Cathode material, cathode sheet, electrochemical device, and preparation method of cathode material

Abstract

The application provides a positive electrode material, a positive electrode sheet, an electrochemical device and a preparation method of the positive electrode material, and belongs to the technical field of secondary batteries.The positive electrode material comprises a first active substance and a second active substance, wherein the first active substance is a ternary material; the second active substance is a lithium iron phosphate material; the mass ratio of the first active substance to the second active substance is (80-95):(5-20); the ratio of the maximum particle size of the first active substance to the maximum particle size of the second active substance is 0.5-1; and the ratio of the minimum particle size of the first active substance to the minimum particle size of the second active substance is 6.4-10.In the positive electrode material, the lithium iron phosphate material is mixed into the ternary material system, and the ternary material and the lithium iron phosphate material are in a mutual coating state, so that the side reaction of the interface of the positive electrode material with electrolyte during charging and discharging is effectively reduced, the loss of active lithium during the charging and discharging process is reduced, and the service life of the secondary battery is prolonged.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to a positive electrode material, a positive electrode sheet, an electrochemical device, and a method for preparing the positive electrode material. Background Technology

[0002] Rechargeable lithium batteries are widely used as energy sources in wireless mobile devices. Simultaneously, they are widely used as power sources for electric vehicles and hybrid electric vehicles, helping to address air pollution caused by petroleum-based gasoline and diesel engines.

[0003] The interior of a secondary battery is a complex chemical reaction system, in which materials such as positive and negative electrode materials, electrolytes, and separators play important roles. In addition to the redox reaction of lithium ions during charging and discharging, there are other side reactions, such as electrolyte decomposition and dissolution of active materials.

[0004] The lifespan of rechargeable batteries is receiving increasing attention. Essentially, a battery's lifespan is determined by its charge-discharge cycles, which involve electrochemical reactions at the positive and negative electrode interfaces. However, these reactions are often accompanied by side reactions. With each charge-discharge cycle, lithium ions are lost, meaning the lithium ion content gradually decreases, leading to capacity degradation and performance decline. According to national standards, batteries with less than 80% capacity should be considered out of service. Therefore, extending battery life requires maximizing the number of cycles before capacity drops to 80%, with the core technology being slowing down the rate of capacity decay.

[0005] Currently, the average cycle life of lithium batteries (lithium nickel cobalt manganese oxide) in the industry is around 1,500 cycles, which increases their application cost. Although ternary lithium batteries have a higher energy density, they have fewer cycle times than lithium iron phosphate batteries, resulting in a significant cost disadvantage and limiting their widespread application.

[0006] Therefore, it is necessary to design a cathode material, a cathode sheet, an electrochemical device, and a method for preparing the cathode material to solve the above problems. Summary of the Invention

[0007] In view of the shortcomings of the prior art, the present invention provides a positive electrode material, a positive electrode sheet, an electrochemical device, and a method for preparing the positive electrode material, in order to solve the technical problem that the positive electrode material in the existing ternary secondary battery system has many side reactions during the charging and discharging process, resulting in a short battery cycle life.

[0008] To achieve the above and other related objectives, the present invention provides a positive electrode material, the positive electrode material comprising: a first active material and a second active material.

[0009] The mass ratio of the first active substance to the second active substance is (80-95):(5-20), the ratio of the maximum particle size of the first active substance to the maximum particle size of the second active substance is 0.5-1, and the ratio of the minimum particle size of the first active substance to the minimum particle size of the second active substance is 6.4-10.

[0010] In one example of the present invention, the ternary material is lithium nickel cobalt manganese oxide.

[0011] In one example of the present invention, a portion of the first active substance is coated on the particle surface of the second active substance, and a portion of the second active substance is coated on the particle surface of the first active substance.

[0012] In one example of the present invention, the particle size of the first active substance is 1.60 μm to 12.50 μm, and the particle size of the second active substance is 0.16 μm to 25.00 μm, wherein the minimum particle size of the second active substance is 0.16 μm to 0.25 μm, and the maximum particle size of the second active substance is 12.50 μm to 25.00 μm.

[0013] The present invention also provides a method for preparing the cathode material described in any of the above examples, the method comprising:

[0014] The first and second active substances are mixed at a mass ratio of (80-95):(5-20) to obtain a mixture.

[0015] The mixture is rolled to obtain the cathode material;

[0016] The ratio of the maximum particle size of the first active substance to the maximum particle size of the second active substance is 0.5 to 1, and the ratio of the minimum particle size of the first active substance to the minimum particle size of the second active substance is 6.4 to 10.

[0017] In one example of the present invention, the particle size of the first active substance is 1.60 μm to 12.50 μm, and the particle size of the second active substance is 0.16 μm to 25.00 μm, wherein the minimum particle size of the second active substance is 0.16 μm to 0.25 μm, and the maximum particle size of the second active substance is 12.50 μm to 25.00 μm.

[0018] The present invention also provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on the positive current collector, the positive active material layer comprising the positive electrode material described in any of the above examples, or the positive electrode material prepared by any of the above preparation methods.

[0019] In one example of the present invention, the positive electrode active material layer contains pores, and the ratio of the maximum pore size of the pores to the maximum particle size of the first active material is 0.04 to 0.1.

[0020] In one example of the present invention, the maximum pore diameter of the pore is 0.5 μm to 2.0 μm.

[0021] The present invention also provides an electrochemical device comprising a negative electrode, a separator, an electrolyte, and a positive electrode as described in any of the above examples.

[0022] In the cathode material of this invention, the ternary material of the first active material and the lithium iron phosphate material of the second active material can be mutually coated at a preset particle size ratio. This mixed-coating state of the cathode material increases the lithium iron phosphate mass content while simultaneously coating the surface of the ternary material with smaller-sized lithium iron phosphate particles. This effectively reduces side reactions between the cathode material interface and the electrolyte during charging and discharging, lowers the loss of active lithium during charging and discharging, and thus extends the lifespan of the secondary battery. Therefore, this invention effectively overcomes some practical problems in the prior art and has high utilization value and practical significance. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figures 1 to 5 These are scanning electron microscope (SEM) images of the positive electrode active material layer on the positive electrode sheet before rolling in Comparative Example 1 and Examples 1 to 4 of the present invention.

[0025] Figures 6 to 10 These are scanning electron microscope (SEM) images of the positive electrode active material layer on the positive electrode sheet after rolling in Comparative Example 2 and Examples 1 to 4 of the present invention.

[0026] Figure 11 This is a trend chart of cycle tests for secondary batteries equipped with positive electrode materials in various embodiments of the present invention.

[0027] Figure 12 This is a schematic flowchart of a method for preparing a positive electrode material in one embodiment of the present invention. Detailed Implementation

[0028] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. It should also be understood that the terminology used in the embodiments of the present invention is for describing specific implementation schemes and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the respective manufacturers.

[0029] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" and "one" used in this specification are only for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered as part of the scope of the invention.

[0030] Electrochemical devices typically include positive electrode materials, negative electrode materials, electrolytes, separators, and corresponding connecting components and circuits. The positive and negative electrode materials can intercalate and deintercalate lithium ions to store and release energy. The electrolyte serves as a carrier for lithium ion transport between the positive and negative electrodes. The separator is permeable to lithium ions but non-conductive, thus separating the positive and negative electrodes to prevent short circuits. The positive and negative electrode materials typically play a decisive role in key performance factors of lithium batteries, such as energy storage capacity, cell energy density, cycle performance, and safety.

[0031] The inventors discovered that mixing lithium iron phosphate (LFP) material into ternary materials can reduce side reactions between the positive electrode and the electrolyte during charging and discharging, thereby improving the cycle life of the secondary battery. However, simply mixing ternary materials and LFP may result in insufficient particle uniformity, leading to excessive exposure of the interface between the ternary material and the electrolyte in the positive electrode. This reduces the effectiveness of LFP in mitigating side reactions at the positive electrode interface, ultimately resulting in a less than ideal improvement in the cycle life of the secondary battery.

[0032] Therefore, to solve the above problems, the present invention provides a cathode material in which the maximum and minimum particle size ratios of the mixed ternary material and lithium iron phosphate material particles are selected. This makes the ternary material and lithium iron phosphate material in the cathode material mutually encapsulate each other, thereby improving the mixing uniformity of the cathode material without increasing the preparation process, reducing the direct contact area between the ternary material and the electrolyte, and further improving and extending the cycle life of the secondary battery.

[0033] The aforementioned cathode material includes a first active material and a second active material. The first active material is a ternary material. Ternary materials, as layered structures, have advantages such as high energy density and high discharge plateau voltage. However, ternary materials have poor thermal stability and are prone to structural collapse during charge and discharge, leading to side reactions at the interface with the electrolyte, thus reducing the battery's cycle life. The second active material is lithium iron phosphate (LFP). LFP is an olivine-structured material where oxygen and phosphorus atoms are strongly covalently bonded, resulting in good structural stability and excellent cycle performance. The addition of LFP to the cathode material effectively improves its structural stability during charge and discharge, reduces interfacial side reactions between the cathode material and the electrolyte, and improves the cycle life of the secondary battery.

[0034] In the cathode material, the mass ratio of the mixed first active material and the second active material is (80-95):(5-20). For example, the mass content of the second active material in the cathode material can be 5 wt%, 10 wt%, 15 wt%, or 20 wt%, preferably 20 wt%. The ratio D1 between the maximum particle size of the first active material and the maximum particle size of the second active material is... max / D2 max The value is 0.5 to 1, for example, it can be 0.5, 0.7 or 0.1; and the ratio D1 of the minimum particle size of the first active substance to the minimum particle size of the second active substance is... min / D2 min The particle size is 6.4–10, for example, 6.4, 8.0, or 10; where D1 represents the particle size of the first active substance. max D1 represents the maximum particle size of the first active substance. min D1 represents the minimum particle size of the first active substance; D2 represents the particle size of the second active substance. max D2 represents the maximum particle size of the second active substance. min This indicates the minimum particle size of the second active substance.

[0035] like Figures 2 to 5 as well as Figures 7 to 10As shown, at the aforementioned particle size ratio, the particle size distributions of the first and second active materials overlap, and the particle size range of the second active material completely encompasses the particle size range of the first active material. Specifically, in the cathode material, the maximum particle size of the second active material is greater than or equal to the maximum particle size of the first active material, and is much larger than the minimum particle size of the first active material; the minimum particle size of the second active material is much smaller than the minimum particle size of the first active material. This results in the first and second active material particles mutually coating each other after the cathode material is rolled, with some smaller-sized first active material particles coating the surface of larger-sized second active material particles, and vice versa.

[0036] In the mixed coating, the first active material coated on the surface of the second active material particles in the cathode material helps to improve the conductivity of the second active material and reduce the impedance of the second active material particles to transport lithium ions with the electrolyte. Meanwhile, the second active material coated on the surface of the first active material particles can isolate the surface of the large first active material particles from direct contact with the electrolyte, reducing the surface area of ​​the first active material directly exposed to the electrolyte. This reduces the interfacial side reactions between the cathode material and the electrolyte during charging and discharging, thereby improving the cycle life of the secondary battery while ensuring the charging and discharging efficiency of the secondary battery.

[0037] It should be noted that the type of ternary material in this invention is not limited. For example, the ternary material can be doped or undoped lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide. Preferably, in some embodiments, lithium nickel cobalt manganese oxide is used as the ternary material.

[0038] In some embodiments, within the defined ranges of the maximum and minimum particle size ratios between the first and second active substances, the particle size range of the first active substance is 1.60 μm to 12.50 μm, and the particle size range of the second active substance is 0.16 μm to 25.00 μm, wherein the minimum particle size of the second active substance is 0.16 μm to 0.25 μm, and the maximum particle size of the second active substance is 12.50 μm to 25.00 μm. For example, in one example, the particle size range of the first active substance is 1.60 μm to 12.50 μm, the particle size range of the second active substance is 0.25 μm to 25.00 μm, and the maximum particle size ratio D1 of the first and second active substances is... max / D2 max The minimum particle size ratio D1 of the first and second active substances is 0.50. min / D2 minThe value is 6.40; in another example, the particle size range of the first active material is 1.60 μm to 12.50 μm, the particle size range of the second active material is 0.20 μm to 12.50 μm, and the maximum particle size ratio D1 of the first active material and the second active material is... max / D2 max The minimum particle size ratio D1 of the first and second active substances is 1.00. min / D2 min It is 10.00.

[0039] Please see Figure 12 The present invention provides a method for preparing the cathode material according to any of the above embodiments, comprising the following steps:

[0040] S1. Mix the first active substance and the second active substance uniformly according to a mass ratio of (80-95):(5-20) to obtain a mixture;

[0041] S2. Roll the mixture to obtain the positive electrode material.

[0042] The mass ratio of the first active substance and the second active substance in the mixture can be 95:5, 90:10, 85:15, or 80:20; the particle sizes of the selected first and second active substances must meet the following condition: the ratio D1 of the maximum particle size of the first active substance to the maximum particle size of the second active substance. max / D2 max The ratio D1 is 0.5 to 1, representing the ratio of the minimum particle size of the first active substance to the minimum particle size of the second active substance. min / D2 min The range is 6.4–10, where D1 represents the particle size of the first active substance. max D1 represents the maximum particle size of the first active substance. min D1 represents the minimum particle size of the first active substance; D2 represents the particle size of the second active substance. max D2 represents the maximum particle size of the second active substance. min This indicates the minimum particle size of the second active substance.

[0043] This preparation method only requires mixing a first active material and a second active material with an appropriate particle size ratio, without any other process steps (such as a coating and sintering step), to achieve mutual coating of the first and second active material particles in the cathode material. Therefore, compared with existing preparation methods, this method eliminates the need for a sintering step, offering advantages such as simple procedures, low cost, and ease of operation.

[0044] In step S1, the first active material used is a ternary material, and the type of ternary material is not limited. For example, the ternary material can be doped or undoped lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide; the second active material used is lithium iron phosphate material.

[0045] In some embodiments, in step S1, the particle size of the first active substance is 1.60 μm to 12.50 μm, and the particle size of the second active substance is 0.16 μm to 25.00 μm, wherein the minimum particle size of the second active substance is 0.16 μm to 0.25 μm, and the maximum particle size of the second active substance is 12.50 μm to 25.00 μm. For example, the particle size of the first active substance is 1.60 μm to 12.50 μm, and the particle size of the second active substance is 0.25 μm to 25.00 μm; or, the particle size of the first active substance is 1.60 μm to 12.50 μm, and the particle size of the second active substance is 0.20 μm to 12.50 μm.

[0046] In step S1, the selected first and second active substances within the specified particle size range can be obtained by screening the raw materials of the first and second active substances through a grading process. The grading process can remove excessively small and excessively large particles from the crushed powder, resulting in pulverized first and second active substance raw materials with a specified particle size range. The grading process can be carried out using equipment and methods known in the art, such as grading sieves, gravity classifiers, and centrifugal classifiers.

[0047] In step S2, the mixture can be rolled using equipment and methods known in the art. For example, in some embodiments, a roller press is used to roll the mixture in step S2.

[0048] The present invention also provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on the positive current collector, the positive active material layer comprising the positive electrode material described in any of the above embodiments of the present invention.

[0049] The preparation process of the positive electrode sheet is as follows:

[0050] The preparation process of the positive electrode sheet is as follows: The positive electrode material, conductive agent, and binder prepared above are mixed in a mass ratio of (90 to 99):1.5:1.5, preferably 97:1.5:1.5. N-methylpyrrolidone (NMP) solvent is added, and the mixture is stirred thoroughly to obtain a positive electrode slurry. The positive electrode slurry is coated onto a positive electrode current collector aluminum foil, and after drying, cold pressing, and slitting, the positive electrode sheet is prepared. The conductive agent can be at least one of the following: Super P (carbon black), acetylene black, carbon nanotubes (CNTs), graphene, and carbon nanofibers (VGCF). For example, the conductive agent can be SP and CNTs, with a mass ratio of SP to CNTs of 1:0.5. The binder can be at least one of PVDF, PTFE, etc.

[0051] In some embodiments, pores exist between the particles in the positive electrode active material layer, and the ratio of the maximum pore size to the maximum particle size of the first active material is 0.04 to 0.1, for example, 0.04, 0.06, 0.07, 0.08, or 0.10. The pores in the positive electrode active material allow the positive electrode material particles in the layer to be fully wetted by the electrolyte, thereby reducing the impedance of ion transport between the positive electrode materials and increasing the ion transport rate between the positive electrode material and the electrolyte. Simultaneously, it facilitates the insertion and extraction of lithium ions on the positive electrode material, preventing side reactions such as lithium plating during charging and discharging, thus significantly reducing the cycle decay rate. Furthermore, within this ratio range, smaller-sized particles of the first active material in the positive electrode material can coat the surface of larger particles of the second active material within the pores, and smaller-sized particles of the second active material can also coat the surface of larger particles of the first active material within the pores, further improving the mutual coating degree between the first and second active materials.

[0052] In some embodiments, after the positive electrode active material layer on the positive electrode sheet is rolled, the maximum pore size of the interparticle pores in the positive electrode active material layer is 0.5μm to 2.0μm. For example, the maximum pore size can be 0.5μm, 0.7μm, 0.8μm, 1.3μm, 1.5μm, 1.8μm or 2.0μm.

[0053] The present invention also provides an electrochemical device, which includes the positive electrode of the present invention. The electrochemical device is, for example, a lithium-ion battery, which includes a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and an electrolyte. The positive and negative electrode materials can intercalate and deintercalate lithium ions to achieve energy storage and release. The electrolyte is the carrier for lithium ion transport between the positive and negative electrodes. The separator is permeable to lithium ions but non-conductive, thereby separating the positive and negative electrodes to prevent short circuits.

[0054] The composition and preparation method of lithium-ion batteries are described in detail below:

[0055] The positive electrode sheet is prepared using the preparation method described above in this invention;

[0056] Negative electrode preparation: Negative electrode materials such as graphite, conductive agent acetylene black, thickener CMC (sodium carboxymethyl cellulose), and binder SBR (styrene-butadiene rubber) are mixed in a mass ratio of 96.4:1:1.2:1.4, and deionized water is added. The mixture is stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, and after drying, cold pressing, and slitting, the negative electrode sheet is prepared.

[0057] Membrane: The membrane is made of PE or PP porous membrane, preferably PP / PE / PP porous membrane, with a thickness of 9μm to 18μm, such as 9μm, 12μm, 16μm or 18μm; air permeability of 180s / 100mL to 380s / 100mL, such as 180s / 100mL, 280s / 100mL or 380s / 100mL; porosity of 30% to 50%, such as 30%, 40% or 50%.

[0058] Electrolyte preparation: EC (ethylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) are mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0059] Battery assembly: The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. Then, an aluminum-plastic film is wrapped around the separator, dried, and the prepared electrolyte is injected. After encapsulation, settling, and formation processes, a 1Ah soft-pack battery (i.e., a lithium-ion battery) is finally produced.

[0060] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available products or can be prepared by conventional methods in the art.

[0061] Example 1

[0062] This embodiment provides a cathode material, the preparation process of which is as follows:

[0063] S1. The first active material and the second active material are uniformly mixed at a mass ratio of 95:5 to obtain a mixture; wherein the first active material is lithium nickel cobalt manganese oxide, and the second active material is lithium iron phosphate. x Co y Mn 1-x-yThe particle size of O2 (NCM) ranges from 1.60 μm to 12.50 μm, and the particle size of lithium iron phosphate (LiFePO4, LFP) ranges from 0.25 μm to 25.00 μm.

[0064] S2. Roller pressing of the mixture yields the cathode material.

[0065] Example 2

[0066] This embodiment prepares a cathode material with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step S1, the first active material and the second active material are uniformly mixed in a mass ratio of 90:10.

[0067] Example 3

[0068] This embodiment prepares a cathode material with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step S1, the first active material and the second active material are uniformly mixed in a mass ratio of 85:15.

[0069] Example 4

[0070] This embodiment prepares a cathode material with the same system as in Example 1. The difference between this embodiment and Example 1 is that in step S1, the first active material and the second active material are uniformly mixed in a mass ratio of 80:20.

[0071] Example 5

[0072] The cathode material prepared in this embodiment is the same system as that in Example 4. The difference between this embodiment and Example 4 is that the particle size of the second active material is 0.20 μm to 18.00 μm.

[0073] Example 6

[0074] The cathode material prepared in this embodiment is the same system as that in Example 4. The difference between this embodiment and Example 4 is that the particle size of the second active material is 0.16 μm to 12.50 μm.

[0075] Comparative Example 1

[0076] This comparative example provides a cathode material, which is lithium nickel cobalt manganese oxide with a particle size of 1.60 μm to 12.50 μm.

[0077] The cathode materials prepared in Examples 1 to 6 and Comparative Example 1 were respectively assembled into lithium-ion batteries to verify the efficacy of the present invention. The preparation process of the lithium-ion battery is as follows:

[0078] (1) Positive electrode sheet: The above-mentioned positive electrode material, conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) solvent is added and the mixture is stirred thoroughly to obtain a positive electrode slurry. The mixture is stirred under vacuum until the system is homogeneous and transparent to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated onto a 16μm aluminum foil current collector. The aluminum foil current collector is then dried at room temperature and transferred to an oven to dry at 80℃~120℃ for 6 hours. After cold pressing and slitting, the positive electrode sheet is obtained.

[0079] (2) Negative electrode sheet: The negative electrode material graphite, conductive agent acetylene black, thickener sodium carboxymethyl cellulose (CMC) and binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96.4:1:1.2:1.4. Deionized water is added to adjust the slurry solid content to 55%. The mixture is then thoroughly stirred and mixed under the action of a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on both sides of an 8μm negative electrode current collector copper foil. After drying at room temperature, it is transferred to an oven for drying. Then, the negative electrode sheet is obtained through cold pressing, slitting and other processes.

[0080] (3) Electrolyte preparation: In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are uniformly mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1mol / L.

[0081] (4) Separator preparation: The diaphragm is a 9μm polyethylene diaphragm with a ceramic coating of 2μm thickness on both sides.

[0082] (5) Battery assembly: The positive electrode, separator, and negative electrode obtained above are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. Then, an aluminum-plastic film is wrapped around the outside, and the battery is transferred to a vacuum oven to dry at 120°C. After injecting 3.0 g / Ah of electrolyte, the battery is sealed. After processes such as standing, hot and cold pressing, formation, clamping, and capacity testing, a soft-pack battery (i.e., a lithium-ion battery) with a capacity of 1Ah is finally obtained.

[0083] The specific steps and conditions for electrolyte formation are as follows: After injecting the electrolyte, maintain a hot pressure environment of 0.1 MPa, charge at 0.02C for 17 minutes at 45°C in a static state, let it stand for 5 minutes, and then charge it to 0.3Ah at 0.02C. After that, cut off the gas bag and vacuum seal it, and let it stand at room temperature for 48 hours to complete the electrolyte formation.

[0084] The parameters and performance of the lithium-ion batteries equipped with the cathode materials of Examples 1 to 6 and Comparative Example 1 were tested. Specifically, the pore size and cycle capacity retention of each prepared lithium-ion battery were tested to verify the effect of the cathode materials in different examples and comparative examples on improving the cycle performance of the electrochemical device.

[0085] The pore size test of the positive electrode active material layer on the positive electrode sheet includes:

[0086] The cross-section of the cathode material before and after rolling was polished using an argon ion beam. The cross-section was then observed under a scanning electron microscope at a magnification of 2000x. The pore sizes between the cathode material particles in the cross-section were marked, and the maximum and minimum pore sizes were selected. The pore size range before and after rolling was then calculated according to the magnification ratio.

[0087] Secondary battery cycle capacity retention testing includes:

[0088] Under a constant temperature environment of 25℃, the lithium-ion battery was first charged to 4.4V (constant voltage cutoff current is 0.05C) with a constant current of 0.21A / g (calculated based on the mass of the positive electrode material). Then, the lithium-ion battery was discharged to 2.5V with a constant current of 0.21A / g (calculated based on the mass of the positive electrode material). This process was repeated for 500 cycles. The ratio of the discharge capacity on the 500th cycle to the discharge capacity on the 1st cycle was recorded as the cycle capacity retention rate.

[0089] The preparation parameters of the cathode materials in Examples 1 to 6 and Comparative Example 1 are shown in Table 1, and the test results of the lithium-ion batteries assembled with the cathode materials are shown in Table 2.

[0090] Table 1: Preparation parameters of cathode materials in Examples 1 to 6 and Comparative Example 1

[0091]

[0092] Where D1 represents the particle size of the first active substance, D1 max D1 represents the maximum particle size of the first active substance. min D1 represents the minimum particle size of the first active substance; D2 represents the particle size of the second active substance. ma x represents the maximum particle size of the second active substance, D2 min This indicates the minimum particle size of the second active substance.

[0093] Table 2: Cycle capacity retention test results of lithium-ion batteries prepared with cathode materials in Examples 1 to 6 and Comparative Example 1

[0094]

[0095]

[0096] like Figures 1 to 11 As can be seen from the preparation parameters of the cathode material in Examples 1 to 6 and Comparative Example 1, as well as the test results of the lithium-ion battery, the present invention provides a cathode material composed of a mixture of ternary material and lithium iron phosphate material. The cathode material is mixed with the two materials within a maximum particle size ratio of 0.5 to 1 and a minimum particle size ratio of 6.4 to 10. This allows the ternary material and lithium iron phosphate material in the cathode material to be mutually coated, thereby increasing the proportion of lithium iron phosphate material while also allowing the surface of the larger ternary material particles to be coated by lithium iron phosphate material. This reduces the side reactions between the cathode material interface and the electrolyte, and improves the cycle life of the lithium-ion battery.

[0097] As can be seen from the preparation parameters of the cathode materials in Examples 1 to 6 and Comparative Example 1 and the test results of the lithium-ion batteries, within an appropriate particle size ratio range, mixing lithium iron phosphate material into ternary materials can ensure conductivity while reducing interfacial side reactions between the cathode material and the electrolyte, thereby effectively slowing down the rate of battery capacity decay. For example, the cathode material of the battery in Comparative Example 1 only contains lithium nickel cobalt manganese oxide, and the battery capacity retention rate decays to 92% after 500 cycles, which is close to the discard capacity of 80%. However, the cathode materials of the batteries in Examples 1 to 6 contain ternary materials and lithium nickel cobalt manganese oxide, and the battery capacity retention rate is still maintained above 95.8% after 500 cycles.

[0098] The test results of lithium-ion batteries prepared by cathode materials in Examples 1 to 4 show that as the proportion of lithium iron phosphate in the cathode material increases, the cycle performance of lithium-ion batteries is further improved. After 500 cycles, the capacity retention rate of lithium-ion batteries increases from 95.8% when the cathode material contains 5 wt% lithium iron phosphate to 97.4% when the cathode material contains 20 wt% lithium iron phosphate.

[0099] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A positive electrode material, characterized in that, Comprising: a first active substance, which is a ternary material; a second active substance, which is a lithium iron phosphate material; wherein part of the first active substance is coated on the surface of the particles of the second active substance, and part of the second active substance is coated on the surface of the particles of the first active substance; The mass ratio of the first active substance and the second active substance is (80-95):(5-20), the ratio of the maximum particle size of the first active substance to the maximum particle size of the second active substance is 0.5-1, and the ratio of the minimum particle size of the first active substance to the minimum particle size of the second active substance is 6.4-10; the particle size of the first active substance is 1.60-12.50 μm, and the particle size of the second active substance is 0.16-25.00 μm, wherein the minimum particle size of the second active substance is 0.16-0.25 μm, and the maximum particle size of the second active substance is 12.50-25.00 μm.

2. The cathode material of claim 1, wherein, The ternary material is lithium nickel cobalt manganese oxide.

3. A method for producing the positive electrode material according to any one of claims 1 to 2, characterized by, Comprising: Mixing a first active substance and a second active substance according to a mass ratio of (80-95):(5-20) to obtain a mixture; Rolling the mixture to obtain a positive electrode material; wherein the ratio of the maximum particle size of the first active substance to the maximum particle size of the second active substance is 0.5-1, and the ratio of the minimum particle size of the first active substance to the minimum particle size of the second active substance is 6.4-10.

4. The preparation method according to claim 3, characterized in that, The particle size of the first active substance selected is 1.60-12.50 μm, and the particle size of the second active substance is 0.16-25.00 μm, wherein the minimum particle size of the second active substance is 0.16-0.25 μm, and the maximum particle size of the second active substance is 12.50-25.00 μm.

5. A positive electrode sheet characterized by comprising: The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, and the positive electrode active material layer comprises a positive electrode material and the positive electrode material of any one of claims 1-2, or the positive electrode material prepared by the preparation method of claims 3 or 4.

6. The positive electrode sheet according to claim 5, wherein The active material layer has pores, and the ratio of the maximum pore size of the pores to the maximum particle size of the first active substance is 0.04-0.

1.

7. The positive electrode sheet according to claim 6, wherein The maximum pore size of the pores is 0.5-2.0 μm.

8. An electrochemical device, characterized by, Comprising, a negative electrode sheet, a separator, an electrolyte, and the positive electrode sheet of any one of claims 5-7.

Images