Battery, positive electrode sheet and preparation method therefor, and electrical apparatus

Abstract

A battery cell (5), a positive electrode sheet (531) and a preparation method therefor, and an electrical apparatus (6). The battery cell (5) comprises the positive electrode sheet (531). The positive electrode sheet (531) comprises: a positive electrode current collector (5311); a first positive electrode active layer (5312) disposed on at least one surface of the positive electrode current collector (5311), the first positive electrode active layer (5312) comprising ternary material A and a lithium iron phosphate material, ternary material A having a Dv50 particle size greater than the Dv50 particle size of the lithium iron phosphate material; and / or, ternary material A having a Dn50 particle size greater than the Dv50 particle size of the lithium iron phosphate material; and a second positive electrode active layer (5313) disposed on the surface of the first positive electrode active layer (5312) away from the positive electrode current collector (5311), the second positive electrode active layer (5313) comprising ternary material B. The positive electrode sheet (531) is applied to the battery cell (5), and can increase the energy density of the battery cell (5) and improve the cycle performance and power stability of the battery cell (5).

Description

Battery, positive electrode sheet, preparation method thereof and electric device

[0001] Related Applications

[0002] The present application claims priority to the Chinese patent application No. 2024113473149, filed on September 26, 2024, and entitled "Battery, positive electrode sheet, preparation method thereof and electric device", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present application relates to the technical field of batteries, in particular to a battery, a positive electrode sheet, a preparation method thereof and an electric device. BACKGROUND

[0004] In recent years, secondary batteries such as lithium ion batteries have developed rapidly, and thus their application range is becoming more and more extensive. For example, secondary batteries such as lithium ion batteries can be widely used in energy storage power supply systems of hydraulic, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc., and thus higher requirements are put forward for their energy density, cycle performance and power performance stability. SUMMARY

[0005] Therefore, it is necessary to provide a battery, a positive electrode sheet, a preparation method thereof and an electric device, which aims to improve the energy density, cycle performance and power performance stability of the battery.

[0006] The present application is realized by the following technical solutions.

[0007] In a first aspect, the present application provides a battery comprising a positive electrode sheet, wherein the positive electrode sheet comprises:

[0008] a positive current collector;

[0009] a first positive active layer disposed on at least one surface of the positive current collector, the first positive active layer comprising a ternary material A and a lithium iron phosphate material, wherein the Dv50 particle size of the ternary material A is greater than the Dv50 particle size of the lithium iron phosphate material, and / or the Dn50 particle size of the ternary material A is greater than the Dn50 particle size of the lithium iron phosphate material; and

[0010] a second positive active layer disposed on a surface of the first positive active layer away from the positive current collector, the second positive active layer comprising a ternary material B;

[0011] wherein the ternary material A and the ternary material B both comprise nickel element, and the molar content of the nickel element in the ternary material A in the total amount of ternary metals is higher than the molar content of the nickel element in the ternary material B in the total amount of ternary metals.

[0012] The ternary material has the advantage of high energy density when applied in the battery, but has some problems at low and medium SOC (state of charge), including large cycle expansion force and obvious power performance deterioration caused by increased cycle resistance DCR.

[0013] Therefore, the application uses a two-layer positive active layer structure design and a lithium iron phosphate and ternary material mixed system. At a high SOC, mainly the ternary material provides lithium ions for discharge, and at a low SOC, mainly the lithium iron phosphate provides lithium ions for discharge. The second positive active layer in the upper layer uses ternary material B to provide good energy density, and the lithium ion transmission speed of the ternary material is fast, so the ternary material B in the second positive active layer in the upper layer can provide good cycle performance. On this basis, the first positive active layer in the lower layer uses ternary material A with a larger particle size and lithium iron phosphate material with a smaller particle size. The mixture of two different particle sizes and different types of positive active materials can improve the compaction density of the first positive active layer. In this way, the lower layer and the upper layer have good compaction density matching, so as to exert the energy density advantage of the ternary material. At the same time, the first positive active layer in the lower layer is added with lithium iron phosphate. The lithium iron phosphate has an olivine structure, and its crystal structure is stable and not easy to collapse, so the material loss is small. Therefore, the cycle resistance of the first positive active layer in the lower layer is low. In addition, the voltage platform of the lithium iron phosphate is lower, and the power performance of the battery at a low and medium SOC is obviously improved due to the elongation of the Fe platform, which makes up for the disadvantage of the rapid increase of DCR of the battery using a single ternary material at a low SOC.

[0014] The ternary material with a higher nickel content has a higher specific capacity. The increase of the nickel content can improve the reversible lithium intercalation capacity of the ternary material, thereby improving the energy density of the battery. However, the increase of the nickel content of the ternary material increases the alkalinity, increases the water absorption, increases the side reaction, and also increases the DCR. Further arranging the ternary material A with a higher nickel content in the first positive active layer in the lower layer can make up for the problem of the increase of the DCR caused by the higher nickel content of the ternary material A, so as to improve the energy density while taking into account the good power performance. The ternary material B with a lower nickel content has a better high-voltage resistance advantage, and has fewer side reactions compared with the high-nickel ternary material, which is beneficial to reduce the DCR of the battery.

[0015] Therefore, the above positive electrode sheet can be applied to the battery, so that the battery has good energy density, cycle performance and power performance stability.

[0016] In some embodiments, at least one of the following characteristics is met:

[0017] (1) the Dv50 particle size of the ternary material A is 2 μm to 5.2 μm, and / or the Dn50 particle size of the ternary material A is 2 μm to 7.2 μm;

[0018] (2) the ternary material A comprises polycrystal particles;

[0019] (3) the difference between the Dv50 particle size of the ternary material A and the Dv50 particle size of the lithium iron phosphate material is 0.8 μm to 4 μm.

[0020] The Dv50 particle size or the Dn50 particle size of the ternary material A is within the above range, so that the gap between the ternary material particles in the first positive electrode active layer is within a more appropriate range, the impedance of lithium ions inside the positive electrode active material particles is lower and the positive electrode active layer has a suitable compactness, thereby improving the conductivity and compactness of the positive electrode active layer, so that the battery can further have good energy density, cycle performance and power performance.

[0021] In some embodiments, the Dv50 particle size of the ternary material A is 3 μm to 4.2 μm, and / or the Dn50 particle size of the ternary material A is 3 μm to 6.2 μm.

[0022] In some embodiments, at least one of the following characteristics is satisfied:

[0023] (1) the Dv50 particle size of the lithium iron phosphate material is 0.8 μm to 1.8 μm, and / or the Dn50 particle size of the lithium iron phosphate material is 0.8 μm to 3.8 μm;

[0024] (2) the lithium iron phosphate material comprises single crystal particles;

[0025] (3) the lithium iron phosphate material comprises one or more of lithium iron phosphate bulk particles, lithium iron phosphate bulk particles coated by a coating layer and a doped modification material of each thereof.

[0026] The Dv50 particle size or the Dn50 particle size of the lithium iron phosphate material is within the above range, and is graded with the ternary material A of the above particle size range, so that the small particle size lithium iron phosphate material is more fully filled in the gap of the large particles, which can further improve the compactness of the lower layer, so that the compactness between the lower active layer and the upper active layer is better matched. In addition, the Dv50 particle size of the lithium iron phosphate material is within the above smaller Dv50 range, Li + The transmission path is smaller, which can improve the conductivity of the first positive electrode active layer, improve the kinetic performance of the battery, especially the power performance at low and medium SOC, improve the power performance of the battery, and thus make the battery have good energy density, cycle performance and power performance.

[0027] The lithium iron phosphate material includes single-crystal particles. Since the crystal structure of the single-crystal particles is perfect or defects are few, the migration resistance of lithium ions in the single-crystal particles is small, so that the solid-phase impedance can be reduced, and the power performance of the battery is improved.

[0028] In some embodiments, the Dv50 particle size of the lithium iron phosphate material is 1.0 μm to 1.5 μm, and / or the Dn50 particle size of the lithium iron phosphate material is 1 μm to 3.5 μm.

[0029] In some embodiments, the mass ratio of the lithium iron phosphate material in the total amount of the lithium iron phosphate material and the ternary material A is 50% to 95%.

[0030] In some embodiments, the mass ratio of the lithium iron phosphate material in the total amount of the lithium iron phosphate material and the ternary material A is 60% to 80%.

[0031] The mass ratio of the lithium iron phosphate material in the above range makes more lithium iron phosphate material more fully filled in the gap of large particles, and more lithium iron phosphate material plays a role in reducing DCR at low SOC in the life cycle, thereby improving the power performance of the battery.

[0032] In some embodiments, at least one of the following characteristics is met:

[0033] (1) The Dv50 particle size of the ternary material B is 4 μm to 13 μm, and / or the Dn50 particle size of the ternary material B is 4 μm to 15 μm.

[0034] (2) The ternary material B includes single-crystal particles.

[0035] The Dv50 particle size or Dn50 particle size of the ternary material B in the upper second positive electrode active layer is in the above range, so that the gap between the ternary material particles in the second positive electrode active layer is in a more appropriate range, the impedance of lithium ions in the positive electrode active material particles is low and has a suitable compaction density, thereby improving the conductivity and compaction density of the second positive electrode active layer, so that the battery can further have good energy density, cycle performance and power performance; In addition, the battery can also achieve 4C fast charging performance while having good cycle life.

[0036] In some embodiments, the Dv50 particle size of the ternary material B is 8 μm to 11 μm, and / or the Dn50 particle size of the ternary material B is 8 μm to 13 μm.

[0037] In some embodiments, at least one of the following characteristics is met:

[0038] (1) the thickness of the first positive electrode active layer is in a range from 15 pm to 150 pm;

[0039] (2) the thickness of the second positive electrode active layer is in a range from 20 pm to 200 pm.

[0040] In some embodiments, the thickness of the first positive electrode active layer is less than the thickness of the second positive electrode active layer.

[0041] In this way, the second positive electrode active layer is the main layer and the first positive electrode active layer is the auxiliary layer. The second positive electrode active layer of the upper layer is the main active layer, which provides the main performance of high energy density and cycle performance of the battery cell. The first positive electrode active layer of the lower layer is the auxiliary layer. In the case of discharging at a low SOC, the main lithium intercalation process is in the first positive electrode active layer of the lower layer because the second positive electrode active layer of the upper layer is close to the full intercalation state. The kinetics of the first positive electrode active layer of the lower layer directly affects the low SOC power of the battery cell. Therefore, the thickness of the first positive electrode active layer of the lower layer is controlled to be small, and the kinetics of the first positive electrode active layer of the lower layer is improved. With the use of the battery, the SOC kinetics advantage (low impedance) of the positive electrode plate will become more and more obvious, reducing the DCR at medium and low SOC in the life cycle, and thus improving the power performance of the battery.

[0042] In some embodiments, at least one of the following characteristics is satisfied:

[0043] (1) the thickness of the first positive electrode active layer is in a range from 30 pm to 60 pm;

[0044] (2) the thickness of the second positive electrode active layer is in a range from 70 pm to 100 pm.

[0045] In some embodiments, the ternary material A and the ternary material B each independently include at least one of a nickel-cobalt-manganese ternary material and a nickel-cobalt-aluminum ternary material.

[0046] In some embodiments, the nickel-cobalt-manganese ternary material includes LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.5 Co 0.25 Mn 0.25 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.68 Co 0.10 Mn 0.22 O2, LiNi 0.8 Co 0.1Mn 0.1 O2and LiNi 0.9 Co 0.05 Mn 0.05 O2;

[0047] The nickel-cobalt-aluminum ternary material includes LiNi 0.8 Co 0.15 Al 0.05 O2.

[0048] In some embodiments, at least one of the following characteristics is satisfied:

[0049] (1) the molar content of nickel element in the ternary material A in the total amount of ternary metals is 33% to 90%;

[0050] (2) the molar content of nickel element in the ternary material B in the total amount of ternary metals is 33% to 90%.

[0051] In some embodiments, at least one of the following characteristics is satisfied:

[0052] (1) the molar content of nickel element in the ternary material A in the total amount of ternary metals is 70% to 90%;

[0053] (2) the molar content of nickel element in the ternary material B in the total amount of ternary metals is 33% to 50%.

[0054] In a second aspect of the present application, a positive electrode sheet is provided, comprising:

[0055] a positive electrode current collector;

[0056] a first positive electrode active layer provided on at least one surface of the positive electrode current collector, the first positive electrode active layer comprising a ternary material A and a lithium iron phosphate material, the Dv50 particle size of the ternary material A being greater than the Dv50 particle size of the lithium iron phosphate material, and / or the Dn50 particle size of the ternary material A being greater than the Dn50 particle size of the lithium iron phosphate material; and

[0057] a second positive electrode active layer provided on the surface of the first positive electrode active layer away from the positive electrode current collector, the second positive electrode active layer comprising a ternary material B;

[0058] wherein the ternary material A and the ternary material B both comprise a nickel element, and the molar content of the nickel element in the ternary material A in the total amount of ternary metals is higher than the molar content of the nickel element in the ternary material B in the total amount of ternary metals.

[0059] In a third aspect, a method for preparing a positive electrode tab is provided. The positive electrode tab is the positive electrode tab provided in the first aspect. The method comprises the following steps:

[0060] forming the first positive electrode active layer on at least one surface of the positive electrode current collector;

[0061] forming the second positive electrode active layer on a surface of the first positive electrode active layer away from the positive electrode current collector.

[0062] In a fourth aspect, a power-using device is provided. The power-using device comprises at least one of the battery provided in the first aspect and the positive electrode tab provided in the second aspect.

[0063] The power-using device provided in the present application comprises the secondary battery provided in the present application, and thus has at least the same advantages as the secondary battery. Details of one or more embodiments of the present application are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the application will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF DRAWINGS

[0064] For better describing and illustrating the embodiments or examples provided in the present application, one or more drawings can be referred to. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any one of the disclosed applications, the presently described embodiments or examples, and the best mode presently contemplated of these applications. Moreover, the same reference numbers are used throughout the drawings to represent the same components. In the drawings:

[0065] FIG. 1 is a schematic view of a positive electrode tab according to an embodiment of the present application.

[0066] FIG. 2 is a schematic view of a battery cell according to an embodiment of the present application.

[0067] FIG. 3 is an exploded view of the battery cell shown in FIG. 2 according to an embodiment of the present application.

[0068] FIG. 4 is a schematic view of a battery module according to an embodiment of the present application.

[0069] FIG. 5 is a schematic view of a battery pack according to an embodiment of the present application.

[0070] FIG. 6 is an exploded view of the battery pack shown in FIG. 5 according to an embodiment of the present application.

[0071] FIG. 7 is a schematic view of a power-using device using a secondary battery as a power source according to an embodiment of the present application.

[0072] FIG. 8 is a cross-sectional scanning electron microscope image of a positive electrode tab prepared in Example 2 of the present application.

[0073] Label explanation: 1, battery pack; 2, upper box body; 3, lower box body; 4, battery module; 5, battery monomer; 51, shell; 52, electrode assembly; 53, cover plate; 531, positive plate; 5311, positive current collector; 5312, first positive active layer; 5313, second positive active layer; 6, electric device. DETAILED DESCRIPTION

[0074] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0075] The "range" disclosed in the present application can be limited in the form of lower limit and upper limit, a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit limit the boundary of the particular range. The range limited in this way can include or not include the end value, either end value can be independently included or not included, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if the ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges of 60-110 and 80-120 are also expected. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4 and 5 are also listed, the following ranges can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "a-b" represents a shorthand representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed herein, and "0-5" is only a shorthand representation of these numerical combinations. In addition, when it is stated that a parameter is an integer ≥2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when it is stated that a parameter is an integer selected from "2-10", it is equivalent to list the integers 2, 3, 4, 5, 6, 7, 8, 9 and 10.

[0076] In the present application, "multiple", "various" and the like are referred to without special limitation, which means greater than or equal to 2 in quantity. For example, "one or more" means one or more than two.

[0077] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0078] Reference to an "embodiment" herein means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment or implementation of the application. The appearances of the phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are they necessarily mutually exclusive of one another. Those skilled in the art will appreciate from the present description that embodiments described herein can be combinable with other embodiments. Reference to an "implementation" herein has a similar understanding.

[0079] Those skilled in the art can understand that, in the method of each implementation or embodiment, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process, and the detailed execution order of each step should be determined according to its function and possible internal logic. If not specifically stated, all steps of the application can be performed in sequence or randomly, and preferably in sequence. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method also includes step (c), which means that step (c) can be added to the method in any order, for example, the method can include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0080] In the present application, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0081] In the present application, "optionally", "optional" and "optional" mean that it can or can not be present, that is, it means to select from any one of the two parallel schemes of "have" or "have". If there are multiple "options" in a technical solution, if there is no special statement and no contradictory relationship or mutual restriction, each "option" is independent.

[0082] Current batteries are difficult to balance high energy density, cycle performance and power performance.

[0083] Ternary lithium batteries are significantly superior to lithium iron phosphate batteries in terms of energy density. Because ternary materials have higher energy density, ternary lithium batteries can provide longer driving range for electric vehicles. This is an important consideration for electric vehicles that pursue high performance and long driving range. However, ternary materials have the problem of large cycle expansion force at low and medium SOC (state of charge) and increasing cycle impedance DCR, which significantly deteriorates the stability of power performance.

[0084] Therefore, how to make the battery have good energy density, cycle performance and power performance stability is a technical problem that needs to be solved.

[0085] Based on this, the application provides a battery, a positive electrode sheet, a preparation method thereof, and a power utilization device, aiming to improve the energy density, cycle performance, and power performance stability of the battery.

[0086] The battery, the positive electrode sheet, and the power utilization device of an embodiment of the application are described below with appropriate reference to the accompanying drawings.

[0087] In some embodiments, the battery is a secondary battery, further a lithium ion battery.

[0088] Generally, the secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charging and discharging of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly playing a role in preventing the short circuit of the positive and negative electrodes, while allowing ions to pass through.

[0089] Positive electrode sheet

[0090] The positive electrode sheet includes a positive current collector, a first positive active layer, and a second positive active layer. The first positive active layer is arranged on at least one surface of the positive current collector, and the first positive active layer includes a ternary material A and a lithium iron phosphate material, the Dv50 particle size of the ternary material A is greater than the Dv50 particle size of the lithium iron phosphate material; and / or, the Dn50 particle size of the ternary material A is greater than the Dn50 particle size of the lithium iron phosphate material. The second positive active layer is arranged on the surface of the first positive active layer away from the positive current collector, and the second positive active layer includes a ternary material B.

[0091] Further, the ternary material A and the ternary material B both include nickel elements, and the molar content of the nickel elements in the ternary material A in the total amount of ternary metals is higher than the molar content of the nickel elements in the ternary material B in the total amount of ternary metals.

[0092] In the positive electrode sheet, the types of the ternary material A and the ternary material B can be the same or different.

[0093] Dv50 is a meaning known in the art, which can be tested by a method known in the art. For example, a laser particle size analyzer (such as Malvern Master Size 3000) is used for determination. Among them, Dv50 represents the particle size corresponding to the cumulative volume percentage of particles of 50% according to the particle size volume distribution, from small particle size.

[0094] The particle size volume distribution can be tested by the following method: take a clean beaker, add an appropriate amount of sample to be tested, and ultrasonic to ensure that the sample is completely dispersed. The testing instrument is Malvern 2000, USA. After the sample is poured into the sample tower and circulated to the test optical system with the solution, the particles are irradiated by the laser beam, and the particle size distribution characteristics can be obtained by receiving and measuring the energy distribution of the scattered light (light intensity: 8-12%). The particle size volume distribution graph is drawn according to the test data.

[0095] It should be noted that in the preparation process of the positive electrode sheet in the present application, the Dv50 particle size of the ternary material A can be controlled to be larger than the Dv50 particle size of the lithium iron phosphate material. When the Dv50 particle size of the ternary material A and the Dv50 particle size of the lithium iron phosphate material are tested in reverse in the battery or the positive electrode sheet, the positive electrode active material can be obtained by peeling off the active layer in the positive electrode sheet and dissolving other materials with a solvent, and then the Dv50 particle size can be measured by a laser particle size analyzer (such as Malvern Master Size 3000). The Dv50 particle size of the positive electrode active material can also be tested by the above-mentioned Dv50 method.

[0096] The Dn50 particle size (number average particle size) is a well-known meaning in the art, which can be tested by known methods in the art. For example, a laser particle size analyzer (such as Malvern Master Size 3000) is used for measurement. Among them, Dn50 represents the average particle size in terms of quantity, which is obtained by multiplying the number of particles of each particle size by its particle size, then adding all the products, and then dividing by the total number of particles, thereby obtaining the average particle size weighted by quantity.

[0097] The Dn50 particle size (number average particle size) can be tested by the following method: take a clean beaker, add an appropriate amount of sample to be tested, and ultrasonic to ensure that the sample is completely dispersed. The testing instrument is Malvern 2000, USA. After the sample is poured into the sample tower and circulated to the test optical system with the solution, the particles are irradiated by the laser beam, and the number average particle size of the particles can be obtained by receiving and measuring the energy distribution of the scattered light (light intensity: 8-12%).

[0098] It should be noted that in the preparation process of the positive electrode sheet in the present application, the Dn50 particle size of the ternary material A can be controlled to be larger than the Dn50 particle size of the lithium iron phosphate material. When the Dv50 particle size of the ternary material A and the Dv50 particle size of the lithium iron phosphate material are tested in reverse in the battery or the positive electrode sheet, the positive electrode active material can be obtained by peeling off the active layer in the positive electrode sheet and dissolving other materials with a solvent, and then the Dv50 particle size can be measured by a laser particle size analyzer (such as Malvern Master Size 3000). The Dv50 particle size of the positive electrode active material can also be tested by the above-mentioned Dv50 method.

[0099] For the detection of the Dn50 particle size (number average particle size) of the positive electrode active material in the positive electrode sheet in the battery, the following method can be used:

[0100] The positive electrode sheet is ion-milled to obtain a cross-section (a cross-section obtained by slicing in the thickness direction), and the cross-section is subjected to optical photograph testing such as SEM (scanning electron microscope). The cross-sections of the first positive electrode active layer and the second positive electrode active layer are respectively subjected to the following tests:

[0101] The positive electrode active material particles in any 5 regions of the same size and shape (actual size: 15 pm x 30 pm square sampling area) in the same scanning electron microscope (SEM) image at 10 kv, 30 k magnification are observed. In this detection process, the characteristic elements of the positive electrode active material particles are detected by EDS to identify whether the positive electrode active material particles are ternary materials or lithium iron phosphate.

[0102] The particle size of each positive electrode active material particle (ternary material or lithium iron phosphate) in the square sampling area is detected, wherein the particle size of each positive electrode active material particle (ternary material or lithium iron phosphate) refers to the longest diameter distance of the particle. According to this, the number of particles of each particle size of the ternary material or lithium iron phosphate in the square sampling area is multiplied by the particle size, then all the products are added, and then divided by the total number of particles, so as to obtain the average particle size weighted by the number, that is, the Dn50 particle size of the ternary material or lithium iron phosphate in the region. The Dn50 particle sizes of the 5 regions are averaged to obtain the Dn50 particle size of the ternary material or lithium iron phosphate in the positive electrode sheet.

[0103] Therefore, the application can provide good energy density by using the ternary material B in the second positive electrode active layer on the upper layer, and the lithium ion transmission speed of the ternary material is relatively fast, and the ternary material B in the second positive electrode active layer on the upper layer can provide good cycle performance; on this basis, the first positive electrode active layer on the lower layer uses the ternary material A with a larger particle size and the lithium iron phosphate material with a smaller particle size, and the mixing and grading of the two different particle sizes and different types of positive electrode active materials can improve the compaction density of the first positive electrode active layer, so that the compaction density between the lower layer and the upper layer of the positive electrode active layer is matched, so as to exert the energy density advantage of the ternary material, and the first positive electrode active layer on the lower layer is added with lithium iron phosphate. The olivine structure of lithium iron phosphate is stable and not easy to collapse, so the material loss is caused. Therefore, the cycle impedance of the first positive electrode active layer on the lower layer is low, and the power performance of the battery in the low SOC state is obviously improved due to the elongation of the Fe platform, which makes up for the disadvantage of the rapid increase of DCR of the battery using a single ternary material in the low SOC state. The ternary material with a higher nickel content has a higher specific capacity. The increase of nickel content can improve the reversible lithium intercalation capacity of the ternary material, thereby improving the energy density of the battery. However, the increase of the nickel content of the ternary material increases the alkalinity, increases the water absorption, increases the side reaction, and also increases the DCR. Further, the ternary material A with a higher nickel content is arranged in the first positive electrode active layer on the lower layer, which can make up for the problem of the increase of DCR caused by the higher nickel content of the ternary material A, so as to improve the energy density while considering the good power performance. The ternary material B with a lower nickel content has a better high-voltage resistance advantage, and has fewer side reactions than the high-nickel ternary material, which is beneficial to reduce the DCR of the battery.

[0104] Therefore, the above-mentioned positive electrode sheet can be applied to the battery, so that the battery has good energy density, cycle performance and power performance stability.

[0105] As a non-limiting example, the positive electrode current collector has two opposite surfaces in the thickness direction of itself, and the positive electrode active layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector.

[0106] In the application, the Dv50 particle size of the ternary material A is greater than the Dv50 particle size of the lithium iron phosphate material, and the Dn50 particle size of the ternary material A is greater than the Dn50 particle size of the lithium iron phosphate material, which can obtain similar technical effects.

[0107] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be obtained by forming a metal material on a polymer material base material. Non-limiting examples of the metal material in the positive current collector can include one or more of aluminum, an aluminum alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, a silver alloy, and the like. Non-limiting examples of the polymer material base material in the positive current collector can include one or more of a polypropylene (PP) base material, a polyethylene terephthalate (PET) base material, a polybutylene terephthalate (PBT) base material, a polystyrene (PS) base material, a polyethylene (PE) base material, and the like.

[0108] Referring to FIG. 1, a positive electrode tab 531 of an embodiment includes a positive current collector 5311, a first positive active layer 5312, and a second positive active layer 5313. The first positive active layer 5312 is disposed on both surfaces of the positive current collector 5311, and the second positive active layer 5313 is disposed on both sides of the first positive active layer 5312 away from the surfaces of the positive current collector 5311. It can be understood that the first positive active layer 5312 and the second positive active layer 5313 described above can be disposed only on one surface of the positive current collector 5311.

[0109] The larger the particle size of the positive active material in the positive active layer means that the gap between the positive active material particles in the positive active layer increases, the migration resistance of lithium ions inside the positive active material particles is greater, thereby increasing the solid-phase impedance of the positive active layer; in addition, the increase in the gap between the positive active material particles also reduces the compaction density of the tab, thereby affecting the energy density of the battery. Smaller particle size of the positive active material generally helps to shorten the diffusion path of lithium ions, reduce the migration resistance of lithium ions inside the positive active material particles, thereby reducing the solid-phase impedance, and improving the conductivity of the positive active layer and the power performance of the battery. However, the smaller the particle size of the positive active material means that the specific surface of the positive active material particles increases, the probability of side reactions of the functional groups on the surface of the positive active material particles increases, which also leads to an increase in the DCR (direct current resistance) of the battery. Therefore, the particle size of the positive active material particles in the positive active layer needs to be controlled within a suitable range, so that the impedance of lithium ions inside the positive active material particles is low and the positive active layer has a suitable compaction density, thereby improving the conductivity of the positive active layer and the compaction density, so that the battery can further have good energy density, cycle performance, and power performance.

[0110] In some embodiments, the difference between the Dv50 particle size of the ternary material A and the Dv50 particle size of the lithium iron phosphate material is in the range of 0.8 μm to 4 μm, for example, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.4 μm, 2.5 μm, 2.8 μm, 3 μm, 3.4 μm, 3.5 μm, 4 μm, or a range defined by any two of the above values as the end values, for example, 2.8 μm to 3.5 μm.

[0111] In some embodiments, the difference between the Dn50 particle size of the ternary material A and the Dn50 particle size of the lithium iron phosphate material is in the range of 0.8 μm to 4 μm, for example, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.4 μm, 2.5 μm, 2.8 μm, 3 μm, 3.4 μm, 3.5 μm, 4 μm, or a range defined by any two of the above values as the end values, for example, 2.8 μm to 3.5 μm.

[0112] Controlling the difference between the Dv50 particle size of the ternary material A and the Dv50 particle size of the lithium iron phosphate material in the above range can make the battery further have good energy density, cycle performance and power performance. In some embodiments, the Dv50 particle size of the ternary material A is in the range of 2 μm to 5.2 μm, for example, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.2 μm. Further, the Dv50 particle size of the ternary material A can be in the range of 3 μm to 4.2 μm, or a range defined by any two of the above values as the end values.

[0113] In some embodiments, the Dn50 particle size of the ternary material A in the first positive electrode active layer is in the range of 2 μm to 7.2 μm, for example, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.2 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.2 μm. Further, the Dn50 particle size of the ternary material A can be in the range of 3 μm to 6.2 μm, 4 μm to 7.2 μm, or 4 μm to 6.2 μm, or a range defined by any two of the above values as the end values.

[0114] In the present application, the Dv50 particle size of the ternary material A is in the range of 2 μm to 5.2 μm, which is equivalent in technical effect to the Dn50 particle size of the ternary material A being in the range of 2 μm to 7.2 μm. Further, in the present application, when the Dv50 particle size of the ternary material A is in the range of 2 μm to 5.2 μm, the Dn50 particle size of the ternary material A is in the range of 2 μm to 7.2 μm.

[0115] In the present application, the Dv50 particle size of the ternary material A is 3 μm to 4.2 μm, which is equivalent in technical effect to the Dn50 particle size of the ternary material A being 3 μm to 6.2 μm. Further, in the present application, when the Dv50 particle size of the ternary material A is 3 μm to 4.2 μm, the Dn50 particle size of the ternary material A is in the range of 3 μm to 6.2 μm.

[0116] The Dv50 particle size or the Dn50 particle size of the ternary material A in the above range makes the gap between the ternary material particles in the first positive electrode active layer in a more appropriate range, the impedance of lithium ions inside the positive electrode active material particles is lower, and the positive electrode active layer has a suitable compaction density, thereby improving the conductivity and compaction density of the positive electrode active layer, and thus the battery can further have good energy density, cycle performance and power performance.

[0117] In some embodiments, the ternary material A includes polycrystalline particles. Single crystal particles are microparticles inside a crystal that are regularly and periodically arranged in three-dimensional space. Polycrystalline particles refer to a collection of single crystals of a large number of oriented grains. The ternary material A with polycrystalline particles, when combined with lithium iron phosphate, has a smaller potential difference between particles during lithium ion extraction, less impact on internal anion crosstalk of the positive electrode, less loss of positive electrode active material, and better long-term cycle performance.

[0118] The Dv50 particle size or the Dn50 particle size of the lithium iron phosphate is relatively small, and if lithium iron phosphate is simply used as the positive electrode active material of the lower layer, the compaction density of the lower layer is lower. The first positive electrode active layer of the lower layer in the present application uses the ternary material with larger particle size and the lithium iron phosphate with smaller particle size, which is beneficial to improve the compaction density of the first positive electrode active layer of the lower layer, so that the compaction density of the first positive electrode active layer of the lower layer and the second positive electrode active layer of the upper layer are better matched, and good energy density is obtained, while good power performance is also considered.

[0119] In some embodiments, the Dv50 particle size of the lithium iron phosphate material is 0.8 μm to 1.8 μm. As an example, the Dv50 particle size of the lithium iron phosphate material can be 0.8 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm. Further, the Dv50 particle size of the lithium iron phosphate material can be 1.0 μm to 1.5 μm, or in a range formed by any two of the above-mentioned point values as end values.

[0120] In some embodiments, the Dn50 particle size of the lithium iron phosphate material in the first positive active layer is 0.8-3.8 μm. For example, the Dn50 particle size of the lithium iron phosphate material can be 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.5 μm, 2.8 μm, 3 μm, 3.5 μm, 3.8 μm. Further, the Dn50 particle size of the lithium iron phosphate material can be 1.0-3.5 μm, or a range defined by any two of the above values as the end values.

[0121] The Dv50 particle size or Dn50 particle size of the lithium iron phosphate material is in the above range, and the lithium iron phosphate material is matched with the ternary material A in the above particle size range, so that the small particle size lithium iron phosphate material is more fully filled in the gap of the large particles, and the compaction density of the lower layer can be further improved, so that the compaction density between the lower active layer and the upper active layer is better matched. In addition, the Dv50 particle size or Dn50 particle size of the lithium iron phosphate material is in the above smaller range, the Li + The transmission path is smaller, the conductivity of the first positive active layer can be improved, the dynamic performance of the battery is improved, the power performance of the battery is improved, and the battery has good energy density, cycle performance and power performance.

[0122] Further, the lithium iron phosphate material includes single crystal particles. The lithium iron phosphate material includes single crystal particles, and the migration resistance of lithium ions in the single crystal particles is small due to the perfect crystal structure or the few defects of the single crystal particles, so that the solid phase impedance can be reduced, and the role of the lower layer in inhibiting the increase of DCR at low SOC can be played, and the power performance of the battery is improved.

[0123] In some embodiments, the lithium iron phosphate material includes one or more of lithium iron phosphate bulk particles, lithium iron phosphate bulk particles coated by a coating layer, and their respective doping modification materials. The lithium iron phosphate bulk particles refer to lithium iron phosphate particles that are not coated or doped or subjected to other modification treatments.

[0124] Further, the coating layer includes but is not limited to carbon coating. In some embodiments, in the first positive active layer, the mass fraction of the lithium iron phosphate material in the total amount of the lithium iron phosphate material and the ternary material A is 50%-95%. For example, the mass fraction of the lithium iron phosphate material in the total amount of the lithium iron phosphate material and the ternary material A can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or a range defined by any two of the above values as the end values, for example, 55%-95%.

[0125] Further, in the total amount of the lithium iron phosphate material and the ternary material A, the mass ratio of the lithium iron phosphate material is 60% to 80%, and can also be within a range formed by any two of the above values as end values. When the mass ratio of the lithium iron phosphate material is within the above range, more lithium iron phosphate material is more fully filled in the gaps of the large particles, and the lithium iron phosphate material plays a greater role in reducing the DCR at low and medium SOC in the life cycle, thereby improving the power performance of the battery.

[0126] The positive electrode sheet is ion-milled to obtain a cross section, and the cross section is tested by optical photography such as SEM (scanning electron microscope). The element distribution results obtained by EDS detection in a rectangular sampling area of 15 microns by 30 microns of the actual size of the cross section are used to identify the relative distribution mass of lithium iron phosphate and ternary material in the sampling area. In this way, the mass ratio of lithium iron phosphate in the total mass of ternary material A and lithium iron phosphate can be detected.

[0127] In some embodiments, the total amount of ternary material A and lithium iron phosphate material is 80% to 98% by mass content in the first positive electrode active layer; as an example, it can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or within a range formed by any two of the above values as end values.

[0128] In some embodiments, the Dv50 particle size of the ternary material B is 4 μm to 13 μm. As an example, the Dv50 particle size of the ternary material B can be 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 9.4 μm, 9.5 μm, 10 μm, 11 μm, 12 μm, 13 μm, or within a range formed by any two of the above values as end values. Further, the Dv50 particle size of the ternary material B can be 8 μm to 11 μm.

[0129] In some embodiments, the Dn50 particle size of the ternary material B in the upper second positive electrode active layer is 4 μm to 15 μm. As an example, the Dn50 particle size of the ternary material B can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 9.4 μm, 9.5 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or within a range formed by any two of the above values as end values. Further, the Dn50 particle size of the ternary material B can be 8 μm to 13 μm.

[0130] In the present application, the Dv50 particle size of the ternary material B is 4-13 μm, which is equivalent in technical effect to the Dn50 particle size of 4-15 μm of the ternary material B. Further, in the present application, when the Dv50 particle size of the ternary material B is 4-13 μm, the Dn50 particle size of the ternary material B is in the range of 4-15 μm.

[0131] In the present application, the Dv50 particle size of the ternary material B is 8-11 μm, which is equivalent in technical effect to the Dn50 particle size of 8-13 μm of the ternary material B. Further, in the present application, when the Dv50 particle size of the ternary material B is 8-11 μm, the Dn50 particle size of the ternary material B is in the range of 8-13 μm.

[0132] The Dv50 particle size or Dn50 particle size of the ternary material B in the upper second positive electrode active layer is in the above range, so that the gap between the ternary material particles in the second positive electrode active layer is in a more appropriate range, the impedance of lithium ions inside the positive electrode active material particles is lower and has a suitable compaction density, thereby improving the conductivity and compaction density of the second positive electrode active layer, so that the battery can further have good energy density, cycle performance and power performance, in addition, the battery can also achieve 4C fast charging performance while having good cycle life, and the life can achieve ≥3000 cls. Further, the ternary material B includes single crystal particles. The ternary material B using single crystal particles has fewer surface defects and provides good cycle performance.

[0133] In some embodiments, the ternary material B is 80-98% by mass content in the second positive electrode active layer; as an example, it can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or in the range constituted by any two of the above as end values.

[0134] In some embodiments, the positive electrode active material in the second positive electrode active layer is the ternary material B.

[0135] In some embodiments, the thickness of the first positive active layer is less than the thickness of the second positive active layer. In this way, the second positive active layer is primary and the first positive active layer is secondary, the second positive active layer of the upper layer is taken as the main active layer to provide the main performance of high energy density and cycle performance of the battery cell; the first positive active layer of the lower layer is taken as the auxiliary, and in the case of discharging at low SOC, the main lithium intercalation process is in the first positive active layer of the lower layer because the second positive active layer of the upper layer is close to the full intercalation state, and the kinetics of the first positive active layer of the lower layer directly affects the low SOC power of the battery cell, so the thickness of the first positive active layer of the lower layer is controlled to be small to improve the kinetics of the first positive active layer of the lower layer, and the SOC kinetics advantage (low impedance) of the positive electrode plate will be more and more obvious with the use of the battery, reducing the DCR at medium and low SOC in the life cycle, and thus improving the power performance of the battery.

[0136] In the present application, the first positive active layer and the second positive active layer have obvious interfaces which can be seen from the optical photograph of the cross section of the positive electrode plate by SEM (scanning electron microscope); then the thickness of the first positive active layer and the second positive active layer at 10 positions of the cross section is measured, and the average of the 10 thickness measurement values is taken as the thickness of the first positive active layer and the second positive active layer.

[0137] In some embodiments, the thickness of the first positive active layer is 15 μm to 150 μm. For example, the thickness of the first positive active layer can be 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, or 150 μm.

[0138] Further, the thickness of the first positive active layer can be 30 μm to 60 μm, or a range formed by any two of the above-mentioned point values as end values. The thickness of the first positive active layer in the range can have better ion conduction performance, and thus can further improve the power performance of the battery.

[0139] In some embodiments, the thickness of the second positive active layer is 20 μm to 200 μm. For example, the thickness of the second positive active layer can be 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, 85 μm, 90 μm, 95 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, or 200 μm.

[0140] Further, the thickness of the second positive electrode active layer can be 70 μm to 100 μm, or a range formed by any two of the above values as end values. The thickness of the second positive electrode active layer in the range can further improve the power performance of the battery.

[0141] In some embodiments, each of the ternary material A and the ternary material B independently comprises at least one of a nickel-cobalt-manganese ternary material and a nickel-cobalt-aluminum ternary material.

[0142] In some embodiments, the mole content of the nickel element in the ternary material A in the total amount of ternary metals is 33% to 90%. For example, the mole content of the nickel element in the ternary material A in the total amount of ternary metals can be 33%, 1 / 3, 50%, 55%, 60%, 65%, 68%, 70%, 75%, 80%, 85%, or 90%.

[0143] For example, LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be referred to as NCM 523 ) has a mole content of the nickel element in the total amount of ternary metals of 0.5, i.e., 50%; and the ternary metal refers to a nickel-cobalt-manganese ternary metal.

[0144] Further, the mole content of the nickel element in the ternary material A in the total amount of ternary metals is 70% to 90%, or a range formed by any two of the above values as end values. The ternary material with a higher nickel content has a higher specific capacity. The increase in the nickel content can improve the reversible lithium intercalation capacity of the ternary material, thereby increasing the energy density of the battery. However, the increase in the nickel content of the ternary material increases the alkalinity and the water absorption, and increases the side reactions, which also increases the DCR. Therefore, the ternary material A with a higher nickel content is arranged in the first positive electrode active layer in the lower layer, and the ternary material A with a smaller particle size is used, which can compensate for the problem of the increased DCR caused by the nickel content of the ternary material A, so as to improve the energy density while taking into account the good power performance.

[0145] In some embodiments, the mole content of the nickel element in the ternary material B in the total amount of ternary metals is 33% to 90%. For example, the mole content of the nickel element in the ternary material B in the total amount of ternary metals can be 33%, 1 / 3, 50%, 55%, 60%, 65%, 68%, 70%, 75%, 80%, 85%, or 90%.

[0146] Further, the mole content of the nickel element in the ternary material B in the total amount of ternary metals is 33% to 50%. The ternary material B in the range has a more excellent high-voltage resistance advantage, and the nickel content thereof is moderate, and the side reactions are less likely to occur compared to the high-nickel ternary material, which is beneficial to reduce the DCR of the battery.

[0147] Non-limiting examples of nickel cobalt manganese ternary materials can include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(also can be referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2(also can be referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2(also can be referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(also can be referred to as NCM 622 ), LiNi 0.68 Co 0.10 Mn 0.22 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2(also can be referred to as NCM 811 ), LiNi 0.9 Co 0.05 Mn 0.05 O2(also can be referred to as NCM 9055 ), etc. Non-limiting examples of nickel cobalt aluminum ternary materials can include LiNi 0.8 Co 0.15 Al 0.05 O2.

[0148] It can be understood that the battery will be accompanied by lithium (Li) deintercalation and consumption during charging and discharging, and the content of Li in the positive active material is different when the battery is discharged to different states. In the enumeration of the positive active material in the present application, the content of Li is the initial state of the material unless otherwise specified. When the positive active material is applied to the positive electrode sheet in the battery system, the content of Li in the positive active material contained in the sheet will usually change after charging and discharging cycles. Among them, the content of Li can be quantified by molar content, but is not limited thereto. As for "the content of Li is the initial state of the material", the initial state of the material refers to the state before being put into the positive electrode slurry. It can be understood that new materials obtained by proper modification of the listed positive active materials are also within the scope of positive active materials, and the foregoing proper modification refers to acceptable modification methods for positive active materials, and non-limiting examples include coating modification.

[0149] In the enumeration of the positive electrode active material in the present application, the content of oxygen (O) is only the theoretical state value, and the release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual content of O will appear to be floating. The content of O can be measured by molar content, but is not limited thereto.

[0150] In some embodiments, the first positive electrode active layer and the second positive electrode active layer also optionally include a binder. As non-limiting examples, the binder can include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0151] In some embodiments, the first positive electrode active layer and the second positive electrode active layer also optionally include a conductive agent. As non-limiting examples, the conductive agent can include one or more of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0152] In some embodiments, the method of preparing the positive electrode tab described above can include the following steps S10-S20:

[0153] S10, forming a first positive electrode active layer on at least one surface of the positive electrode current collector.

[0154] S20, forming a second positive electrode active layer on the surface of the first positive electrode active layer away from the positive electrode current collector.

[0155] Specifically, a first positive electrode slurry and a second positive electrode slurry can be formed respectively, and then the first positive electrode slurry and the second positive electrode slurry are sequentially coated on at least one side surface of the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode tab can be obtained.

[0156] Further, after coating the first positive electrode slurry, drying, and then coating the second positive electrode slurry.

[0157] The first positive electrode slurry described above can be formed by dispersing the components for preparing the first positive electrode active layer, such as the positive electrode active material (including the ternary material A and the lithium iron phosphate material), the conductive agent, the binder, and any other components, in a solvent.

[0158] The second positive electrode slurry described above can be formed by dispersing the components for preparing the second positive electrode active layer, such as the positive electrode active material (including the ternary material B), the conductive agent, the binder, and any other components, in a solvent.

[0159] Further, it is found that the binder is prone to float during the drying process. Therefore, in order to ensure the uniform distribution of the binder in the product, the content of the binder added in the slurry of the first positive active layer is slightly higher. In this way, the content of the binder in the entire positive electrode film is more uniform.

[0160] Further, the mass content of the binder in the slurry of the first positive active layer is greater than the mass content of the binder in the slurry of the second positive active layer. Specifically, the difference between the mass content of the binder in the slurry of the first positive active layer and the mass content of the binder in the slurry of the second positive active layer can be 0.5% to 2%, for example, 0.5%, 1%, 1.5%, 2%, or a range formed by any two of the above point values.

[0161] The type of solvent can be selected from, but not limited to, any of the aforementioned embodiments, for example, N-methyl pyrrolidone (NMP). The positive electrode current collector surface coated with the positive electrode slurry can be on a single surface of the positive electrode current collector, or on both surfaces of the positive electrode current collector.

[0162] Further, the solid content of the first positive electrode slurry can be 40wt% to 80wt%.

[0163] Further, the solid content of the second positive electrode slurry can be 40wt% to 80wt%. For example, the solid content of the first positive electrode slurry and the solid content of the second positive electrode slurry can be 40wt%, 50wt%, 55wt%, 60wt%, 64wt%, 65wt%, 70wt%, 74wt%, 75wt%, 80wt%, or a range formed by any two of the above point values as end values.

[0164] Alternatively, the solid content of the first positive electrode slurry is 64wt% to 74wt%, and the solid content of the second positive electrode slurry is 60wt% to 70wt%.

[0165] When coating the first positive electrode slurry and the second positive electrode slurry, the coating unit area density (excluding solvent) can be 15mg / cm 2 to 35mg / cm 2 . The compaction density of the positive electrode sheet can be 3.0g / cm 3 to 3.6g / cm 3 , optionally 3.3g / cm 3 to 3.5g / cm 3 .

[0166] The negative electrode sheet

[0167] The negative electrode sheet includes a negative electrode current collector. Further, the negative electrode sheet can further include a negative active layer disposed on at least one surface of the negative electrode current collector, and the negative active layer includes a negative active material.

[0168] As a non-limiting example, the negative current collector has two surfaces opposite in the thickness direction thereof, and the negative active layer is disposed on either one or both of the two opposite surfaces of the negative current collector.

[0169] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be obtained by forming a metal material on a polymer material base layer. Non-limiting examples of the metal material in the negative current collector can include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, and the like. Non-limiting examples of the polymer material base layer in the negative current collector can include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

[0170] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As a non-limiting example, the negative active material can include one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, and the like. The silicon-based material can include one or more of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can include one or more of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. The negative active material can be used alone or in combination of two or more.

[0171] In some embodiments, the negative active layer can further optionally include a binder. The binder can include one or more of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0172] In some embodiments, the negative active layer can further optionally include a conductive agent. The conductive agent can include one or more of super P, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0173] In some embodiments, the negative active layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0174] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (a non-limiting example of the solvent is deionized water), to form a negative electrode slurry; coating the negative electrode slurry on at least one side surface of the negative electrode current collector, and after processes such as drying, cold pressing, etc., the negative electrode sheet can be obtained. The surface of the negative electrode current collector to which the negative electrode slurry is coated can be either one surface of the negative electrode current collector or both surfaces of the negative electrode current collector.

[0175] The solid content of the negative electrode slurry can be 40 wt% to 60 wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000 to 10000 mPa·s. When the negative electrode slurry is coated, the coating unit area density, on a dry weight basis (excluding the solvent), can be 75 to 220 g / m 2 . The compaction density of the negative electrode sheet can be 1.0 g / cm 3 to 1.8 g / cm 3 .

[0176] Electrolyte

[0177] The electrolyte has a function of conducting ions between the positive electrode sheet and the negative electrode sheet. The type of the electrolyte is not particularly limited in the present application, and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0178] In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0179] In some embodiments, the electrolyte salt can include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluoro-oxalato-borate (LiDFOB), lithium bis-oxalato-borate (LiBOB), lithium difluoro-bis-oxalato-phosphate (LiDFOP), and lithium tetrafluoro-oxalato-phosphate (LiTFOP).

[0180] In some embodiments, the solvent can include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate, fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, butyrolactone sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0181] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0182] In some embodiments, the additive in the electrolyte solution can include, but is not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0183] Separator film

[0184] In some embodiments, the secondary battery further includes a separator film.

[0185] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator film. During charging and discharging of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The separator film is disposed between the positive electrode sheet and the negative electrode sheet, and functions to prevent short circuiting of the positive and negative electrodes, while allowing ions to pass through.

[0186] The type of the separator film is not particularly limited in the present application, and any known porous structure separator film having good chemical stability and mechanical stability can be used.

[0187] In some embodiments, the material of the separator film can include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0188] In some embodiments, the thickness of the separator film is 6 μm to 40 μm, and can be 12 μm to 20 μm.

[0189] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator film can be made into an electrode assembly through a winding process or a stacking process.

[0190] The secondary battery includes at least one battery cell. The secondary battery can include one or more battery cells.

[0191] In the present application, unless otherwise specified, a "battery cell" refers to a basic unit capable of realizing mutual conversion between chemical energy and electrical energy, and further, generally includes at least a positive electrode sheet, a negative electrode sheet, and an electrolyte. During the charging and discharging of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct the active ions between the positive electrode sheet and the negative electrode sheet.

[0192] The shape of the battery cell is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, the battery cell shown in FIG. 2 is an example of a square structure of a battery cell.

[0193] In some embodiments, the battery cell 5 can include an outer package. The outer package can be used to encapsulate the electrode assembly and the electrolyte described above. In some embodiments, the outer package of the battery cell can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and further, non-limiting examples of the plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, etc.

[0194] In some embodiments, referring to FIG. 3, the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be arranged on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator film can be made into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by a person skilled in the art according to actual needs.

[0195] In some embodiments, the secondary battery can be a battery module or a battery pack. The battery module includes at least one battery cell. The number of battery cells contained in the battery module can be one or more, which can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0196] FIG. 4 is a battery module 4 as an example. Referring to FIG. 4, in the battery module 4, a plurality of battery cells 5 can be arranged in series along a length direction of the battery module 4. Of course, the plurality of battery cells 5 can be arranged in any other manner. The plurality of battery cells 5 can be fixed by fasteners.

[0197] Optionally, the battery module 4 can further include a housing having an accommodation space in which the plurality of battery cells 5 are accommodated.

[0198] In some embodiments, the above-described battery module can be assembled into a battery pack, and the number of battery modules included in the battery pack can be one or more, and a person skilled in the art can select a suitable number according to the application and capacity of the battery pack.

[0199] FIGS. 5 and 6 are a battery pack 1 as an example. Referring to FIGS. 5 and 6, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0200] In addition, an embodiment of the present application further provides a power utilization device including the above-described secondary battery provided by the present application. The secondary battery can be used as a power supply of the power utilization device, or can be used as an energy storage unit of the power utilization device. The power utilization device can include a mobile device, an electric vehicle, an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto. Among them, the mobile device can be a mobile phone, a notebook computer, etc.; the electric vehicle can be a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but is not limited thereto.

[0201] As the power utilization device, the secondary battery can be selected according to the use requirement thereof.

[0202] FIG. 7 is a power utilization device 6 as an example. The power utilization device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of the power utilization device for high power and high energy density of the secondary battery, a battery pack or a battery module can be used.

[0203] As another example of the device, it can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thin and light, and a battery cell can be used as a power supply.

[0204] In order to make the technical problems, technical solutions and beneficial effects solved in the present application clearer, the present application will be further described in detail below in combination with embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way limiting on the application or its applications. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0205] Unless otherwise specified in the embodiments, the techniques or conditions are carried out according to the techniques or conditions described in the literature in the art or according to the product instructions. Unless otherwise specified, the reagents or instruments used are conventional products that can be obtained commercially.

[0206] Embodiment 1

[0207] 1) Preparation of positive electrode sheet

[0208] The positive electrode active materials polycrystalline LiNi 0.8 Co 0.10 Mn 0.1 O2(NCM 811 , ternary material A), lithium iron phosphate (single crystal particles), conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed uniformly in an appropriate amount of solvent N-methyl pyrrolidone (NMP) at a mass ratio of 29.2:68.0:0.7:2.1 to obtain a first positive electrode slurry.

[0209] The specific values of the Dv50 particle size of the ternary material A and the lithium iron phosphate are shown in Table 1. In the total mass of the ternary material A and the lithium iron phosphate, the mass ratio of the lithium iron phosphate is K, and the specific values are shown in Table 1. The positive electrode active material single crystal LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM 523 , ternary material B), conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed uniformly in an appropriate amount of solvent N-methyl pyrrolidone (NMP) at a mass ratio of 96.2:2.7:1.1 to obtain a second positive electrode slurry. The specific values of the Dv50 particle size of the ternary material B are shown in Table 1.

[0210] The first positive electrode slurry was coated on the positive electrode current collector aluminum foil, dried to form a first positive electrode active layer; the second positive electrode slurry was coated on the surface of the dried slurry to form a second positive electrode active layer, and through processes such as drying, cold pressing, slitting, and cutting, a positive electrode sheet was obtained.

[0211] The overall compaction density of the positive electrode sheet was 3.1 g / cm 3.

[0212] 2) Preparation of negative electrode sheet

[0213] The negative active material artificial graphite, conductive agent carbon black (Super P), binder styrene-butadiene rubber (SBR), and carboxymethyl cellulose sodium (CMC) were mixed uniformly in an appropriate amount of solvent deionized water according to a mass ratio of 96.4:0.7:1.8:1.1 to obtain a negative electrode slurry; the negative electrode slurry was coated on a negative current collector copper foil, and through the processes of drying, cold pressing, slitting, and cutting, a negative active material layer with a single side thickness of 54 μm was formed to obtain a negative electrode sheet. The compaction density of the negative electrode sheet was 1.75 g / cm 3 .

[0214] 3) Isolation film

[0215] A 12 μm thick polypropylene isolation film was selected.

[0216] 4) Preparation of electrolyte

[0217] Vinyl carbonate (EC) and methyl ethyl carbonate (EMC) were mixed according to a mass ratio of 30:70 to obtain an organic solvent, 3 wt% of LiFSI in the whole electrolyte was dissolved in the mixed solvent, and LiPF6 that was sufficiently dried was dissolved in the above organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0218] 5) Preparation of secondary battery

[0219] The positive electrode sheet, the isolation film, and the negative electrode sheet were stacked and wound in sequence to obtain an electrode assembly; the electrode assembly was placed in an outer package, and after drying, the electrolyte was injected, and the processes of vacuum packaging, standing, formation, shaping, etc. were performed to obtain a secondary battery; the ratio of the mass of the electrolyte to the capacity of the secondary battery (liquid injection coefficient) was 2.4 g / Ah.

[0220] Examples 2-3

[0221] The secondary batteries of Examples 2-3 were prepared in a similar manner to the secondary battery of Example 1, except that the Dv50 of the ternary material B was different. For specific differences, please refer to Table 1.

[0222] Examples 4-5

[0223] The secondary batteries of Examples 4-5 were prepared in a similar manner to the secondary battery of Example 2, except that the Dv50 of the lithium iron phosphate material was different. For specific differences, please refer to Table 1.

[0224] Examples 6-7

[0225] The secondary batteries of Examples 6-7 were prepared in a similar manner to the secondary battery of Example 2, except that the Dv50 of the ternary material A was different. See Table 1 for the specific differences.

[0226] Examples 8-9

[0227] The secondary batteries of Examples 8-9 were prepared in a similar manner to the secondary battery of Example 2, except that the mass ratio of the lithium iron phosphate in the total mass of the ternary material A and the lithium iron phosphate was different. See Table 1 for the specific differences.

[0228] Examples 10-11

[0229] The secondary batteries of Examples 10-11 were prepared in a similar manner to the secondary battery of Example 2, except that at least one of the specific types of the ternary material A and the ternary material B was different. See Table 1 for the specific differences.

[0230] Comparative Example 1

[0231] The secondary battery of Comparative Example 1 was prepared in a similar manner to the secondary battery of Example 2, except that the first positive electrode slurry was coated on the positive electrode current collector aluminum foil first, and then the second positive electrode slurry was coated. The other processes were the same. The positive electrode sheet formed was different from that of Example 1 in that the positions of the first positive electrode active layer and the second positive electrode active layer in Example 1 were exchanged.

[0232] The following are performance tests.

[0233] (1) Test the compaction density of the entire sheet: disassemble the cell to take the sheet, and punch into a small disc of 1540.25 mm 2 . Measure the weight m and thickness L of the small disc. Take another sheet, wipe off the film layer on the surface to leave the empty current collector foil, and punch into a small disc of 1540.25 mm 2 . Weigh the mass of the empty aluminum foil M0. Then the compaction density PD = (M-M0) / 1.54025 / n / (L-L0), where n is the number of film layers coated on the current collector, which is 1 or 2, single-sided or double-sided coating. L0 is the thickness of the current collector foil.

[0234] (2) Perform SEM (scanning electron microscope) test on the cross-section obtained by slicing the positive electrode sheet of the obtained battery.

[0235] SEM (scanning electron microscope) test was performed on the cross-section of the positive electrode sheet prepared in Example 2, and the SEM image obtained is shown in FIG. 8.

[0236] As can be seen from FIG. 8, the distinct interfaces between the positive current collector 5311 (aluminum foil) and the first positive active layer 5312 and between the first positive active layer 5312 (lower layer) and the second positive active layer 5313 (upper layer) of the positive electrode sheet can be clearly seen. In the first positive active layer 5312 of the lower layer in FIG. 8, the ternary material A is in the A frame and the lithium iron phosphate (LFP) is in the C frame. In the second positive active layer 5313 of the upper layer, the ternary material B is in the B frame.

[0237] The thicknesses of the first positive active layer and the second positive active layer at 10 positions of the cross section were then measured, and the average of the 10 thickness measurement values was taken to obtain the thicknesses of the first positive active layer and the second positive active layer, which were 20 μm and 30 μm, respectively.

[0238] For the detection of the Dn50 particle size (number average particle size) of the positive active material in the positive electrode sheet in the battery, the following method can be used: the positive electrode sheet is ion-milled to obtain a cross section (a cross section obtained by slicing in the thickness direction), and the cross section is tested by optical photography such as SEM (scanning electron microscope). The cross sections of the first positive active layer and the second positive active layer are tested as follows: in the same scanning electron microscope image (SEM) at 10 kv and 30 k magnification, the positive active material particles in any 5 regions with the same size and shape (actual size in a square sampling area of 15 μm by 30 μm) are observed. In this detection process, the characteristic elements of the positive active material particles can be detected by EDS to identify whether the positive active material particles are ternary materials or lithium iron phosphate.

[0239] The particle sizes of each positive active material particle (ternary material or lithium iron phosphate) in the square sampling area are detected, where the particle size of each positive active material particle (ternary material or lithium iron phosphate) refers to the longest diameter distance of the particle. The number of particles of each particle size of the ternary material or lithium iron phosphate in the square sampling area is multiplied by the particle size, all the products are added, and then divided by the total number of particles, to obtain the average particle size weighted by the number, i.e. the Dn50 particle size of the ternary material or lithium iron phosphate in the region. The Dn50 particle sizes of the 5 regions are averaged to obtain the Dn50 particle size of the ternary material or lithium iron phosphate in the positive electrode sheet.

[0240] The Dn50 particle sizes of the ternary material A, the lithium iron phosphate material and the ternary material B are obtained. The Dn50 particle sizes of the ternary material A in Examples 1-11 are 2 μm-7.2 μm, the Dn50 particle sizes of the lithium iron phosphate material are 0.8 μm-3.8 μm, and the Dn50 particle sizes of the ternary material B are 4 μm-15 μm.

[0241] The positive electrode sheet is ion milled to obtain a cross section, and the cross section is tested by optical photograph such as SEM (scanning electron microscope). The relative distribution mass of lithium iron phosphate and ternary material in the sampling area is identified by EDS detection on the actual size of the cross section in a rectangular sampling area of 30 microns by 30 microns. In this way, the mass ratio K of lithium iron phosphate in the total mass of ternary material A and lithium iron phosphate can be detected. The obtained mass ratio K is comparable to the mass ratio K in Table 1.

[0242] (2) Energy density test

[0243] At 25°C, the battery is charged at a current of 0.33C to the designed upper limit voltage with a constant current and constant voltage, the cutoff current is 0.05C, and after standing for 30 min, it is discharged at 0.33C to the designed lower limit voltage, the discharge energy P (Wh) is recorded, and the volume of the battery is recorded as V (L), then the energy density (Wh / L) = P / V.

[0244] The energy density in Example 1 is 510 Wh / L.

[0245] (3) DCR growth rate at medium and low SOC

[0246] Medium and low SOC refers to 0-30% SOC. The DCR value of the battery at medium and low SOC can better reflect the power performance of the battery. In this test, 20% SOC is used.

[0247] The DCR growth rate at medium and low SOC can characterize the power performance decay of the battery at medium and low state of charge. The greater the DCR growth rate at medium and low SOC, the greater the power performance decay of the battery at medium and low state of charge, i.e. the poorer the power performance stability of the battery; on the contrary, the smaller the DCR growth rate at medium and low SOC, the smaller the power performance decay of the battery at medium and low state of charge, i.e. the better the power performance stability of the battery.

[0248] The test method of the DCR of the battery is as follows: the prepared battery is charged at 0.1C to 4.4V in a 25°C environment, then discharged at 0.33C for 144 min to adjust to 20% SOC, at this time the voltage is recorded as V1, discharged at a current of 4C for 10S, and the potential at this time is recorded as V2; DCR1 value = (V1-V2) / 4C corresponding current value.

[0249] Continue to charge at 0.5C to 4.4V in a 25°C environment, then discharge at 0.33C to 2.5V, and so on for 1000 cycles of charge and discharge. The cycled battery is charged at 0.1C to 4.4V in a 25°C environment, then discharged at 0.33C for 144 min to adjust to 20% SOC, at this time the voltage is recorded as V3, discharged at a current of 4C for 10S, and the potential at this time is recorded as V4; DCR2 value = (V3-V4) / 4C corresponding current value.

[0250] DCR growth rate at 20% SOC = (DCR2-DCR1) / DCR1.

[0251] (4) Cycle performance

[0252] The cycle performance was characterized by the cycle number at which the capacity decayed to 80% of the initial capacity at 25°C, and the specific cycle process was as follows: the prepared battery was charged to 4.4V at 0.5C and then discharged to 2.5V at 0.33C in a 25°C environment, and the cycle was repeated until the capacity decayed to 80% of the initial capacity, and the cycle number at this time (the decimal was rounded to the nearest multiple of 10) was recorded, as shown in Table 1.

[0253] The test results of each example and comparative example are shown in Table 1.

[0254] Table 1

[0255] The positive electrode tab of the present application has a double-layer active layer, and the lower layer adopts a mixed system of LFP and NCM, and the upper layer adopts NCM. Therefore, in the case of high SOC, the battery mainly uses NCM to provide lithium ions for discharge; in the case of low SOC, the battery mainly uses LFP to provide lithium ions for discharge.

[0256] Compared with Example 2, Comparative Example 1 uses the second positive electrode active layer containing ternary material B as the lower layer, and uses the first positive electrode active layer containing mixed ternary material A and lithium iron phosphate as the upper layer. Therefore, in the case of high SOC, the lower layer of the second positive electrode active layer mainly uses the lower ternary material B to de-lithiate, and the ternary material B needs to pass through the upper active layer containing LFP to de-lithiate, which increases the lithium ion transmission path and leads to increased impedance. At the same time, the positive electrode is prone to anion crosstalk, which increases the loss of positive electrode material, and further leads to reduced cycle performance. In addition, due to the increased impedance, the rate of lithium intercalation with the negative electrode is inconsistent, which further causes the negative electrode to be prone to lithium precipitation. In the case of low SOC, the positive electrode tab is over-de-lithiated, which leads to increased loss of positive electrode material, and further leads to reduced cycle performance. In addition, the overall impedance is increased due to the increased lithium ion transmission path of the lower layer.

[0257] Each example has good energy density and low DCR growth rate, indicating that it can balance good energy density, cycle performance, and power performance stability.

[0258] Each technical feature of the above-described examples can be combined arbitrarily. To make the description concise, not all possible combinations of each technical feature in the above-described examples are described, but as long as the combinations of these technical features do not exist, they should be considered as the scope of the present disclosure.

[0259] The above embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the application. It should be pointed out that for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, which are all within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims, and the description and drawings can be used to explain the scope of the claims.

Claims

1. A battery comprising a positive electrode tab, the positive electrode tab comprising: a positive electrode current collector; a first positive electrode active layer provided on at least one surface of the positive electrode current collector, the first positive electrode active layer comprising a ternary material A and a lithium iron phosphate material, the ternary material A having a Dv50 particle size greater than a Dv50 particle size of the lithium iron phosphate material, and / or the ternary material A having a Dn50 particle size greater than a Dn50 particle size of the lithium iron phosphate material; and, a second positive electrode active layer provided on a surface of the first positive electrode active layer away from the positive electrode current collector, the second positive electrode active layer comprising a ternary material B; wherein the ternary material A and the ternary material B each comprise a nickel element, and the ternary material A has a molar content of the nickel element in a total amount of ternary metals higher than a molar content of the nickel element in a total amount of ternary metals of the ternary material B. at least one of the following characteristics is satisfied: (1) the ternary material A has a Dv50 particle size of 2 μm to 5.2 μm, and / or the ternary material A has a Dn50 particle size of 2 μm to 7.2 μm; (2) the ternary material A comprises polycrystalline particles; (3) a difference between the Dv50 particle size of the ternary material A and the Dv50 particle size of the lithium iron phosphate material is 0.8 μm to 4 μm. the ternary material A has a Dv50 particle size of 3 μm to 4.2 μm, and / or the ternary material A has a Dn50 particle size of 3 μm to 6.2 μm. at least one of the following characteristics is satisfied: (1) the lithium iron phosphate material has a Dv50 particle size of 0.8 μm to 1.8 μm, and / or the lithium iron phosphate material has a Dn50 particle size of 0.8 μm to 3.8 μm; (2) the lithium iron phosphate material comprises single-crystal particles; (3) the lithium iron phosphate material comprises one or more of a lithium iron phosphate bulk particle, a lithium iron phosphate bulk particle coated by a coating layer, and a doped modification material of each of the lithium iron phosphate bulk particle and the lithium iron phosphate bulk particle coated by the coating layer. the lithium iron phosphate material has a Dv50 particle size of 1.0 μm to 1.5 μm, and / or the lithium iron phosphate material has a Dn50 particle size of 1 μm to 3.5 μm. in a total amount of the lithium iron phosphate material and the ternary material A, the lithium iron phosphate material has a mass ratio of 50% to 95%. in a total amount of the lithium iron phosphate material and the ternary material A, the lithium iron phosphate material has a mass ratio of 60% to 80%. at least one of the following characteristics is satisfied: (1) the ternary material B has a Dv50 particle size of 4 μm to 13 μm, and / or the ternary material B has a Dn50 particle size of 4 μm to 15 μm; (2) the ternary material B comprises single-crystal particles. the ternary material B has a Dv50 particle size of 8 μm to 11 μm, and / or the ternary material B has a Dn50 particle size of 8 μm to 13 μm. at least one of the following characteristics is satisfied: (1) the first positive electrode active layer has a thickness of 15 μm to 150 μm; (2) the second positive electrode active layer has a thickness of 20 μm to 200 μm. the first positive electrode active layer has a thickness less than a thickness of the second positive electrode active layer. ​ ​ ​ ​ ​ 2. The battery of claim 1, wherein, ​ ​ ​ ​ 3. The battery of claim 2, wherein, ​ 4. The battery of any one of claims 1 to 3, wherein, ​ ​ ​ ​ 5. The battery of claim 4, wherein, ​ 6. The battery of any one of claims 1 to 5, wherein, ​ 7. The battery of claim 6, wherein, ​ 8. The battery of any one of claims 1 to 7, wherein, ​ ​ ​ 9. The battery of claim 8, wherein, ​ 10. The battery of any one of claims 1 to 9, wherein, ​ ​ ​ 11. The battery of any one of claims 1 to 10, wherein, ​ 12. The battery of claim 11, wherein, satisfy at least one of the following characteristics: (1) the thickness of the first positive electrode active layer is 30 μm to 60 μm; (2) the thickness of the second positive electrode active layer is 70 μm to 100 μm.

13. The battery of any one of claims 1 to 12, wherein, The ternary material A and the ternary material B each independently include at least one of a nickel-cobalt-manganese ternary material and a nickel-cobalt-aluminum ternary material.

14. The battery of claim 13, wherein, The nickel-cobalt-manganese ternary material includes LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.5 Co 0.25 Mn 0.25 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.68 Co 0.10 Mn 0.22 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, and LiNi 0.9 Co 0.05 Mn 0.05 O2. The nickel-cobalt-aluminum ternary material includes LiNi 0.8 Co 0.15 Al 0.05 O2.

15. The battery of any one of claims 1 to 14, wherein, satisfy at least one of the following characteristics: (1) the molar content of the nickel element in the ternary material A in the total amount of ternary metals is 33% to 90%; (2) the molar content of the nickel element in the ternary material B in the total amount of ternary metals is 33% to 90%.

16. The battery of claim 15, wherein, satisfy at least one of the following characteristics: (1) the molar content of the nickel element in the ternary material A in the total amount of ternary metals is 70% to 90%; (2) the molar content of the nickel element in the ternary material B in the total amount of ternary metals is 33% to 50%.

17. A positive electrode tab, comprising: a positive electrode current collector; a first positive electrode active layer provided on at least one surface of the positive electrode current collector, the first positive electrode active layer including a ternary material A and a lithium iron phosphate material, the Dv50 particle size of the ternary material A being greater than the Dv50 particle size of the lithium iron phosphate material, and / or the Dn50 particle size of the ternary material A being greater than the Dn50 particle size of the lithium iron phosphate material; and, a second positive electrode active layer provided on the surface of the first positive electrode active layer away from the positive electrode current collector, the second positive electrode active layer including a ternary material B; wherein the ternary material A and the ternary material B each include a nickel element, and the molar content of the nickel element in the ternary material A in the total amount of ternary metals is higher than the molar content of the nickel element in the ternary material B in the total amount of ternary metals.

18. A method of making a positive electrode sheet, wherein, The positive electrode tab is the positive electrode tab of claim 17, and the preparation method includes the following steps: forming the first positive electrode active layer on at least one surface of the positive electrode current collector; forming the second positive electrode active layer on the surface of the first positive electrode active layer away from the positive electrode current collector.

19. An electrical device, comprising at least one of the battery of any one of claims 1 to 16 and the positive electrode tab of claim 17.

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