Secondary battery and electric device

Abstract

The invention discloses a secondary battery and an electric device, and belongs to the technical field of batteries. According to the secondary battery provided by the invention, the first positive electrode active material layer is arranged in the thinned region of the positive electrode plate, the second positive electrode active material layer is arranged in the main body region of the positive electrode plate, and the Dv50a of the first positive electrode active material is controlled to be greater than the Dv50b of the second positive electrode active material; the average particle size Da of primary particles of the first conductive agent is smaller than the average particle size Db of primary particles of the second conductive agent; according to the present invention, the lithium precipitation phenomenon at the negative electrode tab side can be effectively relieved, the cycle performance and the safety performance of the secondary battery can be improved, and the prepared secondary battery has the high capacity.

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] With the urgent demand for fast charging capabilities of power batteries in the domestic passenger vehicle market, higher requirements have been placed on their fast charging life performance and safety indicators.

[0003] To address the issue of thick edges on the electrodes caused by surface tension during coating, which can lead to edge bursting during subsequent rolling, secondary batteries employ a common solution: a chamfered coating pad. This chamfer thins the electrode edges, reducing their coating thickness and weight compared to the center. This area is defined as the thinned zone. However, this also leads to lithium plating in the OH (overhang) thinned zone during cycling or at low temperatures, impacting the battery's safety and reliability.

[0004] Therefore, how to solve the lithium plating problem that occurs in the thinned area below the tab while maintaining capacity (e.g.) Figure 1 The issue shown is crucial and remains a challenge for the lithium battery industry. Summary of the Invention

[0005] The purpose of this application is to solve the technical problem in the prior art that lithium deposition occurs on the negative electrode sheet in the thinned area below the tab, resulting in poor cycle performance and safety performance of the secondary battery. The application proposes a secondary battery and power device with low risk of lithium deposition below the negative electrode sheet, low internal resistance, and excellent cycle performance, safety performance and high capacity of the resulting secondary battery.

[0006] To achieve the above objectives, a first aspect of this application provides a secondary battery, the secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive electrode sheet having a first direction, along the first direction, the positive electrode sheet including an adjacently disposed main body region and a thinned region, the positive active material layer including a first positive active material layer and a second positive active material layer, the first positive active material layer being disposed in the thinned region of the positive electrode sheet, and the second positive active material layer being disposed in the main body region of the positive electrode sheet; The first positive electrode active material layer includes a first positive electrode active material and a first conductive agent, and the second positive electrode active material layer includes a second positive electrode active material and a second conductive agent; The first positive electrode active material Dv50a > the second positive electrode active material Dv50b; The average primary particle size Da of the first conductive agent is less than the average primary particle size Db of the second conductive agent.

[0007] As an embodiment of this application, the Dv50a of the first positive electrode active material is 0.95μm ~ 1.55μm.

[0008] As an embodiment of this application, the Dv50b of the second positive electrode active material is 0.70 μm ~ 0.90 μm.

[0009] As an embodiment of this application, the average particle size Da of the primary particles of the first conductive agent satisfies 20nm≤Da≤30nm.

[0010] As an embodiment of this application, the average particle size Db of the primary particles of the second conductive agent satisfies 30nm < Db ≤ 50nm.

[0011] As an embodiment of this application, the specific surface area Ba of the first conductive agent is 120 m². 2 / g~150m 2 / g.

[0012] As an embodiment of this application, the specific surface area Bb of the second conductive agent is 10m². 2 / g~40m 2 / g.

[0013] As an embodiment of this application, the carbon black oil absorption value (DBPa) of the first conductive agent is 200mL / 100g to 300mL / 100g.

[0014] As an embodiment of this application, the carbon black oil absorption value (DBPb) of the second conductive agent is 20 mL / 100g to 100 mL / 100g.

[0015] As an embodiment of this application, the first positive electrode active material satisfies 3.5≤(Dv90a-Dv10a) / Dv50a≤4.3.

[0016] As an embodiment of this application, the second positive electrode active material satisfies 2.7≤(Dv90b-Dv10b) / Dv50b≤3.0.

[0017] In an embodiment of this application, the Dv10a of the first positive electrode active material is 0.35 μm ~ 0.37 μm.

[0018] As an embodiment of this application, the Dv90a of the first positive electrode active material is 3.84 μm ~ 7.00 μm.

[0019] As an embodiment of this application, the Dv99a of the first positive electrode active material is 7.00 μm ~ 13 μm.

[0020] As an embodiment of this application, the Dv10b of the second positive electrode active material is 0.36 μm ~ 0.38 μm.

[0021] As an embodiment of this application, the Dv90b of the second positive electrode active material is 2.30 μm ~ 2.88 μm.

[0022] As an embodiment of this application, the Dv99b of the second positive electrode active material is 4.3 μm ~ 11 μm.

[0023] As an embodiment of this application, based on the total mass of the first positive electrode active material layer, the mass percentage of the first positive electrode active material is 96%~98%, and the mass percentage of the first conductive agent is 0.5%~0.6%.

[0024] As an embodiment of this application, based on the total mass of the second positive electrode active material layer, the mass percentage of the second positive electrode active material is 96%~98%, and the mass percentage of the second conductive agent is 0.5%~0.6%.

[0025] As an embodiment of this application, the first cathode material and the second cathode material each independently include at least one of lithium iron phosphate and lithium manganese oxide.

[0026] As an embodiment of this application, the first conductive agent and the second conductive agent each independently include at least one of Super P, conductive carbon black, acetylene black, and Ketjen black.

[0027] A second aspect of this application provides an electrical device including the secondary battery described in this application.

[0028] Compared with the prior art, the beneficial effects of this application are: The secondary battery provided in this application provides a first positive electrode active material layer in the thinned region of the positive electrode sheet and a second positive electrode active material layer in the main body region of the positive electrode sheet. The first positive electrode active material has a Dv50a greater than the second positive electrode active material's Dv50b; the average primary particle size Da of the first conductive agent is less than the average primary particle size Db of the second conductive agent. This effectively reduces lithium plating caused by excessive lithium intercalation in the negative electrode sheet during charging, and also slows down lithium plating due to insufficient local CB (the ratio of negative electrode capacity to positive electrode capacity) in the later stages of cycling. This effectively improves the cycle performance and safety performance of the secondary battery, and the resulting secondary battery has a high capacity. Attached Figure Description

[0029] Figure 1 A schematic diagram showing the location of lithium deposition on the lower side of the negative electrode tab of a secondary battery; Figure 2 This is a schematic diagram of the coating of the thinned area and the main body area of ​​the positive electrode sheet in Example 1: A - Main area, B - Thinning area; Figure 3 The graph shows the cycle test results of the secondary batteries prepared in Example 1 and Comparative Example 1. Figure 4 The graph shows the expansion force test results of the secondary batteries prepared in Example 1 and Comparative Example 1. Figure 5 The images show lithium deposition on the negative electrode side of the secondary batteries prepared in Example 1 and Comparative Example 1 after disassembly. Detailed Implementation

[0030] 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.

[0031] 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.

[0032] 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.

[0033] In one embodiment of this application, a secondary battery is provided, the secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive electrode sheet having a first direction, along the first direction, the positive electrode sheet including an adjacently disposed main body region and a thinned region, the positive active material layer including a first positive active material layer and a second positive active material layer, the first positive active material layer being disposed in the thinned region of the positive electrode sheet, and the second positive active material layer being disposed in the main body region of the positive electrode sheet; The first positive electrode active material layer includes a first positive electrode active material and a first conductive agent, and the second positive electrode active material layer includes a second positive electrode active material and a second conductive agent; The first positive electrode active material Dv50a > the second positive electrode active material Dv50b; The average primary particle size Da of the first conductive agent is less than the average primary particle size Db of the second conductive agent.

[0034] The secondary battery provided in this application provides a first positive electrode active material layer in the thinned region of the positive electrode sheet and a second positive electrode active material layer in the main body region of the positive electrode sheet. The first positive electrode active material has a Dv50a greater than the second positive electrode active material's Dv50b; the average primary particle size Da of the first conductive agent is less than the average primary particle size Db of the second conductive agent. This effectively reduces lithium plating caused by excessive lithium intercalation in the thinned region of the negative electrode sheet during charging, and also slows down lithium plating caused by insufficient local CB (the ratio of negative electrode capacity to positive electrode capacity) in the thinned region during the later stages of cycling. This effectively improves the cycle performance and safety performance of the secondary battery, and the resulting secondary battery has a high capacity.

[0035] Specifically, in the first aspect, the first positive electrode active material is made of large particles, which, when placed in the thinning region, can effectively resist the relatively drastic volume and stress changes in this relatively high current density area during the cycling process of the secondary battery, reduce crack formation, maintain the stability of the electrode structure, and thus improve the cycle performance of the secondary battery. At the same time, the lithium-ion extraction rate of the large-particle positive electrode active material is slower, which worsens the lithium-ion extraction impedance of the positive electrode, thereby slowing down lithium deposition in the thinning region. The first conductive agent is made of small particles, which is more likely to form a dense, multi-dimensional conductive network, thereby improving electron transport efficiency and uniformity, significantly reducing interface impedance and polarization, and thus improving the cycle performance of the secondary battery. At the same time, the small-particle conductive agent has a strong electrolyte retention capacity, which can reduce lithium deposition caused by excessive electrolyte consumption in the thinning region in the later stages of cycling. The combination of large-particle positive electrode active material and small-particle conductive agent reduces the local lithium insertion pressure and worsens the positive electrode edge insertion / extraction capability. The current density decreases, reducing the formation of lithium dendrites in the thinning region under the negative electrode tab. Secondly, the second positive electrode active material is composed of small particles, which are placed in the main body region. Under the same area, the amount of small particles added is relatively large, providing a larger specific surface area and a relatively high compaction density, thereby maximizing the volumetric energy density of the main body region and ensuring the capacity of the prepared secondary battery. The second conductive agent is composed of large particles, which can ensure overall electron conduction, maintain good conductivity and structural stability, thereby improving the cycle performance of the secondary battery. The combination of small particles of positive electrode active material and large particles of conductive agent can achieve uniform current distribution, reduce lithium deposition, and improve the safety performance of the secondary battery.

[0036] It should be noted that Dv50 refers to the fact that when the material is statistically analyzed according to particle volume, 50% of the total particle volume is contributed by particles with a particle size smaller than or equal to this value.

[0037] It should be noted that a primary particle refers to the smallest independent unit with a complete crystal structure that is initially formed during the material preparation process.

[0038] It should be noted that the testing method for various particle sizes of the first and second positive electrode active materials is as follows: particle size analysis laser diffraction is used for testing, and the specific steps are as follows: The secondary battery was disassembled to obtain the positive electrode sheet. After wiping with DMC (dimethyl carbonate) solvent and drying, the thickness of the positive electrode sheet was measured with a micrometer. Areas showing a significant reduction in thickness were cut using ceramic scissors to obtain the thinned area and the main active material layer. The electrode surface was then scraped at a 45° angle with a blade to obtain the first and second positive electrode active materials. The first and second positive electrode active materials were then pretreated: 5g sample + 5-10 drops of 1% NP40 dispersant, stirred for approximately 1 minute, followed by the addition of 30mL of water. The mixture was then subjected to external sonication at 53kHz and 100% power for 3 minutes; internal sonication power was 15.00kHz for the duration (from the start of sample testing to the end of testing). Sample introduction system: Add 800 mL of water to the Hydro MU2000 and rotate at 2400 r / min; Sample introduction method: Stirred sample introduction.

[0039] It should be noted that the test method for the average particle size of the first and second conductive agents is as follows: particle size analysis laser diffraction is used for testing, and the specific steps are as follows: The secondary battery was disassembled to obtain the positive electrode sheet. After wiping with DMC solvent and drying, the thickness of the positive electrode sheet was measured with a micrometer. Areas showing a significant reduction in thickness were cut using ceramic scissors to obtain the thinned area and the main active material layer. Next, the electrode surface was scraped at a 45° angle with a blade to obtain the first and second positive electrode active materials. The first and second positive electrode active materials were then pretreated: 10g sample + 30mL anhydrous ethanol, external sonication at 53kHz and 100% power for 3 minutes; internal sonication power at 15.00kHz for the duration (from the start of sample testing to the end of testing). Sample introduction system: Add anhydrous ethanol to the injector and set the rotation speed to 2800 r / min; Sample introduction method: Add sample using a dropper, and test after the system has stabilized for about 1 minute.

[0040] In this application, the first direction of the positive electrode sheet can be either the length direction or the width direction of the positive electrode sheet.

[0041] In one embodiment, the thinning region is disposed on at least one side of the main body region in a cross-section perpendicular to the length direction of the positive electrode sheet. In one embodiment, the width of the main body region is 150mm~210mm, and the single-sided thickness of the second positive electrode active material layer on the main body region is 160mm~170mm.

[0042] In one embodiment, the width of the thinned region is 3mm to 5mm, and the single-sided thickness of the first positive electrode active material layer on the thinned region is 10μm to 30μm.

[0043] In one embodiment, the Dv50a of the first positive electrode active material is 0.95 μm ~ 1.55 μm.

[0044] For example, the Dv50a of the first positive electrode active material can be any point value between 0.95μm and 1.55μm or a range value between any two points, such as 0.95μm, 1μm, 1.05μm, 1.1μm, 1.15μm, 1.2μm, 1.25μm, 1.3μm, 1.35μm, 1.4μm, 1.45μm, 1.5μm, 1.55μm, etc.

[0045] In one embodiment, the Dv50a of the first positive electrode active material is 1.15 μm to 1.35 μm. For example, it can be 1.15 μm, 1.18 μm, 1.2 μm, 1.22 μm, 1.25 μm, 1.28 μm, 1.3 μm, 1.32 μm, 1.35 μm, etc.

[0046] This application research found that the range of Dv50a of the first positive electrode active material affects its ability to resist relatively drastic volume and stress changes during secondary battery cycling in the thinned region, thereby affecting its ability to maintain structural stability; in addition, it also affects the ion / electron transport rate; when the Dv50a of the first positive electrode active material is further selected within the above-mentioned range, especially the preferred range, the cycle performance of the obtained secondary battery is better; at the same time, the interaction with the first conductive agent is also better, thereby effectively reducing lithium deposition on the negative electrode side and improving the safety performance of the secondary battery.

[0047] In one embodiment, the Dv50b of the second positive electrode active material is 0.70 μm to 0.90 μm.

[0048] For example, the Dv50b of the second positive electrode active material can be any point value between 0.70μm and 0.90μm or a range value between any two points, such as 0.70μm, 0.72μm, 0.75μm, 0.78μm, 0.80μm, 0.82μm, 0.85μm, 0.88μm, 0.90μm, etc.

[0049] In one embodiment, the Dv50b of the second positive electrode active material is 0.75 μm to 0.80 μm. For example, it can be 0.75 μm, 0.76 μm, 0.77 μm, 0.78 μm, 0.79 μm, 0.80 μm, etc.

[0050] This application research found that the Dv50b of the second positive electrode active material affects the capacity of the secondary battery, as well as the ion / electron transport rate and the interaction with the second conductive agent. When the Dv50b of the second positive electrode active material is selected within the above-mentioned range, especially within a further preferred range, the secondary battery has a higher capacity and better safety and cycle performance.

[0051] In one embodiment, the average particle size Da of the primary particles of the first conductive agent satisfies 20nm ≤ Da ≤ 30nm.

[0052] For example, the average particle size Da of the first conductive agent can be any point value between 20nm and 30nm or a range value between any two points, such as 20nm, 22nm, 24nm, 26nm, 28nm, 30nm, etc.

[0053] In one embodiment, the average particle size Da of the primary particles of the first conductive agent is 22nm to 25nm. For example, it can be 22nm, 23nm, 24nm, 25nm, etc.

[0054] This application research found that the average primary particle size Da of the first conductive agent affects the ability to form a dense, multidimensional conductive network, affects the electron transport efficiency and transport uniformity, thereby affecting the ability to reduce interface impedance and polarization, and thus affecting the driving force for lithium ion deposition. When the average primary particle size Da of the first conductive agent is selected within the above range, especially within a further preferred range, the risk of lithium deposition on the negative electrode side of the secondary battery is lower and the cycle performance is better.

[0055] In one embodiment, the average particle size Db of the primary particles of the second conductive agent satisfies 30nm < Db ≤ 50nm.

[0056] For example, the average particle size Db of the primary particles of the second conductive agent can be any point value or a range value between any two points between 30nm (excluding 30nm) and 50nm, such as 31nm, 32nm, 34nm, 36nm, 38nm, 40nm, 42nm, 44nm, 46nm, 48nm, 50nm, etc.

[0057] In one embodiment, the average particle size Db of the primary particles of the second conductive agent is 35nm to 40nm. For example, it can be 35nm, 36nm, 37nm, 38nm, 39nm, 40nm, etc.

[0058] This application research found that the average particle size Db of the primary particles of the second conductive agent affects the integrity of the electron channel, thereby affecting the uniformity of the current distribution, and consequently affecting the lithium deposition capability and cycle performance. When the average particle size Db of the primary particles of the second conductive agent is selected within the above range, especially within a further preferred range, the risk of lithium deposition on the negative electrode side of the secondary battery is lower, and the cycle performance is better.

[0059] In one embodiment, the specific surface area Ba of the first conductive agent is 120 m². 2 / g~150m 2 / g.

[0060] For example, the specific surface area Ba of the first conductive agent may be 120 m². 2 / g~150m 2 Any point value between / g or a range of values ​​between any two points, for example, 120m 2 / g、125m 2 / g、130m 2 / g、135m 2 / g, 140m 2 / g、145m 2 / g, 150m 2 / g etc.

[0061] In one embodiment, the specific surface area Ba of the first conductive agent is 130 m². 2 / g~140m 2 / g. For example, it can be 130m. 2 / g、132 m 2 / g、134 m 2 / g、136 m 2 / g、138 m 2 / g、140 m 2 / g etc.

[0062] This application research found that the specific surface area of ​​the first conductive agent affects the interfacial contact resistance between the active particles and the conductive agent, thereby affecting the potential safety window on the negative electrode side and the overpotential of lithium ion intercalation. When the specific surface area of ​​the first conductive agent is further selected within the above-mentioned range, especially within the further preferred range, the overall performance of the secondary battery is better.

[0063] In one embodiment, the specific surface area Bb of the second conductive agent is 10m². 2 / g~40m 2 / g.

[0064] For example, the specific surface area Bb of the second conductive agent can be 10m². 2 / g~40m 2 Any point value between / g or a range of values ​​between any two points, for example, 10m. 2 / g, 15m 2 / g、20m 2 / g、25m 2 / g、30m 2 / g、35m 2 / g、40m 2 / g etc.

[0065] In one embodiment, the specific surface area Bb of the second conductive agent is 20m². 2 / g~30m 2 / g. For example, it can be 20m. 2 / g、22m 2 / g、24m 2 / g、26m 2 / g、28m 2 / g、30m 2 / g etc.

[0066] This application research found that the specific surface area of ​​the second conductive agent affects the uniformity of current distribution, thereby reducing lithium ion accumulation and precipitation caused by local non-uniformity; when the specific surface area of ​​the second conductive agent is selected within the above range, especially within a further preferred range, the overall performance of the secondary battery is better.

[0067] It should be noted that the specific surface area of ​​the first and second conductive agents is tested using the gas adsorption BET method to determine the specific surface area of ​​the solid material. The method for obtaining the first and second conductive agents from the secondary battery terminal refers to the aforementioned method for testing the average particle size of the first and second conductive agents. The specific conditions for the BET method are as follows: Sample pretreatment and degassing conditions: 250℃ / 2hrs (vacuum degassing); Sample volume: The sample volume is 2 / 3 of the bulb volume; Determination method: Static volumetric method; Pressure range: 0.05~0.35; Adsorbate: Nitrogen gas; Temperature control medium: liquid nitrogen; Analysis model: Multi-point BET.

[0068] In one embodiment, the carbon black oil absorption value (DBPa) of the first conductive agent is 200 mL / 100 g to 300 mL / 100 g.

[0069] For example, the carbon black oil absorption value (DBPa) of the first conductive agent can be any point value or a range between any two points between 200 mL / 100g and 300 mL / 100g, such as 200 mL / 100g, 210 mL / 100g, 220 mL / 100g, 230 mL / 100g, 240 mL / 100g, 250 mL / 100g, 260 mL / 100g, 270 mL / 100g, 280 mL / 100g, 290 mL / 100g, 300 mL / 100g, etc.

[0070] This study found that the carbon black oil absorption value of the first conductive agent affects the ability to form a three-dimensional chain-like conductive network, thereby affecting the electron transport resistance. When the carbon black oil absorption value of the first conductive agent is further selected within the above range, the overall performance of the secondary battery is better.

[0071] In one embodiment, the carbon black oil absorption value (DBPb) of the second conductive agent is 20 mL / 100 g to 100 mL / 100 g.

[0072] For example, the carbon black oil absorption value DBPb of the second conductive agent can be any point value or a range between any two points between 20mL / 100g and 100mL / 100g, such as 20mL / 100g, 40mL / 100g, 60mL / 100g, 80mL / 100g, 100mL / 100g, etc.

[0073] This study found that the carbon black oil absorption value of the second conductive agent affects the stability of the formed electronic pathway. When the carbon black oil absorption value of the second conductive agent is selected within the above range, the overall performance of the secondary battery is better.

[0074] It should be noted that the test method for the carbon black oil absorption value of the first and second conductive agents is as follows: the test is conducted using an oil absorption value tester; the drop rate is 4 mL / min; and the sample volume is 50 mL of conductive agent.

[0075] In one embodiment, the first positive electrode active material satisfies 3.5≤(Dv90a-Dv10a) / Dv50a≤4.3.

[0076] For example, the (Dv90a-Dv10a) / Dv50a of the first positive electrode active material can be any point value between 3.5 and 4.3 or a range value between any two points, such as 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, etc.

[0077] In one embodiment, the second positive electrode active material satisfies 2.7≤(Dv90b-Dv10b) / Dv50b≤3.0.

[0078] For example, the (Dv90b-Dv10b) / Dv50b of the second positive electrode active material can be any point value between 2.7 and 3.0 or a range value between any two points, such as 2.7, 2.8, 2.9, 3.0, etc.

[0079] This application research found that by controlling the particle size distribution uniformity of the first and second positive electrode active materials to be within the aforementioned ranges, the positive electrode active materials in the thinned region and the main body region can have similar reaction kinetic characteristics. This effectively makes ion transport more uniform, stabilizes the lithium ion flow at the negative electrode, avoids overpotential, and thus effectively suppresses the occurrence of lithium deposition on the negative electrode side. At the same time, with the particle size distribution uniformity values ​​within the aforementioned ranges, the volume changes in the thinned region and the main body region during cycling are more synchronized, reducing internal stress and thus reducing the probability of positive electrode active material particle breakage, thereby improving the cycle performance of the secondary battery.

[0080] In one embodiment, the Dv10a of the first positive electrode active material is 0.35 μm to 0.37 μm.

[0081] For example, the Dv10a of the first positive electrode active material can be any point value between 0.35μm and 0.37μm or a range value between any two points, such as 0.35μm, 0.355μm, 0.34μm, 0.345μm, 0.35μm, 0.355μm, 0.36μm, 0.365μm, 0.37μm, etc.

[0082] In one embodiment, the Dv90a of the first positive electrode active material is 3.84 μm to 7.00 μm.

[0083] For example, the Dv90a of the first positive electrode active material can be any point value between 3.84μm and 7.00μm or a range value between any two points, such as 3.84μm, 4μm, 4.2μm, 4.5μm, 4.8μm, 5μm, 5.2μm, 5.5μm, 5.8μm, 6μm, 6.2μm, 6.5μm, 6.8μm, 7μm, etc.

[0084] In one embodiment, the Dv90a of the first positive electrode active material is 4.4 μm to 5.4 μm. For example, it can be 4.4 μm, 4.6 μm, 4.8 μm, 5 μm, 5.2 μm, 5.4 μm, etc.

[0085] In one embodiment, the Dv99a of the first positive electrode active material is 7.00 μm ~ 13 μm.

[0086] For example, the Dv99a of the first positive electrode active material can be any point value between 7.00μm and 13μm or a range value between any two points, such as 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, etc.

[0087] In one embodiment, the Dv99a of the first positive electrode active material is 9.00 μm to 11 μm. For example, it can be 9 μm, 9.2 μm, 9.5 μm, 9.8 μm, 10 μm, 10.2 μm, 10.5 μm, 10.8 μm, 11 μm, etc.

[0088] This application research found that further controlling the Dv10a, Dv90a and Dv99a of the first positive electrode active material within the above-mentioned range can effectively reduce lithium deposition on the negative electrode side and improve the capacity and cycle performance of the secondary battery.

[0089] In one embodiment, the Dv10b of the second positive electrode active material is 0.36 μm to 0.38 μm.

[0090] For example, the Dv10b of the second positive electrode active material can be any point value between 0.36μm and 0.38μm or a range value between any two points, such as 0.36μm, 0.362μm, 0.364μm, 0.366μm, 0.368μm, 0.37μm, 0.372μm, 0.374μm, 0.376μm, 0.378μm, 0.38μm, etc.

[0091] In one embodiment, the Dv90b of the second positive electrode active material is 2.30 μm ~ 2.88 μm.

[0092] For example, the Dv90b of the second positive electrode active material can be any point value between 2.30μm and 2.88μm or a range value between any two points, such as 2.3μm, 2.4μm, 2.5μm, 2.6μm, 2.7μm, 2.8μm, 2.88μm, etc.

[0093] In one embodiment, the Dv90b of the second positive electrode active material is 2.5 μm to 2.7 μm. For example, it can be 2.5 μm, 2.52 μm, 2.54 μm, 2.56 μm, 2.58 μm, 2.6 μm, 2.62 μm, 2.64 μm, 2.66 μm, 2.68 μm, 2.7 μm, etc.

[0094] In one embodiment, the Dv99b of the second positive electrode active material is 4.3 μm to 11 μm.

[0095] For example, the Dv99b of the second positive electrode active material can be any point value between 4.3μm and 11μm or a range value between any two points, such as 4.3μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, etc.

[0096] In one embodiment, the Dv99b of the second positive electrode active material is 7 μm to 8.2 μm. For example, it can be 7 μm, 7.2 μm, 7.4 μm, 7.6 μm, 7.8 μm, 8 μm, 8.2 μm, etc.

[0097] This study found that selecting Dv10b, Dv90b, and Dv99b as the second positive electrode active materials within the aforementioned ranges can significantly improve the overall performance of the secondary battery.

[0098] In one embodiment, the mass percentage of the first positive electrode active material is 96% to 98%, and the mass percentage of the first conductive agent is 0.5% to 0.6%.

[0099] For example, the mass percentage of the first positive electrode active material can be any point value or any range between two points between 96% and 98%, such as 96%, 96.5%, 97%, 97.5%, 98%, etc.; the mass percentage of the first conductive agent can be any point value or any range between two points between 0.5% and 0.6%, such as 0.5%, 0.52%, 0.54%, 0.56%, 0.58%, 0.6%, etc.

[0100] In one embodiment, the mass percentage of the second positive electrode active material is 96% to 98% based on the total mass of the second positive electrode active material layer, and the mass percentage of the second conductive agent is 0.5% to 0.6%.

[0101] For example, the mass percentage of the second positive electrode active material can be any point value or any range between two points between 96% and 98%, such as 96%, 96.5%, 97%, 97.5%, 98%, etc.; the mass percentage of the second conductive agent can be any point value or any range between two points between 0.5% and 0.6%, such as 0.5%, 0.52%, 0.54%, 0.56%, 0.58%, 0.6%, etc.

[0102] This study found that when the mass percentage of the selected substances is within the above-mentioned range, the secondary battery can have good capacity, excellent cycle performance, and safety performance.

[0103] In one embodiment, the first cathode material and the second cathode material each independently include at least one of lithium iron phosphate and lithium manganese oxide.

[0104] In one embodiment, the first conductive agent and the second conductive agent each independently include at least one of Super P, conductive carbon black, acetylene black, and Ketjen black.

[0105] It should be noted that different Dv50a, Dv10a, Dv90a, Dv99a, Pa=(Dv90a-Dv10a) / Dv50a first positive electrode active materials and different Dv50b, Dv10b, Dv90b, Dv99b, Pb=(Dv90b-Dv10b) / Dv50b can be obtained from commercially available materials or obtained by classifying commercially available positive electrode active materials through air jet milling.

[0106] It should be noted that different Da, Ba, DBPa first conductive agents and different Db, Bb, DBPb second conductive agents can be obtained commercially, or obtained by pulverizing and classifying commercially available conductive agents using a fluidized bed airflow pulverizer and classifier.

[0107] In one embodiment, the coating width of the first positive electrode active material layer is the width of the thinned area; the width of the thinned area is ≤5mm. For example, it can be 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm, etc.

[0108] In one embodiment, the coating length of the first positive electrode active material layer is consistent with the length of the positive electrode sheet.

[0109] In one embodiment, the first positive electrode active material layer further includes a first binder, and the second positive electrode active material layer further includes a second binder.

[0110] This application does not impose any particular limitation on the selection of the first adhesive and the second adhesive, and adhesives conventionally used in the art can be used; for example, the first adhesive and the second adhesive can each be any one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene-butadiene rubber (SBR), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), and polymethyl methacrylate (PAMA).

[0111] In one embodiment, the secondary battery further includes a negative electrode sheet; the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector.

[0112] In one embodiment, the negative electrode active material layer includes a negative electrode active material, a negative electrode binder, a negative electrode thickener, and a negative electrode conductive agent.

[0113] This application does not have any special requirements for the selection of the negative electrode active material; any negative electrode active material conventionally available in the art can be used. For example, the negative electrode active material can be any one of graphite, soft carbon, hard carbon, nano-carbon, or silicon-carbon composite.

[0114] This application does not have any special requirements for the selection of the negative electrode binder, and any negative electrode binder conventionally available in the art can be used. For example, the negative electrode binder can be any one of styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC-Na), gum arabic (GA), polypyrrole (PPy), polyaniline (PANI), polyethylene dioxythiophene (PEDT), polythiophene (PTh), aramid (PPTA), etc.

[0115] This application does not have any special requirements for the selection of the negative electrode thickener; any negative electrode thickener conventionally available in the art can be used. For example, the negative electrode thickener may be any one of sodium carboxymethyl cellulose (CMC-Na), polyacrylic acid (PAA), polyurethane (PU), or guar gum (GG).

[0116] This application does not have any special requirements for the selection of the negative electrode conductive agent; any negative electrode conductive agent conventionally available in the art can be used. For example, the negative electrode conductive agent can be any one of carbon black (SuperP), graphite, carbon nanotubes (CNTs), and graphene.

[0117] In one embodiment, the secondary battery further includes an electrolyte; the electrolyte includes an organic solvent, a lithium salt, and additives.

[0118] This application does not impose any particular limitation on the selection of organic solvents, and organic solvents conventionally used in the art can be used. For example, the organic solvent may be ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl acetate, propyl propionate, ethyl propionate, dimethyl carbonate, etc.

[0119] This application does not impose any particular limitation on the selection of lithium salt, and lithium salts conventionally available in the art can be used. For example, the lithium salt may be lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), etc.

[0120] This application does not impose any particular limitation on the selection of additives, and additives conventionally available in the art can be used. For example, the additives may be vinyl sulfate (DTD), vinylene carbonate (VC), fluoroethylene carbonate (FEC), etc.

[0121] In one embodiment of this application, an electrical device is provided, including the secondary battery described in this application.

[0122] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, 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 and satellites, energy storage systems, etc., but are not limited thereto.

[0123] Example 1 This application provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of positive electrode sheet S1. Preparation of the first positive electrode active material layer slurry: The first positive electrode active material (lithium iron phosphate, Dv50a is 1.15μm, Dv10a is 0.362μm, Dv90a is 4.4μm, Dv99a is 9μm) and the first conductive agent (Super P, Da is 22nm, Ba is 140nm) are prepared. 2 The first positive electrode active material layer slurry is prepared by uniformly dispersing N-methyl-2-pyrrolidone (NMP) with a mass ratio of 96.5:1.5:2 (with a DBPa of 250 mL / 100 g) and the first binder (polyvinylidene fluoride) in N-methyl-2-pyrrolidone (NMP) to form a uniform slurry. S2. Preparation of the second positive electrode active material layer slurry: The second positive electrode active material (lithium iron phosphate, Dv50a is 0.75μm, Dv10a is 0.364μm, Dv90a is 2.5μm, Dv99a is 7μm) and the second conductive agent (Super P, Da is 35nm, Ba is 20nm) are prepared. 2The second binder (polyvinylidene fluoride) and the second binder (polyvinylidene fluoride) are uniformly dispersed in N-methyl-2-pyrrolidone (NMP) at a mass ratio of 96.5:1.5:2 to form a uniform slurry, thus obtaining the second positive electrode active material layer slurry. S3. The first positive electrode active material layer slurry and the second positive electrode active material layer are sequentially and uniformly coated onto the thinned area and the main body area of ​​the 13μm aluminum foil. The electrode coating weight is 0.305mg / 1540.25mm. 2 The compaction density is 2.68 g / cc; after drying, the tabs are cut, stripped and cut to obtain the positive electrode sheet; The schematic diagram of the coating position of the positive electrode sheet is shown below. Figure 2 As shown, B is the thinning region, coated with the first positive electrode active material slurry, and A is the main region, coated with the second positive electrode active material layer slurry. The width of the thinning region is 4 mm, and the single-sided thickness of the coated first positive electrode active material layer is 20 μm. The width of the main region is 180 mm, and the single-sided thickness of the coated second positive electrode active material layer is 165 mm. (2) Preparation of negative electrode sheet S4. The negative electrode active material (graphite), conductive agent (carbon black), negative electrode binder (styrene-butadiene rubber), and negative electrode thickener (sodium carboxymethyl cellulose) are mixed evenly in a mass ratio of 96:1:1.8:1.2, and then evenly dispersed in deionized water to form a uniform slurry. The slurry is then evenly coated onto a 6μm copper foil, with the electrode coating weight controlled at 0.132mg / 1540.25mm. 2 The compaction density is 1.63 g / cc; after drying, the tabs are cut, slits are separated and cut to obtain the negative electrode sheet; (3) Preparation of electrolyte At room temperature, in an argon-filled glove box (H2O < 1 ppm, O2 < 1 ppm), after removing water from the organic solvent, the mixture was stirred according to a mass ratio of ethylene carbonate (EC): ethyl acetate (EA): methyl ethyl carbonate (EMC): diethyl carbonate (DEC) = 3:3:2:2. Lithium hexafluorophosphate (LiPF6) lithium salt was added to the organic solvent, followed by vinylene carbonate (VC) and lithium bis(fluorosulfonyl)imide (LiFSI). The mixture was stirred until homogeneous to obtain the electrolyte. The electrolyte contained 8% lithium hexafluorophosphate (LiPF6) by mass, 0.2% vinylene carbonate by mass, and 1% lithium bis(fluorosulfonyl)imide (LiFSI) by mass. (4) Preparation of secondary batteries A separator is placed between the positive and negative electrode plates, and the cells are formed by winding. The tabs are welded by ultrasonic welding or transfer welding to obtain a dry cell. The dry cell is then injected with electrolyte at a rate of 2.85 g / Ah. After encapsulation and formation, a secondary battery is obtained.

[0124] Example 2 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the parameters of the first positive electrode active material, the second positive electrode active material, the first conductive agent, and the second conductive agent are adjusted to achieve the parameters in Tables 1-2.

[0125] Examples 3-4 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the parameters of the first positive electrode active material are adjusted to achieve the parameters in Tables 1-2.

[0126] Examples 5-6 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the parameters of the second positive electrode active material are adjusted to achieve the parameters in Tables 1-2.

[0127] Examples 7-8 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the parameters of the first conductive agent are adjusted to achieve the parameters in Tables 1-2.

[0128] Examples 9-10 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the parameters of the second conductive agent are adjusted to achieve the parameters in Tables 1-2.

[0129] Example 11 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the types of the first conductive agent and the second conductive agent are adjusted to achieve the parameters in Tables 1-2.

[0130] Example 12 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the type of the first conductive agent is adjusted to achieve the parameters in Tables 1-2.

[0131] Comparative Example 1 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that both the main body region and the thinned region are coated with a first positive electrode active material layer slurry.

[0132] Comparative Example 2 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that both the main body region and the thinned region are coated with a second positive electrode active material slurry.

[0133] Comparative Example 3 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that the thinned area is coated with a second positive electrode active material layer slurry, and the main body area is coated with a first positive electrode active material layer slurry.

[0134] Comparative Example 4 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that the first conductive agent and the second conductive agent are interchanged.

[0135] Comparative Example 5 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that the first positive electrode active material and the second positive electrode active material are interchanged.

[0136] The secondary batteries provided in the examples and comparative examples have the following properties: Dv50a, Dv10a, Dv90a, Dv99a, Pa=(Dv90a-Dv10a) / Dv50a, Dv50b, Dv10b, Dv90b, Dv99b, Pb=(Dv90b-Dv10b) / Dv50b, Da, Ba, DBPa, Db, Bb, DBPb, and the types of the first and second conductive agents are shown in Tables 1-2. Table 1 Parameters of the First Positive Electrode Active Material Layer Table 2 Parameters of the Second Positive Electrode Active Material Layer The performance of the secondary batteries prepared in the examples and comparative examples was tested, including the following aspects: 1. Initial capacity test At room temperature (25±2℃), a test current of 0.33C was used to conduct a simulated constant current and constant voltage charge and discharge test in the voltage range of 2.5V~3.65V, with a cutoff current of 0.05C, to obtain the initial capacity data; 2. SOH check test The test was conducted at (25±2℃). The specific process can be followed as shown in Table 3. Different steps were used for different battery state of charge (SOC) ranges, from 0% to 100%; rest for 120 min, discharge at 1 / 3C constant current to 2.5V, rest for 30 min, charge at 1 / 3C constant current and constant voltage to 3.65V, cut off at I≤0.05C; discharge at 1 / 3C constant current to 2.5V; cycle 3 times; take the last discharge capacity as the SOHcheck capacity; test the SOHcheck capacity once every 100cls for a total of 500cls, record 500cls-SOH; 500cls-SOH = discharge capacity on the 500th cycle / initial capacity 100%; During the SOH check test, three steel clamps were applied, and a 0.5mm*2 foam U-shaped frame was attached. The initial preload was 300kgf. The maximum pressure value observed during the SOH check test was recorded. After the test, the secondary battery was disassembled to observe whether there was lithium plating under the negative electrode tab. Where "none" indicates that no lithium plating was observed, and "slight" indicates 0 mm. 2 ≤Lithium plating area below the tab ≤20mm 2 Severely indicates that the lithium plating area below the electrode tab is ≥20mm². 2 ; Table 3. SOH check test procedure table 3. DCR test At room temperature (25±2℃), after resting for 30 minutes, charge at 1 / 3C constant current and constant voltage to 3.65V, cut off when I≤0.05C, rest for 30 minutes, discharge at 1 / 3C constant current to 50% SOC, and rest for 60 minutes; perform 3C pulse discharge for 10 seconds, rest for 5 minutes, and cycle test for 100cls to test the discharge DCR once. The test results are shown in Table 4. Table 4. Performance Data of Secondary Batteries As can be seen from Table 4, when the technical solution provided in this application is adopted, the secondary battery has high capacity, excellent cycle performance and safety performance, and also has low internal resistance and no lithium plating on the underside of the negative electrode tab; specifically, the initial capacity is above 120.7 Ah, 500cls-SOH is above 93.6%, DCR is below 0.644mΩ, the maximum expansion pressure is below 999kgf, and there is no lithium plating on the underside of the negative electrode tab; As can be seen from Examples 1-12 and Comparative Examples 1-2, using different positive electrode active material layers in the main body region and the thinned region of this application can achieve good overall results; as can be seen from Examples 1-12 and Comparative Examples 3-5, when the coating method of this application and the parameters of the positive electrode active material and the conductive agent satisfy the relationship of this application, significant results can be achieved. The cycle test results, expansion force test results, and lithium deposition photographs on the negative electrode side of the secondary batteries prepared in Example 1 and Comparative Example 1 are shown in the figures below. Figures 3-5 As shown.

[0137] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not 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, 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 surface of the positive current collector, the positive electrode sheet having a first direction, and along the first direction, the positive electrode sheet comprising an adjacently disposed main body region and a thinned region, characterized in that, The positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer. The first positive electrode active material layer is disposed in the thinned area of ​​the positive electrode sheet, and the second positive electrode active material layer is disposed in the main body area of ​​the positive electrode sheet. The first positive electrode active material layer includes a first positive electrode active material and a first conductive agent, and the second positive electrode active material layer includes a second positive electrode active material and a second conductive agent; The first positive electrode active material Dv50a > the second positive electrode active material Dv50b; The average primary particle size Da of the first conductive agent is less than the average primary particle size Db of the second conductive agent.

2. The secondary battery according to claim 1, characterized in that, The Dv50a of the first positive electrode active material is 0.95 μm ~ 1.55 μm; And / or, the Dv50b of the second positive electrode active material is 0.70 μm ~ 0.90 μm.

3. The secondary battery according to claim 1, characterized in that, The average particle size Da of the primary particles of the first conductive agent satisfies 20nm≤Da≤30nm; And / or, the average particle size Db of the primary particles of the second conductive agent satisfies 30nm < Db ≤ 50nm.

4. The secondary battery according to claim 1, characterized in that, The specific surface area Ba of the first conductive agent is 120 m². 2 / g~150m 2 / g; And / or, the specific surface area Bb of the second conductive agent is 10m². 2 / g~40m 2 / g.

5. The secondary battery according to claim 1, characterized in that, The carbon black oil absorption value (DBPa) of the first conductive agent is 200mL / 100g~300mL / 100g; And / or, the carbon black oil absorption value (DBPb) of the second conductive agent is 20 mL / 100 g to 100 mL / 100 g.

6. The secondary battery according to claim 1, characterized in that, The first positive electrode active material satisfies 3.5 ≤ (Dv90a - Dv10a) / Dv50a ≤ 4.3; And / or, the second positive electrode active material satisfies 2.7≤(Dv90b-Dv10b) / Dv50b≤3.

0.

7. The secondary battery according to claim 6, characterized in that, Satisfying at least one of the following (1) to (6): (1) The Dv10a of the first positive electrode active material is 0.35 μm ~ 0.37 μm; (2) The Dv90a of the first positive electrode active material is 3.84 μm ~ 7.00 μm; (3) The Dv99a of the first positive electrode active material is 7.00 μm ~ 13 μm; (4) The Dv10b of the second positive electrode active material is 0.36 μm ~ 0.38 μm; (5) The Dv90b of the second positive electrode active material is 2.30 μm ~ 2.88 μm; (6) The Dv99b of the second positive electrode active material is 4.3 μm ~ 11 μm.

8. The secondary battery according to claim 1, characterized in that, Based on the total mass of the first positive electrode active material layer, the mass percentage of the first positive electrode active material is 96%~98%, and the mass percentage of the first conductive agent is 0.5%~0.6%. And / or, based on the total mass of the second positive electrode active material layer, the mass percentage of the second positive electrode active material is 96%~98%, and the mass percentage of the second conductive agent is 0.5%~0.6%.

9. The secondary battery according to claim 1, characterized in that, The first cathode material and the second cathode material each independently include at least one of lithium iron phosphate and lithium manganese iron phosphate; And / or, the first conductive agent and the second conductive agent each independently include at least one of Super P, conductive carbon black, acetylene black, and Ketjen black.

10. An electrical device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 9.

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