Negative electrode sheet and manufacturing method therefor, and lithium ion battery

Abstract

A negative electrode sheet (10) and a manufacturing method therefor, and a lithium ion battery. The negative electrode sheet (10) comprises a current collector (200) and a negative electrode coating layer (300); the negative electrode coating layer (300) is provided on the current collector (200); the negative electrode coating layer (300) comprises a first negative electrode slurry coating (310) located on the current collector (200) and a second negative electrode slurry coating (320) located on the side of the first negative electrode slurry coating (310) distant from the current collector (200); and a second porosity of the second negative electrode slurry coating (320) is greater than a first porosity of the first negative electrode slurry coating (310). The negative electrode sheet (10) can improve the utilization rate of lithium metal.

Description

Negative electrode sheet, preparation method thereof and lithium ion battery

[0001] Cross-reference to related applications

[0002] The present disclosure claims priority to the Chinese patent application No. 2024104840045, filed on April 19, 2024, and entitled "Negative electrode sheet, preparation method thereof and lithium ion battery", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to the field of new energy technology, in particular to a negative electrode sheet, a preparation method thereof and a lithium ion battery. BACKGROUND

[0004] With higher requirements for battery endurance of electric vehicles, energy storage devices and portable electronic devices, developing lithium ion batteries with high energy density has become a research hotspot. Generally, the energy density of the battery cell is improved by optimizing the system structure of the battery cell and using high-capacity materials. In addition, in the existing lithium battery system, active lithium needs to be consumed on the surface of the negative electrode to generate a solid electrolyte interface (SEI) during the formation process, resulting in low initial efficiency of the battery cell and reduced initial energy density of the battery cell. Therefore, researchers add excess active lithium (pre-lithiation) in the design of the battery cell to compensate for the consumption of active lithium during the formation process, increase the reversible capacity of the first cycle of the battery cell, and improve the actual energy density of the battery cell. The excess active lithium can also be released in the later cycle to supplement the continuous consumption of the SEI film. Currently, lithium supplementing is mainly divided into positive electrode lithium supplementing and negative electrode lithium supplementing. Among them, the negative electrode lithium supplementing mainly adopts a method of rolling a metal lithium sheet and a negative electrode sheet to form a battery cell, and then immersing the battery cell in an electrolyte to form a potential difference (internal short circuit). The metal lithium in the metal lithium sheet is oxidized to produce lithium ions, which enter the electrolyte and then enter (immerse) the negative electrode material, thereby completing the negative electrode lithium supplementing. However, the metal lithium utilization rate of this lithium supplementing method is low, and a large amount of dead lithium is easily formed. SUMMARY

[0005] Therefore, it is necessary to provide a negative electrode sheet to improve the utilization rate of metal lithium.

[0006] A negative electrode sheet, comprising:

[0007] a current collector; and

[0008] a negative electrode coating layer provided on at least one of opposite sides of the current collector, the negative electrode coating layer comprising a first negative electrode slurry coating layer on the current collector and a second negative electrode slurry coating layer, the second negative electrode slurry coating layer being located on a side of the first negative electrode slurry coating layer away from the current collector, and a second porosity of the second negative electrode slurry coating layer being greater than a first porosity of the first negative electrode slurry coating layer.

[0009] In one embodiment, the first porosity is determined by a first particle size distribution span of graphite in a first component category of the first negative electrode slurry coating, and the second porosity is determined by a second particle size distribution span of graphite in a second component category of the second negative electrode slurry coating, wherein the particle size distribution span is (D90-D10) / D50, D10 represents a diameter corresponding to a particle size cumulative distribution percentage of 10%, D50 represents a diameter corresponding to a particle size cumulative distribution percentage of 50%, and D90 represents a diameter corresponding to a particle size cumulative distribution percentage of 90%.

[0010] In one embodiment, the first porosity is 10%-40%, and the second porosity is 12%-45%.

[0011] In one embodiment, the component category and component content of the first negative electrode slurry coating are the same as those of the second negative electrode slurry coating.

[0012] In one embodiment, the first negative electrode slurry coating and the second negative electrode slurry coating each include a negative electrode active material, a conductive agent, a binder, and a dispersant.

[0013] In one embodiment, the mass fraction of the negative electrode active material is 85-98%, the mass fraction of the conductive agent is 1-5%, the mass fraction of the binder is 1-5%, and the mass fraction of the dispersant is 1-5%, with the total amount of slurry solids being 100%.

[0014] In one embodiment, the negative electrode active material includes at least one of a carbon-based negative electrode, a silicon negative electrode, and lithium titanate.

[0015] The conductive agent includes at least one of carbon black, carbon nanotubes, and graphene.

[0016] The binder includes at least one of polyvinylidene fluoride, butadiene rubber, and polyvinyl alcohol.

[0017] The dispersant includes at least one of polyvinylpyrrolidone, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and polyacrylic acid.

[0018] In one embodiment, the negative electrode sheet further includes a lithium metal sheet, which is disposed on a side of the second negative electrode slurry coating away from the current collector.

[0019] The present disclosure also provides a lithium ion battery including the above negative electrode sheet.

[0020] The present disclosure also provides a method for preparing a negative electrode sheet, including the following steps:

[0021] forming an anode coating layer comprising a first anode slurry coating layer on the current collector and a second anode slurry coating layer on the current collector, the second anode slurry coating layer being on a side of the current collector distal to the first anode slurry coating layer, the second anode slurry coating layer having a second porosity greater than a first porosity of the first anode slurry coating layer.

[0022] In one embodiment, in the step of forming the anode coating layer, a double layer coating technique is used to form the first anode slurry coating layer and the second anode slurry coating layer.

[0023] In one embodiment, in the step of forming the anode coating layer, the first anode slurry coating layer is formed from a first anode slurry and the second anode slurry coating layer is formed from a second anode slurry, the first anode slurry and the second anode slurry each comprising an anode active material, a conductive agent, a binder, a dispersant, and a solvent;

[0024] the anode active material has a mass fraction of 85-98%, the conductive agent has a mass fraction of 1-5%, the binder has a mass fraction of 1-5%, and the dispersant has a mass fraction of 1-5%, the mass fractions being based on a total solid content of the slurry being 100%;

[0025] the solvent has a mass ratio of 30-60% to the mass of the anode active material, the conductive agent, the binder, the dispersant, and the solvent as a whole;

[0026] the anode active material comprises at least one of a carbon-based anode, a silicon anode, and lithium titanate;

[0027] the conductive agent comprises at least one of carbon black, carbon nanotubes, and graphene;

[0028] the binder comprises at least one of polyvinylidene fluoride, butadiene rubber, and polyvinyl alcohol;

[0029] the dispersant comprises at least one of polyvinylpyrrolidone, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and polyacrylic acid;

[0030] the solvent comprises at least one of deionized water and N-methylpyrrolidone.

[0031] In one embodiment, the step of forming the anode coating layer is followed by the step of:

[0032] forming a lithium metal sheet on a side of the second anode slurry coating layer distal to the current collector.

[0033] In one embodiment, the lithium metal sheet is compounded with the second anode slurry coating layer of the anode sheet using a calendering method.

[0034] When the lithium metal sheet is arranged on the second negative electrode slurry coating of the negative electrode tab, further, for example, the second porosity of the second negative electrode slurry coating is 35%, the greater the second porosity of the second negative electrode slurry coating, the greater the specific surface area of the second negative electrode slurry coating, the greater the contact area of the second negative electrode slurry coating with the lithium metal sheet, and the more active sites. When the battery cell including the negative electrode tab is soaked in the electrolyte, there are many internal short circuit sites, the local current density formed is low, the metal lithium oxidation reaction is slow and uniform, the metal lithium supplemented through the lithium metal sheet can be more effectively utilized, the active lithium consumed by the formation of the SEI film of the negative electrode can be more effectively compensated, and the capacity retention rate of the lithium ion battery is improved, that is, the cycle life of the lithium ion battery including the above-mentioned battery cell is improved. Moreover, the greater the porosity of the second negative electrode slurry coating, the shorter the path of the lithium ions generated by the oxidation of the metal lithium to be inserted into the first negative electrode slurry coating through the second negative electrode slurry coating, which is more conducive to the lithium compensation of the first negative electrode slurry coating close to the current collector, can more effectively compensate the active lithium consumed by the formation of the SEI, and is conducive to improving the cycle life of the lithium ion battery including the above-mentioned battery cell.

[0035] In addition, since the porosity of the second negative electrode slurry coating is greater than the porosity of the first negative electrode slurry coating, and the smaller the porosity of the first negative electrode slurry coating, for example, the porosity of the first negative electrode slurry coating is 10%, under the same volume, the amount of negative electrode slurry is greater, which can improve the energy density of the battery cell including the above-mentioned negative electrode tab, so that the lithium ion battery including the above-mentioned battery cell has higher gram capacity performance. Therefore, the battery cell including the above-mentioned negative electrode tab can meet the needs of high energy density and high lithium compensation utilization rate. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present disclosure or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present disclosure, and for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

[0037] FIG. 1 is a structural schematic diagram of a negative electrode tab in an embodiment of the present disclosure;

[0038] FIG. 2 is a structural schematic diagram of a negative electrode tab in another embodiment of the present disclosure;

[0039] FIG. 3 is a flow chart of a preparation method of a negative electrode tab in an embodiment of the present disclosure. DETAILED DESCRIPTION

[0040] In order to make the above objectives, features and advantages of the present disclosure more clear and easier to understand, the specific embodiments of the present disclosure will be described in detail below with reference to the drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, the present disclosure can be practiced in many different ways from those described herein, and skilled in the art can make similar improvements without departing from the scope of the present disclosure, therefore the present disclosure is not limited to the specific embodiments disclosed below.

[0041] In the description of the present disclosure, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present disclosure and simplifying the description, and therefore cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present disclosure.

[0042] In addition, the terms "first", "second" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present disclosure, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise explicitly specified and limited.

[0043] In the present disclosure, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting", "fixing" and the like should be understood broadly, for example, it can be fixed connection, or detachable connection, or integral; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, it can be the internal communication of two elements or the interaction relationship between two elements, unless otherwise explicitly limited. For those skilled in the art, the specific meaning of the above terms in the present disclosure can be understood according to the specific circumstances.

[0044] In the present disclosure, unless otherwise explicitly specified and limited, the first feature "on" or "under" the second feature can be that the first and second features are in direct contact, or the first and second features are indirectly in contact through an intermediate medium. Moreover, the first feature "above", "above" and "above" the second feature can be that the first feature is directly above or obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The first feature "below", "below" and "below" the second feature can be that the first feature is directly below or obliquely below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0045] It is to be understood that where an element such as a layer, region or substrate is described as being "on" another element, it can be directly on the other element or intervening elements can also be present. Where an element such as a layer, region or substrate is described as being "connected" to another element, it can be directly connected to the other element or intervening elements can also be present. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises", "comprising", "including", and "having", are inclusive and therefore specify certain embodiments, but do not preclude other embodiments.

[0046] The lithium ion battery mainly includes a positive electrode sheet, a negative electrode sheet, a lithium battery separator and an electrolyte, wherein the positive electrode sheet includes a positive electrode current collector and a positive electrode slurry layer coated on the surface of the positive electrode current collector, and the negative electrode sheet includes a negative electrode current collector and a negative electrode coating layer on the surface of the negative electrode current collector. As shown in FIG. 1, the negative electrode sheet in the embodiment of the present disclosure. The negative electrode sheet 10 includes a current collector 200 and a negative electrode coating layer 300 provided on at least one of the opposite sides of the current collector 200. The negative electrode coating layer 300 includes a first negative electrode slurry coating layer 310 on the current collector 200 and a second negative electrode slurry coating layer 320 on the side of the first negative electrode slurry coating layer 310 away from the current collector 200. The second porosity of the second negative electrode slurry coating layer 320 is greater than the first porosity of the first negative electrode slurry coating layer 310.

[0047] It should be noted that the porosity of the slurry coating layer can be controlled by controlling the particle size distribution of the material in the slurry coating layer. The wider the particle size distribution, the lower the porosity, and the narrower the particle size distribution, the higher the porosity. The particle size distribution is controlled by the particle size distribution span. Particle size distribution span: Span = (D90-D10) / D50, D10 represents the diameter corresponding to 10% of the particle size cumulative distribution percentage (0 to 100%), D50 represents the diameter corresponding to 50% of the particle size cumulative distribution percentage (0 to 100%), and D90 represents the diameter corresponding to 90% of the particle size cumulative distribution percentage (0 to 100%). The larger the particle size distribution span, the wider the particle size distribution, and the lower the porosity; on the contrary, the smaller the particle size distribution span, the narrower the particle size distribution, and the higher the porosity.

[0048] In the present embodiment, the formula of the first negative electrode slurry coating layer 310 and the formula of the second negative electrode slurry coating layer 320 are the same, that is, the component types and component contents of the first negative electrode slurry coating layer 310 and the component types and component contents of the second negative electrode slurry coating layer 320 are all the same. The difference lies in that the particle size distribution of at least one component of the first negative electrode slurry coating layer 310 is different from the particle size distribution of the same at least one component of the second negative electrode slurry coating layer 320.

[0049] Specifically, in the embodiment, the first negative electrode slurry coating 310 and the second negative electrode slurry coating 320 each include a negative electrode active material, a conductive agent, a binder, and a dispersant. Among them, the mass fraction of the negative electrode active material is 85-98%, the mass fraction of the conductive agent is 1-5%, the mass fraction of the binder is 1-5%, and the mass fraction of the dispersant is 1-5%, and the above mass fractions are 100% based on the total solid content of the slurry.

[0050] In the embodiment, the negative electrode active material includes at least one of a carbon-based negative electrode, a silicon negative electrode, and lithium titanate; the conductive agent includes at least one of carbon black, carbon nanotubes, and graphene; the binder includes at least one of polyvinylidene fluoride, butadiene rubber, and polyvinyl alcohol; and the dispersant includes at least one of polyvinylpyrrolidone, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and polyacrylic acid.

[0051] Specifically, in the embodiment, the component types of the first negative electrode slurry coating 310 and the component types of the second negative electrode slurry coating 320 each include graphite. The difference between the first porosity of the first negative electrode slurry coating 310 and the second porosity of the second negative electrode slurry coating 320 is controlled by controlling the particle size distribution span of the graphite in the first negative electrode slurry coating 310 to be different from the particle size distribution span of the graphite in the second negative electrode slurry coating 320. That is, in the embodiment, the first porosity is determined by the first particle size distribution span of the graphite in the first component type of the first negative electrode slurry coating 310, and the second porosity is determined by the second particle size distribution span of the graphite in the second component type of the second negative electrode slurry coating 320. Among them, the particle size distribution span Span = (D90-D10) / D50, D10 represents the diameter corresponding to the particle size cumulative distribution percentage of 10%, D50 represents the diameter corresponding to the particle size cumulative distribution percentage of 50%, and D90 represents the diameter corresponding to the particle size cumulative distribution percentage of 90%.

[0052] Optionally, in the embodiment, the first porosity of the first negative electrode slurry coating 310 is 10%-40%. The second porosity of the second negative electrode slurry coating 320 is 12%-45%. In this way, the second porosity of the second negative electrode slurry coating 320 is greater than the first porosity of the first negative electrode slurry coating 310, and the energy density of the battery cell including the above negative electrode tab 10 can be better balanced.

[0053] Preferably, in the embodiment, the first porosity of the first negative electrode slurry coating 310 is 15%-20%. The second porosity of the second negative electrode slurry coating 320 is 20%-35%. In this way, the first porosity of the first negative electrode slurry coating 310 is smaller, and the second porosity of the second negative electrode slurry coating 320 is larger, and the battery cell including the above negative electrode tab 10 can well balance the requirements of high energy density and high lithium supplement utilization rate.

[0054] When the lithium metal sheet 500 is arranged on the second negative electrode slurry coating 320 of the negative electrode tab 10, further, for example, the second porosity of the second negative electrode slurry coating 320 is 35%, the greater the second porosity of the second negative electrode slurry coating 320, the greater the specific surface area of the second negative electrode slurry coating 320, the greater the contact area between the second negative electrode slurry coating 320 and the lithium metal sheet 500, and the more active sites. When the battery cell including the negative electrode tab 10 is soaked in the electrolyte, the more the internal short circuit sites, the lower the local current density formed, the slower and more uniform the metal lithium oxidation reaction, the more effective the utilization of the metal lithium supplemented by the lithium metal sheet 500, the more effective the compensation for the active lithium consumed by the formation of the SEI film of the negative electrode, and the more conducive to improving the capacity retention rate of the lithium ion battery, i.e., the more conducive to improving the cycle life of the lithium ion battery including the battery cell. Moreover, the greater the porosity of the second negative electrode slurry coating 320, the shorter the path for the lithium ions generated by the oxidation of the metal lithium to be embedded into the first negative electrode slurry coating 310 through the second negative electrode slurry coating 320, which is more conducive to lithium supplementation of the first negative electrode slurry coating 310 close to the current collector 200, more effective compensation for the active lithium consumed by the formation of the SEI of the negative electrode, and more conducive to improving the cycle life of the lithium ion battery including the battery cell.

[0055] In addition, since the porosity of the second negative electrode slurry coating 320 is greater than the porosity of the first negative electrode slurry coating 310, and the smaller the porosity of the first negative electrode slurry coating 310, e.g., the porosity of the first negative electrode slurry coating 310 is 10%, the greater the amount of negative electrode slurry under the same volume, the more the energy density of the battery cell including the negative electrode tab 10 can be improved, and the lithium ion battery including the battery cell has a higher specific capacity to play. Therefore, the battery cell including the negative electrode tab 10 can meet the needs of high energy density and high lithium supplementation utilization rate.

[0056] Optionally, in the present embodiment, the opposite sides of the current collector 200 are provided with the negative electrode coating layer 300. In this way, the battery cell including the negative electrode tab 10 can well meet the needs of high energy density and high lithium supplementation utilization rate. It can be understood that in other embodiments, the negative electrode coating layer 300 can also be arranged on only one side of the current collector 200.

[0057] Optionally, in the present embodiment, the negative electrode tab 10 includes a tab 400 connected to the edge of the current collector 200, and the tab 400 is used to connect the negative electrode tab 10 to the structure of the battery cell, such as a pole, to realize electrical connection. Specifically, in the present embodiment, the current collector 200 and the tab 400 are made of the same material. The current collector 200 and the tab 400 can both be made of metal copper, or both be made of a copper composite current collector.

[0058] As shown in FIG. 2, in the present embodiment, the negative electrode tab 10 further comprises a lithium metal sheet 500. The lithium metal sheet 500 is disposed on the side of the second negative electrode slurry coating layer 320 of the negative electrode tab 10 away from the current collector 200. In this way, a negative electrode tab with a lithium supplement structure can be obtained. As shown in FIG. 2, the lithium metal sheet 500 can be disposed on one side or both sides of the current collector 200.

[0059] In the present embodiment, the lithium metal sheet 500 is a metal lithium foil. In this way, the lithium metal sheet 500 can be compounded with the second negative electrode slurry coating layer 320 of the negative electrode tab 10 in a calendering manner, thereby making it more convenient to prepare the above-mentioned lithium supplement negative electrode structure. It can be understood that in other embodiments, the lithium metal sheet 500 can also be a lithium powder coating layer.

[0060] The present disclosure also provides a lithium ion battery. The lithium ion battery comprises the above-mentioned negative electrode tab.

[0061] As shown in FIG. 3, the present disclosure also provides a preparation method of the above-mentioned negative electrode tab. The preparation method of the negative electrode tab comprises the following steps:

[0062] Step S610, forming a negative electrode coating layer on the current collector, the negative electrode coating layer comprising a first negative electrode slurry coating layer on the current collector and a second negative electrode slurry coating layer on the side of the first negative electrode slurry coating layer away from the current collector, wherein the second porosity of the second negative electrode slurry coating layer is greater than the first porosity of the first negative electrode slurry coating layer.

[0063] In the present embodiment, the first negative electrode slurry coating layer is formed by the first negative electrode slurry coated on the current collector, and the second negative electrode slurry coating layer is formed by the second negative electrode slurry coated on the current collector.

[0064] In the present embodiment, in the step of forming the negative electrode coating layer, optionally, the first negative electrode slurry coating layer and the second negative electrode slurry coating layer are formed by using a double-layer coating technology. In this way, the first negative electrode slurry coating layer and the second negative electrode slurry coating layer can be coated at the same time, without waiting for the first negative electrode slurry coating layer to dry completely before coating the second negative electrode slurry coating layer, thereby saving manpower and resources in the preparation process of the negative electrode tab. It can be understood that in other embodiments, the first negative electrode slurry coating layer can also be dried completely before coating the second negative electrode slurry coating layer.

[0065] In the present embodiment, after the above-mentioned steps are completed, the preparation method of the negative electrode tab further needs to go through an oven drying step, a rolling step, a slitting step, a die cutting or cutting step in sequence, and then the negative electrode tab is prepared.

[0066] In the embodiment, the formulation of the first negative electrode slurry and the formulation of the second negative electrode slurry are the same, and both include negative electrode active material, conductive agent, binder, dispersant and solvent. Among them, the mass fraction of the negative electrode active material is 85-98%, the mass fraction of the conductive agent is 1-5%, the mass fraction of the binder is 1-5%, the mass fraction of the dispersant is 1-5%, and the above mass fractions are 100% based on the total solid content of the slurry; the mass ratio of the solvent to the total components (the whole composed of negative electrode active material, conductive agent, binder, dispersant and solvent) is 30%-60%.

[0067] In the embodiment, the negative electrode active material includes at least one of carbon-based negative electrode, silicon negative electrode and lithium titanate; the conductive agent includes at least one of carbon black, carbon nanotube and graphene; the binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber and polyvinyl alcohol; the dispersant includes at least one of polyvinylpyrrolidone, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose and polyacrylic acid; and the solvent includes at least one of deionized water and N-methyl pyrrolidone.

[0068] The particle size distribution of at least one component of the first negative electrode slurry coating is different from the particle size distribution of at least one component of the second negative electrode slurry coating. Optionally, the porosity of the first negative electrode slurry coating is controlled to be different from the porosity of the second negative electrode slurry coating by controlling the particle size distribution span of graphite in the first negative electrode slurry coating to be different from the particle size distribution span of graphite in the second negative electrode slurry coating. For details, refer to the related description of the negative electrode tab in the foregoing, which will not be described here in detail.

[0069] Optionally, as shown in FIG. 3, after step S610, the method further includes the following step: step S620, forming a lithium metal sheet on the side of the second negative electrode slurry coating away from the current collector.

[0070] Specifically, in the embodiment, the lithium metal sheet is compounded with the second negative electrode slurry coating of the negative electrode tab in a calendering manner, so that the negative electrode tab with lithium supplement structure is more convenient to prepare.

[0071] The following will be further described in combination with specific examples and comparative examples.

[0072] (1) Preparation of a negative electrode tab with lithium supplement structure (1.1) Take carbon-based negative electrode (negative electrode active material), carbon black, styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) according to a mass ratio of 95:2:2:1, wherein the carbon-based negative electrode is specifically graphite, and the particle size distribution span of the graphite is S1; mix the above materials with deionized water (solvent) according to a mass ratio of 52:48 to obtain a first negative electrode slurry.

[0073] (1.2) Take carbon-based negative electrode (negative active material), carbon black (conductive agent), styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) according to the mass ratio of 95:2:2:1, wherein the carbon-based negative electrode is graphite, and the particle size distribution of the graphite is S2; the whole of the negative active material, the conductive agent, the binder and the dispersant is a material, and the material is mixed with deionized water (solvent) according to the mass ratio of 52:48 to obtain a second negative electrode slurry.

[0074] (1.3) The first negative electrode slurry is coated on both sides of the metal copper (current collector) with a thickness of 6 μm to obtain a first intermediate electrode sheet, and the first intermediate electrode sheet is placed in an oven for drying, and the temperature of the oven is 65 ℃, and the drying time is 2 min to obtain a first dried electrode sheet. The porosity a1 of the first negative electrode slurry coating is detected by an image analysis method. The second negative electrode slurry is coated on both sides of the first dried electrode sheet with a thickness of 80 μm, and the second intermediate electrode sheet is placed in an oven for drying, and the temperature of the oven is 65 ℃, and the drying time is 2 min to obtain a second dried electrode sheet. The porosity a2 of the second negative electrode slurry coating is detected by an image analysis method. After the roll pressing step, the slitting step and the die cutting step, the negative electrode sheet is prepared.

[0075] (1.4) The metal lithium foil with a thickness of 2 μm is compounded on both second negative electrode slurry coatings of the negative electrode sheet by calendering, and after the process is completed, the metal lithium foil is completely absorbed into the negative electrode sheet.

[0076] (2) Preparation of lithium ion battery

[0077] The positive electrode sheet, the negative electrode sheet and the separator are laminated and placed in an aluminum shell, and an electrolyte is injected to form a lithium ion battery. The main material of the positive electrode sheet is lithium iron phosphate.

[0078] Examples 1-4 and Comparative Examples 1-2 are prepared according to the above method, and the capacity retention rate test and the positive electrode capacity release test are carried out by using a battery capacity test system, and the test results are as follows:

[0079] According to the above table, compared with Comparative Examples 1 and 2, the negative electrode sheet with high porosity of the second negative electrode slurry coating and low porosity of the first negative electrode slurry coating can improve the utilization rate of lithium supplement, thereby improving the three-hundred-time capacity retention rate and the six-hundred-time capacity retention rate of the lithium ion battery, that is, the cycle life of the lithium ion battery can be provided.

[0080] Compared with Example 1, Example 2 has better cycle life and higher gram capacity, indicating that when the porosities of the first negative electrode slurry coatings are the same, the second negative electrode slurry coating with the negative electrode pole piece with a higher porosity is more conducive to improving the utilization rate of lithium supplement, improving the cycle life, and improving the energy density.

[0081] Compared with Example 2, Example 3 has better cycle life and higher gram capacity, indicating that when the porosities of the second negative electrode slurry coatings are the same, the first negative electrode slurry coating with the negative electrode pole piece with a lower porosity is more conducive to improving the utilization rate of lithium supplement, improving the cycle life, and improving the energy density.

[0082] Compared with Example 3, Example 4 has poorer cycle life and lower gram capacity, indicating that when the porosities of the first negative electrode slurry coatings are the same, the second negative electrode slurry coating with the negative electrode pole piece with a too high porosity is not conducive to improving the utilization rate of lithium supplement, improving the cycle life, and improving the energy density.

[0083] The technical features of the above-described embodiments can be combined arbitrarily. To make the description concise, all possible combinations of the technical features in the above-described embodiments are not described, but as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the disclosure.

[0084] The above-described embodiments only express several implementation manners of the disclosure, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the disclosure, and these all belong to the protection scope of the disclosure. Therefore, the protection scope of the patent of the disclosure should be subject to the appended claims. Industrial applicability

[0085] In the disclosure, when the battery cell including the above-mentioned negative electrode pole piece is soaked in electrolyte, there are many internal short circuit sites, the local current density formed is low, the metal lithium oxidation reaction is slow and uniform, the metal lithium supplemented by the lithium metal piece can be more effectively utilized, the active lithium consumed by the formation of SEI film of the negative electrode can be more effectively made up, which is conducive to improving the capacity retention rate of the lithium ion battery, i.e., conducive to improving the cycle life of the lithium ion battery including the above-mentioned battery cell. Moreover, the greater the porosity of the second negative electrode slurry coating is, the shorter the path of lithium ions generated by the oxidation of metal lithium to be embedded into the first negative electrode slurry coating through the second negative electrode slurry coating, which is more conducive to supplementing lithium by the first negative electrode slurry coating close to the current collector, can more effectively make up the active lithium consumed by the formation of SEI, and is conducive to improving the cycle life of the lithium ion battery including the above-mentioned battery cell.

[0086] In addition, since the porosity of the second negative electrode slurry coating is greater than the porosity of the first negative electrode slurry coating, and the smaller the porosity of the first negative electrode slurry coating, the greater the energy density of the battery cell comprising the above negative electrode plate, so that the lithium ion battery comprising the above battery cell has a higher specific capacity. Therefore, the battery cell comprising the above negative electrode plate can meet the needs of high energy density and high lithium supplement utilization rate.

Claims

1. A negative electrode sheet, characterized by, include: current collector; as well as A negative electrode coating layer is provided on at least one of the two opposite sides of the current collector, the negative electrode coating layer comprising a first negative electrode slurry coating layer and a second negative electrode slurry coating layer located on the current collector, the second negative electrode slurry coating layer being located on a side of the first negative electrode slurry coating layer away from the current collector, and the second porosity of the second negative electrode slurry coating layer being greater than the first porosity of the first negative electrode slurry coating layer.

2. The negative electrode sheet according to claim 1, wherein The first porosity is determined by a first particle size distribution span of graphite in the first component type of the first negative electrode slurry coating, and the second porosity is determined by a second particle size distribution span of graphite in the second component type of the second negative electrode slurry coating, wherein the particle size distribution span is (D90-D10) / D50, D10 represents the diameter corresponding to when the cumulative distribution percentage of the particle size reaches 10%, D50 represents the diameter corresponding to when the cumulative distribution percentage of the particle size reaches 50%, and D90 represents the diameter corresponding to when the cumulative distribution percentage of the particle size reaches 90%.

3. The negative electrode sheet according to claim 1, wherein The first porosity is 10%-40%, and the second porosity is 12%-45%.

4. The negative electrode sheet according to claim 2, wherein The component types and component contents of the first negative electrode slurry coating layer are the same as the component types and component contents of the second negative electrode slurry coating layer.

5. The negative electrode sheet according to claim 4, wherein The first negative electrode slurry coating layer and the second negative electrode slurry coating layer each include a negative electrode active material, a conductive agent, a binder, and a dispersant.

6. The negative electrode sheet according to claim 5, wherein The mass fraction of the negative electrode active material is 85-98%, the mass fraction of the conductive agent is 1-5%, the mass fraction of the binder is 1-5%, and the mass fraction of the dispersant is 1-5%, and the above mass fractions are based on the total solid content of the slurry as 100%.

7. The negative electrode sheet according to claim 5, wherein The negative electrode active material includes at least one of a carbon-based negative electrode, a silicon negative electrode and lithium titanate.

8. The negative electrode sheet according to claim 5, wherein The conductive agent includes at least one of carbon black, carbon nanotubes and graphene.

9. The negative electrode sheet according to claim 5, wherein The binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber and polyvinyl alcohol.

10. The negative electrode sheet according to claim 5, wherein The dispersant includes at least one of polyvinyl pyrrolidone, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose and polyacrylic acid.

11. The negative electrode sheet according to claim 1, wherein The negative electrode plate further includes a lithium metal plate, and the lithium metal plate is disposed on a side of the second negative electrode slurry coating layer away from the current collector.

12. A lithium-ion battery, characterized by, Comprising the negative electrode sheet as described in any one of claims 1 to 11.

13. A method for producing a negative electrode sheet, characterized by The steps include: A negative electrode coating layer is formed on at least one of two opposite sides of the current collector, comprising a first negative electrode slurry coating layer and a second negative electrode slurry coating layer located on the current collector, wherein the second negative electrode slurry coating layer is located on a side of the first negative electrode slurry coating layer away from the current collector, and the second porosity of the second negative electrode slurry coating layer is greater than the first porosity of the first negative electrode slurry coating layer.

14. The method of producing a negative electrode sheet according to claim 13, wherein In the step of forming the negative electrode coating layer, a double-layer coating technology is adopted to form a first negative electrode slurry coating layer and a second negative electrode slurry coating layer.

15. The method of producing a negative electrode sheet according to claim 13, wherein In the step of forming the negative electrode coating layer, the first negative electrode slurry coating layer is formed from a first negative electrode slurry, and the second negative electrode slurry coating layer is formed from a second negative electrode slurry, wherein the first negative electrode slurry and the second negative electrode slurry both include a negative electrode active material, a conductive agent, a binder, a dispersant, and a solvent.

16. The method of producing a negative electrode sheet according to claim 15, wherein The preparation method of the negative electrode sheet satisfies at least one of the following characteristics a-g: a. The mass fraction of the negative electrode active material is 85-98%, the mass fraction of the conductive agent is 1-5%, the mass fraction of the binder is 1-5%, and the mass fraction of the dispersing agent is 1-5%, with the total mass of the slurry solid content being 100%; b. The mass ratio of the solvent to the total mass of the negative electrode active material, the conductive agent, the binder, the dispersing agent, and the solvent is 30%-60%; c. The negative electrode active material includes at least one of a carbon-based negative electrode, a silicon negative electrode, and lithium titanate; d. The conductive agent includes at least one of carbon black, carbon nanotubes, and graphene; e. The binder includes at least one of polyvinylidene fluoride, butadiene rubber, and polyvinyl alcohol; f. The dispersing agent includes at least one of polyvinylpyrrolidone, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and polyacrylic acid; g. The solvent includes at least one of deionized water and N-methyl pyrrolidone.

17. The method of producing a negative electrode sheet according to claim 13, wherein The step of forming the negative electrode coating layer is further followed by the steps of: forming a lithium metal sheet on the side of the second negative electrode slurry coating layer away from the current collector.

18. The method of producing a negative electrode sheet according to claim 17, wherein The lithium metal sheet is compounded with the second negative electrode slurry coating layer of the negative electrode sheet in a calendering manner.

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