Positive electrode material, secondary battery, and electronic device

Abstract

The application provides a positive electrode material, a secondary battery and an electronic device, and particularly relates to the technical field of battery materials. The positive electrode material comprises a first active material and a second active material, the first active material comprises lithium iron manganese phosphate, and the second active material comprises lithium nickel cobalt manganese oxide, wherein the lithium iron manganese phosphate is coated on the outside of the lithium nickel cobalt manganese oxide, the mass of the lithium iron manganese phosphate is M1, the mass of the lithium nickel cobalt manganese oxide is M2, M1 and M2 satisfy formula (1), and the application reasonably designs the ratio of the lithium iron manganese phosphate and the lithium nickel cobalt manganese oxide, optimizes the coating amount of the lithium iron manganese phosphate on the lithium nickel cobalt manganese oxide, so that the secondary battery comprising the positive electrode material can give consideration to the cycle performance and the exertion of the gram capacity.

Description

Technical Field

[0001] This invention relates to the field of battery manufacturing technology, specifically to a cathode material, a secondary battery, and an electronic device. Background Technology

[0002] The rapid development of new energy vehicles has further expanded the market size of power batteries, and as an important component of power batteries, the demand for cathode materials is also increasing. Lithium nickel cobalt manganese oxide (Ni-Co-Mn) cathode materials, through the synergistic effect of the three elements Ni, Co, and Mn, combine the good cycle performance of LiCoO2, the high specific capacity of LiNiO2, and the high safety and low cost of LiMn2O4, making them one of the most promising lithium-ion battery cathode materials. However, with the increasing demand for high energy density, ternary cathode materials are showing a trend towards higher nickel content. The increase in nickel content leads to a decrease in the thermal stability of the material, thus affecting battery safety.

[0003] Lithium iron phosphate (LFP) cathode materials offer good stability and low cost, but they also have some drawbacks: such as a relatively low operating voltage (3.4V) and low battery energy density, making it difficult to meet the varying range requirements of electric vehicles. Lithium manganese iron phosphate (LMP), on the other hand, shares the advantages of safety and low cost with LFP. Compared to LFP, LMP has a higher voltage platform (4.1V), better low-temperature performance, and an energy density approaching that of ternary materials (such as LiNi). 0.5 Co 0.2 Mn 0.3 O2 cathode material), however, lithium manganese iron phosphate material has a low specific capacity.

[0004] Therefore, there is a need to provide a cathode material, a secondary battery, and an electronic device to improve the problems existing in cathode materials. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the present invention provides a cathode material, a secondary battery, and an electronic device to improve the problem that the cycle performance and specific capacity of cathode materials cannot be simultaneously achieved.

[0006] To achieve the above and other related objectives, the present invention provides a cathode material comprising: a first active material and a second active material, wherein the first active material comprises lithium manganese iron phosphate, and the second active material comprises lithium nickel cobalt manganese oxide, wherein the lithium manganese iron phosphate is coated on the outer surface of the lithium nickel cobalt manganese oxide, the mass of the lithium manganese iron phosphate is M1, and the mass of the lithium nickel cobalt manganese oxide is M2, wherein M1 and M2 satisfy formula (1):

[0007]

[0008] In formula (1), the value of a ranges from 0.9 to 1.1; ρ1 is the density of lithium manganese iron phosphate, ρ2 is the density of lithium nickel cobalt manganese oxide, D1 is the median particle size of lithium manganese iron phosphate, and D2 is the median particle size of lithium nickel cobalt manganese oxide.

[0009] In one example of the present invention, the chemical formula of the lithium manganese iron phosphate is LiMn. x Fe (1-x) PO4, wherein 0.6 ≤ x ≤ 0.8; and / or, the lithium nickel cobalt manganese oxide has the chemical formula Li. 1+a Ni x Co y Mn z Me k O 2-b A b Wherein, -0.2≤a≤0.2, 0.8≤x≤0.9, 0<y+z≤0.2, 0≤k≤0.1, x+y+z+k=1, 0≤b≤0.1; Me is selected from one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W, Nb, and A is selected from one or more of N, F, S, Cl.

[0010] In one embodiment of the present invention, D1 < D2.

[0011] In one embodiment of the present invention, the value of D1 ranges from 0.5 μm to 0.9 μm, and the value of D2 ranges from 3 μm to 8 μm.

[0012] In one embodiment of the present invention, the median particle size d of the primary lithium manganese iron phosphate particles ranges from 0.1 μm to 0.4 μm.

[0013] In one embodiment of the present invention, the positive electrode material is used to prepare a positive electrode sheet, the positive electrode sheet is polished with an argon ion cross section, and an elemental energy dispersive spectroscopy (EDS) analysis is performed on a region with an area of ​​A to determine the distribution ratio of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide.

[0014]

[0015] In equation (2), the value of b ranges from 2 to 4.23; S Ni The percentage of Ni element in area A; S Fe ρ1 represents the percentage of Fe element in area A; ρ2 represents the density of lithium manganese iron phosphate; D1 represents the median particle size of lithium manganese iron phosphate; and D2 represents the median particle size of lithium nickel cobalt manganese oxide.

[0016] Another aspect of the present invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the positive electrode material described above in the present invention.

[0017] In one embodiment of the present invention, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one side surface of the negative current collector. The negative active material layer includes a negative electrode material, which includes one or more of graphite material, silicon-carbon negative electrode material, and silicon-oxygen negative electrode material.

[0018] In one embodiment of the present invention, the capacity of the negative electrode material is N, the capacity of the positive electrode material is P, and the value of N / P ranges from 1.02 to 1.12.

[0019] The present invention also provides an electronic device comprising the secondary battery described above.

[0020] The cathode material of this invention uses small-particle lithium manganese iron phosphate and large-particle lithium nickel cobalt manganese oxide. The small-particle lithium manganese iron phosphate is coated on the outside of the large-particle lithium nickel cobalt manganese oxide to isolate the lithium nickel cobalt manganese oxide from the electrolyte and avoid direct contact. This can effectively reduce the occurrence of side reactions and thus improve the overall cycle life. On the other hand, by rationally designing the ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide and optimizing the coating amount of lithium manganese iron phosphate on the outside of lithium nickel cobalt manganese oxide, the situation of local degradation caused by excessive coating material can be avoided, thus balancing the performance of cycle performance and specific capacity. Attached Figure Description

[0021] 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.

[0022] Figure 1 The graphs show the cyclic test curves of Examples 1, 3, and 4 and Comparative Examples 1 to 3 of the present invention. Detailed Implementation

[0023] 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 described therein can be combined with each other.

[0024] 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 described therein can be combined with each other.

[0025] 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 invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0026] Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings:

[0027] In this article, the terms "multiple," "various," and "multiple times" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" means one or more types.

[0028] In this document, terms such as "preferred," "better," and "more effective" are merely descriptions of more effective implementation methods or embodiments, and should be understood not to limit the scope of protection of this invention. If multiple "preferred" terms appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "preferred" term shall be independent.

[0029] In this document, terms such as "further," "even further," and "particularly" are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this invention.

[0030] In this document, when referring to numerical ranges, unless otherwise specified, the distribution of selectable values ​​within a numerical range is considered continuous, including the two endpoints of the range (i.e., the minimum and maximum values), and every value between these two endpoints. When multiple numerical ranges are provided to describe a feature or property, these numerical ranges can be combined.

[0031] To meet the ever-increasing energy density demands of the new energy vehicle sector, lithium manganese iron phosphate (LFMP), which shares advantages with lithium iron phosphate (LFP) such as safety and low cost, has attracted considerable attention. LFMP has a voltage platform of approximately 4.1V, an energy density close to that of pentaneous ternary materials, and superior low-temperature performance compared to LFP. However, its specific capacity is relatively low. Lithium nickel cobalt manganese oxide (NCM) has a high energy density, especially high-nickel materials, but its cycle life is not superior to LFP due to its susceptibility to side reactions in the electrolyte. Combining the high specific energy of NCM with the high safety of LFP can effectively solve the problems existing in cathode materials. Based on this, the present invention provides a cathode material, a secondary battery containing the cathode material, and an electronic device containing the secondary battery.

[0032] The cathode material of the present invention includes a first active material and a second active material. The first active material includes lithium manganese iron phosphate, and the second active material includes lithium nickel cobalt manganese oxide. The lithium manganese iron phosphate coats the outside of the lithium nickel cobalt manganese oxide. The lithium manganese iron phosphate coating the outside of the lithium nickel cobalt manganese oxide can separate the lithium nickel cobalt manganese oxide from the electrolyte, avoid direct contact, effectively reduce the occurrence of side reactions, and thus improve the overall cycle life.

[0033] The inventors discovered in their research that the ratio of lithium manganese iron phosphate (LFP) to lithium nickel manganese cobalt oxide (LCMCO) affects the overall coating effect: excessive LFP coating, i.e., insufficient LCMCO in the cathode material, leads to localized degradation; while insufficient LFP coating, i.e., excessive LCMCO in the cathode material, results in insignificant improvement in the cathode material's cycle life, but high specific capacity. Therefore, the inventors optimized the ratio of LFP to LCMCO through raw material parameter calculations: the mass of LFP is M1, and the mass of LCMCO is M2, where M1 and M2 satisfy formula (1):

[0034]

[0035] In formula (1), a is an empirical constant, with a value ranging from 0.9 to 1.1, and further, a = 1; ρ1 is the density of lithium manganese iron phosphate, ρ2 is the density of lithium nickel cobalt manganese oxide, D1 is the median particle size of lithium manganese iron phosphate, and D2 is the median particle size of lithium nickel cobalt manganese oxide. By controlling the ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide according to formula (1), the excessive or insufficient coating of lithium manganese iron phosphate can be avoided.

[0036] In some embodiments, the median particle size D1 of lithium manganese iron phosphate is smaller than the median particle size D2 of lithium nickel cobalt manganese oxide. The median particle size refers to the particle size corresponding to a cumulative volumetric particle size distribution percentage of 50% for a sample (first active material or second active material). In some embodiments, D1 ranges from 0.5 μm to 0.9 μm; for example, D1 can be 0.5 μm, 0.7 μm, or 0.9 μm, etc.; D2 ranges from 3 μm to 8 μm; for example, D2 can be 3 μm, 5 μm, 7 μm, or 8 μm, etc. Lithium manganese iron phosphate meeting the above particle size requirements can form a better coating effect on the surface of lithium nickel cobalt manganese oxide.

[0037] In this application, the chemical formula of lithium manganese iron phosphate is LiMn. x Fe (1-x) PO4, where x represents the molar content of Mn in lithium manganese iron phosphate, and the value of x ranges from 0.6 to 0.8. In some embodiments, the value of x can be 0.6, 0.7, or 0.8, etc., corresponding to the chemistry of lithium manganese iron phosphate being LiMn. 0.6 Fe 0.4 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.8 Fe 0.2 PO4, etc. In some embodiments, lithium manganese iron phosphate includes primary particles and secondary particles, wherein the secondary particles are formed by the agglomeration of primary particles. Preferably, the median particle size d of the primary particles ranges from 0.1 μm to 0.4 μm. Small particle size materials can reduce polarization and are more conducive to the performance of materials.

[0038] The chemical formula of lithium nickel cobalt manganese oxide is Li 1+a Ni x Co y Mn z Me k O 2-b A bWhere -0.2≤a≤0.2, 0.8≤x≤0.9, 0<y+z≤0.2, 0≤k≤0.1, x+y+z+k=1, 0≤b≤0.1. In this application, the molar content of Ni in the second active material is designed to be between 0.8 and 0.9 (0.8≤x≤0.9), which can effectively improve the specific capacity and cycle performance of the cathode material. In the above formula, both Me and A are doping elements. Me can be selected from one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W, and Nb, and A can be selected from one or more of N, F, S, and Cl. Doping the cathode material with appropriate amounts of Me and the electronegative A element can significantly improve the structural stability of the cathode material, thereby improving the cycle performance. When k=0 and b=0, it indicates that no other elements are doped in lithium nickel cobalt manganese oxide. At this time, its chemical formula is Li 1+a Ni x Co y Mn z O2.

[0039] The cathode material described above in this invention was prepared into a cathode sheet, and the cathode sheet was subjected to argon ion cross-section polishing (CP). Elemental energy dispersive spectroscopy (EDS) mapping analysis was performed on a region with an area of ​​A to obtain the distribution of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide in the cathode material:

[0040]

[0041] In equation (2), b is an empirical coefficient, and its value ranges from 2 to 4.23, for example, b = 3; S Ni S represents the distribution area of ​​Ni element within area A; Fe Let A be the area of ​​Fe distribution within region A; ρ1 is the density of lithium manganese iron phosphate, ρ2 is the density of lithium nickel cobalt manganese oxide, D1 is the median particle size of lithium manganese iron phosphate, and D2 is the median particle size of lithium nickel cobalt manganese oxide. It should be noted that region A ≥ 1 cm. 2 For ease of sample preparation, the area of ​​region A in this application is taken as 1 cm². 2 .

[0042] In formula (2), Ni is selected as the specific element for lithium nickel cobalt manganese oxide and Fe is selected as the specific element for lithium manganese iron phosphate. The distribution of lithium nickel cobalt manganese oxide and lithium manganese iron phosphate can be effectively distinguished by element mapping.

[0043] A second aspect of the present invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The separator is disposed between the positive and negative electrodes, serving as an isolation layer. The electrolyte conducts lithium ions between the positive and negative electrodes. During the charging and discharging process, lithium ions repeatedly insert and extract between the positive and negative electrodes.

[0044] Specifically, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive current collector can be made of a material with good conductivity and mechanical strength, such as aluminum foil. The positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector. The positive active material layer includes a positive electrode material, a positive electrode conductive agent, a positive electrode binder, and a positive electrode dispersant, wherein the positive electrode material is the positive electrode material described above in this invention. The positive electrode conductive agent, positive electrode binder, and positive electrode dispersant can be selected from conventional materials in the art, and no specific limitations are made here. As an example, the positive electrode conductive agent is selected from one or more of conductive carbon black (SP), acetylene black, nano-metal powder, graphene, carbon nanotubes, and carbon nanofibers, or a combination of two or more mixed in any proportion. The positive electrode binder is selected from one or more of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, sodium carboxymethyl cellulose, and styrene-butadiene rubber. The positive electrode dispersant is selected from one or more of polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, and polyacrylate.

[0045] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative current collector can be made of a material with good conductivity and mechanical strength, such as copper foil. The negative current collector has two surfaces opposite each other in its own thickness direction, and the negative active material layer is disposed on either or both of the two opposite surfaces of the negative current collector. The negative active material layer includes a negative electrode material, a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersant. Here, the negative electrode material, negative electrode conductive agent, negative electrode binder, and negative electrode dispersant can be selected from conventional materials in the art, and no specific limitations are made.

[0046] As an example, the negative electrode material is selected from carbon and / or silicon negative electrode materials, such as elemental silicon, silicon oxides, silicon carbide compounds, graphite, etc., where graphite can be natural graphite, artificial graphite, soft carbon, hard carbon, etc. The negative electrode conductive agent is selected from one or a combination of two or more of conductive carbon black, nano silver powder, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc., in any proportion. The negative electrode binder is selected from any one or a combination of several of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR) in any proportion; the negative electrode dispersant can be one or more of polyvinylpyrrolidone (PVP), sodium carboxymethyl cellulose (CMC), and polyvinyl alcohol (PVA).

[0047] When designing lithium batteries, correctly calculating the appropriate ratio of positive and negative electrode capacities is crucial. If the positive electrode has an excess of capacity, during charging, the excess lithium ions released from the positive electrode cannot enter the negative electrode, leading to lithium deposition on the negative electrode surface and the formation of dendrites. This degrades battery cycle performance, can cause internal short circuits, and trigger battery safety issues. Therefore, the negative electrode is usually slightly larger than the positive electrode, but it cannot be excessively so. Excessive capacity will consume lithium in the positive electrode and also waste the negative electrode, reducing battery energy density and increasing battery cost. In this application, the capacity of the negative electrode material is N, and the capacity of the positive electrode material is P. The N / P ratio ranges from 1.02 to 1.12, for example, N / P = 1.02, N / P = 1.08, or N / P = 1.12, etc.

[0048] The membrane is selected from conventional types in the art, such as a 12μm polypropylene (PP) porous membrane.

[0049] The electrolyte can be a combination of conventional types in the art, including organic solvents and lithium salts (selected according to the type of battery). The solvent can be selected from one or a mixture of multiple types of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), etc., in any proportion. The lithium salt is selected from one or a combination of multiple lithium salts in any proportion, including LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate). The lithium salt content in the electrolyte is 5% to 20% by mass. Additives may also be added to the electrolyte, including film-forming additives and functional additives that improve battery performance, such as vinylene carbonate (VC), 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), etc., which can be selected by those skilled in the art according to actual needs. The usual dosage of additives in electrolytes is 1% to 4% of the electrolyte mass, for example, 2%.

[0050] The secondary battery can be assembled using methods commonly used in the art, comprising the aforementioned positive electrode, negative electrode, electrolyte, and separator. Specifically, the positive electrode, separator, and negative electrode are sequentially wound or stacked to form a bare cell, which is then encapsulated in, for example, an aluminum-plastic film. Electrolyte is injected, and after formation, encapsulation, and testing, a secondary battery is obtained. The secondary batteries prepared in this application are all lithium-ion secondary batteries.

[0051] The following describes the preparation method of secondary batteries:

[0052] (1) Preparation of positive electrode sheet

[0053] First, lithium manganese iron phosphate and lithium nickel cobalt manganese oxide are premixed in a mixer until the lithium manganese iron phosphate coats the surface of the lithium nickel cobalt manganese oxide to obtain a positive electrode material. Then, the above-mentioned positive electrode conductive agent, positive electrode binder and positive electrode dispersant are added to the mixer in a certain mass ratio and stirred evenly. Then, N-methylpyrrolidone (NMP) solvent is added at a solid content of 45% to 70% and stirred evenly to obtain a positive electrode slurry. The contents of each component in the positive electrode slurry are as follows: the total mass ratio of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide is 95% to 97.5%, and the amount of lithium manganese iron phosphate used M1 and the amount of lithium nickel cobalt manganese oxide used M2 satisfy formula (1); the mass ratio of positive electrode conductive agent is 1% to 3%; the mass ratio of positive electrode binder is 1.5% to 4%; and the mass ratio of positive electrode dispersant is 0.01% to 1%.

[0054] Then, the positive electrode slurry is prepared at a ratio of 0.3g to 0.4g / 1540.25mm. 2 The areal density is uniformly coated onto the positive electrode current collector, and after drying, rolling, and slitting, a positive electrode sheet is obtained. After rolling and before slitting, the single-sided thickness of the positive electrode sheet is 79 μm to 124 μm, and the compaction density is 2.1 g / cm³. 3 ~2.45g / cm 3 between.

[0055] (2) Preparation of negative electrode sheet

[0056] The aforementioned negative electrode material, negative electrode binder, negative electrode conductive agent, and negative electrode dispersant are added to a mixer in a certain proportion and stirred thoroughly. Then, N-methylpyrrolidone or deionized water is added to the mixture at a solid content of 45%–70% and stirred until homogeneous to obtain a negative electrode slurry. The proportions of each component in the negative electrode slurry are as follows: negative electrode material: 93%–98% by mass; negative electrode conductive agent: 0.2%–1.5% by mass; negative electrode binder: 1%–3% by mass; negative electrode dispersant: 0.01%–1.5% by mass.

[0057] Then, the negative electrode slurry is prepared at a ratio of 0.114–0.167 g / 1540.25 mm. 2 The areal density is uniformly coated onto the negative electrode current collector, and after drying, rolling, and slitting, the negative electrode sheet is obtained. After rolling and before slitting, the single-sided thickness of the negative electrode sheet is 52μm–75μm, and the compaction density is 1.45g / cm³. 3 ~1.8g / cm 3 between.

[0058] (3) Preparation of electrolyte

[0059] In an argon atmosphere glove box with a water content of <10ppm, fully dried lithium salt (LiPF6) was dissolved in an organic solvent and mixed thoroughly to obtain an electrolyte, wherein the concentration of LiPF6 was 1mol / L.

[0060] (4) Preparation of the diaphragm

[0061] Use a 12μm thick porous polymer film of polypropylene (PP) or polyethylene (PE).

[0062] (5) Battery fabrication:

[0063] The positive electrode, separator, and negative electrode obtained above are sequentially stacked or wound to obtain an electrode assembly, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is placed in a packaging shell and thoroughly baked until the water content is below 450 ppm. Electrolyte is then injected, and the battery undergoes formation, sealing, and inspection processes to obtain a secondary battery.

[0064] The specific steps and conditions for 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 formation of the electrolyte.

[0065] Those skilled in the art will understand that the methods for preparing secondary batteries described above are merely embodiments. Other methods commonly used in the art can be employed without departing from the disclosure of this application.

[0066] The present invention also provides an electronic device comprising the secondary battery described above. The secondary battery may be used in the electronic device in the form of a single cell, a battery module, or a battery pack.

[0067] The electronic devices of this invention include, but are not limited to, mobile phones, tablets, laptops, electric toys, electric vehicles, new energy vehicles, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc.

[0068] 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 all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the embodiments are all commercially available.

[0069] Example 1

[0070] This embodiment provides a cathode material, which includes a first active material and a second active material. The first active material includes lithium manganese iron phosphate, with the chemical formula LiMn. 0.6 Fe 0.4 PO4, the second active material includes lithium nickel cobalt manganese oxide, with the chemical formula LiNi 0.83 Co 0.1 Mn 0.07 O2, LiMn 0.6 Fe 0.4 PO4 coated on LiNi 0.83 Co 0.1 Mn 0.07 Outside of O2, the median particle size of lithium manganese iron phosphate is D1 = 0.5 μm, and its density is ρ1 = 3.55 g / cm³. 3 The median particle size of lithium nickel cobalt manganese oxide is D2 = 8 μm, and its density is ρ2 = 4.4 g / cm³. 3 , a=1; Substituting the above parameters into formula (1), we get 7 / 3≥M1 / M2≥0.91; In this embodiment, LiMn in the cathode material 0.6 Fe 0.4 PO4 and LiNi 0.83 Co 0.1 Mn 0.07 The mass ratio of O2 is M1 / M2 = 1.

[0071] This embodiment also provides a secondary battery containing the above-mentioned positive electrode material, and the preparation process of the secondary battery is as follows:

[0072] (1) Preparation of positive electrode sheet

[0073] First, the above-mentioned first active substance (LiMn) 0.6 Fe 0.4 PO4) and the second active substance (LiNi) 0.83 Co 0.1 Mn 0.07 O2) is mixed and stirred evenly in a mixer at a mass ratio of 1:1 to obtain the positive electrode material; then, positive electrode conductive agent acetylene black, positive electrode binder PVDF and positive electrode dispersant (polyvinylpyrrolidone) are added and stirred evenly, wherein the mass ratio of positive electrode material, positive electrode conductive agent, positive electrode conductive agent and positive electrode dispersant is 95:2:2:1; then, solvent NMP is added according to the total mass of positive electrode material, positive electrode conductive agent, positive electrode binder and positive electrode dispersant being 50% of the total mass of positive electrode slurry, and the mixture is stirred evenly to obtain the positive electrode slurry.

[0074] The positive electrode slurry was prepared at a ratio of 0.35g / 1540.25mm. 2The areal density is uniformly coated onto the positive electrode current collector, and after drying, rolling, and slitting, a positive electrode sheet is obtained. Specifically, after rolling and before slitting, the single-sided thickness of the positive electrode sheet is 82 μm, and the compaction density is 2.3 g / cm³. 3 .

[0075] (2) Preparation of negative electrode sheet

[0076] The negative electrode material, negative electrode conductive agent, negative electrode binder, and negative electrode dispersant are thoroughly mixed in a mixer, and then mixed evenly with deionized water to prepare the negative electrode slurry. The mass ratio of the negative electrode material (artificial graphite), negative electrode conductive agent (acetylene black), negative electrode binder (SBR), and negative electrode dispersant (PVP) in the negative electrode slurry is 96:1:1.5:1.5. Deionized water is added at 50% of the total mass of the negative electrode material, negative electrode conductive agent, negative electrode binder, and negative electrode dispersant.

[0077] Then, the negative electrode slurry was prepared at a ratio of 0.135g / 1540.25mm. 2 The areal density is uniformly coated onto the negative electrode current collector, and after drying, rolling, and slitting, the negative electrode sheet is obtained. Specifically, after rolling and before slitting, the single-sided thickness of the negative electrode sheet is 56 μm, and the compaction is 1.6 g / cm³. 3 .

[0078] (3) Preparation of electrolyte

[0079] In an argon atmosphere glove box with a water content of <10ppm, EC, EMC and DEC are 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.

[0080] (4) Preparation of the diaphragm

[0081] Polyethylene (PE) porous polymer film is used as the separator.

[0082] (5) Preparation of secondary batteries

[0083] The positive electrode, separator, and negative electrode obtained above are stacked or wound in sequence to obtain an electrode assembly. The electrode assembly is placed in a packaging shell and fully baked to ensure that the water content is below 450 ppm. Electrolyte is injected for formation, and the battery is then sealed and inspected to produce a secondary battery.

[0084] Example 2

[0085] The preparation method in this embodiment is the same as that in Example 1, except that: LiMn 0.6 Fe 0.4 PO4 and LiNi0.83 Co 0.1 Mn 0.07 The mass ratio of O2 is M1 / M2 = 1.5.

[0086] Example 3

[0087] The preparation method in this embodiment is the same as that in Example 1, except that: LiMn 0.6 Fe 0.4 PO4 and LiNi 0.83 Co 0.1 Mn 0.07 The mass ratio of O2 is M1 / M2 = 7 / 3.

[0088] Comparative Example 1

[0089] The preparation method of this comparative example is the same as that of Example 1, except that: LiMn 0.6 Fe 0.4 PO4 and LiNi 0.83 Co 0.1 Mn 0.07 The mass ratio of O2 is M1 / M2 = 8 / 2.

[0090] Comparative Example 2

[0091] The preparation method of this comparative example is the same as that of Example 1, except that: LiMn 0.6 Fe 0.4 PO4 and LiNi 0.83 Co 0.1 Mn 0.07 The mass ratio of O2 is M1 / M2 = 4 / 6.

[0092] Comparative Example 3

[0093] The preparation method of this comparative example is the same as that of Example 1, except that the cathode material does not contain LiMn. 0.6 Fe 0.4 PO4.

[0094] Example 4

[0095] The preparation method in this embodiment is the same as that in Example 1, except that the chemical formula of the second active substance is LiNi. 0.86 Co 0.05 Mn 0.09 O2; D1=0.85μm, D2=6μm, M1 / M2=55 / 45.

[0096] Example 5

[0097] The preparation method of this embodiment is the same as that of Example 1, and the difference from Example 4 is that: D1 = 0.9 μm, D2 = 3 μm, M1 / M2 = 1.4.

[0098] Example 6

[0099] The preparation method of this embodiment is the same as that of Example 1, and the difference from Example 4 is that D1 = 0.7 μm and D2 = 5 μm.

[0100] Example 7

[0101] The preparation method in this embodiment is the same as that in Example 1, except that the chemical formula of the first active substance is LiMn. 0.7 Fe 0.3 PO4, the second active substance has the chemical formula LiNi 0.8 Co 0.12 Mn 0.07 Zr 0.01 O2.

[0102] Example 8

[0103] The difference between this embodiment and Embodiment 1 is that the chemical formula of the first active substance is LiMn. 0.8 Fe 0.2 PO4, the second active substance has the chemical formula LiNi 0.9 Co 0.05 Mn 0.03 Zn 0.02 O 1.98 F 0.02 .

[0104] The secondary batteries prepared in Examples 1 to 8 and Comparative Examples 1 to 3 were subjected to electrical performance tests. The test methods are shown below, and the test results are shown in Table 1.

[0105] (1) Electrode CP mapping test

[0106] The positive electrode sheets prepared in each embodiment and comparative example were used to prepare samples using argon ion polishing technology (CP cross section), with an area A = 1 cm². 2 EDS mapping analysis was performed on the region to obtain the proportions of Ni and Fe elements in the mapping. Ni / S Fe .

[0107] (2) Gram capacity test

[0108] Five secondary batteries from the comparative example and the embodiment were taken and charged at a constant current rate of 0.33C at room temperature until the voltage reached 4.3V. They were then further charged at a constant voltage of 4.3V until the current dropped below 0.05C, bringing them to a fully charged state of 4.3V. Subsequently, they were discharged at a constant current rate of 0.33C until the voltage reached 2.8V.

[0109] (3) Cyclic performance test of secondary batteries

[0110] Five secondary batteries were taken from each of the comparative and example batteries, and the secondary batteries were repeatedly charged and discharged through the following steps, and the cycle capacity retention rate of the secondary batteries was calculated.

[0111] In an environment of 25°C, the capacitor was charged to 4.35V at a constant current of 0.5C (925mA), then charged to 0.05C (92.5mA) at a constant voltage of 4.35V, and then discharged to 3.0V at a constant current of 0.5C (925mA). The discharge capacity of the first cycle was recorded. Then, 800 charge and discharge cycles were performed, and the discharge capacity of the 800th cycle was recorded.

[0112] Cycle capacity retention = (Discharge capacity of the 800th cycle / Discharge capacity of the first cycle) × 100%.

[0113] Table 1: Experimental results of Examples 1 to 8 and Comparative Examples 1 to 3

[0114]

[0115]

[0116] Comparing Examples 1 to 3 with Comparative Examples 1 to 3, it can be seen that as the mass ratio M1 / M2 of the first active material and the second active material in the cathode material increases (i.e., the proportion of lithium nickel cobalt manganese oxide decreases and the proportion of lithium manganese iron phosphate increases), its specific capacity gradually decreases, and the cycle performance first increases and then decreases. This indicates that excessive coating of lithium manganese iron phosphate will lead to local degradation and weakened cycle performance; insufficient coating of lithium manganese iron phosphate will increase the specific capacity due to the increased proportion of lithium nickel cobalt manganese oxide, but the side reaction between lithium nickel cobalt manganese oxide and the electrolyte will lead to poor cycle performance.

[0117] Comparing Examples 1 and 4 to 6, it can be seen that as the median particle size of the second active material in the cathode material decreases (i.e., lithium nickel cobalt manganese oxide is used in the form of single crystal, polycrystalline or a mixture of single crystal and polycrystalline), M1 / M2 gradually increases, and its specific capacity gradually decreases (due to the low specific capacity of the first active material itself). However, since the selected active material meets the requirements of the formula, all examples demonstrate excellent cycle performance.

[0118] Comparing Examples 1, 7, and 8, lithium manganese iron phosphate with different Mn contents and lithium nickel cobalt manganese oxide with high nickel content are blended at a better mass ratio, and their specific capacity and cycle performance are maintained at a better level. In addition, doping lithium nickel cobalt manganese oxide with a small amount of metal elements and elements with high electronegativity can improve cycle performance.

[0119] The cathode material of this invention employs small-particle lithium manganese iron phosphate and large-particle lithium nickel cobalt manganese oxide. The small-particle lithium manganese iron phosphate coats the surface of the large-particle lithium nickel cobalt manganese oxide, isolating the lithium nickel cobalt manganese oxide from the electrolyte and avoiding direct contact. This effectively reduces side reactions and improves the overall cycle life. Furthermore, by rationally designing the ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide, the problem of excessive coating material leading to localized degradation can be avoided, thus balancing cycle life and specific capacity. Therefore, this invention effectively overcomes some practical problems in the prior art and has high utilization value and practical significance.

[0120] 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, include: The first active material includes lithium manganese iron phosphate; The second active material includes lithium nickel cobalt manganese oxide; Wherein, the lithium manganese iron phosphate is coated on the outside of the lithium nickel cobalt manganese oxide, the mass of the lithium manganese iron phosphate is M1, and the mass of the lithium nickel cobalt manganese oxide is M2, and M1 and M2 satisfy formula (1): ……………（1）； In formula (1), the value of a ranges from 0.9 to 1.1; ρ1 is the density of lithium manganese iron phosphate, ρ2 is the density of lithium nickel cobalt manganese oxide, D1 is the median particle size of lithium manganese iron phosphate, and D2 is the median particle size of lithium nickel cobalt manganese oxide. Wherein, the value of D1 ranges from 0.5μm to 0.9μm, and the value of D2 ranges from 3μm to 8μm.

2. The cathode material according to claim 1, characterized in that, The chemical formula of the lithium manganese iron phosphate is LiMn. x Fe (1-x) PO4, wherein 0.6 ≤ x ≤ 0.8; and / or, the lithium nickel cobalt manganese oxide has the chemical formula Li. 1+ a Ni x Co y Mn z Me k O 2-b A b Wherein, -0.2≤a≤0.2, 0.8≤x≤0.9, 0<y+z≤0.2, 0≤k≤0.1, x+y+z+k=1, 0≤b≤0.1; Me is selected from one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W, Nb, and A is selected from one or more of N, F, S, Cl.

3. The positive electrode material according to claim 1, characterized in that, The median particle size d of the primary lithium manganese iron phosphate particles ranges from 0.1 μm to 0.4 μm.

4. The cathode material according to claim 1, characterized in that, The positive electrode material is used to prepare a positive electrode sheet. The positive electrode sheet is then polished with an argon ion cross-section. An elemental energy dispersive spectroscopy (EDS) analysis is performed on a region with an area of ​​A to determine the distribution ratio of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide. …………（2） In equation (2), the value of b ranges from 2 to 4.23; S Ni The percentage of Ni element in area A; S Fe ρ1 represents the percentage of Fe element in area A; ρ2 represents the density of lithium manganese iron phosphate; D1 represents the median particle size of lithium manganese iron phosphate; and D2 represents the median particle size of lithium nickel cobalt manganese oxide.

5. A secondary battery, characterized in that, include: A positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the positive electrode material according to any one of claims 1 to 4.

6. The secondary battery according to claim 5, characterized in that, The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one side surface of the negative current collector. The negative active material layer includes a negative electrode material, which includes one or more of graphite material, silicon-carbon negative electrode material, and silicon-oxygen negative electrode material.

7. The secondary battery according to claim 6, characterized in that, The capacity of the negative electrode material is N, the capacity of the positive electrode material is P, and the value of N / P ranges from 1.02 to 1.

12.

8. An electronic device, characterized in that, Includes the secondary battery as described in any one of claims 5 to 7.

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