Composite negative electrode active material and method for preparing the same, and negative electrode sheet, secondary battery, and electric device comprising the same

Abstract

The application provides a composite negative electrode active material and a preparation method thereof, a negative electrode sheet containing the composite negative electrode active material, a secondary battery and an electric device. The composite negative electrode active material comprises a negative electrode active material matrix and a conductive polymer layer on the surface of the negative electrode active material matrix, wherein the composite negative electrode active material has an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V in a cyclic voltammetry curve. The composite negative electrode active material provided by the application can balance low volume expansion, high specific capacity and high initial coulomb efficiency, and thus can make the secondary battery balance low volume expansion, high energy density and long cycle life.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a composite negative electrode active material and its preparation method, as well as a negative electrode sheet containing the same, a secondary battery, and an electrical device. Background Technology

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. During the charging and discharging process of secondary batteries, the insertion and extraction of lithium ions into the electrode active materials causes volume changes in the electrode active materials, which in turn cause the battery to expand and contract. Battery expansion has become an important factor affecting the safety and electrochemical performance of secondary batteries. In particular, the volume expansion of the negative electrode active material affects the cycle life and capacity characteristics of secondary batteries. Summary of the Invention

[0003] The purpose of this application is to provide a composite negative electrode active material and its preparation method, as well as a negative electrode sheet, a secondary battery and an electrical device containing the same, wherein the composite negative electrode active material can achieve low volume expansion, high specific capacity and high initial coulombic efficiency, thereby enabling the secondary battery to achieve low volume expansion, high energy density and long cycle life.

[0004] The first aspect of this application provides a composite negative electrode active material, comprising a negative electrode active material matrix and a conductive polymer layer located on the surface of the negative electrode active material matrix, wherein the cyclic voltammetry curve of the composite negative electrode active material has an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V.

[0005] The cyclic voltammetry curve of the composite negative electrode active material was obtained by the following method: (1) A negative electrode sheet containing the composite negative electrode active material was provided; (2) A lithium metal sheet was used as the counter electrode, and 1 mol / L lithium hexafluorophosphate was used as the electrolyte, and a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1 was used as the solvent. Glass fiber was used as the separator. A coin cell was assembled with the negative electrode sheet in a glove box filled with argon gas. The packaged coin cell was placed in the glove box and left to stand for more than 12 hours to age it; (3) Cyclic voltammetry test was performed on the obtained coin cell using an electrochemical workstation to obtain the cyclic voltammetry curve of the composite negative electrode active material, wherein the scanning voltage was 2.0V-4.3V and the scanning rate was 0.1mV / s.

[0006] Through extensive research, the inventors of this application discovered that by setting a conductive polymer layer on the surface of the negative electrode active material matrix and making the cyclic voltammetry curve of the composite negative electrode active material have an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V, the obtained composite negative electrode active material can achieve low volume expansion, high specific capacity and high initial coulombic efficiency.

[0007] Unlike traditional conductive carbon coating layers, this application uses a conductive polymer coating layer. Therefore, the coating layer has lower rigidity and better flexibility, making it less prone to cracking during the charging and discharging process of the secondary battery. This continuously protects the negative electrode active material substrate, preventing direct contact between the substrate and the electrolyte, thus avoiding continuous consumption of active lithium ions and the continuous growth and thickening of the SEI film and by-reaction product layer. The conductive polymer layer in this application comprises a conductive polymer, and unlike traditional conductive polymers (such as polyaniline, polypyrrole, polythiophene, etc.), the conductive polymer used in this application enables the composite negative electrode active material to have higher oxidation and reduction peak potentials. Therefore, the conductive polymer layer in this application also has lithium storage functionality. On the one hand, it can share the lithium storage current, reducing the damage to the negative electrode active material substrate; on the other hand, it can share the lithium intercalation pressure of the negative electrode active material substrate, increasing the energy density of the secondary battery. Furthermore, the conductive polymer layer in this application also helps to reduce the lithium ion concentration on the negative electrode side, reducing concentration polarization. Therefore, by applying the composite negative electrode active material of this application to secondary batteries, the secondary batteries can achieve low volume expansion, high energy density, and long cycle life.

[0008] In any embodiment of this application, the cyclic voltammetry curve of the composite negative electrode active material exhibits an oxidation peak in the range of 3.3V-3.55V. This enables the composite negative electrode active material to possess suitable delithiation capabilities, thereby increasing its specific capacity and initial coulombic efficiency, and consequently, increasing the energy density of the secondary battery.

[0009] In any embodiment of this application, the cyclic voltammetry curve of the composite negative electrode active material exhibits a reduction peak in the range of 2.2V-2.45V. This enables the composite negative electrode active material to possess suitable lithium storage capabilities, thereby increasing its specific capacity and initial coulombic efficiency, and consequently, increasing the energy density of the secondary battery.

[0010] In any embodiment of this application, the oxidation peak potential of the cyclic voltammetry curve of the composite negative electrode active material is greater than the oxidation peak potential of the cyclic voltammetry curve of the negative electrode active material matrix. Optionally, the difference between the oxidation peak potential of the cyclic voltammetry curve of the composite negative electrode active material and the oxidation peak potential of the cyclic voltammetry curve of the negative electrode active material matrix is ​​2.4V-2.8V, more preferably 2.5V-2.75V. This enables the composite negative electrode active material to have suitable delithiation function, which is beneficial to increasing the specific capacity and initial coulombic efficiency of the composite negative electrode active material, and thus beneficial to increasing the energy density of the secondary battery.

[0011] In any embodiment of this application, the reduction peak potential of the cyclic voltammetry curve of the composite negative electrode active material is greater than the reduction peak potential of the cyclic voltammetry curve of the negative electrode active material matrix. Optionally, the difference between the reduction peak potential of the cyclic voltammetry curve of the composite negative electrode active material and the reduction peak potential of the cyclic voltammetry curve of the negative electrode active material matrix is ​​1.5V-2.1V, more preferably 1.7V-1.95V. This enables the composite negative electrode active material to possess suitable lithium storage capabilities, thereby increasing the specific capacity and initial coulombic efficiency of the composite negative electrode active material, and consequently increasing the energy density of the secondary battery.

[0012] In any embodiment of this application, the infrared spectrum of the composite negative electrode active material is at a wavenumber of 1396±41 cm⁻¹. -1 It has the first absorption peak at a wavenumber of 1782±53 cm⁻¹. -1 The first absorption peak has a second absorption peak, and optionally, the height ratio of the first absorption peak to the second absorption peak is 1.75 ± 0.1. The composite negative electrode active material of this application has an absorption peak at 1396 ± 41 cm⁻¹. -1 It has a CN stretching vibration peak at 1782±53 cm⁻¹. -1 It exhibits a C=O stretching vibration peak.

[0013] In any embodiment of this application, the conductive polymer layer comprises a conductive polymer, which includes polyimide. The main chain structure of the polyimide has a ketone carbonyl group, and the molar ratio of the ketone carbonyl C=O to the imide ring C(=O)-NC(=O) is 0.5 or higher, preferably 0.5 to 2, and more preferably 0.5 to 1. This helps the cyclic voltammetric curve of the composite negative electrode active material to have suitable oxidation and reduction peak potentials. On the one hand, it enables rapid lithium storage, shares the lithium storage current, and reduces the damage of the current to the negative electrode active material matrix. On the other hand, it can also share the lithium intercalation pressure of the negative electrode active material matrix and increase the energy density of the secondary battery. In addition, it is beneficial to reduce the lithium ion concentration on the negative electrode side and reduce concentration polarization.

[0014] In any embodiment of this application, the polyimide is obtained by polymerizing a dianhydride monomer with a ketone carbonyl C=O main chain structure with a diamine monomer. This helps the cyclic voltammetric curve of the composite negative electrode active material to have suitable oxidation and reduction peak potentials.

[0015] In any embodiment of this application, the monomer unit of the polyimide is as shown in Formula 1.

[0016]

[0017] X represents at least one of the following groups whose main chain structure has a ketone carbonyl C=O group: alkyl, alicyclic, alicyclic, aromatic, and heteroaromatic groups; Y represents a chain-like aliphatic diamine monomer residue, alicyclic diamine monomer residue, or aromatic diamine monomer residue.

[0018] In any embodiment of this application, X may optionally represent any of the following groups:

[0019]

[0020] R1 and R2 represent R independently. a Substituted or unsubstituted C0-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic, or C2-C30 divalent heteroaromatic, where p represents 0, 1, 2, 3, or 4, and R a Each occurrence independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl, with # indicating the connection position.

[0021] In any embodiment of this application, optionally, Y represents any of the following groups:

[0022]

[0023] R3 to R 12 Each represents R independently. a Substituted or unsubstituted C1-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic or C2-C30 divalent heteroaromatic, R b Each occurrence independently represents -O-, -S-, or -C(=O)-, R a It represents at least one of halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl, and # represents the connection position.

[0024] In any embodiment of this application, optionally, Y represents any of the following groups:

[0025]

[0026]

[0027]

[0028] # indicates the connection position, p independently represents 0, 1, 2, 3 or 4 each time it appears, R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0029] In any embodiment of this application, the weight-average molecular weight of the conductive polymer is 30,000 to 100,000. This allows the conductive polymer layer to achieve a good balance of flexibility, coating effect, and resistance to electrolyte swelling.

[0030] In any embodiment of this application, the glass transition temperature of the conductive polymer is 220℃-260℃, optionally 240℃-260℃. This allows the conductive polymer layer to achieve a balance of good flexibility, coating effect, and resistance to electrolyte swelling.

[0031] In any embodiment of this application, the thickness of the conductive polymer layer is less than 50 nm, optionally less than 10 nm, and more preferably 0.1 nm-7 nm. Thus, the conductive polymer layer can achieve good flexibility, coating effect, and resistance to electrolyte swelling, while also facilitating the composite negative electrode active material to achieve both high specific capacity and high initial coulombic efficiency.

[0032] In any embodiment of this application, the average particle size of the composite negative electrode active material is 3μm-15μm, optionally 5μm-10μm, and more preferably 5μm-8μm. This is beneficial for improving lithium-ion and electron transport performance, thereby further enhancing the kinetic performance of the secondary battery.

[0033] In any embodiment of this application, the negative electrode active material matrix comprises at least one material selected from pre-lithiated or non-pre-lithiated materials: carbon-based materials, silicon-based materials, and tin-based materials. Optionally, the carbon-based material comprises at least one material selected from graphite, soft carbon, and hard carbon; optionally, the silicon-based material comprises at least one material selected from elemental silicon, silicon oxide, and silicon alloy; optionally, the tin-based material comprises at least one material selected from elemental tin, tin oxide, and tin alloy.

[0034] In any embodiment of this application, the negative electrode active material matrix is ​​a pre-lithiated silicon oxide. This can further improve the energy density of the secondary battery.

[0035] In any embodiment of this application, the pre-lithiated silicon oxide comprises: a silicon oxide matrix core; a lithium silicate interlayer located on the surface of the silicon oxide matrix core, comprising lithium silicate grains and silicon and / or silicon dioxide nanocrystals; and a carbon coating layer located on the surface of the lithium silicate interlayer. Optionally, the lithium silicate grains comprise Li₂SiO₃ grains. This can improve the capacity utilization of the secondary battery and reduce the volume expansion of the composite negative electrode active material, thereby also improving the safety performance and cycle life of the secondary battery.

[0036] In any embodiment of this application, the mass ratio of the lithium silicate grains to the silicon and / or silicon dioxide nanocrystals in the lithium silicate intermediate layer is (10-50):(50-90).

[0037] In any embodiment of this application, the thickness of the lithium silicate interlayer is ≤35nm, and can be selected as 15nm-35nm. This can increase the initial coulombic efficiency and specific capacity of the composite anode active material.

[0038] In any embodiment of this application, the thickness of the carbon coating layer is ≤25nm, and can be selected as 15nm-25nm. This can improve the conductivity of the composite negative electrode active material and also suppress the leaching of residual lithium into the negative electrode slurry and electrolyte.

[0039] A second aspect of this application provides a method for preparing the composite negative electrode active material of the first aspect of this application, comprising the following steps: S1, providing a negative electrode active material matrix, a dianhydride monomer with a ketone carbonyl C=O main chain structure, a diamine monomer, and a solvent; optionally, the diamine monomer includes at least one selected from chain aliphatic diamine monomers, alicyclic diamine monomers, and aromatic diamine monomers; S2, dissolving the diamine monomer in the solvent to obtain a solution containing the diamine monomer; S3, under a protective gas atmosphere, adding the negative electrode active material matrix to the solution containing the diamine monomer and stirring to mix. After homogenization, the dianhydride monomer with a ketone carbonyl C=O main chain structure is added and stirred to carry out a polymerization reaction. After the reaction is completed, the prepolymer is obtained by washing and drying. S4, under a protective gas atmosphere, the prepolymer obtained in S3 is subjected to an imidization reaction. After the reaction is completed, a composite negative electrode active material is obtained. The composite negative electrode active material includes a negative electrode active material matrix and a conductive polymer layer located on the surface of the negative electrode active material matrix. The cyclic voltammetry curve of the composite negative electrode active material has an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V.

[0040] This application employs an in-situ polymerization method to prepare composite negative electrode active materials, which results in a more uniform thickness of the conductive polymer layer and better adhesion between the conductive polymer layer and the negative electrode active material matrix. In the preparation method of this application, using dianhydride monomers and diamine monomers with ketone carbonyl C=O main chain structures as raw materials also contributes to the good flexibility of the conductive polymer layer, thereby continuously protecting the negative electrode active material matrix and enabling the secondary battery to have a longer cycle life.

[0041] In any embodiment of this application, the imidization reaction in S4 is carried out using a staged heat preservation process.

[0042] Optionally, in any embodiment of this application, the staged heat preservation process includes: a first stage, heating to 100±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a second stage, heating to 150±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a third stage, heating to 200±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a fourth stage, heating to 250±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; and a fifth stage, heating to 300±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours.

[0043] By adopting the above-mentioned staged heat preservation process, the solvent evaporation, imidization temperature, and movement of conductive polymer molecular chains can be optimally matched, which is conducive to promoting the regular arrangement of conductive polymer molecular chains to form a crystalline structure.

[0044] In any embodiment of this application, the molar ratio of the diamine monomer to the dianhydride monomer having a ketone carbonyl C=O main chain structure is ≥2∶1, and can be selected as (2.1~2.8)∶1.

[0045] In any embodiment of this application, the solvent is an aprotic polar solvent, optionally including at least one selected from N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0046] In any embodiment of this application, the dianhydride monomer having a ketone carbonyl C=O main chain structure comprises at least one selected from the following compounds:

[0047]

[0048] R1 and R2 represent R independently. aSubstituted or unsubstituted C0-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic, or C2-C30 divalent heteroaromatic, where p represents 0, 1, 2, 3, or 4, and R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0049] Optionally, in any embodiment of this application, the dianhydride monomer having a ketone carbonyl C=O main chain structure comprises at least one selected from the following compounds:

[0050]

[0051] In any embodiment of this application, the diamine monomer comprises at least one selected from the following compounds:

[0052]

[0053] R3 to R 12 Each represents R independently. a Substituted or unsubstituted C1-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic or C2-C30 divalent heteroaromatic, R b Each occurrence independently represents -O-, -S-, or -C(=O)-, R a It represents at least one of halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0054] In any embodiment of this application, optionally, the diamine monomer comprises at least one selected from the following compounds:

[0055]

[0056]

[0057] Each occurrence of p independently represents 0, 1, 2, 3, or 4, R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0058] In any embodiment of this application, optionally, the diamine monomer comprises at least one selected from the following compounds:

[0059]

[0060]

[0061] In any embodiment of this application, the negative electrode active material matrix includes at least one of the following materials selected from pre-lithiated or non-pre-lithiated materials: carbon-based materials, silicon-based materials, and tin-based materials, and may be selected as pre-lithiated silicon oxide.

[0062] In any embodiment of this application, optionally, the pre-lithiated silicon oxide comprises: a silicon oxide matrix core; a lithium silicate interlayer located on the surface of the silicon oxide matrix core, comprising lithium silicate grains and silicon and / or silicon dioxide nanocrystals; and a carbon coating layer located on the surface of the lithium silicate interlayer. Optionally, the lithium silicate grains comprise Li2SiO3 grains.

[0063] A third aspect of this application provides a negative electrode sheet, including a negative current collector and a negative electrode film layer located on the surface of the negative current collector, wherein the negative electrode film layer includes a composite negative electrode active material of the first aspect of this application or a composite negative electrode active material prepared by the method of the second aspect of this application, and the mass percentage of the composite negative electrode active material in the negative electrode film layer is 1% to 99%, optionally 5% to 30%, based on the total mass of the negative electrode film layer.

[0064] The fourth aspect of this application provides a secondary battery, including the negative electrode sheet of the third aspect of this application.

[0065] The fifth aspect of this application provides an electrical device, including the secondary battery of the fourth aspect of this application.

[0066] The composite negative electrode active material provided in this application can achieve a balance of low volume expansion, high specific capacity, and high initial coulombic efficiency, thereby enabling the secondary battery to achieve a balance of low volume expansion, high energy density, and long cycle life. The power device of this application includes the secondary battery provided in this application, and therefore has at least the same advantages as the secondary battery. Attached Figure Description

[0067] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0068] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0069] Figure 2 yes Figure 1 An exploded view of the implementation method of the secondary battery.

[0070] Figure 3This is a schematic diagram of one embodiment of the battery module of this application.

[0071] Figure 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0072] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0073] Figure 6 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0074] Figure 7 These are the infrared spectra of the composite negative electrode active materials prepared in Example 1 and Comparative Example 1.

[0075] Figure 8 This is a gel permeation chromatography (GPC) chromatogram of the conductive polymer layer in the composite negative electrode active material prepared in Example 1.

[0076] Figure 9 This is a scanning electron microscope (SEM) image of the composite negative electrode active material prepared in Example 1.

[0077] Figure 10 This is a transmission electron microscope (TEM) image of the composite negative electrode active material prepared in Example 4.

[0078] Figure 11 The cyclic voltammetry curve is shown for the negative electrode active material prepared in Example 3.

[0079] Figure 12 This is a scanning electron microscope (SEM) image of the cross-section of the composite negative electrode active material after the capacity of the secondary battery prepared in Example 1 has decayed to 80% of the capacity of the first discharge cycle.

[0080] Figure 13 This is a scanning electron microscope (SEM) image of the cross-section of the composite negative electrode active material of the secondary battery prepared in Comparative Example 1 after the capacity decayed to 80% of the first discharge capacity.

[0081] The accompanying drawings are not necessarily drawn to scale. The reference numerals are as follows: 1 Battery pack, 2 Upper casing, 3 Lower casing, 4 Battery module, 5 Secondary battery, 51 Housing, 52 Electrode assembly, 53 Cover plate. Detailed Implementation

[0082] The following detailed description, with appropriate reference to the accompanying drawings, discloses the composite negative electrode active material of this application, its preparation method, and embodiments including the negative electrode sheet, secondary battery, and power-consuming device thereof. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for a thorough understanding of this application by those skilled in the art and are not intended to limit the subject matter of the claims.

[0083] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0084] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0085] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0086] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0087] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0088] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0089] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0090] In this application, the representation of a single key passing through a single ring or multiple ring systems means that the single key can be connected to any accessible location in the single ring or multiple ring systems.

[0091] Throughout this specification, substituents of compounds are disclosed by groups or ranges. It is expressly intended that such description include each individual sub-combination of members of these groups and ranges. For example, it is expressly intended that the term "C1-C6 alkyl" individually discloses C1, C2, C3, C4, C5, C6, C1-C6, C1-C5, C1-C4, C1-C3, C1-C2, C2-C6, C2-C5, C2-C4, C2-C3, C3-C6, C3-C5, C3-C4, C4-C6, C4-C5, and C5-C6 alkyl.

[0092] The term "alicyclic group" refers to a carbocyclic system with aliphatic properties, including alkyl, alkenyl, and ynyl groups, whose structure can be monocyclic or polycyclic (e.g., fused ring, bridged ring, spirocyclic). Examples of alicyclic groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclopentenyl, cyclohexenyl, and cyclohexynyl.

[0093] The term "alicyclic group" refers to an alicyclic group in which one or more atoms of the ring are elements other than carbon (e.g., N, O, S, etc.). Examples of alicyclic groups include, but are not limited to, ethylene oxide and azircyclic propane.

[0094] The term "aromatic group" refers to a carbocyclic system with aromatic properties, which can be monocyclic, polycyclic, or fused-ring. Examples of aromatic groups include, but are not limited to, phenyl, biphenyl (e.g., diphenyl, triphenyl), diphenylmethane, naphthyl, and indenyl.

[0095] The term "heteroaromatic group" refers to an aromatic group ring in which one or more atoms are elements other than carbon, such as N, O, S, etc. Examples of heteroaromatic groups include, but are not limited to, pyrrole, furanyl, thiophene, pyridinyl, pyrimidinyl, pyridazinyl, pyrazinyl, benzofuranyl, and benzothiophene.

[0096] Unless otherwise stated, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0097] Unless otherwise stated, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, they can be determined according to the testing methods provided in this application.

[0098] A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to reactivate its active materials and continue to be used. During the charging and discharging process, lithium ions repeatedly insert and extract between the positive and negative electrodes. A separator is placed between the positive and negative electrodes, primarily to prevent short circuits between the positive and negative electrodes while allowing lithium ions to pass through. The positive electrode consists of a positive current collector and a positive electrode film. The positive electrode film is typically formed by coating a positive electrode slurry onto the positive current collector, followed by drying and cold pressing. The positive electrode slurry is usually formed by dispersing positive active materials, conductive agents, binders, and other components in a solvent and stirring them evenly. Similarly, the negative electrode consists of a negative current collector and a negative electrode film. The negative electrode film is typically formed by coating a negative electrode slurry onto the negative current collector, followed by drying and cold pressing. The negative electrode slurry is usually formed by dispersing negative active materials, conductive agents, binders, and other components in a solvent and stirring them evenly.

[0099] During the initial charging of a rechargeable battery, a solid electrolyte interphase (SEI) film forms on the surface of the negative electrode active material. This film protects the negative electrode, preventing direct contact between the active material and the electrolyte, thereby reducing electrolyte decomposition and the consumption of active lithium ions. However, as the battery is charged, the negative electrode active material particles expand in volume. The SEI film on its surface cannot withstand this expansion and cracks, exposing fresh active material to the electrolyte. This leads to continued electrolyte decomposition, continuous consumption of active lithium ions, and the continued growth and thickening of the SEI film and by-reaction product layer.

[0100] To reduce the volume expansion of the negative electrode active material, the commonly used strategy is to coat the surface of the negative electrode active material with a conductive carbon layer. This conductive carbon layer prevents direct contact between the negative electrode active material and the electrolyte, and also suppresses the volume expansion of the negative electrode active material. However, the carbon precursors used to form the conductive carbon layer tend to cause the negative electrode active material to agglomerate during the coating process, forming large secondary particles, thus reducing the compaction density of the negative electrode active material. Furthermore, due to the large particle size of the negative electrode active material, an additional crushing process is required, increasing energy consumption and causing a decrease in the uniformity of particle size distribution. In addition, the conductive carbon material in the conductive carbon layer is usually amorphous carbon, exhibiting short-range order but long-range disorder, and possessing high rigidity. This leads to cracking and even breakage of the conductive carbon layer during the charging and discharging process of the secondary battery. After the conductive carbon layer cracks, the fresh negative electrode active material is exposed in the electrolyte, which will cause the electrolyte to decompose continuously, the active lithium ions to be consumed continuously, and the SEI film and by-reaction product layer to grow and thicken continuously, which in turn leads to an increase in the charge transfer impedance of the secondary battery, and a decrease in fast charge and discharge efficiency and capacity.

[0101] Therefore, coating the surface of the negative electrode active material with a conductive carbon layer cannot effectively improve the volume expansion and capacity utilization of the negative electrode active material, and thus it is difficult to make the secondary battery achieve low volume expansion, high energy density and long cycle life.

[0102] The inventors of this application have proposed a novel composite negative electrode active material through extensive research, which can balance low volume expansion, high specific capacity and high initial coulombic efficiency, thereby enabling secondary batteries to balance low volume expansion, high energy density and long cycle life.

[0103] Composite negative electrode active materials

[0104] Specifically, the first aspect of this application provides a composite negative electrode active material, comprising a negative electrode active material matrix and a conductive polymer layer located on the surface of the negative electrode active material matrix, wherein the cyclic voltammetry curve of the composite negative electrode active material has an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V.

[0105] In this application, the "conductive polymer layer" can completely or partially cover the negative electrode active material matrix.

[0106] Through extensive research, the inventors of this application discovered that by setting a conductive polymer layer on the surface of the negative electrode active material matrix and making the cyclic voltammetry curve of the composite negative electrode active material have an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V, the obtained composite negative electrode active material can achieve low volume expansion, high specific capacity and high initial coulombic efficiency.

[0107] Unlike traditional conductive carbon coating layers, this application uses a conductive polymer coating layer. Therefore, the coating layer has lower rigidity and better flexibility, making it less prone to cracking during the charging and discharging process of the secondary battery. This continuously protects the negative electrode active material substrate, preventing direct contact between the substrate and the electrolyte, thus avoiding continuous consumption of active lithium ions and the continuous growth and thickening of the SEI film and by-reaction product layer. The conductive polymer layer in this application comprises a conductive polymer, and unlike traditional conductive polymers (such as polyaniline, polypyrrole, polythiophene, etc.), the conductive polymer used in this application enables the composite negative electrode active material to have higher oxidation and reduction peak potentials. Therefore, the conductive polymer layer in this application also has lithium storage functionality. On the one hand, it can share the lithium storage current, reducing the damage to the negative electrode active material substrate; on the other hand, it can share the lithium intercalation pressure of the negative electrode active material substrate, increasing the energy density of the secondary battery. Furthermore, the conductive polymer layer in this application also helps to reduce the lithium ion concentration on the negative electrode side, reducing concentration polarization. Therefore, by applying the composite negative electrode active material of this application to secondary batteries, the secondary batteries can achieve low volume expansion, high energy density, and long cycle life.

[0108] In some embodiments, the cyclic voltammetry curve of the composite anode active material exhibits an oxidation peak in the range of 3.3V-3.55V. This enables the composite anode active material to possess suitable delithiation capabilities, thereby increasing its specific capacity and initial coulombic efficiency, and consequently, increasing the energy density of the secondary battery.

[0109] In some embodiments, the cyclic voltammetry curve of the composite anode active material exhibits a reduction peak in the range of 2.2V-2.45V. This enables the composite anode active material to possess suitable lithium storage capabilities, thereby increasing its specific capacity and initial coulombic efficiency, and consequently, increasing the energy density of the secondary battery.

[0110] In some embodiments, the oxidation peak potential of the cyclic voltammetric curve of the composite negative electrode active material is greater than the oxidation peak potential of the cyclic voltammetric curve of the negative electrode active material matrix. Optionally, the difference between the oxidation peak potential of the cyclic voltammetric curve of the composite negative electrode active material and the oxidation peak potential of the cyclic voltammetric curve of the negative electrode active material matrix is ​​2.4V-2.8V, more preferably 2.5V-2.75V. This enables the composite negative electrode active material to possess suitable delithiation capabilities, thereby increasing the specific capacity and initial coulombic efficiency of the composite negative electrode active material, and consequently increasing the energy density of the secondary battery.

[0111] In some embodiments, the reduction peak potential of the cyclic voltammetry curve of the composite negative electrode active material is greater than the reduction peak potential of the cyclic voltammetry curve of the negative electrode active material matrix. Optionally, the difference between the reduction peak potential of the cyclic voltammetry curve of the composite negative electrode active material and the reduction peak potential of the cyclic voltammetry curve of the negative electrode active material matrix is ​​1.5V-2.1V, more preferably 1.7V-1.95V. This enables the composite negative electrode active material to possess suitable lithium storage capabilities, thereby increasing the specific capacity and initial coulombic efficiency of the composite negative electrode active material, and consequently increasing the energy density of the secondary battery.

[0112] The cyclic voltammetry curves of the composite anode active material and the anode active material matrix can be obtained by the following method.

[0113] (1) Provide a negative electrode sheet containing a composite negative electrode active material or a negative electrode active material matrix.

[0114] (2) Preparation of button cell: The counter electrode is a lithium metal sheet, the electrolyte is 1 mol / L lithium hexafluorophosphate (LiPF6) as the solute, and a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 as the solvent. The separator is made of glass fiber. The button cell is assembled with the negative electrode sheet in a glove box filled with argon gas (water and oxygen content are both below 0.1 ppm). The packaged button cell is placed in the glove box and left to stand for more than 12 hours to age it.

[0115] (3) Cyclic voltammetry test: Cyclic voltammetry test was performed on the obtained coin cells using an electrochemical workstation (e.g., Shanghai Chenhua CHI 660 electrochemical workstation). Cyclic voltammetry curves of the composite negative electrode active material and the negative electrode active material matrix were obtained respectively. The scanning voltage of the composite negative electrode active material was 2.0V-4.3V (no peak when it is less than 2.0V), and the scanning voltage of the negative electrode active material matrix was 0V-2.0V (no peak when it is greater than 2.0V). The scanning rate was 0.1mV / s.

[0116] It should be noted that the above-mentioned cyclic voltammetry tests on composite negative electrode active materials and negative electrode active material substrates can be performed by sampling during the preparation of the negative electrode sheet, or by sampling from the prepared secondary battery.

[0117] In one embodiment, the negative electrode sheet in step (1) above can be prepared according to the following steps: the composite negative electrode active material or the negative electrode active material matrix is ​​fully mixed with acetylene black and polyvinylidene fluoride (PVDF) in a mass ratio of 7:2:1, and then added to N-methylpyrrolidone (NMP) and mixed evenly to obtain a slurry. The slurry is evenly coated on copper foil and dried in a vacuum drying oven at 120°C for 12 hours to obtain the negative electrode sheet.

[0118] As an example, when sampling and testing from the prepared secondary battery, the sampling can be performed as follows.

[0119] The secondary battery is discharged (for safety reasons, it is generally left fully discharged); after disassembling the secondary battery, the negative electrode is removed and soaked in dimethyl carbonate (DMC) for a certain period of time (e.g., 2-10 hours); then the negative electrode is removed and dried at a certain temperature and time (e.g., 60°C, 4 hours), and after drying, the negative electrode is removed; the dried negative electrode is then assembled into a coin cell according to the above method and subjected to cyclic voltammetry testing to obtain the cyclic voltammetry curve of the composite negative electrode active material.

[0120] The secondary battery is discharged (for safety reasons, it is generally left fully discharged); after disassembling the secondary battery, the negative electrode is removed and soaked in N,N-dimethylformamide (DMF) for a certain period of time (e.g., more than 12 hours) to remove the conductive polymer layer on the surface of the composite negative electrode active material; then the negative electrode is removed and dried at a certain temperature and time (e.g., 60°C, 4 hours), and the dried negative electrode is removed; the dried negative electrode is then assembled into a coin cell according to the above method and subjected to cyclic voltammetry testing to obtain the cyclic voltammetry curve of the negative electrode active material matrix.

[0121] In some embodiments, the infrared spectrum of the composite negative electrode active material is at a wavenumber of 1396±41 cm⁻¹.-1 It has the first absorption peak at a wavenumber of 1782±53 cm⁻¹. -1 The first absorption peak has a second absorption peak, and optionally, the height ratio of the first absorption peak to the second absorption peak is 1.75 ± 0.1. The composite negative electrode active material of this application has an absorption peak at 1396 ± 41 cm⁻¹. -1 It has a CN stretching vibration peak at 1782±53 cm⁻¹. -1 It exhibits a C=O stretching vibration peak.

[0122] In this application, infrared spectroscopy analysis of composite negative electrode active materials can be performed using instruments and methods known in the art, such as an infrared spectrometer (e.g., Thermo Fisher Nicolet iS10 Fourier transform infrared spectrometer) and tests can be performed in accordance with the general rules of infrared spectroscopy analysis method in GB / T6040-2019.

[0123] In this application, the ratio of the height of the first absorption peak to the height of the second absorption peak may be based on the original peak height or the baseline-corrected peak height.

[0124] In some embodiments, the conductive polymer layer comprises a conductive polymer, which includes polyimide (PI). The main chain structure of the polyimide contains ketone carbonyl groups, and the molar ratio of the ketone carbonyl C=O to the imide ring C(=O)-NC(=O) is greater than 0.5. When the main chain structure of the polyimide contains an appropriate amount of ketone carbonyl C=O, it helps the cyclic voltammetric curve of the composite negative electrode active material to have suitable oxidation and reduction peak potentials. On the one hand, this enables rapid lithium storage, shares the lithium storage current, and reduces the damage of the current to the negative electrode active material matrix. On the other hand, it also helps to share the lithium intercalation pressure of the negative electrode active material matrix and increase the energy density of the secondary battery. Furthermore, when the main chain structure of the polyimide contains an appropriate amount of ketone carbonyl C=O, it also helps to reduce the lithium-ion concentration on the negative electrode side and reduce concentration polarization.

[0125] In some embodiments, optionally, the molar ratio of the ketone carbonyl C=O to the imide ring C(=O)-NC(=O) in the main chain structure of the polyimide is 0.5 to 2, more preferably 0.5 to 1. This avoids the oxidation peak potential of the cyclic voltammetry curve of the composite negative electrode active material shifting to excessively high voltages, which could prevent some lithium ions from being successfully extracted.

[0126] In some embodiments, the polyimide is optionally obtained by polymerizing a dianhydride monomer with a ketone carbonyl C=O group in its main chain structure with a diamine monomer. The diamine monomer may or may not have a ketone carbonyl C=O group in its main chain structure. This helps the cyclic voltammetric curve of the composite negative electrode active material to have suitable oxidation and reduction peak potentials.

[0127] In this application, "polyimide" refers to a class of polymers having an imide ring C(=O)-NC(=O) in the main chain structure, and "ketone carbonyl" refers to a carbonyl group whose carbonyl ends are directly connected to carbon atoms. For example, the carbonyl ends may optionally be directly connected to carbon atoms in alkyl, alicyclic, alicyclic, aromatic, heteroaromatic, etc.

[0128] In some embodiments, the monomer units of the polyimide are shown in Formula 1.

[0129]

[0130] X represents at least one of the following groups whose main chain structure has a ketone carbonyl C=O group: alkyl, alicyclic, aliheterocyclic, aromatic, and heteroaromatic. Y represents a chain-like aliphatic diamine monomer residue, an alicyclic diamine monomer residue, or an aromatic diamine monomer residue. Chain-like aliphatic diamines refer to diamine compounds in which two amino groups (-NH2) are attached to a chain-like hydrocarbon compound or its derivatives. Alicyclic diamines refer to diamine compounds in which two amino groups (-NH2) are attached to an alicyclic compound or its derivatives. Aromatic diamines refer to diamine compounds in which two amino groups (-NH2) are attached to an aromatic compound or its derivatives. A diamine monomer residue refers to the remaining group after a diamine compound loses two amino groups (-NH2).

[0131] In some embodiments, X may optionally represent any of the following groups:

[0132]

[0133] R1 and R2 represent R independently. a Substituted or unsubstituted C0-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic, or C2-C30 divalent heteroaromatic, where p represents 0, 1, 2, 3, or 4, and R a Each occurrence independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl, with # indicating the connection position.

[0134] In this application, when R1 and / or R2 are CO divalent alkyl groups, it means that R1 and / or R2 are not present, i.e.

[0135] It can be

[0136] In some embodiments, optionally, Y represents any of the following groups:

[0137]

[0138] R3 to R 12 Each represents R independently. a Substituted or unsubstituted C1-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic or C2-C30 divalent heteroaromatic, R b Each occurrence independently represents -O-, -S-, or -C(=O)-, R a It represents at least one of halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl, and # represents the connection position.

[0139] In some embodiments, optionally, Y represents any of the following groups:

[0140]

[0141]

[0142] # indicates the connection position, p independently represents 0, 1, 2, 3 or 4 each time it appears, R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0143] In Formula 1, the heteroatoms in the alicyclic and heteroaromatic groups may include at least one selected from N, O, and S; optionally, the heteroatom in the heteroaromatic group is N. The halogen atoms in the halogenated alkyl groups may include at least one selected from F, Cl, and Br.

[0144] In some embodiments, the weight-average molecular weight of the conductive polymer is 30,000-100,000. The conductive polymer used in this application is an oligomer. When its weight-average molecular weight is within a suitable range, the conductive polymer layer can achieve good flexibility, coating effect, and resistance to electrolyte swelling. Furthermore, it effectively avoids the following situations: when the weight-average molecular weight of the conductive polymer is too low, it may dissolve in the electrolyte, thus failing to achieve a coating effect; when the weight-average molecular weight of the conductive polymer is too high, it may transform from an oligomer into a polymer, which may lead to a decrease in the flexibility of the conductive polymer layer and make it prone to cracking.

[0145] In this application, the weight-average molecular weight of the conductive polymer has a meaning known in the art and can be determined using instruments and methods known in the art. For example, the composite negative electrode active material or the negative electrode sheet containing it can be immersed in N,N-dimethylformamide (DMF) to dissolve the conductive polymer in the DMF. The solution is then collected using a syringe, and the weight-average molecular weight is determined by gel permeation chromatography (GPC), high performance liquid chromatography (HPLC), or similar methods.

[0146] In some embodiments, the glass transition temperature (Tg) of the conductive polymer is ≤260℃, optionally 220℃-260℃, and more preferably 240℃-260℃. When the conductive polymer of this application has a suitable Tg, the conductive polymer layer can achieve good flexibility, coating effect, and resistance to electrolyte swelling.

[0147] In this application, the glass transition temperature of the conductive polymer has a meaning known in the art and can be determined using instruments and methods known in the art. For example, a composite negative electrode active material or a negative electrode sheet containing it can be immersed in N,N-dimethylformamide (DMF) to dissolve the conductive polymer in the DMF. Heating is then used to volatilize the DMF, resulting in a conductive polymer test sample. The glass transition temperature is then determined by differential scanning calorimetry (DSC) according to GB / T 29611-2013. A Mettler-Toledo DSC-3 differential scanning calorimeter can be used as the testing instrument.

[0148] In some embodiments, the thickness of the conductive polymer layer is less than 50 nm, optionally less than 10 nm, and more preferably 0.1 nm to 7 nm. When the thickness of the conductive polymer layer is within a suitable range, the conductive polymer layer can achieve good flexibility, coating effect, and resistance to electrolyte swelling, while also facilitating the composite negative electrode active material to achieve both high specific capacity and high initial coulombic efficiency.

[0149] In this application, the thickness of the conductive polymer layer has a meaning known in the art and can be measured using instruments and methods known in the art, such as transmission electron microscopy. This allows for a more accurate determination of the boundary between the conductive polymer layer and the negative electrode active material matrix. To ensure the accuracy of the test results, the thickness can be randomly measured at multiple (e.g., more than 30) different locations in the TEM image obtained using transmission electron microscopy of the sample to be tested (e.g., composite negative electrode active material or negative electrode sheet containing it), and the average value can be taken as the thickness of the conductive polymer layer.

[0150] In some embodiments, the average particle size of the composite negative electrode active material is 3μm-15μm, optionally 5μm-10μm, and more preferably 5μm-8μm. When the average particle size of the composite negative electrode active material is within a suitable range, it is beneficial to improve the lithium-ion and electron transport performance, thereby further improving the kinetic performance of the secondary battery.

[0151] In this application, the average particle size of the composite negative electrode active material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, a scanning electron microscope (e.g., ZEISS Sigma 300) can be used to measure the composite negative electrode active material or the negative electrode sheet containing it to obtain a SEM image. A test area with a length of 100 μm and a width of 100 μm is randomly selected from the SEM image, and the longest diagonal length of all composite negative electrode active material particles in the test area is counted to obtain a particle size distribution curve. The particle size corresponding to the particle size distribution percentage reaching 50% is taken as the average particle size of the composite negative electrode active material.

[0152] In some embodiments, the negative electrode active material matrix comprises at least one material selected from pre-lithiated or non-pre-lithiated materials: carbon-based materials, silicon-based materials, and tin-based materials. Optionally, the carbon-based material comprises at least one material selected from graphite, soft carbon, and hard carbon. Optionally, the silicon-based material comprises at least one material selected from elemental silicon, silicon oxide, and silicon alloy. Optionally, the tin-based material comprises at least one material selected from elemental tin, tin oxide, and tin alloy.

[0153] In some embodiments, the negative electrode active material matrix may optionally include at least one selected from pre-lithiated or unlithiated silicon-based materials and tin-based materials.

[0154] Carbon-based materials are the most commonly used negative electrode active materials in rechargeable batteries, but their theoretical specific capacity is relatively low, resulting in limited room for improvement in the energy density of rechargeable batteries. Silicon-based and tin-based materials have the advantage of high theoretical specific capacity, which can improve the energy density of rechargeable batteries, but they suffer from drawbacks such as severe volume expansion, easy pulverization, and low initial coulombic efficiency. By coating the surface of silicon-based and tin-based materials with the conductive polymer layer of this application, the volume expansion of silicon-based and tin-based materials can be suppressed. At the same time, the conductive polymer layer of this application has good flexibility and is not easy to crack during the charging and discharging process of the rechargeable battery. This can also continuously protect the silicon-based and tin-based materials, avoid the continuous consumption of active lithium ions and the continuous growth and thickening of the SEI film and by-reaction product layer, thereby enabling the rechargeable battery to achieve both high energy density and long cycle life. In addition, the conductive polymer used in the conductive polymer layer of this application also has rapid lithium storage and delithiation functions, which can compensate for the low initial coulombic efficiency of silicon-based and tin-based materials, thereby enabling the rechargeable battery to achieve better capacity utilization.

[0155] In some embodiments, the negative electrode active material matrix is ​​optionally pre-lithiated silicon oxide. Compared to unlithiated silicon oxide, pre-lithiated silicon oxide has a higher initial coulombic efficiency, thereby further improving the energy density of the secondary battery.

[0156] In some embodiments, the pre-lithiated silicon oxide may optionally include a silicon oxide matrix core, a lithium silicate intermediate layer, and a carbon coating layer. The lithium silicate intermediate layer is located on the surface of the silicon oxide matrix core and includes lithium silicate grains and silicon and / or silicon dioxide nanocrystals. The carbon coating layer is located on the surface of the lithium silicate intermediate layer.

[0157] In some embodiments, the pre-lithiated silicon oxide may further include residual lithium, such as at least one selected from LiOH, Li, LiH, Li2O, and Li2CO3. This residual lithium is lithium-containing material remaining after incomplete reaction during the pre-lithiation of the silicon oxide, and is a substance generated during the pre-lithiation process. Residual lithium may dissolve in a solvent (e.g., water) during the preparation of the negative electrode slurry, potentially increasing the pH of the slurry. This increase in pH leads to a decrease in the viscosity of the negative electrode slurry and a reduction in the bonding strength of the negative electrode sheet. Furthermore, during the charging and discharging of the secondary battery, residual lithium may react with the solvent in the electrolyte to produce flammable and explosive hydrogen gas, which could pose a serious safety hazard.

[0158] Further research by the inventors of this application revealed that the conductive polymer used in the conductive polymer layer of this application, in addition to possessing rapid lithium storage and delithiation capabilities, thereby compensating for the low initial coulombic efficiency of silicon oxide, also prevents unreacted residual lithium from reacting with the electrolyte during pre-lithiation, thus further improving the cycle life of the secondary battery. Furthermore, the conductive polymer layer of this application can also protect the carbon coating layer in the pre-lithiated silicon oxide and effectively inhibit its cracking and breakage.

[0159] In some embodiments, the lithium silicate grains optionally include Li2SiO3 grains. This can improve the capacity utilization of the secondary battery and reduce the volume expansion of the composite negative electrode active material, thereby also improving the safety performance and cycle life of the secondary battery.

[0160] In some embodiments, the silicon content in the pre-lithiated silicon oxide can be 50wt%-70wt%.

[0161] In some embodiments, the pre-lithiated silicon oxide may be a commercially available product, or optionally, it may be prepared by mixing silicon oxide with a Li precursor and subjecting it to heat treatment, followed by mixing it with a carbonaceous precursor and subjecting it to heat treatment in a protective atmosphere to obtain the pre-lithiated silicon oxide. Optionally, the Li precursor includes at least one selected from LiOH, Li, LiH, Li₂O, and Li₂CO₃. Optionally, the carbonaceous precursor includes at least one selected from pitch, phenolic resin, epoxy resin, starch, glucose, and cellulose.

[0162] In some embodiments, the mass ratio of the lithium silicate grains to the silicon and / or silicon dioxide nanocrystals in the lithium silicate intermediate layer is (10-50):(50-90), for example, it can be a range consisting of 10:90, 15:85, 20:80, 25:75, 30:70, 35:65, 40:60, 45:55, 50:50 or any of the above values.

[0163] In some embodiments, the thickness of the lithium silicate interlayer is ≤35nm, optionally 15nm-35nm. This can increase the initial coulombic efficiency and specific capacity of the composite anode active material.

[0164] In some embodiments, the thickness of the carbon coating layer is ≤25nm, optionally 15nm-25nm. This can improve the conductivity of the composite negative electrode active material and also suppress the leaching of residual lithium into the negative electrode slurry and electrolyte.

[0165] In some embodiments, the composite negative electrode active material includes a negative electrode active material matrix and a conductive polymer layer located on the surface of the negative electrode active material matrix. The cyclic voltammetry curve of the composite negative electrode active material has an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V. The negative electrode active material matrix is ​​a pre-lithiated silicon-based material, optionally a pre-lithiated silicon oxide. The conductive polymer layer includes a conductive polymer, which includes polyimide. The monomer unit of the polyimide is shown in Formula 1, where X represents at least one of the following groups having a ketone carbonyl C=O main chain structure: alkyl, alicyclic, alicyclic, aromatic, and heteroaromatic groups; Y represents a chain-like aliphatic diamine monomer residue, alicyclic diamine monomer residue, or aromatic diamine monomer residue. The weight-average molecular weight of the conductive polymer is 30,000-100,000. Therefore, the composite negative electrode active material can balance low volume expansion, high specific capacity and high initial coulombic efficiency, thereby enabling secondary batteries to better balance low volume expansion, high energy density and long cycle life.

[0166]

[0167] Preparation method

[0168] The second aspect of this application provides a method for preparing the composite negative electrode active material of the first aspect of this application by in-situ polymerization.

[0169] Specifically, the method includes the following steps: S1, providing a negative electrode active material matrix, a dianhydride monomer with a ketone carbonyl C=O main chain structure, a diamine monomer, and a solvent; S2, dissolving the diamine monomer in the solvent to obtain a solution containing the diamine monomer; S3, under a protective gas atmosphere, adding the negative electrode active material matrix to the solution containing the diamine monomer and stirring until homogeneous, then adding the dianhydride monomer with a ketone carbonyl C=O main chain structure and stirring until homogeneous to carry out a polymerization reaction, and after the reaction is completed, washing and drying to obtain a prepolymer; S4, under a protective gas atmosphere, performing an imidization reaction on the prepolymer obtained in S3, and after the reaction is completed, obtaining a composite negative electrode active material, wherein the composite negative electrode active material includes a negative electrode active material matrix and a conductive polymer layer located on the surface of the negative electrode active material matrix, and the cyclic voltammetry curve of the composite negative electrode active material has an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V.

[0170] In some embodiments, the imidization reaction in S4 employs a staged heat preservation process.

[0171] Optionally, the phased heat preservation process includes: a first stage, heating to 100±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a second stage, heating to 150±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a third stage, heating to 200±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a fourth stage, heating to 250±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; and a fifth stage, heating to 300±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours.

[0172] By adopting the above-mentioned staged heat preservation process, the solvent evaporation, imidization temperature, and movement of conductive polymer molecular chains can be optimally matched, which is conducive to promoting the regular arrangement of conductive polymer molecular chains to form a crystalline structure.

[0173] In the phased heat preservation process of this application, the first stage is mainly used to evaporate the solvent in the system, and the second to fourth stages are mainly used for gradual polycondensation to gradually increase the degree of imidization, thereby ensuring that the imidization reaction is more mild.

[0174] In some embodiments, the molar ratio of the diamine monomer to the dianhydride monomer having a ketone carbonyl C=O main chain structure is ≥2∶1, and can be selected as (2.1~2.8)∶1.

[0175] In some embodiments, the solvent may be an aprotic polar solvent, optionally including at least one selected from N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP).

[0176] In some embodiments, the dianhydride monomer having a ketone carbonyl C=O main chain structure may include at least one selected from the following compounds:

[0177]

[0178] R1 and R2 represent R independently. a Substituted or unsubstituted C0-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic, or C2-C30 divalent heteroaromatic, where p represents 0, 1, 2, 3, or 4, and R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0179] As an example, the dianhydride monomer having a ketone carbonyl C=O main chain structure may include at least one selected from the following compounds:

[0180]

[0181] In some embodiments, the diamine monomer may include at least one selected from aliphatic diamine monomers, alicyclic diamine monomers, and aromatic diamine monomers. Optionally, the diamine monomer is an aromatic diamine monomer.

[0182] In some embodiments, the diamine monomer may include at least one selected from the following compounds:

[0183]

[0184] R3 to R 12 Each represents R independently. a Substituted or unsubstituted C1-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic or C2-C30 divalent heteroaromatic, R b Each occurrence independently represents -O-, -S-, or -C(=O)-, R a It represents at least one of halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0185] In some embodiments, the diamine monomer may optionally include at least one selected from the following compounds:

[0186]

[0187]

[0188]

[0189] Each occurrence of p independently represents 0, 1, 2, 3, or 4, R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

[0190] As an example, the diamine monomer may include at least one selected from the following compounds:

[0191]

[0192]

[0193] In some embodiments, the negative electrode active material matrix includes at least one material selected from pre-lithiated or non-pre-lithiated materials: carbon-based materials, silicon-based materials, and tin-based materials, optionally pre-lithiated silicon oxide. Optionally, the pre-lithiated silicon oxide includes a silicon oxide matrix core, a lithium silicate intermediate layer, and a carbon coating layer. The lithium silicate intermediate layer is located on the surface of the silicon oxide matrix core and includes lithium silicate grains and silicon and / or silicon dioxide nanocrystals. The carbon coating layer is located on the surface of the lithium silicate intermediate layer. Optionally, the lithium silicate grains include Li₂SiO₃ grains.

[0194] This application employs an in-situ polymerization method to prepare composite negative electrode active materials, which results in a more uniform thickness of the conductive polymer layer and better adhesion between the conductive polymer layer and the negative electrode active material matrix. In the preparation method of this application, using dianhydride monomers and diamine monomers with ketone carbonyl C=O main chain structures as raw materials also contributes to the good flexibility of the conductive polymer layer, thereby continuously protecting the negative electrode active material matrix and enabling the secondary battery to have a longer cycle life.

[0195] The preparation method of the second aspect of this application can prepare the composite negative electrode active material of any embodiment of the first aspect of this application. The specific types and contents of each raw material used in the preparation process can be referred to the composite negative electrode active material of the first aspect of this application, and will not be repeated here.

[0196] Negative electrode sheet

[0197] A third aspect of this application provides a negative electrode sheet, including a negative current collector and a negative electrode film layer located on the surface of the negative current collector, wherein the negative electrode film layer includes a composite negative electrode active material of the first aspect of this application or a composite negative electrode active material prepared by the method of the second aspect of this application, and the mass percentage of the composite negative electrode active material in the negative electrode film layer is 1% to 99%, optionally 5% to 30%, based on the total mass of the negative electrode film layer.

[0198] The negative electrode current collector has two surfaces opposite each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0199] The negative electrode film layer does not exclude other negative electrode active materials besides the aforementioned composite negative electrode active material. These other negative electrode active materials may be negative electrode active materials known in the art for use in secondary batteries. As an example, the other negative electrode active materials may include at least one selected from natural graphite, artificial graphite, soft carbon, and hard carbon.

[0200] In some embodiments, the negative electrode film layer may optionally include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include at least one selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0201] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose any particular limitation on the type of negative electrode binder. As an example, the negative electrode binder may include at least one selected from styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0202] In some embodiments, the negative electrode film may optionally include other additives. As an example, the other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, etc.

[0203] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymeric material substrate and a metal material layer formed on at least one surface of the polymeric material substrate. As an example, the metal material may include at least one selected from copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymeric material substrate may include at least one selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0204] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is typically formed by dispersing a composite negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0205] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application further includes a conductive undercoat layer (e.g., composed of a conductive agent and an adhesive) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode film layer.

[0206] Secondary batteries

[0207] This application provides a secondary battery in its fourth aspect. A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to activate its active materials and continue to be used. Typically, a secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. This application does not impose any particular limitation on the type of secondary battery; for example, the secondary battery can be a lithium-ion battery, and specifically, a lithium-ion secondary battery.

[0208] [Negative electrode plate]

[0209] The negative electrode used in the secondary battery of this application is the negative electrode of any embodiment of the third aspect of this application, thereby enabling the secondary battery to achieve low volume expansion, high energy density and long cycle life.

[0210] [Positive electrode plate]

[0211] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector and comprising a positive electrode active material. For example, the positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0212] The positive electrode active material may be any positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include at least one of lithium transition metal oxides, lithium-containing phosphates, and their respective modified compounds. Examples of lithium transition metal oxides include, but are not limited to, at least one of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, and their respective modified compounds. Examples of lithium-containing phosphates include, but are not limited to, at least one of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds. These positive electrode active materials may be used alone or in combination of two or more.

[0213] In some embodiments, to further improve the energy density of the secondary battery, the positive electrode active material may include at least one selected from lithium transition metal oxides and their modified compounds as shown in Formula 1.

[0214] Li a Ni b Co c M d O e A f Formula 1

[0215] In Formula 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is selected from at least one of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from at least one of N, F, S and Cl.

[0216] In this application, the modified compounds of the above-mentioned positive electrode active materials may be those that have undergone doping modification and / or surface coating modification of the positive electrode active materials.

[0217] The positive electrode film typically comprises a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP), but is not limited to this.

[0218] As an example, the binder used for the positive electrode film may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. As an example, the conductive agent used for the positive electrode film may include at least one selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0219] The positive electrode current collector can be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil can be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include at least one selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer material base layer may include at least one selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0220] [Electrolytes]

[0221] The electrolyte acts as a conductor for lithium ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0222] In some embodiments, the electrolyte may be an electrolyte solution comprising an electrolyte salt and a solvent.

[0223] This application does not impose specific limitations on the type of electrolyte salt, which can be selected according to requirements. In some embodiments, as an example, the electrolyte salt may include at least one selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0224] This application does not impose specific limitations on the type of solvent, which can be selected according to requirements. In some embodiments, as an example, the solvent may include at least one selected from ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0225] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature power performance.

[0226] [Isolation membrane]

[0227] Secondary batteries using electrolytes, as well as some secondary batteries using solid electrolytes, also include a separator. The separator is positioned between the positive and negative electrodes, serving a separating function. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0228] In some embodiments, the material of the separator may be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0229] In some embodiments, the positive electrode, the separator, and the negative electrode can be fabricated into an electrode assembly using a winding process and / or a stacking process.

[0230] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0231] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0232] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0233] In some embodiments, such as Figure 2 As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates enclosing a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0234] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding process and / or a stacking process. The electrode assembly is placed in an outer packaging, dried, and then injected with an electrolyte. After vacuum sealing, settling, formation, and shaping processes, a secondary battery is obtained.

[0235] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0236] Figure 3 This is a schematic diagram of battery module 4 as an example. Figure 3 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0237] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0238] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0239] Figure 4 and Figure 5 This is a schematic diagram of battery pack 1 as an example. Figure 4 and Figure 5 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0240] Electrical appliances

[0241] This application provides a fifth aspect of an electrical device, which includes at least one of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0242] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0243] Figure 6 This is a schematic diagram of an example electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0244] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0245] Example

[0246] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0247] Example 1

[0248] (1) Preparation of pre-lithiated silicon oxide

[0249] A mixture of metallic silicon and silicon dioxide was introduced into a reactor and vaporized at 650°C for 4 hours under a vacuum atmosphere of 20 Pa, depositing the mixture onto an adsorption plate. After thorough cooling, the deposit was removed and pulverized using a ball mill. Subsequently, silicon oxide particles with an average particle size of 5 μm were obtained through classification and screening.

[0250] A mixed powder was formed by mixing the prepared silicon oxide particles and LiOH powder to achieve a Li / Si molar ratio of 0.7. The mixed powder was then heat-treated at 600°C for 6 hours under a nitrogen atmosphere, followed by pulverization in a mortar to prepare a lithium-containing silicon oxide. At this point, the silicon oxide had an average particle size of 5 μm.

[0251] The prepared silicon oxide was mixed with 15% by weight of coal-based pitch (based on the mass of silicon oxide) and then heat-treated at 700°C for 3 hours under an argon atmosphere. After the heat treatment, the mixture was cooled to room temperature and sieved to obtain pre-lithiated silicon oxide with an average particle size of 5 μm. The outermost layer of the pre-lithiated silicon oxide is a carbon coating layer with a thickness of approximately 15 nm. The middle layer is a mixed layer of Li₂SiO₃ crystals and silicon and / or silica nanocrystals with a thickness of approximately 30 nm, and the mass ratio of Li₂SiO₃ crystals to silicon and / or silica nanocrystals is approximately 25:75. The core is a mixture of silicon and / or silica nanocrystals.

[0252] (2) Preparation of composite negative electrode active materials

[0253] Under an argon atmosphere, 60 mg of the pre-lithiated silicon oxide (average particle size of 5 μm) prepared above was immersed in 100 mL of N,N-dimethylformamide (DMF) solution containing 30 mg of compound 1-1, and stirred for 30 min. Then, 30 mg of compound 2-1 was added to the above solution, and the polymerization reaction was continued for 12 hours. After the reaction, the obtained precursor was washed three times with DMF at a rate of 8000 rpm and dried in a vacuum drying oven at 80 °C for 12 hours to obtain a prepolymer. The obtained prepolymer was transferred to a tube furnace and heated to 100 °C, 150 °C, 200 °C, 250 °C and 300 °C respectively under an argon atmosphere, and held at each temperature for 1 hour. The heating rate was 3 °C / min. After the reaction, the composite negative electrode active material was obtained.

[0254] Compound 1-1

[0255] Compound 2-1

[0256] (3) Preparation of negative electrode sheet

[0257] The composite negative electrode active material prepared above, artificial graphite, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and dispersant sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 12:85:1:1:1, with deionized water added as a solvent, and then stirred until homogeneous to obtain a negative electrode slurry. The obtained negative electrode slurry was then mixed at a concentration of 9.7 mg / cm³. 2 The areal density is uniformly coated on a 7μm thick copper foil for the negative electrode current collector. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0258] (4) Preparation of positive electrode sheet

[0259] LiNi, the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2 (NCM523), polyvinylidene fluoride (PVDF) binder, and acetylene black conductive agent were mixed at a mass ratio of 98:1:1. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred uniformly under vacuum to obtain a positive electrode slurry. The obtained positive electrode slurry was then mixed at a concentration of 13.7 mg / cm³. 2 The areal density is uniformly coated on a 13μm thick positive electrode current collector aluminum foil. After drying at 140℃, cold pressing, and slitting, the positive electrode sheet is obtained.

[0260] (5) Preparation of electrolyte

[0261] In an argon atmosphere glove box (with water and oxygen content below 0.1 ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed uniformly at a volume ratio of 3:7 as an organic solvent. Then, 12.5% ​​(based on the total mass of the organic solvent) of LiPF6 was added and dissolved in the organic solvent. The mixture was stirred evenly to obtain the electrolyte.

[0262] (6) Separating membrane

[0263] A commercially available PP-PE copolymer microporous film with a thickness of 20μm and an average pore size of 80nm (from Zogo Electronics Technology Co., Ltd., model number 20) was used.

[0264] (7) Preparation of secondary batteries

[0265] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is placed in an outer packaging, injected with the electrolyte, and after processes such as encapsulation, standing, formation, and aging, a secondary battery is obtained.

[0266] Examples 2-15 and Comparative Examples 3-5

[0267] The preparation method of the secondary battery is similar to that in Example 1, except that the preparation parameters of the composite negative electrode active material are different, as detailed in Table 1.

[0268] The molecular formulas of each compound are shown in Table 1.

[0269]

[0270] Comparative Example 1

[0271] The preparation method of the secondary battery is similar to that of Example 1, except that pre-lithiated silicon oxide is directly used to replace the composite negative electrode active material prepared in Example 1 when preparing the negative electrode sheet.

[0272] Comparative Example 2

[0273] The preparation method of the secondary battery is similar to that of Example 1, except that the pre-lithiated silicon oxide coated with polythiophene is used instead of the composite negative electrode active material prepared in Example 1 when preparing the negative electrode sheet.

[0274] Table 1

[0275]

[0276] Test section

[0277] (1) Weight-average molecular weight test of conductive polymers

[0278] The composite negative electrode active material prepared above was immersed in N,N-dimethylformamide (DMF) to dissolve the conductive polymer in DMF. The solution was then collected by syringe, and the weight-average molecular weight of the conductive polymer was tested by gel permeation chromatography. The instrument used was an HLC-8320GPC semi-micro gel permeation chromatograph from Tosoh Corporation.

[0279] (2) Glass transition temperature (Tg) test of conductive polymers

[0280] The composite negative electrode active material prepared above is immersed in N,N-dimethylformamide (DMF) to dissolve the conductive polymer in the DMF. Heating causes the DMF to volatilize, thus obtaining the conductive polymer test sample.

[0281] The Tg of the conductive polymer was tested using a Mettler-Toledo DSC-3 differential scanning calorimeter. The test was performed as follows: approximately 10 mg of the test sample was weighed into a flat-bottomed Al2O3 crucible, leveled, and covered. The temperature was increased from 35 °C to 600 °C at a rate of 10 °C / min. Argon was used as the protective gas, with a purge gas flow rate of 50 mL / min and a protective gas flow rate of 20 mL / min.

[0282] (3) Thickness test of conductive polymer layer

[0283] TEM images of the negative electrode were obtained using a Thermo Fisher Talos transmission electron microscope. Thicknesses at 30 different locations were measured on the TEM images, and the average value was taken as the thickness of the conductive polymer layer.

[0284] (4) Cyclic Voltammetry Testing of Composite Anode Active Materials

[0285] The counter electrode is a lithium metal sheet. The electrolyte is 1 mol / L lithium hexafluorophosphate (LiPF6) as the solute and a 1:1 volume ratio mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the solvent. The separator is made of glass fiber. The CR2032 coin cell is assembled with the prepared negative electrode sheet in a glove box filled with argon gas (water and oxygen content are both below 0.1 ppm). The packaged coin cell is placed in the glove box and left to stand for more than 12 hours to age it.

[0286] Cyclic voltammetry tests were performed on the obtained coin cells using a Shanghai Chenhua CHI 660 electrochemical workstation to obtain cyclic voltammetry curves of the composite negative electrode active material. To better observe the peak positions, the scan voltage for Examples 1-15 and Comparative Examples 3-5 was 2.0V-4.3V, the scan voltage for Comparative Example 1 was 0V-2.0V, and the scan voltage for Comparative Example 2 was 0V-4.0V, with a scan rate of 0.1mV / s. Note that the oxidation peak potential and cyclic peak potential given in Table 2 do not consider the peak position of artificial graphite.

[0287] (5) Initial specific capacity and first coulombic efficiency test of button cell

[0288] At 25°C, the CR2032 coin cell prepared above was first discharged to 0V at a constant current density of 10mA / g, and the first discharge capacity of the coin cell was recorded. Then, it was charged to 2.5V at a constant current density of 10mA / g, and the first charge capacity of the coin cell was recorded. The initial specific capacity of the coin cell (mAh / g) = the first charge capacity of the coin cell / (mass of the composite negative electrode active material + mass of artificial graphite). The initial coulombic efficiency (%) of the coin cell = the first charge capacity of the coin cell / the first discharge capacity of the coin cell × 100%.

[0289] (6) Cycle performance test of secondary batteries

[0290] At 25°C, the prepared secondary battery was charged at a 2C rate and discharged at a 1C rate, undergoing continuous cycling tests within the SOC range of 3% to 97% until the capacity of the secondary battery was less than 80% of the capacity of the first discharge cycle. The number of cycles was recorded. In this application, the capacity retention performance of the secondary battery is evaluated by the number of cycles at 25°C and 2C rate fast charging. The higher the number of cycles, the better the capacity retention performance of the secondary battery.

[0291] Table 2

[0292]

[0293] Based on the test results of Examples 1-15 and Comparative Examples 1-5, it can be seen that by setting a conductive polymer layer on the surface of pre-lithiated silicon oxide and making the cyclic voltammetry curve of the composite negative electrode active material have an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V, the obtained composite negative electrode active material can achieve both high specific capacity and high initial coulombic efficiency, thereby enabling the secondary battery to have a long cycle life.

[0294] Figure 7These are the infrared spectra of the composite negative electrode active materials prepared in Example 1 and Comparative Example 1. The testing instrument was a Thermo Fisher Nicoleti S10 Fourier transform infrared spectrometer, and the testing standard was based on GB / T 6040-2019. Figure 7 It can be seen that the composite negative electrode active material prepared in Example 1 has a 1396±41 cm⁻¹ -1 It has a CN stretching vibration peak at 1782±53 cm⁻¹. -1 The presence of a C=O stretching vibration peak indicates that the conductive polymer was successfully coated onto the pre-lithiated silicon oxide surface using the in-situ polymerization method provided in this application.

[0295] Figure 8 This is a gel permeation chromatography (GPC) chromatogram of the conductive polymer layer in the composite negative electrode active material prepared in Example 1. Figure 9 This is a scanning electron microscope (SEM) image of the composite negative electrode active material prepared in Example 1. Figure 10 This is a transmission electron microscope (TEM) image of the composite negative electrode active material prepared in Example 4. Figure 11 The cyclic voltammetry curve is shown for the negative electrode active material prepared in Example 3.

[0296] from Figure 11 It can be seen that the cyclic voltammetry curve of the composite negative electrode active material provided in this application has an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V. In contrast, the pre-lithiated silicon oxide without a conductive polymer coating in Comparative Example 1 did not show any oxidation or reduction peaks in the scan voltage range of 2.0V-4.3V.

[0297] In the composite negative electrode active materials prepared in Comparative Examples 2-5, the conductive polymer layer uses conventional polythiophene or polyimide with a main chain structure lacking ketone carbonyl groups as the conductive polymer. This can improve the initial coulombic efficiency of the coin cell and the cycle number of the secondary cell to some extent, but the improvement effect is limited. A possible reason is that the pre-lithiated silicon oxide exhibits severe volume expansion. Therefore, using conventional polythiophene or polyimide with a main chain structure lacking ketone carbonyl groups as the conductive polymer cannot continuously protect the pre-lithiated silicon oxide. Simultaneously, using conventional polythiophene or polyimide with a main chain structure lacking ketone carbonyl groups as the conductive polymer has a limited effect on improving the oxidation and reduction peak potentials of the cyclic voltammetry curve of the composite negative electrode active material. Consequently, the conductive polymer layer has a poor effect on sharing the lithium storage current and lithium intercalation pressure, and cannot effectively reduce the damage of the current to the pre-lithiated silicon oxide particles. Furthermore, the conductive polymer layer also has a poor effect on reducing concentration polarization on the negative electrode side. Therefore, it is difficult to achieve a long cycle life for secondary batteries when using conventional polythiophene or polyimide whose main chain structure does not have ketone carbonyl groups as conductive polymers.

[0298] Figure 12 This is a scanning electron microscope (SEM) image of the cross-section of the composite negative electrode active material after the capacity of the secondary battery prepared in Example 1 has decayed to 80% of the capacity of the first discharge cycle. Figure 13 This is a scanning electron microscope (SEM) image of the cross-section of the composite negative electrode active material of the secondary battery prepared in Comparative Example 1 after its capacity decayed to 80% of the first discharge capacity. Figure 12 and Figure 13 It can be seen that the thickness of the by-reaction product layer after cycling of the composite negative electrode active material prepared in Example 1 is approximately 484 nm, while the thickness of the by-reaction product layer after cycling of the pre-lithiated silicon oxide without a conductive polymer layer used in Comparative Example 1 is approximately 1423 nm. Therefore, the conductive polymer layer in the composite negative electrode active material provided in this application can continuously protect the negative electrode active material matrix, thereby effectively avoiding the continuous consumption of active lithium ions and the continuous growth and thickening of the SEI film and by-reaction product layer. Consequently, the secondary battery using it can achieve both high energy density and long cycle life.

[0299] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A composite negative electrode active material, comprising a negative electrode active material matrix and a conductive polymer layer located on the surface of the negative electrode active material matrix, wherein, The cyclic voltammetric curve of the composite negative electrode active material shows an oxidation peak in the range of 3.2V-3.6V and a reduction peak in the range of 2.1V-2.6V. The cyclic voltammetry curve of the composite negative electrode active material was obtained by the following method: (1) A negative electrode sheet containing the composite negative electrode active material was provided; (2) A lithium metal sheet was used as the counter electrode, and 1 mol / L lithium hexafluorophosphate was used as the electrolyte, and a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1 was used as the solvent. Glass fiber was used as the separator. A coin cell was assembled with the negative electrode sheet in a glove box filled with argon gas. The packaged coin cell was placed in the glove box and left to stand for more than 12 hours to age it; (3) Cyclic voltammetry test was performed on the obtained coin cell using an electrochemical workstation to obtain the cyclic voltammetry curve of the composite negative electrode active material, wherein the scanning voltage was 2.0 V - 4.3 V and the scanning rate was 0.1 mV / s.

2. The composite negative electrode active material according to claim 1, wherein, The cyclic voltammetric curve of the composite negative electrode active material exhibits an oxidation peak in the range of 3.3 V - 3.55 V; and / or, The cyclic voltammetry curve of the composite negative electrode active material shows a reduction peak in the range of 2.2 V - 2.45 V.

3. The composite negative electrode active material according to claim 1, wherein, The oxidation peak potential of the cyclic voltammetric curve of the composite negative electrode active material is greater than the oxidation peak potential of the cyclic voltammetric curve of the negative electrode active material matrix; and / or, The reduction peak potential of the cyclic voltammetry curve of the composite negative electrode active material is greater than that of the cyclic voltammetry curve of the negative electrode active material matrix.

4. The composite negative electrode active material according to claim 3, wherein, The difference between the oxidation peak potential of the cyclic voltammetry curve of the composite negative electrode active material and the oxidation peak potential of the cyclic voltammetry curve of the negative electrode active material matrix is ​​2.4 V - 2.8 V.

5. The composite negative electrode active material according to claim 4, wherein, The difference between the oxidation peak potential of the cyclic voltammetry curve of the composite negative electrode active material and the oxidation peak potential of the cyclic voltammetry curve of the negative electrode active material matrix is ​​2.5 V - 2.75 V.

6. The composite negative electrode active material according to claim 3, wherein, The difference between the reduction peak potential of the cyclic voltammetry curve of the composite negative electrode active material and the reduction peak potential of the cyclic voltammetry curve of the negative electrode active material matrix is ​​1.5 V - 2.1 V.

7. The composite negative electrode active material according to claim 6, wherein, The difference between the reduction peak potential of the cyclic voltammetry curve of the composite negative electrode active material and the reduction peak potential of the cyclic voltammetry curve of the negative electrode active material matrix is ​​1.7 V - 1.95 V.

8. The composite negative electrode active material according to claim 1, wherein, The infrared spectrum of the composite negative electrode active material is at a wavenumber of 1396±41 cm⁻¹. -1 It has the first absorption peak at a wavenumber of 1782±53 cm⁻¹. -1 It has a second absorption peak.

9. The composite negative electrode active material according to claim 8, wherein, The ratio of the height of the first absorption peak to the height of the second absorption peak is 1.75 ± 0.

1.

10. The composite negative electrode active material according to claim 1, wherein, The conductive polymer layer comprises a conductive polymer, which includes a polyimide. The main chain structure of the polyimide has a ketone carbonyl group and the molar ratio of the ketone carbonyl group C=O to the imide ring C(=O)-NC(=O) is greater than 0.

5.

11. The composite negative electrode active material according to claim 10, wherein, The polyimide is obtained by polymerizing a dianhydride monomer with a ketone carbonyl C=O group in its main chain structure with a diamine monomer.

12. The composite negative electrode active material according to claim 10, wherein, The main chain structure of the polyimide has a ketone carbonyl group and the molar ratio of the ketone carbonyl group C=O to the imide ring C(=O)-NC(=O) is 0.5 to 2.

13. The composite negative electrode active material according to claim 12, wherein, The main chain structure of the polyimide has a ketone carbonyl group and the molar ratio of the ketone carbonyl group C=O to the imide ring C(=O)-NC(=O) is 0.5 to 1.

14. The composite negative electrode active material according to claim 10, wherein, The monomer units of the polyimide are shown in Formula 1. Formula 1 X represents at least one of the following groups whose main chain structure has a ketone carbonyl C=O group: alkyl, alicyclic, alicyclic, aromatic, and heteroaromatic groups; Y represents a chain-like aliphatic diamine monomer residue, alicyclic diamine monomer residue, or aromatic diamine monomer residue.

15. The composite negative electrode active material according to claim 14, wherein, X represents any one of the following groups: ， R1 and R2 represent R independently. a Substituted or unsubstituted C0-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic, or C2-C30 divalent heteroaromatic, where p represents 0, 1, 2, 3, or 4, and R a Each occurrence independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl, with # indicating the connection position.

16. The composite negative electrode active material according to claim 14, wherein, Y represents any one of the following groups: ， R3 to R 12 Each represents R independently. a Substituted or unsubstituted C1-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic or C2-C30 divalent heteroaromatic, R b Each occurrence independently represents -O-, -S-, or -C(=O)-, R a It represents at least one of halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl, and # represents the connection position.

17. The composite negative electrode active material according to claim 16, wherein, Y represents any one of the following groups: ， # indicates the connection position, p independently represents 0, 1, 2, 3 or 4 each time it appears, R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

18. The composite negative electrode active material according to claim 10, wherein, The weight-average molecular weight of the conductive polymer is 30,000-100,000; and / or, The glass transition temperature of the conductive polymer is 220 ℃ - 260 ℃.

19. The composite negative electrode active material according to claim 18, wherein, The glass transition temperature of the conductive polymer is 240 ℃ - 260 ℃.

20. The composite negative electrode active material according to claim 1, wherein, The thickness of the conductive polymer layer is less than 50 nm; and / or, The average particle size of the composite negative electrode active material is 3 μm - 15 μm.

21. The composite negative electrode active material according to claim 20, wherein, The thickness of the conductive polymer layer is less than 10 nm.

22. The composite negative electrode active material according to claim 21, wherein, The thickness of the conductive polymer layer is 0.1 nm - 7 nm.

23. The composite negative electrode active material according to claim 21, wherein, The average particle size of the composite negative electrode active material is 5 μm - 10 μm.

24. The composite negative electrode active material according to claim 23, wherein, The average particle size of the composite negative electrode active material is 5 μm - 8 μm.

25. The composite negative electrode active material according to claim 1, wherein, The negative electrode active material matrix includes at least one of the following materials selected from pre-lithiated or non-pre-lithiated materials: carbon-based materials, silicon-based materials, and tin-based materials.

26. The composite negative electrode active material according to claim 25, wherein, The carbon-based material includes at least one selected from graphite, soft carbon, and hard carbon.

27. The composite negative electrode active material according to claim 25, wherein, The silicon-based material includes at least one selected from elemental silicon, silicon oxide, and silicon alloy.

28. The composite negative electrode active material according to claim 25, wherein, The tin-based material includes at least one selected from elemental tin, tin oxide, and tin alloy.

29. The composite negative electrode active material according to claim 25, wherein, The substrate of the negative electrode active material is a pre-lithiated silicon oxide.

30. The composite negative electrode active material according to claim 29, wherein, The pre-lithiated silicon oxide includes: Silicon oxide matrix core; A lithium silicate interlayer, located on the surface of the silicon oxide matrix core, comprises lithium silicate grains and silicon and / or silicon dioxide nanocrystals; and A carbon coating layer is located on the surface of the lithium silicate intermediate layer.

31. The composite negative electrode active material according to claim 30, wherein, The lithium silicate grains include Li2SiO3 grains.

32. The composite negative electrode active material according to claim 30, wherein, The mass ratio of lithium silicate grains to silicon and / or silicon dioxide nanocrystals in the lithium silicate intermediate layer is (10~50):(50~90); and / or, The thickness of the lithium silicate interlayer is ≤35 nm; and / or, The thickness of the carbon coating layer is ≤25 nm.

33. The composite negative electrode active material according to claim 32, wherein, The thickness of the lithium silicate intermediate layer is 15 nm - 35 nm.

34. The composite negative electrode active material according to claim 32, wherein, The thickness of the carbon coating is 15 nm - 25 nm.

35. A method for preparing the composite negative electrode active material according to any one of claims 1-34, comprising the following steps: S1 provides a negative electrode active material matrix, a dianhydride monomer with a ketone carbonyl C=O main chain structure, a diamine monomer, and a solvent; S2, dissolve the diamine monomer in the solvent to obtain a solution containing the diamine monomer; S3, Under a protective gas atmosphere, the negative electrode active material matrix is ​​added to the solution containing the diamine monomer and stirred until uniform. Then, the dianhydride monomer with a ketone carbonyl C=O main chain structure is added and stirred until uniform to carry out the polymerization reaction. After the reaction is completed, the prepolymer is obtained by washing and drying. S4. Under a protective gas atmosphere, the prepolymer obtained in S3 is subjected to an imidization reaction. After the reaction is complete, the composite negative electrode active material is obtained. The composite negative electrode active material includes a negative electrode active material matrix and a conductive polymer layer located on the surface of the negative electrode active material matrix. The cyclic voltammetry curve of the composite negative electrode active material has an oxidation peak in the range of 3.2 V - 3.6 V and a reduction peak in the range of 2.1 V - 2.6 V.

36. The method according to claim 35, wherein, The diamine monomer includes at least one selected from chain aliphatic diamine monomers, alicyclic diamine monomers, and aromatic diamine monomers.

37. The method of claim 35, wherein, The imidization reaction in S4 employs a staged heat preservation process.

38. The method according to claim 37, wherein, The phased heat preservation process includes: a first stage, heating to 100±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a second stage, heating to 150±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a third stage, heating to 200±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; a fourth stage, heating to 250±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours; and a fifth stage, heating to 300±15℃ at a rate of 3℃ / min-5℃ / min and holding for 1 hour-1.5 hours.

39. The method according to claim 35, wherein, The molar ratio of the diamine monomer to the dianhydride monomer with a ketone carbonyl C=O group in the main chain structure is ≥2:1; and / or, The solvent is an aprotic polar solvent.

40. The method according to claim 39, wherein, The molar ratio of the diamine monomer to the dianhydride monomer with a ketone carbonyl C=O main chain structure is (2.1~2.8):

1.

41. The method according to claim 40, wherein, The solvent includes at least one selected from N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

42. The method according to claim 35, wherein, The dianhydride monomers having a ketone carbonyl C=O main chain structure include at least one selected from the following compounds: ， R1 and R2 represent R independently. a Substituted or unsubstituted C0-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic, or C2-C30 divalent heteroaromatic, where p represents 0, 1, 2, 3, or 4, and R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

43. The method according to claim 42, wherein, The dianhydride monomers having a ketone carbonyl C=O main chain structure include at least one selected from the following compounds: 。 44. The method of claim 35, wherein, The diamine monomer includes at least one selected from the following compounds: ， R3 to R 12 Each represents R independently. a Substituted or unsubstituted C1-C12 divalent alkyl, C3-C12 divalent alicyclic, C1-C12 divalent alicyclic, C6-C30 divalent aromatic or C2-C30 divalent heteroaromatic, R b Each occurrence independently represents -O-, -S-, or -C(=O)-, R a It represents at least one of halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

45. The method according to claim 44, wherein, The diamine monomer includes at least one selected from the following compounds: ， Each occurrence of p independently represents 0, 1, 2, 3, or 4, R a Each time it appears, it independently represents at least one of the following: halogen atom, C1-C6 alkyl, C1-C6 alkoxy, and C1-C6 haloalkyl.

46. ​​The method according to claim 45, wherein, The diamine monomer includes at least one selected from the following compounds: 。 47. The method of claim 35, wherein, The negative electrode active material matrix includes at least one of the following materials selected from pre-lithiated or non-pre-lithiated materials: carbon-based materials, silicon-based materials, and tin-based materials.

48. The method according to claim 35, wherein, The negative electrode active material matrix includes silicon oxide selected from pre-lithiated silicon oxide.

49. The method according to claim 48, wherein, The pre-lithiated silicon oxide includes: Silicon oxide matrix core; A lithium silicate interlayer, located on the surface of the silicon oxide matrix core, comprises lithium silicate grains and silicon and / or silicon dioxide nanocrystals; and A carbon coating layer is located on the surface of the lithium silicate intermediate layer.

50. The method according to claim 49, wherein, The lithium silicate grains include Li2SiO3 grains.

51. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on the surface of the negative electrode current collector, wherein, The negative electrode film layer comprises the composite negative electrode active material according to any one of claims 1-34 or the composite negative electrode active material prepared by the method according to any one of claims 35-50, wherein the mass percentage of the composite negative electrode active material in the negative electrode film layer is 1% to 99% based on the total mass of the negative electrode film layer.

52. The negative electrode sheet according to claim 51, wherein, The composite negative electrode active material in the negative electrode film layer has a mass percentage content of 5% to 30%, based on the total mass of the negative electrode film layer.

53. A secondary battery comprising the negative electrode sheet as described in claim 51 or 52.

54. An electrical device comprising the secondary battery as described in claim 53.

Images