provides a negative electrode plate, which constructs a layer of polymer film on the surface of the negative electrode active material layer as an artificial SEI film. The polymer film has a special microporous structure, which can prevent the transition metal complexes in the electrolyte from entering the micropores while ensuring the smooth passage of lithium ions, thereby effectively preventing the deposition of transition metal elements at the negative electrode interface. At present, the average pore size of the porous structure of polymer membranes (such as PP, PE, etc.) commonly used in the battery field is at least ten nanometers to hundreds of nanometers, which cannot meet the functional requirements of screening and blocking transition metal complexes in the design of the present invention. However, the applicant has found that the polymer chains in the self-microporous polymers (Polymers of Intrinsic Microporosity, PIMs) have a rigid twisted structure between the aromatic ring planes, which can effectively avoid the accumulation of polymer chains, thereby having an inherent good microporous structure foundation. Therefore, the applicant adjusted parameters such as the degree of polymerization of PIMs to ensure that the average pore size of the polymer film formed by the PIMs material on the negative electrode plate reaches 0.7~1nm, so that the polymer film can achieve the screening protection function of allowing lithium ions to pass through while blocking the passage of transition metal complexes at the interface of the negative electrode plate.

[0027] In the first aspect, Figure 1 As shown, the present invention provides a negative electrode plate, which includes a negative electrode current collector 10, a negative electrode active material layer 20, and a polymer film 30. The negative electrode current collector 10 can be made of a material with good conductivity and mechanical strength, and performs the functions of conduction and current collection. For example, in one example, the negative electrode current collector 10 can be made of copper foil. The negative electrode current collector 10 has two surfaces facing each other in its own thickness direction. The negative electrode active material layer 20 is disposed on at least one side of the negative electrode current collector 10. Specifically, the negative electrode active material layer 20 is disposed on either or both of the two surfaces of the negative electrode current collector 10. The polymer film 30 is disposed on the negative electrode active material layer 20. The polymer film 30 includes a polymer PIMs material and has a microporous structure. The average pore size of the polymer film 30 is any value within the range of 0.7 to 1 nm, for example, 0.7 nm, 0.8 nm, 0.9 nm, or 1 nm. When the average pore size of the polymer membrane 30 is limited to the range of 0.7~1nm, it can play a screening role of blocking the transition metal complexes in the electrolyte from passing through and allowing lithium ions to pass through, preventing the transition metal ions dissolved from the positive electrode from being deposited at the negative electrode interface, thereby effectively inhibiting the increase in the negative electrode interface impedance and improving the storage life and cycle life of lithium-ion batteries under high temperature and high voltage conditions.

[0028] The polymer PIMs material has a polymer chain composed of repeating units bonded to each other, each repeating unit containing at least one aromatic ring and a rigid linker with a twist site. The repeating units are bonded to each other through a rigid linker, which can be a spirocyclic group, a bridged ring group, or a single covalent bond with spatial congestion. The polymer chains in the polymer PIMs effectively avoid chain stacking due to their rigid twisted structure, thereby having a good microporous structure. The present application limits the average pore size of the polymer PIMs to any value within the range of 0.7 to 1 nm, for example, the average pore size of the polymer PIMs can be 0.7 nm, 0.8 nm, 0.9 nm, or 1 nm. Since the microporous structure in the polymer membrane 30 is directly provided by the polymer PIMs, when the average pore size of the polymer PIMs is within the above range, the average pore size of the polymer membrane 30 formed on the negative electrode active material layer 20 can also be limited to the range of 0.7 to 1 nm, thereby enabling the polymer membrane 30 to have a screening function at the negative electrode interface that allows lithium ions to pass through and blocks transition metal complexes. In addition, the polymer PIMs material is highly soluble in common organic solvents and can be conveniently formed into a polymer film 30 on the negative electrode active material layer 20 by coating.

[0029] It should be noted that the type of polymer PIMs is not limited, and any one or more materials in the PIM series of polymers can be used, for example, it can be one or more of PIM-COOH, PIM-1, PIM-2, PIM-3, PIM-4, PIM-5, PIM-6, PIM-7, PIM-8, PIM-9, PIM-10, PIM-11, PIM-12, PIM-13, PIM-14, PIM-15, PIM-16, PIM-17, PIM-18, PIM-19, PIM-20, PIM-21, PIM-22, PIM-23, PIM-24, PIM-25, PIM-26, PIM-27, PIM-28, PIM-29, PIM-30 and PIM-31. In some embodiments, the polymer PIMs are biphenyl-type PIM-PI polymers cross-linked by a first monomer and a second monomer, wherein the first monomer comprises a diamine monomer and the second monomer comprises at least one of a dianhydride monomer and a bismaleimide monomer. In some embodiments, the thickness of the polymer film 30 is any value within the range of 0.5 to 4 μm, for example, the thickness of the polymer film 30 can be 0.5 μm, 0.7 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, or 4 μm. Limiting the thickness of the polymer film 30 within the above range can effectively ensure its own structural strength without excessively increasing the negative electrode interface impedance. If the thickness of the polymer film 30 is too thin, its own structural strength is insufficient and it is easily damaged during battery use, affecting the protection effect of the negative electrode interface. If the thickness of the polymer film 30 is too thick, it will significantly increase the diffusion resistance of lithium ions in the electrolyte at the negative electrode interface, resulting in reduced lithium ion insertion and extraction efficiency during charge and discharge, and easily causing concentration polarization and battery capacity decay.

[0030] The negative electrode active material layer 20 includes a negative electrode material. The type of the negative electrode material is not limited. The negative electrode material can be any conventional negative electrode active material. For example, the negative electrode material can be selected from at least one of tin, carbon-based materials, silicon-based materials and nano-metal oxides. Among them, carbon-based materials include graphite, soft carbon, hard carbon, etc., and nano-metal oxides include nanoparticles such as Fe2O3, CuO, SnO2, and Mn3O4.

[0031] It should be noted that the negative electrode active material layer 20 also includes a negative electrode binder and a negative electrode conductor. The negative electrode binder is selected from at least one of polyimide, styrene-butadiene rubber, polyacrylic acid, polyvinylidene fluoride, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose. The negative electrode conductor is selected from at least one of carbon black, SP, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0032] To further increase battery capacity, the negative electrode material must include at least one silicon-based material, including at least one of pure silicon, silicon-carbon composites, and silicon monoxide. While the introduction of silicon-based materials can effectively increase the specific capacity of the battery, they undergo significant volume expansion during the lithiation process, which can easily cause the SEI film on the surface of the negative electrode sheet, or artificial SEI films of moderate strength, to rapidly rupture. This can lead to severe side reactions at the silicon-based negative electrode interface during charge and discharge, resulting in loss of active lithium in the battery and further deteriorating the battery's cycle life under high-temperature, high-voltage conditions.

[0033] Therefore, in some embodiments, the Young's modulus of the polymer film 30 is any value within the range of 15 to 20 GPa. For example, the Young's modulus of the polymer film 30 can be 15 GPa, 16 GPa, 17 GPa, 18 GPa, 19 GPa, or 20 GPa. The Young's modulus indicates the degree of change in particle length per unit cross-section of a powder after being subjected to a certain tensile stress. The Young's modulus is defined as σ / ε, where σ represents the stress per unit area of the material and ε represents the strain per unit length.

[0034] When the polymer film 30 has a Young's modulus within the aforementioned range, it exhibits both high hardness and toughness, effectively suppressing the volume expansion of the negative electrode sheet caused by the silicon-based material during the charge-discharge process. It also adapts to the high volume expansion of the silicon-based material during the charge-discharge process, preventing the film from tearing and being damaged by the high volume expansion of the silicon-based material. This maintains the screening protection effect on the negative electrode interface and improves the cycle life of the battery. However, if the Young's modulus of the polymer film 30 is too low, the hardness of the polymer film 30 is insufficient, making it unable to effectively suppress the volume expansion of the silicon-based negative electrode. If the Young's modulus of the polymer film 30 is too high, the toughness of the polymer film 30 is reduced, making it unable to adapt to the volume changes of the silicon-based negative electrode during the charge-discharge process and prone to shattering under the influence of large volume deformation.

[0035] In some embodiments, the mass content of silicon in the negative electrode material is 13-100 wt%. For example, the mass content of silicon in the negative electrode material can be 13 wt%, 20 wt%, 26 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%, or 100 wt%. To accommodate the high content of silicon active material introduced into the negative electrode plate, the thickness of the polymer film 30 and the mass content of silicon in the negative electrode material satisfy the relationship of formula (1), which is as follows: -2≤25x-y≤0(1) In formula (1), the thickness of the polymer film 30 is x μm, and the mass content of silicon in the negative electrode material is y wt%. When the thickness of the polymer film 30 and the mass content of silicon in the negative electrode material satisfy the relationship of formula (1), the polymer film 30 can effectively adapt to and suppress the volume expansion caused by the extremely high silicon content in the negative electrode plate within a suitable thickness range (e.g., 0.5-4 μm). Under the relationship of formula (1), on the one hand, the thickness of the polymer film 30 increases with the increase of silicon content in the negative electrode plate, which can effectively suppress and adapt to the increase in volume expansion caused by the gradually increasing silicon content in the negative electrode plate, preventing the polymer film 30 from being torn by the huge volume expansion and affecting the protection effect of the negative electrode interface. On the other hand, even when adapting to pure silicon negative electrode plates, the thickness of the polymer film 30 remains within the suitable thickness range, avoiding the use of an overly thick polymer film 30 when adapting to high-silicon negative electrode plates, which may increase the negative electrode interface impedance and reduce the battery loading capacity and service life.

[0036] In some embodiments, the porosity of the polymer film 30 is any value within the range of 50% to 60%, for example, the porosity of the polymer film 30 can be 50%, 52%, 54%, 55%, 56%, 58%, or 60%. Thus, the polymer film 30 also has a relatively high porosity, which can provide more lithium ion transmission channels at the negative electrode plate interface, thereby promoting uniform diffusion of lithium ions in the electrolyte at the negative electrode interface, reducing the negative electrode interface impedance, lowering the risk of battery polarization, and improving battery cycle performance.

[0037] In some embodiments, the specific surface area of the polymer film 30 is 100-3000 m 2 / g, for example, the specific surface area of the polymer film 30 can be 100m 2 / g、300m 2 / g、500m 2 / g、700m 2 / g、1000m 2 / g、1300m 2 / g、1500m 2 / g、1700m 2 / g、2000m 2 / g、2300m 2 / g、2500m 2 / g、2700m 2 / g or 3000m 2 / g. It can be seen that compared to the insulating polymer films 30 commonly used in the battery field, the polymer film 30 of the present invention has a higher specific surface area, which can effectively improve the electrolyte wettability and lithium ion transmission efficiency at the negative electrode interface, thereby reducing the negative electrode interface impedance, lowering the risk of battery polarization, and improving battery cycle performance. It should be noted that the specific surface area of the polymer film 30 can be measured using a nitrogen adsorption BET test.

[0038] In a second aspect, the present invention also provides a method for preparing a negative electrode sheet. Figure 2 As shown, the method for preparing the negative electrode sheet includes the following steps: S1, dissolving the polymer PIMs in a first solvent to prepare a glue solution; S2. Applying the glue solution to the surface of the bare negative electrode sheet and drying it to form a polymer layer to make a negative electrode sheet; the bare negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector.

[0039] In some embodiments, step S1 includes providing a polymer PIMs powder having an average pore size of 0.7-1 nm, and dissolving the polymer PIMs powder in a first solvent to obtain a glue solution. The first solvent is selected from one of dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, and tetrahydrofuran.

[0040] In some embodiments, step S2 includes placing the bare negative electrode sheet prepared in advance on a spin coating platform, and spin coating the polymer glue on the surface of the bare negative electrode sheet under a low pressure environment, wherein the spin coating speed is 4500~5500rpm, and the spin coating time is 10~80s. The spin coating speed and the spin coating time can be adaptively adjusted according to the thickness of the polymer film to be formed on the surface of the negative electrode sheet. For example, the spin coating speed can be 4500rpm, 4700rpm, 5000rpm, 5200rpm or 5500rpm, and the spin coating time is 10~80s. The drying time can be 10s, 20s, 30s, 40s, 50s, 60s, 70s or 80s; then, the bare negative electrode sheet coated with the glue is placed in a vacuum oven for drying, and the liquid solvent in the glue is dried to form a polymer film on the surface of the negative electrode sheet, wherein the heating temperature of the drying process is 60~80℃, for example, the heating temperature can be 60℃, 65℃, 70℃, 75℃ or 80℃, and the drying time is 4~12 hours, for example, the drying time can be 4 hours, 6 hours, 8 hours, 10 hours or 12 hours.

[0041] In addition, Figure 3 As shown, in some embodiments, the method for preparing the polymer PIMs used in step S1 includes the following steps: S11, placing the first monomer and the second monomer into a second solvent for reaction to obtain a prepolymer solution.

[0042] In step S11 , under a protective gas environment, the first monomer and the second monomer are mixed in a second solvent according to a preset molar ratio and reacted at room temperature for 4 to 6 hours to prepare a prepolymer solution.

[0043] The first monomer comprises a diamine monomer, and the second monomer comprises at least one of a dianhydride monomer and a bismaleimide monomer. The molar ratio of the first monomer to the second monomer is 1:(1-1.05). For example, the mass ratio of the first monomer to the second monomer can be 1:1, 1:1.01, 1:1.02, 1:1.03, 1:1.04, or 1:1.05. By controlling the second monomer to be slightly in excess relative to the first monomer, the first and second monomers can be moderately cross-linked via the terminal carboxyl groups, limiting excessive molecular chain stacking and reducing the average pore size of the formed polymer PIMs to a target range.

[0044] In step S11 , the second solvent used may be a polar organic solvent, such as N-methylpyrrolidone or N,N-dimethylformamide.

[0045] The solid content of the prepolymer solution prepared in step S11 is controlled to be 18-20 wt %, for example, 18 wt %, 18.5 wt %, 19 wt %, 19.5 wt %, or 20 wt %. Increasing the solid content of the prepolymer solution by increasing humidity can reduce the volume of spray-dried droplets in the subsequent atomization step, narrowing the gaps between the prepolymer droplets and facilitating the formation of nanoscale channels between the polymer droplets.

[0046] S12, sequentially performing atomization and imidization or curing treatment on the prepolymer solution to obtain polymer PIMs.

[0047] In step S12, the prepolymer solution is first spray-dried to obtain prepolymer droplets. Specifically, the prepolymer solution is passed into a sprayer and spray-dried on a plane to uniformly form prepolymer droplets on the plane. The inlet temperature of the spray drying is 160-180°C, for example, 160°C, 165°C, 170°C, 175°C, or 180°C; the outlet temperature of the spray drying is 80-90°C, for example, 80°C, 82°C, 84°C, 85°C, 86°C, 88°C, or 90°C; and the atomization pressure of the spray drying is 0.3-0.5 MPa, for example, 0.3 MPa, 0.4 MPa, or 0.5 MPa. By setting the atomization pressure of spray drying to 0.3~0.5 MPa, prepolymer droplets with a particle size of less than or equal to 50 nm can be formed on the plane, thereby further reducing the distance between the prepolymer droplets, so that uniform submicron pores are formed between the PIMs powder particles formed after subsequent drying.

[0048] The prepolymer droplets formed on the plane are then imidized or cured to obtain polymer PIMs powder. Specifically, the prepolymer droplets on the plane are imidized or cured by heating at a temperature of 250-300°C to promote the closure of the imide rings in the prepolymer and the flexibility of the molecular chains, thereby controlling the porosity of the resulting polymer PIMs powder to 50%-60%. The heating temperature for imidization or curing can be any temperature within the range of 250-300°C, for example, 250°C, 255°C, 260°C, 265°C, 270°C, 275°C, 280°C, 285°C, 290°C, 295°C, or 300°C.

[0049] In addition, it should be noted that the bare negative electrode sheet assembly process in step S1 can be prepared using conventional methods in the art. For example, in some embodiments, the bare negative electrode sheet preparation process includes: mixing the negative electrode material, negative electrode conductive agent, negative electrode binder, and negative electrode thickener in a mass ratio of (90-99):(1-10):(1-10):(1-10), optionally in a weight ratio of 97:1:1:1, adding deionized water to adjust the slurry solid content to 40-50wt%, and then stirring and mixing in a vacuum mixer to obtain a negative electrode slurry; uniformly coating the negative electrode slurry on both sides of the negative electrode current collector copper foil; drying at room temperature, transferring to an oven for drying, and then cold pressing, slitting, and other processes to obtain the negative electrode sheet. The negative electrode conductive agent can be selected from at least one conductive material such as carbon black (SuperP), acetylene black, carbon nanotubes (CNT), graphene, and nanofiber carbon fibers (VGCF). The negative electrode binder is selected from at least one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR) and other binding materials; the negative electrode thickener is selected from carboxymethyl cellulose, which can be sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0050] In a third aspect, the present invention also provides an electrochemical device, which can be a liquid lithium-ion secondary battery. The electrochemical device includes a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte. The negative electrode sheet is selected from the negative electrode sheet described in any of the above embodiments, or the negative electrode sheet prepared by the preparation method described in any of the above embodiments. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, and the positive electrode active material layer includes a positive electrode material, a positive electrode conductive agent, and a positive electrode adhesive. Among them, the positive and negative electrode materials can intercalate and deintercalate lithium ions to achieve energy storage and release, the electrolyte is a carrier for the transmission of lithium ions between the positive and negative electrodes, and the separator can pass lithium ions but is non-conductive, thereby separating the positive and negative electrodes to prevent short circuits.

[0051] It should be noted that the preparation of the positive electrode sheet, separator, electrolyte and assembly of the electrochemical device can be prepared using conventional methods in the art. The following describes an example of the preparation method of the electrochemical device: Positive electrode sheet preparation: The positive electrode material, positive electrode conductive agent, and positive electrode binder are mixed in a weight ratio of (90-99):(1-10):(1-10), with an optional weight ratio of 95:2.5:2.5. N-methylpyrrolidone (NMP) solvent is added and stirred in a vacuum mixer until the mixture becomes homogeneous and transparent to obtain a positive electrode slurry. This positive electrode slurry is evenly coated on a positive electrode current collector aluminum foil. The positive electrode current collector aluminum foil is then air-dried at room temperature and then transferred to an oven for drying. The positive electrode sheets are then cold-pressed and slit.

[0052] The positive electrode material is selected from one or more of lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP), lithium cobalt oxide (LCO), lithium nickel oxide (LNO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), and lithium rich manganese-based oxide (LRMO). The positive electrode conductive agent can be selected from at least one conductive material such as carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and nanofiber carbon fibers (VGCF). The positive electrode binder can be selected from at least one of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), for example, PVDF can be used as the positive electrode binder.

[0053] Preparation of electrolyte: The electrolyte in this application can be selected from conventional types of electrolytes in the field. For example, a small molecule plasticizer and a lithium salt are mixed in a mass ratio of (8-9):(1-2) to prepare the electrolyte. The small molecule plasticizer is selected from one or more combinations of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, fluorinated ethylene carbonate, dipropyl carbonate, dimethyl sulfoxide dimethoxyethane, N-methyl-2-pyrrolidone, γ-butyrolactone, and polyethylene glycol dimethyl ether; the lithium salt is selected from one or more combinations of LiBF4, LiBF6, LiAsF6, LiPF6, LiClO4, LiFSI, LiTFSI, LiB(C6H5)4, LiAlCl4, LiBr, LiCF3SO3, LiN(CF3SO2)2, and LiC(CF3SOSO2)3.

[0054] Preparation of diaphragm: The diaphragm is selected from conventional porous polymer films in the field. For example, the diaphragm material can be selected from one or more combinations of polyvinylidene fluoride, polystyrene, polyarylethersulfone, polyvinyl chloride, polypropylene, polyethylene, polyamide, polyimide, polyacrylic acid, polyacetal, polycarbonate, polyester, polyetherimide, polyimide, polyketone, polyphenylene ether, polyphenylene sulfide, polymethylpentene, polysulfone non-woven glass, glass fiber material, ceramics, metal oxides, and composites of organic and inorganic substances. For example, in one example, the diaphragm is selected from polyethylene (PE) or polypropylene (PP) porous membrane, and PP / PE / PP porous membrane can be selected, with a thickness of 9μm to 18μm, such as 9μm, 12μm, 16μm or 18μm; the air permeability is 180s / 100mL to 380s / 100mL, such as 180s / 100mL, 280s / 100mL or 380s / 100mL; the porosity is 30% to 50%, such as 30%, 40% or 50%.

[0055] Battery assembly: Battery assembly is performed according to conventional methods. For example, after preparation, the negative electrode sheet, separator, and positive electrode sheet are stacked in sequence and placed in an aluminum-plastic film to form a dry cell. The dry cell is then baked to remove water. The prepared electrolyte is injected into the dry cell and encapsulated. After formation, venting, and aging, a lithium-ion battery with a preset capacity is obtained, such as a finished soft-pack lithium-ion battery with a capacity of 1.2Ah.

[0056] The formation, exhaust and aging conditions are as follows: charging to a cutoff voltage at a rate of 0.1 to 0.33 C, an exhaust vacuum of -40 to -98 kPa, and an exhaust time of more than 10 s; an aging temperature of 40 to 50° C., and an aging time of 24 to 72 h.

[0057] The technical solutions of the present invention are described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art.

[0058] Example 1 This embodiment provides a negative electrode plate, which includes a bare negative electrode plate containing a silicon-based negative electrode material and a polymer film coated on the surface of the bare negative electrode plate. The polymer PIMs material in the polymer film is spirobifluorene-based polyimide, the thickness of the polymer film is 1 μm, the average pore size of the polymer film is 0.8 nm, the Young's modulus of the polymer film is 20 GPa, and the specific surface area of the polymer film is 600 m 2 / g, and the porosity of the polymer membrane is 60%.

[0059] The preparation method of the negative electrode sheet is as follows: (1) The preparation process of the bare negative electrode sheet includes: mixing the negative electrode material, negative electrode conductive agent (acetylene black), negative electrode binder (styrene-butadiene rubber) and negative electrode thickener (sodium carboxymethyl cellulose) in a mass ratio of 97:1:1:1, adding deionized water, adjusting the slurry solid content to 50wt%, and then fully stirring and mixing under the action of a vacuum mixer to obtain a negative electrode slurry; evenly coating the negative electrode slurry on both sides of the negative electrode current collector copper foil; after drying at room temperature, transferring to an oven for drying, and then cold pressing, slitting and other processes to obtain a bare negative electrode sheet. Among them, the negative electrode material is a silicon-carbon alloy sourced from Guangdong Candlelight New Energy Technology Co., Ltd., model number is silicon-carbon alloy 1200, and the silicon content is 26%.

[0060] (2) The preparation process of polymer PIMs includes: placing a first monomer, spirobifluorene diamine (such as 2,7-diamino-9,9'-spirobifluorene) and a second monomer, pyromellitic dianhydride (PMDA), in NMP at a mass ratio of 1:1 and mixing them evenly, so that the first monomer and the second monomer are condensed at room temperature of 25°C for 6 hours to form a prepolymer solution, which is a polyamic acid solution with a solid content of 20wt%. The prepolymer solution is spray-dried at an atomization pressure of 0.3 MPa (inlet 180°C / outlet 80°C) to form densely arranged prepolymer droplets on a plane; the prepolymer droplets are placed at 300°C for thermal imidization for 2 hours to obtain spirobifluorene-based polyimide powder.

[0061] (3) The process of forming a polymer film on the surface of a bare negative electrode includes: dissolving polymer PIMs (spirobifluorenyl polyimide powder) in DMF to prepare a glue solution; placing the bare negative electrode on a spin coating platform, and spin coating the glue solution on the surface of the bare negative electrode under low pressure, wherein the spin coating speed is controlled to be 5000 rpm and the spin coating time is 20 s during the spin coating process; finally, placing the bare negative electrode with the glue solution coated on the surface in a vacuum oven, heating and drying at 80 ° C for 12 h to obtain a negative electrode with a surface coated with a polymer film.

[0062] This embodiment also assembles the negative electrode plate into the lithium-ion battery to facilitate subsequent battery performance testing. The assembly process of the lithium-ion battery is as follows: Positive electrode sheet preparation: The positive electrode material NCM811, the positive electrode conductive agent SP, and the positive electrode binder PVDF were mixed in a mass ratio of 97:1.8:1.2 (a total of 100 parts by mass). 82 parts by mass of NMP were added and thoroughly stirred to obtain a positive electrode slurry. The positive electrode slurry was coated onto the positive electrode current collector aluminum foil and dried, roll-pressed, and cut into pieces to produce the positive electrode sheet. The positive electrode material NCM811 was sourced from Xiamen Tungsten Co., Ltd., model number M821A.

[0063] Diaphragm preparation: A polyethylene porous film with a thickness of 11 μm was used as the diaphragm. The air permeability of the diaphragm was 230 s / 100 mL, and the porosity of the diaphragm was 40%.

[0064] Electrolyte preparation: The electrolyte contained 60 g diethyl carbonate, 10 g fluoroethylene carbonate, 10 g difluoroethyl acetate, 6 g dimethyl carbonate and 14 g LiPF6, which were mixed evenly.

[0065] Battery assembly: The negative electrode sheet, separator, and positive electrode sheet are stacked in sequence and placed in an aluminum-plastic film to obtain a dry cell; 3.4 g of electrolyte is injected into the dry cell with a dew point of -60°C and a vacuum degree of -98 kPa. After injection, the cell is allowed to stand at room temperature for 48 hours; the dry cell after injection is formed, degassed, and aged to obtain a soft-pack lithium-ion battery. Specifically, after constant capacity, the cell is charged to 4.3 V at a charge rate of 1 C, with a degassing time of 20 seconds and a degassing vacuum degree of -98 kPa; the aging temperature is 45°C and the aging time is 24 hours.

[0066] Example 2 This embodiment provides a negative electrode sheet of the same system as that of Example 1, and assembles the negative electrode sheet into a lithium-ion battery of the same system as that of Example 1. This embodiment differs from Example 1 in that a copolymer of biphenyl dianhydride and diphenyl diamine is used as the polymer PIMs. The resulting polymer membrane has an average pore size of 0.8 nm, a Young's modulus of 18 GPa, and a specific surface area of 300 m 2 / g, porosity is 50%.

[0067] The preparation method of polymer PIMs includes placing a first monomer, biphenyl diamine, and a second monomer, biphenyl dianhydride, in a mass ratio of 1:1 in NMP and mixing them uniformly. The first and second monomers are polycondensed at room temperature (25°C) for 6 hours to form a prepolymer solution, which is a polyamic acid solution with a solid content of 20wt%. The prepolymer solution is spray-dried at an atomization pressure of 0.3 MPa (inlet 180°C / outlet 80°C) to form densely distributed prepolymer droplets on a plane. The prepolymer droplets are then thermally imidized at 300°C for 2 hours to produce a copolymer powder of biphenyl diamine and biphenyl diamine.

[0068] Example 3 This example provides a negative electrode sheet of the same system as Example 1, and the negative electrode sheet is assembled into a lithium-ion battery of the same system as Example 1. The difference between this example and Example 1 is that a BMI-PIMs crosslinked product is used as the polymer PIMs. The formed polymer membrane has an average pore size of 0.7 nm, a Young's modulus of 15 GPa, and a specific surface area of 400 m 2 / g, porosity is 50%.

[0069] The preparation method of polymer PIMs includes placing a first monomer, spirobifluorene diamine, and a second monomer, bismaleimide monomer (such as 4,4'-bismaleimide diphenylmethane), in a mass ratio of 1:1 in DMF and mixing them evenly. The first and second monomers are then polycondensed at room temperature (25°C) for 6 hours to form a prepolymer solution, which is a polyamic acid solution with a solid content of 20wt%. The prepolymer solution is then spray-dried (inlet 180°C / outlet 80°C) at an atomization pressure of 0.3 MPa to form densely packed prepolymer droplets on a flat surface. The prepolymer droplets are then thermally imidized at 250°C for 2 hours to produce a cross-linked BMI-PIM powder.

[0070] Example 4 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. The difference between this embodiment and Example 1 is that in step (1), a silicon-carbon alloy with a silicon content of 13% by mass is selected as the negative electrode material, which is sourced from Guangdong Candlelight New Energy Technology Co., Ltd. and is model number silicon-carbon alloy 650; in step (3), the spin coating time for coating the glue on the bare negative electrode plate is 10 seconds, so that the thickness of the formed polymer film is 0.5 μm.

[0071] Example 5 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. The difference between this embodiment and Example 1 is that in step (1), a silicon-carbon alloy with a silicon content of 50% by mass is selected as the negative electrode material, which is sourced from Changzhou Silicon Source New Energy Materials Co., Ltd. and is model NSC9100; and in step (3), the spin coating time for applying the glue solution on the bare negative electrode plate is 40 seconds, so that the thickness of the formed polymer film is 2 μm.

[0072] Example 6 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. The difference between this embodiment and Example 1 is that in step (1), pure silicon is selected as the negative electrode material, which is sourced from Guangzhou Hongwu Materials Technology Co., Ltd. and is model A212; and in step (3), the spin coating time for applying the glue solution on the bare negative electrode plate is 80 seconds, so that the thickness of the formed polymer film is 4 μm.

[0073] Example 7 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. This embodiment differs from Example 1 in that the spin coating time for applying the glue solution on the bare negative electrode plate in step (3) is 4 seconds, so that the thickness of the formed polymer film is 0.2 μm.

[0074] Example 8 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. This embodiment differs from Example 1 in that the spin coating time for applying the glue solution on the bare negative electrode plate in step (3) is 10 seconds, so that the thickness of the formed polymer film is 0.5 μm.

[0075] Example 9 This embodiment provides a negative electrode sheet of the same system as that of Example 1, and assembles the negative electrode sheet into a lithium-ion battery of the same system as that of Example 1. This embodiment differs from Example 1 in that the spin coating time for applying the glue solution on the bare negative electrode sheet in step (3) is 40 seconds, so that the thickness of the formed polymer film is 2 μm.

[0076] Example 10 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. This embodiment differs from Example 1 in that the spin coating time for applying the glue solution on the bare negative electrode plate in step (3) is 80 seconds, so that the thickness of the formed polymer film is 4 μm.

[0077] Example 11 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. The difference between this embodiment and Example 1 is that in step (1), a silicon-carbon alloy with a silicon content of 13% by mass is selected as the negative electrode material, which is sourced from Guangdong Candlelight New Energy Technology Co., Ltd. and is model number silicon-carbon alloy 650.

[0078] Example 12 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. The difference between this embodiment and Example 1 is that in step (1), a silicon-carbon alloy with a silicon content of 50% by mass is selected as the negative electrode material, which is sourced from Changzhou Silicon Source New Energy Materials Co., Ltd. and is model NSC9100.

[0079] Example 13 This embodiment provides a negative electrode plate of the same system as that of Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as that of Example 1. The difference between this embodiment and Example 1 is that in step (1), the negative electrode material is a combination of silicon dioxide and graphite, the silicon content in the negative electrode material is 25% by mass, and the combination of silicon dioxide and graphite is sourced from Beitui, model number BTR-SiO-25.

[0080] Example 14 This embodiment provides a negative electrode sheet of the same system as that of Example 1, and assembles the negative electrode sheet into a lithium-ion battery of the same system as that of Example 1. The difference between this embodiment and Example 1 is that LNMO is used as the positive electrode material in the positive electrode sheet, and LNMO is specifically a single crystal LiNi coated with Li2ZrO3. 0.5 Mn 1.5 O4, from Xiamen Tungsten Co., Ltd., model number is XW46.

[0081] Comparative Example 1 This comparative example provides a negative electrode sheet of the same system as Example 1, and assembles the negative electrode sheet into a lithium-ion battery of the same system as Example 1. The difference between this comparative example and Example 1 is that no polymer film is provided on the surface of the negative electrode sheet.

[0082] Comparative Example 2 This comparative example provides a negative electrode plate of the same system as Example 1, and assembles the negative electrode plate into a lithium-ion battery of the same system as Example 1. This comparative example differs from Example 1 in that no polymer film is provided on the surface of the negative electrode plate; and in step (1), a silicon-carbon alloy having a silicon content of 13% by mass is selected as the negative electrode material, which is sourced from Guangdong Candlelight New Energy Technology Co., Ltd. and is designated as Silicon-Carbon Alloy 650.

[0083] Comparative Example 3 This comparative example provides a negative electrode sheet of the same system as Example 1, and assembles the negative electrode sheet into a lithium-ion battery of the same system as Example 1. This comparative example differs from Example 1 in that the polymer film on the surface of the negative electrode sheet uses a PVDF porous membrane of the same thickness, with a Young's modulus of 2 GPa and a porosity of 40%.

[0084] Comparative Example 4 This comparative example provides a negative electrode sheet of the same system as Example 1, and assembles the negative electrode sheet into a lithium-ion battery of the same system as Example 1. This comparative example differs from Example 1 in that the polymer film on the surface of the negative electrode sheet uses a PVDF-HFP porous membrane of the same thickness, with a Young's modulus of 1.5 GPa and a porosity of 40%.

[0085] The polymer films on the surface of the negative electrode plates prepared in Examples 1 to 14 and Comparative Examples 1 to 4 were subjected to parameter tests, and the high-temperature cycle performance and storage performance tests were performed on the lithium-ion batteries prepared and assembled in Examples 1 to 14 and Comparative Examples 1 to 4 to verify the improvement effect of the negative electrode plates of the present application on the battery cycle life, storage life and rate performance. The test results are shown in Table 1.

[0086] Polymer film parameter test: The specific surface area, average pore size and porosity of the polymer film on the surface of the negative electrode sheet in each embodiment and comparative example were measured by using a BET (Brunauer-Emmett-Teller) analyzer.

[0087] The specific surface area test involves freezing the negative electrode in liquid nitrogen for 5 minutes to embrittle the polymer film on the surface. The polymer film is then peeled off using precision tweezers or scraped horizontally with an ultra-thin blade. The exfoliated polymer film fragments are cut into 1–2 mm² pellets and degassed at 90°C for 10 hours to remove gas and moisture adsorbed on the sample surface. To test the specific surface area, the polymer film pellets are placed under a load and immersed in liquid nitrogen. A nitrogen adsorption-desorption curve at 77K is plotted, and the specific surface area (S) is calculated using the BET method.

[0088] The average pore size and porosity test specifically involves placing a negative electrode coated with a polymer film in a DMF solution and stirring at room temperature for 24 hours to dissolve the polymer components in the polymer film. A portion of the liquid is then centrifuged and the supernatant is dried to obtain a microporous polymer powder. The pore size and porosity of the microporous polymer powder are measured according to the national standard GB / T 21650.1-2008, "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption." It should be noted that since the polymer powder extracted through dissolution and drying has the same material properties as when it was in the polymer film, and the microporous structure in the polymer film is directly provided by the polymer PIMs material, the average pore size of the polymer powder microporous structure measured by this test method can be used to represent the average pore size of the polymer film microporous structure.

[0089] The Young's modulus test of the polymer film is specifically as follows: the Young's modulus of the polymer film on the surface of the negative electrode is tested using the nanoindentation method, and a quasi-static test is performed under a load of 0.5 mN using a Berkovich diamond indenter. The contact stiffness is obtained by analyzing the initial slope of the unloading curve using the Oliver-Pharr model, and the Young's modulus is calculated by combining the indenter geometry correction and the PIMs Poisson's ratio (0.35-0.40).

[0090] Battery performance test: Battery high-temperature cycle life test: First, the rated capacity is calculated based on the surface capacity of the electrode in the lithium-ion batteries prepared in each embodiment and comparative example, and the rated current of each lithium-ion battery is obtained based on the rated capacity; the lithium-ion battery is charged and discharged at a constant current using a current value of 0.33× the rated current as the constant-capacity charge and discharge current to obtain the constant-capacity capacity of the lithium-ion batteries in each embodiment and comparative example. Then, at 45°C, the lithium-ion batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 4 are charged and discharged at a current rate of 1C / 1C in the test voltage range of 2.8V (discharge cut-off voltage) to 4.3V (charge cut-off voltage) until the capacity is less than or equal to 80% of the constant-capacity discharge capacity, and the number of cycles is recorded. For the lithium-ion battery prepared in Example 14, the battery was charged and discharged in a test voltage range of 2 V (discharge cut-off voltage) to 4.85 V (charge cut-off voltage) at a current rate of 1 C / 1 C until the capacity was less than or equal to 80% of the initial capacity, and the number of cycles was recorded.

[0091] Battery High-Temperature Storage DCR Test: For the lithium-ion batteries prepared in each embodiment and comparative example, the initial DCR of the lithium-ion batteries was tested. The lithium-ion batteries were then placed in a 60°C environment for 90 days. The DCR of the batteries after 90 days of storage at 60°C was recorded, and the DCR increase of the batteries was calculated.

[0092] Table 1: Negative electrode plate parameters prepared in Examples 1 to 14 and Comparative Examples 1 to 4 and assembled battery performance test results

[0093] By comparing the test results of Examples 1 to 14 and Comparative Examples 1 to 2, it can be seen that compared with the negative electrode plates without surface protection in Comparative Examples 1 and 2, the battery provided by the embodiment of the present invention has a polymer film with an average pore size of 0.7~1nm attached to the surface of the negative electrode plate, which can effectively inhibit the deposition of transition metal ions in the electrolyte on the negative electrode interface, thereby slowing down the increase in impedance of the negative electrode interface during use and storage, and reducing the occurrence of side reactions between the negative electrode interface and the electrolyte in a high temperature environment, effectively improving the cycle life, storage life and rate performance of the battery under high temperature and high voltage conditions.

[0094] Comparing the test results of Examples 1 to 3 and Comparative Examples 3 to 4, it can be seen that although Comparative Examples 3 and 4 also form a polymer protective film on the surface of the negative electrode, the PVDF and PVDF-HFP materials used in the polymer film have large pores and cannot effectively prevent transition metal ions in the electrolyte from depositing on the negative electrode interface. As a result, the lithium-ion battery's high-temperature cycle life and storage impedance performance are slightly improved compared to Comparative Examples 1 and 2. The polymer film on the surface of the negative electrode in the embodiment of the present invention, due to its specific microporous structure, can block transition metal ions from entering the negative electrode interface while allowing lithium ions to pass through. Compared with Comparative Examples 3 and 4, it can effectively suppress the increase in the battery's storage impedance under high temperature conditions and significantly improve the battery's cycle life under high temperature conditions.

[0095] At the same time, because the Young's modulus of the polymer films in Comparative Examples 3 and 4 is too small, it is unable to effectively suppress and adapt to the huge volume expansion of the silicon-based negative electrode during the charge and discharge process, and the polymer film has difficulty maintaining protection for the negative electrode interface during battery use. In contrast, the polymer film on the surface of the negative electrode plate in the embodiment of the present invention has a higher Young's modulus and is more capable of suppressing the volume expansion of the silicon-based negative electrode under the same thickness conditions, thereby effectively maintaining protection for the negative electrode interface during battery use, reducing the occurrence of side reactions at the negative electrode interface, and inhibiting the deposition of metal layers at the negative electrode interface, thereby effectively improving the high-temperature cycle life of the silicon-based negative electrode battery and suppressing the increase in storage impedance of the silicon-based negative electrode battery.

[0096] Comparing the test results of Examples 1, 4 to 6 and Examples 7, 8, and 12, it can be seen that when the thickness of the polymer film (x μm) and the mass content of silicon in the negative electrode material (y wt%) satisfy the relationship of formula (1) -2≤25x-y≤0, the polymer film grows with an appropriate thickness to adapt to the larger negative electrode volume expansion brought about by the high silicon content, thereby effectively maintaining the protection of the negative electrode interface during battery use without increasing the lithium ion diffusion resistance at the negative electrode interface too much and occupying too much internal space of the battery, thereby improving the high-temperature cycle life of the silicon-based negative electrode battery and effectively suppressing the increase in storage impedance of the silicon-based negative electrode battery. If, as in Examples 7, 8, and 12, the polymer film is too thin, the polymer film is easily damaged during battery use due to its insufficient structural strength, making it difficult to maintain protection for the negative electrode interface, resulting in a relative decrease in the high-temperature cycle life of the battery and a relative increase in storage impedance.

[0097] Comparing the test results of Examples 1, 4 to 6 and Examples 9 to 11, it can be seen that if the thickness of the polymer film is too thick, as in Examples 9 to 11, the diffusion resistance of lithium ions in the electrolyte at the negative electrode interface will be significantly increased, resulting in a decrease in the lithium ion deintercalation efficiency during the charge and discharge process, and easily causing concentration polarization and battery capacity decay.

[0098] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone familiar with the technology may modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention shall still be covered by the claims of the present invention. Anyone familiar with the technology may modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A negative electrode plate, characterized in that: include: negative electrode current collector; A negative electrode active material layer is provided on the negative electrode current collector; A polymer membrane is provided on the negative electrode active material layer, wherein the polymer membrane comprises polymer PIMs and has an average pore size of 0.7-1 nm.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material layer includes a negative electrode material, the negative electrode material includes a silicon-based material, and the Young's modulus of the polymer film is 15-20 GPa.

3. The negative electrode sheet according to claim 2, characterized in that: The thickness of the polymer film is 0.5-4 μm.

4. The negative electrode sheet according to claim 2 or 3, characterized in that: The mass content of silicon in the negative electrode material is 13-100 wt %; the thickness of the polymer film and the mass content of silicon in the negative electrode material satisfy the relationship of formula (1): -2≤25x-y≤0(1) The thickness of the polymer film is x μm, and the mass content of silicon in the negative electrode material is y wt%.

5. The negative electrode sheet according to claim 1, characterized in that: The porosity of the polymer is 50% to 60%.

6. The negative electrode sheet according to claim 1, characterized in that: The polymer PIMs include a polymer of a first monomer and a second monomer, wherein the first monomer includes a diamine monomer, and the second monomer includes at least one of a dianhydride monomer and a bismaleimide monomer.

7. The negative electrode sheet according to claim 2, characterized in that: The silicon-based material includes at least one of silicon, a silicon-carbon composite and silicon monoxide.

8. A method for preparing the negative electrode sheet according to any one of claims 1 to 7, characterized in that: include: dissolving the polymer PIMs in a first solvent to prepare a glue solution; The glue is applied to the surface of a bare negative electrode plate and dried to form a polymer layer to prepare a negative electrode plate; the bare negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector.

9. The preparation method according to claim 8, characterized in that The preparation process of the polymer PIMs includes: placing a first monomer and a second monomer into a second solvent for reaction to obtain a prepolymer solution; wherein the molar ratio of the first monomer to the second monomer is 1:(1-1.05), the first monomer comprises a diamine monomer, the second monomer comprises at least one of a dianhydride monomer and a bismaleimide monomer, and the solid content of the prepolymer solution is 18-20 wt %; The prepolymer solution is subjected to atomization and imidization or curing treatment in sequence to obtain polymer PIMs.

10. The preparation method according to claim 9, characterized in that The step of atomizing and imidizing or curing the prepolymer solution to obtain polymer PIMs comprises: The prepolymer solution is spray-dried to obtain prepolymer droplets; wherein the inlet temperature of the spray drying is 160-180° C., the outlet temperature of the spray drying is 80-90° C., and the atomization pressure of the spray drying is 0.3-0.5 MPa; The prepolymer droplets are subjected to imidization or curing treatment to obtain polymer PIMs powder; wherein the heating temperature of the imidization or curing treatment is 250-300°C.

11. An electrochemical device, characterized in that: The invention comprises a positive electrode sheet, a separator, an electrolyte and a negative electrode sheet according to any one of claims 1 to 7, or a negative electrode sheet prepared by the preparation method according to any one of claims 8 to 10.

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