Pole piece and cylindrical lithium ion battery

Abstract

The invention relates to a pole piece and a cylindrical lithium ion battery. The pole piece comprises a current collector and active substance layers positioned on two sides of the current collector; in the length direction of the pole piece, the content of at least one active substance in the active substance layer is gradually reduced from the roll head to the roll tail of the pole piece. The expansion difference of the inner and outer ring pole pieces is improved and the battery performance is improved by regulating and controlling the formulas of the upper and lower slurry layers and the coating thickness in the length direction.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and more particularly to a cylindrical battery and an electrode. Background Technology

[0002] Cylindrical lithium-ion batteries are primarily composed of a positive electrode, a negative electrode, and a separator that separates the positive and negative electrodes, wound together to form a cylindrical core. This core is then placed in a cylindrical casing, filled with electrolyte, and sealed to form the cylindrical lithium-ion battery. The positive and negative electrodes have identical active layers from beginning to end along their length; for example, the active layers contain the same types and proportions of active materials, conductive agents, and binders. Due to the cylindrical structure, the inner (beginning) and outer (end) layers of the positive and negative electrodes experience different stresses during charging and discharging, resulting in inconsistent rebound along the electrode thickness, especially in the negative electrode. This ultimately leads to performance differences between the inner and outer layers of the cylindrical cell, thus accelerating performance degradation. Summary of the Invention

[0003] The following is an overview of the subject matter described in detail in this application. This overview is not intended to limit the scope of protection of this application.

[0004] The first aspect of this application provides an electrode sheet comprising a current collector and active material layers located on both sides of the current collector.

[0005] In one exemplary embodiment, along the length direction of the electrode sheet, from the beginning to the end of the roll, the total content of at least one active substance in the active substance layer gradually decreases; for example, along the length direction of the electrode sheet, from the beginning to the end of the roll, the active substance mass per unit area (i.e., areal density) or overall distribution density of at least one active substance gradually decreases.

[0006] In one exemplary embodiment, the active material layer is a bilayer structure containing active material, comprising a bottom layer close to the current collector and an upper layer away from the current collector.

[0007] In one exemplary embodiment, the content of active substances in the upper layer and the lower layer are different.

[0008] In one exemplary embodiment, the bottom layer is a high-content active substance layer, and the top layer is a medium-to-low-content active substance layer.

[0009] In one exemplary embodiment, the thickness of the bottom layer and the top layer gradually changes along the length direction of the electrode sheet, from the beginning to the end of the roll.

[0010] In one exemplary embodiment, along the length of the electrode sheet, from the beginning to the end of the roll, the thickness of the bottom layer or the top layer gradually decreases, while the thickness of the other layer gradually increases.

[0011] In one exemplary embodiment, along the length of the electrode sheet, from the beginning to the end of the roll, the total thickness of the active material layer remains essentially constant, the thickness of the bottom layer gradually decreases, and the thickness of the top layer gradually increases. That is, this is achieved by reducing the proportion of the high-activity material layer thickness and increasing the proportion of the low-activity material layer thickness during double-layer coating.

[0012] In one exemplary embodiment, the slurry formulations of the bottom layer and the top layer are different.

[0013] In one exemplary embodiment, the electrode is one or both of a negative electrode and a positive electrode.

[0014] In one exemplary embodiment, the electrode is a positive electrode or a negative electrode.

[0015] In one exemplary embodiment, when the electrode is a negative electrode, the active material includes a silicon-based material.

[0016] In one exemplary embodiment, the silicon-based material is selected from one or more of nano-silicon, silicon suboxide, pre-lithiated silicon oxide, pre-magnesiated silicon oxide, and silicon-carbon composite materials.

[0017] In one exemplary embodiment, the mass ratio of silicon-based material in the bottom layer is 10%-20% (e.g., 20%), and the mass ratio of silicon-based material in the top layer is 1%-5% (e.g., 5%).

[0018] In one exemplary embodiment, the active material further includes graphite.

[0019] In one exemplary embodiment, the graphite is selected from one or more of natural graphite and artificial graphite.

[0020] In one exemplary embodiment, the proportion of graphite is (1 - proportion of silicon-based material) × 100%.

[0021] In one exemplary embodiment, when the electrode is a positive electrode, the active material includes a ternary material.

[0022] In one exemplary embodiment, the ternary material in the underlying layer is a high-nickel ternary active material; preferably, it is an 8-series (nickel molar ratio ≥80% and <90%) to a 9-series (nickel molar ratio ≥90%) ternary material, for example, a 9-series ternary material.

[0023] In an exemplary embodiment, the ternary material in the upper layer is a medium-nickel ternary active material, preferably a 5-7 series ternary material; for example, a 6 series ternary material.

[0024] In one exemplary embodiment, the raw materials in the active material layer further include a conductive agent, a binder, and a solvent.

[0025] In one exemplary embodiment, the conductive agent is selected from one or more of carbon-based materials, metal-based materials, and conductive polymers.

[0026] In one exemplary embodiment, the carbon-based material is selected from one or more of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.

[0027] In one exemplary embodiment, the metal-based material is metal powder or metal fiber, etc.; the metal is selected from one or more of copper, nickel, aluminum, silver, etc.

[0028] In one exemplary embodiment, the conductive polymer is a polyphenylene derivative.

[0029] In one exemplary embodiment, the adhesive is selected from one or more of the following: polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc.

[0030] In one exemplary embodiment, the solvent is deionized water or NMP.

[0031] In one exemplary embodiment, the mass ratio of active material, conductive agent, and binder in the active material layer is (90-98):(0.5-5):(1-10); for example, 90:5:5, 95:2:3, or 97:1:2.

[0032] In one exemplary embodiment, along the length direction of the electrode sheet, from the beginning to the end of the coil, the thickness percentage of the bottom layer decreases linearly from 65%-51% to 35%-49%, and the thickness percentage of the top layer increases linearly from 35%-49% to 51%-65%.

[0033] A second aspect of this application provides a cylindrical lithium-ion battery, including the aforementioned electrode.

[0034] In one exemplary embodiment, the cylindrical lithium-ion battery further includes a separator.

[0035] In one exemplary embodiment, the diaphragm includes a substrate layer and a surface treatment layer.

[0036] In one exemplary embodiment, the substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide.

[0037] In one exemplary embodiment, the surface treatment layer is a ceramic layer, and the material of the ceramic layer is at least one of alumina, boehmite, etc.

[0038] Compared with existing related technologies, this application has the following technical effects: 1) This application provides a positive or negative electrode sheet, both of which adopt a double-coating design with different slurry formulations for the two coatings; and the thickness of the two coatings gradually changes along the length of the electrode sheet.

[0039] 2) This application provides a cylindrical battery and electrode sheet. Along the length of the negative electrode sheet, from the beginning to the end of the roll, the silicon content decreases, and the thickness expansion value of the negative electrode sheet gradually decreases. Along the thickness direction of the negative electrode sheet, a high-silicon layer is located at the bottom, and a low-silicon layer is located at the top. The top layer restricts the expansion of part of the bottom layer. Along the length of the electrode sheet, from the beginning to the end of the roll, the thickness expansion of the outer electrode sheet decreases. This application improves the difference in expansion between the inner and outer electrode sheets by controlling the formulation of the upper and lower slurry layers and the coating thickness along the length direction, thereby improving battery performance.

[0040] Other features and advantages of this application will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the application. Other advantages of this application can be realized and obtained by means of the embodiments described in the description and the accompanying drawings. Attached Figure Description

[0041] The accompanying drawings are used to provide an understanding of the technical solutions of this application and constitute a part of the specification. They are used together with the embodiments of this application to explain the technical solutions of this application and do not constitute a limitation on the technical solutions of this application.

[0042] Figure 1 This is a comparison chart of the battery cycle life of Comparative Example 1 and Example 1 of this application; Figures 2A to 2B This is a schematic diagram showing the thickness variation of the active material layers in the negative and positive electrodes of this application; Figures 3A to 3B This is a schematic diagram of the negative electrode and positive electrode of this application. Detailed Implementation

[0043] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all 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. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be arbitrarily combined with each other.

[0044] The present application will be further described in detail below with reference to specific embodiments, but these embodiments should not be construed as limiting the present application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this invention.

[0045] The raw materials used in this application are all conventional products on the market.

[0046] Unless otherwise specified, all materials and reagents used in the embodiments of this invention are commercially available.

[0047] Experimental methods not specified in the examples are generally performed under standard conditions or as recommended by the manufacturer.

[0048] Example 1 1) Composition and preparation of electrodes: 1.1 Negative electrode: The raw materials for the negative electrode active material layer are graphite and silicon carbide, conductive agent SP (carbon black), binder PAA (polyacrylic acid), and solvent. Deionized water is used as the solvent.

[0049] The formulation of slurry I for bottom layer A is as follows: by mass percentage, negative electrode active material: conductive agent: binder = 90%: 5%: 5%, of which silicon-based material accounts for 20% of the negative electrode active material; The formulation of slurry II in the upper layer B is as follows: by mass percentage, negative electrode active material: conductive agent: binder = 95%: 2%: 3%, of which silicon-based material accounts for 5% of the negative electrode active material.

[0050] The negative electrode active material, conductive agent, binder and solvent are mixed and stirred in the corresponding proportions to form a uniform and dispersed slurry to obtain the active slurry. The active slurry is coated using a double-layer coating machine. The bottom layer is coated on the current collector and the top layer is coated on the bottom layer. The active material layers are coated on both sides of the current collector. After drying, the negative electrode sheet is obtained.

[0051] From beginning to end, the thickness percentage of slurry I decreases linearly from 60% to 40%, while the thickness percentage of slurry II increases linearly from 40% to 60%, with the total thickness of slurry I and slurry II remaining unchanged.

[0052] 1.2 Positive electrode: The raw materials for the positive electrode active material layer are ternary positive electrode active material, conductive agent SP, binder PVDF (polyvinylidene fluoride), and solvent; among which, the solvent is NMP (N-methylpyrrolidone).

[0053] The formulation of slurry III for the bottom layer C is as follows: by mass percentage, positive electrode active material: conductive agent: binder = 95%: 3%: 2%, where the positive electrode active material is a 9-series ternary material; The formulation of slurry IV in the upper layer D is as follows: by mass percentage, positive electrode active material: conductive agent: binder = 97%: 2%: 1%, wherein the positive electrode active material is a 6-series ternary material.

[0054] The positive electrode active material, conductive agent, binder and solvent are mixed and stirred in the corresponding proportions to form a uniform and dispersed slurry to obtain the active slurry. The active slurry is coated using a double-layer coating machine. The bottom layer is coated on the current collector and the top layer is coated on the bottom layer. The active material layers are coated on both sides of the current collector. After drying, the positive electrode sheet is obtained.

[0055] From beginning to end, the thickness percentage of slurry III decreases linearly from 55% to 45%, while the thickness percentage of slurry IV increases linearly from 45% to 55%, with the total thickness of slurry III and slurry IV remaining unchanged.

[0056] 2) Cylindrical lithium-ion batteries The cylindrical lithium-ion battery includes a positive electrode, a negative electrode, and a separator. The separator includes a PE substrate layer and an aluminum oxide surface treatment layer.

[0057] Comparative Example 1 1) Composition and preparation of electrodes: 1.1 Negative electrode plate: The raw materials for the negative electrode active material layer are graphite and silicon carbide, conductive agent SP, binder PAA, and solvent. Deionized water is used as the solvent.

[0058] Negative electrode slurry formulation: Negative electrode active material: Conductive agent: Binder = 90%: 5%: 5%, of which silicon-based materials account for 15% of the negative electrode active material.

[0059] The thickness of the electrode sheets is consistent from beginning to end of the roll.

[0060] 1.2 Positive electrode: The raw materials for the positive electrode active material layer are a ternary positive electrode active material, a conductive agent SP, a binder PVDF, and a solvent. NMP is used as the solvent.

[0061] Positive electrode slurry formulation: Positive electrode active material: Conductive agent: Binder = 94%:3%:3%, wherein the positive electrode active material is a 9-series ternary material.

[0062] The thickness of the electrode sheets is consistent from beginning to end of the roll.

[0063] Comparative Example 2 The positive and negative electrodes use the formulations of Example 1, but the thickness of the double active material layer remains unchanged along the length of the electrode.

[0064] From the beginning to the end of the roll, the thickness of slurry I accounts for 60%, the thickness of slurry II accounts for 40%, and the total thickness of slurry I and slurry II remains unchanged; from the beginning to the end of the roll, the thickness of slurry III accounts for 55%, the thickness of slurry IV accounts for 45%, and the total thickness of slurry III and slurry IV remains unchanged.

[0065] The variations in electrode thickness in Examples 1, 1, and 2 are shown in Table 1. Figures 2A to 3B As shown.

[0066] Table 1. Variation of Electrode Thickness The battery performance in Example 1 and Comparative Example 1 was tested, and the results are as follows: Figure 1 As shown in the figure. The results show that the lithium-ion battery prepared using the electrode of this application can significantly improve the battery cycle performance.

[0067] In summary, 1) This application provides a positive or negative electrode sheet, both of which employ a double-coating design with inconsistent slurry formulations for the two coatings; and the thickness of the two coatings gradually changes along the length of the electrode sheet. 2) This application provides a cylindrical battery and electrode sheet, wherein the silicon content of the negative electrode sheet decreases from the beginning to the end along its length, and the expansion value in the thickness direction of the negative electrode sheet gradually decreases; along the thickness direction of the negative electrode sheet, a high-silicon layer is located at the bottom, and a low-silicon layer is located at the top, with the top layer restricting the expansion of part of the bottom layer; along the length direction of the electrode sheet, the expansion in the thickness direction of the outer ring electrode sheet decreases from the beginning to the end. This application improves the difference in expansion between the inner and outer ring electrode sheets by controlling the slurry formulations of the upper and lower layers and the coating thickness in the length direction, thereby improving battery performance.

[0068] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. An electrode, the electrode comprising a current collector and active material layers located on both sides of the current collector; wherein, Along the length of the electrode sheet, from the beginning to the end of the coil, the total content of at least one active substance in the active substance layer gradually decreases.

2. The electrode according to claim 1, wherein, Along the length of the electrode sheet, from the beginning to the end of the coil, the mass of at least one active substance per unit area gradually decreases.

3. The electrode according to claim 1 or 2, wherein, The active material layer is a two-layer structure containing active materials, including a bottom layer close to the current collector and an upper layer away from the current collector; The bottom layer is a high-content active substance layer, and the top layer is a medium-to-low-content active substance layer; Along the length of the electrode sheet, from the beginning to the end of the coil, the total thickness of the active material layer remains constant, the thickness of the bottom layer gradually decreases, and the thickness of the top layer gradually increases. Optionally, the electrode is one or both of a negative electrode and a positive electrode.

4. The electrode according to claim 3, wherein, When the electrode is a negative electrode, the active material includes silicon-based materials; Optionally, the silicon-based material is selected from one or more of nano-silicon, silicon suboxide, pre-lithiated silicon oxide, pre-magnesiated silicon oxide, and silicon-carbon composite materials; Optionally, the mass ratio of silicon-based material in the bottom layer is 10%-20%, and the mass ratio of silicon-based material in the top layer is 1%-5%.

5. The electrode according to claim 4, wherein, The active material also includes graphite; Optionally, the graphite is selected from one or more of natural graphite and artificial graphite; Optionally, the proportion of graphite is (1 - proportion of silicon-based material) × 100%.

6. The electrode according to claim 3, wherein, When the electrode is a positive electrode, the active material includes ternary materials; Optionally, the ternary material in the bottom layer is a high-nickel ternary active material; the ternary material in the top layer is a medium-nickel ternary active material. Optionally, the high-nickel ternary active material is an 8-series or 9-series ternary material; Optionally, the nickel-containing ternary active material is a 5-series to 7-series ternary material.

7. The electrode according to claim 3, wherein, The raw materials in the active material layer also include conductive agents, binders, and solvents; Optionally, the conductive agent is selected from one or more of carbon-based materials, metal-based materials, and conductive polymers; Optionally, the adhesive is selected from one or more of the following: polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon. Optionally, the solvent is deionized water or NMP.

8. The electrode according to claim 7, wherein, In the bottom layer or the top layer, the mass ratio of active material, conductive agent, and binder is (90-98):(0.5-5):(1-10); and / or Along the length of the electrode sheet, from the beginning to the end of the roll, the thickness percentage of the bottom layer decreases linearly from 65%-51% to 35%-49%, while the thickness percentage of the top layer increases linearly from 35%-49% to 51%-65%.

9. A cylindrical lithium-ion battery comprising the electrode as described in any one of claims 1 to 8.

10. The cylindrical lithium-ion battery according to claim 9, wherein, The cylindrical lithium-ion battery also includes a separator, which includes a substrate layer and a surface treatment layer; Optionally, the substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Optionally, the surface treatment layer is a ceramic layer, and the material of the ceramic layer is at least one of alumina and boehmite.

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