Secondary battery, method for preparing same, and device including the secondary battery

Abstract

The present application relates to a secondary battery, a method for preparing the same, and a device containing the secondary battery. The negative electrode sheet of the secondary battery of the present application includes a negative electrode current collector and a negative electrode film layer, wherein the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, wherein the first negative electrode film layer is disposed on at least one surface of the negative electrode current collector and includes a first negative electrode active material, and the second negative electrode film layer is disposed on the first negative electrode film layer and includes a second negative electrode active material, wherein the first negative electrode active material includes artificial graphite, and the second negative electrode active material includes natural graphite, and the compacted density of the negative electrode film layer is less than or equal to 1.6 g / cm 3 .

Description

Technical Field

[0001] The present application belongs to the field of electrochemical technology, and more specifically, relates to a secondary battery, a preparation method thereof, and a device containing the secondary battery. Background Art

[0002] Secondary batteries are widely used in various consumer electronic products and electric vehicles due to their outstanding features such as light weight, no pollution, and no memory effect.

[0003] With the development of the new energy industry, people are placing higher demands on the energy density of secondary batteries, and at the same time, on their electrochemical performance. However, current methods for improving the energy density of secondary batteries often do not balance the other electrochemical properties of secondary batteries. Therefore, how to ensure that secondary batteries have high energy density while balancing other electrochemical properties is a key challenge in the field of battery design.

[0004] In view of this, it is necessary to provide a secondary battery that can solve the above problems. Summary of the Invention

[0005] In view of the technical problems existing in the background technology, the present application provides a secondary battery, a preparation method thereof and a device containing the secondary battery, aiming to enable the secondary battery to have a high energy density while also taking into account good low-temperature power performance.

[0006] In order to achieve the above-mentioned object, the first aspect of the present application provides a secondary battery, which includes a negative electrode plate, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer, the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, the first negative electrode film layer is arranged on at least one surface of the negative electrode current collector and includes a first negative electrode active material, the second negative electrode film layer is arranged on the first negative electrode film layer and includes a second negative electrode active material, the first negative electrode active material includes artificial graphite, the second negative electrode active material includes natural graphite, and the compaction density of the negative electrode film layer is less than or equal to 1.6 g / cm 3 .

[0007] In any embodiment, the compaction density of the negative electrode film layer is 1.3 g / cm 3 -1.6g / cm 3 , optional 1.4g / cm 3 -1.55g / cm 3 .

[0008] In any embodiment, the powder compaction density of the second negative electrode active material under a pressure of 30,000 N is greater than the powder compaction density of the first negative electrode active material under a pressure of 30,000 N.

[0009] In any embodiment, the volume average particle size D of the second negative electrode active material V 50 is greater than the volume average particle size D of the first negative electrode active material V 50.

[0010] In any embodiment, the specific surface area of the second negative electrode active material is greater than the specific surface area of the first negative electrode active material.

[0011] In any embodiment, the second negative electrode active material has a greater degree of graphitization than the first negative electrode active material.

[0012] In any embodiment, the porosity of the second negative electrode film layer is greater than the porosity of the first negative electrode film layer.

[0013] In any embodiment, the natural graphite has a morphology of one or more of spherical and quasi-spherical.

[0014] In any embodiment, the artificial graphite has a morphology of one or more of block and flake.

[0015] In any embodiment, the mass proportion of the natural graphite in the second negative electrode active material is greater than or equal to 60%, and can be optionally 80%-100%.

[0016] In any embodiment, the mass proportion of the artificial graphite in the first negative electrode active material is greater than or equal to 60%, and can be optionally 80%-100%.

[0017] In any embodiment, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:0.9-1:1.5, and can be optionally 1:01-1:1.2.

[0018] In any embodiment, the surface density CW of the negative electrode film layer satisfies: 9 mg / cm 2 ≤CW≤13mg / cm 2 , optionally, 10.0 mg / cm 2 ≤CW≤11.5mg / cm 2 .

[0019] In any embodiment, the first negative electrode film layer and / or the second negative electrode film layer comprises a silicon-based material.

[0020] In any embodiment, the first negative electrode film layer includes a silicon-based material, and the mass proportion of the silicon-based material in the first negative electrode film layer is less than or equal to 60%, and can be optionally 1%-30%.

[0021] In any embodiment, the second negative electrode film layer includes a silicon-based material, and the mass proportion of the silicon-based material in the second negative electrode film layer is less than or equal to 10%, and can be optionally 1%-5%.

[0022] In any embodiment, the first negative electrode film layer and the second negative electrode film layer both include silicon-based materials, and the mass proportion of the silicon-based material in the first negative electrode film layer is recorded as W1, and the mass proportion of the silicon-based material in the second negative electrode film layer is recorded as W2, then W1≥W2.

[0023] In any embodiment, negative electrode film layers are provided on both surfaces of the negative electrode current collector, and the negative electrode film layers on the two surfaces have different thicknesses.

[0024] In any embodiment, negative electrode film layers are provided on both surfaces of the negative electrode current collector, and the negative electrode film layers on the two surfaces have different surface densities.

[0025] In any embodiment, negative electrode film layers are provided on both surfaces of the negative electrode current collector, and the negative electrode film layers on the two surfaces have different compaction densities.

[0026] In any embodiment, the secondary battery includes a positive electrode plate, the positive electrode plate includes a positive electrode collector and a positive electrode film layer arranged on at least one surface of the positive electrode collector and including a positive electrode active material, the positive electrode active material including a lithium transition metal oxide, an olivine-structured lithium-containing phosphate and their respective modified compounds. One or more.

[0027] In any embodiment, optionally, the positive electrode active material includes one or more of an olivine-structured lithium-containing phosphate and a modified compound thereof.

[0028] A second aspect of the present application provides a method for preparing a secondary battery, comprising preparing a negative electrode sheet of the secondary battery by the following steps:

[0029] 1) forming a first negative electrode film layer comprising a first negative electrode active material on at least one surface of a negative electrode current collector, wherein the first negative electrode active material comprises artificial graphite;

[0030] 2) forming a second negative electrode film layer comprising a second negative electrode active material on the first negative electrode film layer, wherein the second negative electrode active material comprises natural graphite;

[0031] 3) The compaction density of the negative electrode film is controlled at 1.6g / cm through the cold pressing process 3 the following.

[0032] The third aspect of the present application provides a device, which includes the secondary battery described in the first aspect of the present application or the secondary battery prepared by the method described in the second aspect of the present application.

[0033] Compared with the prior art, the present application includes at least the following beneficial effects: the negative electrode plate of the secondary battery of the present application includes a first negative electrode film layer and a second negative electrode film layer, a first negative electrode active material of a specific composition is selected in the first negative electrode film layer, a second negative electrode active material of a specific composition is selected in the second negative electrode film layer, and the compaction density of the negative electrode film layer is controlled within a specific range, thereby enabling the secondary battery of the present application to have a higher energy density while also taking into account better low-temperature power performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] To more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on the drawings without inventive effort.

[0035] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of the present application.

[0036] Figure 2 It is a schematic diagram of one embodiment of the negative electrode sheet in the secondary battery of the present application.

[0037] Figure 3 It is a schematic diagram of another embodiment of the negative electrode sheet in the secondary battery of the present application.

[0038] Figure 4 This is an exploded schematic diagram of an embodiment of the secondary battery of the present application.

[0039] Figure 5 is a schematic diagram of one embodiment of a battery module.

[0040] Figure 6 is a schematic diagram of one embodiment of a battery pack.

[0041] Figure 7 yes Figure 6 Schematic diagram of the decomposition.

[0042] Figure 8 This is a schematic diagram of one embodiment of a device including the secondary battery of the present application as a power source.

[0043] Figure 9 This is a scanning electron microscope (SEM) image of the cross section of the negative electrode sheet prepared in Example 1.

[0044] The reference numerals are as follows: 1 battery pack, 2 upper case, 3 lower case, 4 battery module, 5 secondary battery, 51 shell, 52 electrode assembly, 53 cover plate, 10 negative electrode sheet, 101 negative electrode current collector, 102 second negative electrode film layer, 103 first negative electrode film layer. DETAILED DESCRIPTION

[0045] Below, the embodiments of the secondary battery of the present application, its preparation method and the device containing the secondary battery are described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0046] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0048] Unless otherwise specified, all steps of the present 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 may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0049] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0050] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: 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).

[0051] In this application, the terms "plurality" and "multiple" refer to two or more.

[0052] Unless otherwise specified, the terms used in this application have the commonly understood meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0053] secondary batteries

[0054] The first aspect of the present application provides a secondary battery. The secondary battery includes a positive electrode sheet, a negative electrode sheet and an electrolyte. During the charge and discharge process of the battery, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet. The present application has no particular restrictions on the shape of the secondary battery, which can be cylindrical, square or any other shape. Figure 1 A secondary battery 5 having a square structure is shown as an example.

[0055] [Negative electrode]

[0056] The negative electrode sheet in the secondary battery of the present application includes a negative electrode current collector and a negative electrode film layer, wherein the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, wherein the first negative electrode film layer is arranged on at least one surface of the negative electrode current collector and includes a first negative electrode active material, and the second negative electrode film layer is arranged on the first negative electrode film layer and includes a second negative electrode active material, wherein the first negative electrode active material includes artificial graphite, and the second negative electrode active material includes natural graphite, and the compaction density of the negative electrode film layer is less than or equal to 1.6 g / cm 3 .

[0057] Because the negative electrode sheet of the present application utilizes a double-layer negative electrode film layer, and the first and second negative electrode film layers utilize specific first and second negative electrode active materials, respectively, the battery can achieve both high energy density and good low-temperature power performance. The inventors have discovered that when the first negative electrode active material of the present application comprises artificial graphite, the second negative electrode active material comprises natural graphite, and the compaction density of the negative electrode film layers is controlled within a given range, the active sites in the first and second negative electrode film layers are optimally matched, thereby facilitating improved low-temperature power performance of the battery. Simultaneously, the porosity of the first and second negative electrode film layers is also optimally optimized, thereby facilitating electrolyte infiltration and increasing the battery's cycle life. Natural graphite has good kinetic properties and is located in the second negative electrode film layer, away from the negative electrode current collector, which facilitates active ion transport. However, natural graphite is not pressure-resistant. Therefore, controlling the compaction density of the negative electrode film layer within a given range can help fully utilize the active ion transport properties of natural graphite, thereby enabling the battery to achieve high energy density while also maintaining good low-temperature power performance. Artificial graphite has good cycling performance and is located in the first negative electrode film layer, close to the negative electrode current collector, which helps improve the battery's cycle life and energy density.

[0058] It should be noted that the compaction density of the negative electrode film layer refers to the compaction density of the entire negative electrode film layer, rather than the compaction density of each of the first negative electrode film layer and the second negative electrode film layer.

[0059] In some embodiments, the compaction density of the negative electrode film layer is 1.3 g / cm 3 -1.6g / cm 3 , optional 1.4g / cm 3 -1.55g / cm 3 .

[0060] The compacted density of the negative electrode film layer has a well-known meaning in the art and can be measured using methods known in the art. The compacted density of the negative electrode film layer = the surface density of the negative electrode film layer / the thickness of the negative electrode film layer.

[0061] The surface density of the negative electrode film layer has a well-known meaning in the art and can be tested using methods known in the art. For example, take a coated and cold-pressed negative electrode sheet, punch it into a small disc with an area of S1, weigh its mass and record it as M1, then take a small disc with the same area of S1, wipe off the negative electrode film layer, weigh the mass of the negative electrode current collector and record it as M0. The surface density of the negative electrode film layer = (mass of the negative electrode sheet M1 - mass of the negative electrode current collector M0) / S1. To ensure the accuracy of the test results, multiple groups (for example, 10 groups) of test samples can be tested and the average value is taken as the test result.

[0062] The thickness of the negative electrode film layer can be measured using a micrometer, for example, a Mitutoyo 293-100 with an accuracy of 0.1 μm. In this application, the thickness of the negative electrode film layer refers to the sum of the thicknesses of the first negative electrode film layer and the second negative electrode film layer.

[0063] The inventors of the present application have found through in-depth research that when the negative electrode plate of the present application satisfies the above-mentioned design and optionally satisfies one or more of the following parameters, the performance of the battery can be further improved.

[0064] In some embodiments, the powder compaction density of the second negative electrode active material under a pressure of 30,000 N is greater than the powder compaction density of the first negative electrode active material under a pressure of 30,000 N.

[0065] The compacted density of a material powder has a well-known meaning in the art and can be measured using methods known in the art. For example, referring to GB / T 24533-2009, an electronic pressure testing machine (such as the UTM7305) can be used for testing: a certain amount of powder is placed on a dedicated compaction mold, and different pressures are set. The thickness of the powder at different pressures can be read on the machine, and the compacted density at different pressures can be calculated. In this application, the pressure is set to 30,000 N.

[0066] In some embodiments, the volume average particle size D of the second negative electrode active material is V 50 is greater than the volume average particle size D of the first negative electrode active material V 50.

[0067] Volume average particle size D of the material V 50 has a well-known meaning in the art and can be measured using methods known in the art. For example, it can be measured using a laser diffraction particle size distribution measuring instrument (such as Mastersizer 3000) according to the particle size distribution laser diffraction method (for details, please refer to GB / T19077-2016). V 50 refers to the particle size corresponding to when the cumulative volume percentage of the material reaches 50%.

[0068] In some embodiments, the specific surface area (SSA) of the second negative active material is greater than the specific surface area (SSA) of the first negative active material.

[0069] The specific surface area (SSA) of a material has a meaning well known in the art and can be measured using methods known in the art, such as nitrogen adsorption specific surface area analysis and calculation using the BET (Brunauer Emmett Teller) method. Nitrogen adsorption specific surface area analysis can be performed using a NOVA 2000e surface area and pore size analyzer available from Quantachrome.

[0070] In some embodiments, the second negative electrode active material has a greater degree of graphitization than the first negative electrode active material.

[0071] The graphitization degree of a material has a well-known meaning in the art and can be tested using methods known in the art. For example, an X-ray diffractometer (such as Bruker D8 Discover) can be used for testing. The test can refer to JIS K 0131-1996 and JB / T 4220-2011, and the d 002 The size of the G is then calculated according to the formula G = (0.344-d 002 ) / (0.344-0.3354)×100% to calculate the graphitization degree, d 002 It is the distance between layers in the crystal structure of a material expressed in nanometers (nm).

[0072] In some embodiments, the porosity of the second negative electrode film layer is greater than the porosity of the first negative electrode film layer.

[0073] The porosity of a membrane layer has a well-known meaning in the art and can be measured using methods known in the art. For example, the true volume of the membrane layer can be determined using a Micromeritics AccuPyc II 1340 fully automatic true density tester using a gas (e.g., helium or nitrogen) displacement method in accordance with GB / T 24586-2009. The porosity of the membrane layer is calculated as follows: (apparent volume of the membrane layer - true volume of the membrane layer) / apparent volume of the membrane layer.

[0074] In some embodiments, the natural graphite has a morphology of one or more of spherical and quasi-spherical.

[0075] In some embodiments, the artificial graphite has a morphology of one or more of block and flake.

[0076] The morphology of a material is well-known in the art and can be measured using methods known in the art. For example, the material can be attached to a conductive adhesive and the morphology of the material can be measured using a scanning electron microscope (e.g., a ZEISS Sigma 300). For testing, see JY / T010-1996.

[0077] In some embodiments, the natural graphite has a porous structure inside, so that the natural graphite and artificial graphite in the negative electrode film layer can be distinguished by optical microscopy or scanning electron microscopy, and the position of the boundary area between the first negative electrode film layer and the second negative electrode film layer can be easily determined.

[0078] In some embodiments, the mass proportion of the natural graphite in the second negative electrode active material is greater than or equal to 60%, and can be optionally 80%-100%.

[0079] In some embodiments, the mass proportion of the artificial graphite in the first negative electrode active material is greater than or equal to 60%, and can be optionally 80%-100%.

[0080] In some embodiments, a thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:0.9-1:1.5, and can be optionally 1:1.01-1:1.2.

[0081] When the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is within the given range, it is beneficial to form a gradient pore distribution in the negative electrode film layer, so that the liquid phase conduction resistance of the active ions released from the positive electrode on the surface of the negative electrode film layer is reduced, and the active ions will not accumulate on the surface of the negative electrode film layer to cause lithium precipitation problems. At the same time, the uniform diffusion of active ions in the negative electrode film layer is also beneficial to reducing polarization, further improving the low-temperature power performance and cycle performance of the battery.

[0082] In this application, the thickness of each of the first negative electrode film layer and the second negative electrode film layer can be measured using a scanning electron microscope (such as ZEISS Sigma 300). The sample preparation is as follows: first, the negative electrode sheet is cut into a sample of a certain size (for example, 2 cm × 2 cm), and the negative electrode sheet is fixed on the sample stage with paraffin wax; then the sample stage is placed in the sample holder and locked, and the power of the argon ion cross-section polisher (for example, IB-19500CP) is turned on and the vacuum is applied (for example, 10 -4Pa), set the argon flow rate (e.g. 0.15MPa) and voltage (e.g. 8KV) and polishing time (e.g. 2 hours), adjust the sample stage to the swing mode and start polishing. For sample testing, please refer to JY / T010-1996. In order to ensure the accuracy of the test results, multiple (e.g. 10) different areas can be randomly selected from the sample to be tested for scanning testing, and at a certain magnification (e.g. 500 times), read the thickness of the first negative electrode film layer and the second negative electrode film layer in the scale test area, and take the average value of the test results of multiple test areas as the thickness of the first negative electrode film layer and the second negative electrode film layer.

[0083] In some embodiments, the surface density CW of the negative electrode film layer satisfies: 9 mg / cm 2 ≤CW≤13mg / cm 2 , optionally, 10.0 mg / cm 2 ≤CW≤11.5mg / cm 2 .

[0084] It should be noted that the areal density CW of the negative electrode film layer refers to the areal density of the entire negative electrode film layer, that is, the sum of the areal densities of the first negative electrode film layer and the second negative electrode film layer.

[0085] In the secondary battery of the present application, the first negative electrode film layer and / or the second negative electrode film layer generally contain a negative electrode active material and an optional binder, an optional conductive agent, and other optional additives, and are generally formed by coating and drying the respective film layer slurries. The negative electrode film layer slurry is generally formed by dispersing the negative electrode active material and the optional conductive agent and binder in a solvent and stirring them evenly. The solvent may be, for example, N-methylpyrrolidone (NMP) or deionized water. Other optional additives may be, for example, a thickener (e.g., sodium carboxymethyl cellulose CMC-Na), a PTC thermistor material, and the like.

[0086] As an example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0087] As an example, the binder may include one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0088] In the secondary battery of the present application, in addition to the graphite material mentioned above, the first negative electrode active material and / or the second negative electrode active material may optionally include a certain amount of other commonly used negative electrode active materials, such as one or more of soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys. The tin-based material may include one or more of elemental tin, tin oxide compounds, and tin alloys. The silicon-based material may be a pre-lithiated silicon-based material or a non-pre-lithiated silicon-based material. The tin-based material may be a pre-lithiated tin-based material or a non-pre-lithiated tin-based material. The preparation methods of these materials are well known and can be obtained commercially. Those skilled in the art can make appropriate choices based on the actual use environment.

[0089] In some embodiments, the first negative electrode film layer and / or the second negative electrode film layer comprises a silicon-based material. Optionally, the silicon-based material is a pre-lithiated silicon-based material, thereby reducing the volume expansion of the negative electrode plate and compensating for the loss of active ions.

[0090] In some embodiments, the first negative electrode film layer includes a silicon-based material, and the mass proportion of the silicon-based material in the first negative electrode film layer is less than or equal to 60%, and can be optionally 1%-30%.

[0091] In some embodiments, the second negative electrode film layer includes a silicon-based material, and the mass proportion of the silicon-based material in the second negative electrode film layer is less than or equal to 10%, and can be optionally 1%-5%.

[0092] In some embodiments, the first negative electrode film layer and the second negative electrode film layer both include silicon-based materials, and the mass proportion of the silicon-based material in the first negative electrode film layer is recorded as W1, and the mass proportion of the silicon-based material in the second negative electrode film layer is recorded as W2, then W1≥W2.

[0093] The above-mentioned various parameter tests on the first negative electrode active material and the second negative electrode active material can be conducted by sampling and testing before slurry coating, or by sampling and testing from the negative electrode film layer after cold pressing.

[0094] When the test sample is sampled from the negative electrode film layer after cold pressing, as an example, the sampling can be performed according to the following steps.

[0095] (1) First, randomly select a cold-pressed negative electrode film layer and sample the second negative electrode active material (for example, a blade can be used to scrape the powder for sampling). The scraping depth does not exceed the boundary area between the first negative electrode film layer and the second negative electrode film layer.

[0096] (2) Secondly, the first negative electrode active material is sampled. During the cold pressing process of the negative electrode film layer, there may be an interfusion layer in the boundary area between the first negative electrode film layer and the second negative electrode film layer (that is, the first active material and the second active material are simultaneously present in the interfusion layer). For the accuracy of the test, when sampling the first negative electrode active material, the interfusion layer can be scraped off first, and then the first negative electrode active material can be sampled by scraping powder.

[0097] (3) The first negative electrode active material and the second negative electrode active material collected above are placed in deionized water respectively, and the first negative electrode active material and the second negative electrode active material are filtered and dried. Then, the dried negative electrode active materials are sintered at a certain temperature and time (for example, 400°C, 2h) to remove the binder and the conductive agent, thereby obtaining test samples of the first negative electrode active material and the second negative electrode active material.

[0098] During the sampling process, an optical microscope or a scanning electron microscope may be used to assist in determining the position of the boundary region between the first negative electrode film layer and the second negative electrode film layer.

[0099] The natural graphite and artificial graphite used in this application can be obtained through commercial channels.

[0100] In the secondary battery of the present application, the negative electrode current collector may be a metal foil or a composite current collector, and the composite current collector may be formed by placing a metal material on a polymer substrate. As an example, the negative electrode current collector may be a copper foil.

[0101] In the secondary battery of the present application, the negative electrode film layer may be provided on one surface of the negative electrode current collector, or may be provided on both surfaces of the negative electrode current collector.

[0102] Figure 2 The schematic diagram of an embodiment of the negative electrode plate 10 in the secondary battery of the present application is shown. The negative electrode plate 10 is composed of a negative electrode current collector 101, a first negative electrode film layer 103 respectively disposed on both surfaces of the negative electrode current collector 101, and a second negative electrode film layer 102 disposed on the first negative electrode film layer 103.

[0103] Figure 3 A schematic diagram of another embodiment of the negative electrode sheet 10 in the secondary battery of the present application is shown. The negative electrode sheet 10 is composed of a negative electrode current collector 101, a first negative electrode film layer 103 disposed on one surface of the negative electrode current collector 101, and a second negative electrode film layer 102 disposed on the first negative electrode film layer 103.

[0104] In the secondary battery of the present application, when the negative electrode film layer is arranged on both surfaces of the negative electrode current collector, the parameter ranges (such as thickness, surface density, compaction density, etc.) of the negative electrode film layers on both sides of the negative electrode current collector can be the same or different.

[0105] In some embodiments, negative electrode film layers are disposed on both surfaces of the negative electrode current collector, and the negative electrode film layers on the two surfaces have different thicknesses.

[0106] In some embodiments, negative electrode film layers are disposed on both surfaces of the negative electrode current collector, and the negative electrode film layers on the two surfaces have different surface densities.

[0107] In some embodiments, negative electrode film layers are disposed on both surfaces of the negative electrode current collector, and the negative electrode film layers on the two surfaces have different compaction densities.

[0108] When one or more parameters of the thickness, surface density, and compaction density of the negative electrode film layer located on the two surfaces of the negative electrode current collector are different, the differentiated design of the negative electrode film layer can play a role in reducing the internal resistance of the battery, improving the battery safety performance, and extending the battery cycle life.

[0109] It should be noted that the parameter ranges for each negative electrode film layer (such as thickness, surface density, and compacted density) given in this application refer to the parameter ranges for the negative electrode film layer on a single side of the negative electrode current collector. When the negative electrode film layer is provided on both surfaces of the negative electrode current collector, the parameters of the negative electrode film layer on either surface satisfy the requirements of this application and are considered to fall within the scope of protection of this application. The parameter ranges for the negative electrode film layer thickness, surface density, and compacted density described in this application refer to the parameter ranges for the negative electrode film layer after cold pressing and used for battery assembly.

[0110] In addition, in the secondary battery of the present application, the negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode plate described in the present application further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the first negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode plate described in the present application further includes a protective layer covering the surface of the second negative electrode film layer.

[0111] [Positive electrode]

[0112] In the secondary battery of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector and including a positive electrode active material.

[0113] It can be understood that the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer can be provided on either or both of the two opposite surfaces of the positive electrode current collector.

[0114] In the secondary battery of the present application, the positive electrode current collector can be a conventional metal foil or a composite current collector, and the composite current collector can be formed by placing a metal material on a polymer substrate. As an example, the positive electrode current collector can be aluminum foil.

[0115] In the secondary battery of the present application, the positive electrode active material may include one or more of a lithium transition metal oxide, an olivine-structured lithium-containing phosphate, and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of lithium phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds. The present application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for secondary batteries may also be used.

[0116] In some embodiments, to improve the energy density of the battery, the positive electrode active material may include one or more of the lithium transition metal oxides represented by Formula 1 and modified compounds thereof.

[0117] Li a Ni b Co c M d O e A f Formula 1

[0118] 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 one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from one or more of N, F, S and Cl.

[0119] In some embodiments, in order to improve the safety performance of the battery, the positive electrode active material may include one or more of olivine-structured lithium-containing phosphates and their modified compounds, for example, may include one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon and their respective modified compounds.

[0120] In the present application, the modified compounds of the above materials may be doping-modified and / or surface-coated modified materials.

[0121] In the secondary battery of the present application, the positive electrode film layer may optionally further include a binder and / or a conductive agent.

[0122] As an example, the binder for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0123] As an example, the conductive agent used for the positive electrode film layer may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0124] [Electrolytes]

[0125] The electrolyte conducts active ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte; it can be selected based on needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte solution).

[0126] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0127] In some embodiments, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium bisoxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP).

[0128] In some embodiments, the solvent may include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene 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), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE).

[0129] 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, or additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature performance.

[0130] [Isolation film]

[0131] Secondary batteries using electrolytes and some secondary batteries using solid electrolytes also include an isolating membrane. The isolating membrane is arranged between the positive electrode plate and the negative electrode plate to play an isolating role. The present application has no particular restrictions on the type of isolating membrane, and any well-known porous structure membrane with good chemical stability and mechanical stability can be selected. In some embodiments, the material of the isolating membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolating membrane can be a single-layer film or a multi-layer composite film. When the isolating membrane is a multi-layer composite film, the materials of each layer can be the same or different.

[0132] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process and / or a lamination process.

[0133] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0134] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0135] In some embodiments, as Figure 4 As shown, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can be formed into an electrode assembly 52 through a winding process and / or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be adjusted according to demand.

[0136] In some embodiments, secondary batteries may be assembled into a battery module. The battery module may contain multiple secondary batteries, and the specific number may be adjusted according to the application and capacity of the battery module.

[0137] Figure 5 As an example, the battery module 4 is shown. Figure 5As shown, in the battery module 4, the plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0138] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0139] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0140] Figure 6 and Figure 7 The battery pack 1 is used as an example. Figure 6 and Figure 7 As shown, a battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0141] Method for preparing secondary battery

[0142] In a second aspect of the present application, a method for preparing a secondary battery is provided, comprising preparing a negative electrode sheet of the secondary battery by the following steps:

[0143] 1) forming a first negative electrode film layer comprising a first negative electrode active material on at least one surface of a negative electrode current collector, wherein the first negative electrode active material comprises artificial graphite;

[0144] 2) forming a second negative electrode film layer comprising a second negative electrode active material on the first negative electrode film layer, wherein the second negative electrode active material comprises natural graphite;

[0145] 3) The compaction density of the negative electrode film is controlled at 1.6g / cm through the cold pressing process 3 the following.

[0146] During the preparation process of the secondary battery, by controlling and adjusting the respective compositions of the first negative electrode active material and the second negative electrode active material of the negative electrode plate and the negative electrode plate preparation process, the secondary battery of the present application can have a higher energy density while also taking into account better low-temperature power performance.

[0147] In the method for preparing the secondary battery of the present application, the first negative electrode slurry for forming the first negative electrode film layer and the second negative electrode slurry for forming the second negative electrode film layer can be coated simultaneously at one time or in two separate coatings.

[0148] In some embodiments, the first negative electrode slurry and the second negative electrode slurry are applied simultaneously. This simultaneous application can improve the adhesion between the first negative electrode film layer and the second negative electrode film layer, thereby further improving the low-temperature power performance and cycle performance of the battery.

[0149] In addition to the preparation method of the negative electrode sheet of the present application, the other structures and preparation methods of the secondary battery of the present application are well known. For example, the positive electrode sheet of the present application can be prepared as follows: the positive electrode active material and the optional conductive agent (such as carbon black), the binder (such as PVDF), etc. are mixed and dispersed in a solvent (such as NMP), stirred evenly and coated on the positive electrode collector, and the positive electrode sheet is obtained after drying. Metal foil such as aluminum foil can be used as the positive electrode collector. When preparing the positive electrode sheet, the positive electrode tab can be obtained by punching or laser die cutting in the uncoated area of the positive electrode collector.

[0150] Finally, the secondary battery can be prepared as follows: stack the positive electrode sheet, the isolation membrane, and the negative electrode sheet in order, so that the isolation membrane is placed between the positive and negative electrode sheets and the negative electrode sheet to play an isolating role, and then obtain the electrode assembly through a winding (and / or lamination) process; place the electrode assembly in an outer package, inject the electrolyte after drying, and obtain a secondary battery through vacuum packaging, standing, formation, shaping and other processes.

[0151] The method for preparing a secondary battery provided in the second aspect of this application is capable of preparing the secondary battery provided in the first aspect of this application. The parameters of the raw materials and their contents used in the preparation of the secondary battery can be referred to those of the secondary battery provided in the first aspect of this application and will not be described in detail here. Unless otherwise specified, all raw materials used in the preparation of the secondary battery provided in this application can be obtained commercially.

[0152] Device

[0153] The third aspect of the present application provides a device. The device comprises the secondary battery of the first aspect of the present application or a secondary battery prepared using the method of the second aspect of the present application. The secondary battery can be used as a power source for the device or as an energy storage unit for the device. The device of the present application utilizes the secondary battery provided herein and therefore has at least the same advantages as the secondary battery.

[0154] The device may be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0155] The device can select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0156] Figure 8 This is an example device. The device is a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module can be used.

[0157] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0158] The beneficial effects of the present application are further illustrated below with reference to the examples.

[0159] Example

[0160] In order to make the invention purpose, technical scheme and beneficial technical effect of this application clearer, this application is further described in detail below with reference to the examples. However, it should be understood that the examples of this application are only for the purpose of explaining this application and are not intended to limit this application, and the examples of this application are not limited to the examples given in the specification. Specific experimental conditions or operating conditions not specified in the examples can be prepared under conventional conditions or under conditions recommended by the material supplier.

[0161] Preparation of 1. Secondary Battery

[0162] Example 1

[0163] 1) Preparation of positive electrode sheet

[0164] The positive electrode active material, lithium iron phosphate, the conductive agent, conductive carbon black Super-P, and the binder, polyvinylidene fluoride (PVDF), were mixed thoroughly in N-methylpyrrolidone solvent at a mass ratio of 97:1:2 and then uniformly stirred to prepare a positive electrode slurry. The positive electrode slurry was then coated on both surfaces of the positive electrode current collector aluminum foil. The positive electrode sheets were obtained through drying, cold pressing, slitting, and cutting. The surface density of the positive electrode film layer on one side of the positive electrode current collector was 22.7 mg / cm 2 , compacted density is 2.6g / cm 3 .

[0165] 2) Preparation of negative electrode sheet

[0166] The first step is to prepare the first negative electrode slurry: the first negative electrode active material artificial graphite, binder SBR, thickener sodium carboxymethyl cellulose (CMC-Na), and conductive agent conductive carbon black (Super P) are mixed thoroughly in deionized water at a mass ratio of 96.2:1.8:1.2:0.8 to prepare the first negative electrode slurry. The powder compaction density of artificial graphite under a pressure of 30,000N is 1.7g / cm 3 , volume average particle size DV 50 is 12.5μm and the specific surface area is 1.2m 2 / g, a graphitization degree of 93.5%, and a morphology of one or more of block and flake.

[0167] The second step is to prepare the second negative electrode slurry: the second negative electrode active material natural graphite, binder SBR, thickener sodium carboxymethyl cellulose (CMC-Na), and conductive agent conductive carbon black (Super P) are mixed in a deionized water solvent at a mass ratio of 96.2:1.8:1.2:0.8 and then stirred thoroughly to prepare the second negative electrode slurry. The powder compaction density of natural graphite under a pressure of 30,000N is 1.8g / cm 3 , volume average particle size D V 50 is 16.5μm and the specific surface area is 3.0m 2 / g, a graphitization degree of 97.5%, and a morphology of one or more of spherical and quasi-spherical shapes.

[0168] In the third step, the first and second negative electrode slurries are extruded simultaneously through a dual-chamber coating device to form a wet film. The first negative electrode slurry is coated on both surfaces of the negative electrode current collector copper foil to form the first negative electrode film layer, and the second negative electrode slurry is coated on the first negative electrode film layer to form the second negative electrode film layer.

[0169] In the fourth step, the wet film is dried in different temperature zones of an oven and then cold pressed to obtain a negative electrode film layer with the required compaction density. The negative electrode sheet is then obtained through slitting and cutting. The thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:1.01, and the surface density of the negative electrode film layer is 10.5 mg / cm 2 The compaction density of the negative electrode film is 1.6g / cm 3 .

[0170] 3) Isolation film

[0171] PE film is selected as the isolation film.

[0172] 4) Preparation of electrolyte

[0173] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the above organic solvent at a ratio of 1 mol / L to prepare an electrolyte.

[0174] 5) Preparation of secondary batteries

[0175] The positive electrode sheet, separator, and negative electrode sheet are stacked in order and wound to obtain an electrode assembly, which is then placed in an outer package and the electrolyte is added. After packaging, standing, formation, aging, and other processes, a secondary battery is obtained.

[0176] Examples 2-8 and Comparative Examples 1-2

[0177] The secondary battery was prepared using a method similar to that of Example 1, but the composition and product parameters of the negative electrode were adjusted, as shown in Table 1 for details.

[0178] 2. Performance Testing Method

[0179] 1) Energy density test

[0180] In a 25°C environment, the secondary battery is discharged at a constant current of 1.0C (i.e., the current value that completely discharges the theoretical capacity within 1 hour) to a cut-off voltage of 2.0V. Then, it is charged at a constant current of 1.0C to a cut-off voltage of 3.8V. Constant voltage charging is continued until the current reaches 0.05C, at which point the secondary battery is fully charged. After the fully charged secondary battery is allowed to rest for 5 minutes, it is discharged at a constant current of 1.0C to a cut-off voltage of 2.0V. The discharge capacity at this point is the actual capacity of the secondary battery at 1.0C, recorded as C0.

[0181] In an environment of 25°C, the secondary battery is charged at a constant current of 0.33C0 to a cut-off voltage of 3.8V, and then continued to be charged at a constant voltage to a current of 0.05C. At this time, the secondary battery is fully charged. After the fully charged secondary battery is allowed to stand for 5 minutes, it is discharged at a constant current of 0.33C0 to a cut-off voltage of 2.0V, and the discharge energy Q of the secondary battery is obtained.

[0182] Energy density of a secondary battery (Wh / Kg) = discharge energy Q of the secondary battery / mass m of the secondary battery.

[0183] 2) Low temperature power density test

[0184] The actual capacity C0 of the secondary battery at 1.0C and the discharge energy Q of the secondary battery are obtained according to the above energy density test method.

[0185] In a 25°C environment, a secondary battery was charged at a constant current of 0.33C0 to a cutoff voltage of 3.8V. Constant voltage charging continued until the current reached 0.05C, at which point the secondary battery was fully charged. After the fully charged secondary battery rested for 5 minutes, it was discharged at a constant current of 0.33C0 for 90 minutes (i.e., adjusted to 50% SOC). The secondary battery was then placed in a -20°C environment and allowed to rest for 120 minutes. The voltage at this point, U1, was recorded. The secondary battery was then discharged at a constant current of 0.36C0 for 10 seconds, and the voltage at this point, U2, was recorded. The internal resistance of the secondary battery, R, is (U1-U2) / 0.36 / C0. According to the HPPC (Hybrid Pulse Power Characteristic) test method, the power of the secondary battery, P, was calculated as [lower cutoff voltage × (U1-lower cutoff voltage) / R] × 1000. The power density (W / Wh) of the secondary battery at -20°C is calculated as the power P of the secondary battery divided by the discharge energy Q of the secondary battery. In this application, the lower cut-off voltage is 2.0V.

[0186] 3. Test results of various embodiments and comparative examples

[0187] The secondary batteries prepared in the embodiments and comparative examples were tested according to the above performance test method. The test results are shown in Table 1.

[0188] Table 1

[0189]

[0190] From the test data of Examples 1-8 and Comparative Examples 1-2 in Table 1, it can be seen that only when the first negative electrode active material is artificial graphite and the second negative electrode active material is natural graphite, and the compaction density of the negative electrode film is controlled at 1.6 g / cm 3 When the density of the negative electrode film is 1.3 g / cm2, the secondary battery can have a high energy density and good low-temperature power performance. 3 -1.6g / cm 3 , especially 1.4 g / cm 3 -1.55g / cm 3 When the secondary battery is used, the overall performance is better.

[0191] Figure 9 This is a scanning electron microscope (SEM) image of the cross section of the negative electrode sheet prepared in Example 1. Figure 9 It can be seen that the active sites and porosity of the first and second negative electrode film layers have been reasonably optimized, so that the active ions can diffuse evenly and quickly in the negative electrode film layer, which is also beneficial to reducing the negative electrode polarization. Figure 9It can be seen that natural graphite has an internal pore structure, which allows the distinction between natural graphite and artificial graphite in the negative electrode film layer, and easily determines the location of the boundary between the first and second negative electrode films. Natural graphite is located in the second negative electrode film layer, away from the negative electrode current collector, which can maintain its structural integrity better, thereby facilitating active ion transport. Artificial graphite is located in the first negative electrode film layer, closer to the negative electrode current collector, which is beneficial for improving the battery's cycle life and energy density.

[0192] In the negative electrode sheet prepared in Comparative Example 1, the natural graphite is located in the first negative electrode film layer close to the negative electrode current collector, and the artificial graphite is located in the second negative electrode film layer away from the negative electrode current collector. This is not conducive to the active ion transport characteristics of natural graphite, and is also not conducive to the improvement of the cycle life and energy density of the secondary battery by artificial graphite. Therefore, the energy density and low-temperature power performance of the secondary battery prepared in Comparative Example 1 are poor.

[0193] In the negative electrode sheet prepared in Comparative Example 2, the compaction density of the negative electrode film layer is greater than 1.6 g / cm 3 , which results in poor matching of the active sites and porosity of the first negative electrode film layer and the second negative electrode film layer, and the secondary battery prepared in Comparative Example 2 is difficult to have a high energy density while also having good low-temperature power performance.

[0194] It should also be noted that, based on the disclosure and guidance of the above description, those skilled in the art to which this application belongs may also make appropriate changes and modifications to the above embodiments. Therefore, this application is not limited to the specific embodiments disclosed and described above, and some modifications and changes to this application also fall within the scope of protection of the claims of this application. In addition, although some specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to this application.

Claims

1. A secondary battery comprising a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer, the negative electrode film layer comprising a first negative electrode film layer and a second negative electrode film layer, the first negative electrode film layer being disposed on at least one surface of the negative electrode current collector and comprising a first negative electrode active material, the second negative electrode film layer being disposed on the first negative electrode film layer and comprising a second negative electrode active material, The first negative electrode active material includes artificial graphite, the second negative electrode active material includes natural graphite, and the volume average particle size D of the second negative electrode active material is V 50 is greater than the volume average particle size D of the first negative electrode active material V 50, and the compaction density of the negative electrode film layer is less than or equal to 1.6g / cm 3 .

2. The secondary battery according to claim 1, wherein The compaction density of the negative electrode film layer is 1.3 g / cm 3 -1.6g / cm 3 .

3. The secondary battery according to claim 2, wherein The compaction density of the negative electrode film layer is 1.4 g / cm 3 -1.55g / cm 3 .

4. The secondary battery according to claim 1, wherein The powder compaction density of the second negative electrode active material under a pressure of 30,000 N is greater than the powder compaction density of the first negative electrode active material under a pressure of 30,000 N.

5. The secondary battery according to claim 1, wherein The specific surface area of the second negative electrode active material is greater than the specific surface area of the first negative electrode active material.

6. The secondary battery according to claim 1, wherein The second negative electrode active material has a greater degree of graphitization than the first negative electrode active material.

7. The secondary battery according to claim 1, wherein The porosity of the second negative electrode film layer is greater than the porosity of the first negative electrode film layer.

8. The secondary battery according to claim 1, wherein The natural graphite has a morphology of one or more of spherical and quasi-spherical; and / or, The artificial graphite has a shape of one or more of block and flake.

9. The secondary battery according to claim 1, wherein The mass proportion of the natural graphite in the second negative electrode active material is greater than or equal to 60%.

10. The secondary battery according to claim 9, wherein The mass proportion of the natural graphite in the second negative electrode active material is 80%-100%.

11. The secondary battery according to claim 1, wherein The mass proportion of the artificial graphite in the first negative electrode active material is greater than or equal to 60%.

12. The secondary battery according to claim 11, wherein The artificial graphite accounts for 80% to 100% by mass of the first negative electrode active material.

13. The secondary battery according to claim 1, wherein The thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:0.9-1:1.

5.

14. The secondary battery according to claim 13, wherein The thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 1:01-1:1.

2.

15. The secondary battery according to claim 1, wherein The surface density CW of the negative electrode film layer satisfies: 9 mg / cm 2 ≤CW≤13mg / cm 2 .

16. The secondary battery according to claim 15, wherein The surface density CW of the negative electrode film layer satisfies: 10.0 mg / cm 2 ≤CW≤11.5mg / cm 2 .

17. The secondary battery according to claim 1, wherein The first negative electrode film layer and / or the second negative electrode film layer comprises a silicon-based material.

18. The secondary battery according to claim 1, wherein The first negative electrode film layer includes a silicon-based material, and the mass proportion of the silicon-based material in the first negative electrode film layer is less than or equal to 60%.

19. The secondary battery according to claim 18, wherein The silicon-based material accounts for 1% to 30% by mass in the first negative electrode film layer.

20. The secondary battery according to claim 1, wherein The second negative electrode film layer includes a silicon-based material, and the mass proportion of the silicon-based material in the second negative electrode film layer is less than or equal to 10%.

21. The secondary battery according to claim 20, wherein The mass proportion of the silicon-based material in the second negative electrode film layer is 1%-5%.

22. The secondary battery according to claim 1, wherein The first negative electrode film layer and the second negative electrode film layer both include silicon-based materials, and the mass proportion of the silicon-based material in the first negative electrode film layer is recorded as W1, and the mass proportion of the silicon-based material in the second negative electrode film layer is recorded as W2, then W1≥W2.

23. The secondary battery according to claim 1, wherein Both surfaces of the negative electrode current collector are provided with negative electrode film layers, and the negative electrode film layers on the two surfaces have different thicknesses; and / or, Both surfaces of the negative electrode current collector are provided with negative electrode film layers, and the negative electrode film layers on the two surfaces have different surface densities; and / or, Both surfaces of the negative electrode current collector are provided with negative electrode film layers, and the negative electrode film layers on the two surfaces have different compaction densities.

24. The secondary battery according to claim 1, wherein The secondary battery includes a positive electrode plate, which includes a positive electrode collector and a positive electrode film layer arranged on at least one surface of the positive electrode collector and including a positive electrode active material. The positive electrode active material includes one or more of a lithium transition metal oxide, an olivine-structured lithium-containing phosphate, and their respective modified compounds.

25. The secondary battery according to claim 24, wherein The positive electrode active material includes one or more of an olivine-structured lithium-containing phosphate and a modified compound thereof.

26. A method for preparing a secondary battery, comprising preparing a negative electrode sheet of the secondary battery by the following steps: 1) forming a first negative electrode film layer comprising a first negative electrode active material on at least one surface of a negative electrode current collector, wherein the first negative electrode active material comprises artificial graphite; 2) forming a second negative electrode film layer comprising a second negative electrode active material on the first negative electrode film layer, wherein the second negative electrode active material comprises natural graphite, and the volume average particle size D of the second negative electrode active material is V 50 is greater than the volume average particle size D of the first negative electrode active material V 50; 3) The compaction density of the negative electrode film is controlled at 1.6g / cm through the cold pressing process 3 the following.

27. A device comprising the secondary battery according to any one of claims 1 to 25 or the secondary battery prepared by the method according to claim 26.

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