Secondary battery, method for manufacturing the same, and electric device

Abstract

This invention relates to the field of battery technology, and particularly to a secondary battery, its preparation method, and an electrical device thereof. The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative current collector and an A-film layer and a B-film layer disposed on opposite surfaces of the negative current collector. The concentration of active metal elements per unit area of ​​the A-film layer is greater than that of the B-film layer. The ratio of the negative electrode active material capacity per unit area of ​​the A-film layer to that of the B-film layer is 0.5 to 1.13. Meeting these conditions can reduce the problem of metal deposition on the negative electrode side.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a secondary battery, its preparation method, and an electrical device thereof. Background Technology

[0002] With the rapid growth of portable electronic devices, electric vehicles, and other technologies, the demand for power batteries is also constantly increasing. Among these, the electrochemical performance of batteries is receiving increasing attention.

[0003] When metal deposits form on the surface of the negative electrode during the charging and discharging process, it can easily lead to a decrease in battery performance. For example, in lithium-ion batteries, lithium plating on the negative electrode side can affect battery performance. Summary of the Invention

[0004] The main objective of this application is to provide a secondary battery that aims to reduce the problem of metal deposition on the negative electrode side.

[0005] To achieve the above objectives, this application proposes a secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator.

[0006] The negative electrode sheet includes a negative current collector and an A film layer and a B film layer disposed on opposite surfaces of the negative current collector. The concentration of active metal element per unit area of ​​the A film layer is greater than the concentration of active metal element per unit area of ​​the B film layer.

[0007] The ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 0.5 to 1.13.

[0008] In this application, when the concentration of active metal elements in the A film layer of the negative electrode is higher than that in the B film layer of the negative electrode, the active metal elements in the A film layer tend to migrate to the B film layer. Based on the fact that the ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer satisfies the above-mentioned range, the B film layer has sufficient negative electrode active material to provide capacity to accommodate the active metal elements migrating from the A film layer side to the B film layer side. This reduces the problem of active metal elements precipitating in metal form when the capacity of the B film layer side is insufficient, thereby reducing the problem of metal precipitation on the negative electrode side.

[0009] It is understandable that, considering the concentration difference of active metal elements in the A and B film layers on both sides of the negative electrode, a potential difference will exist on both sides of the negative electrode after electrolyte wetting. This potential difference acts as a driving force, causing active metal elements to migrate from the side with a higher concentration of active metal elements in the A film layer to the side with a lower concentration in the B film layer, thus bringing the active metal elements in the A and B film layers into equilibrium. If the capacity of the negative electrode active material in the B film layer is insufficient to provide space for the migrating active metal elements to intercalate, this can easily lead to the precipitation of the migrating active metal elements on the surface of the B film layer in the form of metal. To solve this problem, the ratio of the negative electrode active material capacity per unit area of ​​the A film layer to that of the B film layer is within the aforementioned range, so that the overall negative electrode active material capacity in the secondary battery is reasonably distributed. This helps the B film layer to have sufficient negative electrode active material capacity to provide space for the migrating active metal elements to intercalate, reducing the problem of metal precipitation on the negative electrode side.

[0010] Optionally, the ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 1.09 to 1.13.

[0011] Theoretically, the ratio of the capacity of the negative electrode active material per unit area of ​​layer A to that of layer B should be less than 1.13. However, if the ratio is too low, the capacity of layer B will be too large, resulting in excess negative electrode capacity in layer B. This excess negative electrode capacity does not have corresponding lithium ion insertion, which increases the mass and volume of the battery and reduces its energy density. Therefore, to balance reducing metal deposition on the negative electrode side and mitigating the decrease in battery energy density, the ratio of the capacity of the negative electrode active material per unit area of ​​layer A to that of layer B should be between 1.09 and 1.13.

[0012] Optionally, the A film layer includes a negative electrode active layer and an active metal element supplementation layer;

[0013] The B film layer is the negative electrode active layer.

[0014] In this scheme, active metal elements are added to one side of the negative electrode sheet. For example, in a lithium-ion battery, lithium is added only to one side of the negative electrode sheet. This helps to reduce the modulus of the negative electrode sheet, making it easier to deform. It also reduces the gap between the positive and negative electrodes at the corner when the negative electrode is bent, thus improving the battery dynamics. It is understood that wound cells have a structure where the electrode sheets are bent to form corners.

[0015] Optionally, the active metal element supplement layer is located on the side of the negative electrode active layer away from the negative electrode current collector, and there is a gap between the side of the active metal element supplement layer and the side of the negative electrode current collector.

[0016] If the active metal element supplement layer extends beyond the negative electrode, there is a risk of puncturing the separator and short-circuiting the positive electrode. Therefore, there is a gap between the side of the active metal element supplement layer and the side of the negative electrode current collector.

[0017] Optionally, the distance between the side of the active metal element replenishment layer and the side of the negative electrode current collector is 1.5 mm to 2.5 mm.

[0018] Taking lithium-ion batteries as an example, when lithium strips are used as a lithium replenishment layer (active metal element replenishment layer) in the negative electrode, due to the good ductility of lithium strips, if the replenishment width is too wide during the replenishment stage, the width of the lithium strip after winding and cold pressing will exceed that of the anode, which may puncture the separator and cause a short circuit with the positive electrode. Therefore, the distance between the side of the active metal element replenishment layer and the side of the negative electrode current collector is kept at 1.5mm to 2.5mm to reduce the risk of puncturing the separator caused by the width of the active metal element replenishment layer exceeding that of the negative electrode current collector after cold pressing.

[0019] Optionally, the negative electrode sheet includes a bending region and a straight region, the convex surface of the bending region includes the A film layer, and the concave surface of the bending region includes the B film layer.

[0020] Theoretically, in the scheme of supplementing active metal elements on one side of the negative electrode, the active metal elements can be supplemented on one side in the bending region or on the straight region. Considering that supplementing the positive electrode active material on the convex side can reduce the hardness of the concave side, thereby reducing the overall hardness of the electrode, it is easier to reduce the gap between the positive and negative electrode in the bending region and improve the battery dynamics.

[0021] Optionally, the ratio of the unit area mass of the negative electrode active layer of the A film layer to the unit area mass of the negative electrode active layer of the B film layer is 1.09 to 1.13.

[0022] When the ratio of the unit area mass of the negative electrode active layer in film layer A to the unit area mass of the negative electrode active layer in film layer B meets the above range, it indicates that the capacity of the negative electrode active material is reasonably distributed, which helps film layer B to have enough capacity of negative electrode active material to provide space for the embedded positive electrode active material that has migrated over, thereby reducing the problem of metal deposition on the negative electrode side.

[0023] Optionally, the negative electrode active layer of the A film layer has a unit area mass of 110.389 g / m³. 2 Up to 116.883 g / m 2 ;

[0024] And / or, the area mass per unit area of ​​the negative electrode active layer of the B film is 103.247 / m². 2 Up to 110.389 g / m 2 .

[0025] In one embodiment, the unit area mass of the negative electrode active layer of film layer A meets the above-mentioned range. In another embodiment, the unit area mass of the negative electrode active material film layer of film layer B meets the above-mentioned range.

[0026] Optionally, the first adhesive force between the diaphragm and the A membrane layer is greater than the second adhesive force between the diaphragm and the B membrane layer.

[0027] To improve the adhesion between the separator and the electrode, the separator surface is generally covered with an adhesive. The adhesive is sticky. Considering that the negative electrode coating is generally porous, if the adhesive on the separator surface is too sticky, the adhesive may be embedded in the negative electrode pores, affecting the transport of positive electrode active ions and deteriorating the kinetics.

[0028] One surface of the separator faces membrane layer A, which includes an active metal element supplement layer. The other surface of the separator faces membrane layer B, which does not include the active metal element supplement layer. The first adhesive force between the separator and membrane layer A is greater than the second adhesive force between the separator and membrane layer B. This can improve the adhesion between the separator and the active metal element supplement layer, while reducing the embedding of the adhesive on the separator into the negative electrode pores of membrane layer B.

[0029] Optionally, the mass of the adhesive on the surface of the diaphragm facing the A membrane layer is greater than the mass of the adhesive on the surface of the diaphragm facing the B membrane layer.

[0030] The mass of the adhesive on the surface of the diaphragm facing membrane layer A is greater than the mass of the adhesive on the surface of the diaphragm facing membrane layer B. This helps to make the first adhesive force between the diaphragm and membrane layer A greater than the second adhesive force between the diaphragm and membrane layer B.

[0031] Optionally, the mass of the adhesive per unit area on the surface of the diaphragm facing the A membrane layer is 0.09 mg / cm². 2 Up to 0.103 mg / cm 2 The mass of the adhesive per unit area on the surface of the diaphragm facing the B membrane layer is 0.02 mg / cm². 2 Up to 0.082 mg / cm 2 .

[0032] Meeting the above-mentioned range for the mass of adhesive per unit area on the side of the diaphragm facing membrane layer A helps to improve the adhesion between the diaphragm and membrane layer A. Meeting the above-mentioned range for the mass of adhesive per unit area on the side of the diaphragm facing membrane layer B helps to reduce the embedment of adhesive on the diaphragm into the negative electrode pores of membrane layer B.

[0033] Optionally, the ratio of the second adhesive force to the first adhesive force is 0.25 to 0.8.

[0034] In one embodiment, the ratio of the second adhesive force to the first adhesive force is 0.25 to 0.8.

[0035] Optionally, the first adhesive force is 5 N / m to 8 N / m;

[0036] And / or, the second adhesive force is 2 N / m to 4 N / m.

[0037] Optionally, the electrolyte includes an electrolyte solution, the secondary battery is a lithium-ion battery, the electrolyte includes a lithium salt and an additive, and the concentration range of the lithium salt is 0.05 mol / L to 2 mol / L;

[0038] And / or, the additive includes at least one of fluoroethylene carbonate, vinylene carbonate, and propylene carbonate, wherein the concentration of the fluoroethylene carbonate is from 0.05% wt to 1.5% wt; the concentration of the vinylene carbonate is from 0.05% wt to 1% wt; and the concentration of the propylene carbonate is from 0.05% wt to 1.2% wt.

[0039] In designs where the negative electrode includes a lithium replenishment layer, cell lifespan is improved, but the amount of lithium salt in the electrolyte also needs to be increased. Understandably, as the cell cycles, lithium salt and additives are consumed. Insufficient amounts of lithium salt and additives can lead to cell degradation. Therefore, the concentrations of lithium salt and additives in the electrolyte need to be increased. Maintaining lithium salt and additive concentrations within the aforementioned range helps improve battery cycle performance.

[0040] Optionally, the A film layer includes a first negative electrode active material at the portion near the negative electrode current collector, and a second negative electrode active material at the portion away from the negative electrode current collector, wherein the conductivity of the second negative electrode active material is in the range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s, the capacity range of the first negative electrode active material is 370mAh / g to 372mAh / g;

[0041] And / or, the B film layer includes a first negative electrode active material at a portion near the negative electrode current collector, and a second negative electrode active material at a portion away from the negative electrode current collector, wherein the conductivity of the second negative electrode active material is in the range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s, the capacity range of the first negative electrode active material is 370mAh / g to 372mAh / g.

[0042] The second negative electrode active material is closer to the negative electrode surface. The conductivity of the second negative electrode active material meets the above range, indicating that its conductivity is relatively large, which helps to improve the kinetics of the negative electrode. Taking lithium-ion batteries as an example, the improvement of negative electrode kinetics helps lithium ions to quickly insert into the negative electrode active layer and reduce the risk of lithium plating.

[0043] If the capacity of the first negative electrode active material meets the above range, it indicates that its capacity is relatively large. Taking lithium-ion batteries as an example, it can provide more lithium vacancies to store lithium ions.

[0044] Optionally, the first negative electrode active material includes graphite, wherein the particle size Dv50 of the graphite is 11 μm to 13 μm;

[0045] And / or, the second negative electrode active material includes graphite, wherein the graphite has a particle size Dv50 of 8.6 μm to 10.7 μm.

[0046] Reducing the graphite particle size facilitates the rapid embedding of active metal ions into the graphite, which helps improve the kinetics of the negative electrode. The second negative electrode active material is closer to the negative electrode surface, and the improved negative electrode kinetics help lithium ions to be rapidly embedded into the negative electrode active layer, reducing the risk of lithium plating.

[0047] Optionally, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery is 1.07 to 1.10.

[0048] In a secondary battery, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material must satisfy the aforementioned range to ensure that the negative electrode has sufficient capacity for the insertion of active metal elements. In other words, the secondary battery as a whole has sufficient negative electrode active material capacity to provide space for the insertion of active metal elements.

[0049] Optionally, this application also provides a method for preparing a secondary battery, comprising:

[0050] Prepare the negative electrode slurry;

[0051] The negative electrode slurry is coated on both sides of the negative electrode current collector, dried, and extruded to obtain a negative electrode active layer on the two opposite surfaces of the negative electrode current collector.

[0052] An active metal element supplement layer is provided on the surface of the active layer of the negative electrode current collector on at least one side, so that an A film layer and a B film layer are provided on the opposite two surfaces of the negative electrode current collector, and the concentration of active metal element per unit area of ​​the A film layer is greater than the concentration of active metal element per unit area of ​​the B film layer.

[0053] The ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 1.09 to 1.13.

[0054] In the scheme of supplementing the positive electrode active material to the negative electrode sheet, when the concentration of active metal elements per unit area on the two opposite surfaces of the negative electrode sheet is different, the ratio of the negative electrode active material capacity per unit area of ​​film layer A to the negative electrode active material capacity per unit area of ​​film layer B meets the above range, which helps to reduce the problem of metal deposition on the negative electrode side.

[0055] Optionally, the step of providing an active metal element supplement layer on the surface of the negative electrode active layer on at least one side of the negative electrode current collector, so that A film layer and B film layer are provided on opposite surfaces of the negative electrode current collector, wherein the concentration of active metal element per unit area of ​​the A film layer is greater than the concentration of active metal element per unit area of ​​the B film layer, includes:

[0056] An active metal element supplement layer is provided on the surface of the negative electrode active layer on one side of the negative electrode current collector. The side with the active metal element supplement layer is called film layer A, and the side without the active metal element supplement layer is called film layer B. The active metal element concentration per unit area of ​​film layer A is greater than that of film layer B.

[0057] In the scheme of supplementing active metal elements to the negative electrode sheet, active metal elements can be supplemented to one side of the negative electrode sheet.

[0058] Optionally, the step of providing an active metal element supplement layer on the surface of the negative electrode active layer on at least one side of the negative electrode current collector includes:

[0059] An active metal element supplement sheet is placed on the surface of the negative electrode active layer and rolled to obtain an active metal element supplement layer on the surface of the negative electrode active layer.

[0060] An active metal element supplement layer can be obtained on the surface of the negative electrode active layer by placing an active metal element supplement sheet on it and then rolling it. For example, in a lithium-ion battery, a lithium sheet is used as the active metal element supplement sheet.

[0061] Optionally, the process of preparing the secondary battery further includes: preparing a positive electrode sheet, wherein the capacity of the active metal element supplement layer accounts for 10% to 13% of the total capacity of the positive electrode active material in the positive electrode sheet.

[0062] During the fabrication of a secondary battery, when active metal elements are added to one side of the negative electrode surface, a concentration difference exists between the active metal elements on both sides of the negative electrode. After the negative electrode is immersed in the electrolyte, a potential difference exists on both sides of the negative electrode, causing the active metal elements on both sides to migrate and drive them to reach equilibrium. The percentage of the capacity of the active metal elements added to the negative electrode sheet relative to the total capacity of the positive electrode active material meets the above-mentioned range. The ratio of the unit area capacity of the negative electrode active material in the A film layer to the unit area capacity of the negative electrode active material in the B film layer meets the above-mentioned range. The ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery meets the above-mentioned range. The reasonable distribution of the overall negative electrode active material capacity in the secondary battery helps the B film layer to have sufficient negative electrode active material capacity to provide space for the migrating active metal elements to embed, reducing the problem of metal deposition on the negative electrode side.

[0063] Optionally, after the step of setting an active metal element supplement layer on the surface of the negative electrode active layer on at least one side of the negative electrode current collector, the method further includes using a detection mechanism to detect the quality of the active metal element supplement layer on the surface of the negative electrode active layer.

[0064] If the quality of the added active metal elements does not meet the preset requirements during the process of replenishing active metal elements on the negative electrode side, it may affect the battery life. Therefore, after the step of setting an active metal element replenishment layer on the surface of the negative electrode active layer on at least one side, the method further includes using a detection mechanism to detect the quality of the active metal element replenishment layer on the surface of the negative electrode active layer.

[0065] Optionally, the detection mechanism includes a camera, which is directed toward the active metal element supplementation layer to acquire quality information of the active metal element supplementation layer.

[0066] In the step of setting an active metal element replenishment layer on the surface of the negative electrode active material, a camera can be used to photograph the active metal element replenishment layer to obtain quality information, thereby determining whether the active metal element replenishment layer meets the preset requirements. Taking lithium secondary battery lithium replenishment as an example, lithium metal is silvery-white, and the negative electrode active material is black. The replenishment area can be distinguished by the color of the photographed image. When the replenishment area does not meet the preset requirements, it can be considered that the lithium replenishment quality is insufficient, and lithium replenishment can be performed again when the replenishment is insufficient.

[0067] Optionally, the step of coating the negative electrode slurry on both sides of the negative electrode current collector, drying, and extruding to obtain a negative electrode active layer on the opposite two surfaces of the negative electrode current collector includes:

[0068] Prepare a first negative electrode slurry, the first negative electrode slurry comprising a first negative electrode active material, the capacity of the first negative electrode active material being in the range of 370 mAh / g to 372 mAh / g;

[0069] Prepare a second negative electrode slurry, which includes a second negative electrode active material with a conductivity range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s;

[0070] The first negative electrode slurry is coated on one side of the negative electrode current collector, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry layer; and / or, the first negative electrode slurry is coated on the other side B of the negative electrode current collector, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry layer.

[0071] In the step of setting the negative electrode active layer on one side (side A) and the other side (side B) of the negative electrode current collector, the negative electrode active layer can be prepared by double coating on side A. A first slurry is first coated on the surface of the negative electrode current collector, followed by a second slurry. The second negative electrode active material is closer to the negative electrode surface. The conductivity of the second negative electrode active material meets the above-mentioned range, indicating that its conductivity is relatively high, which helps improve the kinetics of the negative electrode. Taking a lithium-ion battery as an example, improved negative electrode kinetics helps lithium ions to quickly embed into the negative electrode active layer, reducing the risk of lithium plating. The capacity of the first negative electrode active material meets the above-mentioned range, indicating that its capacity is relatively high. Taking a lithium-ion battery as an example, it can provide more lithium vacancies for storing lithium ions.

[0072] Optionally, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery is 1.07 to 1.10.

[0073] Optionally, the process of preparing the secondary battery further includes:

[0074] Prepare an electrolyte. The secondary battery is a lithium-ion battery. The electrolyte includes a lithium salt. The concentration range of the lithium salt is X1×(1+Y1), where X1 is from 0.88mol / L to 1.82mol / L, and Y1 is the percentage of the capacity of the active metal element supplement layer to the total capacity of the positive electrode active material in the positive electrode sheet.

[0075] In designs where the negative electrode includes a lithium replenishment layer, cell lifespan is improved, but the amount of lithium salt in the electrolyte also needs to be increased. Understandably, lithium salt is consumed during cell cycling; insufficient lithium salt can lead to cell degradation. Therefore, the concentration of lithium salt in the electrolyte needs to be increased. A lithium salt concentration within the aforementioned range helps improve battery cycle performance.

[0076] Under normal circumstances, in a scheme without lithium replenishment, the concentration of lithium salt in the electrolyte can be considered as X1. As the capacity of lithium replenishment increases, the concentration of lithium salt also increases. Y1 is the percentage of the capacity of lithium replenishment to the total capacity of the positive electrode active material in the positive electrode sheet. Therefore, the concentration of replenished lithium salt is X1×Y1.

[0077] This application also provides an electrical device, which includes a secondary battery as described above.

[0078] In this application, the secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative current collector and an A film layer and a B film layer disposed on opposite surfaces of the negative current collector. The concentration of active metal elements per unit area of ​​the A film layer is greater than the concentration of active metal elements per unit area of ​​the B film layer. The ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 0.5 to 1.13.

[0079] In a secondary battery, when the concentration of active metal elements in the A film layer of the negative electrode is higher than that in the B film layer, the active metal elements in the A film layer tend to migrate to the B film layer. Since the ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer satisfies the above-mentioned range, the B film layer has sufficient negative electrode active material to provide capacity to accommodate the active metal elements migrating from the A film layer side to the B film layer side. This reduces the problem of active metal elements precipitating in metallic form when the capacity of the B film layer side is insufficient, thereby reducing the problem of metal precipitation on the negative electrode side. Attached Figure Description

[0080] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0081] Figure 1 This is a schematic diagram of the structure of the negative electrode sheet according to an embodiment of this application;

[0082] Figure 2 This is a schematic diagram of the negative electrode sheet according to another embodiment of this application;

[0083] Figure 3 This is a schematic diagram of the negative electrode sheet according to another embodiment of this application;

[0084] Figure 4 This is a schematic diagram of the negative electrode sheet in another embodiment of this application;

[0085] Figure 5 This is a schematic diagram of the structure of the negative electrode sheet in yet another embodiment of this application;

[0086] Figure 6 This is a schematic diagram of the structure of a battery cell according to one embodiment of this application;

[0087] Figure 7 yes Figure 6 An exploded view of a battery cell according to one embodiment of this application is shown.

[0088] Figure 8 This is a schematic diagram of a battery module according to one embodiment of this application;

[0089] Figure 9 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0090] Figure 10 yes Figure 9 An exploded view of a battery pack according to one embodiment of this application is shown;

[0091] Figure 11 This is a schematic diagram of an electrical device in which a single battery cell is used as a power source according to one embodiment of this application;

[0092] Figure 12 This is a morphology diagram of the negative electrode B film layer according to an embodiment of this application;

[0093] Figure 13 This is a topographic image of the negative electrode B film layer according to another embodiment of this application.

[0094] Explanation of icon numbers:

[0095]

[0096]

[0097] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0098] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0099] The secondary battery, its preparation method, and its electrical device are disclosed in detail below with appropriate reference to the accompanying drawings. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0100] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0101] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0102] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0103] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0104] During the cycling process of secondary batteries, the loss of positive electrode active material leads to a decline in battery cycle performance. To address this issue, replenishing the positive electrode active material is typically used to improve cycle performance. For example, commercial vehicles currently suffer from rapid battery degradation and insufficient lifespan, resulting in short vehicle replacement cycles. Therefore, developing long-life battery cells is crucial. Taking lithium-ion secondary batteries as an example, negative electrode lithium replenishment is a common method to improve cell lifespan. By pre-retaining lithium at the negative electrode, the lifespan of the cell can be effectively improved.

[0105] During the lithium replenishment process at the negative electrode, when there is a difference in lithium replenishment between the two surfaces of the negative electrode, there is a potential difference between the negative electrode with high lithium concentration and the negative electrode with low lithium concentration after being wetted by the electrolyte. The potential difference acts as a driving force to cause lithium from the negative electrode with high concentration to migrate to the negative electrode with low concentration. When the negative electrode with low lithium concentration cannot accommodate the migrated lithium, the lithium exists on the surface of the negative electrode in the form of lithium plating.

[0106] To address the aforementioned problems, this application proposes a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode comprises a negative current collector and an A film layer and a B film layer disposed on opposite surfaces of the negative current collector. The concentration of active metal elements per unit area of ​​the A film layer is greater than the concentration of active metal elements per unit area of ​​the B film layer. The ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 0.5 to 1.13.

[0107] The negative electrode includes a negative current collector and film layers A and B disposed on opposite surfaces of the negative current collector, such as... Figure 1 The diagram shown is a schematic diagram of the structure of a negative electrode sheet in an embodiment. A film layer 20 and B film layer 30 are respectively provided on opposite sides of the negative electrode current collector 10. Both film layer 20 and B film layer 30 include negative electrode active material to provide capacity for the insertion of positive electrode active material.

[0108] Active metal elements, for example, in lithium-ion batteries, the active metal element is lithium metal, and in sodium-ion batteries, the active metal element is sodium metal. It can be understood that active metal elements refer to metal elements that can be inserted and extracted into the positive and negative electrodes in secondary batteries.

[0109] A film layer and a film layer B are respectively provided on opposite sides of the negative electrode current collector. The concentration of active metal element per unit area of ​​film layer A is greater than that of film layer B. In one embodiment, taking a lithium-ion battery as an example, film layer A includes a lithium replenishment layer, that is, film layer A includes lithium (active metal element) replenished on the negative electrode side. The concentration of lithium in film layer A is greater than that in film layer B. Film layer B may or may not contain lithium. The difference in lithium concentration between film layer A and film layer B makes film layer A... After being immersed in the electrolyte, there is a potential difference between the A film layer and the B film layer. This potential difference acts as a driving force to promote the migration of lithium ions from the A film layer to the B film layer. When the lithium ion concentration in the A film layer is higher than that in the B film layer under the influence of the electrolyte, lithium ions in the A film layer are more likely to migrate to the B film layer. At this time, if the capacity of the negative electrode active material in the B film layer is insufficient to accommodate the migration of lithium ions, that is, the excess lithium ions cannot be accommodated by the negative electrode active material in the B film layer. The excess lithium ions exist on the surface of the B film layer in the form of lithium plating, causing the phenomenon of lithium plating.

[0110] Test method and procedure for the concentration of active metal element per unit area of ​​the film layer: For example, taking a lithium-ion battery as an example, the active metal element is lithium. The battery is disassembled to obtain the negative electrode sheet. Coating samples are taken from opposite sides of the negative electrode sheet. For example, the sampling area is S, the sample on one surface of the negative electrode sheet is sample 1, and the sample on the other surface is sample 2. Sample 1 and sample 2 are placed in polytetrafluoroethylene (PTFE) digestion containers, and an appropriate amount of aqua regia (usually a mixture of nitric acid and hydrochloric acid) is added for digestion treatment to ensure that the lithium element in the sample can be completely dissolved. After digestion, the solution is... Transfer the sample to a centrifuge tube, bring the volume to a certain level using deionized water, and filter to remove undissolved solids. Introduce the treated sample solution into an ICP-OES or ICP-MS instrument for analysis. By establishing a standard curve, determine the lithium concentration in the sample solution and calculate the lithium content in sample 1 and sample 2. For example, if the mass of lithium in sample 1 and sample 2 is M1 and M2 respectively, the formula for calculating the concentration of positive electrode active material per unit area of ​​the membrane layer is M1 / S. Based on the test results, the side with the higher concentration of positive electrode active material per unit area is membrane layer A, and the side with the lower concentration is membrane layer B.

[0111] The testing method and steps for the negative electrode active material capacity per unit area of ​​the film layer: For example, taking a lithium-ion battery as an example, based on the previous test of the concentration of active metal elements per unit area of ​​the film layer, after determining film layer A and film layer B, the negative electrode sheet is split into two parts. One part has film layer B removed, and the capacity of film layer A is tested. That is, the negative electrode sheet with film layer A is assembled into a battery, the battery is fully charged and then fully discharged, and the discharge capacity is the negative electrode active material capacity per unit area of ​​film layer A; similarly, the capacity of the other negative electrode sheet with film layer B is tested.

[0112] In this application, when the concentration of active metal elements in the A film layer of the negative electrode is higher than that in the B film layer of the negative electrode in a secondary battery, the active metal elements in the A film layer tend to migrate to the B film layer. Based on the fact that the ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer satisfies the above-mentioned range, the B film layer has sufficient negative electrode active material to provide capacity to accommodate the active metal elements migrating from the A film layer side to the B film layer side. This reduces the problem of active metal elements precipitating in metal form when the capacity of the B film layer side is insufficient, thereby reducing the problem of metal precipitation on the negative electrode side.

[0113] It is understandable that, considering the concentration difference of active metal elements in the A and B film layers on both sides of the negative electrode, a potential difference will exist on both sides of the negative electrode after electrolyte wetting. This potential difference acts as a driving force, causing active metal elements to migrate from the side with a higher concentration of active metal elements in the A film layer to the side with a lower concentration in the B film layer, thus bringing the active metal elements in the A and B film layers into equilibrium. If the capacity of the negative electrode active material in the B film layer is insufficient to provide space for the migrating active metal elements to intercalate, this can easily lead to the precipitation of the migrating active metal elements on the surface of the B film layer in the form of metal. To solve this problem, the ratio of the negative electrode active material capacity per unit area of ​​the A film layer to that of the B film layer is within the aforementioned range, so that the overall negative electrode active material capacity in the secondary battery is reasonably distributed. This helps the B film layer to have sufficient negative electrode active material capacity to provide space for the migrating active metal elements to intercalate, reducing the problem of metal precipitation on the negative electrode side.

[0114] In one embodiment, the ratio of the negative electrode active material capacity per unit area of ​​film layer A to the negative electrode active material capacity per unit area of ​​film layer B is 1.09 to 1.13.

[0115] Theoretically, the ratio of the capacity of the negative electrode active material per unit area of ​​layer A to that of layer B should be less than 1.13. However, if the ratio is too low, the capacity of layer B will be too large, resulting in excess negative electrode capacity in layer B. This excess negative electrode capacity does not have corresponding lithium ion insertion, which increases the mass and volume of the battery and reduces its energy density. Therefore, to balance reducing metal deposition on the negative electrode side and mitigating the decrease in battery energy density, the ratio of the capacity of the negative electrode active material per unit area of ​​layer A to that of layer B should be between 1.09 and 1.13.

[0116] The values ​​from 0.5 to 1.13 mentioned above include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 0.5, 0.8, 1, 1.05, 1.1, 1.13, and the range values ​​between any two of the above point values.

[0117] In one embodiment, film layer A includes a negative electrode active layer and an active metal element supplement layer; film layer B is a negative electrode active layer.

[0118] The negative electrode active layer includes the negative electrode active material.

[0119] An active metal element replenishment layer refers to a layer that includes active metal elements. For example, in a lithium-ion battery, the active metal element replenishment layer includes a lithium replenishment layer. For instance, the lithium replenishment layer can compensate for the active lithium consumed by the formation of a solid electrolyte interface film on the negative electrode surface during the first charge of the battery.

[0120] For example, such as Figure 2 The diagram shows a schematic of the negative electrode sheet in one embodiment. A film layer 20 and a film layer 30 are provided on opposite sides of the negative electrode current collector 10. The A film layer 20 includes a negative electrode active layer 21 and an active metal element supplement layer 22, while the B film layer 30 is the negative electrode active layer. That is, in one embodiment, the active metal element supplement layer is only provided in the A film layer 20.

[0121] In this scheme, active metal elements are added to one side of the negative electrode sheet. For example, in a lithium-ion battery, lithium is added only to one side of the negative electrode sheet. This helps to reduce the modulus of the negative electrode sheet, making it easier to deform. It also reduces the gap between the positive and negative electrodes at the corner when the negative electrode is bent, thus improving the battery dynamics. It is understood that wound cells have a structure where the electrode sheets are bent to form corners.

[0122] Understandably, taking lithium-ion batteries as an example, in the solution for lithium replenishment of the negative electrode sheet, a single-sided lithium replenishment scheme is adopted. In traditional schemes, a double-sided lithium replenishment scheme is used, that is, lithium is replenished on both opposite surfaces of the negative electrode current collector. Double-sided lithium replenishment tends to increase the hardness of the negative electrode sheet, making it difficult to bend at the corners during the electrode sheet winding process, resulting in a larger gap between the negative electrode sheet and the positive electrode sheet. The larger gap deteriorates the battery dynamics. Therefore, in order to improve the battery dynamics, a single-sided lithium replenishment scheme can be adopted here.

[0123] In one embodiment, the active metal element supplement layer is located on the side of the negative electrode active layer away from the negative electrode current collector, and there is a gap between the side of the active metal element supplement layer and the side of the negative electrode current collector.

[0124] If the active metal element supplement layer extends beyond the negative electrode, there is a risk of puncturing the separator and short-circuiting the positive electrode. For example, in one embodiment, the active metal element supplement layer is located in the middle of the negative electrode current collector, so that there is a gap between the side of the active metal element supplement layer and the side of the negative electrode current collector.

[0125] like Figure 5 As shown, this is a schematic diagram of the structure of the negative electrode sheet in one embodiment. The negative electrode current collector has an A film layer 20 and a B film layer 30 on its two opposite surfaces. The A film layer 20 includes a negative electrode active layer 21 and an active metal element supplement layer 22. The width of the active metal element supplement layer 22 on both sides does not exceed the width of the negative electrode current collector 10.

[0126] In one embodiment, the distance between the side of the active metal element replenishment layer and the side of the negative electrode current collector is 1.5 mm to 2.5 mm.

[0127] Taking lithium-ion batteries as an example, when lithium strips are used as the lithium replenishment layer in the negative electrode, due to the good ductility of lithium strips, if the lithium replenishment width is too wide during the lithium replenishment stage, the width of the lithium strip after winding and cold pressing will exceed that of the anode, which may puncture the separator and cause a short circuit with the positive electrode. Therefore, the distance between the side of the positive electrode active material replenishment layer and the side of the negative electrode current collector is kept at 1.5mm to 2.5mm to reduce the risk of puncturing the separator caused by the width of the positive electrode active material replenishment layer exceeding that of the negative electrode current collector after cold pressing.

[0128] The values ​​in the range of 1.5mm to 2.5mm include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 1.5mm, 2mm, 2.5mm, etc., and the range values ​​between any two of the above point values.

[0129] In one embodiment, the negative electrode sheet includes a bending region and a straight region, the convex surface of the bending region includes a film layer A, and the concave surface of the bending region includes a film layer B.

[0130] The bending region refers to the structural area of ​​the negative electrode sheet that has a bend, such as... Figure 3 As shown, this is the bending shape of the negative electrode sheet in one embodiment. The side of the bending area 40 that is concave inward is the concave surface 41, and the side of the bending area that is convex outward is the convex surface 42.

[0131] A flat region, a region that extends linearly, such as... Figure 3 As shown, there are no bends in the 50 straight sections.

[0132] Theoretically, in the scheme of supplementing active metal elements on one side of the negative electrode, the active metal elements can be supplemented on one side in the bending region or on the straight region. Considering that supplementing active metal elements on the convex surface can reduce the hardness of the concave surface, thereby reducing the overall hardness of the electrode, it is easier to reduce the gap between the positive and negative electrode in the bending region and improve the battery dynamics.

[0133] Understandably, taking lithium-ion batteries as an example, the negative electrode sheet has a certain degree of hardness. During the cell winding process, the negative electrode sheet needs to be bent. During the bending process, the gap between the concave side of the negative electrode and the positive electrode is larger than the gap between the convex side of the negative electrode and the positive electrode, which makes it easy for lithium to be deposited on the concave side of the negative electrode. Placing the lithium replenishment layer on the convex side of the negative electrode, which is less prone to lithium deposition, can improve the problem of easy lithium deposition on the concave side of the negative electrode. At the same time, placing the lithium replenishment layer on the convex side of the negative electrode will not increase the hardness of the concave side of the negative electrode, thus reducing the problem that when the lithium replenishment layer is placed on the concave side of the negative electrode, the hardness of the concave side of the negative electrode is high, and the gap between the concave side of the negative electrode and the positive electrode will be further increased when the negative electrode is bent.

[0134] In one embodiment, the ratio of the unit area mass of the negative electrode active layer of film A to the unit area mass of the negative electrode active layer of film B is 1.09 to 1.13.

[0135] In general, during the preparation of negative electrode sheets, in order to simplify the process, the negative electrode active layers on both sides of the negative electrode sheet are usually the same, that is, the composition of the negative electrode active layer is the same, and the same negative electrode active material slurry is usually used to prepare the negative electrode active layer.

[0136] It is understandable that the capacity of the negative electrode active layer is provided by the negative electrode active material. When the composition of the negative electrode active layer on both opposite surfaces of the negative electrode is the same, the capacity of the negative electrode active material can be measured by the unit area mass of the negative electrode active layer on both opposite surfaces of the negative electrode sheet. Thus, when the ratio of the unit area mass of the negative electrode active layer of film A to the unit area mass of the negative electrode active layer of film B meets the above range, it indicates that the capacity of the negative electrode active material is reasonably distributed. This helps film B to have sufficient negative electrode active material capacity to provide space for the intercalation of the migrating positive electrode active material, reducing the problem of metal deposition on the negative electrode side.

[0137] Methods and steps for determining the composition of the negative electrode active layer: Disassemble the battery, remove the negative electrode sheet, scrape off the coating on the surface of the negative electrode current collector, and determine the specific structure of the inorganic components of the coating by XRD first, and then determine the constituent elements and their corresponding contents by XRF or EDS; for organic components, their composition and relative content can be determined by Raman spectroscopy.

[0138] In one embodiment, the unit area mass of the negative electrode active layer of film A is 110.389 g / m³. 2 Up to 116.883 g / m2 .

[0139] In one embodiment, the negative electrode active layer of the B film has a unit area mass of 103.247 g / m³. 2 Up to 110.389 g / m 2 .

[0140] In one embodiment, the first adhesive force between the diaphragm and membrane layer A is greater than the second adhesive force between the diaphragm and membrane layer B.

[0141] Adhesion strength test method and procedure: Tensile test method. 1. Using a tensile testing machine, take the negative electrode sheet and diaphragm (30mm x 100mm in size), fix the electrode sheet, and place the diaphragm on the tensile testing machine clamp. 2. Click the test button and slowly move the clamp at a certain speed to promote the separation of the diaphragm from the negative electrode sheet. 3. Record the tensile force and the displacement data of the clamp as the result of the adhesion strength.

[0142] To improve the adhesion between the separator and the electrode, the separator surface is generally covered with an adhesive. The adhesive is sticky. Considering that the negative electrode coating is generally porous, if the adhesive on the separator surface is too sticky, the adhesive may be embedded in the negative electrode pores, affecting the transport of positive electrode active ions and deteriorating the kinetics.

[0143] In a design where layer A includes a negative electrode active layer and an active metal element replenishment layer, with the replenishment layer located on the surface of the negative electrode active layer, and layer B does not include the replenishment layer, for example, in a lithium-ion battery, the replenishment layer serves as a lithium replenishment layer. After lithium replenishment, the presence of lithium on the surface of the negative electrode reduces its adhesion to the separator. Therefore, to improve the adhesion between the separator and the replenishment layer, the viscosity of the separator surface facing layer A needs to be relatively higher. However, if a higher viscosity is also used on the separator surface facing layer B, it could lead to a significant amount of adhesive remaining on the negative electrode surface after winding and cold pressing. This adhesive might embed into the pores of the negative electrode, affecting lithium-ion transport and deteriorating kinetics. Therefore, the viscosity of the two sides of a single-sided lithium-replenishing separator should be different.

[0144] Therefore, the membrane includes a first surface and a second surface facing each other. The first surface faces the A membrane layer, which includes an active metal element supplement layer. The second surface faces the B membrane layer, which does not include the active metal element supplement layer. The viscosity of the first surface is greater than that of the second surface, which can improve the adhesion between the membrane and the active metal element supplement layer, while reducing the embedding of the adhesive on the membrane into the negative electrode pores of the B membrane layer.

[0145] In one embodiment, the mass of the adhesive on the surface of the diaphragm facing the A membrane layer is greater than the mass of the adhesive on the surface of the diaphragm facing the B membrane layer.

[0146] The mass of the adhesive on the surface of the diaphragm facing membrane layer A is greater than the mass of the adhesive on the surface of the diaphragm facing membrane layer B. This helps to make the first adhesive force between the diaphragm and membrane layer A greater than the second adhesive force between the diaphragm and membrane layer B.

[0147] In one embodiment, the mass of adhesive per unit area on the surface of the diaphragm facing the A membrane layer is 0.09 mg / cm². 2 Up to 0.103 mg / cm 2 The mass of adhesive per unit area on the surface of the diaphragm facing the B membrane layer is 0.02 mg / cm². 2 Up to 0.082 mg / cm 2 .

[0148] The above 0.09 mg / cm 2 Up to 0.103 mg / cm 2 In this context, the values ​​include the minimum and maximum values ​​within the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 0.09 mg / cm³. 2 0.1 mg / cm 2 0.103 mg / cm 2 And so on, as well as the range of values ​​between any two of the above point values.

[0149] The above 0.02 mg / cm 2 Up to 0.082 mg / cm 2 In this context, the values ​​include the minimum and maximum values ​​within the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 0.02 mg / cm³. 2 0.04 mg / cm 2 0.06 mg / cm 2 0.082 mg / cm 2 And so on, as well as the range of values ​​between any two of the above point values.

[0150] In one embodiment, the ratio of the second adhesive force to the first adhesive force is 0.25 to 0.8.

[0151] The values ​​from 0.25 to 0.8 mentioned above include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 0.25, 0.4, 0.5, 0.6, 0.7, 0.8, etc., as well as the range values ​​between any two of the above point values.

[0152] In one embodiment, the first adhesive force is 5 N / m to 8 N / m.

[0153] In one embodiment, the second adhesive force is 2 N / m to 4 N / m.

[0154] The values ​​from 5 N / m to 8 N / m include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 5 N / m, 7 N / m, 8 N / m, etc., and the range values ​​between any two of the above point values.

[0155] The values ​​from 2N / m to 4N / m include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 2N / m, 3N / m, 4N / m, etc., and the range values ​​between any two of the above point values.

[0156] In one embodiment, the electrolyte includes an electrolyte solution, the secondary battery is a lithium-ion battery, and the electrolyte includes a lithium salt and an additive, wherein the concentration of the lithium salt ranges from 0.05 mol / L to 2 mol / L.

[0157] In one embodiment, the additive includes at least one of fluoroethylene carbonate, vinylene carbonate, and propylene carbonate, wherein the concentration of fluoroethylene carbonate is from 0.05% wt to 1.5% wt; the concentration of vinylene carbonate is from 0.05% wt to 1% wt; and the concentration of propylene carbonate is from 0.05% wt to 1.2% wt.

[0158] In designs where the negative electrode includes a lithium replenishment layer, cell lifespan is improved, but the amount of lithium salt and additives in the electrolyte also needs to be increased. Understandably, lithium salt and additives are consumed during cell cycling; insufficient amounts can lead to cell degradation. Therefore, the concentration of lithium salt and additives in the electrolyte needs to be increased. Concentrations within the aforementioned range help improve battery cycle performance.

[0159] Methods for detecting lithium salt concentration and additive concentration: Qualitative analysis of lithium salt and additives in the electrolyte is performed using GC-MS to confirm the presence of the lithium salt and additives described in this application. Specifically, the electrolyte is diluted with a solvent (e.g., dichloromethane) to obtain a sample to be tested. The sample is injected into the GC-MS for analysis to obtain a mass spectrum. The substances of each component are identified based on the mass spectrum of the detected components and the corresponding spectral library search results. For example, it is confirmed that the electrolyte contains the target substances, including dehydration additives, deacidification additives, and film-forming additives. The concentrations of the dehydration additives, deacidification additives, and film-forming additives can be detected by high-performance liquid chromatography (HPLC). Standard solutions with different concentration gradients of the above-mentioned target substances are prepared. Using the electrolyte to be tested as the sample, the standard solutions and the sample to be tested are analyzed separately by liquid chromatography. The mass of the target substance in the sample to be tested is calculated based on the standard curve to obtain the concentration of the target substance.

[0160] In one embodiment, the A film layer includes a first negative electrode active material at the portion near the negative electrode current collector, and a second negative electrode active material at the portion of the A film layer away from the negative electrode current collector, wherein the conductivity of the second negative electrode active material is in the range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s, the capacity range of the first negative electrode active material is 370mAh / g to 372mAh / g.

[0161] In one embodiment, the B film layer includes a first negative electrode active material at the portion near the negative electrode current collector, and a second negative electrode active material at the portion of the B film layer away from the negative electrode current collector, wherein the conductivity of the second negative electrode active material is in the range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s, the capacity range of the first negative electrode active material is 370mAh / g to 372mAh / g.

[0162] like Figure 4 As shown, this is a schematic diagram of the structure of the negative active layer on one side of the negative electrode sheet in one embodiment. The negative active layer 21 on one side surface of the negative current collector 10 includes a portion 211 close to the negative current collector and a portion 212 away from the negative current collector.

[0163] Electrical conductivity is an important physical quantity that describes the electrical conductivity of a material. It reflects the material's ability to transmit current under the influence of an external electric field.

[0164] Test methods and procedures for the conductivity of negative electrode active material: Ionic conductivity can be tested using the AC impedance method. The active material is prepared into electrode sheets and assembled into a coin cell. Then, by applying a small-amplitude AC voltage signal across the coin cell, the response of the coin cell to the AC voltage (i.e., impedance) is measured, thereby indirectly calculating the conductivity.

[0165] The capacity of the first negative electrode active material is the amount of electricity that a unit mass of the first negative electrode material can store and release.

[0166] The method for testing the capacity of the first negative electrode active material involves disassembling the battery to obtain the negative electrode sheet. After determining the composition of the negative electrode active material, the negative electrode active material is prepared into a negative electrode sheet and assembled into a battery. After full charging and full discharging, the discharge capacity is divided by the mass of the negative electrode material in the negative electrode sheet to obtain the capacity of the negative electrode material.

[0167] like Figure 4As shown, in the portion 211 near the negative electrode current collector, the second negative electrode active material is closer to the negative electrode surface. The conductivity of the second negative electrode active material meets the above range, indicating that its conductivity is relatively large, which helps to improve the kinetics of the negative electrode. Taking lithium-ion batteries as an example, the improvement of negative electrode kinetics helps lithium ions to quickly embed into the negative electrode active layer, reducing the risk of lithium plating.

[0168] If the capacity of the first negative electrode active material meets the above range, it indicates that its capacity is relatively large. Taking lithium-ion batteries as an example, it can provide more lithium vacancies to store lithium ions.

[0169] For example, in one embodiment, taking a lithium-ion battery as an example, since the lifespan of lithium-replenishing cells is longer, the internal resistance increases more significantly in the later stages of long-term cell cycling, which can easily lead to lithium plating and a drop in capacity. The second negative electrode active layer is closer to the negative electrode surface. Using fast-charging materials with higher conductivity can effectively improve the dynamics of the negative electrode and reduce the risk of lithium plating. At the same time, using high-capacity materials can provide more lithium vacancies to store lithium ions.

[0170] In one embodiment, the first negative electrode active material comprises graphite with a particle size Dv50 of 11 μm to 13 μm.

[0171] In one embodiment, the second negative electrode active material includes graphite with a particle size Dv50 of 8.6 μm to 10.7 μm.

[0172] Reducing the graphite particle size facilitates the rapid embedding of active metal ions into the graphite, which helps improve the kinetics of the negative electrode. The second negative electrode active material is closer to the negative electrode surface, and the improved negative electrode kinetics help lithium ions to be rapidly embedded into the negative electrode active layer, reducing the risk of lithium plating.

[0173] Dv50 is the particle size at which the cumulative particle size distribution percentage of a sample reaches 50%. Physically, it means that 50% of the particles are larger than Dv50, and 50% are smaller. It is typically characterized using transmission electron microscopy (TEM). By statistically analyzing the particle diameter distribution in different test areas and plotting the particle size distribution, the particle size distribution range can be obtained.

[0174] Under normal circumstances, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in a secondary battery is greater than or equal to 1.

[0175] In one embodiment, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery is 1.07 to 1.10.

[0176] For example, in one embodiment, the negative electrode of a lithium secondary battery has lithium metal added to one side of its two opposing surfaces, while the other side does not. The capacity of the negative electrode active material on the lithium-added side is defined as Q(lithium-added); the capacity of the negative electrode active material on the non-lithium-added side is defined as Q(non-lithium-added); the capacity of lithium added on the lithium-added side is defined as q(lithium-added); and the total capacity of the positive electrode active material is defined as q(total). The ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery is m, which satisfies Equation 1: Q(lithium-added) + Q(non-lithium-added) = m × q(total).

[0177] In addition, the lithium concentration on the negative electrode side with lithium replenishment and the side without lithium replenishment eventually tends to be in equilibrium. That is, it is equivalent to distributing the lithium on the negative electrode side with lithium replenishment evenly to the two sides of the negative electrode under ideal conditions, which satisfies Equation 2: Q(lithium replenishment) - Q(no lithium replenishment) = q(lithium replenishment) / 2.

[0178] Adding and subtracting equations 1 and 2 above, we get:

[0179] Q(lithium replenished) / Q(unreplenished)=(m×q(total)+q(replenished) / 2) / (m×q(total)-q(replenished) / 2).

[0180] That is, the final capacity of the negative electrode active material on both sides of the negative electrode is the relationship between the total capacity of the positive electrode active material and the lithium replenishment capacity. Based on the lithium replenishment scheme of the negative electrode, the percentage of the lithium replenishment capacity of the negative electrode to the total capacity of the positive electrode active material is 10% to 13%, and the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material of the secondary battery is m (range value from 1.07 to 1.10), so the range value of Q (lithium replenishment) / Q (no lithium replenishment) can be obtained as 1.09 to 1.13.

[0181] The total capacity of the negative electrode active materials refers to the electrical energy that all the negative electrode active materials in the battery can store and release, such as the capacity provided by all the negative electrode active materials on the negative electrode side.

[0182] The total capacity of the positive electrode active material refers to the electrical energy that all the positive electrode active materials in the battery can store and release. For example, all positive electrode active materials include all the positive electrode active materials on the positive electrode side.

[0183] The test method and steps for the total capacity of the negative electrode active material are as follows: Cut the negative electrode sheet (area S) into the shape of a coin cell (area s1) using a punch. Assemble the punched negative electrode with lithium metal to form a coin cell. Then perform 3 charge and discharge cycles (charge and discharge voltage is 0V-2V, charge and discharge current is 0.05C). Test the charge and discharge capacity Q1 of the coin cell. The total capacity of the negative electrode active material is (Q1 / s1)S.

[0184] The test method and steps for the total capacity of the positive electrode active material are as follows: The positive electrode sheet (area S0) is punched into the shape of a coin cell (area S2). The punched positive electrode is assembled with lithium metal to form a coin cell. Then, it is charged and discharged 3 times (charge and discharge voltage is 2.5V-3.7V, charge and discharge current is 0.05C). The charge and discharge capacity Q2 of the coin cell is tested. Then the total capacity of the positive electrode active material is (Q2 / s2)S0.

[0185] In a secondary battery, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material satisfies the above-mentioned range, ensuring that the negative electrode has sufficient capacity for the positive electrode active material to intercalate. In other words, the secondary battery as a whole has sufficient negative electrode active material capacity to provide space for the intercalation of active metal elements.

[0186] In one embodiment, this application also provides a method for preparing a secondary battery, comprising: preparing a negative electrode slurry; coating the negative electrode slurry on both sides of a negative electrode current collector, drying and pressing to obtain a negative electrode active layer on the opposite two surfaces of the negative electrode current collector; providing an active metal element supplement layer on the surface of the negative electrode active layer on at least one side of the negative electrode current collector, such that an A film layer and a B film layer are provided on the opposite two surfaces of the negative electrode current collector, wherein the concentration of active metal element per unit area of ​​the A film layer is greater than the concentration of active metal element per unit area of ​​the B film layer; and the ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 1.09 to 1.13.

[0187] In the scheme of supplementing active metal elements to the negative electrode sheet, when the concentration of active metal elements per unit area on the two opposite surfaces of the negative electrode sheet is different, the ratio of the negative electrode active material capacity per unit area of ​​film layer A to the negative electrode active material capacity per unit area of ​​film layer B meets the above range, which helps to reduce the problem of metal deposition on the negative electrode side.

[0188] In one embodiment, the step of providing an active metal element supplement layer on the surface of the active layer of at least one side of the negative electrode current collector, and providing an A film layer and a B film layer on opposite surfaces of the negative electrode current collector, wherein the concentration of active metal element per unit area of ​​the A film layer is greater than the concentration of active metal element per unit area of ​​the B film layer, includes: providing an active metal element supplement layer on the surface of the active layer of the negative electrode current collector on one side, wherein the side with the active metal element supplement layer is the A film layer, and the side without the active metal element supplement layer is the B film layer, wherein the concentration of active metal element per unit area of ​​the A film layer is greater than the concentration of active metal element per unit area of ​​the B film layer.

[0189] In the scheme of supplementing active metal elements to the negative electrode sheet, active metal elements can be supplemented to one side of the negative electrode sheet.

[0190] In one embodiment, the step of setting an active metal element supplement layer on the surface of the active layer of the negative electrode current collector on at least one side includes: setting an active metal element supplement sheet on the surface of the active layer of the negative electrode, rolling it, and obtaining an active metal element supplement layer on the surface of the active layer of the negative electrode.

[0191] An active metal element supplement layer can be obtained on the surface of the negative electrode active layer by placing an active metal element supplement sheet on it and then rolling it. For example, in a lithium-ion battery, a lithium sheet is used as the active metal element supplement sheet.

[0192] In one embodiment, the preparation process of the secondary battery further includes: preparing a positive electrode sheet, wherein the capacity of the active metal element supplement layer accounts for 10% to 13% of the total capacity of the positive electrode active material in the positive electrode sheet.

[0193] During the fabrication of a secondary battery, when active metal elements are added to one side of the negative electrode surface, a concentration difference exists between the active metal elements on both sides of the negative electrode. After the negative electrode is immersed in the electrolyte, a potential difference exists on both sides of the negative electrode, causing the active metal elements on both sides to migrate and drive them to reach equilibrium. The percentage of the capacity of the active metal elements added to the negative electrode sheet relative to the total capacity of the positive electrode active material meets the above-mentioned range. The ratio of the unit area capacity of the negative electrode active material in the A film layer to the unit area capacity of the negative electrode active material in the B film layer meets the above-mentioned range. The ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery meets the above-mentioned range. The reasonable distribution of the overall negative electrode active material capacity in the secondary battery helps the B film layer to have sufficient negative electrode active material capacity to provide space for the migrating active metal elements to embed, reducing the problem of metal deposition on the negative electrode side.

[0194] In one embodiment, after the step of providing an active metal element supplement layer on the surface of the active layer on at least one side of the negative electrode current collector, the method further includes using a detection mechanism to detect the quality of the active metal element supplement layer on the surface of the negative electrode active layer.

[0195] If the quality of the added active metal elements does not meet the preset requirements during the process of replenishing active metal elements on the negative electrode side, it may affect the battery life. Therefore, after the step of setting an active metal element replenishment layer on the surface of the negative electrode active layer on at least one side, the method further includes using a detection mechanism to detect the quality of the active metal element replenishment layer on the surface of the negative electrode active layer.

[0196] In one embodiment, the detection mechanism includes a camera, which is directed toward the active metal element replenishment layer to acquire quality information of the active metal element replenishment layer.

[0197] In the step of setting an active metal element replenishment layer on the surface of the negative electrode active material, a camera can be used to photograph the active metal element replenishment layer to obtain quality information, thereby determining whether the active metal element replenishment layer meets the preset requirements. Taking lithium secondary battery lithium replenishment as an example, lithium metal is silvery-white, and the negative electrode active material is black. The replenishment area can be distinguished by the color of the photographed image. When the replenishment area does not meet the preset requirements, it can be considered that the lithium replenishment quality is insufficient, and lithium replenishment can be performed again when the replenishment is insufficient.

[0198] Reducing the amount of missing lithium coating within a preset range can effectively improve the lithium replenishment effect and enhance its uniformity. The more missing lithium coating, the less total lithium replenishment is needed, thus diminishing the effect of lithium replenishment on extending cell life. Using methods such as CCD monitoring cameras, the amount of missing lithium coating per unit area can be effectively monitored in real time, playing a preventative and mitigating role.

[0199] In one embodiment, the step of coating a negative electrode slurry on both sides of a negative electrode current collector, drying, and extruding to obtain a negative electrode active layer on the opposite two surfaces of the negative electrode current collector includes: preparing a first negative electrode slurry, the first negative electrode slurry comprising a first negative electrode active material having a capacity range of 370 mAh / g to 372 mAh / g; and preparing a second negative electrode slurry, the second negative electrode slurry comprising a second negative electrode active material having a conductivity range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s; Coating the first negative electrode slurry onto one side of the negative electrode current collector, and coating the second negative electrode slurry onto the surface of the first negative electrode slurry layer; and / or, coating the first negative electrode slurry onto the other side of the negative electrode current collector, and coating the second negative electrode slurry onto the surface of the first negative electrode slurry layer.

[0200] In the step of setting the negative electrode active layer on one side (side A) and the other side (side B) of the negative electrode current collector, the negative electrode active layer can be prepared by double coating on side A. A first slurry is first coated on the surface of the negative electrode current collector, followed by a second slurry. The second negative electrode active material is closer to the negative electrode surface. The conductivity of the second negative electrode active material meets the above-mentioned range, indicating that its conductivity is relatively high, which helps improve the kinetics of the negative electrode. Taking a lithium-ion battery as an example, improved negative electrode kinetics helps lithium ions to quickly embed into the negative electrode active layer, reducing the risk of lithium plating. The capacity of the first negative electrode active material meets the above-mentioned range, indicating that its capacity is relatively high. Taking a lithium-ion battery as an example, it can provide more lithium vacancies for storing lithium ions.

[0201] In one embodiment, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery is 1.07 to 1.10.

[0202] In the preparation process of secondary batteries, the test steps and methods for determining the percentage of the capacity of the active metal element added to the negative electrode sheet relative to the total capacity of the positive electrode active material are as follows: For example, taking a lithium secondary battery as an example, the active metal element added to the negative electrode side is lithium strip with a mass of M3, and the specific capacity of lithium is fixed at 3860mAh / g.

[0203] In the secondary battery manufacturing process, the mass M4 of the positive electrode active material coated on the surface of the positive electrode current collector is known, and the specific capacity C1 of the positive electrode active material is also known. Then the total capacity of the positive electrode active material is C1×M4. The percentage of the capacity of the lithium added to the negative electrode to the total capacity of the positive electrode active material is (3860mAh / g×M3) / (C1×M4)×100%.

[0204] In one embodiment, the preparation process of the secondary battery further includes: preparing an electrolyte, wherein the secondary battery is a lithium-ion battery, the electrolyte includes a lithium salt, the concentration range of the lithium salt is X1×(1+Y1), X1 is 0.88mol / L to 1.82mol / L, and Y1 is the percentage of the capacity of the active metal element supplement layer to the total capacity of the positive electrode active material in the positive electrode sheet.

[0205] In designs where the negative electrode includes a lithium replenishment layer, cell lifespan is improved, but the amount of lithium salt in the electrolyte also needs to be increased. Understandably, lithium salt is consumed during cell cycling; insufficient lithium salt can lead to cell degradation. Therefore, the concentration of lithium salt in the electrolyte needs to be increased. A lithium salt concentration within the aforementioned range helps improve battery cycle performance.

[0206] Under normal circumstances, in a scheme without lithium replenishment, the concentration of lithium salt in the electrolyte can be considered as X1. As the capacity of lithium replenishment increases, the concentration of lithium salt also increases. Y1 is the percentage of the capacity of lithium replenishment to the total capacity of the positive electrode active material in the positive electrode sheet. Therefore, the concentration of replenished lithium salt is X1×Y1.

[0207] In addition, the secondary battery, battery module, battery pack and power device of this application will be described below with appropriate reference to the accompanying drawings.

[0208] In one embodiment of this application, a secondary battery is provided.

[0209] Typically, a secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the electrodes while allowing ions to pass through. The separator described above is the type used in this application.

[0210] The positive electrode includes a positive current collector and a positive coating disposed on at least one surface of the positive current collector.

[0211] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive coating is disposed on either or both of the two opposite surfaces of the positive current collector.

[0212] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0213] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0214] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of cathode materials in this application, the molar Li content refers to the initial state of the material, i.e., the state before feeding. When the cathode material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0215] In the examples of cathode materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0216] In some embodiments, the positive electrode coating may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0217] In some embodiments, the positive electrode coating may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0218] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0219] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0220] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0221] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0222] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0223] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0224] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0225] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0226] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0227] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements.

[0228] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0229] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0230] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0231] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0232] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0233] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 The example shown is a square-structured battery cell 5.

[0234] In some implementations, refer to Figure 7 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0235] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0236] Figure 8 This is battery module 4, used as an example. (See reference...) Figure 8 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0237] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0238] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0239] Figure 9 and Figure 10 This is battery pack 1 as an example. (See reference...) Figure 9 and Figure 10 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0240] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0241] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0242] Figure 11 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of individual battery cells, a battery pack or battery module can be used.

[0243] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0244] Example

[0245] Example 1

[0246] Preparation of negative electrode sheet:

[0247] Artificial graphite, conductive carbon black, carboxymethyl cellulose (CMC) binder, polystyrene-butadiene rubber, and water solvent were uniformly mixed in a weight ratio of 250:1:15:200. After stirring, a uniform negative electrode slurry with a viscosity of 6500 mPa / s was obtained. The negative electrode slurry was uniformly coated on both sides of a copper foil, pre-dried, and then rolled to a compaction density of 1.6 g / cm³. 3 The negative electrode sheet is obtained by drying it in a 40m long oven at 110℃ at a baking speed of 5m / min, and controlling the water content of the electrode sheet to be below 300ppm.

[0248] A single-sided lithium-filled electrode is prepared by bonding lithium strips to the surface of the negative electrode sheet using a rolling process (the overhang of the lithium strip and the head and bottom of the anode electrode sheet is 1.5 mm). This results in a negative electrode sheet with two surfaces: an A film layer and a B film layer. The A film layer includes the negative electrode active layer and the lithium strip on its surface, while the B film layer is the negative electrode active layer itself. The concentration of positive electrode active material (lithium concentration) per unit area in the A film layer is greater than that in the B film layer.

[0249] The total capacity of the negative electrode active material is 248.52 Ah. The ratio of the negative electrode active material capacity per unit area of ​​film layer A to that per unit area of ​​film layer B is 1.13.

[0250] The capacity of the positive electrode active material provided by the lithium supplement belt is 23Ah.

[0251] Preparation of the positive electrode sheet:

[0252] The cathode material (lithium iron phosphate): conductive carbon black: PVDF: surfactant were mixed in a mass ratio of 1000:20:1:10, and then N-methylpyrrolidone solvent was added. After stirring, a uniform cathode slurry was obtained. The viscosity of the cathode slurry was 6700 mPa / s. The cathode slurry was evenly coated on both sides of aluminum foil, pre-dried, and then rolled to a compaction density of 2.6 g / cm³. 3 The positive electrode is obtained by drying it in a 40m x 110℃ oven at a baking speed of 5m / min, and controlling the water content of the electrode to be below 250ppm.

[0253] The total capacity of the positive electrode active material is 228 Ah.

[0254] The ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material is 1.07.

[0255] Preparation of the diaphragm:

[0256] 84 wt% alumina, 10 wt% styrene-butadiene rubber (SBR) binder, and 6 wt% carboxymethyl cellulose (CMC) dispersant were dispersed in water, stirred evenly, and coated onto both sides of a 7 μm thick polyethylene membrane substrate. After drying, a membrane with a ceramic coating was obtained for later use. Polyvinylidene fluoride (PVDF) binder was dispersed in water, stirred evenly, and coated onto both sides of a membrane with a ceramic coating on one side. After drying, the final membrane was obtained.

[0257] Electrolyte preparation:

[0258] Ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1. LiPF6 was then dissolved in this solution to obtain an electrolyte. The concentration of LiPF6 in this electrolyte was 1 mol / L.

[0259] Battery assembly:

[0260] The cells are arranged in the order of "diaphragm-negative electrode sheet-diaphragm-positive electrode sheet". One end of the positive electrode sheet, negative electrode sheet and two diaphragms is fixed to the discharge roller, and the other end is stacked together and fixed to the winding shaft. The positive electrode sheet, negative electrode sheet and two diaphragms are wound. After winding, the cells undergo cold pressing, shelling, baking, liquid injection, wetting, high-temperature standing, formation, liquid replenishment and aging to complete the preparation of the hard-shell battery cell.

[0261] Examples 2 to 3

[0262] Based on Example 1, the ratio of the negative electrode active material capacity per unit area of ​​film layer A to the negative electrode active material capacity per unit area of ​​film layer B was adjusted.

[0263] Examples 4 to 5

[0264] Based on Example 1, the ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material was adjusted.

[0265] Comparative Example 1

[0266] Based on Example 1, the ratio of the negative electrode active material capacity per unit area of ​​membrane layer A to that of membrane layer B was adjusted. It is understood that the negative electrode slurry composition of membrane layers A and B is the same, and the ratio of the coating weights of the negative electrode slurries of membrane layers A and B is equal to the ratio of the negative electrode active material capacity per unit area of ​​membrane layer A to that of membrane layer B; that is, the ratio of the negative electrode active material capacity per unit area of ​​membrane layer A to that of membrane layer B is adjusted by adjusting the coating weights per unit area of ​​membrane layers A and B.

[0267] Lithium plating test: The cells of Comparative Example 1 and Examples 1 to 5 were subjected to a high-current lithium plating test at 25°C. After the test, the cells were disassembled to observe the interface and the improvement of lithium plating on one side was observed.

[0268] The lithium plating test conditions are shown in Table 1 below: where Cn is the battery capacity and 1C is the current at which the battery is fully charged in 1 hour.

[0269] Table 1 Lithium plating test conditions

[0270] step process 1) 1.43C constant current charging 0.7CnAh 2) 1.22C constant current charging 0.1CnAh 3) 1.01C constant current charging 0.1CnAh 4) 0.84C constant current charging to 3.65V 5) Let stand for 5 minutes 6) 1C constant current discharge to 2.5V 7) 1 / 3C constant current discharge to 2.5V 8) Let stand for 60 minutes 9) Repeat steps 1-8 10 times.

[0271] Table 2. List of Experimental Data

[0272]

[0273] As shown in Table 2, when the concentration of positive active material per unit area of ​​film layer A is greater than that of film layer B, the ratio of the total capacity of negative active material to the total capacity of positive active material is greater than or equal to 1, and the ratio of the capacity of negative active material per unit area of ​​film layer A to that of film layer B meets the range (0.5 to 1.13). No lithium plating was found in film layer B.

[0274] In Comparative Example 1, when the concentration of positive active material per unit area of ​​film layer A is greater than that of film layer B, the ratio of the total capacity of negative active material to the total capacity of positive active material is greater than or equal to 1. However, the ratio of the capacity of negative active material per unit area of ​​film layer A to that of film layer B does not meet the range (0.5 to 1.13), resulting in lithium plating on film layer B.

[0275] This indicates that when the concentration of positive electrode active material per unit area of ​​film layer A is greater than that of film layer B, the ratio of the capacity of negative electrode active material per unit area of ​​film layer A to that of film layer B meets the range (0.5 to 1.13), which helps to reduce the phenomenon of lithium plating in film layer B.

[0276] Example 6

[0277] Based on Example 1, the adhesion between the two sides of the diaphragm and the negative electrode was adjusted.

[0278] Battery cycle performance test: As shown in Table 3, Cn is the battery capacity, and 1C is the current required for the battery to be fully charged in 1 hour. The capacity retention rate can be obtained by comparing the data from step 6 for different number of cycles.

[0279] Table 3 Cyclic Performance Test Procedure

[0280]

[0281] Table 4. List of Experimental Data

[0282]

[0283] Whether the separator adhesive is embedded in the pores on the B-film side of the negative electrode: Disassemble the battery, remove the negative electrode, and observe whether the separator adhesive is embedded in the pores on the B-film side using SEM (scanning electron microscope). Specifically, such as... Figure 12 and Figure 13 As shown, Figure 12 The image shows the morphology of the negative electrode B film layer in Example 6. The morphology of the surface layer of the negative electrode is significantly different from that of the deeper layer, indicating that the surface layer of the negative electrode is blocked by the binder and the original morphology cannot be seen. Figure 13 The image shows the morphology of the negative electrode B film layer in Example 1. The surface morphology of the negative electrode is basically the same as that of the deep layer, and the binder is not embedded in the negative electrode to block the lithium ion transport channel.

[0284] Furthermore, as can be seen from Table 4, the capacity retention rate of Example 1 is higher than that of Example 6, indicating that the adhesion force of the separator towards the B membrane layer helps to reduce the embedding of the adhesive on the separator surface into the B membrane layer within a certain range, thereby improving the cycle performance of the battery.

[0285] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural transformations made using the contents of the specification and drawings of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the scope of patent protection of the present invention.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator; The negative electrode sheet includes a negative current collector and an A film layer and a B film layer disposed on opposite surfaces of the negative current collector. The concentration of active metal element per unit area of ​​the A film layer is greater than the concentration of active metal element per unit area of ​​the B film layer. The ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 0.5 to 1.

13.

2. The secondary battery as described in claim 1, characterized in that, The ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 1.09 to 1.

13.

3. The secondary battery as described in claim 1 or 2, characterized in that, The A film layer includes a negative electrode active layer and an active metal element supplementation layer; The B film layer is the negative electrode active layer.

4. The secondary battery as described in claim 3, characterized in that, The active metal element supplement layer is located on the side of the negative electrode active layer away from the negative electrode current collector, and there is a gap between the side of the active metal element supplement layer and the side of the negative electrode current collector.

5. The secondary battery as described in claim 4, characterized in that, The distance between the side of the active metal element replenishment layer and the side of the negative electrode current collector is 1.5 mm to 2.5 mm.

6. The secondary battery as described in any one of claims 1 to 5, characterized in that, The negative electrode sheet includes a bending region and a straight region. The convex surface of the bending region includes the A film layer, and the concave surface of the bending region includes the B film layer.

7. The secondary battery as described in any one of claims 1 to 6, characterized in that, The ratio of the unit area mass of the negative electrode active layer of the A film layer to the unit area mass of the negative electrode active layer of the B film layer is 1.09 to 1.

13.

8. The secondary battery as described in any one of claims 3 to 7, characterized in that, The unit area mass of the negative active layer of the A film layer is 110.389 g / m 2 to 116.883 g / m 2 ; And / or, the unit area mass of the negative active layer of the B film layer is 103.247 g / m 2 to 110.389 g / m 2 .

9. The secondary battery as described in any one of claims 3 to 8, characterized in that, The first adhesive force between the diaphragm and the A membrane layer is greater than the second adhesive force between the diaphragm and the B membrane layer.

10. The secondary battery as described in any one of claims 3 to 9, characterized in that, The mass of the adhesive on the surface of the diaphragm facing the A membrane layer is greater than the mass of the adhesive on the surface of the diaphragm facing the B membrane layer.

11. The secondary battery as described in any one of claims 3 to 10, characterized in that, The mass of the binder per unit area of the surface of the separator toward the side of the A film layer is 0.09 mg / cm 2 to 0.103 mg / cm 2 The mass of the binder per unit area of the surface of the separator toward the side of the B film layer is 0.02 mg / cm 2 to 0.082 mg / cm 2 .

12. The secondary battery as described in any one of claims 9 to 11, characterized in that, The ratio of the second adhesive force to the first adhesive force is 0.25 to 0.

8.

13. The secondary battery as described in any one of claims 9 to 12, characterized in that, The first adhesive force is 5 N / m to 8 N / m; And / or, the second adhesive force is 2 N / m to 4 N / m.

14. The secondary battery according to any one of claims 1 to 13, characterized in that, The electrolyte includes an electrolyte solution, the secondary battery is a lithium-ion battery, and the electrolyte solution includes a lithium salt and additives, wherein the concentration of the lithium salt ranges from 0.05 mol / L to 2 mol / L. ； And / or, the additive includes at least one of fluoroethylene carbonate, vinylene carbonate, and propylene carbonate, wherein the concentration of the fluoroethylene carbonate is from 0.05% wt to 1.5% wt; the concentration of the vinylene carbonate is from 0.05% wt to 1% wt; and the concentration of the propylene carbonate is from 0.05% wt to 1.2% wt.

15. The secondary battery as described in any one of claims 1 to 14, characterized in that, The A film layer includes a first negative electrode active material at the portion near the negative electrode current collector, and a second negative electrode active material at the portion away from the negative electrode current collector. The conductivity of the second negative electrode active material ranges from 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s, the capacity range of the first negative electrode active material is 370mAh / g to 372mAh / g; And / or, the B film layer includes a first negative electrode active material at a portion near the negative electrode current collector, and a second negative electrode active material at a portion away from the negative electrode current collector, wherein the conductivity of the second negative electrode active material is in the range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s, the capacity range of the first negative electrode active material is 370mAh / g to 372mAh / g.

16. The secondary battery as described in claim 15, characterized in that, The first negative electrode active material includes graphite, wherein the particle size Dv50 of the graphite is 11 μm to 13 μm; And / or, the second negative electrode active material includes graphite, wherein the graphite has a particle size Dv50 of 8.6 μm to 10.7 μm.

17. The secondary battery as described in any one of claims 1 to 16, characterized in that, The ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery is 1.07 to 1.

10.

18. A method for preparing a secondary battery, characterized in that, include: Prepare the negative electrode slurry; The negative electrode slurry is coated on both sides of the negative electrode current collector, dried, and extruded to obtain a negative electrode active layer on the two opposite surfaces of the negative electrode current collector. An active metal element supplement layer is provided on the surface of the active layer of the negative electrode current collector on at least one side, so that an A film layer and a B film layer are provided on the two opposite surfaces of the negative electrode current collector, and the active metal element concentration per unit area of ​​the A film layer is greater than the active metal element concentration per unit area of ​​the B film layer. The ratio of the negative electrode active material capacity per unit area of ​​the A film layer to the negative electrode active material capacity per unit area of ​​the B film layer is 1.09 to 1.

13.

19. The method for preparing a secondary battery as described in claim 18, characterized in that, The step of providing an active metal element supplement layer on the surface of the negative electrode active layer on at least one side of the negative electrode current collector, such that A film layer and B film layer are provided on opposite surfaces of the negative electrode current collector, wherein the active metal element concentration per unit area of ​​the A film layer is greater than the active metal element concentration per unit area of ​​the B film layer, includes: An active metal element supplement layer is provided on the surface of the negative electrode active layer on one side of the negative electrode current collector. The side with the active metal element supplement layer is called film layer A, and the side without the active metal element supplement layer is called film layer B. The active metal element concentration per unit area of ​​film layer A is greater than that of film layer B.

20. The method for preparing a secondary battery as described in claim 18 or 19, characterized in that, The step of providing an active metal element supplement layer on the surface of the negative electrode active layer on at least one side of the negative electrode current collector includes: An active metal element supplement sheet is placed on the surface of the negative electrode active layer and rolled to obtain an active metal element supplement layer on the surface of the negative electrode active layer.

21. The method for preparing a secondary battery according to any one of claims 18 to 20, characterized in that, The process of preparing a secondary battery also includes: Prepare a positive electrode sheet, wherein the capacity of the active metal element supplement layer accounts for 10% to 13% of the total capacity of the positive electrode active material in the positive electrode sheet.

22. The method for preparing a secondary battery according to any one of claims 18 to 21, characterized in that, After the step of setting an active metal element supplement layer on the surface of the negative electrode active layer on at least one side of the negative electrode current collector, the method further includes using a detection mechanism to detect the quality of the active metal element supplement layer on the surface of the negative electrode active layer.

23. The method for preparing a secondary battery as described in claim 22, characterized in that, The detection mechanism includes a camera, which is directed toward the active metal element supplementation layer to obtain quality information of the active metal element supplementation layer.

24. The method for preparing a secondary battery according to any one of claims 18 to 23, characterized in that, The step of coating the negative electrode slurry on both sides of the negative electrode current collector, drying, and extruding to obtain a negative electrode active layer on the opposite two surfaces of the negative electrode current collector includes: Prepare a first negative electrode slurry, the first negative electrode slurry comprising a first negative electrode active material, the capacity of the first negative electrode active material being in the range of 370 mAh / g to 372 mAh / g; Prepare a second negative electrode slurry, which includes a second negative electrode active material with a conductivity range of 0.8 × 10⁻⁶. -6 Up to 10 -5 cm 2 / s; The first negative electrode slurry is coated on one side of the negative electrode current collector, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry layer; and / or, the first negative electrode slurry is coated on the other side of the negative electrode current collector, and the second negative electrode slurry is coated on the surface of the first negative electrode slurry layer.

25. The method for preparing a secondary battery according to any one of claims 18 to 24, characterized in that, The ratio of the total capacity of the negative electrode active material to the total capacity of the positive electrode active material in the secondary battery is 1.07 to 1.

10.

26. The method for preparing a secondary battery according to any one of claims 18 to 25, characterized in that, The process of preparing a secondary battery also includes: Prepare an electrolyte. The secondary battery is a lithium-ion battery. The electrolyte includes a lithium salt. The concentration range of the lithium salt is X1×(1+Y1), where X1 is from 0.88mol / L to 1.82mol / L, and Y1 is the percentage of the capacity of the active metal element supplement layer to the total capacity of the positive electrode active material in the positive electrode sheet.

27. An electrical appliance, characterized in that, The electrical device includes a secondary battery as described in any one of claims 1 to 17.

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