Positive electrode material, secondary battery, and electronic device

Abstract

The invention provides a positive electrode material, a secondary battery and electronic equipment, and particularly relates to the technical field of battery materials. The positive electrode material comprises a first active substance and a second active substance, the first active substance comprises lithium manganese iron phosphate, the second active substance comprises nickel cobalt lithium manganate, the lithium manganese iron phosphate is coated on the outer side of the nickel cobalt lithium manganate, the mass of the lithium manganese iron phosphate is M1, the mass of the nickel cobalt lithium manganate is M2, and the mass of the lithium manganese iron phosphate is M1. M1 and M2 meet the formula (1), the coating amount of the lithium manganese iron phosphate on the lithium nickel cobalt manganese oxide is optimized by reasonably designing the ratio of the lithium manganese iron phosphate to the lithium nickel cobalt manganese oxide, so that the secondary battery containing the positive electrode material can give consideration to the cycle performance and gram volume.

Description

Technical Field

[0001] The present invention relates to the technical field of battery manufacturing, and in particular to a positive electrode material, a secondary battery and an electronic device. Background Art

[0002] The rapid development of new energy vehicles has driven the further expansion of the power battery market. As an important component of power batteries, the demand for cathode materials is also increasing. Nickel cobalt manganese oxide cathode materials combine LiCoO 2 Good cycle performance, LiNiO 2 High specific capacity, LiMn 2 O 4 With its advantages of high safety and low cost, it has become one of the most promising cathode materials for lithium-ion batteries. However, with the demand for high energy density, ternary cathode materials are trending towards high nickel content. The increase in nickel content leads to a decrease in the thermal stability of the material, which in turn affects the safety of the battery.

[0003] Lithium iron phosphate cathode material has good stability and low cost, but it also has some problems: for example, the working voltage is low (3.4V), the battery energy density is low, and it is difficult to meet the different people's requirements for the range of electric vehicles. Lithium iron manganese phosphate cathode material has the same advantages as lithium iron phosphate, safety and low cost. Compared with lithium iron phosphate material, lithium iron manganese phosphate material has a higher voltage platform (4.1V), better low temperature performance, and energy density close to the five series ternary materials (for example, LiNi 0.5 Co 0.2 Mn 0.3 O 2 Positive electrode material), however, the gram capacity of lithium manganese iron phosphate material is lower.

[0004] Therefore, it is necessary to provide a positive electrode material, a secondary battery and an electronic device to improve the problems existing in the positive electrode material. Summary of the invention

[0005] In view of the above shortcomings of the prior art, the present invention provides a positive electrode material, a secondary battery and an electronic device to improve the problem that the cycle performance and gram capacity of the positive electrode material cannot be taken into account at the same time.

[0006] To achieve the above-mentioned object and other related objects, the present invention provides a positive electrode material, the positive electrode material comprising: a first active material and a second active material, the first active material comprising lithium manganese iron phosphate, the second active material comprising lithium nickel cobalt manganese oxide, wherein the lithium manganese iron phosphate is coated on the outer surface of the lithium nickel cobalt manganese oxide, and the mass of the lithium manganese iron phosphate is M 1 , the mass of the lithium nickel cobalt manganese oxide is M2 , M 1 and M 2 satisfy formula (1):

[0007]

[0008] In formula (1), the value range of a is from 0.9 to 1.1; ρ 1 is the density of lithium iron manganese phosphate, ρ 2 is the density of lithium nickel cobalt manganese oxide, D 1 is the median particle size of lithium iron manganese phosphate, D 2 is the median particle size of lithium nickel cobalt manganese oxide.

[0009] In an example of the present invention, the chemical formula of the lithium iron manganese phosphate is LiMn x Fe (1-x) PO 4 , where 0.6 ≤ x ≤ 0.8; and / or, the chemical formula of the lithium nickel cobalt manganese oxide is Li 1+a Ni x Co y Mn z Me k O 2-b A b , where -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; A is selected from one or more of N, F, S, Cl.

[0010] In an example of the present invention, D 1 <D 2 .

[0011] In an example of the present invention, the value range of the D 1 is from 0.5 μm to 0.9 μm, and the value range of the D 2 is from 3 μm to 8 μm.

[0012] In an example of the present invention, the value range of the median particle size d of the primary particles of the lithium iron manganese phosphate is from 0.1 μm to 0.4 μm.

[0013] In an example of the present invention, the positive electrode material is used to prepare a positive electrode plate, the positive electrode plate is polished by argon ion cross-section, and an area of A is taken for elemental energy spectrum analysis to obtain the distribution ratio of the lithium iron manganese phosphate and the lithium nickel cobalt manganese oxide:

[0014]

[0015] In formula (2), the value range of b is from 2 to 4.23; S Ni is the proportion of Ni element in area A; S Fe is the proportion of Fe element in area A; ρ 1 is the density of lithium iron manganese phosphate, ρ 2 is the density of lithium nickel cobalt manganese oxide, D 1 is the median particle size of lithium iron manganese phosphate, D 2 is the median particle size of lithium nickel cobalt manganese oxide.

[0016] On the other hand, the present invention provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. Among them, the positive electrode sheet includes the positive electrode material described above in the present invention.

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

[0018] In an example of the present invention, the capacity of the negative electrode material is N, and the capacity of the positive electrode material is P. The value range of N / P is from 1.02 to 1.12.

[0019] The present invention also provides an electronic device, which includes the secondary battery described above in the present invention.

[0020] The positive electrode material of the present invention uses small particles of lithium iron manganese phosphate and large particles of lithium nickel cobalt manganese oxide, and coats the small particles of lithium iron manganese phosphate on the outside of the large particles of lithium nickel cobalt manganese oxide to isolate the lithium nickel cobalt manganese oxide from the electrolyte and avoid direct contact, which can effectively reduce the occurrence of side reactions and thus improve the overall cycle life; on the other hand, by reasonably designing the ratio of lithium iron manganese phosphate and lithium nickel cobalt manganese oxide and optimizing the coating amount of lithium iron manganese phosphate on the outside of lithium nickel cobalt manganese oxide, the situation of local attenuation deterioration caused by excessive coating material can be avoided, thus taking into account the cycle performance and the utilization of specific capacity. Description of the Drawings

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 It is a cyclic test curve graph of Embodiment 1, Embodiment 3, Embodiment 4 and Comparative Examples 1 to 3 of the present invention. Detailed Implementation Modes

[0023] The following specific examples illustrate the implementation modes of the present invention. Those skilled in the art can easily understand the 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 implementation modes. 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, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0024] The following specific examples illustrate the implementation modes of the present invention. Those skilled in the art can easily understand the 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 implementation modes. 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, without conflict, the following embodiments and the features in the embodiments 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 those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0026] Unless otherwise stated or there is a contradiction, the terms or phrases used herein have the following meanings:

[0027] As used herein, "a plurality of", "a variety of", "multiple times", etc., unless otherwise specified, mean greater than 2 or equal to 2 in quantity. For example, "one or more" means one or greater than or equal to two.

[0028] As used herein, "preferred", "better", "more preferable" are only used to describe implementation modes or embodiments with better effects, and it should be understood that they do not constitute a limitation on the protection scope of the present invention. If "preferred" appears multiple times in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction relationship, each "preferred" is independent of each other.

[0029] As used herein, "further", "even further", "especially", etc. are used for descriptive purposes and indicate differences in content, but should not be construed as a limitation on the protection scope of the present invention.

[0030] In this text, when it comes to numerical ranges, unless otherwise specified, the distribution of optional numerical values within the numerical range is considered continuous, including the two numerical endpoints of the numerical range (i.e., the minimum value and the maximum value), as well as each numerical value between these two numerical endpoints. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined.

[0031] To meet the increasing demand for energy density in the field of new energy vehicles, lithium iron manganese phosphate (LFMP) materials, which have the same advantages as lithium iron phosphate (LFP) such as safety and low cost, have attracted much attention. The voltage platform of lithium iron manganese phosphate materials is about 4.1V, the energy density is close to that of five-series ternary materials, and the low-temperature performance is better than that of lithium iron phosphate materials. However, the specific capacity of lithium iron manganese phosphate materials is not high. Lithium nickel cobalt manganese oxide (NCM) has a high energy density, especially high-nickel materials. However, due to the easy occurrence of side reactions between its surface and the electrolyte, its cycle life has no advantage compared with lithium iron phosphate. Combining the high specific energy of lithium nickel cobalt manganese oxide with the high safety of lithium iron manganese phosphate can effectively solve the problems existing in the cathode material. Based on this, the present invention provides a cathode material, a secondary battery including the cathode material, and an electronic device including 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 iron manganese phosphate, and the second active material includes lithium nickel cobalt manganese oxide. Among them, lithium iron manganese phosphate is coated on the outside of lithium nickel cobalt manganese oxide. The lithium iron manganese phosphate coated on the outside of lithium nickel cobalt manganese oxide can separate lithium nickel cobalt manganese oxide from the electrolyte, avoid direct contact, and effectively reduce the occurrence of side reactions, thereby improving the overall cycle life.

[0033] The inventors found in the research that the ratio of lithium iron manganese phosphate to lithium nickel cobalt manganese oxide will affect the overall coating effect: if the coating amount of lithium iron manganese phosphate is too much, that is, the ratio of lithium nickel cobalt manganese oxide in the cathode material is too small, it will lead to the deterioration of local attenuation; if the coating amount of lithium iron manganese phosphate is insufficient, that is, the ratio of lithium nickel cobalt manganese oxide in the cathode material is too high, the improvement of the cycle of the cathode material is not obvious, but the specific capacity of the material is high. Therefore, the inventors optimized the ratio of lithium iron manganese phosphate to lithium nickel cobalt manganese oxide through the parameter calculation of raw materials: the mass of lithium iron manganese phosphate is M 1 , the mass of lithium nickel cobalt manganese oxide is M 2 , M 1 and M 2 satisfy formula (1):

[0034]

[0035] In formula (1), a is an empirical constant, and the value range of a is from 0.9 to 1.1. Further, a = 1; ρ 1 is the density of lithium iron manganese phosphate, ρ 2 is the density of lithium nickel cobalt manganese oxide, D1 is the median particle size of lithium iron manganese phosphate, D 2 is the median particle size of lithium nickel cobalt manganese oxide. According to formula (1), the ratio of lithium iron manganese phosphate to lithium nickel cobalt manganese oxide can be adjusted to avoid excessive or insufficient coating amount of lithium iron manganese phosphate.

[0036] In some embodiments, the median particle size D of lithium iron manganese phosphate 1 is less than the median particle size D of lithium nickel cobalt manganese oxide 2 . Among them, the median particle size refers to the particle size corresponding to when the cumulative volume particle size distribution percentage of a sample (the first active material or the second active material) reaches 50%. In some embodiments, the value range of D 1 is from 0.5 μm to 0.9 μm. For example, D 1 can take values such as 0.5 μm, 0.7 μm, or 0.9 μm, etc.; the value range of D 2 is from 3 μm to 8 μm. For example, D 2 can take values such as 3 μm, 5 μm, 7 μm, or 8 μm, etc. Lithium iron manganese phosphate satisfying the above particle size can form a good coating effect on the surface of lithium nickel cobalt manganese oxide.

[0037] In this application, the chemical formula of lithium iron manganese phosphate is LiMn x Fe (1-x) PO 4 , where x represents the molar content of Mn element in lithium iron manganese phosphate, and the value range of x is 0.6 ≤ x ≤ 0.8. In some embodiments, the value of x can be 0.6, 0.7, or 0.8, etc. Correspondingly, the chemical formula of lithium iron manganese phosphate is LiMn 0.6 Fe 0.4 PO 4 , LiMn 0.7 Fe 0.3 PO 4 , LiMn 0.8 Fe 0.2 PO 4 etc. In some embodiments, lithium iron manganese phosphate includes primary particles and secondary particles. Among them, the secondary particles are formed by the aggregation of primary particles. Preferably, the value range of the median particle size d of the primary particles is from 0.1 μm to 0.4 μm. The material with a small particle size can reduce polarization and is more conducive to the performance of the material.

[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 b, where, -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 cycling performance of the cathode material. In the above formula, both the Me element and the A element are doping elements. Me can be selected from one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W, Nb, and A can be selected from one or more of N, F, S, Cl. Doping an appropriate amount of Me element and A element with strong electronegativity in the cathode material can significantly improve the structural stability of the cathode material, thereby improving the cycling performance. When k = 0 and b = 0, it means that no other elements are doped in the lithium nickel cobalt manganese oxide, and at this time, its chemical formula is Li 1+a Ni x Co y Mn z O 2 .

[0039] Prepare the cathode electrode sheet with the cathode material described above in the present invention, and perform argon ion cross-section polishing (CP) on the cathode electrode sheet. Take a region with an area of A for elemental energy spectrum mapping analysis to obtain the distribution of lithium iron phosphate manganese and lithium nickel cobalt manganese oxide in the cathode material:

[0040]

[0041] In formula (2), b is an empirical coefficient, and its value range is 2 to 4.23. For example, b = 3; S Ni is the distribution area of Ni element in the area of A; S Fe is the distribution area of Fe element in the area of A; ρ 1 is the density of lithium iron phosphate manganese, ρ 2 is the density of lithium nickel cobalt manganese oxide, D 1 is the median particle size of lithium iron phosphate manganese, D 2 is the median particle size of lithium nickel cobalt manganese oxide. It should be noted that the above-mentioned region A ≥ 1 cm 2 , for the convenience of sample preparation, the area of region A in this application is taken as 1 cm 2 .

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

[0043] In a second aspect of the present invention, a secondary battery is provided. The secondary battery includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. The separator is disposed between the positive electrode plate and the negative electrode plate and serves to isolate them; the electrolyte serves to conduct lithium ions between the positive electrode plate and the negative electrode plate. During the charge and discharge process of the battery, lithium ions are inserted into and extracted from between the positive electrode plate and the negative electrode plate back and forth.

[0044] Specifically, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of a material with good electrical conductivity and mechanical strength, such as aluminum foil. The positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active material layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector. The positive electrode active material layer includes a positive electrode material, a positive electrode conductive agent, a positive electrode binder, and a positive electrode dispersant. Among them, the positive electrode material is the positive electrode material described above in the present invention. The positive electrode conductive agent, the positive electrode binder, and the positive electrode dispersant can be selected from the conventional material types in the art and are not specifically limited herein. As an example, the positive electrode conductive agent is selected from one of conductive carbon black (SP), acetylene black, nano metal powder, graphene, carbon nanotubes, carbon nanofibers, or a combination of two or more thereof in any proportion. The positive electrode binder is selected from one or a mixture of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, sodium carboxymethyl cellulose, 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 plate includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of a material with good electrical conductivity and mechanical strength, such as copper foil. The negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector. The negative electrode 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, the negative electrode conductive agent, the negative electrode binder, and the negative electrode dispersant can be selected from the conventional material types in the art and are not specifically limited herein.

[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. Graphite can be natural graphite, artificial graphite, soft carbon, hard carbon, etc. The negative electrode conductive agent is selected from one of conductive carbon black, nano silver powder, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. or a combination of two or more in any proportion. The negative electrode binder is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), etc. or several combinations 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 a lithium battery, it is very important to correctly calculate the reasonable ratio coefficient of the positive and negative electrode capacities. If the positive electrode is in excess, during charging, the excess lithium ions coming out of the positive electrode cannot enter the negative electrode, and lithium deposition will occur on the surface of the negative electrode, resulting in the formation of dendrites, which will deteriorate the battery cycle performance and also cause internal short circuit of the battery, triggering battery safety problems. Therefore, usually the negative electrode is slightly more than the positive electrode, but it cannot be excessive by too much. Excessive amount will consume the lithium in the positive electrode; in addition, it will also cause waste of the negative electrode, reduce the battery energy density, and increase the 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 value range of N / P is from 1.02 to 1.12. For example, N / P = 1.02 or N / P = 1.08 or N / P = 1.12, etc.

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

[0049] The electrolyte can be selected from conventional combination types in the art, which includes organic solvents and lithium salts (selected according to the battery type). Among them, the solvent can be selected from one 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. or a variety of them mixed in any proportion. The lithium salt is selected from LiPF 6 (lithium hexafluorophosphate), LiBF 4 (lithium tetrafluoroborate) LiClO 4 (lithium perchlorate), LiAsF 6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonimide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium bis(oxalato)borate), LiPO 2 F 2(Lithium difluorophosphate), LiDFOP (lithium difluoro(oxalato)phosphate), LiTFOP (lithium tetrafluoro(oxalato)phosphate), or a combination of multiple ones in any proportion. The mass content of the lithium salt in the electrolyte is 5% - 20%. Additives can also be added to the electrolyte. The additives include film-forming additives and functional additives that can improve battery performance. For example, they can be vinylene carbonate (VC), 1,3-propane sultone (PS), ethylene sulfate (DTD), etc. Those skilled in the art can select according to actual needs. The conventional dosage of the additives in the electrolyte is 1% - 4% of the mass of the electrolyte, such as 2%, etc.

[0050] The secondary battery can assemble the above-mentioned positive electrode sheet, negative electrode sheet, electrolyte, and separator together by using the methods commonly used in the art. Among them, the positive electrode sheet, separator, negative electrode sheet, etc. are wound or stacked in sequence to form a bare battery cell, and then it is installed in, for example, an aluminum-plastic film for encapsulation, the 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 preparation method of the secondary battery is described below:

[0052] (1) Preparation of the positive electrode sheet

[0053] First, lithium iron manganese phosphate and lithium nickel cobalt manganese oxide are premixed in a blender until lithium iron manganese phosphate is coated on the surface of 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 blender according to a certain mass ratio and stirred evenly, and then solvent N-methylpyrrolidone (NMP) is added according to a solid content of 45% - 70% and stirred and mixed evenly to obtain a positive electrode paste; among them, the contents of each component in the positive electrode paste are: the total mass ratio of lithium iron manganese phosphate and lithium nickel cobalt manganese oxide is 95% - 97.5%, and the usage amount M of lithium iron manganese phosphate 1 and the usage amount M of lithium nickel cobalt manganese oxide 2 satisfy formula (1); the mass ratio of the positive electrode conductive agent is 1% - 3%; the mass ratio of the positive electrode binder is 1.5% - 4%; the mass ratio of the positive electrode dispersant is 0.01 - 1%.

[0054] Then, the positive electrode paste is uniformly coated on the positive electrode current collector at a surface density of 0.3g - 0.4g / 1540.25mm 2 , and after drying, rolling, and slitting, a positive electrode sheet is obtained. Among them, after rolling and before slitting, the single-sided thickness of the positive electrode sheet is 79μm - 124μm, and the compaction density is between 2.1g / cm 3 and 2.45g / cm 3 .

[0055] (2) Preparation of the negative electrode sheet

[0056] The above-mentioned anode material, anode binder, anode conductive agent, and anode dispersant are added to a blender in a certain proportion and stirred evenly. Then, a solvent of N-methylpyrrolidone or deionized water is added according to a solid content of 45% to 70% and stirred and mixed evenly to obtain an anode slurry. Among them, the proportions of the components in the anode slurry are as follows: the mass ratio of the anode material is 93% to 98%; the mass ratio of the anode conductive agent is 0.2% to 1.5%, the mass ratio of the anode binder is 1% to 3%; the mass ratio of the anode dispersant is 0.01 to 1.5%.

[0057] Then, the anode slurry is uniformly coated on the anode current collector at a surface density of 0.114 to 0.167 g / 1540.25 mm 2 . After drying, rolling, and slitting, an anode plate is obtained. Among them, after rolling and before slitting, the single-sided thickness of the anode plate is 52 μm to 75 μm, and the compression density is between 1.45 g / cm 3 and 1.8 g / cm 3 .

[0058] (3) Preparation of electrolyte

[0059] In an argon atmosphere glove box with a water content of <10 ppm, a fully dried lithium salt (LiPF 6 ) is dissolved in an organic solvent, and after mixing evenly, an electrolyte is obtained. Among them, the concentration of LiPF 6 is 1 mol / L.

[0060] (4) Preparation of separator

[0061] A 12-μm-thick polypropylene (PP) or polyethylene (PE) porous polymer film is selected.

[0062] (5) Preparation of battery:

[0063] The above-prepared positive electrode plate, separator, and negative electrode plate are stacked or wound in sequence to obtain an electrode assembly, and the separator is placed in the middle of the positive and negative electrode plates to play a role in isolation. The electrode assembly is placed in a packaging shell, and after sufficient baking to make the water content below 450 ppm, the electrolyte is injected, and after formation, sealing, inspection and other processes, a secondary battery is manufactured.

[0064] The specific steps and conditions of formation are as follows: After injecting the electrolyte, maintain a hot pressing environment of 0.1 MPa, charge at 45°C at 0.02C for 17 min in a static state, stand for 5 min and then charge to 0.3 Ah at 0.02C. After that, cut off the air bag and vacuum package, and stand at room temperature for 48 h, so that the electrolyte is completed for formation.

[0065] Those skilled in the art will understand that the preparation method of the secondary battery described above is only an example. Other methods commonly used in the art can be adopted without departing from the content disclosed in this application.

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

[0067] The electronic devices of the present invention include, but are not limited to, mobile phones, tablets, laptop computers, electric toys, battery cars, new energy vehicles, ships, spacecraft, etc. Among them, the electric toys can include fixed or mobile electric toys, such as game consoles, electric vehicle toys, electric ship toys, electric aircraft toys, etc., and the spacecraft can include airplanes, rockets, space shuttles, spaceships, etc. The new energy vehicle can be a pure electric vehicle, a hybrid vehicle or an extended-range vehicle, etc.

[0068] The technical solutions of the present invention will be described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples can all be commercially purchased.

[0069] Example 1

[0070] This example provides a positive electrode material, which includes a first active material and a second active material. The first active material includes lithium manganese iron phosphate, and its chemical formula is LiMn 0.6 Fe 0.4 PO 4 , and the second active material includes lithium nickel cobalt manganese oxide, and its chemical formula is LiNi 0.83 Co 0.1 Mn 0.07 O 2 , LiMn 0.6 Fe 0.4 PO 4 is coated on the outside of LiNi 0.83 Co 0.1 Mn 0.07 O 2 . Among them, the median particle size D 1 of lithium manganese iron phosphate is 0.5 μm, and the density ρ 1 is 3.55 g / cm 3 ; the median particle size D 2 of lithium nickel cobalt manganese oxide is 8 μm, and the density ρ 2 is 4.4 g / cm 3 , a = 1; substituting the above parameters into formula (1) gives 7 / 3 ≥ M 1 / M2 ≥0.91; In this embodiment, in the positive electrode material, LiMn 0.6 Fe 0.4 PO 4 and LiNi 0.83 Co 0.1 Mn 0.07 O 2 The mass ratio M 1 / M 2 = 1.

[0071] This embodiment also provides a secondary battery including the above positive electrode material. The preparation process of the secondary battery is as follows:

[0072] (1) Preparation of the positive electrode sheet

[0073] First, mix and stir the above first active material (LiMn 0.6 Fe 0.4 PO 4 ) and the second active material (LiNi 0.83 Co 0.1 Mn 0.07 O 2 ) in a mixer at a mass ratio of 1:1 to obtain a positive electrode material; then add acetylene black as the positive electrode conductive agent, PVDF as the positive electrode binder, and (polyvinylpyrrolidone) as the positive electrode dispersant and stir evenly. Among them, the mass ratio of the positive electrode material, the positive electrode conductive agent, the positive electrode conductive agent, and the positive electrode dispersant is 95:2:2:1; then add the solvent NMP according to the total mass of the positive electrode material, the positive electrode conductive agent, the positive electrode binder, and the positive electrode dispersant accounting for 50% of the total mass of the positive electrode slurry, and mix and stir evenly to obtain the positive electrode slurry.

[0074] Coat the positive electrode slurry evenly on the positive electrode current collector at a surface density of 0.35 g / 1540.25 mm 2 , and after drying, rolling, and slitting, obtain the positive electrode sheet. Among them, 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 / cm 3 .

[0075] (2) Preparation of the negative electrode sheet

[0076] Fully stir the negative electrode material, the negative electrode conductive agent, the negative electrode binder, and the negative electrode dispersant in a mixer, and then stir and mix evenly with deionized water to prepare the negative electrode slurry. Among them, the mass ratio of artificial graphite as the negative electrode material, acetylene black as the negative electrode conductive agent, SBR as the negative electrode binder, and PVP as the negative electrode dispersant in the negative electrode slurry is 96:1:1.5:1.5, and deionized water is added according to the percentage of the total mass of the negative electrode material, the negative electrode conductive agent, the negative electrode binder, and the negative electrode dispersant accounting for 50% of the total mass of the negative electrode slurry.

[0077] Then, the negative electrode paste is uniformly coated on the negative electrode current collector at a surface density of 0.135 g / 1540.25 mm 2 , and after drying, rolling, and slitting, a negative electrode sheet is obtained. Among them, after rolling and before slitting, the single-sided thickness of the negative electrode sheet is 56 μm, and the compaction density is 1.6 g / cm 3 .

[0078] (3) Preparation of the electrolyte

[0079] In an argon atmosphere glove box with a water content of <10 ppm, EC, EMC, and DEC are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, the fully dried lithium salt LiPF 6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / l.

[0080] (4) Preparation of the separator

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

[0082] (5) Preparation of the secondary battery

[0083] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked or wound in sequence to obtain an electrode assembly. The electrode assembly is placed in a packaging shell, and after sufficient baking to make the water content below 450 ppm, the electrolyte is injected, sealed, and inspected to prepare a secondary battery.

[0084] Example 2

[0085] The preparation method of this example is the same as that of Example 1, except that: the mass ratio M 0.6 of LiMn 0.4 Fe 4 PO 0.83 and LiNi 0.1 Co 0.07 Mn 2 O 1 / M 2 = 1.5.

[0086] Example 3

[0087] The preparation method of this example is the same as that of Example 1, except that: the mass ratio M 0.6 of LiMn 0.4 Fe 4 PO 0.83 and LiNi 0.1 Co 0.07 Mn 2 O 1 / M 2 = 7 / 3.

[0088] Comparative Example 1

[0089] The preparation method of this comparative example is the same as that of Example 1, except that: the mass ratio M of LiMn 0.6 Fe 0.4 PO 4 and LiNi 0.83 Co 0.1 Mn 0.07 O 2 is M 1 / M 2 = 8 / 2.

[0090] Comparative Example 2

[0091] The preparation method of this comparative example is the same as that of Example 1, except that: the mass ratio M of LiMn 0.6 Fe 0.4 PO 4 and LiNi 0.83 Co 0.1 Mn 0.07 O 2 is M 1 / M 2 = 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 positive electrode material does not contain LiMn 0.6 Fe 0.4 PO 4 .

[0094] Example 4

[0095] The preparation method of this example is the same as that of Example 1, except that the chemical formula of the second active substance is LiNi 0.86 Co 0.05 Mn 0.09 O 2 ; D 1 = 0.85μm, D 2 = 6μm, M 1 / M 2 = 55 / 45.

[0096] Example 5

[0097] The preparation method of this example is the same as that of Example 1, and the difference from Example 4 is that: D 1 = 0.9μm, D 2 = 3μm, M 1 / M 2 = 1.4.

[0098] Example 6

[0099] The preparation method of this example is the same as that of Example 1, and the difference from Example 4 is that: D 1 = 0.7 μm, D 2 = 5 μm.

[0100] Example 7

[0101] The preparation method of this example is the same as that of Example 1, and the difference is that: the chemical formula of the first active material is LiMn 0.7 Fe 0.3 PO 4 , and the chemical formula of the second active material is LiNi 0.8 Co 0.12 Mn 0.07 Zr 0.01 O 2 .

[0102] Example 8

[0103] The difference between this example and Example 1 is that: the chemical formula of the first active material is LiMn 0.8 Fe 0.2 PO 4 , and the chemical formula of the second active material is 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 method is as follows, and the test results are shown in Table 1.

[0105] (1) CP mapping test of the electrode

[0106] Take the positive electrode sheets prepared in each example and comparative example, and use argon ion polishing technology (CP cross-section) to prepare samples. Take an area A = 1 cm 2 for EDS mapping analysis and testing: obtain the Ni element ratio and Fe element ratio S Ni / S Fe .

[0107] (2) Specific capacity test

[0108] Five secondary batteries from the comparative examples and the examples were each taken, and at room temperature, they were charged at a constant current of 0.33C until the voltage reached 4.3V, and then further charged at a constant voltage of 4.3V until the current was lower than 0.05C, so that they were in a fully charged state of 4.3V. Subsequently, they were discharged at a constant current of 0.33C until the voltage reached 2.8V and then stopped.

[0109] (3) Cycle performance test of secondary battery

[0110] Five secondary batteries prepared in the comparative examples and the examples were each taken, 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, it was charged at a constant current of 0.5C (925 mA) until 4.35V, then charged at a constant voltage of 4.35V until 0.05C (92.5 mA), and then discharged at a constant current of 0.5C (925 mA) until 3.0V, and the discharge capacity of the first cycle was recorded; then 800 charge and discharge cycles were carried out, and the discharge capacity of the 800th cycle was recorded.

[0112] Cycle capacity retention rate = (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 and Comparative Examples 1 to 3, it can be seen that as the mass ratio M 1 / M 2 of the first active material and the second active material in the positive electrode material increases (that is, the proportion of lithium nickel cobalt manganese oxide decreases and the proportion of lithium iron manganese phosphate increases), its specific capacity gradually decreases, and the cycle performance first increases and then decreases; this shows that: too much coating amount of lithium iron manganese phosphate will lead to deterioration of local attenuation and weakening of the cycle; insufficient coating of lithium iron manganese phosphate, due to the increase in the proportion of lithium nickel cobalt manganese oxide, its specific capacity increases, but due to the side reaction between lithium nickel cobalt manganese oxide and the electrolyte, the cycle becomes worse.

[0117] Comparing Example 1 and Examples 4 to 6, it can be seen that as the median particle size of the second active material in the positive electrode material decreases (that is, lithium nickel cobalt manganese oxide is used in the form of single crystal, polycrystalline or a mixture of single crystal and polycrystalline), M 1 / M 2Gradually increasing, its specific capacity gradually decreases (due to the low specific capacity of the first active material itself), but because the selected active material materials meet the formula requirements, the examples all exhibit excellent cycling performance.

[0118] Comparing Comparative Example 1, Example 7 and Example 8, lithium iron manganese phosphate with different Mn contents and lithium nickel cobalt manganese oxide with a high nickel content are blended in a better mass ratio, and both their specific capacity and cycling performance remain at a better level; in addition, doping a small amount of metal elements and elements with higher electronegativity in lithium nickel cobalt manganese oxide can improve the cycling performance.

[0119] The cathode material of the present invention uses small-particle lithium iron manganese phosphate and large-particle lithium nickel cobalt manganese oxide. The small-particle lithium iron manganese phosphate is coated on 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, which can effectively reduce the occurrence of side reactions, thereby improving the overall cycle life; on the other hand, by reasonably designing the ratio of lithium iron manganese phosphate and lithium nickel cobalt manganese oxide, the situation of local attenuation deterioration caused by excessive coating material can be avoided, so as to balance the performance of cycling and specific capacity. Therefore, the present invention effectively overcomes some practical problems in the prior art and thus has high utilization value and practical significance.

[0120] The above embodiments are only illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A positive electrode material, characterized in that, it includes: a first active material, the first active material including lithium iron manganese phosphate; a second active material, the second active material including lithium nickel cobalt manganese oxide; Among them, the lithium iron manganese phosphate is coated on the outer side of the lithium nickel cobalt manganese oxide, and the mass of the lithium iron manganese phosphate is M 1 , and the mass of the lithium nickel cobalt manganese oxide is M 2 , M 1 and M 2 satisfy formula (1): In formula (1), the value range of a is from 0.9 to 1.1; ρ 1 is the density of lithium iron manganese phosphate, ρ 2 is the density of lithium nickel cobalt manganese oxide, D 1 is the median particle size of lithium iron manganese phosphate, D 2 is the median particle size of lithium nickel cobalt manganese oxide.

2. The positive electrode material according to claim 1, characterized in that, the chemical formula of the lithium iron manganese phosphate is LiMn x Fe (1-x) PO 4 , wherein, 0.6 ≤ x ≤ 0.8; and / or, the chemical formula of the lithium nickel cobalt manganese oxide is 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, D 1 <D 2 。 4. The positive electrode material according to claim 3, characterized in that, The said D 1 has a value range of 0.5 μm to 0.9 μm, and the said D 2 has a value range of 3 μm to 8 μm.

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

6. The positive electrode material according to claim 1, characterized in that, preparing a positive electrode sheet from the positive electrode material, performing argon ion cross-section polishing on the positive electrode sheet, and taking a region with an area of A for elemental energy spectrum analysis to obtain the distribution ratio of the lithium iron manganese phosphate to the lithium nickel cobalt manganese oxide: In formula (2), the value range of b is from 2 to 4.23; S Ni is the proportion of Ni element in area A; S Fe is the proportion of Fe element in area A; ρ 1 is the density of lithium iron manganese phosphate, ρ 2 is the density of lithium nickel cobalt manganese oxide, D 1 is the median particle size of lithium iron manganese phosphate, D 2 is the median particle size of lithium nickel cobalt manganese oxide.

7. A secondary battery, characterized in that, it includes: a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the positive electrode sheet includes the positive electrode material according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, characterized in that, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, the negative electrode active material layer includes a negative electrode material, and the negative electrode material includes one or more of a graphite material, a silicon-carbon negative electrode material, and a silicon-oxygen negative electrode material.

9. The secondary battery according to claim 8, characterized in that, the capacity of the negative electrode material is N, the capacity of the positive electrode material is P, and the value range of N / P is from 1.02 to 1.

12.

10. An electronic device, characterized in that, it includes the secondary battery according to any one of claims 7 to 9.

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