Positive electrode sheet, battery, and electric device

Abstract

The application provides a positive electrode sheet, a secondary battery and an electric device. The positive electrode sheet provided by the application comprises a current collector, and a first active material layer and a second active material layer which are arranged on at least one side surface of the current collector in a laminated manner, the first active material layer is arranged close to the current collector, the first active material layer and the second active material layer independently comprise a positive electrode active material, the thickness of the first active material layer is H1 μm, and the thickness of the second active material layer is H2 μm; the positive electrode active material in the first active material layer satisfies 0 Span +D1 mo )*h1<2, and the positive electrode active material in the second active material layer satisfies 2 Span +D2 mo )*h2<6, wherein h1=H1 / (H1+H2) and h2=H2 / (H1+H2). The positive electrode sheet provided by the application can guarantee a high compaction density of the electrode sheet and a good liquid absorption capacity of the electrode sheet, and the prepared battery has excellent cycle performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] To improve the energy density of the battery, the method of improving the compaction density is generally adopted at present, thereby improving the volume energy density. With the increase of the compaction density, the liquid absorption capacity of the electrode sheet and the cycle performance of the battery will decrease. At present, although the methods such as vacuum injection, high-temperature aging and multiple injection can make the electrolyte gradually infiltrate the electrode sheet, the longer the path of electrolyte penetration is, the greater the compaction density is, and the more difficult the electrolyte is to fully infiltrate. SUMMARY

[0003] In view of this, the present application provides a positive electrode sheet, a secondary battery and an electric device. The positive electrode sheet provided by the present application can not only ensure the high compaction density of the electrode sheet, but also ensure the good liquid absorption capacity of the electrode sheet, and the battery prepared thereby has excellent cycle performance.

[0004] A first aspect of an embodiment of the present application provides a positive electrode sheet, comprising a current collector and a first active material layer and a second active material layer which are stacked on at least one side surface of the current collector, the first active material layer is arranged close to the current collector, the first active material layer and the second active material layer independently comprise a positive electrode active material, the thickness of the first active material layer is H1 μm, and the thickness of the second active material layer is H2 μm.

[0005] The positive electrode active material in the first active material layer satisfies 0 < (D1 Span + D1 mo )*h1 < 2,

[0006] The positive electrode active material in the second active material layer satisfies 2 < (D2 Span + D2 mo )*h2 < 6,

[0007] Wherein, D1 Span = (D1 V90 -D1 V10 ) / D1 V50 , D2 Span = (D2 V90 -D2 V10 ) / D2 V50 , D1 mo is the most frequent particle size of the positive electrode active material in the first active material layer, D2 mo is the most frequent particle size of the positive electrode active material in the second active material layer, h1 = H1 / (H1+H2), and h2 = H2 / (H1+H2).

[0008] The positive electrode tab provided in the present application can control the particle size and distribution of the material, thereby regulating the compaction density of the tab and optimizing the liquid absorption capacity of the tab, by reasonably designing and adjusting the particle size distribution of the positive electrode active material and regulating the thickness of the active material layer.

[0009] The first active material layer and the second active material layer in the positive electrode tab provided in the present application have different liquid absorption capacities and lithium ion migration rates. The electrolyte first contacts the second active material layer, and the electrolyte can be fully absorbed by the second active material layer, thereby ensuring that the tab has good liquid absorption capacity. As the path of the electrolyte becomes longer, the electrolyte can be gradually absorbed by the first active material layer, and the first active material layer is designed to have high liquid absorption capacity, so that the electrolyte can still be well absorbed, and the tab has strong liquid absorption capacity. Meanwhile, the compaction density of the first active material layer is lower than that of the second active material layer. The electrolyte first contacts the second active material layer, and the electrolyte is sufficient and the path is short, so that the tab has good liquid absorption capacity under high compaction density. As the path becomes longer, the compaction density of the tab is lower than that of the first active material layer, so that the tab has good liquid absorption capacity under the condition of less electrolyte.

[0010] Therefore, the positive electrode tab provided in the present application can ensure high compaction density and strong liquid absorption capacity of the tab, and the lithium ion battery prepared therefrom has excellent cycle performance.

[0011] The second aspect of the embodiment of the present application provides a secondary battery comprising the positive electrode tab provided in the first aspect of the embodiment of the present application. Since the positive electrode tab provided in the embodiment of the present application is used, the secondary battery can have good liquid absorption capacity and excellent cycle performance.

[0012] The third aspect of the embodiment of the present application provides a power consumption device comprising the secondary battery provided in the second aspect of the embodiment of the present application. Since the secondary battery provided in the embodiment of the present application is used to supply power, the power consumption device has strong endurance and strong market competitiveness. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 The structure schematic diagram of the cross section of the positive electrode tab provided in an embodiment of the present application is shown in the figure;

[0014] The figure number explanation: 1-positive electrode tab; 10-current collector; 20-first active material layer; 30-second active material layer. DETAILED DESCRIPTION

[0015] Under the background of rapid development of lithium-ion battery technology, improving battery performance to meet the growing demand for energy storage has become a research hotspot. In particular, in electric vehicles and large-scale energy storage systems, the energy density, power density, cycle stability, and safety of the battery are key indicators of battery performance. However, traditional battery design often struggles to balance these performance indicators, especially in the selection and preparation of positive electrode materials, where there are some contradictions and challenges.

[0016] In the background art, to improve the energy density of the battery, researchers usually adopt the method of increasing the compaction density of the positive electrode material, which can increase the content of active substances in a unit volume, but also leads to a decrease in the porosity of the electrode sheet, the permeability and liquid absorption capacity of the electrolyte, thereby affecting the migration of lithium ions and the charge and discharge performance of the battery. In addition, high compaction density can also exacerbate the volume expansion of the battery during charging and discharging, causing stress and cracking of the material structure, affecting the cycle stability and life of the battery.

[0017] To solve these problems, researchers have tried various methods, such as optimizing the composition, structure, and preparation process of the positive electrode material, using nanotechnology, carbon coating, ion doping, and other means to improve the electrochemical performance of the material. Although these methods have improved battery performance to some extent, they still face challenges in terms of cost, process complexity, and large-scale production.

[0018] See Figure 1 The embodiments of the present application provide a positive electrode sheet 1, comprising a current collector and a first active material layer and a second active material layer stacked on at least one side surface of the current collector, the first active material layer being arranged close to the current collector, the first active material layer and the second active material layer independently comprising a positive electrode active material, the thickness of the first active material layer being H1 μm, and the thickness of the second active material layer being H2 μm.

[0019] The positive electrode active material in the first active material layer satisfies 0 < (D1 Span + D1 mo ) * h1 < 2,

[0020] The positive electrode active material in the second active material layer satisfies 2 < (D2 Span + D2 mo ) * h2 < 6,

[0021] wherein D1 Span = (D1 V90 - D1 V10 ) / D1 V50 , and D2 Span = (D2 V90 - D2 V10 ) / D2 V50, D1 mo D2 mo is the most frequent particle size of the positive active material in the second active material layer, h1 = H1 / (H1+H2), h2 = H2 / (H1+H2).

[0022] In this application, D Span is the particle size distribution half-width of the positive active material, defined as D Span = (D V90 -D V10 ) / D V50 . Dv 90 , Dv 50 and Dv 10 are parameters of the particle size distribution of the positive material, representing the particle size at which the cumulative volume is 90%, 50% and 10%, respectively. Dv 90 90%: the particle size at which the cumulative volume is 90%, meaning that in the particle size distribution of the positive material, 90% of the particles have a diameter less than this value. Dv 50 50%: the particle size at which the cumulative volume is 50%, i.e. the median particle size or median diameter, representing the average particle size of the powder. Dv 10 10%: the particle size at which the cumulative volume is 10%, meaning that in the particle size distribution of the positive material, only 10% of the particles have a diameter less than this value. These parameters provide important information about the particle size distribution of the positive material, which is crucial for the physical and electrochemical properties of the reaction material. For example, the difference between Dv 90 and Dv 10 can reflect the uniformity of the material, while Dv 50 is often used to represent the average particle size of the powder, D Span reflects the width of the particle size distribution, i.e. the degree of dispersion of particle size, and D mo is the most frequent particle size, which is the particle size with the highest frequency of occurrence in the active material layer. In this application, for convenience of description and differentiation, the particle size distribution half-width of the positive active material in the first active material layer is denoted as D1 Span , the parameters of the positive material particle size distribution Dv 90 , Dv 50 and Dv 10 are denoted as D1v 90 , D1v 50 and D1v 10 , and the most frequent particle size is denoted as D1 mo ; the particle size distribution half-width of the positive active material in the second active material layer is denoted as D2 Span , the parameters of the positive material particle size distribution D2v 90 , D2v 50 and D2v 10 are denoted as D1v 90D1v 50 and D1v 10 D1v mo

[0023] The smaller the Dv50, the smaller the flatness of the particles, which helps to increase the specific surface area of the material, thereby enhancing the liquid absorption capacity, accelerating the migration of lithium ions, improving the electrochemical reaction rate, and reducing stress concentration, thereby improving the cycle stability and safety of the battery; the smaller D mo is associated with a higher specific surface area, which not only enhances the activity of the electrochemical reaction and the liquid absorption capacity, but also helps to improve the migration rate of lithium ions, thereby improving the charge and discharge performance of the battery; in addition, the consistency of D mo helps to improve the cycle stability of the battery and reduce the problems of structural stress and local overheating caused by inconsistent particle sizes. By adjusting appropriate Dv50, D mo , and D span , the size of the material particles and their matching can be controlled, and then dense packing between particles can be achieved, increasing the compaction density of the material and thereby improving the energy density of the battery.

[0024] The first positive electrode active material layer satisfies the condition 0 < (D1 Span + D1 mo )*h1 < 2, controls the combination of the particle size distribution and the most frequent particle size of the positive electrode active material in the first positive electrode active material layer, and the thickness ratio in the positive electrode sheet, i.e., the value of (D1 Span + D1 mo )*h1 is less than 2, at which time the first positive electrode active material layer has better liquid absorption capacity, which helps the penetration of the electrolyte and the migration of lithium ions. At this time, the first active material layer has a higher compaction density, which helps to improve the charge and discharge capacity of the battery. Optionally, the value of (D1 Span + D1 mo )*h1 can be one of 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.8.

[0025] The second positive electrode active material layer satisfies the condition 2 < (D2 Span + D2 mo )*h2 < 6, controls the combination of the particle size distribution and the most frequent particle size of the positive electrode active material in the second positive electrode active material layer, and the thickness ratio in the positive electrode sheet, i.e., the value of 2 < (D2 Span + D2 mo )*h2 < 6 is less than 2, at which time the second active material layer has a higher compaction density, which helps to improve the energy density of the battery. Optionally, the value of (D2 Span + D2 moThe value of h2 can be one of 2.2, 2.5, 2.7, 2.9, 3.2, 3.5, 3.7, 3.9, 4.1, 4.3, 4.5, 4.9, 5.1, 5.5, 5.7.

[0026] In the present application, by controlling D mo and the combination of D Span , in combination with the optimization of the thickness proportion of each layer of active material layer h1 and h2, the performance of the positive electrode material of the battery can be regulated, not only improving the liquid absorption capacity of the material, but also maintaining a relatively high compaction density of the positive electrode material, ensuring the performance of the battery under high energy density and high power density, and improving the cycle stability of the battery.

[0027] In the present application, the test method of D Span , D mo value can refer to GB / T 19077-2016 / ISO 13320:2009 Particle Size Distribution Laser Diffraction Method, and the test instrument is a laser particle size analyzer (such as Malvern 3000).

[0028] In the present application, the current collector of the positive electrode sheet can be aluminum foil, carbon-coated aluminum foil, or other materials that can be used as current collector.

[0029] In some embodiments of the present application, h1 < h2. The thickness of the second active material layer is greater than that of the first active material layer, which can provide better liquid absorption capacity and faster lithium ion migration, helping to improve the power performance and cycle stability of the battery.

[0030] In some embodiments of the present application, the thickness of the first active material layer satisfies 20 μm < H1 < 300 μm, and the thickness of the second active material layer satisfies 20 μm < H2 < 300 μm. In this way, the performance of the battery can be better exerted. Specifically, the thickness of the first active material layer can be but is not limited to 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 90 μm, 100 μm, 150 μm, 200 μm; and the thickness of the second active material layer can be but is not limited to 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 90 μm, 100 μm, 150 μm, 200 μm.

[0031] In some embodiments of the application, h2 / h1=(2-4). The first active material layer is close to the current collector, which is beneficial to improve the liquid absorption capacity and electrochemical reaction rate; while the thicker second active material layer provides additional storage capacity, while helping to structural stability and cycle life of the battery. Alternatively, the value of h2 / h1may be 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.6, 4.0.

[0032] In some embodiments of the application, 1 Span <6, 1 Span <6, 0.2 μm mo <3 μm, 0.2 μm mo <3 μm. In the present application, the D1 Span , D2 Span of the positive active material is selected to be between 1 and 6, and D1 mo , D2 mo is in the range of 0.2 μm to 3 μm, in order to comprehensively optimize the battery performance. The moderate D1 Span , D2 Span ensures the consistency of the particle size distribution, which helps to improve the cycle stability and reliability of the battery; while the appropriate D mo range ensures sufficient specific surface area, which promotes the activity of electrochemical reaction and liquid absorption capacity. This material selection balances the energy density and power density of the battery, while taking into account the cost-effectiveness, thermal management, mechanical strength and long-term stability of the battery.

[0033] In the present application, the positive active material has the formula LiFe 1-x M x PO4, wherein 0≤x Span and D mo and the control of the active thickness, the comprehensive performance of the battery is improved. In one embodiment, the positive active material is selected from lithium iron phosphate materials with or without doping.

[0034] In the present application, the first active material layer and the second active material layer each independently further comprise a conductive agent and a binder, and the mass ratio of the positive electrode active material, the conductive agent and the binder in the first active material layer is (85-99):(1-14):(1-14); and / or the mass ratio of the positive electrode active material, the conductive agent and the binder in the second active material layer is (85-99):(1-14):(1-14). The conductive agent is selected from at least one of conductive carbon black, graphite, conductive ink, graphene, ketjen black, acetylene black, carbon nanotube, conductive carbon fiber and other conductive carbon materials. The binder is selected from at least one of polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyolefin, SBR rubber, polyurethane and related modified materials. The positive electrode active material layer design proposed in the present application realizes the overall optimization of battery performance by adding a conductive agent and a binder in the first and / or second active material layer and controlling the mass ratio of the positive electrode active material, the conductive agent and the binder in the first and / or second active material layer to be within the range of (85-99):(1-14):(1-14). Such a composite ratio not only improves the electrical conductivity and structural stability of the electrode, but also improves the cycle life, ensures process compatibility and cost-effectiveness.

[0035] It can be understood that in the present application, at least one side surface of the current collector can also be provided with three or more active material layers, and any two adjacent active material layers meet the rules described in the present application, which are all within the protection scope of the present application.

[0036] In some embodiments of the present application, the preparation of the positive electrode sheet comprises: S01, forming a first active material layer and a second active material layer arranged in layers on the surface of the current collector. Specifically, the process of forming the positive electrode active material layer described above comprises coating, and the specific process of coating can be a process well known to those skilled in the art.

[0037] In the embodiments of the present application, the current collector can be any current collector suitable for the positive electrode of a lithium ion battery, for example, an aluminum foil or a carbon-coated aluminum foil.

[0038] In some cases, a multi-layer coating process, for example, a double-layer coating process, can be used. Specifically, a coating device that can simultaneously realize double-layer coating is used to simultaneously form the first active material layer and the second active material layer on the surface of the current collector. In other cases, the first active material layer and the second active material layer can also be sequentially coated on the surface of the current collector.

[0039] In some embodiments of the present application, the first active material layer slurry includes, but is not limited to, a first solvent, a first binder, a first conductive agent, and a first positive electrode active material. The second active material layer slurry includes, but is not limited to, a second solvent, a second binder, a second conductive agent, and a second positive electrode active material. The first solvent and the second solvent described above can be agents suitable for positive electrode slurry known in the art, including but not limited to N-methyl pyrrolidone (NMP).

[0040] The embodiments of the present application also provide a secondary battery including the aforementioned positive electrode sheet provided by the embodiments of the present application. Due to the adoption of the positive electrode sheet provided by the embodiments of the present application, the secondary battery can have both high energy density and excellent cycle performance.

[0041] In the embodiments of the present application, the secondary battery can be a liquid secondary battery using a liquid electrolyte, a solid-state secondary battery using a solid-state electrolyte, a semi-solid secondary battery using a gel-state electrolyte, or a semi-solid secondary battery including both a solid-state electrolyte and a liquid electrolyte.

[0042] The embodiments of the present application also provide an electric device including the secondary battery provided by the embodiments of the present application. Due to the adoption of the secondary battery provided by the embodiments of the present application, the electric device has strong endurance and strong market competitiveness.

[0043] In some embodiments of the present application, the electric device includes, but is not limited to, a 3C electronic device, a power vehicle, an energy storage system, etc. The power vehicle includes, but is not limited to, a new energy vehicle, a power-assisted bicycle, etc.

[0044] The technical solutions of the present application are further described in the following embodiments.

[0045] Embodiment 1

[0046] (1) Screen different D span and different D mo positive electrode active materials, and add the positive electrode active material lithium iron phosphate, the conductive agent, and the binder according to the mass ratio of 90:5:5 into the solvent N-methyl pyrrolidone (NMP) to uniformly stir to obtain a positive electrode slurry;

[0047] (2) The positive electrode slurry is coated into a positive electrode sheet by using a coating machine;

[0048] (3) The first active material lithium iron phosphate and the second active material lithium iron phosphate are both prepared according to the above method. The thickness of the positive electrode sheet is 150 μm;

[0049] (4) The positive electrode sheet and the negative electrode sheet are rolled, and the positive electrode sheet can have a compaction density of 2.73 g / cm3;

[0050] (5) The positive electrode sheet, the negative electrode sheet (graphite: conductive agent carbon black: binder SBR = 90:5:5) are rolled, cut, and then the positive electrode sheet, the separator film, and the negative electrode sheet are stacked in order. The stacked core is baked and assembled to form a soft package battery. Then, the battery is prepared by liquid injection (the electrolyte is a lithium hexafluorophosphate organic solution with a concentration of 1.0 mol / L, wherein the solvent is EC: DMC: EMC with a mass ratio of 1:1:1), formation, aging, and sealing;

[0051] (5) The liquid absorption time of the positive electrode sheet and the cycle capacity retention rate of the battery are tested.

[0052] Examples 2-7

[0053] The example 1 is basically the same, except that the related parameters of the active material and the electrode sheet compaction are different. The specific parameters are shown in Table 1.

[0054] Comparative Examples 1-2

[0055] The example 1 is basically the same, except that the related parameters of the active material and the electrode sheet compaction are different. The specific parameters are shown in Table 1.

[0056] Performance test

[0057] Test of compaction density:

[0058] The coated electrode sheet is rolled in a roller press with a pressure of 1 MPa. A 30 mm sampler is used to cut small round pieces. The mass and thickness are measured, and the compaction density is calculated.

[0059] Liquid absorption time:

[0060] The positive electrode sheet is placed in a glove box, and 0.02 ml of electrolyte is injected into the center of the electrode sheet using a disposable syringe. Timing starts until the electrolyte is completely absorbed (no excess electrolyte is observed on the surface), and the time is recorded as the liquid absorption time.

[0061] Cycle capacity retention rate:

[0062] The prepared battery is tested for charge and discharge at 25°C. The battery is charged at 1C constant current to 3.8V, and then charged at constant voltage of 3.8V to the cutoff current of 0.2C. The battery is discharged at 1C constant current to 2V. The capacity retention rate is calculated after 1000 cycles according to the above test system. The average value of 2 batteries is taken.

[0063] Table 1 Active material and electrode sheet parameters

[0064]

[0065] Table 2 Liquid absorption and cycle test results

[0066] Examples Liquid absorption time (s) Cycle retention rate @ 1000 times Example 1 369 95.20％ Example 2 395 94.50％ Example 3 425 92.00％ Example 4 498 90.30％ Example 5 560 89.20％ Example 6 632 87.70％ Example 7 785 85.40％ Comparative Example 1 930 80.20％ Comparative Example 2 1050 79.00％

[0067] According to the test data in Table 1 and Table 2, when the positive active material in the first active material layer satisfies 0 < (D1 Span + D1 mo )*h1< 2, and the positive active material in the second active material layer satisfies 2 < (D2 Span + D2 mo )*h2< 6, the liquid absorption capacity of the electrode sheet and the battery cycle performance are excellent.

[0068] Meanwhile, through the comparison of Examples 1-6 and Example 7, it can be found that when h1< h2, the liquid absorption capacity of the electrode sheet and the battery cycle performance are further improved. Further, when h2 / h1 = (2-4), the liquid absorption capacity of the electrode sheet and the battery cycle performance are better. In the present application, by controlling D1 mo , D2 mo and the combination of D1 Span , D2 Span , combined with the optimization of the thickness ratio h1 and h2 of each active material layer, the performance of the battery positive material can be controlled, not only the liquid absorption capacity of the material is improved, but also the positive material has a higher compaction density, which ensures the performance of the battery under high energy density and high power density, and improves the cycle stability of the battery.

[0069] The above is an exemplary embodiment of the present application. It should be pointed out that for ordinary skilled in the art, without departing from the principles of the present application, some improvements and refinements can be made, which are also considered within the protection scope of the present application.

Claims

1. A positive electrode sheet, characterized by, The positive electrode plate comprises a current collector and first and second active material layers laminated on at least one side surface of the current collector, the first active material layer being disposed close to the current collector, the first and second active material layers independently comprising a positive electrode active material, the thickness of the first active material layer being H1 μm, and the thickness of the second active material layer being H2 μm; The positive active material in the first active material layer satisfies 0 < (D1 Span + D1 mo )*h1 < 2. The positive active material in the second active material layer satisfies 2 < (D2 Span + D2 mo )* h2 < 6. wherein D1 Span = (D1 V90 - D1 V10 ) / D1 V50 , D2 Span = (D2 V90 - D2 V10 ) / D2 V50 , D1 mo is the most frequent particle diameter of the positive active material in the first active material layer, D2 mo is the most frequent particle diameter of the positive active material in the second active material layer, h1 = H1 / (H1 + H2), h2 = H2 / (H1 + H2).

2. The cathode electrode of claim 1, wherein, h1 < h2.

3. The cathode electrode of claim 2, wherein, h2 / h1 = (2-4).

4. The positive electrode sheet according to any one of claims 1 to 3, characterized by, 20 μm < H1 < 300 μm, 20 μm < H2 < 300 μm.

5. The positive electrode sheet according to any one of claims 1 to 3, characterized by Satisfy: 1 < D1 Span <6, 1 < D2 Span <6, 0.2 μm < D1 mo <3 μm, 0.2 μm < D2 mo <3 μm.

6. The positive electrode plate of any one of claims 1-3, wherein, LiFe 1-x M x PO4, wherein 0≤x<1, M is selected from one or more of Mn, Mg, Co, Ni, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, or Ti.

7. The positive electrode plate of any one of claims 1-3, wherein, The positive electrode active material is lithium iron phosphate.

8. The positive electrode plate of any one of claims 1-3, wherein, The positive electrode plate further comprises a conductive agent and a binder, and in the first active material layer, the mass ratio of the positive electrode active material, the conductive agent and the binder is (85-99):(1-14):(1-14). And / or, in the second active material layer, the mass ratio of the positive electrode active material, the conductive agent and the binder is (85-99):(1-14):(1-14).

9. A secondary battery characterized by comprising: The positive electrode plate according to any one of claims 1-8.

10. An electric device, characterized by The secondary battery according to claim 9.

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