Positive electrode sheet, method for manufacturing positive electrode sheet, and secondary battery

Abstract

The application provides a positive electrode sheet, a preparation method of the positive electrode sheet and a secondary battery, and relates to the technical field of batteries. The positive electrode sheet provided by the application comprises a positive electrode current collector, a first coating layer and a second coating layer which are arranged on at least one side of the positive electrode current collector in a laminated mode, the first coating layer is arranged close to the positive electrode current collector, and the second coating layer is arranged on the side of the first coating layer away from the positive electrode current collector. The first coating layer comprises an active additive, a first active material, a first conductive agent, a first adhesive and a first solvent. The second coating layer comprises a second active material, a second conductive agent, a second adhesive and a second solvent. In this way, the active additive in the first coating layer can compensate for the current-carrying ions, improve the cycle performance of the secondary battery, and reduce the possibility of side reactions between the active additive and the electrolyte and gas production.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a positive electrode sheet, a method for preparing the positive electrode sheet, and a secondary battery. Background Technology

[0002] Due to their advantages such as rechargeability, environmental friendliness, and high flexibility, rechargeable batteries are widely used in new energy vehicles, energy storage systems, and other fields.

[0003] However, the formation of an SEI film on the negative electrode surface during lithium-ion battery formation leads to the loss of active lithium, resulting in an unnecessary loss of battery energy density. Therefore, lithium replenishment technology has become a research hotspot for improving battery energy density.

[0004] The lithium replenishment technology proposed in the relevant technologies is not reasonable enough, which leads to accelerated decay in the later stage of the cycle and is also very easy to trigger side reactions, which may lead to increased gas production. Summary of the Invention

[0005] This application provides a positive electrode sheet, a method for preparing the positive electrode sheet, and a secondary battery, which can improve the cycle performance of the secondary battery and reduce the possibility of side reactions and gas generation caused by contact between active additives and electrolyte.

[0006] In a first aspect, this application provides a positive electrode sheet, which includes a positive current collector and a first coating and a second coating stacked on at least one side of the positive current collector. The first coating is disposed close to the positive current collector, and the second coating is disposed on the side of the first coating away from the positive current collector.

[0007] The first coating comprises an active additive, a first active material, a first conductive agent, a first adhesive, and a first solvent. The second coating comprises a second active material, a second conductive agent, a second adhesive, and a second solvent.

[0008] In this way, the active additives in the first coating can compensate for the current-carrying ions, and this process, as well as the inert materials remaining from this process, are located on the side closer to the positive electrode current collector. This can significantly reduce the possibility of inert materials hindering the uniform transport and intercalation / deintercalation of current-carrying ions, improve the cycle performance of the secondary battery, and also reduce the possibility of side reactions occurring when the active additives come into contact with the electrolyte, thus reducing the possibility of gas generation.

[0009] Optionally, the active additives include any one or at least two of Li2NiO2, Li5FeO4, Li2O, Li2O2, Li3N, Li2C2O4, and Li2C4O4.

[0010] In this way, active additives can be configured in different ways, providing a wide range of choices.

[0011] Optionally, the mass of the active additive is M1, and the mass of the first active material is M2. The ratio of M1 to M2 is 0.5% to 30%.

[0012] In this way, the quality of the active additive can be controlled when setting the first coating, and the amount of active additive in the first coating set at this mass ratio can be more appropriate.

[0013] Optionally, the positive electrode sheet has a first orientation. On one side of the positive current collector, the first coating has a first size H1 in the first orientation, and the second coating has a second size H2 in the first orientation, wherein H1 and H2 satisfy: 20μm≤H1+H2≤200μm. Here, the first orientation is the stacking orientation of the first and second coatings.

[0014] In this way, the total thickness of the first and second coatings on the positive electrode current collector can be made more reasonable.

[0015] Optionally, H1 and H2 also satisfy: 0.5:99.5≤H1:H2≤99.5:0.5.

[0016] This reduces the possibility that the first coating is too thin, resulting in a poor replenishment effect of the first coating on the current carrier ions. It also reduces the possibility that the first coating is too thick, resulting in the inert substances remaining after the reaction of the active additives approaching the main reaction interface and interfering with the insertion and extraction of current carrier ions at the second coating.

[0017] Optionally, the first active material is any one of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary cathode materials. The second active material is the same substance as the first active material.

[0018] Thus, the ion insertion and extraction synchronization of the first and second coatings is good, which can reduce the risk of local polarization, capacity inhomogeneity or lithium dendrites caused by material differences.

[0019] Optionally, the first active material is a first lithium iron phosphate material, and the second active material is a second lithium iron phosphate material.

[0020] The first lithium iron phosphate material differs from the second lithium iron phosphate material in the following ways: The doping elements are different; the crystal structure is different; the particle size is different; and the carbon coating content is different.

[0021] In the embodiments of this application, the first active material and the second active material can be lithium iron phosphate obtained by different modification processes, so that the first active material in the first coating and the second active material in the second coating can achieve synergistic complementarity in performance, overcome the problem of mutual constraint between performance caused by a single modification strategy, and improve the comprehensive electrochemical performance of the positive electrode.

[0022] Secondly, this application provides a method for preparing a positive electrode sheet, used to prepare any of the positive electrode sheets mentioned in the first aspect above.

[0023] The preparation method includes: providing an active additive, a first active material, a first conductive agent, a first adhesive, and a first solvent, and mixing them to obtain a first slurry.

[0024] A second active material, a second conductive agent, a second adhesive, and a second solvent are provided and mixed to obtain a second slurry.

[0025] The first slurry and the second slurry are coated on the same side of the positive electrode current collector in a double-layer coating manner to form a wet electrode sheet.

[0026] The wet electrode sheet is dried and rolled to obtain the positive electrode sheet.

[0027] In this process, after the wet electrode is dried and rolled, the first slurry forms the first coating, and the second slurry forms the second coating.

[0028] Thirdly, this application also provides a secondary battery, which includes a negative electrode, a separator, and a positive electrode as described in the first aspect above, wherein the separator is disposed between the positive electrode and the negative electrode.

[0029] Optionally, the secondary battery also includes a package containing a mounting cavity. The positive electrode, separator, and negative electrode are all disposed in the mounting cavity, and an electrolyte is injected into the mounting cavity, which wets the positive and negative electrode.

[0030] The beneficial effects of the positive electrode preparation method provided in the second aspect and the various possible designs of the second aspect, and the secondary battery provided in the third aspect and the various possible designs of the third aspect, can be found in the beneficial effects of the first aspect and the various possible embodiments of the first aspect, and will not be repeated here. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of a positive electrode sheet according to an embodiment of this application.

[0032] Figure 2 This is a flowchart illustrating a method for preparing a positive electrode sheet according to an embodiment of this application.

[0033] Explanation of reference numerals in the attached figures: 100: Positive electrode sheet; 10: Positive current collector; 20: First coating; 30: Second coating. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims and drawings of this application are intended to cover non-exclusive inclusion.

[0036] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of the phrase "embodiment" in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0037] In this article, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists, A and B exist simultaneously, or B exists. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0038] The directional terms appearing in the following description refer to the directions shown in the figures and are not intended to limit the specific structure of this application. For example, in the description of this application, terms such as "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the figures. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0039] Furthermore, the terms "first," "second," etc., in the specification and claims of this application or in the aforementioned drawings are used to distinguish different objects rather than to describe a specific order, and may explicitly or implicitly include one or more of the features.

[0040] In the description of this application, unless otherwise stated, "multiple" means two or more (including two), and similarly, "multiple groups" means two or more (including two groups).

[0041] It should be understood that the phrase "one embodiment" or "an embodiment" throughout the specification means that a specific feature, structure, or characteristic related to the embodiment is included in at least one embodiment of this application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. Furthermore, these specific features, structures, or characteristics can be combined in any suitable manner in one or more embodiments.

[0042] With the development of new energy technologies, secondary batteries have developed rapidly, especially lithium-ion batteries, which have been widely used in recent years due to their advantages such as high energy density and low failure rate.

[0043] For example, lithium-ion batteries, as a power source, are now widely used in new energy vehicles, electric bicycles, laptops, mobile phones and other electrical devices to provide power for these devices and maintain their normal operation.

[0044] However, issues such as the formation of the solid electrolyte interphase (SEI) film, electrolyte decomposition, and irreversible consumption of active lithium in lithium-ion batteries lead to low initial efficiency and initial capacity loss. Currently, the industry commonly improves the initial efficiency and early cycle capacity of batteries by adding lithium replenishing agents to the slurry to replenish the lithium consumed by SEI formation and side reactions.

[0045] However, while adding lithium additives can improve early-stage cycling, large lithium additive particles form inert sites within the active material in later stages of cycling. These sites lack lithium insertion / extraction sites, hindering uniform lithium-ion transport and insertion / extraction. This leads to kinetic imbalances and decreased utilization of active materials, accelerating overall capacity decay. Therefore, the effect of lithium additives on improving the cycle performance of lithium-ion batteries is limited. Furthermore, this uneven lithium insertion state may exacerbate electrolyte side reactions, resulting in increased gas production in the secondary battery.

[0046] Based on the above problems, this application proposes a secondary battery, which includes a negative electrode, a separator, and other components. Figure 1The positive electrode 100 is shown. The positive electrode 100 and the negative electrode can work together to achieve reversible migration and energy conversion of current-carrying ions, realizing the mutual conversion of electrical energy and chemical energy.

[0047] The separator is located between the positive electrode 100 and the negative electrode. It provides a transport channel for current-carrying ions, enabling efficient transport between them. Furthermore, the separator prevents direct contact between the positive and negative electrodes, thus reducing the likelihood of short circuits.

[0048] The arrangement of the positive electrode in this application is beneficial to improving the coulombic efficiency and early cycle capacity of the secondary battery in the first cycle. It can also reduce the possibility of kinetic imbalance and decreased utilization of active materials in the later stages of cycling, and reduce the possibility of gas generation in the secondary battery.

[0049] It should be noted that the separator membrane may include a substrate and a coating, with the coating applied to the substrate. The substrate may be polypropylene (PP), polyethylene (PE), or a PP / PE composite substrate. The substrate thickness may be between 5-20 μm, specifically 5 μm, 10 μm, 15 μm, or 20 μm, etc.

[0050] The coating can be an alumina, a binder, or a composite coating of alumina and a binder. The coating thickness can be 1-6 μm.

[0051] It should also be noted that, in the embodiments of this application, the negative electrode sheet may include a negative electrode current collector and a negative electrode coating. The negative electrode current collector is copper foil or composite copper foil, and the negative electrode coating includes a negative electrode active material. The negative electrode active material can be a traditional graphite material, including artificial graphite, natural graphite, or a mixture of artificial and natural graphite. The thickness of the negative electrode sheet can be between 10 μm and 200 μm, specifically 10 μm, 25 μm, 90 μm, 150 μm, or 200 μm, etc.

[0052] In the negative electrode sheet, the negative electrode active material satisfies the following condition: the coating weight of the negative electrode active material per unit area = X * the total weight of the negative electrode coating per unit area. Here, X represents the mass percentage of the negative material in the total material of the negative electrode coating, ranging from 92% to 99%, and the areal density of the active material is greater than or equal to 8 mg / cm².

[0053] In some embodiments, the secondary battery may further include a package containing a mounting cavity, in which a positive electrode 100, a separator, and a negative electrode are disposed. An electrolyte is also injected into the mounting cavity, which wets the positive electrode 100 and the negative electrode.

[0054] In this way, the encapsulation can hold the electrolyte, protect and support the positive electrode 100, the separator and the negative electrode, and block external moisture, dust and other impurities, so that the secondary battery can work normally and reliably.

[0055] Here, this application will provide a detailed description of the positive electrode 100 provided in the embodiments of this application with reference to the accompanying drawings.

[0056] Reference Figure 1 As shown, this application provides a positive electrode 100, which includes a positive current collector 10 and a first coating 20 and a second coating 30 stacked on at least one side of the positive current collector 10. The first coating 20 is disposed close to the positive current collector 10, and the second coating 30 is disposed on the side of the first coating 20 away from the positive current collector 10. The first coating 20 includes an active additive, a first active material, a first conductive agent, and a first binder. The second coating 30 includes a second active material, a second conductive agent, and a second binder.

[0057] In this embodiment, the first coating 20 and the second coating 30 are stacked, with the first coating 20 disposed close to the positive current collector 10 and the second coating 30 disposed away from the positive current collector 10. The first coating 20 and the second coating 30 may be disposed on only one side of the positive current collector 10, or on both sides of the positive current collector 10.

[0058] The first coating 20 contains a first active material, and the second coating 30 contains a second active material, so that both the first coating 20 and the second coating 30 can reversibly release or receive current-carrying ions to achieve the mutual conversion of chemical energy and electrical energy.

[0059] The first coating 20 also contains an active additive, which can release and replenish current-carrying ions to compensate for the loss of current-carrying ions during the first cycle and alleviate the loss of current-carrying ions during long-term cycles. This allows more current-carrying ions to participate in subsequent charge-discharge cycles, thereby improving the first-cycle coulombic efficiency and early-cycle capacity of the secondary battery equipped with the positive electrode 100.

[0060] In this example, the first coating 20 includes an active additive, a first active material, a first conductive agent, a first adhesive, and a first solvent. The second coating 30 includes a second active material, a second conductive agent, a second adhesive, and a second solvent. It is evident that the active additive is only present in the first coating 20, and no similar additive is present in the second coating 30.

[0061] With the above setup, during the cycle of the secondary battery equipped with the positive electrode 100, the compensation reaction for current-carrying ions triggered by the active additive is concentrated on the side close to the positive electrode current collector 10. Thus, the inert substances remaining from the compensation reaction are also located on the side close to the positive electrode current collector 10.

[0062] In this way, the inert material has a small chance of causing physical interference and chemical erosion to the structural integrity and chemical stability of the second active material in the second coating 30. It can greatly reduce the possibility of the inert material hindering the uniform transport and insertion / extraction of current-carrying ions, improve the cycle performance of the secondary battery, and reduce the possibility of side reactions and gas generation caused by the contact between active additives and electrolyte.

[0063] The first coating 20 also includes a first conductive agent and a first adhesive. These are the inactive parts of the first coating 20. The first conductive agent can construct an electron transport channel, and the first adhesive can bond the first active material, active additive and the first conductive agent together and attach them to the surface of the positive electrode current collector 10.

[0064] The first conductive agent can be acetylene black, graphite, or carbon black, etc., and the first adhesive can be polyacrylic acid or polyvinylidene fluoride.

[0065] In other embodiments, the first coating 20 may also include a first dispersant, such as sodium polyacrylate, alkyl phosphate, or polyethylene glycol, etc.

[0066] Similarly, the second coating 30 includes several inactive components: a second conductive agent, a second adhesive, and a second solvent. The materials and functions of the second conductive agent, the second adhesive, and the second solvent can be referred to the aforementioned first conductive agent, the first adhesive, and the first solvent, and will not be repeated here in the embodiments of this application.

[0067] Alternatively, the second coating 30 may also include a second dispersant similar to the first dispersant.

[0068] It is understandable that the second coating 30 is located on the side away from the positive current collector 10. When the positive electrode 100 is placed in the package, the second coating 30 will be in direct contact with the electrolyte, making the transport path of current-carrying ions at the second coating 30 shorter, serving as the "main reaction interface".

[0069] The first coating 20, which is far from the positive electrode current collector 10, requires the carrier ions to pass through the pores of the entire second coating 30 to reach it, thus becoming a "reaction-weak region".

[0070] To make the solution and beneficial effects of this application clearer, this application will be described in detail here in conjunction with relevant technologies. For ease of explanation, the side containing the first coating 20 will be referred to as the bottom layer, and the side containing the second coating 30 will be referred to as the top layer.

[0071] In related technologies, the lithium replenishing agent is uniformly filled into the coating of the current collector. Although this can improve the early cycle life, there is an accelerated degradation trend in the later stages of cycling. Specifically, large-particle lithium replenishing agents form inert sites within the active material, without lithium intercalation sites.

[0072] On the one hand, large inert sites form physical barriers in the active material, blocking the continuous transport paths of electrons and current-carrying ions. While they do not provide lithium intercalation sites themselves, they permanently occupy space, reducing the proportion of active material per unit volume and thus diluting the effective capacity of the electrode. On the other hand, during cycling, current and lithium flow bypass these inert sites, causing the active particles around these sites to experience higher local current densities, exacerbating uneven lithium intercalation and capacity decay.

[0073] Furthermore, most lithium replenishing agents, and the solid byproducts remaining after lithium release, catalyze the oxidative decomposition of the electrolyte, generating large amounts of gas (such as CO2 and O2) and consuming active lithium. In addition, metal ions in the solid byproducts may dissolve and migrate to the negative electrode, damaging the SEI film and forming a "shuttle effect," further consuming lithium and electrolyte.

[0074] The above problems can cause a vicious cycle of "gas production - increased pressure - contact deterioration - increased resistance", which is one of the reasons for the capacity drop and battery swelling in the later stages of secondary battery cycles.

[0075] In this embodiment, active additives are provided only in the first coating 20 near the positive current collector 10, and the advantages are mainly reflected in the following aspects.

[0076] (1) Isolation of inert substances. The compensation reaction of the active additive to the current-carrying ions is confined to the bottom layer, and the residual inert substances are also confined to the bottom layer. In this way, the possibility of direct physical interference and chemical corrosion of the compensation reaction and the residual substances generated by the compensation reaction on the structural integrity and chemical stability of the second coating 30 can be reduced.

[0077] (2) Improve the uniformity of carrier ion embedding. The conductive network of the first conductive agent in the bottom layer can ensure that the active additive can react fully. The carrier ions released by the active additive can be efficiently transported to the surface layer, so that the carrier ions can be uniformly "spread" into the upper second coating layer 30.

[0078] Obviously, through the configuration of this application, the first positive electrode active material can release current-carrying ions at the bottom layer, causing the current-carrying ions to diffuse away from the positive electrode current collector 10, while the current-carrying ions in the electrolyte diffuse towards the positive electrode current collector 10. In this way, the two sides of the second active material can receive current-carrying ions simultaneously, which is more uniform than the unidirectional embedding of current-carrying ions from the separator side.

[0079] The addition of current-carrying ions at the bottom layer in this application is equivalent to a "built-in current-carrying ion buffer layer", which can alleviate the uneven lithium intercalation caused by uneven current distribution to a certain extent, and is particularly beneficial to the thicker positive electrode 100.

[0080] (3) Optimize electron and ion transport channels. Since the inert substances remaining after the reaction of the active additives are confined to the bottom layer, the second coating 30 on the surface is relatively pure and has a continuous and efficient electron transport path (conductive agent network) and ion transport path (pores in the second coating 30), and is less likely to be blocked by large particles of inert substances.

[0081] (4) Suppressing gas production and side reactions. This application confines the catalytically active inert material to the bottom layer, significantly reducing its contact area with the electrolyte (especially the upper layer of fresh electrolyte). This makes the first coating 20, where the active additive is located, relatively stable.

[0082] On the one hand, even if gas generation occurs, the generated gas is confined to the bottom layer, which is closer to the positive electrode current collector 10 and the casing, thus facilitating heat and pressure management. On the other hand, the reaction of the active additives to replenish the current-carrying ions mainly occurs during the formation stage. In subsequent cycles, the interface between the relatively pure upper second coating 30 and the electrolyte is more stable, and side reactions are significantly reduced.

[0083] It is evident that the configuration of this application can improve the cycle performance of the secondary battery with the positive electrode 100, and reduce the energy decay and gas production of the secondary battery.

[0084] In this embodiment, the secondary battery can be a lithium-ion battery, in which case the deintercalable current-carrying ions in the lithium-ion battery are lithium ions. The first active material can specifically be any one of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary cathode materials, and the second active material is the same substance as the first active material.

[0085] With the above settings, the first active material in the first coating 20 is the same as the second active material in the second coating 30. In this way, the physicochemical properties (such as ion diffusion rate and electronic conductivity) of the first active material and the second active material are consistent.

[0086] In this way, the ion insertion and extraction synchronization of the first coating 20 and the second coating 30 can be good during the charging and discharging process of the secondary battery, which can reduce the risk of local polarization, capacity unevenness or lithium dendrites caused by material differences.

[0087] Understandably, in practical applications, the aforementioned lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary cathode materials are widely used. Taking lithium iron phosphate as an example, the following explanation clarifies that lithium iron phosphate not only encompasses pure-phase lithium iron phosphate but also includes substances obtained through modification of pure-phase lithium iron phosphate.

[0088] In the embodiments of this application, the first active material and the second active material can be the same substance. For example, both can be pure-phase lithium iron phosphate, or both can be lithium iron phosphate obtained through the same modification process. The modified lithium iron phosphate materials have different specifications in terms of doping elements, crystal structure, particle size, and carbon coating amount.

[0089] For example, introducing doping elements into pure-phase lithium iron phosphate can optimize the electronic conductivity and ion diffusion rate of the material, thereby improving the rate performance, cycle life, and high and low temperature adaptability of the secondary battery with positive electrode 100.

[0090] In terms of crystal structure, lithium iron phosphate has different crystal structures such as single crystal, core-shell structure, and nanocrystalline.

[0091] In terms of the geometric size of lithium iron phosphate grains or particles, lithium iron phosphate can be divided into micron-scale, submicron-scale, and nano-scale. The particle size directly determines the crystal characteristics and the length of the ion diffusion path.

[0092] In terms of carbon coating content, lithium iron phosphate can be divided into low carbon coating lithium iron phosphate with a carbon coating content of 0.5% to 1.0%, medium carbon coating lithium iron phosphate with a carbon coating content of 1.0% to 2.0%, and high carbon coating lithium iron phosphate with a carbon coating content of 2.0% to 5.0%, etc.

[0093] In this application, when the first active material and the second active material are the same substance, the doping elements, crystal structure, particle size and carbon coating amount in the first active material and the second active material can all be the same.

[0094] In addition to the above settings, the first active material and the second active material can also be different. Specifically, the first active material is a first lithium iron phosphate material, and the second active material is a second lithium iron phosphate material.

[0095] The first lithium iron phosphate material differs from the second lithium iron phosphate material in that it contains different doping elements. These doping elements may include any one or a combination of at least two of the elements selected from Ti, Al, Mg, V, Ni, or Mn.

[0096] In this example, the content of each dopant element in the first lithium iron phosphate material and the second lithium iron phosphate material is no higher than 5000 ppm. This can reduce the possibility that excessive dopant elements may cause lattice distortion or collapse of the first lithium iron phosphate and the second lithium iron phosphate, or even block the lithium ion migration channels.

[0097] And / or, the first lithium iron phosphate material has a different crystal structure than the second lithium iron phosphate material. Thus, the first lithium iron phosphate material can be any one of single crystal, core-shell structure, and nanocrystalline, and the second lithium iron phosphate material can be another one of single crystal, core-shell structure, and nanocrystalline.

[0098] And / or, the first lithium iron phosphate material has a different crystal structure than the second lithium iron phosphate material. Thus, the particle size of the first lithium iron phosphate material can be any one of micrometer, submicrometer, and nanometer scales, and the particle size of the second lithium iron phosphate material can be another one of micrometer, submicrometer, and nanometer scales.

[0099] And / or, the carbon coating content differs between the first lithium iron phosphate material and the second lithium iron phosphate material. Thus, the first lithium iron phosphate material can be a low-carbon-coating lithium iron phosphate, the second lithium iron phosphate material can be a high-carbon-coating lithium iron phosphate, and so on.

[0100] In the embodiments of this application, the first active material and the second active material are lithium iron phosphate obtained by different modification processes, so that the first active material in the first coating 20 and the second active material in the second coating 30 can achieve synergistic complementarity in performance, overcome the problem of mutual constraint between performance caused by a single modification strategy, and improve the comprehensive electrochemical performance of the positive electrode 100.

[0101] In the embodiments of this application, the active additive is a substance that can replenish lithium ions, and there are multiple options for its configuration.

[0102] In one alternative embodiment, the active additive may be a binary lithium-rich inorganic compound, such as Li2O, Li2O2, or Li3N.

[0103] In another alternative embodiment, the active additive can be a ternary / multi-component lithium-rich oxide, such as Li2NiO2 or Li5FeO4.

[0104] In other alternative embodiments, the active additive may be an organolithium sacrificial salt, such as Li2C2O4 or Li2C4O4.

[0105] All of the active additives listed above can release lithium ions to compensate for the loss of active lithium during the charging and discharging process of secondary batteries.

[0106] In other embodiments, the active additive may also include a combination of multiple substances, such as a combination of Li2NiO2 and Li5FeO4, a combination of Li2O and Li5FeO4, a combination of Li3N and Li2NiO2, or a combination of three substances, such as Li3N, Li2O and Li5FeO4, etc. The embodiments of this application do not specifically limit this.

[0107] When the active additives include Li₂NiO₂ and Li₅FeO₄, the mass ratio of Li₂NiO₂ to Li₅FeO₄ can be between 0.5 and 10. When the active additives include Li₂O and Li₅FeO₄, the mass ratio of Li₂O to Li₅FeO₄ can be between 0.25 and 0.4.

[0108] It is understood that the above only describes the proportion of each component in certain combinations of active additives. For other combinations, the component ratios of each substance will not be repeated here. The selection can be based on whether the performance of each component is synergistic and complementary, and whether the side effects caused by the conflict of physicochemical properties are avoided.

[0109] Furthermore, the materials that can be selected as active additives listed in the above description of this application are merely illustrative examples. The range of materials that can be selected as active additives in the embodiments of this application includes, but is not limited to, the aforementioned substances, and may also be other substances that can supplement lithium ions.

[0110] In some embodiments, the mass of the active additive is M1, and the mass of the first active material is M2. The value of M1 / M2 is 0.5% to 30%.

[0111] This allows for quality control of the active additive during the application of the first coating. A first coating applied at this specific mass ratio ensures a suitable amount of active additive. On the one hand, it reduces the possibility of insufficient replenishment of charge-carrying ions due to inadequate active additive.

[0112] On the other hand, excessive active additives can lead to increased gas production from their reactions and a smaller amount of the first active material. This results in less of the positive electrode sheet per unit mass / volume that can participate in the reaction, potentially reducing the energy density of the secondary battery equipped with the positive electrode sheet. In this application, the value of M1 / M2 is 0.5% to 30%, which can effectively reduce the possibility of the above problems.

[0113] In the embodiments of this application, the value of M1 / M2 can be 0.5%, 1%, 3.5%, 6%, 18%, 26.5%, 30%, or 20%~25%, etc. The specific value of M1 / M2 is not specifically limited in the embodiments of this application.

[0114] like Figure 1 As shown, the positive electrode 100 has a first orientation. On one side of the positive current collector 10, the first coating 20 has a first dimension H1 in the first orientation, which is also commonly referred to as the thickness of the first coating 20. The second coating 30 has a second dimension H2 in the first orientation, which is also commonly referred to as the thickness of the second coating 30.

[0115] Wherein, H1 and H2 satisfy: 20μm≤H1+H2≤200μm. The first direction refers to the stacking direction of the first coating 20 and the second coating 30.

[0116] In this way, the total thickness of the first coating 20 and the second coating 30 on the positive electrode current collector 10 can be made more reasonable. On the one hand, it can avoid the possibility that a small total thickness of the first coating 20 and the second coating 30 would lead to insufficient loading of the first active material and the second active material, thereby reducing the energy density of the secondary battery.

[0117] On the other hand, it can also avoid the total thickness of the first coating 20 and the second coating 30 being too large, which would result in the migration path of the current-carrying ions in the first coating 20 and / or the second coating 30 being too long, potentially increasing the resistance to electron transport.

[0118] In this application, 20μm≤H1+H2≤200μm. Under this setting, the energy density of the secondary battery equipped with the positive electrode and the optimization of the electron transport path can be taken into account.

[0119] It should be noted that the value of H1+H2 can be 20μm, 30μm, 80μm, 150μm, 200μm, or 100μm~180μm, etc. The specific value of H1+H2 is not specifically limited in this embodiment of the application.

[0120] Furthermore, in this embodiment, the thickness ratio of the first coating 20 to the second coating 30 is defined as follows: H1:H2 is specifically between 0.5:99.5 and 99.5:0.5. This allows for control over the thickness of both the first and second coatings.

[0121] The specific value of H1:H2 can be 0.5:99.5, 1:99, 50:50, 60:40, 90:10, or 99.5:0.5, etc. The specific value of H1:H2 is not specifically limited in this embodiment of the application.

[0122] By using the design of this application, the possibility that the thickness of the first coating 20 is too small, resulting in a poor replenishment effect of the first coating 20 on the current carrier ions, can be reduced. It can also reduce the possibility that the thickness of the first coating 20 is too large, resulting in the inert substances remaining after the reaction of the active additive approaching the main reaction interface and interfering with the insertion and extraction of current carrier ions at the second coating 30.

[0123] In addition, this application also provides a method for preparing a positive electrode sheet, such as... Figure 2 As shown, the method includes the following steps.

[0124] Step 201: Provide an active additive, a first active material, a first conductive agent, a first adhesive, and a first solvent, and mix them to obtain a first slurry.

[0125] Step 202: Provide a second active material, a second conductive agent, a second adhesive, and a second solvent, and mix them to obtain a second slurry.

[0126] Through steps 201 and 202, differentiated first and second slurries can be obtained respectively, so that the active additive exists only in the first slurry.

[0127] The first solvent is used to ensure uniform distribution of the components in the first coating, and the second solvent is used to ensure uniform distribution of the components in the second coating. Both the first and second solvents can be deionized water or N-methylpyrrolidone.

[0128] Step 203: Apply the first slurry and the second slurry to the same side of the positive current collector in a double-layer coating manner to form a wet electrode sheet.

[0129] In this application, a first slurry and a second slurry can be coated simultaneously using a dual-head coating machine. This results in a strong bond between the first coating 20 and the second coating 30, high production efficiency, and easy and precise control of the thickness of the first coating 20 and the second coating 30.

[0130] In this way, the first slurry and the second slurry can be coated on the positive electrode current collector in layers, avoiding the mixing of the first slurry and the second slurry with different formulation components, so that the active additive can be located only on the side close to the positive electrode current collector.

[0131] Furthermore, in the first coating 20 or the second coating 30, the active material satisfies the following condition: coating weight of active material per unit area = A * total weight of coating per unit area. Here, A represents the mass percentage of the active material in the total material of the coating it is in, ranging from 92% to 99%, and the areal density of the active material is greater than or equal to 16 mg / cm³. 2 The compaction density of the positive electrode sheet is ≥2.4 g / cm³. 3 .

[0132] In the above description, for the first coating 20, the active material is the first active material, and for the second coating 30, the active material is the second active material.

[0133] Of course, in addition to the above methods, the first slurry can be coated onto the positive current collector first, and then dried and rolled to form an electrode with the first coating. Then, the second slurry is coated onto the first coating, and then dried and rolled again to form the positive electrode.

[0134] Step 204: Dry and roll the wet electrode sheet to obtain the positive electrode sheet.

[0135] In this step, the wet electrode sheet can be dried by hot air drying or infrared drying to form a dry electrode sheet, so that the first solvent and the second solvent evaporate, and the active additives, the first active material, the first conductive agent and the first binder in the first slurry form a dry coating with a certain strength, and the second active additives, the second active material, the second conductive agent and the second binder in the second slurry also form a dry coating with a certain strength.

[0136] The dry electrode sheet is then rolled using a roller press to compress the internal pores of the first coating 20 and the second coating 30, reducing contact resistance and increasing volumetric energy density. This yields the positive electrode sheet 100.

[0137] In this process, after the wet electrode is dried and rolled, the first slurry forms the first coating 20, and the second slurry forms the second coating 30.

[0138] It should be noted that the active additives, the first active material, etc. have been described in detail in the previous examples, and will not be repeated here.

[0139] To make the objectives, technical solutions, and beneficial effects of this application clearer, the present invention is further described below with reference to embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of this application.

[0140] Example 1 (1) Preparation of positive electrode sheet Lithium iron phosphate, active additive (Li5FeO4), polyvinylidene fluoride (PVDF), acetylene black, and N-methylpyrrolidone (NMP) are stirred and dispersed to obtain a first slurry. Lithium iron phosphate, PVDF, acetylene black, and NMP are then stirred and dispersed to obtain a second slurry.

[0141] The first and second slurries are coated onto the surface of the positive electrode current collector using a double-layer coating process, and the thickness is controlled. After drying, the material is rolled and then die-cut and slit to obtain the positive electrode sheet.

[0142] (2) Preparation of secondary batteries Take the negative electrode sheet and dissolve it in a mixed solvent of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 5:3:2 using 1 mol / L lithium hexafluorophosphate as the solute to form an electrolyte. Then, perform stacking and electrolyte injection processes to obtain a lithium-ion battery.

[0143] The difference between Examples 2-6 and Example 1 is that the mass ratio of lithium iron phosphate to active additive is different.

[0144] The difference between Examples 7-8 and Example 6 is that the thickness ratio of the first coating to the second coating is different.

[0145] The difference between Comparative Example 1 and Example 1 is that no active additives are provided in the positive electrode sheet.

[0146] The difference between Comparative Example 2 and Example 1 is that the active additive is uniformly disposed on the positive electrode current collector.

[0147] Performance tests were conducted on the above embodiments and comparative examples to test the 0.33C discharge capacity, 45℃ 1000-cycle capacity retention rate, and volume change percentage of the lithium-ion batteries in each embodiment and comparative example. The data obtained are shown in the table below.

[0148] Table 1

[0149] Based on the data above, it can be seen that when the active additive is incorporated into the first coating, the discharge capacity and capacity retention of the lithium-ion battery are improved. Furthermore, compared to the case where the active additive is uniformly disposed in the coating of the positive electrode current collector, gas generation is also improved.

[0150] Comparing Examples 1-9, Comparative Examples 1 and 2, it is evident that without active additives, the discharge capacity of the secondary battery is relatively low. Adding active additives significantly increases the discharge capacity, but the improvement in high-temperature cycling is not significant, and there is excessive gas generation during storage. Combining Comparative Examples 1-9 and Comparative Example 2, double-layer coating can improve gas generation. For example, in Example 5, the gas generation is only 40% of that in Comparative Example 2, showing a significant improvement in gas generation and a marked enhancement in cycle performance.

[0151] Comparing Examples 2-6, it can be seen that as the content of the active additive increases, the discharge capacity initially increases, reaching 150 mAh / g before stabilizing. The cycle life, however, initially increases and then decreases. This indicates that when the amount of active additive reaches 3%, it can compensate for the loss of active lithium caused by the formation of the SEI film during the initial charge-discharge cycle. Thus, in the initial stage of cycling, the increased lithium replenishment leads to an increase in active lithium, thereby improving cycle performance.

[0152] Comparing Examples 4, 8, and 9, it can be seen that when the ratio of the thickness of the first coating to the thickness of the second coating is 1:1, the discharge capacity and 1000-cycle capacity retention are both high, and gas generation is not excessive. Therefore, a 1:1 ratio of the thickness of the first coating to the thickness of the second coating is preferred.

[0153] Finally, it should be noted that the above embodiments are merely specific implementations of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions within the technical scope disclosed in this application should be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A positive electrode plate, characterized in that, The positive electrode includes a positive current collector and a first coating and a second coating stacked on at least one side of the positive current collector. The first coating is disposed close to the positive current collector, and the second coating is disposed on the side of the first coating away from the positive current collector. The first coating comprises an active additive, a first active material, a first conductive agent, and a first adhesive; The second coating comprises a second active material, a second conductive agent, and a second adhesive.

2. The positive electrode sheet according to claim 1, characterized in that, The active additives include any one or at least two of Li2NiO2, Li5FeO4, Li2O, Li2O2, Li3N, Li2C2O4, and Li2C4O4.

3. The positive electrode sheet according to claim 1, characterized in that, The mass of the active additive is M1, and the mass of the first active material is M2; The value of M1 / M2 is 0.5% to 30%.

4. The positive electrode sheet according to any one of claims 1-3, characterized in that, The positive electrode sheet has a first direction. On one side of the positive current collector, the first coating has a first size H1 in the first direction, and the second coating has a second size H2 in the first direction. H1 and H2 satisfy: 20μm≤H1+H2≤200μm. Wherein, the first direction is the stacking direction of the first coating and the second coating.

5. The positive electrode sheet according to claim 4, characterized in that, H1 and H2 also satisfy: 0.5:99.5≤H1:H2≤99.5:0.

5.

6. The positive electrode sheet according to claim 1, characterized in that, The first active material is any one of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary cathode materials; The second active material is the same substance as the first active material.

7. The positive electrode sheet according to claim 1, characterized in that, The first active material is a first lithium iron phosphate material, and the second active material is a second lithium iron phosphate material; The first lithium iron phosphate material has different doping elements than the second lithium iron phosphate material. And / or, the first lithium iron phosphate material has a different crystal structure than the second lithium iron phosphate material; And / or, the first lithium iron phosphate material has a different particle size than the second lithium iron phosphate material; And / or, the carbon coating amount in the first lithium iron phosphate material is different from that in the second lithium iron phosphate material.

8. A method for preparing a positive electrode sheet, characterized in that, The preparation method for the positive electrode sheet according to any one of claims 1-7 comprises: An active additive, a first active material, a first conductive agent, a first binder, and a first solvent are provided and mixed to obtain a first slurry; A second active material, a second conductive agent, a second adhesive, and a second solvent are provided and mixed to obtain a second slurry; The first slurry and the second slurry are coated on the same side of the positive current collector in a double-layer coating manner to form a wet electrode sheet; The wet electrode sheet is dried and rolled to obtain a positive electrode sheet; In this process, after the wet electrode is dried and rolled, the first slurry forms a first coating, and the second slurry forms a second coating.

9. A secondary battery, characterized in that, The secondary battery includes a negative electrode, a separator, and a positive electrode as described in any one of claims 1-7; The separator is disposed between the positive electrode and the negative electrode.

10. The secondary battery according to claim 9, characterized in that, The secondary battery also includes a package, which has a mounting cavity, and the positive electrode, the separator, and the negative electrode are all disposed in the mounting cavity; The mounting cavity is also filled with electrolyte, which wets the positive electrode and the negative electrode.

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