Winding battery cell and application thereof

Abstract

The invention discloses a winding battery cell and application thereof, and belongs to the technical field of battery material preparation. The invention provides different negative electrode formulas for coating the A surface and the B surface of the negative electrode plate, the A surface is coated with the first negative electrode formula, the R angle of the corresponding winding cell is not easy to fall off, the negative electrode formula with low flexibility requirement and low adhesive strength requirement can be adopted, and the negative electrode scheme beneficial to reducing the direct current internal resistance of the cell can be designed. And the surface B is coated with the second negative electrode formula, so that the problem that the R angle of the winding cell is easy to fall off can be solved, and the negative electrode formula with both flexibility and adhesive force can be adopted.

Description

Technical Field

[0001] The invention relates to a wound battery core and application thereof, belonging to the technical field of batteries. Background Art

[0002] In order to increase the energy density of the battery cell, the size of the battery cell is getting larger and larger, and the thickness of the battery cell is getting thicker and thicker, and the corresponding negative electrode sheet is also getting thicker. Relatively speaking, for the wound battery cell, the outer negative electrode sheet at the R corner has a smoother curvature change, so it is not easy to have the problem of material falling off when the battery cell is hot pressed (corresponding to the negative electrode sheet A side), while the inner side of the R corner has a higher deformation curvature when the battery cell is hot pressed, which is prone to material falling off (corresponding to the negative electrode sheet B side). Moreover, compared to the B side, the A side is closer to the outer layer of the wound battery cell as a whole, and its outer electrode sheet is easier to be infiltrated with the electrolyte and easier to have good adhesion with the coated diaphragm, while the inner electrode sheet is more difficult to be infiltrated with the electrolyte and more difficult to have good adhesion with the coated diaphragm. Therefore, the risk of interfacial lithium plating in the inner layer of the core due to poor wettability is relatively high, and the bonding effect of the negative electrode sheet of the core will be deviated. Summary of the Invention

[0003] To solve the above problems, the present invention provides a double-sided asymmetric negative electrode sheet and a secondary battery containing a wound battery cell prepared based on the same, which has the advantages of low internal resistance of the battery cell and good cycle performance.

[0004] An object of the present invention is to provide a wound battery cell, wherein the wound battery cell (1) comprises a negative electrode sheet (2), the negative electrode sheet (2) comprises a current collector (20) and a first negative electrode active layer (21) arranged on one side of the current collector (20) in a thickness direction and a second negative electrode active layer (22) arranged on the other side, wherein the first negative electrode active layer (21) is arranged away from a winding center (5), and the second negative electrode active layer (22) is arranged toward the winding center (5);

[0005] The first negative electrode active layer (21) contains a first binder, which includes a binder 1 and a binder 2; the second negative electrode active layer (22) contains a second binder, which includes a binder A;

[0006] The mass proportion of the binder 1 in the first negative electrode active layer (21) is lower than the mass proportion of the binder A in the second negative electrode active layer (22); wherein the swelling rates of the binder 1 and the binder A are both 43%-75%, and the swelling rate of the binder 2 is 0%-20%.

[0007] Furthermore, the adhesive 1 accounts for 0.1%-50% of the mass of the first adhesive, and the adhesive A accounts for 50%-100% of the mass of the second adhesive.

[0008] Furthermore, the second negative electrode active layer (22) further contains a binder B, and the swelling rate of the binder B is 0%-20%.

[0009] Furthermore, the binder 2 accounts for 50%-99.9% of the mass of the first binder; and the binder B accounts for 0.1%-50% of the mass of the second binder.

[0010] Furthermore, the binder 1 and the binder A are each independently selected from at least one of 2-ethylhexyl acrylate-modified styrene-butadiene rubber, acrylamide-modified styrene-butadiene rubber, butyl acrylate-modified styrene-acrylic rubber, acrylonitrile-modified styrene-acrylic rubber and acrylamide-modified styrene-acrylic rubber.

[0011] Furthermore, the binder 2 and the binder B are each independently selected from at least one of acrylic acid-acrylamide-butyl acrylate, acrylic acid-acrylonitrile-acrylamide, methyl acrylate-butadiene-acrylic acid-acrylonitrile, and acrylic acid-acrylonitrile-butyl acrylate.

[0012] Furthermore, the mass proportion of the first binder in the first negative electrode active layer (21) is smaller than the mass proportion of the second binder in the second negative electrode active layer (22).

[0013] Furthermore, the mass of the first binder in the first negative electrode active layer (21) is 1.0-3.2 parts by weight; and the mass of the second binder in the second negative electrode active layer (22) is 1.0-3.5 parts by weight.

[0014] Furthermore, the negative electrode active layer may or may not contain a thickener.

[0015] Furthermore, when the negative electrode active layer contains a thickener, the thickener includes at least one of carboxymethyl cellulose or its sodium salt or lithium salt, carboxyethyl cellulose or its sodium salt or lithium salt, and acrylate-modified sodium carboxymethyl cellulose or its sodium salt or lithium salt.

[0016] Furthermore, when the negative electrode active layer contains a thickener: by weight, the first negative electrode active layer (21) contains 0.01-1.5 parts of the thickener; by weight, the second negative electrode active layer (22) contains 0.1-1.5 parts of the thickener.

[0017] Another object of the present invention is to provide a secondary battery comprising the wound battery cell and an electrolyte.

[0018] Another object of the present invention is to provide an electrical device comprising the above-mentioned secondary battery.

[0019] Beneficial effects of the present invention:

[0020] The present invention provides a negative electrode sheet with different negative electrode formulas coated on the A side and the B side, wherein the A side is coated with a first negative electrode formula (corresponding to a negative electrode formula that is not prone to material shedding problems at the R angle of a wound battery cell, and has low flexibility requirements and low adhesion requirements. The overall performance of the battery cell can be improved by designing a negative electrode scheme that is beneficial to reducing the DC internal resistance of the battery cell, such as reducing the amount of negative electrode binder added or increasing the proportion of negative electrode binders with good kinetics. In addition, the temperature rise of the A side is greater than that of the B side during hot pressing, and the negative electrode sheet as a whole can be more easily bonded to the diaphragm during hot pressing. Therefore, the total amount of binder added is reduced, but the proportion of negative electrode binders that are more prone to hot pressing deformation is increased, and better adhesion can still be obtained), and the B side is coated with a second negative electrode formula (corresponding to a negative electrode formula that is prone to material shedding problems at the R angle of a wound battery cell, and a negative electrode formula that requires both flexibility and adhesion strength). In combination with appropriate formula raw material selection, the electrochemical performance is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the wound battery cell structure of the present invention.

[0022] Figure 2 Schematic diagram of the double-sided asymmetric negative electrode sheet structure of the present invention.

[0023] Among them, 1 represents a wound battery cell, 2 represents a negative electrode sheet, 3 represents a positive electrode sheet, 4 represents a separator, 5 represents a winding center, 20 represents a current collector, 21 represents a first negative electrode active layer, and 22 represents a second negative electrode active layer. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0025] Currently, the formula of the outer layer (negative electrode sheet A side) and the inner layer (negative electrode sheet B side) of the negative electrode sheet in the battery cell are the same, and the coating process is also the same. Then, when the core is hot-pressed, the glue coating on the outer diaphragm is heated first, and under the action of pressure, it is easier to bond with the glue layer on the diaphragm. The glue layer of the diaphragm near the inner layer of the core is relatively lower in temperature, and the adhesion between the coating and the negative electrode sheet is relatively poor. Moreover, after injection, the electrolyte preferentially infiltrates the negative electrode sheet near the outer layer of the core, while the electrolyte diffusion in the inner layer becomes more difficult and more difficult to infiltrate the negative electrode sheet. This makes it relatively difficult for the electrolyte to penetrate the negative electrode sheet in the inner layer, and the adhesion between the inner negative electrode sheet and the glue-coated diaphragm is also poor, but the adhesion of the outer layer will be relatively strong, resulting in a large disparity in adhesion across the entire thickness and relatively high internal stress. Therefore, at this stage, negative electrode sheets with the same formula on the AB side need to take into account various advantages such as flexibility, adhesion, and dynamics, which is relatively complex and difficult to design.

[0026] Based on the above problems, the present invention designs a negative electrode sheet with different A and B side formulas for preparing wound battery cells. Specifically:

[0027] A wound battery cell, the wound battery cell (1) comprising a negative electrode sheet (2), the negative electrode sheet (2) comprising a current collector (20) and a first negative electrode active layer (21) arranged on one side of the current collector (20) in a thickness direction and a second negative electrode active layer (22) arranged on the other side, the first negative electrode active layer (21) being arranged away from a winding center (5), and the second negative electrode active layer (22) being arranged toward the winding center (5);

[0028] The first negative electrode active layer (21) contains a first binder, which includes a binder 1 and a binder 2; the second negative electrode active layer (22) contains a second binder, which includes a binder A;

[0029] The mass proportion of the binder 1 in the first negative electrode active layer (21) is lower than the mass proportion of the binder A in the second negative electrode active layer (22); wherein the swelling rates of the binder 1 and the binder A are both 43%-75%, and the swelling rate of the binder 2 is 0%-20%.

[0030] In the present invention, considering the differences between the A and B sides of the negative electrode sheet, different negative electrode formulations are applied to the A side (corresponding to the first negative electrode active layer) and the B side (corresponding to the second negative electrode active layer). Side A, corresponding to the R-angle corners of wound cells, is less susceptible to material shedding and is coated with a negative electrode formulation with lower requirements for flexibility and adhesion. Side B, corresponding to the R-angle corners of wound cells, is susceptible to material shedding and is coated with a negative electrode formulation that balances flexibility and adhesion. Therefore, the mass proportion of binder 1 on side A is lower than that of binder A on side B. More importantly, the present invention specifically selects a binder with an expansion coefficient of 43%-75%. This is not only because its proper expansion can buffer the volume change of the active material, thereby reducing active material shedding, maintaining the integrity of the electrode structure, and improving the cycle life of the battery, but also because its suitable expansion coefficient and adhesion enable simultaneous coating on both sides. Using the same binder reduces complexity in the production process and ensures high consistency in electrode performance on both sides, which is crucial for uniform charging and discharging and overall performance stability of the battery. Different binders can lead to interfacial compatibility issues, which can be avoided by using a single binder.

[0031] Preferably, the expansion rate may be 43%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 63%, 65%, 67%, 70%, 71%, 72%, 73%, 74%, 75%, etc. or within the range formed by any two of the above point values, including but not limited to the above point values ​​or ranges.

[0032] In the wound battery cell of the present invention, in each winding layer, the first negative electrode active layer (21) is further away from the winding center (5) than the second negative electrode active layer (22). In this structure, the formulation of the second negative electrode active layer (22) can overcome the problems of difficulty in wetting the inner electrode with the electrolyte and difficulty in achieving good adhesion with the coated separator, and can improve the interfacial lithium deposition caused by poor wettability of the inner layer of the wound core.

[0033] The schematic diagram of the wound battery structure of the present invention is shown in Figure 1 , 1 represents the wound cell, 2 represents the negative electrode sheet, 3 represents the positive electrode sheet, 4 represents the separator, and 5 represents the winding center. Among them, the specific structural parts of the negative electrode sheet Figure 2 , 20 represents a current collector, 21 represents a first negative electrode active layer, and 22 represents a second negative electrode active layer. As can be seen from the structural diagram, the wound battery cell contains a negative electrode sheet (2), a positive electrode sheet (3) and a separator (4), and the separator (4) is located between the negative electrode sheet (2) and the positive electrode sheet (3).

[0034] In the negative electrode sheet of the present invention, the binder used on side A (first binder) and the binder used on side B (second binder) are both compositions. Preferably, the binder 1 accounts for 0.1%-50% of the mass of the first binder, and the binder A accounts for 50%-100% of the mass of the second binder. Different binders can combine their respective advantages. Their combination can simultaneously improve the bonding strength and flexibility of the electrode sheet. Their mutual connection may form a specific structure that better adapts to the volume change of the electrode material during the charge and discharge process. At the same time, the combination of multiple binders can optimize the electrochemical performance of the electrode.

[0035] Preferably, the binder 1 accounts for 0.5%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc. of the mass of the first binder, including but not limited to the above proportions.

[0036] Preferably, binder A accounts for 50.1%, 51%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, 99.5%, 99.9%, etc. of the mass of the second binder, including but not limited to the above proportions.

[0037] More preferably, in the binder composition, the first negative electrode active layer (21) further comprises a binder 2, the swelling rate of the binder 2 being 0%-20%; the second negative electrode active layer (22) further comprises a binder B, the swelling rate of the binder B being 0%-20%. The present invention combines two binders with different swelling rates and applies them to different sides of the negative electrode sheet by adjusting the ratio of the two components. The similar composition optimizes the interfacial affinity of the electrode, promotes the diffusion of lithium ions, and thus improves the overall performance of the battery. Moreover, experimental verification has found that the bonding force of only the first negative electrode binder is low, its swelling rate is large, the cycle performance is worse, and the bonding force is biased; the rigidity of only the second negative electrode binder is biased, the sheet is less flexible, and it is easy to crack during processing and coating, and the corresponding DCR is larger. The mixed use of the two has a better effect. Furthermore, the present invention combines a binder with a swelling rate of 0%-20% with a binder with a swelling rate of 43%-75%. Binder 1, with a swelling rate of 0%-20%, exhibits superior adhesion. Its long-chain molecules are better able to coat the surface of the negative electrode particles and form a mutually affinity bond with the copper foil, resulting in a larger effective bonding area and a more effective bonding network. This effectively maintains electrode stability, buffers volume changes, and prevents the active material from separating from the current collector during repeated cycles. However, its flexibility is relatively poor. Binder 2, with a swelling rate of 43%-75%, has a spherical structure, resulting in relatively poor adhesion between the negative electrode particles and the copper foil. Direct contact between negative electrode particles facilitates lithium ion and electron transport, while the bonding area through negative electrode binder 1 is relatively small, thus further reducing the DCR. Furthermore, negative electrode binder 2 exhibits greater flexibility, which improves the flexibility of the negative electrode sheet and mitigates local stress concentrations during charge and discharge. The combination of the two binders can superimpose point bonding in the network bonding structure formed by the binder 1, thereby obtaining a negative electrode structure with better bonding strength, better flexibility and better electrolyte wettability.

[0038] Preferably, the swelling ratio is 1%, 5%, 8%, 10%, 12%, 15%, 17%, 18%, 19%, etc., including but not limited to the above values.

[0039] Preferably, the binder 2 accounts for 50%-99.9% of the first binder mass, and the binder B accounts for 0.1%-50% of the second binder mass. The temperature rise of surface A during hot pressing is greater than that of surface B. This arrangement also allows surface A to better "resist" the effects of temperature on the active layer.

[0040] Preferably, the adhesive 2 accounts for 50.1%, 51%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, 99.5%, 99.9%, etc. of the mass of the first adhesive, including but not limited to the above proportions.

[0041] Preferably, binder B accounts for 0.5%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc. of the mass of the second binder, including but not limited to the above proportions.

[0042] Preferably, the binder 1 and the binder A are each independently selected from at least one of 2-octyl acrylate-modified styrene-butadiene rubber, acrylamide-modified styrene-butadiene rubber, butyl acrylate-modified styrene-acrylic rubber, acrylonitrile-modified styrene-acrylic rubber and acrylamide-modified styrene-acrylic rubber;

[0043] The binder 2 and the binder B are each independently selected from at least one of acrylic acid-acrylamide-butyl acrylate, acrylic acid-acrylonitrile-acrylamide, methyl acrylate-butadiene-acrylic acid-acrylonitrile, and acrylic acid-acrylonitrile-butyl acrylate.

[0044] In the present invention, the combination of the first binder and the second binder is selected from the above-mentioned components. This is because, in addition to the properties mentioned above, these components can react during the process to form a network structure (such as the amino groups in acrylamide and the carboxyl groups in acrylic acid), which can better buffer the volume changes of the active material. This network structure may also form on the surface of the active material, thereby forming a dense film, further stabilizing the electrode structure.

[0045] Preferably, the mass proportion of the first binder in the first negative electrode active layer (21) is less than the mass proportion of the second binder in the second negative electrode active layer (22). When the mass proportion of the binder 1 in the first negative electrode active layer (21) is less than the mass proportion of the binder A in the second negative electrode active layer (22), the mass proportion of the first binder can be less than or equal to the mass proportion of the second binder. Of course, in order to reduce the DC internal resistance of the battery cell, the overall performance of the battery cell can also be improved by reducing the amount of negative electrode binder added, that is, the above preferred solution. In addition, the temperature rise of the A side is greater than that of the B side during hot pressing, and the negative electrode sheet as a whole can be more easily bonded to the diaphragm during hot pressing. Therefore, reducing the total amount of binder added, but increasing the proportion of negative electrode binder that is more prone to hot pressing deformation can still achieve better bonding strength.

[0046] Preferably, the mass of the first binder in the first negative electrode active layer (21) is 1.0-3.2 parts by weight; and the mass of the second binder in the second negative electrode active layer (22) is 1.0-3.5 parts by weight.

[0047] Preferably, the mass of the first binder in the first negative electrode active layer (21) is 1.1 parts, 1.3 parts, 1.5 parts, 1.8 parts, 2.0 parts, 2.5 parts, 3.0 parts, 3.1 parts, etc., including but not limited to the above masses.

[0048] Preferably, the mass of the second binder in the second negative electrode active layer (22) is 1.1 parts, 1.3 parts, 1.5 parts, 1.8 parts, 2.0 parts, 2.5 parts, 3.0 parts, 3.1 parts, 3.2 parts, 3.3 parts, 3.4 parts, etc., including but not limited to the above masses.

[0049] Preferably, the negative electrode active layer contains or does not contain a thickener; when the negative electrode active layer contains a thickener: the thickener includes at least one of carboxymethyl cellulose or its sodium salt or lithium salt, carboxyethyl cellulose or its sodium salt or lithium salt, and acrylate-modified sodium carboxymethyl cellulose or its sodium salt or lithium salt. Adding a thickener can improve the viscosity of the electrode slurry, making it more suitable for the coating process and enabling the components therein to be evenly distributed. When the above thickeners are selected, the slurry can maintain good uniformity and stability during coating. Moreover, the above thickeners have certain bonding properties, which can tightly bond the active material, conductive agent and current collector together, and evenly disperse the active material and other particles through electrostatic repulsion to form a stable electrode structure.

[0050] Preferably, when the negative electrode active layer contains a thickener: the first negative electrode active layer (21) contains 0.01-1.5 parts of the thickener by weight; the second negative electrode active layer (22) contains 0.1-1.5 parts of the thickener by weight. In one embodiment of the present invention, the thickener in the B side can be appropriately greater than that in the A side. This is because the thickener can improve the wettability of the electrode to the electrolyte, promote the penetration of the electrolyte, thereby forming a channel that is conducive to ion transmission and improving the rate performance of the battery.

[0051] Preferably, the first negative electrode active layer (21) contains 0.02 parts, 0.05 parts, 0.1 parts, 0.2 parts, 0.3 parts, 0.4 parts, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, 1.0 parts, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, etc. of thickener, including but not limited to the above parts by mass.

[0052] Preferably, the second negative electrode active layer (22) contains 0.15 parts, 0.2 parts, 0.3 parts, 0.4 parts, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, 1.0 parts, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, etc. of thickener, including but not limited to the above parts by mass.

[0053] The present invention also provides a secondary battery (which may be a lithium ion secondary battery or a sodium ion secondary battery) containing the wound battery core.

[0054] Preferably, for the double-sided asymmetric negative electrode sheet in the wound battery cell, the first negative electrode formula corresponding to the A side is 95-99 parts of negative electrode material, 0.5-2.0 parts of negative electrode conductor, 0.01-1.5 parts of thickener, the total proportion of negative electrode binder 1 + negative electrode binder 2 is 1.0-3.2 parts, and the proportion of negative electrode binder 1 is 0.1-50%; the second negative electrode formula corresponding to the B side is 95-99 parts of negative electrode material: 0.5-2.0 parts of negative electrode conductor, 0.1-1.5 parts of thickener, 1-3.5 parts (by mass) of negative electrode binder A + negative electrode binder B, and the proportion of negative electrode binder A is 50%-100%.

[0055] Preferably, in the first negative electrode formula, the negative electrode material is 95.2-99 parts, the negative electrode conductive agent is 0.5-1.5 parts, the thickener is 0.01-1.2 parts, and the negative electrode binder 1 + the negative electrode binder 2 is 1.0-3.0 parts.

[0056] Preferably, in the second negative electrode formula, the negative electrode material accounts for 95-98.8 parts, the negative electrode conductor accounts for 0.5-1.5 parts, the thickener accounts for 0.1-1.2 parts, and the negative electrode binder A + negative electrode binder B accounts for 1.0-3.3 parts.

[0057] Preferably, the negative electrode active material comprises a graphite negative electrode material, which is at least one of needle coke, pitch tar, petroleum coke, etc., obtained by shaping, graphitizing, granulating, and coating.

[0058] Preferably, the negative electrode conductive agent is selected from one or more of conductive carbon black, conductive graphite, carbon nanotubes and graphene.

[0059] Preferably, the secondary battery is a liquid battery containing an electrolyte. The electrolyte is any electrolyte well known to those skilled in the art, without particular limitation, and the solvent is at least one of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, and diethylene glycol dimethyl ether. When the secondary battery is a sodium ion secondary battery, the electrolyte salt is at least one of sodium perchlorate, sodium bis(oxalate)borate, and sodium hexafluorophosphate. When the secondary battery is a lithium ion secondary battery, the electrolyte salt is at least one of lithium perchlorate, lithium bis(oxalate)borate, and lithium hexafluorophosphate.

[0060] Preferably, the secondary battery further comprises a separator and a positive electrode sheet. The negative electrode sheet prepared above is rolled up with the positive electrode sheet and the separator to prepare a secondary battery.

[0061] Preferably, the type of the diaphragm is any diaphragm well known to those skilled in the art, without any special restrictions, such as one or more of polyethylene, polypropylene, polyacrylonitrile, polyacrylonitrile fiber, polyamic acid, polyimide, polyamide, polydopamine, polyarylethersulfone, polyvinylidene fluoride, polyethylene oxide, polyethylene terephthalate, polyester, non-woven fabric membrane and cellulose paper-based isolation membrane.

[0062] Preferably, the positive electrode sheet contains a positive electrode active material layer, and the positive electrode active material layer comprises the following components in mass percentage: 85-99.5% of positive electrode active material, 0.3-8% of positive electrode conductor and 0.3-7% of positive electrode binder.

[0063] Preferably, when the secondary battery is a lithium ion secondary battery, the positive electrode active material is at least one of lithium cobaltate, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese phosphate, lithium iron manganese phosphate, and lithium iron phosphate.

[0064] Preferably, the positive electrode binder is any binder well known to those skilled in the art, without particular limitation, such as one or more of chitosan, xanthan gum, gellan gum, gum arabic, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, polymethacryloyl, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polyacrylamide, styrene-butadiene rubber, sodium alginate, polyethylene glycol, guar gum, guar gum polymer, and guar gum copolymer. In the embodiment of the present invention, PVDF is selected.

[0065] The preparation methods involved in the above content are as follows:

[0066] Lithium-ion secondary battery: The negative electrode is cut and die-cut to remove some blank foil, and the negative electrode, separator, and positive electrode are wound / stacked to obtain a single bare cell. The single bare cell is placed in a shell (such as aluminum-plastic film, aluminum-steel shell, etc.), dried, injected with electrolyte, packaged, formed, and capacity divided to obtain a lithium-ion secondary battery.

[0067] Positive electrode: The positive electrode active material, conductive agent, and binder are mixed with N-methylpyrrolidone (NMP) in a mass ratio of 85-99.5:0.3-8:0.3-7 and stirred evenly to produce a positive electrode slurry. This slurry is then coated onto aluminum foil, dried, roll-pressed, die-cut, and welded to produce the positive electrode.

[0068] The materials involved in the following examples are as follows:

[0069] 1. Negative electrode binder 1 / negative electrode binder A (emulsion type negative electrode binder):

[0070] Isooctyl acrylate modified styrene-butadiene rubber: swelling rate 49%-52%,

[0071] Acrylamide modified styrene butadiene rubber: swelling rate 43%-47%,

[0072] Butyl acrylate modified styrene acrylic rubber: swelling rate 70%-75%,

[0073] Acrylonitrile modified styrene acrylic rubber: swelling rate 57%-64%,

[0074] Acrylamide modified styrene acrylic rubber: swelling rate is 65%-69%.

[0075] 2. Negative electrode binder 2 / negative electrode binder B (aqueous negative electrode binder):

[0076] Acrylic acid-acrylamide-butyl acrylate ternary copolymer adhesive: swelling rate is 15%-20%,

[0077] Acrylic acid-acrylonitrile-acrylamide ternary copolymer adhesive: swelling rate 0%-3%,

[0078] Methyl acrylate-butadiene-acrylic acid-acrylonitrile quaternary copolymer adhesive: swelling rate is 3%-6%,

[0079] Acrylic acid-acrylonitrile-butyl acrylate terpolymer adhesive: the swelling rate is 9%-15%.

[0080] Example 1

[0081] A double-sided asymmetric negative electrode secondary battery comprises the following steps:

[0082] 1. Preparation method of double-sided asymmetric negative electrode sheet:

[0083] (1) The first negative electrode formula is a mixture of 96.7 parts of negative electrode material (graphite), 1 part of conductive agent, 1.3 parts of negative electrode binder 1 + negative electrode binder 2, and 1 part of CMC, wherein the negative electrode binder 1 accounts for 50%, the negative electrode binder 1 is acrylamide-modified styrene-butadiene rubber, and the negative electrode binder 2 is acrylic acid-acrylamide-butyl acrylate ternary copolymer binder. After mixing and stirring, 100 parts of deionized water are added to the solid content of 50%, and the viscosity is controlled to 1800mPa·s and the fineness is 30μm to obtain the first negative electrode formula slurry;

[0084] (2) The second negative electrode formulation is 96.5 parts of negative electrode material, 1 part of conductive agent, 1 part of CMC, 0.5 parts of negative electrode binder 1, and 1.0 parts of negative electrode binder 2. 100 parts of deionized water are added to the mixture until the solid content is 50% and the viscosity is controlled to 2000 mPa·s and the fineness is 30 μm to obtain the second negative electrode formulation slurry;

[0085] (3) The first negative electrode formula slurry and the second negative electrode formula slurry are gradually extruded and coated on the electrode foil (copper foil), dried and rolled to obtain the negative electrode. On the complete electrode roll, the first negative electrode area on the A side and the second negative electrode formula area on the B side are formed, and then the electrode sheets of the set size are formed by striping and slicing.

[0086] 2. Preparation method of positive electrode sheet:

[0087] The positive electrode sheets were the same type. Specifically, the positive electrode formula was as follows: 97.3 parts of positive electrode material (LFP lithium iron material), 1 part of conductive agent, and 1.5 parts of PVDF were mixed, deionized water was added to a solid content of 63.2%, and the viscosity was controlled to 9240 mPa·s and the fineness was 10 μm to obtain a positive electrode slurry;

[0088] 3. Battery assembly:

[0089] The positive electrode sheet, negative electrode sheet, and separator are wound in a 9+3+3+3 pattern (3um alumina ceramic on one side, 3um PVDF coated on both sides) to obtain a bare cell, the positive and negative electrode tabs are welded, the bare cell is placed in an aluminum-plastic film battery case, liquid is injected (the electrolyte is lithium hexafluorophosphate), encapsulated, allowed to stand, formed, and capacity divided to obtain a lithium-ion secondary battery.

[0090] Example 2

[0091] The negative electrode binder 1 was replaced by butyl acrylate modified styrene acrylic rubber, and the negative electrode binder 2 was replaced by acrylic acid-acrylonitrile-acrylamide ternary copolymer binder. The rest was the same as in Example 1.

[0092] Example 3

[0093] The negative electrode binder 1 was replaced by styrene-butadiene rubber modified with isooctyl acrylate, and the negative electrode binder 2 was replaced by acrylic acid-acrylonitrile-butyl acrylate ternary copolymer binder. The rest was the same as in Example 1.

[0094] Example 4

[0095] The negative electrode binder 1 was replaced by acrylonitrile-modified styrene-propylene rubber, and the negative electrode binder 2 was replaced by a methyl acrylate-butadiene-acrylic acid-acrylonitrile quaternary copolymer binder. The rest was the same as in Example 1.

[0096] Example 5

[0097] The negative electrode binder 1 was replaced by acrylamide-modified styrene-acrylic rubber, and the negative electrode binder 2 was replaced by acrylic acid-acrylonitrile-acrylamide ternary copolymer binder. The rest was the same as in Example 1.

[0098] Example 6

[0099] Only in the second negative electrode formulation, the negative electrode binder 1 was replaced with butyl acrylate modified styrene acrylic rubber, and the negative electrode binder 2 was replaced with acrylic acid-acrylonitrile-acrylamide ternary copolymer binder. The rest was the same as in Example 1.

[0100] Example 7

[0101] The relative mass percentages of the negative electrode binder 1 and the negative electrode binder 2 in the first negative electrode formulation were adjusted to 10% for the negative electrode binder 1 and 90% for the negative electrode binder 2. The rest was the same as in Example 1.

[0102] Example 8

[0103] The relative mass percentages of negative electrode binder 1 and negative electrode binder 2 in the second negative electrode formulation were adjusted to 45% for negative electrode binder 1 and 55% for negative electrode binder 2. The total amount remained unchanged, and the rest was the same as in Example 1.

[0104] Example 9

[0105] The negative electrode binder 2 in the first negative electrode formulation was completely replaced with the negative electrode binder 1, and the rest was the same as in Example 1.

[0106] Example 10

[0107] The amount of the negative electrode binder in the first negative electrode formulation was adjusted to 1.5 parts, the relative ratio of the negative electrode binder 1 and the negative electrode binder 2 remained unchanged, and the amount of the negative electrode material was adjusted to 96.5 parts. The rest was the same as in Example 1.

[0108] Comparative Example 1

[0109] The first negative electrode formulation slurry was coated on both sides of the negative electrode sheet, and the rest was the same as in Example 1.

[0110] Comparative Example 2

[0111] The second negative electrode formulation slurry was coated on both sides of the negative electrode sheet, and the rest was the same as in Example 1.

[0112] Comparative Example 3

[0113] The binder 1 in the negative electrode formulation was changed to styrene-acrylic rubber modified with isooctyl acrylate (swelling rate 76-80%), and the rest was the same as in Example 1.

[0114] Comparative Example 4

[0115] The binder 2 in the negative electrode formulation was changed to acrylic acid-acrylamide-isooctyl acrylate, with a swelling rate of 21% to 25%. The rest was the same as in Example 1.

[0116] Test Case

[0117] The battery systems assembled from all examples and comparative examples were subjected to the following tests:

[0118] (1) Negative electrode wettability test:

[0119] Take the negative electrode sheet and use a micropipette to drop 10μL of electrolyte on the negative electrode sheet. Test the time it takes for the electrolyte to completely disappear from the electrode sheet. The longer the time, the worse the wettability of the negative electrode sheet with the electrolyte.

[0120] (2) Electrode peeling force test:

[0121] The negative electrode sheet was cut into 25 mm wide samples, and tape (25 cm wide) was pasted on the surface. A 1.5 kg roller was used to roll the tape back and forth three times to ensure good adhesion between the tape and the negative electrode sheet area. One end was then gently peeled off and vertically clamped on a tensile fixture at a peeling speed of 50 mm / min to test the adhesion between the negative electrode sheet area and the copper foil at 180°.

[0122] (3) Flexibility test:

[0123] Take steel needles of different diameters, cut the electrode into 10mm wide strips, wrap the steel needle around it, and observe the corresponding diameter of the steel needle when cracks appear. The smaller the diameter of the steel needle, the better the flexibility of the electrode.

[0124] (4) DC internal resistance test

[0125] At room temperature, each embodiment and comparative example was charged to 3.65V at 1C, cut off at 0.05C, and then discharged to 50% SOC at 1C. The ambient temperature was adjusted to -20°C, and the discharge DC internal resistance was recorded at 0.36 for 30s.

[0126] (5) Cycle test:

[0127] After standing for 2 days at 45°C, the lithium-ion secondary battery was discharged at 1C to 2.0V. The battery was then clamped in a double-sided clamp with a clamping force of 3000N at 25°C. The charging process was as follows: 1C constant current charging to 3.65V, constant voltage charging at 3.65V until the current was less than 0.5C, and then the battery was left standing for 5 minutes. Then, 1C constant current charging to 3.65V, constant voltage charging at 3.65V until the current was less than 0.1C, and then the battery was left standing for 5 minutes. Then, 1C constant current charging to 3.65V, constant voltage charging at 3.65V until the current was less than 0.05C. The discharge process was as follows: 5 minutes of rest, then 1C constant current discharge to 2.0V, and this cycle continued until the discharge capacity reached 80% of the initial capacity at 1C constant current discharge to 2.0V.

[0128] The test results are shown in the table below.

[0129] Table 1 Test results

[0130]

[0131]

[0132]

[0133] As can be seen from Table 1, both side A and side B in Comparative Example 2 are negative electrode formulas that take into account flexibility and adhesion. In Example 1, side A (corresponding to the outer side of the R corner, which is not easy to fall off) has a higher proportion of negative electrode binder 2 with good wettability with the electrolyte, which will cause its adhesion to deviate. In addition, the total amount of negative electrode binder added is reduced. Therefore, the peeling force between the negative electrode sheet area on side A and the copper foil is significantly reduced. After the total amount of binder added is reduced, the flexibility of the negative electrode sheet will also be slightly worse. However, the corresponding DCR can be significantly reduced, and a smaller DCR is conducive to reducing polarization accumulation during the cycle, thereby improving the cycle performance. Therefore, the cycle performance in Example 1 is relatively better. Furthermore, the binder corresponding to negative electrode binder 2 has relatively high adhesion. Its long-chain molecules are better able to coat the surface of the negative electrode particles and form an affinity bond with the copper foil. This results in a larger effective bonding area and a more effective bonding network. This effectively maintains electrode stability, buffers volume changes, and prevents the active material from separating from the current collector during repeated cycling. However, its flexibility is relatively poor. Binder 1 generally has a spherical structure, resulting in relatively poor adhesion between the negative electrode particles and the copper foil. Direct contact between negative electrode particles facilitates lithium ion and electron transport, while the bonding area through negative electrode binder 1 is relatively small, thus further reducing the DCR. Furthermore, negative electrode binder 2 has greater flexibility, which improves the flexibility of the negative electrode sheet and mitigates local stress concentrations during charge and discharge. Combining these two binders allows for point bonding within the network-like bonding structure formed by binder 1, resulting in a negative electrode structure with improved adhesion, flexibility, and electrolyte wettability. It should be pointed out that the negative electrode binder 2 with a high swelling rate needs to be compounded with the negative electrode binder 1 with a low swelling rate, so as to take into account the adhesion, flexibility, DCR and cycle performance.

[0134] Comparing Example 1 and Example 2, the swelling rate of the negative electrode binder 1 in Example 2 is significantly larger, its adhesion is relatively small and its flexibility is very good, but the swelling rate of the negative electrode binder 2 is significantly smaller and its rigidity is greater, and its adhesion is relatively good (both are composed of polar functional monomers, which significantly improve the adhesion). Overall, the liquid absorption time of the negative electrode sheet is shorter, and the corresponding electrolyte wettability is better, which is beneficial to reducing DCR. A smaller DCR is beneficial to reducing polarization accumulation during the cycle, thereby improving the cycle performance.

[0135] The rules of Examples 3 and 4 compared with Example 1 are similar.

[0136] Compared with Example 5, in the second negative electrode formula of Example 1, the swelling rate of the negative electrode binder 1 is lower, and it is modified with polar functional groups, and has better adhesion, while the swelling rate of the negative electrode binder 2 is higher. Therefore, the wettability corresponding to the electrode of Example 1 is slightly worse (the shorter the liquid absorption time, the better the electrolyte wettability), and the flexibility of its electrode is almost the same (the larger the winding needle diameter, the worse the corresponding electrode flexibility), so its DCR is slightly increased, but its adhesion is higher, and the corresponding cycle performance will be more advantageous.

[0137] Comparing Examples 1, 2, and 6, it can be seen that Example 6 is the same as Example 1 except that the second negative electrode formulation (B) is replaced with the same formulation as that of the B side of Example 2. It can be seen that its performance is between that of Examples 1 and 2.

[0138] Compared with Example 1, Example 7 only changes the ratio of negative electrode binders 1 and 2, and the proportion of negative electrode binder 1 is reduced from 50% to 10%. The flexibility of negative electrode binder 1 is good, but the adhesion is low, and the electrolyte wettability is good. The negative binder 2 has good adhesion, but poor flexibility, and relatively poor electrolyte wettability. It can be seen that compared with the A side of Example 1, the negative electrode sheet corresponding to the A side of Example 7 has higher adhesion and relatively poorer flexibility, but the corresponding electrolyte wettability deviates a little. Overall, its DCR is higher than that of Example 1, but due to the The binder corresponding to the negative electrode binder 2 in Example 2 has a relatively high adhesion force and a higher proportion. Its long-chain molecules can better coat the surface of the negative electrode particles and have mutual affinity bonding with the copper foil. The effective bonding area is larger and the formed bonding network is more effective. Therefore, it can more effectively maintain the stability of the electrode, buffer the volume change, and prevent the active material from separating and falling off from the current collector during repeated cycles, which is beneficial to improving the cycle. Overall, the increase in its DCR is not conducive to improving the cycle, but its high adhesion is beneficial to improving the cycle. Overall, its cycle performance is slightly improved.

[0139] Comparing Example 1 and Example 8, the proportion of negative electrode binder 1 in the B side is increased, and the rest is the same, wherein binder 1 is generally a spherical structure, and its negative electrode particles and copper foil are relatively point-bonded, so the adhesion is relatively poor. Direct contact between negative electrode particles is more conducive to lithium ion transmission and electron transmission, and the relative bonding area through negative electrode binder 1 is smaller, so it is more conducive to reducing DCR. The increase in the proportion of negative electrode binder 1 will reduce the adhesion to a certain extent, but because it is compounded with binder 2 with good adhesion, the decrease in its adhesion is relatively small, but it will improve flexibility and improve electrolyte wettability, so its corresponding DCR is lower and the cycle is relatively better.

[0140] Comparing Example 1 and Example 9, in Example 9, the A surface is completely replaced with the negative electrode binder 1, and the corresponding adhesion will be significantly reduced, the electrolyte wettability will be improved, and the flexibility will also be significantly improved, so its corresponding DCR will be reduced. However, since its A surface is entirely made of the negative electrode binder 1, it lacks the negative electrode binder 2 that can more effectively limit the displacement of the negative electrode particles, and its cycle performance will be relatively slightly worse.

[0141] Comparing Example 1 and Example 10, the amount of binder added to surface A in Example 10 increases, and its adhesion is improved, and its flexibility is improved, but the corresponding electrolyte wettability is deteriorated. This is because the additional negative electrode binder will make the structure between the negative electrode particles tighter to a certain extent, which is not conducive to the infiltration of the electrolyte. Therefore, its corresponding DCR will become larger and the cycle performance will deteriorate.

[0142] Comparing Example 1 and Comparative Example 1, the formula of side A and side B in Comparative Example 1 is the same as that of side A in Example 1. This will result in insufficient adhesion (the total amount of adhesive added will be relatively small) and poor flexibility (there is a certain risk of material falling off when used for side B) of the comparative example B (the side with higher requirements for adhesion and flexibility). Although the DCR can be significantly reduced, there is a certain possibility of material falling off during the processing, which is not conducive to long-term circulation. Therefore, the cycle of comparative example 1 is poor.

[0143] Compared with Example 1 and Comparative Example 3, in Comparative Example 3, the negative electrode binder 1 is replaced with a binder with a higher swelling rate, and its bonding force is relatively poor. Although it can significantly reduce the DCR, it is not conducive to its cycle performance.

[0144] Compared with Example 1 and Comparative Example 4, in Comparative Example 4, replacing the negative electrode binder 2 with a binder with a higher swelling rate will also significantly reduce the cycle performance.

[0145] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A wound battery cell, characterized in that: The wound battery cell (1) comprises a negative electrode sheet (2), the negative electrode sheet (2) comprising a current collector (20) and a first negative electrode active layer (21) arranged on one side of the current collector (20) in a thickness direction and a second negative electrode active layer (22) arranged on the other side, the first negative electrode active layer (21) being arranged away from the winding center (5), and the second negative electrode active layer (22) being arranged toward the winding center (5); The first negative electrode active layer (21) contains a first binder, which includes a binder 1 and a binder 2; the second negative electrode active layer (22) contains a second binder, which includes a binder A; The mass proportion of the binder 1 in the first negative electrode active layer (21) is lower than the mass proportion of the binder A in the second negative electrode active layer (22); wherein the swelling rates of the binder 1 and the binder A are both 43%-75%, and the swelling rate of the binder 2 is 0%-20%.

2. The wound battery cell according to claim 1, characterized in that The adhesive 1 accounts for 0.1%-50% of the mass of the first adhesive, and the adhesive A accounts for 50%-100% of the mass of the second adhesive.

3. The wound battery cell according to claim 2, characterized in that: The mass proportion of the first binder in the first negative electrode active layer (21) is smaller than the mass proportion of the second binder in the second negative electrode active layer (22).

4. The wound battery cell according to claim 3, characterized in that Contains at least one of the following characteristics: (1) The first binder in the first negative electrode active layer (21) has a mass of 1.0-3.2 parts by weight; (2) The mass of the second binder in the second negative electrode active layer (22) is 1.0-3.5 parts by weight.

5. The wound battery cell according to claim 1, characterized in that: The binder 1 and the binder A are each independently selected from at least one of 2-octyl acrylate-modified styrene-butadiene rubber, acrylamide-modified styrene-butadiene rubber, butyl acrylate-modified styrene-acrylic rubber, acrylonitrile-modified styrene-acrylic rubber and acrylamide-modified styrene-acrylic rubber.

6. The wound battery cell according to any one of claims 1 to 5, characterized in that: Contains at least one of the following characteristics: (1) The binder 2 accounts for 50%-99.9% of the total mass of the first binder; (2) The binder 2 is selected from at least one of acrylic acid-acrylamide-butyl acrylate, acrylic acid-acrylonitrile-acrylamide, methyl acrylate-butadiene-acrylic acid-acrylonitrile, and acrylic acid-acrylonitrile-butyl acrylate; (3) The second negative electrode active layer (22) further contains a binder B, and the swelling rate of the binder B is 0%-20%.

7. The wound battery cell according to claim 6, characterized in that: Contains at least one of the following characteristics: (1) The binder B accounts for 0.1% to 50% of the total mass of the second binder; (2) The binder B is selected from at least one of acrylic acid-acrylamide-butyl acrylate, acrylic acid-acrylonitrile-acrylamide, methyl acrylate-butadiene-acrylic acid-acrylonitrile, and acrylic acid-acrylonitrile-butyl acrylate.

8. The wound battery cell according to claim 1, characterized in that: The negative electrode active layer may or may not contain a thickener. When the negative electrode active layer contains a thickener, it has at least one of the following characteristics: (1) The thickener comprises at least one of carboxymethyl cellulose or its sodium salt or lithium salt, carboxyethyl cellulose or its sodium salt or lithium salt, and acrylate-modified sodium carboxymethyl cellulose or its sodium salt or lithium salt; (2) The first negative electrode active layer (21) contains 0.01-1.5 parts of a thickener by weight; (3) The second negative electrode active layer (22) contains 0.1-1.5 parts of thickener by weight.

9. A secondary battery, characterized in that: The secondary battery contains the wound battery cell according to any one of claims 1 to 8 and an electrolyte.

10. An electrical device, characterized in that: The electric device includes the secondary battery according to claim 9.

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