Secondary battery and electric device

Abstract

This application provides a secondary battery and an electrical device. The secondary battery described in this application uses a lithium phosphate containing an olivine structure as the positive electrode active material. In the SEM image of the positive electrode cross-section of the secondary battery, the number N of positive electrode active material with a particle size less than 0.2 μm per unit cross-sectional area is shown. S The number N of positive electrode active materials with a particle size greater than 1 μm B The number N of positive electrode active materials with a particle size less than or equal to 1 μm and greater than or equal to 0.2 μm. M Satisfies: 0.57 ≤ (N) S +N B ) / N M With a value of ≤50.1, the energy density, high and low temperature cycle performance, and rate performance of the secondary battery are effectively improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0002] Lithium phosphates with an olivine structure have been widely used in new energy vehicles and large-scale energy storage industries due to their advantages of low cost, long lifespan, and high safety. Despite the broad market demand, compaction density and low-temperature performance remain significant shortcomings of lithium phosphates, hindering the development of the lithium battery industry. As consumers demand higher energy density and superior high and low temperature performance from lithium batteries, next-generation power and energy storage batteries are placing even greater demands on the compaction and high and low temperature performance of lithium phosphates. Therefore, developing lithium phosphate materials with ultra-high compaction and excellent high and low temperature performance is extremely urgent and has broad application prospects, which is of great significance for promoting the development of both new energy vehicles and energy storage industries. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and an electrical device to improve the energy density, high and low temperature cycle performance and rate performance of a secondary battery that uses lithium phosphate as the positive electrode active material.

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one side of the positive current collector, the positive active material layer comprising a positive active material, the positive active material comprising a lithium phosphate having an olivine structure;

[0005] The positive electrode plate satisfies: 0.57 ≤ N ≤ 50.1, N = (N S +N B ) / N M , where N S per μm 2 The SEM image of the cross-section of the positive electrode sheet shows the number of positive electrode active materials with a particle size less than 0.2 μm per unit cross-sectional area; N B per μm 2 The number of positive electrode active materials with a particle size greater than 1 μm per unit cross-sectional area in the SEM image of the positive electrode sheet; N M per μm 2 The SEM image of the cross-section of the positive electrode sheet shows the number of positive electrode active materials with a particle size less than or equal to 1 μm and greater than or equal to 0.2 μm per unit cross-sectional area.

[0006] As an embodiment of this application, the positive electrode sheet satisfies: 5 ≤ N S ≤50.

[0007] As an embodiment of this application, the positive electrode sheet satisfies: 1 ≤ N M ≤10.

[0008] As an embodiment of this application, the positive electrode sheet satisfies: 0 <N B ≤0.4.

[0009] As an embodiment of this application, the film resistance of the positive electrode is R, which satisfies: R≤580mΩ.

[0010] As an embodiment of this application, the bonding force of the positive electrode sheet is A, which satisfies: A≥12N / m.

[0011] As an embodiment of this application, the porosity P of the positive electrode sheet satisfies: 20% ≤ P ≤ 40%.

[0012] As an embodiment of this application, the positive electrode sheet satisfies: 92≤N*P*R≤3510, where P is the porosity of the positive electrode sheet; and R mΩ is the film resistance of the positive electrode sheet.

[0013] A second aspect of this application provides an electrical device comprising the secondary battery described in the first aspect of this application.

[0014] Compared with the prior art, the beneficial effects of this application are:

[0015] This application provides a secondary battery that uses lithium phosphate with an olivine structure as the positive electrode active material, and controls the number N of positive electrode active material with a particle size of less than 0.2 μm per unit cross-sectional area in the SEM image of the positive electrode sheet of the secondary battery. S The number N of positive electrode active materials with a particle size greater than 1 μm B The number N of positive electrode active materials with a particle size less than or equal to 1 μm and greater than or equal to 0.2 μm. M Satisfies: 0.57 ≤ (N) S +N B ) / N M With a value of ≤50.1, the energy density, high and low temperature cycle performance, and rate performance of the secondary battery are effectively improved. Attached Figure Description

[0016] Figure 1 This is a SEM image of the cross-section of the positive electrode sheet prepared in Example 1. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0019] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0020] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0021] An embodiment of this application provides a secondary battery, the secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one side of the positive current collector, the positive active material layer comprising a positive active material, the positive active material comprising a lithium phosphate having an olivine structure;

[0022] The positive electrode plate satisfies: 0.57 ≤ N ≤ 50.1, N = (N S +N B ) / N M , where N S per μm 2 The SEM image of the cross-section of the positive electrode sheet shows the number of positive electrode active materials with a particle size less than 0.2 μm per unit cross-sectional area; N B per μm 2 N represents the number of positive electrode active materials with a particle size greater than 1 μm per unit cross-sectional area in the SEM image of the positive electrode cross-section. M per μm 2 The SEM image of the cross-section of the positive electrode sheet shows the number of positive electrode active materials with a particle size less than or equal to 1 μm and greater than or equal to 0.2 μm per unit cross-sectional area.

[0023] The inventors of this application have discovered that, for secondary batteries using lithium phosphate as the positive electrode active material, by controlling the number of particles with a particle size within a certain range per unit cross-sectional area in the positive electrode sheet, the compaction density of the positive electrode sheet can be improved. This results in denser particle contact, lower electrode porosity, and lower membrane resistance, reducing the amount of conductive agent and binder required, increasing the proportion of active material in the positive electrode sheet, and thus improving the energy density of the secondary battery. Furthermore, it can shorten the diffusion path of active ions, improve the kinetics of the positive electrode sheet, and thereby improve the low-temperature performance of the secondary battery.

[0024] In this application, the method for testing the number of particles per unit cross-sectional area of ​​the positive electrode sheet includes the following steps: After the positive electrode sheet is ion-cut, the cross-section of the electrode sheet is subjected to SEM detection to obtain an SEM image of the electrode sheet at 30K; the number of particles per unit cross-sectional area in the positive electrode sheet is calculated using the following formula: N x =n x / (h / p*w / p), where n x This indicates the number of particles (individuals) within different particle size ranges in the 30K magnification SEM image of the positive electrode cross-section; h and w represent the actual measured length and width of the 30K magnification SEM image of the positive electrode cross-section, respectively, in cm; p represents the actual measured length with a scale bar of 0.2 μm in the 30K magnification SEM image of the positive electrode cross-section, in cm / μm. Since the positive electrode active material particles are irregularly shaped, the particle size is based on the longest diameter; also, if only half a particle is visible in the image, it is counted as one particle.

[0025] In some embodiments, the positive electrode plate satisfies: 5 ≤ N S ≤50. For example, in the SEM image of the positive electrode cross-section, the number N of positive electrode active material with a particle size less than 0.2 μm per unit cross-sectional area. S It can be a value within the range of 5, 10, 15, 18, 20, 22, 25, 27, 30, 35, 38, 40, 45, 50, or any two of the above. The number N of positive electrode active materials with a particle size less than 0.2 μm. S Within the aforementioned range, it can effectively fill the gaps in the positive electrode sheet, improve the compaction ability of the positive electrode sheet, and does not affect the processing performance of the positive electrode sheet. In some embodiments, the positive electrode sheet satisfies: 18 ≤ N S ≤32.

[0026] In some embodiments, the positive electrode plate satisfies: 1 ≤ N M ≤10. For example, in the SEM image of the positive electrode cross-section, the number N of positive electrode active material with a particle size less than or equal to 1 μm and greater than or equal to 0.2 μm per unit cross-sectional area. MIt can be a value from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or any two of the above points. In some embodiments, the positive electrode plate satisfies: 0 <N B ≤0.4. For example, in the SEM image of the positive electrode cross-section, the number N of positive electrode active material with a particle size greater than 1 μm per unit cross-sectional area. B It can be a value within the range of 0.1, 0.2, 0.3, 0.4, or any two of the above points. N M and N B Within the aforementioned range, the compaction capability of the positive electrode sheet can be improved, while ensuring that active ions have suitable diffusion paths, thus guaranteeing superior low-temperature performance of the electrode sheet. In some embodiments, the positive electrode sheet satisfies: 2 ≤ N M ≤7.

[0027] In some embodiments, the film resistance of the positive electrode is R, satisfying: R ≤ 580mΩ. For example, the film resistance R of the positive electrode can be 280mΩ, 300mΩ, 310mΩ, 320mΩ, 350mΩ, 380mΩ, 400mΩ, 450mΩ, 460mΩ, 480mΩ, 500mΩ, 550mΩ, 560mΩ, 580mΩ, or a value within a range consisting of any two of the above. In some embodiments, the film resistance R of the positive electrode satisfies: 280mΩ ≤ R ≤ 580mΩ.

[0028] In some embodiments, the bonding force of the positive electrode sheet is A, which satisfies: A≥12N / m. For example, the bonding force A of the positive electrode sheet can be 12N / m, 13N / m, 14N / m, 15N / m, 16N / m, 17N / m, 18N / m, 19N / m, 20N / m, or a value within the range formed by any two of the above points.

[0029] In some embodiments, 12 N / m ≤ A ≤ 20 N / m.

[0030] In some embodiments, the compaction density of the positive electrode sheet is 2.6 g / cm³. 3 above.

[0031] In some embodiments, the porosity P of the positive electrode sheet satisfies: 20% ≤ P ≤ 40%. For example, the porosity P of the positive electrode sheet can be 20%, 23%, 26%, 29%, 32%, 38%, 40%, or a value within a range consisting of any two of the above points.

[0032] In some embodiments, the porosity P of the positive electrode sheet satisfies: 25% ≤ P ≤ 36%.

[0033] In some embodiments, the positive electrode sheet satisfies: 92 ≤ N*P*R ≤ 3510, where P is the porosity of the positive electrode sheet; and R mΩ is the film resistance of the positive electrode sheet. In some embodiments, the positive electrode sheet satisfies: 370 ≤ N*P*R ≤ 1660.

[0034] In some embodiments, the positive electrode plate satisfies: 2≤N M ≤7, 18≤N S ≤32, 370≤N*P*R≤1660, where P is the porosity of the positive electrode sheet; R mΩ is the film resistance of the positive electrode sheet. When the positive electrode sheet meets the above conditions, the energy density, high and low temperature cycle performance, and rate performance of the secondary battery can be further balanced.

[0035] In some embodiments, the secondary battery further includes a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer comprising a negative electrode material.

[0036] The types of anode materials are not specifically limited and can be selected according to actual needs. For example, anode materials can be artificial graphite, natural graphite, silicon-carbon composite materials, silicon suboxide, hard carbon, lithium metal, and lithium titanate, etc.

[0037] The type of negative electrode current collector is not specifically limited and can be selected according to actual needs. Copper foil or carbon-coated copper foil is preferred.

[0038] In some embodiments, the secondary battery further includes a separator. The type of separator is not particularly limited and can be selected according to actual needs. The separator can be a polypropylene membrane, polyethylene membrane, polyvinylidene fluoride membrane, spandex membrane, aramid membrane, or a multilayer composite membrane modified with a coating.

[0039] In some embodiments, the preparation of a secondary battery includes: stacking a positive electrode, a separator, and a negative electrode in sequence, with the separator acting as a separator between the positive and negative electrodes; winding the separator into a square bare cell; inserting it into a battery casing; baking it at 65–95°C to remove water; injecting electrolyte; sealing the casing; and then performing processes such as settling, hot and cold pressing, formation, clamping, and capacity testing to obtain a secondary battery.

[0040] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft pack, such as a pouch-type soft pack, and the material of the soft pack may be plastic, such as one or more of polypropylene, polybutylene terephthalate, polybutylene succinate, etc.

[0041] In some embodiments, the shape of the secondary battery is not particularly limited, and it can be cylindrical, square or other arbitrary shapes.

[0042] A second aspect of this application provides an electrical device comprising the secondary battery described in the first aspect of this application. The electrical device can be an application device such as a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, and power tool. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. This application does not impose special limitations on the above-described device.

[0043] The following are specific embodiments of this application, and the technical solutions of this application are further described in conjunction with the embodiments. However, this application is not limited to these embodiments. Unless otherwise specified, the reagents, methods, and equipment used in this application are all conventional reagents, methods, and equipment in this technical field.

[0044] Example 1

[0045] Example 1 provides a method for preparing a positive electrode sheet, comprising the following steps:

[0046] (1) D 50 Large particles of 1μm, D 50 Medium particles of 0.5 μm, D 50 Small LiFePO4 particles with a diameter of 0.2 μm were added to a high-speed mixer at a mass ratio of 10:10:80 and mixed evenly. Then, conductive agent SP and binder PVDF were added at a mass ratio of 97:1:2. Finally, N-methylpyrrolidone (NMP) was added. The mixture was stirred at 8000 rpm for 24 hours to obtain a uniformly mixed positive electrode slurry.

[0047] (2) The obtained slurry is prepared according to 200g / m 2 The areal density of (single-sided) coating is applied to both sides of the aluminum foil and dried at 120℃ for 5 minutes;

[0048] (3) The obtained dried positive electrode sheet is prepared according to 2.6 g / cm 3 The electrode sheets are compacted and rolled, and then slit to obtain positive electrode sheets.

[0049] Example 2

[0050] The preparation method of the positive electrode sheet in Example 2 is basically the same as that in Example 1, except that the ratio of large, medium and small particles is 15:15:70.

[0051] Example 3

[0052] The preparation method of the positive electrode sheet in Example 3 is basically the same as that in Example 1, except that the ratio of large, medium and small particles is 20:20:60.

[0053] Example 4

[0054] The preparation method of the positive electrode sheet in Example 4 is basically the same as that in Example 1, except that the ratio of large, medium and small particles is 25:25:50.

[0055] Example 5

[0056] The preparation method of the positive electrode sheet in Example 5 is basically the same as that in Example 1, except that the ratio of large, medium and small particles is 10:50:40.

[0057] Example 6

[0058] The preparation method of the positive electrode sheet in Example 6 is basically the same as that in Example 1, except that the ratio of large, medium and small particles is 10:60:30.

[0059] Example 7

[0060] The preparation method of the positive electrode sheet in Example 7 is basically the same as that in Example 1, except that the ratio of large, medium and small particles is 10:70:20.

[0061] Example 8

[0062] The preparation method of the positive electrode sheet in Example 8 is basically the same as that in Example 1, except that the ratio of large, medium and small particles is 10:80:10.

[0063] Example 9

[0064] The preparation method of the positive electrode sheet in Example 9 is basically the same as that in Example 1, except that D is used. 50 Replace 1μm large particles with D 50 It is a large-particle LiFePO4 material with a particle size of 2μm.

[0065] Example 10

[0066] The preparation method of the positive electrode sheet in Example 10 is basically the same as that in Example 2, except that D is used. 50 Replace 1μm large particles with D 50It is a large-particle LiFePO4 material with a particle size of 2μm.

[0067] Example 11

[0068] The preparation method of the positive electrode sheet in Example 11 is basically the same as that in Example 3, except that D is used. 50 Replace 1μm large particles with D 50 It is a large-particle LiFePO4 material with a particle size of 2μm.

[0069] Example 12

[0070] The preparation method of the positive electrode sheet in Example 12 is basically the same as that in Example 4, except that D is used. 50 Replace 1μm large particles with D 50 It is a large-particle LiFePO4 material with a particle size of 2μm.

[0071] Example 13

[0072] The preparation method of the positive electrode sheet in Example 13 is basically the same as that in Example 5, except that D is used. 50 Replace 1μm large particles with D 50 It is a large-particle LiFePO4 material with a particle size of 2μm.

[0073] Example 14

[0074] The preparation method of the positive electrode sheet in Example 14 is basically the same as that in Example 6, except that D is used. 50 Replace 1μm large particles with D 50 It is a large-particle LiFePO4 material with a particle size of 2μm.

[0075] Example 15

[0076] The preparation method of the positive electrode sheet in Example 15 is basically the same as that in Example 7, except that D is used. 50 Replace 1μm large particles with D 50 It is a large-particle LiFePO4 material with a particle size of 2μm.

[0077] Example 16

[0078] The preparation method of the positive electrode sheet in Example 16 is basically the same as that in Example 8, except that D is used. 50 Replace 1μm large particles with D 50 It is a large-particle LiFePO4 material with a particle size of 2μm.

[0079] Example 17

[0080] The preparation method of the positive electrode sheet in Example 17 is basically the same as that in Example 1, except that D is used. 50Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0081] Example 18

[0082] The preparation method of the positive electrode sheet in Example 18 is basically the same as that in Example 2, except that D is used. 50 Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0083] Example 19

[0084] The preparation method of the positive electrode sheet in Example 19 is basically the same as that in Example 3, except that D is used. 50 Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0085] Example 20

[0086] The preparation method of the positive electrode sheet in Example 20 is basically the same as that in Example 4, except that D is used. 50 Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0087] Example 21

[0088] The preparation method of the positive electrode sheet in Example 21 is basically the same as that in Example 5, except that D is used. 50 Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0089] Example 22

[0090] The preparation method of the positive electrode sheet in Example 22 is basically the same as that in Example 6, except that D is used. 50 Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0091] Example 23

[0092] The preparation method of the positive electrode sheet in Example 23 is basically the same as that in Example 7, except that D is used. 50 Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0093] Example 24

[0094] The preparation method of the positive electrode sheet in Example 24 is basically the same as that in Example 8, except that D is used. 50 Replace 0.2μm small particles with D 50 It is a small-particle LiFePO4 material with a particle size of 0.1 μm.

[0095] Comparative Example 1

[0096] The preparation method of the positive electrode sheet in Comparative Example 1 is basically the same as that in Example 1, except that the positive electrode active material is only D. 50 It is a large-particle LiFePO4 material with a particle size of 1μm.

[0097] Comparative Example 2

[0098] The preparation method of the positive electrode in Comparative Example 2 is basically the same as that in Example 1, except that the positive electrode active material is only D. 50 It is a medium-particle LiFePO4 material with a particle size of 0.5 μm.

[0099] Comparative Example 3

[0100] The preparation method of the positive electrode in Comparative Example 3 is basically the same as that in Example 1, except that the positive electrode active material is only D. 50 It is a small-particle LiFePO4 material with a particle size of 0.2 μm.

[0101] Comparative Example 4

[0102] The preparation method of the positive electrode sheet in Comparative Example 4 is basically the same as that in Example 1, except that the positive electrode active material is D. 50 Large particles of 1μm and D 50 The two materials are medium-sized LiFePO4 particles with a particle size of 0.5 μm and a mass ratio of 50:50.

[0103] Comparative Example 5

[0104] The preparation method of the positive electrode sheet in Comparative Example 5 is basically the same as that in Example 1, except that the positive electrode active material is D. 50 Medium particles of 0.5 μm and D 50 The two materials are small LiFePO4 particles with a size of 0.2 μm and a mass ratio of 50:50.

[0105] Comparative Example 6

[0106] The preparation method of the positive electrode sheet in Comparative Example 6 is basically the same as that in Example 1, except that the positive electrode active material is D. 50 Large particles of 1μm and D 50 The two materials are small LiFePO4 particles with a size of 0.2 μm and a mass ratio of 50:50.

[0107] Comparative Example 7

[0108] The preparation method of the positive electrode sheet in Comparative Example 7 is basically the same as that in Example 1, except that the mass ratio of large, medium and small particles is 80:10:10.

[0109] The physical properties of the positive electrode sheets in Examples 1-24 and Comparative Examples 1-7 are shown in Table 1; taking Example 1 as an example, the SEM image of the cross-section of the positive electrode sheet is shown in Table 1. Figure 1 The relevant particle size characteristics per unit cross-sectional area can be obtained from the figure. The specific characteristics are listed in Table 1. The SEM images of the positive electrode cross-sections prepared in other embodiments and comparative examples are similar. Therefore, they will not be listed and shown one by one in the following. All relevant information can be found in Table 1.

[0110] Table 1

[0111]

[0112]

[0113] Electrochemical performance testing:

[0114] Graphite, thickener CMC, binder SBR, and conductive agent SP are mixed evenly in a ratio of 97:1:1:1. Deionized water is added as a solvent, and the mixture is stirred thoroughly to ensure uniform dispersion of the slurry. The negative electrode slurry is then evenly coated on both sides of the current collector copper foil, dried at 120°C, cold-pressed, slit, and cut into sheets to obtain the negative electrode sheet.

[0115] After the positive electrode, negative electrode, and separator (a 12μm thick polypropylene film) are wound into a bare cell, they are placed in an aluminum-plastic film, baked, and then injected with electrolyte (EC / EMC = 3 / 7, 1M lithium hexafluorophosphate). After formation, sealing, and capacity testing, a 3Ah soft-pack battery is obtained. The following performance tests are then conducted, and the test results are shown in Table 2.

[0116] Specific capacity: At room temperature, charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage of 3.65V until the current is less than 0.05C. After standing for 5 minutes, discharge at a current of 0.33C to 2.5V to obtain the initial discharge specific capacity.

[0117] Low temperature: At room temperature, the battery is charged at a constant current of 1C to 3.65V, then charged at a constant voltage until the current is less than 0.05C. After standing for 5 minutes, it is discharged at a current of 1C to 2.5V, yielding the secondary battery capacity C0. At room temperature, the battery is charged at a constant current of 1C to 3.65V, then charged at a constant voltage until the current is less than 0.05C. After standing for 5 minutes, the temperature is adjusted to -20℃, and the battery is stood for 120 minutes. It is then discharged at 1C to 2.5V, yielding the low-temperature capacity C. Low-temperature capacity retention = C / C0.

[0118] Rate performance: At room temperature, the capacitor is charged at a constant current of 1C to 3.65V, then charged at a constant voltage of 3.65V until the current is less than 0.05C. After resting for 5 minutes, it is discharged at a current of 4C to 2.5V to obtain the 4C discharge capacity. 4C rate = 4C discharge capacity / 1C discharge capacity × 100%.

[0119] Cyclic performance test: At 45℃, 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. After resting for 5 minutes, discharge at a current of 1C to 2.5V. Repeat this charge-discharge cycle. The capacity retention rate after 900 cycles = (capacity after 900 cycles / capacity after the first cycle) × 100%.

[0120] Table 2

[0121]

[0122]

[0123] As can be seen from the above embodiments and comparative examples, this application controls the number N of positive electrode active materials with a particle size of less than 0.2 μm per unit cross-sectional area in the SEM image of the positive electrode cross-section of the secondary battery. S The number N of positive electrode active materials with a particle size greater than 1 μm B The number N of positive electrode active materials with a particle size less than or equal to 1 μm and greater than or equal to 0.2 μm. M Satisfies: 0.57 ≤ (N) S +N B ) / N M The value ≤50.1 effectively improves the energy density, high and low temperature cycle performance, and rate performance of the secondary battery. Furthermore, comparing Examples 1-24, it can be seen that by controlling the positive electrode of the secondary battery to satisfy: 370≤N*P*R≤1660, the energy density, high and low temperature cycle performance, and rate performance of the secondary battery can be further balanced.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, characterized in that, The positive electrode includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector. The positive active material layer contains a positive active material, which contains a lithium phosphate having an olivine structure. The positive electrode plate satisfies: 0.57 ≤ N ≤ 50.1, N = (N S +N B ) / N M , where N S per μm 2 The SEM image of the cross-section of the positive electrode sheet shows the number of positive electrode active materials with a particle size less than 0.2 μm per unit cross-sectional area; N B per μm 2 The number of positive electrode active materials with a particle size greater than 1 μm per unit cross-sectional area in the SEM image of the positive electrode sheet; N M per μm 2 The number of positive electrode active materials with a particle size less than or equal to 1 μm and greater than or equal to 0.2 μm per unit cross-sectional area in the SEM image of the positive electrode sheet; 5≤N S ≤50;1≤N M ≤10;0<N B ≤0.4; The method for testing the number of particles per unit cross-sectional area of ​​the positive electrode sheet includes the following steps: After ion cutting, the cross-section of the positive electrode sheet is subjected to SEM examination to obtain an SEM image of the electrode sheet at 30K; the number of particles per unit cross-sectional area in the positive electrode sheet is calculated using the following formula: N x =n x / (h / p*w / p), where n x The number of particles in different size ranges is represented in the SEM image of the positive electrode cross-section at a magnification of 30K; h and w represent the actual measured length and width of the positive electrode cross-section at a magnification of 30K, respectively, in cm; p represents the actual measured length with a scale bar of 0.2 μm in the SEM image of the positive electrode cross-section at a magnification of 30K, in cm / μm.

2. The secondary battery according to claim 1, characterized in that, The film resistance of the positive electrode is R, which satisfies: R≤580mΩ.

3. The secondary battery according to claim 1, characterized in that, The bonding force of the positive electrode sheet is A, which satisfies: A≥12N / m.

4. The secondary battery according to claim 1, characterized in that, The porosity of the positive electrode sheet is P, which satisfies: 20%≤P≤40%.

5. The secondary battery according to claim 1, characterized in that, The positive electrode sheet satisfies: 92≤N*P*R≤3510, where P is the porosity of the positive electrode sheet; RmΩ is the film resistance of the positive electrode sheet.

6. The secondary battery according to claim 1, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: （1）2≤N M ≤7； （2）18≤N S ≤32； (3) 370≤N*P*R≤1660, where P is the porosity of the positive electrode and RmΩ is the film resistance of the positive electrode.

7. An electrical device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 6.

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