Positive pole piece, preparation method thereof and battery

Abstract

The invention relates to the technical field of batteries, in particular to a positive pole piece, a preparation method thereof and a battery. The positive pole piece comprises a current collector; the first active layer is arranged on the surface of at least one side of the current collector in the thickness direction, and the first active layer comprises a first active substance, a first binder and a first conductive agent; the second active layer is arranged on the surface, far away from the current collector, of the first active layer, and the second active layer comprises a second active substance and a second binder; wherein the crystallinity of the first binder is greater than the crystallinity of the second binder, and the average particle size Dv50 of the first active substance is smaller than the average particle size Dv50 of the second active substance. The positive pole piece has high surface density so as to improve the energy density, and also can effectively inhibit the rising of ohmic impedance and charge transfer impedance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a positive electrode sheet, a method for preparing the same, and a battery. Background Technology

[0002] In electrochemical energy storage devices such as lithium-ion batteries, energy density is one of the core performance indicators. One traditional method to improve energy density is to increase the coating areal density of the electrode, i.e., to load more active material (such as positive or negative electrode material) onto a unit area of ​​current collector while reducing the proportion of inactive components (such as binders and conductive agents), thereby improving the utilization rate of electrode material per unit mass or volume. However, as the coating areal density increases, the electrode thickness also increases, leading to a longer ion transport path and increased charge transport resistance within the electrode. This phenomenon is mainly reflected in two key parameters: ohmic resistance, as increased electrode thickness increases the conduction path length of electrons and ions within the electrode and in the electrolyte, resulting in increased ohmic resistance, especially when the conductive network between active material particles is insufficient, leading to decreased electronic conductivity and further exacerbating the internal resistance problem; and charge transfer resistance, as the reaction kinetics at the electrode / electrolyte interface are limited by the exposed area of ​​active sites and the ion diffusion rate. High areal density electrodes may experience difficulties in electrolyte penetration and decreased interfacial charge transfer efficiency due to dense particle packing or reduced porosity.

[0003] The aforementioned increase in impedance significantly deteriorates the battery's rate performance (i.e., its charge / discharge capability at high current densities). When the battery operates at high rates, the polarization voltage drop caused by internal resistance increases, resulting in a sharp drop in capacity, voltage plateau collapse, and increased heat generation. For example, in fast charging or high-power output scenarios, the actual usable capacity of the battery may be significantly lower than the theoretical value due to impedance limitations. Therefore, how to effectively suppress the rise in ohmic impedance and charge transfer impedance while increasing areal density to improve energy density has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] In view of this, the present invention aims to at least partially solve one of the technical problems in the related art. To this end, the present invention provides a positive electrode sheet, a method for preparing the same, and a battery, wherein the positive electrode sheet, while having a high areal density to improve energy density, can also effectively suppress the rise in ohmic impedance and charge transfer impedance.

[0005] To solve the above-mentioned technical problems, this application is implemented as follows: According to one aspect of this application, an embodiment of this application provides a positive electrode sheet, the positive electrode sheet comprising: current collector; A first active layer is disposed on at least one side surface of the current collector along the thickness direction, and the first active layer includes a first active material, a first binder, and a first conductive agent; A second active layer is disposed on the surface of the first active layer away from the current collector, and the second active layer includes a second active substance and a second binder; Wherein, the crystallinity of the first binder is greater than that of the second binder, and the average particle size Dv50 of the first active material is less than that of the second active material.

[0006] In some embodiments, the crystallinity of the first adhesive is C1; the crystallinity of the second adhesive is C2; The C1 and C2 satisfy the following relationship: 8% ≤ C1 - C2 ≤ 18%.

[0007] In some embodiments, the average particle size Dv50 of the second active material is D2; and the crystallinity of the second binder is C2. The C2 and D2 satisfy the following relationship: 40≤C2×D2×100≤70.

[0008] In some of these embodiments, the crystallinity C1 of the first adhesive is 45% to 50%.

[0009] In some of these embodiments, the crystallinity C2 of the second binder is 32% to 37%.

[0010] In some embodiments, the average particle size Dv50 of the first active material is 0.5~0.9 μm.

[0011] In some of these embodiments, the average particle size Dv50 of the second active material is 1~3 μm.

[0012] In some embodiments, the mass ratio of the first active substance, the first binder, and the first conductive agent in the first active layer is (96~98):(1.5~2.5):(0.5~1.5).

[0013] In some of these embodiments, the first active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide.

[0014] In some embodiments, the first adhesive includes at least one of polyvinylidene fluoride, carboxylic acid-modified polyvinylidene fluoride, and acrylic acid-modified polyvinylidene fluoride.

[0015] In some embodiments, the first conductive agent includes at least one of carbon black, carbon nanotubes, carbon nanofibers, and graphene.

[0016] In some of these embodiments, the thickness of the first active layer is 60-120 μm.

[0017] In some embodiments, the second active layer further includes a second conductive agent.

[0018] In some of these embodiments, the mass ratio of the second active material, the second binder, and the second conductive agent in the second active layer is (96~98):(1.5~3):(0~1).

[0019] In some of these embodiments, the second active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide.

[0020] In some embodiments, the second adhesive includes at least one of polyvinylidene fluoride, carboxylic acid-modified polyvinylidene fluoride, and acrylic acid-modified polyvinylidene fluoride.

[0021] In some embodiments, the second conductive agent includes at least one of carbon black, carbon nanotubes, carbon nanofibers, and graphene.

[0022] In some of these embodiments, the thickness of the second active layer is 60-120 μm.

[0023] According to another aspect of this application, embodiments of this application provide a method for preparing a positive electrode sheet, comprising the following steps: A first active slurry is coated on at least one side of the current collector to obtain a first active layer; A second active slurry is coated onto the surface of the first active layer to obtain a second active layer; The current collector coated with the first active layer and the second active layer is dried and rolled to obtain the positive electrode sheet.

[0024] In some embodiments, the preparation of the first active slurry includes: mixing the first active substance, the first binder and the first conductive agent uniformly in a solvent to obtain the first active slurry.

[0025] In some embodiments, the preparation of the second active slurry includes: mixing the second active substance, the second binder, and the second conductive agent uniformly in a solvent to obtain the second active slurry.

[0026] In some of these embodiments, the drying temperature is 70~110°C.

[0027] In some embodiments, the compaction density of the roller press is 2.4~2.8 g / cm³. 3 .

[0028] According to another aspect of this application, an embodiment of this application provides a battery including a positive electrode sheet, the positive electrode sheet including the aforementioned positive electrode sheet, or including a positive electrode sheet prepared according to the aforementioned preparation method.

[0029] Implementing the technical solution of the present invention has at least the following beneficial effects: In the embodiments of this application, the provided positive electrode sheet can control the pore size of the second active layer to be larger than that of the first active layer by controlling the particle size of the second active material to be no smaller than that of the first active material; furthermore, the binder of the second active layer has lower crystallinity (where crystallinity represents the degree of order of molecular chains, the lower the crystallinity, the closer the material is to an amorphous state, and the easier it is to swell and dissolve in organic solvents), and the binder swells more during the liquid injection and resting period, thus the pore size of the second active layer is larger; the porosity of the second active material layer is comprehensively improved, thereby improving the rate performance of the positive electrode sheet.

[0030] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0031] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0032] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0033] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0034] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0035] Unless otherwise specified, all steps in this application may 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 it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0036] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0037] One method to improve battery energy density is to increase the coating areal density and reduce the proportion of inactive materials. However, as the areal density increases, ohmic resistance and charge transfer resistance increase significantly, leading to a deterioration in battery rate performance.

[0038] In view of this, embodiments of this application provide a positive electrode sheet, the positive electrode sheet comprising: current collector; A first active layer is disposed on at least one side surface of the current collector along the thickness direction, and the first active layer includes a first active material, a first binder, and a first conductive agent; A second active layer is disposed on the surface of the first active layer away from the current collector, and the second active layer includes a second active substance and a second binder; Wherein, the crystallinity of the first binder is greater than that of the second binder, and the average particle size Dv50 of the first active material is less than that of the second active material.

[0039] In this application, the particle size of the second active material is controlled to be no smaller than that of the first active material, and the pore size of the second active layer is controlled to be larger than that of the first active layer. Furthermore, the binder of the second active layer has a lower crystallinity (where crystallinity represents the degree of order of the molecular chain; the lower the crystallinity, the closer the material is to an amorphous state, and the easier it is to swell and dissolve in organic solvents). The binder swells more during the liquid injection and resting period, thus the pore size of the second active layer is larger. This comprehensively improves the porosity of the second active material layer, thereby improving the rate performance of the positive electrode.

[0040] In some embodiments, the crystallinity of the first adhesive is C1; the crystallinity of the second adhesive is C2; and C1 and C2 satisfy the following relationship: 8% ≤ C1 - C2 ≤ 18%. The crystallinity of the first and second adhesives satisfying this relationship ensures the bonding effect between the first and second active layers. If the crystallinity of the second adhesive is too low, such as the difference between the crystallinity of the first and second adhesives being greater than 18%, although the second active layer may have higher porosity, the bonding strength will be too low, making it easier for the first and second active layers to peel off.

[0041] In some embodiments, the average particle size Dv50 of the second active material is D2; the crystallinity of the second binder is C2; and C2 and D2 satisfy the following relationship: 40 ≤ C2 × D2 × 100 ≤ 70. The inventors have discovered that the average particle size Dv50 of the second active material and the crystallinity of the second binder have a certain synergistic effect. Specifically, the higher the crystallinity of the second binder, the smaller its swelling and the smaller the pores generated by the swelling; the larger the average particle size Dv50 of the second active material, the larger the corresponding pores of the second active layer. When the product of the two is within the above range, the porosity can be maximized while ensuring that the ionic solid-phase impedance is within an acceptable range. When the value of the above formula is too large, the increase in material particle size will increase the solid-phase diffusion distance inside the material. This effect outweighs the improvement in ion-liquid phase transport brought about by the increase in the pore size of the second active layer, and instead deteriorates the rate performance of the battery. When the value of the above formula is too small, the effect of the increase in internal diffusion brought about by the decrease in particle size is not significant. The influence of ion-liquid phase transport brought about by the increase in the pore size of the second active layer on the rate performance of the battery is dominant. At this time, if the value of the formula continues to decrease, the pore size of the second active layer decreases, and the rate performance of the battery also deteriorates.

[0042] In some embodiments, the crystallinity C1 of the first adhesive is 45% to 50%. As an example, the crystallinity C1 of the first adhesive can be 45%, 48%, 50%, etc., or other values ​​within the above range, which are not limited here.

[0043] In some embodiments, the crystallinity C2 of the second adhesive is 32% to 37%. As an example, the crystallinity C2 of the second adhesive can be 32%, 35%, 37%, etc., or other values ​​within the above range, which are not limited here.

[0044] In this application, the crystallinity range of the first binder is within the conventional range of crystallinity used in the art, which can provide sufficient adhesion between the first active layer and the current collector. Simultaneously, the second binder within the aforementioned range can exhibit high swelling, which is beneficial for improving the pore size of the second active layer, thereby improving the overall rate performance of the positive electrode. Furthermore, the crystallinity of both the first and second binders within the aforementioned range satisfies the requirements of the aforementioned relationship, enabling the positive electrode to have superior rate performance while also ensuring good adhesion between the current collector and the first active layer, and between the first and second active layers. Specifically, some of the first binder in the first active layer will float to the surface of the first active layer, and the side of the first active layer away from the current collector has a rougher surface compared to the current collector. Therefore, even with a lower adhesion force from the second binder, a relatively good adhesion effect can still be maintained between the first and second active layers.

[0045] In some embodiments, the average particle size Dv50 of the first active material is 0.5~0.9 μm. As an example, the average particle size Dv50 of the first active material can be 0.5 μm, 0.7 μm, 0.9 μm, etc., or other values ​​within the above range, which are not limited here.

[0046] In some embodiments, the average particle size Dv50 of the second active material is 1~3 μm. As an example, the average particle size Dv50 of the second active material can be 1 μm, 2 μm, 3 μm, etc., or other values ​​within the above range, which are not limited here.

[0047] In some embodiments, the testing methods for the average particle size Dv50 of the first active material and the average particle size Dv50 of the second active material include: placing the material to be tested in a Malvern laser particle size analyzer and reading the particle size value when the particle volume distribution accounts for 50%.

[0048] In some embodiments, the mass ratio of the first active material, the first binder, and the first conductive agent in the first active layer is (96~98):(1.5~2.5):(0.5~1.5). As an example, the mass ratio of the first active material, the first binder, and the first conductive agent in the first active layer can be 96:1.5:0.5, 97:2:1, 98:2.5:1.5, etc., and of course, other ratios within the above range are also possible, without limitation. If the proportion of the first binder is too low, the electrode peeling force is poor, and the first active layer is easily peeled off, affecting battery performance; if the proportion of the first binder is too high, the proportion of active material in the first active layer is too low, affecting the battery's energy density. Similarly, if the proportion of the first conductive agent is too low, the overall electronic conductivity of the positive electrode is poor, affecting the battery's rate performance; if the proportion of the first conductive agent is too high, the proportion of active material in the first active layer is too low, affecting the battery's energy density.

[0049] In some embodiments, the first active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide. As an example, the first active material may be lithium iron phosphate or lithium manganese iron phosphate.

[0050] In some embodiments, the first adhesive includes at least one of polyvinylidene fluoride (PVDF), carboxylic acid-modified PVDF, and acrylic acid-modified PVDF. As an example, the first adhesive may be PVDF or carboxylic acid-modified PVDF.

[0051] In some embodiments, the first conductive agent includes at least one of carbon black, carbon nanotubes, carbon nanofibers, and graphene. As an example, the first conductive agent may be carbon black or carbon nanotubes.

[0052] In some embodiments, the thickness of the first active layer is 60~120μm. As an example, the thickness of the first active layer can be 60μm, 80μm, 100μm, 120μm, etc., or other values ​​within the above range, and is not limited here. When the thickness of the first active layer is too low, the battery energy density is insufficient and it is not conducive to processing and manufacturing; when the thickness of the first active layer is too high, the battery rate performance is poor, which is not conducive to practical applications.

[0053] In some embodiments, the second active layer further includes a second conductive agent.

[0054] In some embodiments, the mass ratio of the second active material, the second binder, and the second conductive agent in the second active layer is (96~98):(1.5~3):(0~1). As an example, the mass ratio of the second active material, the second binder, and the second conductive agent in the second active layer can be 96:3:1, 97:2.5:0.5, 98:2, etc., and of course, other ratios within the above range are also possible, without limitation. If the proportion of the second binder is too low, the number of pores produced by the binder will be insufficient, affecting the rate performance of the battery; if the proportion of the second binder is too high, the proportion of active material in the second active layer will be too low, affecting the energy density of the battery.

[0055] In this application, the second active layer is located on the surface of the first active layer. During charging and discharging, it only needs to conduct electrons of its own layer, while the first active layer needs to conduct electrons shared by the first and second active layers. Therefore, its amount added is reduced compared to the first conductive agent, ensuring the maximum utilization of energy density.

[0056] In some embodiments, the second active material includes at least two of lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide. As an example, the second active material may be lithium iron phosphate or lithium manganese iron phosphate.

[0057] In some embodiments, the second adhesive comprises at least two of polyvinylidene fluoride (PVDF), carboxylic acid-modified PVDF, and acrylic acid-modified PVDF. As an example, the second adhesive may be PVDF or carboxylic acid-modified PVDF.

[0058] In some embodiments, the second conductive agent includes at least two of carbon black, carbon nanotubes, carbon nanofibers, and graphene. As an example, the second conductive agent may be carbon black or carbon nanotubes.

[0059] In some embodiments, the thickness of the second active layer is 60-120 μm. As an example, the thickness of the second active layer can be 60 μm, 80 μm, 100 μm, 120 μm, etc., or other values ​​within the above range, and is not limited here. When the thickness of the second active layer is too low, the battery energy density is insufficient and it is not conducive to processing and manufacturing; when the thickness of the second active layer is too high, the battery rate performance is poor, which is not conducive to practical applications.

[0060] Based on the same inventive concept, this application provides a method for preparing a positive electrode sheet, including the following steps: A first active slurry is coated on at least one side of the current collector to obtain a first active layer; A second active slurry is coated onto the surface of the first active layer to obtain a second active layer; The current collector coated with the first active layer and the second active layer is dried and rolled to obtain the positive electrode sheet.

[0061] It should be understood that all the features and advantages described above regarding the "positive electrode sheet" also apply to the "preparation method of the positive electrode sheet", and will not be repeated here.

[0062] In some embodiments, the preparation of the first active slurry includes: mixing the first active substance, the first binder and the first conductive agent uniformly in a solvent to obtain the first active slurry.

[0063] In some embodiments, the preparation of the second active slurry includes: mixing the second active substance, the second binder, and the second conductive agent uniformly in a solvent to obtain the second active slurry.

[0064] In some embodiments, after coating the first active layer, the process further includes a step of drying the current collector coated with the first active layer. The drying temperature is 70~110°C. Examples include 70°C, 90°C, and 110°C, but other values ​​within the above range are also possible and are not limited here.

[0065] In some embodiments, the drying temperature after applying the second coating is 70~110°C. As examples, drying temperatures of 70°C, 90°C, 110°C, etc., are possible, but other values ​​within the above range are not limited here.

[0066] In some embodiments, the compaction density of the roller press is 2.4~2.8 g / cm³. 3 As an example, the compaction density of roller pressing can be 2.4 g / cm³. 3 2.5g / cm 3 2.6g / cm 3 2.8g / cm 3 "etc." can also be other values ​​within the above range, and no restrictions are imposed here.

[0067] Based on the same inventive concept, this application provides a battery including a positive electrode sheet, which includes the aforementioned positive electrode sheet or a positive electrode sheet prepared according to the aforementioned preparation method.

[0068] In some embodiments, the battery further includes a negative electrode, an electrolyte, and a separator. That is, the battery includes a positive electrode, a negative electrode, an electrolyte, and a separator.

[0069] In this embodiment, the materials and structures of the negative electrode current collector, the conductive agent and the binder in the negative electrode active material layer are not limited, and negative electrode structures and components known in the art that can be used in secondary batteries can be selected.

[0070] In this embodiment, the specific material or type of the separator is not limited, and any separator known in the art that can be used in secondary batteries can be selected.

[0071] It should also be noted that the battery in this application does not limit the specific material or type of electrolyte. Any components and types known in the art that can be used in secondary batteries can be selected, as long as the purpose of this application can be achieved.

[0072] Since the battery provided in this embodiment of the invention adopts all the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, and will not be described in detail here.

[0073] The following describes the implementation methods of this application. The implementation methods described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the implementation methods, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents, materials, or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0074] Example 1 Preparation of the first active slurry: The first lithium iron phosphate, the first binder polyvinylidene fluoride (PVDF), and conductive carbon black (Super-P) were placed in a stirred tank in a ratio of 97.5:1.5:1. After adding N-methylpyrrolidone and stirring evenly, the first active slurry was obtained. The average particle size Dv50 of the first lithium iron phosphate was 0.51 μm, and the crystallinity of the first binder PVDF was 45.4%.

[0075] Preparation of the second active slurry: Lithium iron phosphate (LiFePO4), polyvinylidene fluoride (PVDF), and conductive carbon black (Super-P) were placed in a stirred tank at a ratio of 97:2:1. N-methylpyrrolidone was added and the mixture was stirred until homogeneous to obtain the second active slurry. The average particle size (Dv50) of the LiFePO4 was 1.32 μm, and the crystallinity of the PVDF was 32.5%.

[0076] A first active slurry was coated onto the surface of a 13µm aluminum foil and dried at 80°C to obtain a first active layer. A second active slurry was then coated onto the surface of the first active layer and dried at 80°C to obtain a second active layer. The aluminum foil coated with the first and second active layers was then rolled to obtain a positive electrode sheet with a compaction density of 2.5 g / cm³. 3 .

[0077] Examples 2-6 The specific differences between Examples 2-6 and Example 1 are shown in Table 1. All contents not mentioned in Table 1 are the same as those in Example 1.

[0078] Comparative Examples 1-7 The specific differences between Comparative Examples 1-7 and Example 1 are shown in Table 1. All contents not covered in Table 1 are the same as those in Example 1.

[0079] Table 1 Performance testing 1. Peel force test Cut the electrode to be tested into strips 25mm wide and fix them to the test mold with double-sided tape. Then, use tape to connect the universal tensile testing machine to the test surface of the electrode to be tested. After the tensile force stabilizes, read the tensile force value at this time, which is the electrode peeling force.

[0080] 2. Electrical performance testing Battery manufacturing Negative electrode preparation: Artificial graphite, carboxymethyl cellulose, styrene-butadiene rubber, and conductive carbon black (SP) are mixed with deionized water in a stirring tank at a ratio of 95.5:1.5:2:1 to form a homogeneous slurry; the negative electrode slurry is coated on an 8µm copper foil (with a single-sided active material layer thickness of 170µm), and dried to obtain a negative electrode roll; the negative electrode roll is rolled to 110µm on a roller press to obtain a rolled negative electrode sheet.

[0081] Separator: A 12μm thick polypropylene (PP) membrane is selected as the separator.

[0082] Electrolyte: In a glove box filled with inert gas, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate are mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, lithium salt LiPF6 is dissolved in the organic solvent with a concentration of 1.5 mol / L to obtain the electrolyte.

[0083] Battery assembly: The rolled positive and negative electrode sheets are cut into the required sizes and stacked with 12μm thick PP separators to obtain dry cells; the dry cells are placed in perforated aluminum-plastic film and sealed to obtain pouch cells. The pouch cells are dried in a vacuum drying oven, injected with electrolyte, and after formation and capacity testing, the pouch battery to be tested is obtained.

[0084] 5C Ratio Performance Test The battery was placed in a 25℃ incubator for 6 hours to allow the internal temperature of the battery to reach equilibrium with the environment. It was then discharged at 1C constant current to 2.5V and allowed to rest for 10 minutes. Next, it was charged at 1C constant current to 3.65V, then charged at constant voltage until the current ≤0.05C, and allowed to rest for 10 minutes. Finally, it was discharged at 1C constant current to 2.5V, and the capacity was recorded as C0. After resting for 10 minutes, it was charged at 1C constant current to 3.65V, then charged at constant voltage until the current ≤0.05C, and allowed to rest for 10 minutes. Finally, it was discharged at 5C constant current to 2.5V, and the capacity was recorded as C1. The 5C discharge capacity of the battery was calculated as C1 / C0 × 100%.

[0085] The positive electrode sheets of each embodiment and comparative example, and the test results of the batteries prepared therefrom are shown in Table 2.

[0086] Table 2 A comparison of Examples 1 and 2 shows that reducing the material particle size and increasing the crystallinity of the binder ensures that the value of the formula does not change significantly, resulting in little difference in rate performance. This is mainly due to the combined effect of reducing the particle size to improve the rate and increasing the crystallinity to reduce the pore size, indicating a synergistic effect between particle size and crystallinity. A comparison of Examples 3 and 4 shows that reducing the material particle size and increasing the crystallinity of the binder ensures that the value of the formula does not change significantly, resulting in little difference in rate performance. This is mainly due to the combined effect of reducing the particle size to improve the rate and increasing the crystallinity to reduce the pore size, indicating a synergistic effect between particle size and crystallinity. A comparison between Examples 1-2 and Examples 3-4 shows that the values ​​of the formula are within a certain range. Due to the synergistic effect of particle size and binder crystallinity, the rate performance of the battery can be guaranteed.

[0087] As can be seen from the comparison of Examples 4 and 5, within the formula range, reducing the crystallinity of the binder is beneficial to improving the rate performance, mainly because the reduction in crystallinity increases the pore size of the electrode.

[0088] As can be seen from the comparison of Examples 1 and 6, under the limit of the amount of binder and conductive agent, the rate performance is slightly reduced, but the difference is not significant. This is mainly due to the poor electronic conductivity caused by the large amount of binder added.

[0089] As can be seen from the comparison between Example 1 and Comparative Example 1, further reducing the particle size will worsen the rate performance when the crystallinity is consistent; this is because smaller particle size has a smaller improvement on the rate, while reducing the electrode pore size has a greater deterioration on the rate performance.

[0090] As can be seen from the comparison between Example 3 and Comparative Example 2, when the particle size is large, increasing the crystallinity of the binder or increasing the particle size when the crystallinity of the binder is relatively high will deteriorate the rate performance. This is because the improvement in rate performance caused by the increase in electrode pore size due to crystallinity is not as great as the deterioration in rate performance caused by the increase in particle size, resulting in an overall deterioration in rate performance.

[0091] A comparison of Example 1 and Comparative Example 3 shows that increasing the particle size of the lower layer and reducing crystallinity has little effect on the rate performance, but the deterioration of the electrode peeling force due to the decrease in crystallinity is relatively large, resulting in the peeling of the active material layer.

[0092] As can be seen from the comparison between Example 1 and Comparative Example 4, when the crystallinity of the binder is relatively low, further reducing the particle size will worsen the rate performance, because smaller particle size has less effect on the rate improvement, while reducing the electrode pore size has a greater effect on the rate performance deterioration.

[0093] As can be seen from the comparison between Example 1 and Comparative Example 5, when the particle size is consistent, excessively increasing the crystallinity of the binder will deteriorate the rate performance, because the increase in crystallinity leads to the reduction of pore size and the deterioration of rate performance.

[0094] As can be seen from the comparison between Example 1 and Comparative Example 6, when the crystallinity is consistent, excessively increasing the particle size will deteriorate the rate performance because the increase in particle size leads to the deterioration of the bulk phase transport of the material and the deterioration of the rate performance.

[0095] As can be seen from the comparison between Example 1 and Comparative Example 7, the increase in particle size leads to the deterioration of the solid phase transport capability of the material, the increase in crystallinity leads to less swelling of the binder and lower porosity of the electrode, and the rate performance of the battery deteriorates more.

[0096] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0097] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0098] It should be noted that the terms "and / or" or " / " used herein are merely descriptions of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The singular forms "a," "described," and "the" used in the embodiments of the invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0099] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode plate, characterized in that, The positive electrode sheet includes: current collector; A first active layer is disposed on at least one side surface of the current collector along the thickness direction, and the first active layer includes a first active material, a first binder, and a first conductive agent; A second active layer is disposed on the surface of the first active layer away from the current collector, and the second active layer includes a second active substance and a second binder; Wherein, the crystallinity of the first binder is greater than that of the second binder, and the average particle size Dv50 of the first active material is less than that of the second active material.

2. The positive electrode sheet according to claim 1, characterized in that, The first adhesive has a crystallinity of C1; the second adhesive has a crystallinity of C2. The C1 and C2 satisfy the following relationship: 8% ≤ C1 - C2 ≤ 18%.

3. The positive electrode sheet according to claim 1, characterized in that, The average particle size Dv50 of the second active material is D2; the crystallinity of the second binder is C2; The C2 and D2 satisfy the following relationship: 40≤C2×D2×100≤70.

4. The positive electrode sheet according to claim 1, characterized in that, The crystallinity C1 of the first adhesive is 45%~50%; And / or, the crystallinity C2 of the second adhesive is 32%~37%.

5. The positive electrode sheet according to claim 1, characterized in that, The average particle size (Dv50) of the first active substance is 0.5~0.9 μm; And / or, the average particle size Dv50 of the second active substance is 1~3 μm.

6. The positive electrode sheet according to claim 1, characterized in that, The mass ratio of the first active material, the first binder, and the first conductive agent in the first active layer is (96~98):(1.5~2.5):(0.5~1.5). And / or, the first active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide; And / or, the first adhesive comprises at least one of polyvinylidene fluoride, carboxylic acid-modified polyvinylidene fluoride, and acrylic acid-modified polyvinylidene fluoride; And / or, the first conductive agent includes at least one of carbon black, carbon nanotubes, carbon nanofibers, and graphene; And / or, the thickness of the first active layer is 60~120μm.

7. The positive electrode sheet according to claim 1, characterized in that, The second active layer further includes a second conductive agent; Preferably, the mass ratio of the second active substance, the second binder, and the second conductive agent in the second active layer is (96~98):(1.5~3):(0~1); And / or, the second active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide; And / or, the second adhesive comprises at least one of polyvinylidene fluoride, carboxylic acid-modified polyvinylidene fluoride, and acrylic acid-modified polyvinylidene fluoride; And / or, the second conductive agent includes at least one of carbon black, carbon nanotubes, carbon nanofibers, and graphene; And / or, the thickness of the second active layer is 60~120μm.

8. The method for preparing the positive electrode sheet according to any one of claims 1 to 7, characterized in that, Includes the following steps: A first active slurry is coated on at least one side of the current collector to obtain a first active layer; A second active slurry is coated onto the surface of the first active layer to obtain a second active layer; The current collector coated with the first active layer and the second active layer is dried and rolled to obtain the positive electrode sheet.

9. The method for preparing the positive electrode sheet according to claim 8, characterized in that, The preparation of the first active slurry includes: mixing the first active substance, the first binder and the first conductive agent uniformly in a solvent to obtain the first active slurry; The preparation of the second active slurry includes: mixing the second active substance, the second binder, and the second conductive agent uniformly in a solvent to obtain the second active slurry; And / or, the drying temperature is 70~110℃; And / or, the compaction density of the roller press is 2.4~2.8 g / cm³. 3 .

10. A battery, comprising a positive electrode, characterized in that, The positive electrode sheet includes the positive electrode sheet according to any one of claims 1 to 7, or the positive electrode sheet prepared by the preparation method according to any one of claims 7 to 9.

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