Positive pole piece and lithium battery

Abstract

The invention relates to the technical field of lithium batteries, and discloses a positive pole piece and a lithium battery. The positive electrode material layer comprises a first positive electrode material sub-layer and a second positive electrode material sub-layer, the second positive electrode material sub-layer is arranged on one side, far away from the positive electrode current collector, of the first positive electrode material sub-layer, the first positive electrode material sub-layer comprises a positive electrode active material, and the second positive electrode material sub-layer comprises a negative electrode active material. The first positive electrode material sub-layer comprises a positive electrode active material, the second positive electrode material sub-layer comprises a positive electrode active material, the positive electrode active material comprises lithium manganese iron phosphate and a ternary material, the mass ratio of the lithium manganese iron phosphate in the first positive electrode material sub-layer is x1, and the mass ratio of the ternary material in the first positive electrode material sub-layer is y1; the mass ratio of the lithium manganese iron phosphate in the second positive electrode material sub-layer is x2, and the mass ratio of the ternary material in the second positive electrode material sub-layer is y2, x1gt; x1, x2, y1lt; y2. And the positive pole piece layer has relatively high and relatively consistent ion migration rate and relatively good quick charge performance.

Description

Technical Field

[0001] The present application relates to the technical field of lithium batteries, and in particular to positive electrode sheets and lithium batteries. Background Art

[0002] Energy conservation and emission reduction are the key to the sustainable development of the automotive industry. In this case, electric vehicles have become an important part of the sustainable development of the automotive industry due to their advantages of energy conservation and environmental protection. For electric vehicles, battery technology is an important factor in their development. Lithium-ion batteries are widely used in electric vehicles, hybrid vehicles and energy storage systems due to their high energy density, long cycle life and environmental protection. Summary of the invention

[0003] In a first aspect of the present application, the present application proposes a positive electrode sheet, comprising:

[0004] A positive electrode current collector; a positive electrode material layer, wherein the positive electrode material layer is arranged on at least one side of the positive electrode current collector; the positive electrode material layer includes a first positive electrode material sublayer and a second positive electrode material sublayer, wherein the second positive electrode material sublayer is arranged on a side of the first positive electrode material sublayer away from the positive electrode current collector, wherein the first positive electrode material sublayer includes a positive electrode active material, the second positive electrode material sublayer includes a positive electrode active material, the positive electrode active material includes lithium iron manganese phosphate and a ternary material, and based on the mass of the positive electrode active material of the first positive electrode material sublayer, the mass proportion of lithium iron manganese phosphate in the first positive electrode material sublayer is x 1 , the mass proportion of the ternary material in the first positive electrode material sublayer is y 1 Based on the mass of the positive electrode active material of the second positive electrode material sublayer, the mass proportion of lithium manganese iron phosphate in the second positive electrode material sublayer is x 2 , the mass proportion of the ternary material in the second positive electrode material sublayer is y 2 , x 1 >x 2 ,y 1 <y 2 .

[0005] In the positive electrode sheet proposed in the present application, the positive electrode material layer has a multilayer structure, and each positive electrode material sublayer contains two types of positive electrode active materials, lithium iron manganese phosphate and ternary materials. In different positive electrode material sublayers, the mixing ratio of the two types of positive electrode active materials is different, wherein the mass ratio of lithium iron manganese phosphate to ternary materials in the positive electrode material sublayer relatively close to the separator is relatively high, while the mass ratio of lithium iron manganese phosphate to ternary materials in the positive electrode material sublayer relatively close to the positive electrode current collector is relatively low. As a result, the overall positive electrode material layer has a high and relatively consistent ion migration rate and good fast charging performance.

[0006] In some embodiments, 0.2 ≥ x1 -x 2 >0, 0.2 ≥ y 2 -y 1 ≥ 0. Thus, the difference in the mass fraction of lithium iron manganese phosphate and the difference in the mass fraction of the ternary material between two adjacent positive electrode material sub-layers are within the foregoing ranges, and the positive electrode material layer has good fast charging performance.

[0007] In some embodiments, based on the mass of the positive electrode active material of the first positive electrode material sub-layer, the mass fraction of the lithium iron manganese phosphate is 40% - 100%. Thus, within the foregoing range of the mass fraction of the lithium iron manganese phosphate in the first positive electrode material sub-layer, the first positive electrode material sub-layer has good safety performance.

[0008] In some embodiments, based on the mass of the positive electrode active material of the second positive electrode material sub-layer, the mass fraction of the lithium iron manganese phosphate is 40% - 100%. Thus, within the foregoing range of the mass fraction of the lithium iron manganese phosphate in the second positive electrode material sub-layer, the second positive electrode material sub-layer has good safety performance.

[0009] In some embodiments, based on the mass of the positive electrode active material of the first positive electrode material sub-layer, the mass fraction of the ternary material is 10% - 70%. Thus, within the foregoing range of the mass fraction of the ternary material in the first positive electrode material sub-layer, the first positive electrode material sub-layer has high ionic conductivity.

[0010] In some embodiments, based on the mass of the positive electrode active material of the second positive electrode material sub-layer, the mass fraction of the ternary material is 10% - 70%. Thus, within the foregoing range of the mass fraction of the ternary material in the second positive electrode material sub-layer, the second positive electrode material sub-layer has high ionic conductivity.

[0011] In some embodiments, based on the mass of the positive electrode active material of the positive electrode material layer, the mass fraction of the ternary material is 10% - 70%. Thus, within the foregoing range of the ternary material in the positive electrode material layer, the positive electrode material layer has high energy density.

[0012] In some embodiments, the chemical formula of the lithium iron manganese phosphate satisfies LiMn (1-w) Fe w PO 4 , where 0.5 < w < 1. Thus, on the premise of satisfying the foregoing chemical formula, the lithium iron manganese phosphate materials in each positive electrode material sub-layer can be the same or different. If the manganese to iron ratio in the lithium iron manganese phosphate material is relatively high, the specific capacity of the material is relatively high.

[0013] In some embodiments, the ternary material includes lithium nickel cobaltate, and the chemical formula of the lithium nickel cobaltate satisfies LiNiz Co t M 1-z-t O 2 wherein, M includes at least one of Mn and Al, where 0.5 ≤ z < 1 and 0 < t < 0.5. Thus, in each positive electrode material sub-layer, the ternary material includes at least one of lithium nickel cobalt manganate (NCM) and lithium nickel cobalt aluminate (NCA), and NCA has a higher specific capacity.

[0014] In some embodiments, the particle size Dv50 of the lithium iron manganese phosphate is 0.5 μm - 2 μm. Thus, the porosity of each positive electrode material sub-layer relatively decreases as the mass ratio of the lithium iron manganese phosphate material therein increases, so that the porosity of the positive electrode material sub-layer can be adjusted by controlling the content of the lithium iron manganese phosphate.

[0015] In some embodiments, the particle size Dv50 of the ternary material is 2 μm - 6 μm. Thus, the porosity of each positive electrode material sub-layer relatively increases as the mass ratio of the ternary material therein increases, so that the porosity of the positive electrode material sub-layer can be adjusted by controlling the content of the ternary material.

[0016] In some embodiments, the porosity of the first positive electrode material sub-layer is 30% - 60%. Within the foregoing porosity range, the first positive electrode material sub-layer has a relatively large contact area with the electrolyte, which helps the electrolyte to be fully infiltrated.

[0017] In some embodiments, the porosity of the second positive electrode material sub-layer is 30% - 60%. Within the foregoing porosity range, the second positive electrode material sub-layer has a relatively large contact area with the electrolyte, which helps the electrolyte to be fully infiltrated.

[0018] In some embodiments, the porosity of the positive electrode material layer is 30% - 60%. Within the foregoing porosity range, the entire positive electrode material layer has a good contact with the electrolyte, which helps the electrolyte to be fully infiltrated.

[0019] In some embodiments, the thickness of the first positive electrode material sub-layer is 50 μm - 200 μm. Thus, the first positive electrode material sub-layer can have good cycling performance.

[0020] In some embodiments, the thickness of the second positive electrode material sub-layer is 50 μm - 200 μm. Thus, the second positive electrode material sub-layer can have good cycling performance.

[0021] In some embodiments, the thickness of the positive electrode material layer is 100 μm - 400 μm. Thus, by providing multiple positive electrode material sub-layers, the positive electrode material layer can have a high ion migration rate and fast charging performance.

[0022] In some embodiments, the first positive electrode material sublayer further includes a binder, and the mass fraction of the binder is 1%-2.5% based on the mass of the first positive electrode material sublayer; the second positive electrode material sublayer further includes a binder, and the mass fraction of the binder is 1%-2.5% based on the mass of the second positive electrode material sublayer. Thus, the binder can improve the structural stability of the positive electrode material sublayer and reduce undesirable conditions such as powder shedding.

[0023] In some embodiments, the mass fraction of the binder in the first positive electrode material sublayer is greater than the mass fraction of the binder in the second positive electrode material sublayer. Thus, the positive electrode material layer has better safety performance and better cycle performance.

[0024] In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl alcohol (PVA). Thus, the polymer binder used has good adhesion.

[0025] In the second aspect of the present application, the present application proposes a lithium battery, including the positive electrode sheet proposed in the present application. Therefore, the lithium battery proposed in the present application has good cycle performance, safety performance and fast charging performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0027] Figure 1 This is a schematic diagram of the structure of a lithium battery in one embodiment of the present application.

[0028] Description of reference numerals:

[0029] 1-negative electrode current collector; 2-negative electrode material layer; 3-separator; 4-second positive electrode material sublayer; 5-first positive electrode material sublayer; 6-positive electrode current collector. DETAILED DESCRIPTION

[0030] The embodiments of the present application are described in detail below. Examples of the embodiments are shown in the drawings, but unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of actually identical structures may be omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0031] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0032] The terms "including" and "having" in the specification and claims of the present application and any modifications thereof are open expressions, that is, including the contents specified in the present application but not excluding other contents.

[0033] In the description of this application, regardless of whether the words "about" or "approximately" are used, all the numbers disclosed herein are approximate values. The value of each number may differ by less than 10% or a reasonable difference considered by those skilled in the art, such as 1%, 2%, 3%, 4% or 5%.

[0034] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is 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 listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0035] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0036] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0037] In the positive electrode sheet of commercial lithium-ion batteries, lithium manganese iron phosphate (LMFP) is one of the positive electrode active materials, which has a higher voltage platform and is conducive to improving the endurance of electric vehicles. At the same time, LMFP has good safety performance, and the raw material source is abundant and the price is low. As a positive electrode active material, its cost is relatively controllable for large-scale production and application. However, the electronic conductivity and lithium ion diffusion coefficient of LMFP are low, and the rate of lithium ion escape from the positive electrode sheet of the battery is low, resulting in low fast charging capacity and unable to meet the demand for fast charging. In contrast, the specific capacity of ternary materials is higher and has a higher energy density, but its safety performance as a positive electrode active material is poor, the cycle life is short, and the cost is high. The positive electrode sheet proposed in this application rationally designs LMFP and ternary materials under a multilayer structure, which can maximize the advantages of the two types of positive electrode active materials, while making the entire positive electrode sheet have better performance.

[0038] In a first aspect of the present application, the present application proposes a positive electrode sheet, comprising:

[0039] A positive electrode current collector; a positive electrode material layer, wherein the positive electrode material layer is arranged on at least one side of the positive electrode current collector; the positive electrode material layer includes a first positive electrode material sublayer and a second positive electrode material sublayer, wherein the second positive electrode material sublayer is arranged on a side of the first positive electrode material sublayer away from the positive electrode current collector, wherein the first positive electrode material sublayer includes a positive electrode active material, the second positive electrode material sublayer includes a positive electrode active material, the positive electrode active material includes lithium iron manganese phosphate and a ternary material, and based on the mass of the positive electrode active material of the first positive electrode material sublayer, the mass proportion of lithium iron manganese phosphate in the first positive electrode material sublayer is x 1 , the mass proportion of the ternary material in the first positive electrode material sublayer is y 1 Based on the mass of the positive electrode active material of the second positive electrode material sublayer, the mass proportion of lithium manganese iron phosphate in the second positive electrode material sublayer is x 2 , the mass proportion of the ternary material in the second positive electrode material sublayer is y 2 , x 1 >x 2 ,y 1 <y 2 .

[0040] In the positive electrode sheet proposed in the present application, the positive electrode material layer has a multi-layer structure, and each positive electrode material sub-layer contains two types of positive electrode active materials: lithium manganese iron phosphate and ternary material. Figure 1In a lithium battery, the positive electrode sheet includes a first positive electrode material sublayer 5, a second positive electrode material sublayer 4, and a positive electrode current collector 6. In different positive electrode material sublayers, the mixing ratios of the two types of positive electrode active materials are different. In the positive electrode material sublayer relatively close to the positive electrode current collector, the mass ratio of lithium iron manganese phosphate to the ternary material is relatively high, while in the positive electrode material sublayer relatively close to the separator, the mass ratio of lithium iron manganese phosphate to the ternary material is relatively low. Ternary materials have higher ion conductivity and higher energy density than lithium iron manganese phosphate. Therefore, the ion migration ability of the first positive electrode material sublayer relatively close to the positive electrode current collector is stronger than that of the second positive electrode material sublayer, so that the resistance to the diffusion of lithium ions in the positive electrode material sublayer and the positive electrode material layer is reduced, which facilitates the deintercalation of lithium ions in the positive electrode material sublayer relatively close to the current collector. In addition, the particle size of lithium iron manganese phosphate is relatively small, and it can be filled in the gaps between the particles of the ternary material to form a relatively stable positive electrode material layer structure, and at the same time constitute an ion transmission path, so that the cycle performance and power performance of the positive electrode material layer are improved. In addition, compared with ternary materials, lithium manganese iron phosphate has a more stable crystal structure and better thermal stability. During the charging and discharging process, it can balance the heat accumulation effect of the ternary material during charging and discharging, thereby improving the safety performance of the positive electrode material layer. As a result, the ion transmission capabilities of different positive electrode material sublayers are relatively consistent, so that the overall positive electrode material layer has a higher energy density, a higher and relatively consistent ion migration rate, better fast charging performance and better safety performance.

[0041] In some embodiments, 0.2 ≥ x 1 -x 2 >0,0.2≥y 2 -y 1 ≥0. Therefore, the difference in the mass proportion of lithium iron manganese phosphate and the mass proportion of ternary materials in the two adjacent positive electrode material sublayers are within the aforementioned range. Since there is a certain difference in the particle size of lithium iron manganese phosphate and the ternary material, the particle size of the ternary material is larger than that of lithium iron manganese phosphate, and the positive electrode material sublayer with a relatively small mass ratio of lithium iron manganese phosphate and ternary material has more pores. The positive electrode material sublayer relatively close to the separator is provided with a relatively low mass ratio of positive electrode active materials of lithium iron manganese phosphate and ternary materials, which makes the infiltration of the electrolyte easier and more sufficient, and facilitates the freer migration and diffusion of lithium ions before and after deintercalation in the positive electrode material sublayer closer to the positive electrode current collector, thereby reducing the polarization of lithium ions in the positive electrode of the battery. The mass ratio of the two positive electrode material sublayers is within the aforementioned difference range, and the deintercalation and migration ability of lithium ions in the positive electrode material layer is stronger, which is conducive to the positive electrode material layer having greater power performance and better fast charging performance, and can shorten the time for the positive electrode of the battery to reach the cut-off voltage.

[0042] In some embodiments, based on the mass of the positive active material of the first positive electrode material sublayer, the mass fraction of the lithium manganese iron phosphate is 40%-100%. Thus, in the first positive electrode material sublayer, the mass fraction of the lithium manganese iron phosphate is within the aforementioned range, so that the first positive electrode material sublayer has better safety performance.

[0043] In some embodiments, based on the mass of the positive active material of the second positive electrode material sublayer, the mass fraction of the lithium iron manganese phosphate is 40%-100%. Thus, in the second positive electrode material sublayer, the mass fraction of the lithium iron manganese phosphate is within the aforementioned range, so that the second positive electrode material sublayer has good safety performance. It should be noted that the mass fractions of the lithium iron manganese phosphate in the first positive electrode material sublayer and the second positive electrode material sublayer may be the same or different.

[0044] In some embodiments, based on the mass of the positive active material of the first positive electrode material sublayer, the mass fraction of the ternary material is 10%-70%. Thus, in the first positive electrode material sublayer, the mass fraction of the ternary material is within the aforementioned range, so that the first positive electrode material sublayer has a higher ionic conductivity.

[0045] In some embodiments, based on the mass of the positive active material of the second positive electrode material sublayer, the mass fraction of the ternary material is 10%-70%. Thus, in the second positive electrode material sublayer, the mass fraction of the ternary material is within the aforementioned range, so that the second positive electrode material sublayer has a higher ionic conductivity. It should be noted that the mass fractions of the ternary material in the first positive electrode material sublayer and the second positive electrode material sublayer may be the same or different. At the same time, the addition of lithium iron manganese phosphate in each positive electrode material sublayer can slow down the heat accumulation during the charging and discharging process of the ternary material, thereby improving the safety performance of the overall positive electrode material layer.

[0046] In some embodiments, based on the mass of the positive active material of the positive electrode material layer, the mass fraction of the ternary material is 10%-70%. Thus, the ternary material in the overall positive electrode material layer is within the aforementioned range, and the positive electrode material layer has a higher capacity density and better cycle performance.

[0047] In some embodiments, the chemical formula of the lithium iron manganese phosphate satisfies LiMn (1-w) Fe w PO 4, where 0.5 < w < 1. Thus, on the premise of satisfying the foregoing chemical formula, the lithium iron manganese phosphate materials in each positive electrode material sub-layer can be the same or different. The change in the manganese-to-iron ratio of the lithium iron manganese phosphate material can adjust the battery performance. Within a certain range of the manganese-to-iron ratio, a higher plateau voltage can be achieved, and at the same time, it has better safety performance. The manganese-to-iron ratio of lithium iron manganese phosphate can also be selected according to the performance of the mixed ternary material.

[0048] In some embodiments, the ternary material includes lithium nickel cobaltate, and the chemical formula of the lithium nickel cobaltate satisfies LiNi z Co t M 1-z-t O 2 , where M includes at least one of Mn and Al, where 0.5 ≤ z < 1 and 0 < t < 0.5. Thus, in each positive electrode material sub-layer, the ternary material includes at least one of lithium nickel cobalt manganese oxide (NCM) and lithium nickel cobalt aluminum oxide (NCA). NCA has a higher energy density than NCM, and the deficiency in its safety performance can be improved by adjusting the mixing ratio with lithium iron manganese phosphate. NCM has better thermal stability and relatively stable electrical properties, and specific properties can be improved by mixing with NCA and / or mixing with lithium iron manganese phosphate, so that the battery achieves higher safety and fast charging performance.

[0049] In some embodiments, the Dv50 particle size of the lithium iron manganese phosphate is 0.5 μm - 2 μm. Thus, the porosity of each positive electrode material sub-layer decreases relatively as the mass ratio of the lithium iron manganese phosphate material to the ternary material therein increases. By mixing with a ternary material with a larger particle size, the infiltration of the electrolyte is relatively easy, thereby reducing the migration resistance of lithium ions.

[0050] In some embodiments, the Dv50 particle size of the ternary material is 2 μm - 6 μm. Thus, the porosity of the positive electrode material sub-layer increases relatively as the mass ratio of the lithium iron manganese phosphate material to the ternary material therein decreases. At the same time, when the porosity is relatively large, there are more active sites in the positive electrode material layer, which is beneficial to the insertion and extraction of lithium ions, improving the fast charging performance of the battery.

[0051] In some embodiments, the porosity of the first positive electrode material sub-layer is 30% - 60%. Within the foregoing porosity range, the first positive electrode material sub-layer has a relatively large contact area with the electrolyte, which helps the full infiltration of the electrolyte.

[0052] In some embodiments, the porosity of the second positive electrode material sublayer is 30%-60%. Within the aforementioned porosity range, the second positive electrode material sublayer has a larger contact area with the electrolyte, which is conducive to the full infiltration of the electrolyte. The degree of infiltration of the electrolyte in different positive electrode material sublayers can be adjusted according to the mass ratio of the lithium iron manganese phosphate material to the ternary material, so that the migration resistance of lithium ions in the positive electrode material layer is reduced and the internal resistance of the battery is reduced.

[0053] In some embodiments, the porosity of the positive electrode material layer is 30%-60%. Within the aforementioned porosity range, the positive electrode material layer as a whole has good contact with the electrolyte. This enables the positive electrode material layer to have good electrolyte storage capacity, so that it maintains a high electrolyte absorption speed, which is beneficial to improving the ion migration capacity in the positive electrode sheet.

[0054] In some embodiments, the thickness of the first cathode material sublayer is 50 μm-200 μm. Thus, the first cathode material sublayer can have good cycle performance.

[0055] In some embodiments, the thickness of the second positive electrode material sublayer is 50 μm-200 μm. Thus, the second positive electrode material sublayer can have good cycle performance. The path of ion migration in the positive electrode material sublayer is short, the ion migration ability is strong, and the power performance is good.

[0056] In some embodiments, the thickness of the positive electrode material layer is 100 μm-400 μm. Thus, the positive electrode material layer can have a higher ion migration rate, and the battery has better cycle performance.

[0057] In some embodiments, the first positive electrode material sublayer further includes a binder, and the mass fraction of the binder is 1%-2.5% based on the mass of the first positive electrode material sublayer; the second positive electrode material sublayer further includes a binder, and the mass fraction of the binder is 1%-2.5% based on the mass of the second positive electrode material sublayer. Thus, the binder can improve the structural stability of the positive electrode material sublayer and reduce undesirable conditions such as powder shedding.

[0058] In some embodiments, the mass fraction of the binder in the first positive electrode material sublayer is greater than the mass fraction of the binder in the second positive electrode material sublayer. Thus, the binder can better fix the lithium iron manganese phosphate and the ternary material in the positive electrode material sublayer. When the mass of the lithium iron manganese phosphate and the ternary material is high, the positive electrode active material in the positive electrode material sublayer has better structural stability, and the structural degradation after cyclic charge and discharge is less, so that the positive electrode material layer has better safety performance and better cycle performance.

[0059] In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl alcohol (PVA). Thus, the polymer binder used has good adhesion.

[0060] In a second aspect of the present application, the present application proposes a lithium battery, including the positive electrode sheet proposed in the present application. Figure 1 In the lithium battery, the positive electrode sheet includes a first positive electrode material sublayer 5, a second positive electrode material sublayer 4, and a positive electrode current collector 6, the negative electrode sheet includes a negative electrode current collector 1 and a negative electrode material layer 2, and the lithium battery includes a separator 3. Therefore, the lithium battery proposed in the present application has good cycle performance, safety performance, and fast charging performance.

[0061] The scheme of the present application is described below by specific examples. It should be noted that the following examples are only used to illustrate the present application and should not be considered as limiting the scope of the present application. If no specific technology or conditions are specified in the examples, the technology or conditions described in the literature in this area or the product instructions are used. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained commercially.

[0062] Example 1

[0063] In the first positive electrode material sublayer: lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 The mass of LiNi accounts for 76.5%. 0.8 Co 0.1 Mn 0.1 O 2 The mass percentage of the positive electrode material is 19.1%, the rest is 2.2% polyvinylidene fluoride (PVDF), 1.7% conductive carbon black (SP), and 0.5% carbon nanotube (CNT). The thickness of the first positive electrode material sublayer after rolling is 63 μm.

[0064] In the second positive electrode material sublayer: lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 The mass of LiNi accounts for 57.7%. 0.8 Co 0.1 Mn 0.1 O 2 The mass proportion is 38.5%, the rest is 1.8% polyvinylidene fluoride (PVDF) by mass, 1.5% conductive carbon black (SP) by mass, 0.5% carbon nanotubes (CNT) by mass, and the thickness of the second positive electrode material sublayer is 63μm after rolling.

[0065] Example 2

[0066] Example 2 is consistent with Example 1, except that: lithium manganese iron phosphate is LiMn 0.5 Fe 0.5 PO 4 .

[0067] Comparative Example 1

[0068] In the positive electrode material layer: lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 The mass proportion of is 67.1%, the mass proportion of ternary materials is 28.7%, and the rest is polyvinylidene fluoride (PVDF) (2.1%), conductive carbon black (SP) (1.6%), and carbon nanotubes (CNT) (0.5%). The thickness of the positive electrode material layer is 126μm.

[0069] Test method:

[0070] After assembling the positive electrode plates into lithium batteries, they are then divided into different capacities.

[0071] 1. Discharge specific capacity and discharge voltage test at 25°C:

[0072] 0.33C discharge to 2.5V, let stand for 1h; 0.33C cc to 4.25V, 4.25V cv, 0.05C cut-off, let stand for 1h, 0.33C discharge to 2.5V, let stand for 1h, repeat three times; take the third discharge capacity as c 0 The third charge capacity is 0.33C constant current constant voltage mode (cc-cv) and the capacity is c 1 , take the average discharge voltage as 0.33C rate discharge voltage.

[0073] 2. Ratio test

[0074] Charge to 4.25V at 0.33C constant current, let stand for 1h, the charge capacity is c 2 ;

[0075] Charge to 4.25V at 1C constant current, let stand for 1h, the charge capacity is c 3 , discharge to 2.5V at 0.33C, and let stand for 1h;

[0076] Charge to 4.25V at 2C constant current, let stand for 1h, the charge capacity is c 4 , discharge to 2.5V at 0.33C, and let stand for 1h;

[0077] Charge to 4.25V at 3C constant current, let stand for 1h, the charge capacity is c 5, discharge to 2.5V at 0.33C, and let stand for 1h; the constant current ratios of the rate charge are (0.33C, 1C, 2C, 3C): c 2 / c 1 , c 3 / c 1 , c 4 / c 1 , c 5 / c 1 According to c 0 , and the weight of the pole sheet area of ​​the positive electrode material are used to calculate the first discharge specific capacity at a rate of 0.33C.

[0078] Test results: see Table 1

[0079] The ratio of the charging current in the constant current charging stage to the rated capacity of the battery is 0.33 times.

[0080] Table 1

[0081]

[0082] It can be seen from Table 1 that the positive electrode plate proposed in the present application has improved constant current ratios obtained in constant current charging tests at different rates compared with comparative example 1 with the same positive electrode material layer thickness, indicating that the positive electrode plate proposed in the present application has better fast charging performance.

[0083] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. "First feature" and "second feature" may include one or more of the features.

[0084] In the description of the present application, a first feature being “on” or “under” a second feature may include that the first and second features are directly in contact with each other, or may include that the first and second features are not in direct contact with each other but are in contact with each other via another feature therebetween.

[0085] In the description of the present application, “above”, “over” and “above” a first feature to a second feature includes the first feature being directly above and obliquely above the second feature, or simply means that the first feature is horizontally higher than the second feature.

[0086] In the description of the present application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for examples and may be any technical feature connected by "and / or" in the present application.

[0087] In the present application, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of each step should be determined by its function and possible internal logic. If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) can be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0088] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode sheet, characterized in that: include: Positive electrode current collector; A positive electrode material layer, the positive electrode material layer being disposed on at least one side of the positive electrode current collector; The positive electrode material layer includes a first positive electrode material sublayer and a second positive electrode material sublayer, wherein the second positive electrode material sublayer is arranged on a side of the first positive electrode material sublayer away from the positive electrode current collector, wherein the first positive electrode material sublayer includes a positive electrode active material, the second positive electrode material sublayer includes a positive electrode active material, the positive electrode active material includes lithium iron manganese phosphate and a ternary material, based on the mass of the positive electrode active material of the first positive electrode material sublayer, the mass proportion of lithium iron manganese phosphate in the first positive electrode material sublayer is x1, and the mass proportion of the ternary material in the first positive electrode material sublayer is y1; based on the mass of the positive electrode active material of the second positive electrode material sublayer, the mass proportion of lithium iron manganese phosphate in the second positive electrode material sublayer is x2, and the mass proportion of the ternary material in the second positive electrode material sublayer is y2, x1>x2, y1 <y2。 2. The positive electrode sheet according to claim 1, characterized in that: 0.2≥x1-x2>0, 0.2≥y2-y1≥0.

3. The positive electrode sheet according to claim 1, characterized in that: Based on the mass of the positive electrode active material of the first positive electrode material sublayer, the mass fraction of the lithium manganese iron phosphate is 40%-100%; and / or, Based on the mass of the positive electrode active material of the second positive electrode material sublayer, the mass fraction of the lithium manganese iron phosphate is 40%-100%; and / or, Based on the mass of the positive electrode active material of the first positive electrode material sub-layer, the mass fraction of the ternary material is 10%-70%; and / or, Based on the mass of the positive electrode active material of the second positive electrode material sub-layer, the mass fraction of the ternary material is 10%-70%; and / or, Based on the mass of the positive electrode active material of the positive electrode material layer, the mass fraction of the ternary material is 10%-70%.

4. The positive electrode sheet according to claim 1, characterized in that: The chemical formula of the lithium iron manganese phosphate satisfies LiMn w Fe w PO4, where 0.5 < w < 1; and / or, The ternary material includes lithium nickel cobalt oxide, and the chemical formula of the lithium nickel cobalt oxide satisfies LiNi z Co t M 1-z-t O2, wherein M includes at least one of Mn and Al, wherein 0.5≤z<1,0 <t<0.5。 5. The positive electrode sheet according to claim 1, characterized in that: The particle size Dv50 of the lithium manganese iron phosphate is 0.5 μm-2 μm; and / or, The particle size Dv50 of the ternary material is 2 μm-6 μm.

6. The positive electrode sheet according to claim 1, characterized in that: The porosity of the first cathode material sublayer is 30%-60%; and / or, The porosity of the second positive electrode material sublayer is 30%-60%; and / or, The porosity of the positive electrode material layer is 30%-60%.

7. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: The thickness of the first positive electrode material sublayer is 50 μm-200 μm; and / or, The thickness of the second cathode material sublayer is 50 μm-200 μm; and / or, The thickness of the positive electrode material layer is 100 μm-400 μm.

8. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: The first positive electrode material sublayer further includes a binder, and the mass fraction of the binder is 1%-2.5% based on the mass of the first positive electrode material sublayer; The second positive electrode material sub-layer further includes a binder. Based on the mass of the second positive electrode material sub-layer, the mass fraction of the binder is 1%-2.5%.

9. The positive electrode sheet according to claim 8, characterized in that: The mass fraction of the binder in the first positive electrode material sublayer is greater than the mass fraction of the binder in the second positive electrode material sublayer; and / or, The binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and polyvinyl alcohol.

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

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