Positive electrode material, preparation method thereof, secondary battery and electric device

Abstract

The application relates to a positive electrode sheet and a preparation method thereof, a secondary battery and an electric device. The positive electrode sheet comprises a positive electrode current collector and a first positive electrode film layer and a second positive electrode film layer which are sequentially stacked on the surface of the positive electrode current collector; the first positive electrode film layer comprises a first positive electrode active material, the second positive electrode film layer comprises a second positive electrode active material, the Dv50 particle size of the first positive electrode active material is smaller than that of the second positive electrode active material, the first positive electrode active material and the second positive electrode active material both contain a conductive coating layer, the conductive coating layer comprises a conductive material, and the mass content of the conductive material in the first positive electrode active material is greater than that in the second positive electrode active material.

Description

Technical Field

[0001] This application belongs to the field of secondary battery technology, specifically relating to a positive electrode material and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] Secondary batteries are widely used in various consumer electronics products and electric vehicles due to their outstanding characteristics such as light weight, no pollution, and no memory effect.

[0003] Currently used secondary batteries, such as lithium-ion batteries, mainly rely on Li. + It operates by moving between the positive and negative electrodes. During charging and discharging, Li... + It intercalates and deintercalates back and forth between the two electrodes. During charging, Li... + The lithium is extracted from the positive electrode and inserted into the negative electrode through the electrolyte, putting the negative electrode in a lithium-rich state; the opposite occurs during discharge.

[0004] In actual production and application, it has been found that current lithium-ion batteries and other secondary batteries have difficulty simultaneously achieving both energy density and power performance. Summary of the Invention

[0005] Therefore, it is necessary to provide a cathode material that can balance the energy density and functional performance of a battery, as well as its preparation method, a secondary battery, and an electrical device.

[0006] In a first aspect, this application provides a positive electrode sheet, comprising a positive current collector and a first positive electrode film layer and a second positive electrode film layer sequentially stacked on the surface of the positive current collector;

[0007] The first positive electrode film layer includes a first positive electrode active material, and the second positive electrode film layer includes a second positive electrode active material. The Dv50 particle size of the first positive electrode active material is smaller than that of the second positive electrode active material. Both the first positive electrode active material and the second positive electrode active material contain a conductive coating layer, which includes a conductive material. The mass content of the conductive material in the first positive electrode active material is greater than that in the second positive electrode active material.

[0008] Not wishing to be limited by any theory, the positive electrode sheet described in this application comprises two positive electrode film layers with different Dv50 particle sizes and conductive material contents of the positive electrode active material. The first positive electrode film layer, which is closer to the positive electrode current collector, uses a first positive electrode active material with a smaller Dv50 particle size and a higher conductive material content. This results in better conductivity of the first positive electrode active material, a denser packing of the first positive electrode film layer with a higher compaction density, and thus better electrical contact with the positive electrode current collector. Furthermore, both the first and second positive electrode active materials use conductive coating materials, which helps reduce internal resistance, increase battery voltage, and consequently improve power performance. The second positive electrode active material, with its larger Dv50 particle size, has larger pores, allowing the electrolyte to fully wet the first positive electrode film layer and be transported to the second positive electrode film layer. This facilitates the full utilization of the discharge capacity of the positive electrode active material in the positive electrode sheet and improves the energy density of the battery.

[0009] In summary, the above-mentioned positive electrode sheet, when applied to secondary batteries, enables the secondary batteries to simultaneously possess good energy density and power performance.

[0010] In any embodiment of this application, the positive electrode sheet satisfies at least one of the following conditions:

[0011] (1a) The Dv50 particle size of the first positive electrode active material is ≤1500nm, and can be selected as 100nm~1000nm, or more preferably 100nm~900nm;

[0012] (1b) The Dv50 particle size of the second positive electrode active material is 2μm to 6μm, and can be selected as 2μm to 4.5μm.

[0013] In any embodiment of this application, the difference between the mass content of the conductive material in the first positive electrode active material and the mass content of the conductive material in the second positive electrode active material is 0.5% to 2.7%; optionally, it is 0.5% to 1.5%.

[0014] In any embodiment of this application, the positive electrode sheet satisfies at least one of the following conditions:

[0015] (2a) The mass content of the conductive material in the first positive electrode active material is 2% to 4%;

[0016] (2b) The mass content of the conductive material in the second positive electrode active material is 0.8% to 3%;

[0017] (2c) The conductive material includes conductive carbon.

[0018] In any embodiment of this application, both the first positive electrode active material and the second positive electrode active material include a core, and the conductive coating layer covers at least a portion of the surface of the core;

[0019] Both the core of the first positive electrode active material and the core of the second positive electrode active material contain lithium phosphate.

[0020] In any embodiment of this application, the core of the first positive electrode active material is doped with manganese, and the second positive electrode active material may or may not be doped with manganese.

[0021] The mass content of manganese in the core of the first positive electrode active material is less than the mass content of manganese in the core of the second positive electrode active material.

[0022] In any embodiment of this application, the core of the first positive electrode active material comprises lithium iron phosphate; the core of the second positive electrode active material comprises lithium manganese iron phosphate.

[0023] In any embodiment of this application, the thickness of the first positive electrode film is less than the thickness of the second positive electrode film.

[0024] In any embodiment of this application, the difference between the thickness of the second positive electrode film and the thickness of the first positive electrode film is 10 μm to 100 μm.

[0025] In any embodiment of this application, the positive electrode sheet satisfies at least one of the following conditions:

[0026] (3a) The thickness of the first positive electrode film is 60 μm to 100 μm;

[0027] (3b) The thickness of the second positive electrode film is 100 μm to 200 μm.

[0028] In any embodiment of this application, the positive electrode sheet satisfies at least one of the following conditions:

[0029] (4a) In the first positive electrode film layer, the mass content of the first positive electrode active material is 90% to 98%;

[0030] (4b) In the second positive electrode film layer, the mass content of the second positive electrode active material is 90% to 98%.

[0031] A second aspect of this application provides a method for preparing the positive electrode sheet provided in the first aspect of this application, the method comprising the following steps:

[0032] Prepare a first positive electrode slurry containing the first positive electrode active material and a second positive electrode slurry containing the second positive electrode active material;

[0033] The first positive electrode slurry and the second positive electrode slurry are sequentially coated on the positive electrode current collector to form the first positive electrode film layer and the second positive electrode film layer, respectively.

[0034] A third aspect of this application provides a secondary battery comprising the positive electrode provided in the first aspect of this application.

[0035] A fourth aspect of this application provides an electrical device that includes the secondary battery provided in the third aspect of this application. Attached Figure Description

[0036] To more clearly illustrate the technical solution of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without any creative effort.

[0037] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application;

[0038] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of this application is shown.

[0039] Figure 3 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application;

[0040] Explanation of reference numerals in the attached figures:

[0041] 1. Secondary battery; 11. Housing; 12. Electrode assembly; 13. Cover plate; 2. Electrical device. Detailed Implementation

[0042] The present application will be further described below with reference to specific embodiments. It should be understood that these specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0043] For the sake of brevity, this document only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is expected that ranges of 60–110 and 80–120 are also to be understood. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all to be expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range “a–b” represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0044] In the description of this article, it should be noted that, unless otherwise stated, "above" and "below" include the number itself, and "several" in "one or more" means two or more.

[0045] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0046] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0047] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0048] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0049] In this description, unless otherwise stated, the term "or" is inclusive. That is, the phrase "A or (or) B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist). Unless otherwise stated, the terms used in this application have their commonly known meanings as understood by those skilled in the art. Unless otherwise stated, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0050] Secondary batteries

[0051] A rechargeable battery is a battery that can be recharged after it has been discharged, allowing the active materials to be activated and the battery to continue to be used.

[0052] Typically, a secondary battery includes a positive electrode, a negative electrode, and an electrolyte. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor of ions between the positive and negative electrodes. Furthermore, a secondary battery may also include a separator.

[0053] One embodiment of this application provides a positive electrode sheet. The positive electrode sheet includes a positive current collector and a first positive electrode film layer and a second positive electrode film layer sequentially stacked on at least one surface of the positive current collector.

[0054] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and a first positive electrode film layer and a second positive electrode film layer are disposed on either or both of the two opposite surfaces of the positive current collector.

[0055] The first positive electrode film layer includes a first positive electrode active material, and the second positive electrode film layer includes a second positive electrode active material. The Dv50 particle size of the first positive electrode active material is smaller than that of the second positive electrode active material. Both the first and second positive electrode active materials contain a conductive coating layer, which includes a conductive material. The mass content of the conductive material in the first positive electrode active material is greater than that in the second positive electrode active material.

[0056] The volume average particle size (Dv50) is a well-known concept in the art and can be measured using methods known in the art. Dv50 represents the particle size at which the cumulative volume distribution percentage of particles reaches 50%. For example, in this paper, a laser particle size analyzer, specifically a Malvern 3000 (MasterSizer 3000), is used, and the measurement is performed according to the standard procedure: GB / T19077-2016 / ISO13320:2009. Since the primary particles in the positive electrode active material are relatively small, the particle size at which the cumulative volume distribution percentage of particles reaches 50% essentially corresponds to the particle size at which the cumulative volume distribution percentage of secondary particles reaches 50%.

[0057] The specific testing procedure can be as follows: Take an appropriate amount of the sample to be tested, and ensure that the sample concentration is 8% to 12% opacity. Add 20 mL of deionized water and simultaneously sonicate for 5 minutes at an ultrasonic frequency of 53 kHz and an ultrasonic power of 120 W to ensure that the sample is completely dispersed. Then, measure the sample according to the standard GB / T19077-2016 / ISO 13320:2009.

[0058] Not wishing to be limited by any theory, the positive electrode sheet described in this application comprises two positive electrode film layers with different Dv50 particle sizes and conductive material contents of the positive electrode active material. The first positive electrode film layer, which is closer to the positive electrode current collector, uses a first positive electrode active material with a smaller Dv50 particle size and a higher conductive material content. This results in better conductivity of the first positive electrode active material, a denser packing of the first positive electrode film layer with a higher compaction density, and thus better electrical contact with the positive electrode current collector. Furthermore, both the first and second positive electrode active materials use conductive coating materials, which helps reduce internal resistance, increase battery voltage, and consequently improve power performance. The second positive electrode active material, with its larger Dv50 particle size, has larger pores, allowing the electrolyte to fully wet the first positive electrode film layer and be transported to the second positive electrode film layer. This facilitates the full utilization of the discharge capacity of the positive electrode active material in the positive electrode sheet and improves the energy density of the battery.

[0059] In summary, the above-mentioned positive electrode sheet, when applied to secondary batteries, enables the secondary batteries to simultaneously possess good energy density and power performance.

[0060] In some embodiments, the Dv50 particle size of the first positive electrode active material is ≤1500nm, for example 100nm~1500nm, or 100nm~1000nm, or 300nm~1000nm; it can be <1000nm, for example 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm; it can be 100nm~900nm, 100nm~800nm, or 200nm~900nm.

[0061] Furthermore, the specific surface area of ​​the first positive electrode active material is 10 m². 2 / g~27m 2 / g; or 10m 2 / g~16m 2 / g. In this paper, the specific surface area can be obtained by the BET test method. The first positive electrode active material has a large specific surface area, and the particles have good contact, which can improve conductivity.

[0062] In some embodiments, the Dv50 particle size of the second positive electrode active material is 2 μm to 6 μm, for example, it can be 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, or 6 μm. It can be selected as 2 μm to 4.5 μm.

[0063] Furthermore, the specific surface area of ​​the second positive electrode active material is 5m². 2 / g~10m 2 / g, its specific surface area is smaller than that of the first positive electrode active material.

[0064] By using nanoscale particles with a Dv50 particle size of <1000nm for the first positive electrode active material and microscale particles with a Dv50 particle size range for the second positive electrode active material, the energy density of the battery can be further improved.

[0065] In some embodiments, the difference between the mass content of the conductive material in the first positive electrode active material and the mass content of the conductive material in the second positive electrode active material is 0.5% to 2.7%; optionally, it is 0.5% to 1.5%. By controlling the difference in the mass content of the conductive material in the first and second positive electrode active materials, a balanced improvement in conductivity and energy density is further achieved.

[0066] In some embodiments, the mass content of conductive material in the first positive electrode active material is 2% to 4%, for example, 2%, 2.5%, 3%, 3.5%, or 4%.

[0067] In some embodiments, the mass content of conductive material in the second positive electrode active material is 0.8% to 3%, for example, 0.8%, 1%, 1.5%, 2%, 2.5%, or 3%.

[0068] In some embodiments, the conductive material in the first positive electrode active material and the second positive electrode active material includes conductive carbon.

[0069] In some embodiments, both the first positive electrode active material and the second positive electrode active material include a core, and a conductive coating layer covers at least a portion of the surface of the core.

[0070] In some embodiments, the core of both the first positive electrode active material and the core of the second positive electrode active material include lithium phosphate.

[0071] Lithium-containing phosphates include, but are not limited to, at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium nickel phosphate, lithium cobalt phosphate, and lithium vanadium phosphate.

[0072] Furthermore, the core of the first positive electrode active material is doped with manganese, while the second positive electrode active material may or may not be doped with manganese. The mass content of manganese in the core of the first positive electrode active material is lower than that in the core of the second positive electrode active material. This is beneficial for increasing the voltage plateau of the battery, thereby facilitating the achievement of higher energy density.

[0073] In some embodiments, the secondary battery is a lithium-ion battery.

[0074] It is understood that in other embodiments, the positive electrode active material of the above-described positive electrode film may also contain other positive electrode active materials for batteries known in the art. As an example, the positive electrode active material may also include at least one of the following materials: lithium transition metal oxides and modified compounds thereof.

[0075] Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05O2 and its modified compounds.

[0076] In some embodiments, the core of the first positive electrode active material includes lithium iron phosphate; the core of the second positive electrode active material includes lithium manganese iron phosphate. The lithium iron phosphate and lithium manganese iron phosphate are layered to form the positive electrode sheet. The upper second positive electrode film layer provides higher energy density and power performance, while the lower first positive electrode film layer provides good cycle performance. Therefore, the energy density, cycle performance, and especially the low-temperature power performance of the battery can be further improved.

[0077] Furthermore, the chemical formula of lithium manganese iron phosphate is LiMn. x Fe 1-x PO4, 0 < x < 1. Alternatively, 0.1 ≤ x ≤ 0.9; as examples, x = 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, or 0.9.

[0078] In some embodiments, the thickness of the first positive electrode film is less than the thickness of the second positive electrode film. Controlling the thickness of the second positive electrode film to be relatively large is beneficial for further improving the low-temperature power performance of the battery.

[0079] Furthermore, the difference between the thickness of the second positive electrode film and the thickness of the first positive electrode film is 10 μm to 100 μm, and can be selected as 10 μm to 50 μm. As an example, this difference can be 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, or 100 μm.

[0080] In some embodiments, the thickness of the first positive electrode film is 60 μm to 100 μm; as an example, the thickness of the first positive electrode film can be 60 μm, 70 μm, 80 μm, 90 μm, or 100 μm.

[0081] In some embodiments, the thickness of the second positive electrode film is 100 μm to 200 μm; as examples, the thickness of the second positive electrode film is 100 μm, 120 μm, 140 μm, 160 μm, 180 μm, or 200 μm.

[0082] In some embodiments, the mass content of the first positive electrode active material in the first positive electrode film layer is 90% to 98%. As an example, the mass content of the first positive electrode active material in the first positive electrode film layer can be 90%, 92%, 94%, 96%, or 98%.

[0083] In some embodiments, the mass content of the second positive electrode active material in the second positive electrode film layer is 90% to 98%. As examples, the mass content of the second positive electrode active material in the second positive electrode film layer can be 90%, 92%, 94%, 96%, or 98%.

[0084] In some embodiments, the first positive electrode film layer and the second positive electrode film layer may also each independently and optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, styrene-butadiene rubber (SBR), water-based acrylic resin, ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0085] Furthermore, in the first positive electrode film layer and / or the second positive electrode film layer, the mass content of the binder is 0.5% to 10%.

[0086] In some embodiments, the first positive electrode film and the second positive electrode film may also each optionally include a conductive agent independently. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0087] Furthermore, in the first positive electrode film layer and / or the second positive electrode film layer, the mass content of the conductive agent is 0.5% to 10%.

[0088] In some embodiments, the positive electrode sheet can be prepared by: preparing a positive electrode slurry containing the above-mentioned positive electrode active material; coating the positive electrode slurry onto the positive electrode current collector to form a positive electrode film layer.

[0089] One embodiment of this application also provides a method for preparing the above-mentioned positive electrode sheet, comprising the following steps S1 to S2:

[0090] S1. Prepare a first positive electrode slurry containing the first positive electrode active material and a second positive electrode slurry containing the second positive electrode active material.

[0091] S2. First positive electrode slurry and second positive electrode slurry are sequentially coated on the positive electrode current collector to form the first positive electrode film layer and the second positive electrode film layer, respectively.

[0092] As an example, the positive electrode sheet can be prepared as follows: The first positive electrode active material, conductive agent, binder, and optionally other components are dispersed in a solvent such as N-methylpyrrolidone to form a first positive electrode slurry; the first positive electrode slurry is coated onto a positive electrode current collector and dried. Then, the second positive electrode active material, conductive agent, binder, and optionally other components are dispersed in a solvent such as N-methylpyrrolidone to form a second positive electrode slurry; the second positive electrode slurry is coated onto the dried first positive electrode slurry, and after drying, cold pressing, and other processes, the positive electrode sheet is obtained.

[0093] In some implementations, the positive current collector may be a metal foil or a composite current collector.

[0094] For example, aluminum foil can be used as the metal foil. Optionally, the thickness of the positive electrode current collector is 7μm to 20μm. For example, aluminum foil with a thickness of 7μm to 20μm can be used.

[0095] For example, a composite current collector may include a polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector can be formed by forming a metal material on the polymeric material substrate. The metal material includes, but is not limited to, at least one of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys; the polymeric material substrate includes, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0096] Negative electrode sheet

[0097] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0098] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0099] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material on the polymer material substrate. The metal material includes, but is not limited to, at least one of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys; the polymer material substrate includes, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0100] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0101] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0102] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0103] In some embodiments, the negative electrode film layer may optionally include other additives, such as thickeners, for example, sodium carboxymethyl cellulose (CMC-Na).

[0104] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent such as deionized water to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0105] electrolytes

[0106] The electrolyte acts as a conductor of ions between the negative and positive electrodes. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0107] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0108] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0109] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0110] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0111] Separating membrane

[0112] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0113] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0114] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0115] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0116] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0117] This application does not impose any particular limitation on the shape of the metal battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 Here is an example of a square-structured metal battery 1.

[0118] In some implementations, refer to Figure 2 The outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The shell 11 has an opening communicating with the receiving cavity, and the cover plate 13 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator can be formed into an electrode assembly 12 by a winding process or a stacking process. The electrode assembly 12 is encapsulated within the receiving cavity. The aforementioned gel electrolyte is impregnated in the electrode assembly 12. The secondary battery 1 may contain one or more electrode assemblies 12, which can be selected by those skilled in the art according to specific practical needs.

[0119] In addition, the present invention also provides an electrical device, which includes at least one of a secondary battery, a battery module, or a battery pack provided by the present invention. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Among them, mobile devices may be, for example, mobile phones, laptops, etc.; electric vehicles may be, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to these.

[0120] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0121] Figure 3 Here is an example of an electrical device 2. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc.

[0122] Another example device could be a mobile phone, tablet, laptop, etc.

[0123] To make the objectives, technical solutions, and advantages of this invention clearer and more concise, the invention is described using the following specific embodiments, but the invention is by no means limited to these embodiments. The embodiments described below are merely preferred embodiments of the invention and can be used to describe the invention, but should not be construed as limiting the scope of the invention. It should be noted that any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the protection scope of this invention.

[0124] To better illustrate the present invention, the following embodiments are provided for further explanation. The specific embodiments are as follows. Unless otherwise specified, all raw materials are commercially available.

[0125] I. Battery Manufacturing

[0126] Comparative Example 1

[0127] (1) Preparation of positive electrode sheet

[0128] The positive electrode active materials (carbon-coated lithium iron phosphate and carbon-coated lithium manganese iron phosphate, LiMn) x Fe 1-x PO4 (x = 0.5, the same below), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are dissolved in solvent N-methylpyrrolidone (NMP) at a mass ratio of 48.5:48.5:0.8:2.2. After thorough stirring and mixing, a positive electrode slurry is obtained. The above positive electrode slurry is coated onto the positive electrode current collector aluminum foil to form a positive electrode film. After coating, it is dried, cold-pressed, slit, and the positive electrode sheet is prepared. The total thickness of the positive electrode film is 240 μm.

[0129] (2) Preparation of negative electrode sheet

[0130] Artificial graphite (anode active material), acetylene black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (thickener) were dissolved in deionized water at a mass ratio of 96.5:0.7:1.8:1. After being mixed evenly, a negative electrode slurry was obtained. The negative electrode slurry was then coated onto a copper foil (anode current collector). After coating, the foil was dried, cold-pressed, slit, and prepared into a negative electrode sheet.

[0131] (3) Preparation of electrolyte

[0132] Ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate were mixed in a volume ratio of 20:20:60. Thoroughly dried LiPF6 was then dissolved in this mixture and thoroughly mixed to obtain the electrolyte. The concentration of LiPF6 was 1 mol / L. The entire operation was conducted in an argon atmosphere glove box with a water content of <10 ppm.

[0133] (4) Separating membrane

[0134] A polyethylene film with a thickness of 12μm was used as the separator.

[0135] (5) Assembly of battery cells

[0136] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to isolate them. The stacked components are then wound to form an electrode assembly. The electrode assembly is placed in a housing, dried, and then injected with electrolyte. After formation and settling processes, a battery is obtained.

[0137] Comparative Example 2

[0138] Compared with Comparative Example 1, all the positive electrode active materials were carbon-coated lithium iron phosphate, except that they were all carbon-coated lithium iron phosphate. The mass ratio of carbon-coated lithium iron phosphate, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) was 97:0.8:2.2.

[0139] Comparative Example 3

[0140] Compared with Comparative Example 1, all the positive electrode active materials were carbon-coated lithium manganese iron phosphate, except that they were all carbon-coated lithium manganese iron phosphate. The mass ratio of carbon-coated lithium manganese iron phosphate, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) was 97:0.8:2.2.

[0141] Example 1

[0142] It is basically the same as Comparative Example 1, except that the only difference is in the preparation of the positive electrode sheet.

[0143] The specific steps are as follows:

[0144] The positive electrode active material (carbon-coated lithium iron phosphate), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are dissolved in solvent N-methylpyrrolidone (NMP) at a mass ratio of 97:0.8:2.2. After thorough stirring and mixing, a first positive electrode slurry is obtained. The first positive electrode slurry is then coated onto the positive electrode current collector aluminum foil. After coating, a first positive electrode film layer is formed.

[0145] The positive electrode active material (carbon-coated lithium manganese iron phosphate, LiMn) x Fe 1-x PO4 (x=0.5), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were dissolved in solvent N-methylpyrrolidone (NMP) at a mass ratio of 97:0.8:2.2. After thorough mixing, a second positive electrode slurry was obtained. The second positive electrode slurry was then coated onto the first positive electrode film. After coating, the film was dried to form the second positive electrode film. The film was then cold-pressed, slit, and prepared to obtain the positive electrode sheet.

[0146] Comparative Example 4

[0147] It is basically the same as Example 1, except that the Dv50 particle size, BET and carbon coating amount of the first positive electrode active material are different, as shown in Table 1.

[0148] Example 2

[0149] The process is essentially the same as in Example 1, except that the types of positive electrode active materials in the first and second positive electrode films are interchanged. The preparation steps for this positive electrode are as follows:

[0150] The positive electrode active material (carbon-coated lithium manganese iron phosphate, the same as in Example 1), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were dissolved in the solvent N-methylpyrrolidone (NMP) at a mass ratio of 97:0.8:2.2. After thorough stirring and mixing, a first positive electrode slurry was obtained. The first positive electrode slurry was then coated onto the positive electrode current collector aluminum foil. After coating, a first positive electrode film layer was formed. Note: The coating amount and thickness of the first positive electrode film layer are comparable to those of the second positive electrode film layer in Example 1.

[0151] The positive electrode active material (carbon-coated lithium iron phosphate, the same as in Example 1), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 97:0.8:2.2. After thorough mixing, a second positive electrode slurry was obtained. This second positive electrode slurry was then coated onto the first positive electrode film. After coating, it was dried to form the second positive electrode film, which was then cold-pressed, slit, and prepared to obtain the positive electrode sheet. Note: The coating amount and thickness of the second positive electrode film are comparable to those of the first positive electrode film in Example 1.

[0152] Examples 3-10

[0153] The results are basically the same as in Example 1, except that the Dv50 particle size, BET, and carbon coating amount of the first positive electrode active material and / or the second positive electrode active material are different, as shown in Table 1.

[0154] It should be noted that BET in Table 1 refers to specific surface area.

[0155] II. Battery Performance Testing

[0156] (I) Energy density testing

[0157] Charge the capacitor to 3.65V at 0.33C standard at 25℃, then charge it to 0.05C at 3.65V constant voltage, let it stand for 10 minutes, and then discharge it to 2.5V at 0.33C. Record its discharge capacity and then calculate the energy density during discharge.

[0158] Energy density (Wh / L) = Discharge capacity (Wh) / Mass of lithium-ion secondary battery (kg).

[0159] (II) Low-Temperature Power Performance Testing

[0160] 1. Discharge the battery from 100% SOC for 30 minutes at room temperature (25℃) until it reaches 50% SOC;

[0161] 2. Set the temperature of the battery chamber to -30℃ and let it stand for 2 hours;

[0162] 3. Discharge for 120 seconds using a 3C rate current;

[0163] 4. Divide the voltage difference by the current to obtain the DCR under -30℃ conditions (as shown in Table 1), which is the DC internal resistance of the battery at 50% SOC.

[0164] (III) The cyclic testing process is as follows:

[0165] At 25°C, the battery is charged to 4.4V at a rate of 0.5C, then charged at a constant voltage until the current is below 0.05C, and then discharged to 2.8V at a rate of 1C. This full charge and discharge cycle constitutes one charge-discharge cycle of the battery. This cycle test is repeated continuously until the battery's discharge capacity decays to 70% of its initial capacity. The number of cycles at this point is recorded, which is the cycle life, measured in cls, as shown in Table 1.

[0166] Table 1

[0167]

[0168] As shown in Table 1, Comparative Example 1, by mixing two positive electrode active materials without layered coating, failed to fully utilize the advantages of lithium manganese iron phosphate. Comparative Examples 2 and 3 used only a single positive electrode active material; the former resulted in a battery with a higher DCR, indicating poor low-temperature power performance, while the latter resulted in a battery with poor cycle performance. In Comparative Example 4, the conductivity content of the first positive electrode active material in the first positive electrode film layer near the positive electrode current collector was much lower than that in the upper second positive electrode film layer, resulting in performance comparable to Comparative Example 1. Therefore, the secondary batteries prepared in Comparative Examples 1 to 4 cannot simultaneously achieve good energy density, low-temperature performance, and cycle life.

[0169] The batteries prepared in each embodiment all exhibit good energy density, low-temperature performance, and cycle life. Specifically, compared to Example 2, Example 1 shows that placing carbon-coated lithium iron phosphate in the lower layer closer to the positive electrode current collector, rather than in the upper layer further away from the positive electrode current collector, further improves the battery's energy density, low-temperature performance, and cycle life.

[0170] Compared to Example 7, Examples 1 and 3-6 show that the Dv50 of the first positive electrode active material is <1000nm, which can further improve the energy density, low-temperature performance and cycle life of the battery.

[0171] As can be seen from Examples 1 and 8-9 compared to Example 10, under the same conditions, the Dv50 of the second positive electrode active material is between 2μm and 4.5μm, which can further improve the energy density, low-temperature performance and cycle life of the battery.

[0172] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0173] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims, and the specification and drawings can be used to interpret the content of the claims.

Claims

1. A positive electrode plate, characterized in that, It includes a positive current collector and a first positive electrode film layer and a second positive electrode film layer sequentially stacked on the surface of the positive current collector; The first positive electrode film layer includes a first positive electrode active material, and the second positive electrode film layer includes a second positive electrode active material. The Dv50 particle size of the first positive electrode active material is smaller than that of the second positive electrode active material. Both the first positive electrode active material and the second positive electrode active material contain a conductive coating layer, which includes a conductive material. The mass content of the conductive material in the first positive electrode active material is greater than that in the second positive electrode active material.

2. The positive electrode sheet as described in claim 1, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (1a) The Dv50 particle size of the first positive electrode active material is ≤1500nm; (1b) The Dv50 particle size of the second positive electrode active material is 2μm~6μm.

3. The positive electrode sheet as described in claim 2, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (1) The Dv50 particle size of the first positive electrode active material is 100nm~1000nm; (2) The particle size of the second positive electrode active material Dv50 is 2μm~4.5μm.

4. The positive electrode sheet as described in claim 3, characterized in that, The Dv50 particle size of the first positive electrode active material is 100nm~900nm.

5. The positive electrode sheet as described in claim 1, characterized in that, The difference between the mass content of the conductive material in the first positive electrode active material and the mass content of the conductive material in the second positive electrode active material is 0.5% to 2.7%.

6. The positive electrode sheet as described in claim 5, characterized in that, The difference between the mass content of the conductive material in the first positive electrode active material and the mass content of the conductive material in the second positive electrode active material is 0.5% to 1.5%.

7. The positive electrode sheet as described in claim 1, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (2a) The mass content of the conductive material in the first positive electrode active material is 2%~4%; (2b) The mass content of the conductive material in the second positive electrode active material is 0.8%~3%; (2c) The conductive material includes conductive carbon.

8. The positive electrode sheet according to any one of claims 1 to 7, characterized in that, Both the first positive electrode active material and the second positive electrode active material include a core, and the conductive coating layer covers at least a portion of the surface of the core; Both the core of the first positive electrode active material and the core of the second positive electrode active material contain lithium phosphate.

9. The positive electrode sheet as described in claim 8, characterized in that, The core of the second positive electrode active material is doped with manganese, while the first positive electrode active material may or may not be doped with manganese. The mass content of manganese in the core of the first positive electrode active material is less than the mass content of manganese in the core of the second positive electrode active material.

10. The positive electrode sheet as described in claim 9, characterized in that, The core of the first positive electrode active material includes lithium iron phosphate; the core of the second positive electrode active material includes lithium manganese iron phosphate.

11. The positive electrode sheet according to any one of claims 1 to 7, 9 to 10, characterized in that, The thickness of the first positive electrode film is less than the thickness of the second positive electrode film.

12. The positive electrode sheet as described in claim 11, characterized in that, The difference between the thickness of the second positive electrode film and the thickness of the first positive electrode film is 10 μm to 100 μm.

13. The positive electrode sheet according to any one of claims 1 to 7, 9 to 10, and 12, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (3a) The thickness of the first positive electrode film is 60 μm to 100 μm; (3b) The thickness of the second positive electrode film is 100μm~200μm.

14. The positive electrode sheet according to any one of claims 1 to 7, 9 to 10, and 12, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (4a) In the first positive electrode film layer, the mass content of the first positive electrode active material is 90%~98%; (4b) In the second positive electrode film layer, the mass content of the second positive electrode active material is 90%~98%.

15. A method for preparing a positive electrode sheet as described in any one of claims 1 to 14, characterized in that, The preparation method includes the following steps: Prepare a first positive electrode slurry containing the first positive electrode active material and a second positive electrode slurry containing the second positive electrode active material; The first positive electrode slurry and the second positive electrode slurry are sequentially coated on the positive electrode current collector to form the first positive electrode film layer and the second positive electrode film layer, respectively.

16. A secondary battery, characterized in that, It includes the positive electrode sheet as described in any one of claims 1 to 14.

17. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 16.

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