Negative pole piece, lithium ion secondary battery and electric equipment

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to a negative pole piece, a lithium ion secondary battery and electric equipment. According to the negative pole piece, the multi-layer coating design is adopted, meanwhile, through the gradient design of the particle size of the negative pole active material, the arrangement of the holes in the coating layers and the special regulation and control of the porosity of the coating layers, the infiltration performance and the dynamic performance of the electrolyte are improved, and particularly, the charging capacity of the pole piece under the global SOC is guaranteed.

Description

Technical Field

[0001] This application belongs to the technical field of secondary batteries, and particularly relates to a negative electrode sheet, a lithium-ion secondary battery, and an electrical device. Background Art

[0002] With the wide application of lithium-ion batteries in fields such as electric vehicles, rail transit, and energy storage, the demand for lithium-ion battery technologies with high energy density and long cycle life is increasing continuously. And with the increase in energy density, the charging rate of the battery has also received more and more extensive attention. Currently, the fast charging ability of the battery is mainly limited by the negative electrode. If the high-energy density negative electrode is not specially modified, its thickness will affect the infiltration of the electrolyte and the migration rate of lithium ions. How to develop a lithium-ion battery that can simultaneously achieve high energy density, efficient fast charging, and long life is a difficult point that the current industry focuses on.

[0003] Some prior arts disclose negative electrode sheets, including a current collector and two coatings provided on the current collector. By providing through holes in the outer coating, the problems of insufficient electrolyte infiltration performance and kinetic performance caused by the excessive thickness of the negative electrode sheet are solved.

[0004] However, these methods in the prior art only perform rough regulation based on experience, and often can only improve the charging ability of the negative electrode at low SOC (state of charge). However, as the charging reaches the high SOC stage, the charging ability of the negative electrode will still be limited. Summary of the Invention

[0005] Therefore, the technical problem to be solved by this application is to overcome the defects in the prior art that the excessive thickness of the electrode sheet will cause insufficient electrolyte infiltration performance and kinetic performance, and it is difficult to improve the charging ability of the negative electrode at high SOC, so as to provide a negative electrode sheet, a lithium-ion secondary battery, and an electrical device.

[0006] To this end, this application provides the following technical solutions:

[0007] According to one aspect of this application, there is provided a negative electrode sheet, including:

[0008] A current collector having two opposite surfaces in its own thickness direction;

[0009] An active layer provided on at least one surface of the current collector, wherein the active layer includes a first coating layer, a second coating layer, and a third coating layer arranged in sequence, the first coating layer is close to the current collector, and holes are respectively provided on the first coating layer, the second coating layer, and the third coating layer;

[0010] The average particle sizes of the active materials in the first coating layer, the second coating layer, and the third coating layer are D1 μm, D2 μm, and D3 μm, respectively, where 12 ≤ D1 ≤ 18, 8 ≤ D2 ≤ 14, 4 ≤ D3 ≤ 10, and D1 > D2 > D3 is satisfied;

[0011] The porosities of the first coating layer, the second coating layer, and the third coating layer are A1%, A2%, and A3%, respectively, where 32 ≤ A1 ≤ 36, 34 ≤ A2 ≤ 38, 36 ≤ A3 ≤ 40, and A1 < A2 < A3 is satisfied.

[0012] As an example, the average particle size of the active material in the first coating layer can be 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, or within the range composed of any of the above values; the average particle size of the active material in the second coating layer can be 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or within the range composed of any of the above values; the average particle size of the active material in the third coating layer can be 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or within the range composed of any of the above values; as long as D1 > D2 > D3 is satisfied.

[0013] As an example, the porosity of the first coating layer can be 32%, 33%, 34%, 35%, 36%, or within the range composed of any of the above values; the porosity of the second coating layer can be 34%, 35%, 36%, 37%, 38%, or within the range composed of any of the above values; the porosity of the third coating layer can be 36%, 37%, 38%, 39%, 40%, or within the range composed of any of the above values; as long as A1 < A2 < A3 is satisfied.

[0014] In some alternative embodiments, the diameters of the holes in the first coating layer, the second coating layer, and the third coating layer are d1 μm, d2 μm, and d3 μm, respectively, where D2 > d1, D3 > d2, and d1 > d2 > d3 is satisfied.

[0015] In some alternative embodiments, the diameters of the holes in the first coating layer, the second coating layer, and the third coating layer satisfy: 6 ≤ d1 ≤ 12, 3 ≤ d2 ≤ 8, 2 ≤ d3 ≤ 5. As an example, the diameter of the holes in the first coating layer can be 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, or within the range composed of any of the above values; the diameter of the holes in the second coating layer can be 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, or within the range composed of any of the above values; the diameter of the holes in the third coating layer can be 2μm, 3μm, 4μm, 5μm, or within the range composed of any of the above values.

[0016] In some alternative embodiments, the depths of the holes in each coating layer respectively account for 40% - 80% of the thickness of the first coating layer, the second coating layer, or the third coating layer. As an example, the depths of the holes in each coating layer respectively account for 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80% of the thickness of the coating layer where they are located, or within the range composed of any of the above values. Those skilled in the art can understand that if the depth of the holes is too shallow, the improvement effect is not obvious, and if the depth reaches 100%, it will affect the adhesion between the coating layers.

[0017] In some alternative embodiments, the thickness of the active layer is 150μm - 400μm; as an example, the thickness of the active layer can be 150μm, 200μm, 250μm, 300μm, 350μm, 400μm, or within the range composed of any of the above values.

[0018] And / or, the ratio of the thickness of the second coating layer to the thickness of the first coating layer is (0.5 - 0.7):1; as an example, the ratio of the thickness of the second coating layer to the thickness of the first coating layer can be 0.5:1, 0.55:1, 0.6:1, 0.65:1, 0.7:1, or within the range composed of any of the above values.

[0019] And / or, the ratio of the thickness of the third coating layer to the thickness of the first coating layer is (0.3 - 0.5):1; as an example, the ratio of the thickness of the third coating layer to the thickness of the first coating layer can be 0.3:1, 0.35:1, 0.4:1, 0.45:1, 0.5:1, or within the range composed of any of the above values.

[0020] In some optional embodiments, the initial porosities of the first coating layer, the second coating layer, and the third coating layer before punching and rolling are a1%, a2%, and a3% respectively, where 30 ≤ a1 ≤ 32, 28 ≤ a2 ≤ 30, 26 ≤ a3 ≤ 28, and a1 > a2 > a3. As an example, the initial porosity of the first coating layer can be 30%, 30.5%, 31%, 31.5%, 32%, or within the range composed of any of the above values; the initial porosity of the second coating layer can be 28%, 28.5%, 29%, 29.5%, 30%, or within the range composed of any of the above values; the initial porosity of the third coating layer can be 26%, 26.5%, 27%, 27.5%, 28%, or within the range composed of any of the above values; as long as a1 > a2 > a3 is satisfied. In the present application, since the step of punching each coating layer is included before rolling, the porosities of the first coating layer, the second coating layer, and the third coating layer after rolling will be higher than the initial porosities of each coating layer before punching and rolling.

[0021] In some optional embodiments, the first coating layer includes a first active material, a first binder, and a first conductive agent with a mass ratio of (90 - 98):(1 - 5):(1 - 5);

[0022] and / or, the second coating layer includes a second active material, a second binder, and a second conductive agent with a mass ratio of (90 - 98):(1 - 5):(1 - 5);

[0023] and / or, the third coating layer includes a third active material, a third binder, and a third conductive agent with a mass ratio of (90 - 98):(1 - 5):(1 - 5).

[0024] Those skilled in the art can understand that due to the capillary phenomenon during the drying process of the electrode sheet, the binder will float up with the evaporation of the solvent. Therefore, the proportion of the binder in the coating layer located in the lower layer is slightly higher than that in the coating layer located in the upper layer, so as to ensure the adhesion between the coating layers.

[0025] In some optional embodiments, the first active material, the second active material, and the third active material each independently include at least one of natural graphite, artificial graphite, hard carbon, soft carbon, mesocarbon microbeads (MCMB), nano - silicon, silicon alloy, silicon - carbon composite material, SiO x materials;

[0026] And / or, the first binder, the second binder, and the third binder each independently include at least one of polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), polyacrylic acid (PAA), styrene-butadiene rubber (SBR), and polyimide (PI);

[0027] And / or, the first conductive agent, the second conductive agent, and the third conductive agent each independently include at least one of carbon black, Ketjen black, carbon nanotubes, and graphene.

[0028] According to another aspect of the present application, a lithium-ion secondary battery is provided, comprising the above-mentioned negative electrode plate.

[0029] According to another aspect of the present application, there is provided an electrical device comprising the above-mentioned lithium-ion secondary battery.

[0030] In this application, the porosity of each coating layer can be tested by existing methods and equipment in the field. As an example, the mercury injection method can be used for testing, that is, mercury is pressed into the pores in the electrode by pressure, and the porosity calculation formula is: porosity = volume of injected mercury / total volume of sample.

[0031] It is understood by those skilled in the art that the lithium-ion secondary battery provided in the present application includes not only the above-mentioned negative electrode sheet, but also structural parts such as positive electrode sheet, electrolyte, diaphragm and shell. During the battery charging and discharging process, lithium ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet, the electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet, and the composite diaphragm is arranged between the positive electrode sheet and the negative electrode sheet, mainly playing the role of preventing the positive and negative electrodes from short-circuiting, while allowing lithium ions to pass through.

[0032] As an example, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active layer is arranged on any one or both of the two opposite surfaces of the negative electrode current collector. The material, composition and manufacturing method of the positive electrode sheet used in the lithium-ion battery of the present application may include any technology disclosed in the prior art.

[0033] As an example, the material, composition and manufacturing method of the negative electrode sheet used in the lithium-ion secondary battery of the present application may include any technology disclosed in the prior art.

[0034] The material and shape of the separator used in the lithium ion secondary battery of the present application are not particularly limited, and may include any technology disclosed in the prior art.

[0035] The electrolyte used in the lithium ion secondary battery of the present application may include any technology disclosed in the prior art.

[0036] The present application does not specifically limit the preparation method of the lithium-ion secondary battery, and the lithium-ion secondary battery can be prepared by using conventional preparation methods in the art. For example, the positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence, with the separator located between the positive electrode sheet and the negative electrode sheet, and the battery cell is obtained through a stacking or winding process, and then the lithium-ion secondary battery of the present application can be obtained through processes such as baking, liquid injection, formation, and packaging.

[0037] It can be understood that in the electrical device provided by the present application, the lithium-ion secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can 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. The electrical device has the same advantages as the above-mentioned lithium-ion secondary battery compared with the prior art, which will not be elaborated here.

[0038] The technical solution of the present application has the following advantages:

[0039] The negative electrode sheet provided by the present application includes: a current collector having two opposite surfaces in its own thickness direction; an active layer provided on at least one surface of the current collector, wherein the active layer includes a first coating layer, a second coating layer, and a third coating layer arranged in sequence, the first coating layer is close to the current collector, and holes are respectively provided on the first coating layer, the second coating layer, and the third coating layer; the particle sizes of the active materials in the first coating layer, the second coating layer, and the third coating layer are D1 μm, D2 μm, and D3 μm respectively, 12 ≤ D1 ≤ 18, 8 ≤ D2 ≤ 14, 4 ≤ D3 ≤ 10, and D1 > D2 > D3; the porosities of the first coating layer, the second coating layer, and the third coating layer after rolling are A1%, A2%, and A3% respectively, 32 ≤ A1 ≤ 36, 34 ≤ A2 ≤ 38, 36 ≤ A3 ≤ 40, and A1 < A2 < A3. The negative electrode sheet of the present application adopts a multi-layer design, and at the same time, through the gradient design of the particle size of the negative electrode active material, the setting of holes on each coating layer, and the special regulation of the porosity of each coating layer, the electrolyte infiltration performance and kinetic performance are improved, especially the charging ability of the electrode sheet under the full range of SOC is ensured.

[0040] The negative electrode sheet provided by the present application can avoid the extension of the active material during the rolling process through the special regulation of the hole diameters on two adjacent coating layers and the particle size of the active material in the coating layer. This can create cavities between the multi-layer slurries to capture and store excess electrolyte during electrolyte infiltration, thereby solving the problem of electrolyte dryness during the cycling process and further improving the electrical performance.

[0041] Additional aspects and advantages of the present application will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0043] Figure 1 is a schematic structural diagram of the negative electrode plate in the embodiment of the present application;

[0044] Reference numerals:

[0045] 1, the first coating layer; 2, the second coating layer; 3, the third coating layer; 4, the current collector; 5, the hole. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] The following embodiments are provided to better further understand the present application, which is not limited to the described best embodiment, and does not limit the content and protection scope of the present application. Any product that is the same as or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior art features falls within the protection scope of the present application.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the text of this application are intended to cover non-exclusive inclusion.

[0048] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features.

[0049] Referring to "embodiments" herein means that the specific features, structures or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0050] The "range" disclosed in this application is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a specific range. The range defined in this way can include or exclude the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. In this application, unless otherwise stated, the numerical range "a - b" represents an abbreviated representation of any real number combination between a and b, where both a and b are real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" are fully listed herein, and "0 - 5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥ 2, it is equivalent to disclosing that the parameter can be, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0051] In the description of the embodiments of this application, the term "and / or" is merely a description of the association relationship between associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone.

[0052] In the description of the embodiments of this application, the term "at least one" refers to one or more than two (including two).

[0053] For those not specifying the specific experimental steps or conditions in the embodiments, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For those reagents or instruments without indicating the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0054] The following describes this application with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit this application in any way.

[0055] Embodiment 1

[0056] This embodiment provides a negative electrode plate, the structural schematic diagram of which is as Figure 1 shown, including a current collector 4 having two opposite surfaces in its own thickness direction; an active layer provided on at least one surface of the current collector 4, wherein the active layer includes a first coating layer 1, a second coating layer 2, and a third coating layer 3 arranged in sequence, the first coating layer 1 is arranged close to the current collector 4, and holes 5 are respectively provided on the first coating layer 1, the second coating layer 2, and the third coating layer 3;

[0057] The specific preparation method of the above negative electrode plate includes the following steps:

[0058] First Coating Layer: The first active material artificial graphite with an average particle size of 18 μm, the first binder (CMC and SBR with a mass ratio of 2:2.2), and the first conductive agent SP are dispersed in the solvent N-methylpyrrolidone in a mass ratio of 93.8:4.2:2, stirred and mixed to form a uniform slurry; the obtained slurry is coated on the surface of the negative current collector copper foil (with a thickness of 8 μm), dried, and the initial porosity of the first coating layer is tested. In this embodiment, the initial porosity of the first coating layer is 31.5%; then laser drilling treatment is carried out, the pore diameter of the holes is 10 μm, and the depth of the holes is 80 μm; roll pressing, and the porosity after roll pressing can be adjusted by adjusting the number of holes;

[0059] Second Coating Layer: The second active material artificial graphite with an average particle size of 13 μm, the second binder (CMC and SBR with a mass ratio of 1.6:1.6), and the second conductive agent SP are dispersed in the solvent N-methylpyrrolidone in a mass ratio of 95.8:3.2:1, stirred and mixed to form a uniform slurry; the obtained slurry is coated on the surface of the first coating layer, dried, and the initial porosity of the second coating layer is tested. In this embodiment, the initial porosity of the second coating layer is 29.5%; then laser drilling treatment is carried out, the pore diameter of the holes is 6 μm, and the depth of the holes is 30 μm; roll pressing, and the porosity after roll pressing can be adjusted by adjusting the number of holes;

[0060] Third Coating Layer: The third active material artificial graphite with an average particle size of 8 μm, the third binder (CMC and SBR with a mass ratio of 1:1), and the third conductive agent SP are dispersed in the solvent N-methylpyrrolidone in a mass ratio of 97:2:1, stirred and mixed to form a uniform slurry; the obtained slurry is coated on the surface of the second coating layer, dried, and the initial porosity of the third coating layer is tested. In this embodiment, the initial porosity of the third coating layer is 27.4%; then laser drilling treatment is carried out, the pore diameter of the holes is 4 μm, and the depth of the holes is 16 μm; roll pressing, and the porosity after roll pressing can be adjusted by adjusting the number of holes to obtain the negative electrode plate.

[0061] In this embodiment, the porosity after roll pressing of the first coating layer is adjusted to 33.6%, and the thickness is 100 μm; the porosity after roll pressing of the second coating layer is adjusted to 35.8%, and the thickness is 60 μm; the porosity after roll pressing of the third coating layer is adjusted to 38.6%, and the thickness is 40 μm.

[0062] Examples 2 - 5

[0063] This embodiment provides a negative electrode plate, which is different from Example 1 as shown in Table 1.

[0064] Example 6

[0065] This embodiment provides a negative electrode plate. Compared with Embodiment 1, D2 = d1 and D3 = d2. For specific parameters, see Table 1.

[0066] Comparative Example 1

[0067] This comparative example provides a negative electrode plate that only includes a first coating layer, and its thickness is equal to the thickness of the active layer in the embodiment. For specific parameters, see Table 1.

[0068] Comparative Example 2

[0069] This comparative example provides a negative electrode plate. Compared with Embodiment 3, no holes are provided on each coating layer. For specific parameters, see Table 1.

[0070] Comparative Examples 3 - 6

[0071] This comparative example provides a negative electrode plate. Compared with Embodiment 1, the differences are shown in Table 1.

[0072] Comparative Example 7

[0073] This comparative example provides a negative electrode plate. Compared with Embodiment 1, no holes are provided on the first coating layer, that is, laser drilling is not performed. For specific parameters, see Table 1.

[0074] Table 1

[0075] Group D1, μm D2, μm D3, μm a1,％ a2,％ a3,％ A1，％ A2，％ A3，％ Example 1 18 13 8 31.5 29.5 27.4 33.6 35.8 38.6 Example 2 12 10 6 30.8 28.9 27.8 32.8 34.9 36.7 Example 3 17 9 7 31.7 28.1 26.6 35.5 37.4 39.8 Example 4 16 12 5 30.6 29.7 26.9 34.1 35.7 38.2 Example 5 18 14 4 31.1 28.5 27.8 33.6 36.3 37.4 Example 6 14 10 8 30.2 28.6 26.1 30.5 29.4 38.1 Comparative Example 1 15 / / 31.5 / / 28.4 / / Comparative Example 2 17 9 7 31.2 29.3 27.1 27.1 23.4 20.9 Comparative Example 3 18 13 8 31.5 29.5 27.4 28.4 30.3 32.1 Comparative Example 4 18 13 8 31.5 29.5 27.4 38.2 41.5 42.4 Comparative Example 5 10 7 3 28.4 24.2 19.8 25.9 22.1 17.7 Comparative Example 6 20 16 12 33.8 31.4 29.1 31.3 28.4 25.3

[0076] Test Example

[0077] 1. Membrane resistance

[0078] The membrane resistance of the electrode plate is directly measured using a membrane resistance meter.

[0079] 2. Tortuosity

[0080] The tortuosity of the electrode plate is directly measured using a mercury porosimeter.

[0081] 3. Ohmic impedance and ion transfer impedance:

[0082] Two identical negative electrode plates are made into a simple single - cell soft - pack battery and an appropriate amount of electrolyte is injected. The composition and preparation method of the electrolyte are as follows: Ethylene carbonate (EC), dimethylcarbamide (DMC), and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, dry LiPF6 is dissolved in the above solvent to prepare an electrolyte with a concentration of 1 mol / L. After sealing, an electrochemical impedance spectroscopy (EIS) test is performed using an electrochemical workstation to obtain the ohmic impedance and ion transfer impedance.

[0083] 4. Charging ability under full - range SOC

[0084] Generally, the fast charging performance of a battery is evaluated by its fast charging ability within the SOC range of 10% to 80%. In this application, only the charging ability at high SOC (80% to 100% SOC) is compared for the charging ability across the entire SOC range. At low SOC, since lithium ions are first intercalated into the graphite on the surface of the negative electrode tab, and due to the stacked punched process of small particle size graphite, the charging ability differences are not significant, so no comparison is made in this regard.

[0085] Test method: Considering the general vehicle charging conditions, fast charging is carried out within the SOC range of 10% to 80%, and small rate charging is carried out within the range of 80% to 100%. Therefore, 1C rate charging is performed for this high SOC range, and the ratio of the charging capacity to the total rated capacity after the battery cells are charged to the same upper limit voltage is compared. Generally, due to issues such as ion transport and electrolyte wetting in the battery cells (i.e., the problems to be solved in the present invention), polarization occurs in the battery cells, meaning that the voltage of the battery reaches the upper limit while the actual charging capacity does not reach 100% SOC. To ensure the same initial conditions and only compare high SOC, the battery is charged to 80% SOC with a 0.1C current and then compared. The closer the SOC under this condition is to 100%, the better the charging ability across the entire SOC range.

[0086] Rate C: The current value = the rated capacity of the battery, which is 1C, that is, the current that can charge the battery to 100% SOC in 1 hour; by analogy, 2C means the current value = 2 × the rated capacity, and the full charge time is 1 / 2 hour; 3C means the current value = 3 × the rated capacity, and the full charge time is 1 / 3 hour.

[0087] Calculation method of SOC (state of charge): SOC = charging capacity / rated capacity, or charging current × charging time / rated capacity.

[0088] The preparation method of the battery used in the test is as follows:

[0089] Positive electrode tab: The thickness of the aluminum foil is 15um. It is mixed with the solvent NMP according to the mass ratio of the positive electrode main material NCM811: conductive agent carbon black: binder PVDF = 97.5:1:1.5, homogenized and coated; and then dried and roll-pressed subsequently to obtain the positive electrode tab;

[0090] Negative electrode tab: The negative electrode tabs provided in each example and comparative example;

[0091] Separator: A porous polyethylene-based separator with a base film thickness of 9um;

[0092] Electrolyte: Ethylene carbonate (EC), dimethylcarbamide (DMC), and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then dried LiPF6 is dissolved in the above solvent to prepare an electrolyte with a concentration of 1mol / L;

[0093] Battery assembly: Stack the positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator positioned between the positive electrode sheet and the negative electrode sheet. A battery cell is obtained through the lamination process, and then a soft-pack battery can be obtained after processes such as baking, liquid injection, formation, and encapsulation.

[0094] 5. Cycling performance

[0095] Form a soft-pack battery (with the same composition as above) using the above negative electrode sheet and the same positive electrode (either ternary positive electrode material or lithium iron phosphate as the active material), apply the same electrolyte as above, and perform normal-temperature cycling at 1C. Collect the SOH after 1000 cycles.

[0096] SOH (state of health), SOH = (current effective capacity of the battery / rated capacity of the battery) × 100%.

[0097] 6. Fast charging performance

[0098] Form a soft-pack battery (with the same composition as above) using the above negative electrode sheet and the positive electrode sheet. Cycle it once at a rate of 0.1C, and collect the voltage values V1 and V2 corresponding to 10% and 80% SOC. Then charge the battery at 4C within the voltage range [V1, V2] and discharge it at a rate of 1C for 50 cycles. The closer the capacity retention rate is to 70%, the better the fast charging performance.

[0099] The specific test results are shown in the following table:

[0100] Table 2

[0101]

[0102]

[0103] From the data in the above table, it can be seen that in the embodiments of the present application, by adopting a multi-layer coating design for the negative electrode sheet, and at the same time through the gradient design of the particle size of the negative active material, the setting of holes on each coating layer, and the special regulation of the porosity of each coating layer, the electrolyte infiltration performance and kinetic performance are improved. In particular, the charging ability of the electrode sheet under the full-range SOC is ensured. For traditional thick electrodes, without performing particle size gradient design, multi-layer coating, or laser drilling, the film resistance, ohmic impedance, and ion transfer impedance of the electrode sheet are all relatively large, which is not conducive to the fast charging and cycle life of the battery (Comparative Example 1); applying multi-layer coating can slightly improve the overall performance of the electrode sheet; applying multi-layer coating and laser drilling, if the porosity is still small, the improvement effect is not obvious, and if the porosity is too large, it will affect the connection of the conductive network between each coating layer and affect the fast charging ability, but the electrolyte infiltration effect still exists. If only the upper coating layer is drilled and the coating layer close to the current collector is not provided with holes, the effect of improving the cycle will be reduced.

[0104] Obviously, the above embodiments are merely examples given for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A negative electrode plate, characterized in that, Comprising: A current collector having two opposite surfaces in its own thickness direction; An active layer disposed on at least one surface of the current collector, wherein the active layer includes a first coating layer, a second coating layer, and a third coating layer disposed in sequence, the first coating layer is disposed close to the current collector, and holes are respectively disposed on the first coating layer, the second coating layer, and the third coating layer; The average particle diameters of the active materials in the first coating layer, the second coating layer, and the third coating layer are D1 μm, D2 μm, and D3 μm respectively, 12 ≤ D1 ≤ 18, 8 ≤ D2 ≤ 14, 4 ≤ D3 ≤ 10, and D1 > D2 > D3 is satisfied; The porosities of the first coating layer, the second coating layer, and the third coating layer are A1%, A2%, and A3% respectively, 32 ≤ A1 ≤ 36, 34 ≤ A2 ≤ 38, 36 ≤ A3 ≤ 40, and A1 < A2 < A3 is satisfied.

2. The negative electrode sheet according to claim 1, characterized in that, The diameters of the holes on the first coating layer, the second coating layer, and the third coating layer are d1 μm, d2 μm, and d3 μm respectively, D2 > d1, D3 > d2, and d1 > d2 > d3 is satisfied.

3. The negative electrode sheet according to claim 1, wherein, The diameters of the holes on the first coating layer, the second coating layer, and the third coating layer satisfy: 6 ≤ d1 ≤ 12, 3 ≤ d2 ≤ 8, 2 ≤ d3 ≤ 5.

4. The negative electrode sheet according to claim 1, characterized in that, The depths of the holes on each coating layer respectively account for 40% - 80% of the thickness of the first coating layer, the second coating layer, or the third coating layer.

5. The negative electrode sheet according to claim 1, characterized in that, The thickness of the active layer is 150 μm - 400 μm; And / or, the thickness ratio of the second coating layer to the first coating layer is (0.5 - 0.7):1; And / or, the thickness ratio of the third coating layer to the first coating layer is (0.3 - 0.5):

1.

6. The negative electrode sheet according to claim 1, characterized in that, The initial porosities of the first coating layer, the second coating layer, and the third coating layer before punching and rolling are a1%, a2%, and a3% respectively, 30 ≤ a1 ≤ 32, 28 ≤ a2 ≤ 30, 26 ≤ a3 ≤ 28, and a1 > a2 > a3 is satisfied.

7. The negative electrode sheet according to any one of claims 1-6, characterized in that, The first coating layer includes a first active material, a first binder, and a first conductive agent with a mass ratio of (90 - 98):(1 - 5):(1 - 5); And / or, the second coating layer includes a second active material, a second binder, and a second conductive agent with a mass ratio of (90 - 98):(1 - 5):(1 - 5); And / or, the third coating layer includes a third active material, a third binder, and a third conductive agent with a mass ratio of (90 - 98):(1 - 5):(1 - 5).

8. The negative electrode sheet according to claim 7, wherein The first active material, the second active material, and the third active material each independently include at least one of natural graphite, artificial graphite, hard carbon, soft carbon, mesocarbon microbeads, nano-silicon, silicon alloy, silicon-carbon composite material, and SiO x material; And / or, the first binder, the second binder, and the third binder each independently include at least one of polyvinylidene fluoride, carboxymethyl cellulose, polyvinyl alcohol, polyacrylic acid, styrene-butadiene rubber, and polyimide; And / or, the first conductive agent, the second conductive agent, and the third conductive agent each independently include at least one of carbon black, Ketjen black, carbon nanotubes, and graphene.

9. A lithium ion secondary battery, characterized in that, Comprising the negative electrode sheet according to any one of claims 1 - 8.

10. An electrical device, characterized in that, Comprising the lithium ion secondary battery according to claim 9.

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