Secondary battery and electric device

Abstract

The invention discloses a secondary battery and a power utilization device, and belongs to the technical field of batteries, the number of unit area pores in an SEM graph of the cross section of a positive pole piece, the porosity and the mass percentage content of an element M in a positive active material are controlled to meet the following conditions: 0.4 * 103 < = N * P * R < = 1053 * 103; the dynamic performance of the positive pole piece can be remarkably improved, electron transmission between positive active materials is facilitated, the infiltration effect of an electrolyte is improved, the diffusion path of lithium ions is improved, the de-intercalation speed of the lithium ions is increased, the gram volume of the secondary battery is effectively improved, and the cycle performance and the storage performance of the secondary battery are remarkably improved; the impedance of the secondary battery is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a secondary battery and an electrical device using the same. Background Art

[0002] With the development of technology, secondary batteries are increasingly widely used in fields such as electric vehicles, hybrid vehicles, and renewable energy storage. However, existing secondary batteries still have deficiencies in terms of cycle performance (especially high and low temperature cycle performance).

[0003] In view of this, the present application is proposed. Summary of the Invention

[0004] An object of the present invention is to overcome the deficiencies of the prior art and provide a secondary battery and an electrical device using the same, wherein the secondary battery has excellent cycle performance and storage performance.

[0005] To achieve the above object, a first aspect of the present application provides a secondary battery, including a positive electrode plate, the positive electrode plate including a positive current collector and a positive active material layer provided on at least one surface of the positive current collector; the positive active material layer includes a positive active material, the positive active material includes an M element, and the M element includes at least one of Al, Mn, Ni, Co, W, Mo, Ti, and V;

[0006] The positive electrode plate satisfies: 0.4×10 3 ≤N×P×R≤1053×10 3 ;

[0007] wherein, N pieces / μm 2 is the number of pores per unit area in the SEM image of the cross-section of the positive electrode plate;

[0008] P is the porosity of the positive electrode plate;

[0009] R ppm is the mass percentage content of the M element in the positive active material.

[0010] As an embodiment of the present application, the positive electrode plate satisfies: 1.8×10 3 ≤N×P×R≤36.5×10 3 .

[0011] As an embodiment of the present application, at least one of the following (1) to (3) is satisfied:

[0012] (1) 1≤N≤60;

[0013] (2) 10%≤P≤60%;

[0014] (3) 100≤R≤300000.

[0015] As an embodiment of the present application, the positive electrode sheet satisfies: 1 ≤ N×L ≤ 57.3;

[0016] Wherein, L is the mass percentage content of the positive electrode active material in the positive electrode sheet.

[0017] As an embodiment of the present application, the L satisfies: 95.5% ≤ L ≤ 99%.

[0018] As an embodiment of the present application, the positive electrode sheet satisfies: 1×10 3 ≤ N×C ≤ 15×10 3 ;

[0019] Wherein, C g / m 2 is the single-sided surface density of the positive electrode sheet.

[0020] As an embodiment of the present application, the C satisfies: 60 ≤ C ≤ 320.

[0021] As an embodiment of the present application, the positive electrode sheet satisfies: 0.4 ≤ N / D ≤ 23.1;

[0022] Wherein, D g / cm 3 is the compaction density of the positive electrode sheet.

[0023] As an embodiment of the present application, the D satisfies: 2.5 ≤ D ≤ 2.9.

[0024] As an embodiment of the present application, the positive electrode active material includes a lithium-containing phosphate, and the lithium-containing phosphate includes at least one of the compounds represented by the chemical general formula Li x Fe 1-y M y PO 4 wherein, 0.9 ≤ x ≤ 1.1, 0 < y ≤ 0.8, and M includes at least one of Al, Mn, Ni, Co, W, Mo, Ti, and V.

[0025] As an embodiment of the present application, the positive electrode active material includes a first positive electrode active material, a second positive electrode active material, and a third positive electrode active material; the particle size Dv50 of the first positive electrode active material is D1, the particle size Dv50 of the second positive electrode active material is D2; the particle size Dv50 of the third positive electrode active material is D3, satisfying: D1 > D2 > D3.

[0026] As an embodiment of the present application, at least one of the following (4) to (6) is satisfied:

[0027] (4) 0.8 μm < D1 ≤ 2 μm;

[0028] (5) 0.2 μm < D2 ≤ 0.8 μm;

[0029] (6) 0.01 μm ≤ D3 ≤ 0.2 μm.

[0030] As an embodiment of the present application, at least one of the following (7) to (9) is satisfied:

[0031] (7) The mass percentage of the first positive electrode active material in the positive electrode active material is 1% to 30%;

[0032] (8) The mass percentage of the second positive electrode active material in the positive electrode active material is 10% to 80%;

[0033] (9) The mass percentage of the third positive electrode active material in the positive electrode active material is 10% to 80%.

[0034] The second aspect of the present application provides an electrical device, including the secondary battery described above, and the secondary battery serves as the power supply of the electrical device.

[0035] The beneficial effects of the present invention are as follows: By controlling the number of pores per unit area, the porosity, and the mass percentage of element M in the positive electrode active material in the SEM image of the cross-section of the positive electrode sheet of the present application to satisfy: 0.4 * 10 3 ≤ N × P × R ≤ 1053 * 10 3 ; it can significantly improve the kinetic performance of the positive electrode sheet, facilitate the electron transfer between the positive electrode active materials, improve the wetting effect of the electrolyte, improve the diffusion path of lithium ions, increase the deintercalation speed of lithium ions, effectively improve the specific capacity of the secondary battery, significantly improve the cycle performance and storage performance of the secondary battery, and reduce the impedance of the secondary battery. Description of the Drawings

[0036] Figure 1 It is a SEM image of the cross-section (profile) of the positive electrode sheet of Example 1 at a magnification of 30K. Detailed Embodiments

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Apparently, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the scope of protection of the present application.

[0038] In the present application, among the technically characterized descriptions in an open-ended manner, it includes a closed technical solution composed of the listed features, and also includes an open technical solution containing the listed features.

[0039] In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are considered continuous and include the minimum and maximum values of the range, as well as each value between such minimum and maximum values. Further, when the range refers to integers, it includes each integer between the minimum and maximum values of the range. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0040] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0041] An embodiment of the present application provides a secondary battery, including a positive electrode plate, the positive electrode plate including a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector; the positive electrode active material layer includes a positive electrode active material, the positive electrode active material includes an M element, and the M element includes at least one of Al, Mn, Ni, Co, W, Mo, Ti, and V;

[0042] The positive electrode plate satisfies: 0.4*10 3 ≤N×P×R≤1053*10 3 For example, it can be 0.4, 0.5, 0.8, 1, 2, 4, 5, 6, 8, 10, 20, 40, 50, 60, 80, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1050, 1053 or a range composed of any two of these numerical values;

[0043] wherein, the number of pores per unit area in the SEM image of the cross-section of the positive electrode plate is N per μm 2 ;

[0044] P is the porosity of the positive electrode plate;

[0045] R ppm is the mass percentage content of the M element in the positive electrode active material.

[0046] The inventors of the present application have found through research that the performance of secondary batteries has an obvious correlation with the number of pores per unit area, the porosity, and the mass percentage content of element M in the positive active material in the SEM image of the cross-section of the positive electrode plate. The present application controls the number of pores per unit area, the porosity, and the mass percentage content of element M in the positive active material in the SEM image of the cross-section of the positive electrode plate to satisfy: 0.4 * 10 3 ≤N×P×R≤1053 * 10 3 ; which can significantly improve the kinetic performance of the positive electrode plate, facilitate the electron transfer between the positive active materials, improve the infiltration effect of the electrolyte, improve the diffusion path of lithium ions, increase the deintercalation speed of lithium ions, effectively increase the specific capacity of the secondary battery, significantly improve the cycle performance and storage performance of the secondary battery, and reduce the impedance of the secondary battery.

[0047] It should be noted that the test method for the number of pores per unit area in the SEM image of the cross-section of the positive electrode plate of the present application is as follows: (1) After ion cutting the positive electrode plate, perform SEM detection on the cross-section of the electrode plate to obtain the SEM image of the electrode plate at 30K; (2) Calculate the number of particles per unit area in the positive electrode plate through the following formula: N = n / (h / p * w / p).

[0048] Among them, n represents the number of pores (pieces) in the SEM image of the cross-section (profile) of the positive electrode plate at 30K magnification. The pores are irregular in shape, and a continuous black area can be considered as 1 pore.

[0049] h and w respectively represent the actually measured lengths of the length and width of the SEM image of the cross-section (profile) of the positive electrode plate at 30K magnification, with the unit of cm.

[0050] p represents the actually measured length of the scale of 0.2μm in the SEM image of the cross-section (profile) of the positive electrode plate at 30K magnification, with the unit of cm / μm.

[0051] It should be noted that the porosity of the positive electrode plate of the present application can be tested by the cetane absorption method. For specific details, refer to GB / T 33052-2016 Porosity Determination Method.

[0052] It should be noted that the test method for element M in the positive electrode plate of the present application is as follows: Cut the positive electrode plate into small round pieces (the diameter of the round pieces can be 1 - 100mm), take 20 small round pieces for ICP testing, referring to EPA 6010D - 2018 Inductively Coupled Plasma Atomic Emission Spectrometry.

[0053] In one of the embodiments, the positive electrode plate satisfies: 1.8 * 10 3 ≤N×P×R≤36.5 * 10 3 .

[0054] In one embodiment, 1 ≤ N ≤ 60. For example, it can be 1, 2, 4, 5, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, or a range composed of any two of these values. By controlling N within this range, the ion transport kinetics can be further improved, the ion intercalation / deintercalation path can be improved, the stability of the positive electrode sheet and the wetting effect of the electrolyte can be enhanced, the lithium ion transport rate can be increased, and the cycle performance and storage performance of the secondary battery can be further improved.

[0055] In one embodiment, 10% ≤ P ≤ 60%. For example, it can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or a range composed of any two of these values. By controlling P within this range, the diffusion path of lithium ions can be improved, the migration rate of lithium ions can be increased, the wetting effect of the electrolyte can be enhanced, the electrolyte can be promoted to infiltrate into the interior of the positive electrode sheet, the resistance of the positive electrode film can be reduced, and the cycle performance and storage performance of the secondary battery can be further improved.

[0056] In one embodiment, 100 ≤ R ≤ 20000. For example, it can be 100, 200, 400, 500, 800, 1000, 1500, 2000, 2500, 3000, or a range composed of any two of these values. By controlling R within this range, the structural stability of the positive electrode active material can be improved, the conductivity of the positive electrode active material can be increased, the electron conductivity and ion diffusivity can be enhanced, the side reactions at the contact interface with the electrolyte can be reduced, and the cycle performance and storage performance of the secondary battery can be further improved.

[0057] In one embodiment, the positive electrode sheet satisfies: 1 ≤ N × L ≤ 57.3; for example, it can be 1, 2, 5, 10, 20, 30, 40, 50, 55, 57.3, or a range composed of any two of these values. By controlling N × L within this range, the cycle performance and storage performance of the secondary battery can be further improved.

[0058] Wherein, L is the mass percentage content of the positive electrode active material in the positive electrode sheet.

[0059] In one embodiment, L satisfies: 95.5% ≤ L ≤ 99%; for example, it can be 95.5, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, or a range composed of any two of these values.

[0060] As an embodiment of the present application, the positive electrode sheet satisfies: 0.2 * 10 3 ≤ N * C ≤ 12 * 10 3 For example, it can be 0.2 * 10 3 0.5 * 103 、 1 * 10 3 、 2 * 10 3 、 4 * 10 3 、 5 * 10 3 、 8 * 10 3 、 10 * 10 3 、 12 * 10 3 or a range composed of any two of these values.

[0061] wherein, C g / m 2 is the single - sided areal density of the positive electrode sheet.

[0062] wherein, the test method for the single - sided areal density of the positive electrode sheet is to cut the positive electrode sheet into multiple (the number can be 3 to 100) small circular pieces with a small area (the area can be 1540.25 mm 2 ), weigh the mass of the small circular pieces with a high - precision balance, subtract the mass of the positive electrode current collector, divide by the area of the circular piece, and then divide by 2 to calculate the single - sided areal density of the positive electrode sheet.

[0063] In one embodiment, the C satisfies: 60 ≤ C ≤ 320, for example, it can be 60, 80, 100, 120, 150, 180, 200, 250, 300, 320 or a range composed of any two of these values.

[0064] In one embodiment, the positive electrode sheet satisfies: 0.4 ≤ N / D ≤ 23.1, for example, it can be 0.4, 0.5, 0.8, 1, 2, 4, 6, 10, 12, 15, 20, 23, 23.1 or a range composed of any two of these values. By controlling N / D within this range, the cycle performance and storage performance of the secondary battery can be further improved.

[0065] wherein, D g / cm 3 is the tap density of the positive electrode sheet, wherein, the tap density of the positive electrode sheet = the areal density of the positive electrode sheet / (the thickness of the positive electrode sheet after rolling - the thickness of the positive electrode current collector).

[0066] In one embodiment, the D satisfies: 2.5 ≤ D ≤ 2.9, for example, it can be 2.5, 2.55, 2.6, 2.65, 2.7, 2.75, 2.8, 2.85, 2.9 or a range composed of any two of these values.

[0067] In one embodiment, the positive electrode active material includes lithium - containing phosphate, and the lithium - containing phosphate includes a chemical general formula of Li x Fe 1-y M y PO 4At least one of the shown compounds, where 0.9 ≤ x ≤ 1.1, 0 < y ≤ 0.8, and M includes at least one of Al, Mn, Ni, Co, W, Mo, Ti, and V.

[0068] In one embodiment, the positive electrode active material includes a first positive electrode active material, a second positive electrode active material, and a third positive electrode active material; the Dv50 particle size of the first positive electrode active material is D1, the Dv50 particle size of the second positive electrode active material is D2; the Dv50 particle size of the third positive electrode active material is D3, satisfying: D1 > D2 > D3. Controlling the particle sizes of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material to satisfy this relationship is beneficial to improving the compaction density of the positive electrode sheet, enhancing the wetting effect of the electrolyte, and increasing the lithium ion transmission rate. At the same time, it can improve the processing performance of the positive electrode slurry and the positive electrode sheet, which is beneficial to improving the capacity and rate performance of the secondary battery.

[0069] Among them, the Dv50 particle size represents the particle size value corresponding to when the volume cumulative distribution percentage of the positive electrode active material reaches 50%. The Dv50 of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material of the present invention is tested with reference to GB-T 19077-2016 using a particle size analysis laser diffractometer.

[0070] In one embodiment, 0.8 μm < D1 ≤ 2 μm, for example, it can be 0.801 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm or the range composed of any two of these values.

[0071] In one embodiment, 0.2 μm < D1 ≤ 0.8 μm, for example, it can be 0.201 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm or the range composed of any two of these values.

[0072] In one embodiment, 0.01 μm ≤ D1 ≤ 0.2 μm, for example, it can be 0.01 μm, 0.02 μm, 0.04 μm, 0.05 μm, 0.06 μm, 0.08 μm, 0.1 μm, 0.12 μm, 0.15 μm, 0.16 μm, 0.2 μm or the range composed of any two of these values.

[0073] By controlling the Dv50 of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material within the above ranges, the particle size distribution of the positive electrode active material presents a multi-stage gradation, the particle pores of the positive electrode active material are maximally filled, the wettability of the electrolyte to the secondary particles is effectively improved, the electron transfer between the positive electrode active materials is further enhanced, the stability of the positive electrode active material is improved, and the situation of breakage during the tablet pressing process is avoided. Furthermore, the cycle performance and storage stability of the secondary battery are improved.

[0074] In one embodiment, the mass percentage of the first positive electrode active material in the positive electrode active material is 1 to 30%, for example, it can be 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, or a range composed of any two of these values.

[0075] In one embodiment, the mass percentage of the second positive electrode active material in the positive electrode active material is 10 to 80%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or a range composed of any two of these values.

[0076] In one embodiment, the mass percentage of the third positive electrode active material in the positive electrode active material is 10 to 80%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or a range composed of any two of these values.

[0077] By controlling the contents of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material in the positive electrode active material, the insertion and extraction of lithium ions can be further promoted, the electron transfer between the positive electrode active materials can be enhanced, and they can be in more sufficient contact with the electrolyte. Furthermore, the cycle performance and storage stability of the secondary battery are improved.

[0078] In one embodiment, the type of the positive electrode current collector is not particularly limited, and it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc., and carbon materials such as carbon cloth and carbon paper.

[0079] Among them, the form of the positive electrode current collector is not particularly limited. When the positive electrode current collector is a metal material, the form of the positive electrode current collector can be a metal foil, a metal cylinder, a metal strip roll, a metal plate, a metal foil, a metal plate mesh, a stamped metal, a foamed metal, etc. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector can include, but is not limited to, a carbon plate, a carbon film, a carbon cylinder, etc.

[0080] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder.

[0081] In one embodiment, the secondary battery further includes a negative electrode plate, which 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, and the negative electrode active material layer includes a negative electrode active material.

[0082] In the present application, there is no particular limitation on the negative electrode current collector, as long as the object of the present application can be achieved. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or composite current collector, etc.

[0083] In one embodiment, the negative electrode active material can be natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO 2 , lithium titanate Li 4 Ti 5 O 12 , at least one of Li-Al alloy and metallic lithium, etc.

[0084] In one embodiment, the negative electrode active material layer further includes a conductive agent and a binder.

[0085] In one embodiment, there is no limitation on the type of the conductive agent mentioned in the present application, and any known conductive agent can be used.

[0086] In one embodiment, the conductive agent includes at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0087] In one embodiment, there is no limitation on the type of the binder mentioned in the present application, and any known positive electrode binder can be used.

[0088] In one embodiment, the binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer or its hydride, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene difluoride, and polytetrafluoroethylene-ethylene copolymer.

[0089] In the secondary battery mentioned in the present application, in order to prevent short - circuit, a separator is usually provided between the positive electrode and the negative electrode. There are no particular limitations on the material and shape of the separator, as long as the effects of the present application are not significantly impaired.

[0090] In one embodiment, the separator includes a porous sheet - like or non - woven fabric - like material with excellent liquid - retaining properties, etc. The materials of the resin or glass fiber separator include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc.

[0091] In one embodiment, the polyolefin is polyethylene or polypropylene. The materials of the above - mentioned separator can be used alone or in any combination.

[0092] In one embodiment, the secondary battery may include an outer package, which can be used to encapsulate the above - mentioned electrode assembly and electrolyte.

[0093] In one embodiment, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch - type soft package. The material of the soft package can be plastic. As plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. can be cited.

[0094] In one embodiment, the electrolyte further includes a lithium salt and an organic solvent. The specific types of the lithium salt and the organic solvent are not particularly limited and can be selected according to actual needs.

[0095] Among them, the lithium salt can include lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, etc., and the solvent can include ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and propyl propionate, etc.

[0096] The present application has no particular limitations on the shape of the secondary battery, which can be cylindrical, square, or any other shape.

[0097] The present application has no particular limitations on the shape of the secondary battery, which can be cylindrical, square, or any other shape.

[0098] One embodiment of the present application provides an electrical device, including the above - mentioned secondary battery, and the secondary battery serves as the power supply of the electrical device.

[0099] Exemplarily, the above - mentioned electrical device can include mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug - in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but is not limited thereto.

[0100] The following further elaborates the present application with specific examples:

[0101] Example 1

[0102] A method for preparing a secondary battery, comprising the following steps:

[0103] (1) Preparation of the positive electrode sheet

[0104] Preparation of the positive electrode active material: Weigh lithium carbonate, iron phosphate, glucose, and alumina and add them to a mixed solution, stir evenly, add ammonia water solution to adjust the pH to 9.8, carry out hydrothermal reaction at 180 °C for 5 h, naturally cool to room temperature, carry out suction filtration, washing (washed 3 times with distilled water and then 3 times with ethanol), and drying (dried in vacuum at 80 °C) to obtain a precursor. The masses of lithium carbonate and iron phosphate are calculated according to the molecular formula LiFePO 4 and the mass of glucose is calculated according to 4.5% of the mass of iron phosphate, and the mass of alumina is calculated according to 0.593% of the mass of iron phosphate. Sinter the above precursor at 750 °C for 6 h under a nitrogen protection atmosphere to obtain a positive electrode active material with an aluminum element content of 3000 ppm;

[0105] The prepared positive electrode active material is subjected to ball milling and screening to obtain a first positive electrode active material with Dv50 of 1 μm, a second positive electrode active material with Dv50 of 0.5 μm, and a third positive electrode active material with Dv50 of 0.2 μm;

[0106] Add the first positive electrode active material with Dv50 of 1 μm, the second positive electrode active material with Dv50 of 0.5 μm, and the third positive electrode active material with Dv50 of 0.2 μm into a sand mill and mix evenly according to a mass ratio of 5:20:75 to obtain a high-compaction positive electrode active material.

[0107] Add the above high-compaction positive electrode active material (corresponding to the active substance in Table 2), conductive agent SP, and binder PVDF according to a mass ratio of 97:1:2, and finally add N-methylpyrrolidone (NMP), stir at a speed of 8000 rpm for 24 h to obtain a uniformly mixed positive electrode slurry;

[0108] Coat the obtained positive electrode slurry on both sides of the aluminum foil at a surface density of 200 g / m 2 (single-sided), and dry at 120 °C for 5 min;

[0109] Roll press the obtained dried positive electrode sheet at a pole piece compaction of 2.6 g / cm 3 , and cut into strips to obtain the positive electrode sheet.

[0110] (2) Preparation of the negative electrode sheet

[0111] Graphite, thickener CMC, binder SBR, and conductive agent SP were mixed evenly in a ratio of 97:1:1:1, and then deionized water was added as a solvent. After sufficient stirring, the negative electrode slurry was evenly dispersed. The negative electrode slurry was evenly coated on both sides of the current collector copper foil, and then dried at 120 °C. After cold pressing, slitting, and blanking, the negative electrode sheet was obtained.

[0112] (3) Preparation of electrolyte

[0113] Organic solvents ethylene carbonate and ethyl methyl carbonate were mixed in a mass ratio of 3:7. Then, in a glove box with an argon atmosphere with a water content of <10 ppm, the fully dried lithium hexafluorophosphate was dissolved in the above organic solvents and mixed evenly to obtain an electrolyte, where the concentration of lithium hexafluorophosphate was 1 M.

[0114] (4) Assembly of secondary battery:

[0115] The positive electrode sheet, negative electrode sheet, and separator (a polypropylene film with a thickness of 12 μm) were wound into a bare battery core, then placed into an aluminum-plastic film. After baking, the electrolyte was injected, and then through formation, sealing, and grading, a 3 Ah secondary battery was obtained.

[0116] Among them, the parameters of Example 1 are shown in Table 1.

[0117] Among them, the SEM image of the cross-section (profile) of the positive electrode sheet of Example 1 at 30K magnification is as Figure 1 shown.

[0118] Example 2

[0119] The preparation method of the secondary battery of Example 2 was basically the same as that of Example 1, except that the mass ratio of the active material, conductive agent SP, and binder PVDF in the positive electrode sheet was 96:2:2.

[0120] Example 3

[0121] The preparation method of the secondary battery of Example 3 was basically the same as that of Example 1, except that the mass ratio of the active material, conductive agent SP, and binder PVDF in the positive electrode sheet was 96.5:1.5:2.

[0122] Example 4

[0123] The preparation method of the secondary battery of Example 4 was basically the same as that of Example 1, except that the mass ratio of the active material, conductive agent SP, and binder PVDF in the positive electrode sheet was 97:1:2.

[0124] Example 5

[0125] The preparation method of the secondary battery of Example 5 is basically the same as that of Example 1, except that the mass ratio of the active material, the conductive agent SP, and the binder PVDF in the positive electrode sheet is 97.5:0.5:2.

[0126] Example 6

[0127] The preparation method of the secondary battery of Example 6 is basically the same as that of Example 1, except that the mass ratio of the active material, the conductive agent SP, and the binder PVDF in the positive electrode sheet is 98:1:1.

[0128] Example 7

[0129] The preparation method of the secondary battery of Example 7 is basically the same as that of Example 1, except that the mass ratio of the active material, the conductive agent SP, and the binder PVDF in the positive electrode sheet is 98.5:0.5:1.

[0130] Example 8

[0131] The preparation method of the secondary battery of Example 8 is basically the same as that of Example 1, except that the mass ratio of the active material, the conductive agent SP, and the binder PVDF in the positive electrode sheet is 99:0.5:0.5.

[0132] Example 9

[0133] The preparation method of the secondary battery of Example 9 is basically the same as that of Example 5, except that the surface density (single-sided) of the positive electrode sheet is 60 g / m 2 。

[0134] Example 10

[0135] The preparation method of the secondary battery of Example 10 is basically the same as that of Example 5, except that the surface density (single-sided) of the positive electrode sheet is 100 g / m 2 。

[0136] Example 11

[0137] The preparation method of the secondary battery of Example 11 is basically the same as that of Example 5, except that the surface density (single-sided) of the positive electrode sheet is 140 g / m 2 。

[0138] Example 12

[0139] The preparation method of the secondary battery of Example 12 is basically the same as that of Example 5, except that the surface density (single-sided) of the positive electrode sheet is 180 g / m 2 。

[0140] Example 13

[0141] The preparation method of the secondary battery of Example 13 is basically the same as that of Example 5, except that the areal density (single-sided) of the positive electrode sheet is 220 g / m 2 .

[0142] Example 14

[0143] The preparation method of the secondary battery of Example 14 is basically the same as that of Example 5, except that the areal density (single-sided) of the positive electrode sheet is 260 g / m 2 .

[0144] Example 15

[0145] The preparation method of the secondary battery of Example 15 is basically the same as that of Example 5, except that the areal density (single-sided) of the positive electrode sheet is 300 g / m 2 .

[0146] Example 16

[0147] The preparation method of the secondary battery of Example 16 is basically the same as that of Example 5, except that the areal density (single-sided) of the positive electrode sheet is 320 g / m 2 .

[0148] Example 17

[0149] The preparation method of the secondary battery of Example 17 is basically the same as that of Example 5, except that the compaction density of the positive electrode sheet is 2.50 g / cm 3 .

[0150] Example 18

[0151] The preparation method of the secondary battery of Example 18 is basically the same as that of Example 5, except that the compaction density of the positive electrode sheet is 2.55 g / cm 3 .

[0152] Example 19

[0153] The preparation method of the secondary battery of Example 19 is basically the same as that of Example 5, except that the compaction density of the positive electrode sheet is 2.60 g / cm 3 .

[0154] Example 20

[0155] The preparation method of the secondary battery of Example 20 is basically the same as that of Example 5, except that the compaction density of the positive electrode sheet is 2.65 g / cm 3 .

[0156] Example 21

[0157] The preparation method of the secondary battery of Example 21 is basically the same as that of Example 5, except that the compaction density of the positive electrode sheet is 2.70 g / cm3 .

[0158] Example 22

[0159] The preparation method of the secondary battery of Example 22 is basically the same as that of Example 5, except that the positive electrode plate is compacted to 2.75 g / cm 3 .

[0160] Example 23

[0161] The preparation method of the secondary battery of Example 23 is basically the same as that of Example 5, except that the positive electrode plate is compacted to 2.80 g / cm 3 .

[0162] Example 24

[0163] The preparation method of the secondary battery of Example 24 is basically the same as that of Example 5, except that the positive electrode plate is compacted to 2.90 g / cm 3 .

[0164] Examples 25 to 28

[0165] The preparation method of the secondary battery of Examples 25 to 28 is basically the same as that of Example 1, except that by changing the mass ratio of iron phosphate and alumina, the value of R is changed accordingly.

[0166] Among them, the mass of alumina in Example 25 is calculated according to 0.0198% of the mass of iron phosphate.

[0167] Among them, the mass of alumina in Example 26 is calculated according to 0.198% of the mass of iron phosphate.

[0168] Among them, the mass of alumina in Example 27 is calculated according to 1.977% of the mass of iron phosphate.

[0169] Among them, the mass of alumina in Example 28 is calculated according to 59.321% of the mass of iron phosphate.

[0170] Examples 29 to 37

[0171] The preparation methods of Examples 29 to 31 are basically the same as that of Example 1, except that by changing the grinding and screening, the Dv50 of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material is changed.

[0172] The preparation methods of Examples 32 to 37 are basically the same as that of Example 1, except that the ratios of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material are changed.

[0173] Comparative Example 1

[0174] The preparation method of the secondary battery of Comparative Example 1 is basically the same as that of Example 5, except that the mass ratio of the active material, conductive agent SP and binder PVDF in the positive electrode sheet is 99:0.5:0.5, and the positive electrode sheet is compacted to 2.95 g / cm 3 , and by changing the mass ratio of iron phosphate and alumina, the value of R is further changed.

[0175] The mass of alumina in Comparative Example 1 is calculated according to 79.095% of the mass of iron phosphate.

[0176] Comparative Example 2

[0177] The preparation method of the secondary battery of Comparative Example 2 is basically the same as that of Example 5, except that the positive electrode sheet is compacted to 2.1 g / cm 3 , and by changing the mass ratio of iron phosphate and alumina, the value of R is further changed.

[0178] The mass of alumina in Comparative Example 2 is calculated according to 0.0158% of the mass of iron phosphate.

[0179] Table 1 Parameter Table

[0180]

[0181]

[0182] Table 2 Parameter Table

[0183]

[0184]

[0185] Table 3

[0186]

[0187]

[0188] Performance Test

[0189] 1. Specific capacity: At room temperature, charge at a constant current of 0.33C until 3.65V, then charge at a constant voltage of 3.65V until the current is less than 0.05C. After standing for 5 min, discharge at a current of 0.33C until 2.5V to obtain the first discharge specific capacity.

[0190] 2. Low-temperature performance: At room temperature, charge at a constant current of 1C to 3.65V, then charge at a constant voltage until the current is less than 0.05C, let it stand for 5 minutes, then discharge at a current of 1C to 2.5V to obtain the cell capacity C0. At room temperature, charge at a constant current of 1C to 3.65V, then charge at a constant voltage until the current is less than 0.05C, let it stand for 5 minutes, then adjust the temperature to -20°C, let it stand for 120 minutes, and discharge at 1C to 2.5V to obtain the low-temperature capacity C. The low-temperature capacity retention rate = C / C0.

[0191] 3. Cycle performance test: At 45°C, charge at a constant current of 4C to 3.65V, then charge at a constant voltage of 3.65V until the current is less than 0.05C, let it stand for 5 minutes, and then discharge at a current of 1C to 2.5V. Repeat the charge and discharge cycle like this. The capacity retention rate of the 9000th cycle = the capacity of the 1000th cycle / the capacity of the first cycle × 100%.

[0192] 4. Storage performance test: Charge at a constant current of 1C to 3.65V, then charge at a constant voltage of 3.65V until the current is less than 0.05C. Place the fully charged battery in an oven at 60°C, take out the battery every 15 days, move it to room temperature and let it stand for 2 hours. Discharge at a current of 1C to 2.5C, let it stand for 10 minutes, charge at a current of 1C to 3.65V, then charge at a constant voltage of 3.65V until the current is less than 0.05C, let it stand for 5 minutes, and then discharge at a current of 1C to 2.5V to obtain the restored capacity.

[0193] 5. DCR test: At room temperature, charge at a constant current of 1C to 3.65V, then charge at a constant voltage until the current is less than 0.05C, let it stand for 5 minutes, then discharge at a current of 1C for 30 minutes to adjust the battery to 50% SOC. Discharge the battery at a rate of 5C for 10s and record the voltage drop ΔV. From DCR = ΔV / 5C, the DCR of the battery can be calculated.

[0194] Table 4 Performance test results

[0195]

[0196]

[0197] As can be seen from Table 3, in the present invention, by controlling the number of pores per unit area, the porosity, and the mass percentage content of element M in the positive active material in the SEM image of the cross-section of the positive electrode sheet, it satisfies: 0.4*10 3 ≤N×P×R≤1053*10 3; It can significantly improve the kinetic performance of the positive electrode sheet, facilitate the electron transfer between the positive active materials, enhance the wetting effect of the electrolyte, improve the diffusion path of lithium ions, increase the insertion / extraction speed of lithium ions, effectively improve the specific capacity of the secondary battery, significantly improve the cycle performance and storage performance of the secondary battery, and reduce the impedance of the secondary battery.

[0198] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery, characterized in that: The invention comprises a positive electrode sheet, wherein the positive electrode sheet comprises 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 active material layer comprises a positive electrode active material, wherein the positive electrode active material comprises an M element, and the M element comprises at least one of Al, Mn, Ni, Co, W, Mo, Ti, and V; The positive electrode sheet meets the following requirements: 0.4*10 3 ≤N×P×R≤1053*10 3 ; Among them, N / μm 2 is the number of pores per unit area in the SEM image of the cross section of the positive electrode sheet; P is the porosity of the positive electrode sheet; R ppm is the mass percentage of the M element in the positive electrode active material.

2. The secondary battery according to claim 1, characterized in that: The positive electrode sheet meets the following requirements: 1.8*10 3 ≤N×P×R≤36.5*10 3 .

3. The secondary battery according to claim 1, characterized in that: Satisfy at least one of the following (1) to (3): (1)1≤N≤60； (2)10％≤P≤60％； (3)100≤R≤300000。 4. The secondary battery according to claim 1, characterized in that: The positive electrode sheet satisfies: 1≤N×L≤57.3; Wherein, L is the mass percentage of the positive electrode active material in the positive electrode sheet.

5. The secondary battery according to claim 4, characterized in that: The L satisfies: 95.5%≤L≤99%.

6. The secondary battery according to claim 1, characterized in that: The positive electrode sheet meets the following requirements: 0.2*10 3 ≤N*C≤12*10 3 ; Where, C g / m 2 It is the single-sided surface density of the positive electrode sheet.

7. The secondary battery according to claim 6, characterized in that: The C satisfies: 60≤C≤320.

8. The secondary battery according to claim 1, characterized in that: The positive electrode sheet satisfies: 0.4≤N / D≤23.1; Where, D g / cm 3 is the compaction density of the positive electrode.

9. The secondary battery according to claim 8, characterized in that: The D satisfies: 2.5≤D≤2.

9.

10. The secondary battery according to claim 1, characterized in that: The positive electrode active material includes a lithium-containing phosphate, and the lithium-containing phosphate includes a compound represented by the chemical general formula Li x Fe 1-y M y PO4, where 0.9 ≤ x ≤ 1.1, 0 < y ≤ 0.8, and M includes at least one of Al, Mn, Ni, Co, W, Mo, Ti, and V.

11. The secondary battery according to claim 1, characterized in that: The positive electrode active material includes a first positive electrode active material, a second positive electrode active material and a third positive electrode active material; the particle size Dv50 of the first positive electrode active material is D1, the particle size Dv50 of the second positive electrode active material is D2; the particle size Dv50 of the third positive electrode active material is D3, satisfying: D1>D2>D3.

12. The secondary battery according to claim 11, characterized in that: Satisfy at least one of the following (4) to (6): (4) 0.8 μm <D1≤2μm; (5) 0.2 μm <D2≤0.8μm; (6)0.01μm≤D3≤0.2μm.

13. The secondary battery according to claim 11, characterized in that: Satisfy at least one of the following (7) to (9): (7) The mass percentage of the first positive electrode active material to the positive electrode active material is 1 to 30%; (8) The mass percentage of the second positive electrode active material to the positive electrode active material is 10 to 80%; (9) The mass percentage of the third positive electrode active material to the positive electrode active material is 10 to 80%.

14. An electrical device, characterized in that: It comprises the secondary battery according to any one of claims 1 to 13, wherein the secondary battery is used as a power supply for the electrical device.

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