A positive electrode sheet and its application

Abstract

The present invention provides a positive electrode sheet and its application. The positive electrode sheet of the present invention comprises a first positive electrode active layer, a second positive electrode active layer, and a third positive electrode active layer stacked in the direction away from the positive electrode current collector. The first positive electrode active layer comprises a first lithium cobaltate, the second positive electrode active layer comprises a second lithium cobaltate, and the third positive electrode active layer comprises a third lithium cobaltate. The aluminum content of the third lithium cobaltate is greater than the aluminum content of the second lithium cobaltate, and / or the aluminum content of the first lithium cobaltate is greater than the aluminum content of the second lithium cobaltate. This positive electrode sheet enables a battery to achieve relatively excellent energy density, rate capability, cycle expansion performance, and cycle performance.

Description

Technical Field

[0001] The invention relates to a positive electrode sheet and application thereof, belonging to the field of batteries. Background Art

[0002] As various fields have increasingly higher requirements for the energy density of consumer polymer lithium-ion batteries, existing technologies usually increase the energy density of lithium-ion batteries by making the electrodes in lithium-ion batteries thicker. However, thick electrodes are difficult to have excellent rate performance, and as the thickness of the electrodes increases, the cycle performance of the lithium-ion battery is attenuated to a certain extent, and the cycle expansion rate of the lithium-ion battery increases.

[0003] Therefore, it is necessary to develop a battery electrode that can provide the battery with excellent energy density, rate performance, cycle expansion performance and cycle performance. Summary of the Invention

[0004] The present invention provides a positive electrode sheet, which can enable a battery to have relatively excellent energy density, rate performance, cycle expansion performance and cycle performance.

[0005] The present invention provides a battery, which includes the above-mentioned positive electrode sheet, and thus has relatively excellent energy density, rate performance, cycle expansion performance and cycle performance.

[0006] The present invention provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active layer provided on at least one functional surface of the positive electrode current collector;

[0007] The positive electrode active layer includes a first positive electrode active layer, a second positive electrode active layer and a third positive electrode active layer stacked in sequence in a direction away from the positive electrode current collector;

[0008] The first positive electrode active layer includes a first lithium cobaltate, the second positive electrode active layer includes a second lithium cobaltate, and the third positive electrode active layer includes a third lithium cobaltate;

[0009] The aluminum content in the third lithium cobaltate is greater than the aluminum content in the second lithium cobaltate; and / or the aluminum content in the first lithium cobaltate is greater than the aluminum content in the second lithium cobaltate.

[0010] The positive electrode sheet as described above, wherein the first positive electrode active layer further comprises a first conductive agent, the second positive electrode active layer further comprises a second conductive agent, and the third positive electrode active layer further comprises a third conductive agent;

[0011] The content of the second conductive agent in the second positive electrode active layer is greater than the content of the first conductive agent in the first positive electrode active layer; and / or the content of the second conductive agent in the second positive electrode active layer is greater than the content of the third conductive agent in the third positive electrode active layer.

[0012] The positive electrode sheet as described above, wherein the aluminum content in the third lithium cobaltate is greater than the aluminum content in the first lithium cobaltate; and / or,

[0013] The content of the first conductive agent in the first positive electrode active layer is greater than the content of the third conductive agent in the third positive electrode active layer.

[0014] The positive electrode sheet as described above, wherein the first positive electrode active layer further comprises a first binder;

[0015] Based on the total mass of the first positive electrode active layer, the mass percentage of the first lithium cobaltate is 92-98%, the mass percentage of the first conductive agent is 0.05-4%, and the mass percentage of the first binder is 0.1-4%;

[0016] And / or, the second positive electrode active layer further includes a second binder;

[0017] Based on the total mass of the second positive electrode active layer, the mass percentage of the second lithium cobaltate is 92-98%, the mass percentage of the second conductive agent is 0.05-4%, and the content of the second binder is 0.1-4%;

[0018] And / or, the third positive electrode active layer further includes a third binder;

[0019] Based on the total mass of the third positive electrode active layer, the mass percentage of the third lithium cobaltate is 92-98%, the mass percentage of the third conductive agent is 0.05-4%, and the content of the third binder is 0.1-4%.

[0020] The positive electrode sheet as described above, wherein the aluminum content in the second lithium cobaltate is 4000-6000 ppm, and the content of the second conductive agent in the second positive electrode active layer is 1.6-2 wt%.

[0021] The positive electrode sheet as described above, wherein the aluminum content in the first lithium cobaltate is 6000-8000 ppm, and the content of the first conductive agent in the first positive electrode active layer is 1.2-1.6 wt %.

[0022] The positive electrode sheet as described above, wherein the aluminum content in the third lithium cobaltate is 8000-10000 ppm, and the content of the third conductive agent in the third positive electrode active layer is 0.8-1.2 wt%.

[0023] The positive electrode sheet as described above, wherein the thickness of the active layer of the positive electrode sheet is 40-75 μm.

[0024] The positive electrode sheet as described above, wherein the surface density of the positive electrode active layer is 20-35 mg / cm2 .

[0025] The positive electrode sheet as described above, wherein the ratio of the area density of the first positive electrode active layer, the area density of the second positive electrode active layer and the area density of the third positive electrode active layer is (10-40%): (10-40%): (10-40%).

[0026] The present invention provides a battery, wherein the battery includes the positive electrode sheet as described above.

[0027] The positive electrode sheet provided by the present invention has a positive electrode active layer that includes a first positive electrode active layer, a second positive electrode active layer, and a third positive electrode active layer that are stacked in sequence along a direction away from the positive electrode current collector, and the aluminum content in the third lithium cobaltate of the third positive electrode active layer and / or the aluminum content in the first lithium cobaltate of the first positive electrode active layer is greater than the aluminum content in the second lithium cobaltate of the second positive electrode active layer. The positive electrode sheet of the present invention includes three active layers, which helps to improve the energy density of the battery. Furthermore, the aluminum content in the third lithium cobaltate and / or the aluminum content in the first lithium cobaltate is greater than the aluminum content in the second lithium cobaltate, which helps to balance the polarization potential of the first positive electrode active layer, the second positive electrode active layer, and the third positive electrode active layer, thereby improving the rate performance, cycle expansion performance, and cycle performance of the battery. Therefore, the positive electrode sheet of the present invention can enable the battery to have relatively excellent energy density, rate performance, cycle expansion performance, and cycle performance.

[0028] The battery provided by the present invention includes the above-mentioned positive electrode sheet, and therefore has relatively excellent energy density, rate performance, cycle expansion performance and cycle performance, and is suitable for wide application and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the following briefly introduces the drawings required for use in the embodiments of the present invention or related technologies. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort.

[0030] Figure 1 Schematic diagram of the structure of the positive electrode sheet of some embodiments of the present invention;

[0031] Figure 2 This is a cross-sectional SEM image of the positive electrode sheet in Example 2 of the present invention.

[0032] Description of reference numerals:

[0033] 1: positive electrode current collector;

[0034] 2: first positive electrode active layer;

[0035] 3: second positive electrode active layer;

[0036] 4: The third positive electrode active layer. DETAILED DESCRIPTION

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0038] Figure 1 Schematic diagram of the structure of the positive electrode sheet of some embodiments of the present invention. Figure 1 As shown, the present invention provides a positive electrode sheet, comprising a positive electrode current collector 1 and a positive electrode active layer provided on at least one functional surface of the positive electrode current collector 1;

[0039] The positive electrode active layer includes a first positive electrode active layer 2, a second positive electrode active layer 3 and a third positive electrode active layer 4 stacked in sequence in a direction away from the positive electrode current collector 1;

[0040] The first positive electrode active layer 2 includes a first lithium cobaltate, the second positive electrode active layer 3 includes a second lithium cobaltate, and the third positive electrode active layer 4 includes a third lithium cobaltate;

[0041] The aluminum content in the third lithium cobaltate is greater than the aluminum content in the second lithium cobaltate; and / or the aluminum content in the first lithium cobaltate is greater than the aluminum content in the second lithium cobaltate.

[0042] The present invention does not impose any particular limitations on the positive electrode current collector 1; any commonly used positive electrode current collector 1 in the art may be used. For example, the positive electrode current collector 1 may be a conventional metal foil current collector or a metal foil current collector provided with a primer layer. In the present invention, the functional surfaces of the positive electrode current collector 1 refer to the two surfaces of the positive electrode current collector 1 that have the largest area and are disposed opposite each other.

[0043] The present invention can form a positive electrode sheet by setting the positive electrode active layer on one functional surface of the positive electrode collector 1. In this case, the positive electrode sheet includes, in sequence, the positive electrode collector 1, the first positive electrode active layer 2, the second positive electrode active layer 3, and the third positive electrode active layer 4, which are stacked together. The positive electrode active layer can also be set on two functional surfaces of the positive electrode collector to form a positive electrode sheet. In this case, the positive electrode sheet includes, in sequence, the third positive electrode active layer 4, the second positive electrode active layer 3, the first positive electrode active layer 2, the positive electrode collector 1, the first positive electrode active layer 2, the second positive electrode active layer 3, and the third positive electrode active layer 4, which are stacked together.

[0044] In the present invention, the first positive electrode active layer 2 includes a first lithium cobaltate, the second positive electrode active layer 3 includes a second lithium cobaltate, and the third positive electrode active layer 4 includes a third lithium cobaltate. The first lithium cobaltate, the second lithium cobaltate, and the third lithium cobaltate are respectively the positive electrode active materials in the first positive electrode active layer 2, the second positive electrode active layer 3, and the third positive electrode active layer 4, which can respectively ensure that the first positive electrode active layer 2, the second positive electrode active layer 3, and the third positive electrode active layer 4 perform normal lithium intercalation and deintercalation, thereby ensuring the normal operation of the positive electrode sheet. Therefore, the performance of the first lithium cobaltate, the second lithium cobaltate, and the third lithium cobaltate will directly affect the performance of the positive electrode sheet.

[0045] The first, second, and third lithium cobaltates of the present invention refer to lithium cobaltates each doped with a small amount of aluminum. By doping the first, second, and third lithium cobaltates with aluminum, the stability of the first, second, and third lithium cobaltates can be improved, thereby improving the cycle stability of the battery. In some embodiments, the first, second, and third lithium cobaltates can be obtained by commercially purchasing lithium cobaltates doped with different amounts of aluminum, as needed.

[0046] In the present invention, the positive electrode active layer (the first positive electrode active layer 2, the second positive electrode active layer 3 and the third positive electrode active layer 4) may also include other positive electrode active materials in addition to lithium cobalt oxide (the first lithium cobalt oxide, the second lithium cobalt oxide and the third lithium cobalt oxide). For example, the positive electrode active material may also include lithium iron phosphate, lithium manganese oxide or at least one of a ternary material.

[0047] The present invention provides a positive electrode sheet that can simultaneously improve the cycle performance, cycle expansion performance, rate performance and energy density of a battery by making the positive electrode active layer include a first positive electrode active layer 2, a second positive electrode active layer 3 and a third positive electrode active layer 4 that are stacked, and making the aluminum content in the third lithium cobaltate and / or the aluminum content in the first lithium cobaltate greater than the aluminum content in the second lithium cobaltate.

[0048] The inventors have analyzed the reasons for the above results and speculated that the reasons are as follows:

[0049] Since the positive electrode active layer of the present invention includes the first positive electrode active layer 2 , the second positive electrode active layer 3 and the third positive electrode active layer 4 which are stacked, the arrangement of the multiple active layers helps to improve the energy density of the battery.

[0050] Furthermore, since the second positive electrode active layer 3 is located between the first positive electrode active layer 2 and the third positive electrode active layer 4, according to the porous electrode theory, during battery operation, the polarization potential of the third positive electrode active layer 4 and the polarization potential of the first positive electrode active layer 2 are greater than the polarization potential of the second positive electrode active layer 3. By ensuring that the aluminum content of the third lithium cobalt oxide in the third positive electrode active layer 4 with a large polarization potential and / or the aluminum content of the first lithium cobalt oxide in the first positive electrode active layer 2 with a large polarization potential are greater than the aluminum content of the second lithium cobalt oxide in the second positive electrode active layer 3, the present invention can balance the polarization potentials of the second positive electrode active layer 3, the first positive electrode active layer 2, and the third positive electrode active layer 4, thereby improving the battery's cycle performance, cycle expansion performance, and rate performance.

[0051] Therefore, the positive electrode sheet of the present invention can enable the battery to have relatively excellent cycle performance, cycle expansion performance, rate performance and energy density.

[0052] In some embodiments of the present invention, the first positive electrode active layer 2 further includes a first conductive agent, the second positive electrode active layer 3 further includes a second conductive agent, and the third positive electrode active layer 4 further includes a third conductive agent;

[0053] The content of the second conductive agent in the second positive electrode active layer 3 is greater than the content of the first conductive agent in the first positive electrode active layer 2 ; and / or the content of the second conductive agent in the second positive electrode active layer 3 is greater than the content of the third conductive agent in the third positive electrode active layer 4 .

[0054] In the present invention, the conductive agent (first conductive agent, second conductive agent and third conductive agent) can increase the conductivity of the positive electrode active layer and can also store electrolyte. Therefore, the content of the conductive agent (first conductive agent, second conductive agent and third conductive agent) will also directly affect the performance of the positive electrode sheet.

[0055] The present invention does not particularly limit the first conductive agent, the second conductive agent, and the third conductive agent, and conductive agents commonly used in the art can be selected. In some embodiments, the first conductive agent, the second conductive agent, and the third conductive agent can independently be selected from at least one of carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and porous carbon.

[0056] The present invention can further improve the cycle performance, cycle expansion performance, rate performance and energy density of the battery by making the content of the second conductive agent in the second positive electrode active layer 3 greater than the content of the first conductive agent in the first positive electrode active layer 2 and / or the content of the third conductive agent in the third positive electrode active layer 4.

[0057] The inventors have analyzed the reasons for the above results and speculated that the reasons are as follows:

[0058] Since the second positive electrode active layer 3 is located between the first positive electrode active layer 2 and the third positive electrode active layer 4, according to the porous electrode theory, during the operation of the battery, the reaction activity of the second positive electrode active layer 3 is less than the reaction activity of the first positive electrode active layer 2 and the reaction activity of the third positive electrode active layer 4. By maximizing the content of the second conductive agent in the second positive electrode active layer 3, the present invention can, on the one hand, increase the conductivity and the reaction activity of the second positive electrode active layer 3, thereby improving the energy density and rate performance of the battery. On the other hand, the high content of the second conductive agent in the second positive electrode active layer 3 can also preserve the electrolyte in the battery as much as possible, thereby providing more lithium ion transmission channels for the first positive electrode active layer 2, ensuring that the first positive electrode active layer 2 will not fail in cycling due to electrolyte bridge breakage, thereby improving the energy density, cycling performance, and cycling expansion performance of the battery.

[0059] In some embodiments of the present invention, the aluminum content in the third lithium cobaltate is greater than the aluminum content in the first lithium cobaltate; and / or the content of the first conductive agent in the first positive electrode active layer 2 is greater than the content of the third conductive agent in the third positive electrode active layer 4.

[0060] It can be understood that in the positive electrode sheet of the present invention, the aluminum content in the third lithium cobaltate, the aluminum content in the first lithium cobaltate, and the aluminum content in the second lithium cobaltate decrease in sequence; and / or, the content of the third conductive agent in the third positive electrode active layer 4, the content of the first conductive agent in the first positive electrode active layer 2, and the content of the second conductive agent in the second positive electrode active layer 3 increase in sequence.

[0061] The present invention can further improve the cycle performance, rate performance, cycle expansion performance and energy density of the battery by making the aluminum content in the first lithium cobaltate, the second lithium cobaltate and the third lithium cobaltate satisfy the above relationship, and / or making the content of the first conductive agent in the first positive electrode active layer 2, the content of the second conductive agent in the second positive electrode active layer 3 and the content of the third conductive agent in the third positive electrode active layer 4 satisfy the above relationship.

[0062] The inventors speculate that this is due to the following reasons:

[0063] According to the porous electrode theory, during the operation of the battery, the polarization potential of the third positive electrode active layer 4, the polarization potential of the first positive electrode active layer 2, and the polarization potential of the second positive electrode active layer 3 decrease successively, and the reaction activity of the third positive electrode active layer 4, the reaction activity of the first positive electrode active layer 2, and the reaction activity of the second positive electrode active layer 3 decrease successively.

[0064] The present invention can maximize the balance of the polarization potentials of the first positive electrode active layer 2, the second positive electrode active layer 3, and the third positive electrode active layer 4 by ensuring that the aluminum content in the first lithium cobaltate, the second lithium cobaltate, and the third lithium cobaltate satisfies the above-mentioned relationship; and / or, by ensuring that the content of the first conductive agent in the first positive electrode active layer 2, the content of the second conductive agent in the second positive electrode active layer 3, and the content of the third conductive agent in the third positive electrode active layer 4 satisfy the above-mentioned relationship, the reaction activities of the first positive electrode active layer 2, the second positive electrode active layer 3, and the third positive electrode active layer 4 can be maximized, thereby further improving the cycle performance, rate performance, cycle expansion performance, and energy density of the battery.

[0065] In some embodiments of the present invention, the first positive electrode active layer 2 further includes a first binder;

[0066] Based on the total mass of the first positive electrode active layer 2, the mass percentage of the first lithium cobaltate is 92-98%, the mass percentage of the first conductive agent is 0.05-4%, and the mass percentage of the first binder is 0.1-4%;

[0067] And / or, the second positive electrode active layer 3 further includes a second binder;

[0068] Based on the total mass of the second positive electrode active layer 3, the mass percentage of the second lithium cobaltate is 92-98%, the mass percentage of the second conductive agent is 0.05-4%, and the content of the second binder is 0.1-4%;

[0069] And / or, the third positive electrode active layer 4 further includes a third binder;

[0070] Based on the total mass of the third positive electrode active layer 4 , the mass percentage of the third lithium cobaltate is 92-98%, the mass percentage of the third conductive agent is 0.05-4%, and the mass percentage of the third binder is 0.1-4%.

[0071] The present invention does not particularly limit the first binder, the second binder, and the third binder, and any binder commonly used in the art may be used. For example, the first binder, the second binder, and the third binder may be independently selected from at least one of polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyethylene oxide (PEO), SBR, and polyacrylates.

[0072] In the present invention, when at least one of the first positive electrode active layer 2, the second positive electrode active layer 3 and the third positive electrode active layer 4 meets the above parameter limitations, the cycle performance, energy density, cycle expansion performance and rate performance of the battery can be better improved.

[0073] In some embodiments of the present invention, the aluminum content in the first lithium cobaltate and the content of the first conductive agent, the aluminum content in the second lithium cobaltate and the content of the second conductive agent, and the aluminum content in the third lithium cobaltate and the content of the third conductive agent can be further limited in order to further improve the cycle performance, rate performance, cycle expansion performance and energy density of the battery.

[0074] For example, the aluminum content in the second lithium cobaltate is 4000-6000 ppm, and the content of the second conductive agent in the second positive electrode active layer 3 is 1.6-2 wt %; and / or,

[0075] The aluminum content in the first lithium cobaltate is 6000-8000 ppm, and the content of the first conductive agent in the first positive electrode active layer 2 is 1.2-1.6 wt %; and / or,

[0076] The aluminum content in the third lithium cobaltate is 8000-10000 ppm, and the content of the third conductive agent in the third positive electrode active layer 4 is 0.8-1.2 wt %.

[0077] In some embodiments of the present invention, a method for preparing a positive electrode sheet includes the following steps:

[0078] Disposing the first positive electrode active slurry on at least one functional surface of the positive electrode current collector 1 and drying it to form a first positive electrode active layer 2 on the functional surface of the positive electrode current collector 1;

[0079] The second positive electrode active slurry is placed on the surface of the first positive electrode active layer 2 away from the positive electrode current collector 1, and dried to form the second positive electrode active layer 3 on the surface of the first positive electrode active layer 2 away from the positive electrode current collector 1;

[0080] The third positive electrode active slurry is placed on the surface of the second positive electrode active layer 3 away from the first positive electrode active layer 2 and dried to form the third positive electrode active layer 4 on the surface of the second positive electrode active layer 3 away from the first positive electrode active layer 2, and a positive electrode sheet is formed.

[0081] In some other embodiments of the present invention, the method for preparing the positive electrode sheet includes the following steps:

[0082] Disposing the first positive electrode active slurry on at least one functional surface of the positive electrode current collector 1 and drying it to form a first positive electrode active layer 2 on the functional surface of the positive electrode current collector 1;

[0083] A double-cavity coating die is used to arrange the second positive electrode active slurry and the third positive electrode active slurry on the surface of the first positive electrode active layer 2 away from the positive electrode current collector 1, and drying is performed to obtain a positive electrode sheet comprising the second positive electrode active layer 3 and the third positive electrode active layer 4, wherein the second positive electrode active layer 3 is arranged on the surface of the first positive electrode active layer 2 away from the positive electrode current collector 1, and the third positive electrode active layer 4 is arranged on the surface of the second positive electrode active layer 3 away from the first positive electrode active layer 2.

[0084] It can be understood that in the present invention, a three-cavity die head can also be used to set the first positive electrode active slurry, the second positive electrode active slurry and the third positive electrode active slurry on the functional surface of the positive electrode collector 1, and dry them to obtain a positive electrode sheet comprising a first positive electrode active layer 2, a second positive electrode active layer 3 and a third positive electrode active layer 4, wherein the first positive electrode active layer 2, the second positive electrode active layer 3 and the third positive electrode active layer 4 are stacked in sequence in a direction away from the positive electrode current collector 1.

[0085] In the above-mentioned preparation method of the positive electrode sheet of the present invention, the first positive electrode active layer 2 includes a first lithium cobalt oxide and a first conductive agent, the second positive electrode active layer 3 includes a second lithium cobalt oxide and a second conductive agent, and the third positive electrode active layer 4 includes a third lithium cobalt oxide and a third conductive agent;

[0086] The aluminum content in the third lithium cobaltate is greater than the aluminum content in the second lithium cobaltate; and / or the aluminum content in the first lithium cobaltate is greater than the aluminum content in the second lithium cobaltate.

[0087] Furthermore, in the above-mentioned method for preparing the positive electrode sheet, in order to avoid sedimentation of the first positive electrode active slurry, the second positive electrode active slurry and the third positive electrode active slurry, which affects the performance of the battery, the storage time of the first positive electrode active slurry, the second positive electrode active slurry and the third positive electrode active slurry is less than or equal to 24 hours.

[0088] In the above-mentioned preparation method of the positive electrode sheet of the present invention, the drying time of each drying can be 5-8 hours; the solid content of the first positive electrode active slurry, the second positive electrode active slurry and the third positive electrode active slurry can be 60-80%, and the viscosity can be 2000-7000 mPa.s.

[0089] In some embodiments of the present invention, the thickness of the active layer of the positive electrode sheet is 40-75 μm. The thickness defined by the above parameters can further increase the energy density of the battery while ensuring that the battery has relatively excellent cycle performance and rate performance.

[0090] In the present invention, the thickness of the positive electrode active layer refers to the thickness of the positive electrode active layer disposed on one functional surface of the positive electrode current collector 1. If the positive electrode sheet includes the positive electrode current collector 1 and the positive electrode active layers disposed on both functional surfaces of the positive electrode current collector, the sum of the thicknesses of the two positive electrode active layers in the positive electrode sheet is 80-150 μm.

[0091] In the present invention, the surface density of the positive electrode active layer can be limited in order to further increase the energy density of the battery while maintaining relatively excellent cycle performance, cycle expansion performance and rate performance. In some embodiments of the present invention, the surface density of the positive electrode active layer is 20-35 mg / cm 2 .

[0092] In the present invention, the surface density of the positive electrode active layer refers to the surface density of the positive electrode active layer provided on one functional surface of the positive electrode current collector 1. If the positive electrode sheet includes the positive electrode current collector 1 and the positive electrode active layer provided on two functional surfaces of the positive electrode current collector, the surface density of the positive electrode sheet is 40-70 mg / cm 2 .

[0093] In a specific embodiment, the surface density of the positive electrode active layer is 23 mg / cm 2 It not only enables the battery to have relatively excellent cycle performance, cycle expansion performance, energy density and rate performance, but also facilitates processing and saves production costs.

[0094] Furthermore, the ratio of the area density of the first positive electrode active layer 2 , the area density of the second positive electrode active layer 3 , and the area density of the third positive electrode active layer 4 is (10-40%): (10-40%): (10-40%).

[0095] In the present invention, the positive electrode active layer includes a first positive electrode active layer 2, a second positive electrode active layer 3, and a third positive electrode active layer 4 in the thickness direction of the positive electrode active layer. The areal density of the positive electrode active layer can be understood as the sum of the areal density of the first positive electrode active layer 2, the areal density of the second positive electrode active layer 3, and the areal density of the third positive electrode active layer 4. By ensuring that the ratio of the areal density of the first positive electrode active layer 2, the areal density of the second positive electrode active layer 3, and the areal density of the third positive electrode active layer 4 meets the above-mentioned range, the present invention not only enables the battery to have excellent energy density, cycle performance, cycle expansion performance, and rate performance, but also facilitates production and processing, which is conducive to broadening the application of positive electrode sheets.

[0096] In a specific embodiment, the surface density ratio of the first positive electrode active layer 2 , the second positive electrode active layer 3 and the third positive electrode active layer 4 is 1:1:1, or 3:4:3, which can make the positive electrode sheet more convenient for production and processing.

[0097] A second aspect of the present invention provides a battery comprising the above-mentioned positive electrode sheet.

[0098] In the present invention, the positive electrode sheet, separator and negative electrode sheet are stacked or wound to obtain a battery core, and the battery core is placed in an outer package, injected with electrolyte, formed, repackaged and divided into different volumes to obtain the battery of the present invention.

[0099] The separator can be a commercially available one. The outer packaging can be an aluminum-plastic film. The electrolyte can be a commercially available one, which can include a lithium salt and a non-aqueous solvent. In the present invention, the lithium salt is not particularly limited, and any lithium salt known in the art can be used, as long as the objectives of the present invention are achieved. For example, the lithium salt can include at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, or LiPO2F2. In the present invention, the non-aqueous solvent is not particularly limited, as long as the objectives of the present invention are achieved. For example, the non-aqueous solvent can include at least one of a carbonate compound, a carboxylate compound, an ether compound, a nitrile compound, or other organic solvents.

[0100] Since the battery of the present invention includes the above-mentioned positive electrode sheet, it can have relatively excellent cycle performance, energy density, cycle expansion performance and rate performance.

[0101] The technical solutions of the present invention are further illustrated below with reference to specific examples. All parts, percentages, and ratios described in the following examples are based on weight. All reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further treatment. The instruments used in the examples are also commercially available.

[0102] Example 1

[0103] The lithium ion battery of this embodiment is prepared by the following steps:

[0104] 1) Positive electrode

[0105] The first positive electrode active slurry is placed on two functional surfaces of the aluminum foil using an extrusion coater and then dried to obtain a first positive electrode active layer;

[0106] Using a double-cavity die head, a second positive electrode active slurry and a third positive electrode active slurry are disposed on a surface of the first positive electrode active layer away from the aluminum foil, and dried to obtain a positive electrode sheet comprising the second positive electrode active layer and the third positive electrode active layer, wherein the second positive electrode active layer is disposed on a surface of the first positive electrode active layer, and the third positive electrode active layer is disposed on a surface of the second positive electrode active layer away from the first positive electrode active layer;

[0107] In the first positive electrode active layer, the mass ratio of the first lithium cobaltate, carbon black, and PVDF binder is 97.6%:1.4%:1.0%, and the aluminum content in the first lithium cobaltate is 7000 ppm;

[0108] In the second positive electrode active layer, the mass ratio of the second lithium cobalt oxide, carbon black, and PVDF binder is 97.2%:1.8%:1.0%, and the aluminum content in the second lithium cobalt oxide is 5000 ppm;

[0109] In the third positive electrode active layer, the mass ratio of the third lithium cobalt oxide, carbon black, and PVDF binder is 98%:1.0%:1.0%, and the aluminum content in the third lithium cobalt oxide is 9000 ppm;

[0110] The surface densities of the first positive electrode active layer, the second positive electrode active layer, and the third positive electrode active layer are 7.67 mg / cm 2 ;

[0111] The thickness of the aluminum foil is 9 μm, the compaction density of the positive electrode sheet is 4.15 g / cc, and the thickness of the positive electrode sheet is 120 μm.

[0112] 2) Negative electrode

[0113] The negative electrode active slurry is placed on two functional surfaces of the copper foil using an extrusion coater and then dried to obtain a negative electrode sheet containing a negative electrode active layer;

[0114] The negative electrode active layer includes graphite, a binder of styrene-butadiene rubber, a dispersant of sodium carboxymethyl cellulose, and a conductive agent of carbon black. The mass ratio of graphite, a binder of styrene-butadiene rubber, a dispersant of sodium carboxymethyl cellulose, and a conductive agent of carbon black is 96.5%:1.5%:1.5%:0.5%. The surface density of the negative electrode active layer is 12.3 mg / cm 2 .

[0115] 3) Lithium-ion batteries

[0116] The positive electrode sheet of step 1), the negative electrode sheet of step 2) and the separator are wound into a core, the core is placed in an outer package, an electrolyte is injected, and the battery is formed, repackaged and divided into different capacities to obtain a lithium-ion battery;

[0117] Among them, the diaphragm includes a stacked ceramic layer, a PVDF coating layer, a polypropylene base film, a PVDF coating layer and a ceramic layer. The thickness of the diaphragm is 9μm, the thickness of the polypropylene base film is 5μm, the thickness of the two ceramic layers is 2μm in total, and the thickness of the two coating layers is 2μm in total.

[0118] Example 2

[0119] The preparation method of the lithium ion battery of this embodiment is basically the same as that of Example 1, except that:

[0120] In step 1), the surface density of the first positive electrode active layer and the second positive electrode active layer are 6.9 mg / cm 2 The surface density of the third positive electrode active layer is 9.2 mg / cm 2 .

[0121] Figure 2 This is a cross-sectional SEM image of the positive electrode sheet in Example 2 of the present invention. Figure 2 As shown, the difference between the first positive electrode active layer, the second positive electrode active layer and the third positive electrode active layer cannot be clearly observed in the cross section of the positive electrode sheet. The reason is that the morphologies of the first lithium cobalt oxide, the second lithium cobalt oxide and the third lithium cobalt oxide are similar.

[0122] Example 3

[0123] The preparation method of the lithium ion battery of this embodiment is basically the same as that of Example 1, except that:

[0124] In step 1), the aluminum content in the first lithium cobaltate is 5000 ppm, the aluminum content in the second lithium cobaltate is 3000 ppm; and the aluminum content in the third lithium cobaltate is 7000 ppm.

[0125] Comparative Example 1

[0126] The preparation method of the lithium ion battery of this comparative example is basically the same as that of Example 1, except that:

[0127] The first positive electrode active slurry in step 1) is placed on two functional surfaces of the aluminum foil to form a positive electrode sheet including a positive electrode active layer;

[0128] Among them, the surface density of the positive electrode active layer is 23 mg / cm 2 ,The thickness of the positive electrode sheet is 120μm.

[0129] Comparative Example 2

[0130] The preparation method of the lithium ion battery of this comparative example is basically the same as that of comparative example 1, except that:

[0131] The second positive electrode active slurry in step 1) is placed on two functional surfaces of the aluminum foil to form a positive electrode sheet including a positive electrode active layer.

[0132] Comparative Example 3

[0133] The preparation method of the lithium ion battery of this comparative example is basically the same as that of comparative example 1, except that:

[0134] The third positive electrode active slurry in step 1) is placed on two functional surfaces of the aluminum foil to form a positive electrode sheet including a positive electrode active layer.

[0135] Comparative Example 4

[0136] The preparation method of the lithium ion battery of this comparative example is basically the same as that of Example 1, except that:

[0137] The aluminum content in the second lithium cobaltate in step 1) is set to 9000 ppm.

[0138] Comparative Example 5

[0139] The preparation method of the lithium ion battery of this comparative example is basically the same as that of the embodiment, except that:

[0140] The content of the conductive agent in the second positive electrode active slurry in step 1) is 1.0%, and a positive electrode sheet including a positive electrode active layer is formed.

[0141] Performance Testing

[0142] The following tests were performed on the batteries of the embodiment and the comparative example. The test results are shown in Table 1:

[0143] 1) Gram capacity (q)

[0144] During the cell manufacturing process, the positive electrode sheets are weighed and their weight is recorded. Generally, 32 positive electrode sheets are measured for each experimental condition, and the average weight of the 32 positive electrode sheets is taken as m. The length of the active layer covered by the adhesive tape in each positive electrode sheet is measured with a steel ruler, and the average length of the active layer covered by the adhesive tape in the 32 positive electrode sheets is taken as L1. The average length of the 32 positive electrode sheets is recorded as L0. The mass percentage of the active material in the positive electrode active layer is recorded as a. The average capacity of the 32 battery cells is recorded as Q. The gram capacity q is:

[0145] q=Q*L0 / m×a×(L0-L1).

[0146] 2) Energy density calculation

[0147] The capacity of the lithium-ion batteries of the embodiment of the present invention and the comparative example was tested using a sorting cabinet. The battery energy density was calculated using the formula energy density = capacity * voltage platform / mass.

[0148] 3) Cyclic performance, cyclic expansion performance and rate performance

[0149] Place the battery in a (25±3)℃ environment and let it stand for 3 hours. When the battery cell reaches (25±3)℃, charge the battery to 4.25V at 1C, then charge it to 4.48V at 0.7C, then charge it to a cutoff current of 0.05C at 4.48V, and then discharge it to 3V at 0.5C. Record the initial capacity Q0 and initial thickness h0. When the cycle reaches the required number of times or the capacity attenuation rate is less than 70% or the thickness exceeds the test requirement, use the previous discharge capacity as the battery capacity Q2, and simultaneously test the thickness h2 of the battery cell to calculate the capacity retention rate (%) and thickness expansion rate (%):

[0150] Capacity retention rate (%) = Q2 / Q0×100%

[0151] Thickness expansion rate (%) = h2 / h0×100%.

[0152] Table 1

[0153]

[0154] It can be seen from Table 1 that the lithium-ion battery prepared in the embodiment of the present invention has relatively excellent cycle performance, rate performance, cycle expansion performance and energy density.

[0155] Furthermore, when the aluminum content in the first lithium cobaltate is 6000-8000 ppm and the content of the conductive agent in the first positive electrode active layer is 1.2-1.6 wt%; the aluminum content in the second lithium cobaltate is 4000-6000 ppm and the content of the conductive agent in the second positive electrode active layer is 1.6-2 wt%; and the aluminum content in the third lithium cobaltate is 8000-10000 ppm and the content of the conductive agent in the third positive electrode active layer is 0.8-1.2 wt%, the lithium-ion battery has more excellent cycle performance, rate performance, cycle expansion performance and energy density.

[0156] Each embodiment in this specification is described in a related manner. Similar parts between the embodiments can be referred to in conjunction with each other. Each embodiment focuses on the differences from other embodiments. The above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of protection of the present invention.

Claims

1. A positive electrode sheet, characterized in that: comprising a positive electrode current collector and a positive electrode active layer disposed on at least one functional surface of the positive electrode current collector; The positive electrode active layer includes a first positive electrode active layer, a second positive electrode active layer and a third positive electrode active layer stacked in sequence in a direction away from the positive electrode current collector; The first positive electrode active layer includes a first lithium cobaltate, the second positive electrode active layer includes a second lithium cobaltate, and the third positive electrode active layer includes a third lithium cobaltate; The aluminum content of the third lithium cobaltate is greater than the aluminum content of the second lithium cobaltate, and the aluminum content of the first lithium cobaltate is greater than the aluminum content of the second lithium cobaltate; The aluminum content in the first lithium cobaltate is 6000-8000ppm; The aluminum content in the second lithium cobaltate is 4000-6000ppm; The aluminum content in the third lithium cobaltate is 8000-10000 ppm.

2. The positive electrode sheet according to claim 1, characterized in that The first positive electrode active layer further includes a first conductive agent, the second positive electrode active layer further includes a second conductive agent, and the third positive electrode active layer further includes a third conductive agent; The content of the second conductive agent in the second positive electrode active layer is greater than the content of the first conductive agent in the first positive electrode active layer; and / or the content of the second conductive agent in the second positive electrode active layer is greater than the content of the third conductive agent in the third positive electrode active layer.

3. The positive electrode sheet according to claim 2, characterized in that: The aluminum content in the third lithium cobaltate is greater than the aluminum content in the first lithium cobaltate; and / or, The content of the first conductive agent in the first positive electrode active layer is greater than the content of the third conductive agent in the third positive electrode active layer.

4. The positive electrode sheet according to claim 2, characterized in that The first positive electrode active layer further includes a first binder; Based on the total mass of the first positive electrode active layer, the mass percentage of the first lithium cobaltate is 92-98%, the mass percentage of the first conductive agent is 0.05-4%, and the mass percentage of the first binder is 0.1-4%; And / or, the second positive electrode active layer further includes a second binder; Based on the total mass of the second positive electrode active layer, the mass percentage of the second lithium cobaltate is 92-98%, the mass percentage of the second conductive agent is 0.05-4%, and the content of the second binder is 0.1-4%; And / or, the third positive electrode active layer further includes a third binder; Based on the total mass of the third positive electrode active layer, the mass percentage of the third lithium cobaltate is 92-98%, the mass percentage of the third conductive agent is 0.05-4%, and the content of the third binder is 0.1-4%.

5. The positive electrode sheet according to claim 2, characterized in that: The content of the second conductive agent in the second positive electrode active layer is 1.6-2 wt %.

6. The positive electrode sheet according to claim 2, characterized in that: The content of the first conductive agent in the first positive electrode active layer is 1.2-1.6 wt %; and / or, The content of the third conductive agent in the third positive electrode active layer is 0.8-1.2 wt %.

7. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: The thickness of the positive electrode active layer is 40-75 μm.

8. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: The surface density of the positive electrode active layer is 20-35 mg / cm 2 .

9. The positive electrode sheet according to claim 8, characterized in that: The ratio of the area density of the first positive electrode active layer, the area density of the second positive electrode active layer and the area density of the third positive electrode active layer is (10-40%): (10-40%): (10-40%).

10. A battery, characterized in that: The battery comprises the positive electrode sheet according to any one of claims 1 to 9.

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