Secondary battery and electric device

Abstract

The invention discloses a secondary battery and an electric device, and relates to the technical field of batteries. According to the secondary battery provided by the invention, the absolute value of the Zeta potential of the positive electrode active material layer is controlled to be within a certain range, the roundness and the average particle size of the primary particles of the positive electrode active material are controlled to be within a certain range, and meanwhile, the mass percent of carbon in the positive electrode active material is controlled to be within a certain range; the energy density and the power density of the secondary battery can be effectively improved, so that the prepared secondary battery has excellent low-temperature high-power discharge capability and high-temperature cycle performance.

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] Lead-acid batteries have been used in automotive start-stop batteries for over a century due to their low cost and mature technology. However, they suffer from low energy density, short cycle life, and significant environmental pollution, causing substantial environmental damage from production to disposal, which contradicts the current global trend towards green and low-carbon development. As automobiles become increasingly intelligent and connected, the power consumption of electrical accessories is gradually increasing, and more and more intelligent equipment requires more power. 12V lithium iron phosphate batteries, with their high energy density, long cycle life, good safety performance, low cost, and environmental friendliness, are more in line with the demands of energy conservation and emission reduction. The shift from lead-acid to lithium-ion batteries in the start-stop power supply field is an inevitable trend.

[0003] However, current 12V lithium iron phosphate batteries still face the following challenges: the lithium iron phosphate cathode material suffers from low conductivity and ion diffusion rate. It must not only meet the power requirements for high-rate discharge at -20℃ or even lower temperatures, but also withstand high-temperature cycling at 45℃ and long-term storage at 60℃ or even higher. Therefore, it is necessary to develop lithium iron phosphate cathode materials that can meet both high-power discharge and high-temperature cycling performance requirements. This will be of great significance for the development and application of start-stop high-power batteries. Summary of the Invention

[0004] The purpose of this application is to solve the technical problems of poor low-temperature high-power discharge performance and poor high-temperature cycling performance of existing secondary batteries, and to propose a secondary battery and power device with excellent low-temperature high-power discharge capability and high-temperature cycling performance.

[0005] To achieve the above objectives, a first aspect of this application provides a secondary battery, including 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 active material layer comprising a positive active material, the positive active material comprising carbon. The absolute value of the Zeta potential Z of the positive electrode active material layer satisfies: Z > 10mV; The roundness R of the primary particles of the positive electrode active material satisfies: R > 0.6; The average particle size D of the primary particles of the positive electrode active material satisfies: 50nm≤D≤200nm; The mass percentage C of carbon element in the positive electrode active material satisfies: 1.2% ≤ C ≤ 1.8%.

[0006] As an embodiment of this application, the secondary battery satisfies 1≤μ×υ×U≤2; μ is the ratio of the 3C first-cycle rate discharge capacity of the positive electrode active material under the conditions of voltage 2.0 V~3.7V and temperature 25℃ to the 0.1C first-cycle rate discharge capacity of the positive electrode active material under the conditions of voltage 2.0 V~3.7V and temperature 25℃. υ is the ratio of the discharge capacity of the positive electrode active material at 0.33C and -20℃ to the discharge capacity of the positive electrode active material at 0.1C under the condition of 25℃; U is the average voltage of the positive electrode active material under 3C first-cycle rate discharge conditions of 2.0 V to 3.7 V and 25 °C, in V.

[0007] As an embodiment of this application, 0.75 ≤ μ ≤ 0.9.

[0008] As an implementation scheme of this application, 0.4≤υ≤0.7.

[0009] As an implementation scheme of this application, U>3.10.

[0010] As an embodiment of this application, the oil absorption value W of the positive electrode active material satisfies: 20mL / 100g < W < 50mL / 100g.

[0011] As an embodiment of this application, the iron dissolution rate P of the positive electrode active material satisfies: P < 200 ppm.

[0012] As an embodiment of this application, the proportion of particles with a primary particle size of less than 50 nm in the positive electrode active material is less than 20%, and the proportion of particles with a primary particle size of greater than 200 nm in the positive electrode active material is less than 10%.

[0013] As an embodiment of this application, the positive electrode active material further includes element M, and the mass content of element M is 1000ppm~6000ppm; The element M includes at least one of Mg, Ti, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0014] As an embodiment of this application, the positive electrode active material includes lithium iron phosphate.

[0015] As an embodiment of this application, the method for preparing the positive electrode sheet includes the following steps: (1) Mix the iron source, lithium source and M source according to the dosage ratio, then add the carbon source and water to mix and obtain a slurry; (2) The slurry is ball-milled and sand-milled in sequence. After sand milling, it is stirred and reacted, and then spray-dried to obtain precursor powder. (3) Sinter the precursor powder, and after sintering, allow it to cool naturally. Then crush, sieve, and remove iron to obtain the positive electrode active material. (4) Mix the positive electrode active material, positive electrode binder, positive electrode conductive agent and positive electrode dispersant and then add them to an organic solvent for dispersion. After dispersion, defoam and adjust the pH value with a pH adjuster to obtain positive electrode slurry. (5) The positive electrode slurry is coated on at least one surface of the positive electrode current collector, then dried, cold-pressed and cut to obtain a positive electrode sheet.

[0016] As an embodiment of this application, the areal density of the positive electrode sheet is 7.5 mg / cm³. 2 ~10mg / cm 2 .

[0017] As an embodiment of this application, the film resistance of the positive electrode is 100mΩ~500mΩ.

[0018] As an embodiment of this application, the specific surface area of ​​the positive electrode sheet is 5m². 2 / g~10m 2 / g.

[0019] A second aspect of this application provides an electrical device including the secondary battery described in this application.

[0020] Compared with the prior art, the beneficial effects of this application are: The secondary battery provided in this application effectively improves the energy density and power density of the secondary battery by controlling the absolute value of the Zeta potential of the positive electrode active material layer within a certain range, the roundness and average particle size of the primary particles of the positive electrode active material within a certain range, and controlling the mass percentage of carbon elements in the positive electrode active material within a certain range. This results in a secondary battery with excellent low-temperature high-power discharge capability and high-temperature cycling performance. Attached Figure Description

[0021] Figure 1 Here is a SEM image of the positive electrode active material in Example 1; Figure 2 for Figure 1 The SEM image obtained is analyzed using visualization software; Figure 3 This is a size distribution diagram of the primary particles of the positive electrode active material in Example 1 (the horizontal axis represents the particle size range in nm, and the vertical axis represents the number of particles in the corresponding particle size range in the unit). Detailed Implementation

[0022] 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.

[0023] 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.

[0024] 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.

[0025] In one embodiment of this application, a secondary battery is provided, comprising 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 active material layer comprising a positive active material, the positive active material comprising carbon. The absolute value of the Zeta potential Z of the positive electrode active material layer satisfies: Z > 10mV; The roundness R of the primary particles of the positive electrode active material satisfies: R > 0.6; The average particle size D of the primary particles of the positive electrode active material satisfies: 50nm≤D≤200nm; The mass percentage C of carbon element in the positive electrode active material satisfies: 1.2% ≤ C ≤ 1.8%.

[0026] This application research found that the secondary battery provided by this application can effectively improve the energy density and power density of the secondary battery by controlling the absolute value of the Zeta potential of the positive electrode active material layer within a certain range, the roundness and average particle size of the primary particles of the positive electrode active material within a certain range, and controlling the mass percentage of carbon elements in the positive electrode active material within a certain range. This results in the prepared secondary battery having excellent low-temperature high-power discharge capability and high-temperature cycling performance.

[0027] In one embodiment, the absolute value Z of the Zeta potential of the positive electrode active material layer is 14mV~28mV.

[0028] For example, the absolute value Z of the zeta potential of the positive electrode active material layer can be any point value between 14mV and 28mV or a range value between any two points, such as 14mV, 15mV, 18mV, 20mV, 22mV, 25mV, 28mV, etc.

[0029] It should be noted that the test method for the absolute value of the zeta potential of the positive electrode active material layer is as follows: The determination is carried out in accordance with the national standard GB / T 32668-2016 "General Rules for Electrophoretic Method of Zeta Potential Analysis of Colloidal Particles". Specifically, after disassembling a clean, fully discharged secondary battery, the positive electrode sheet is obtained. The positive electrode active material layer on the positive electrode sheet is scraped off and mixed with a solvent (one or more of water, ethanol, and NMP) at a mass ratio of 1:200. The mixture is ultrasonically dispersed for 30 minutes to form a solution. The zeta potential of the solution is then measured using an electrophoretic light scattering instrument to obtain the absolute value of the zeta potential of the positive electrode active material layer.

[0030] This application research found that limiting the absolute value of the Zeta potential of the positive electrode active material layer to a certain range can enhance the electrostatic repulsion between particles in the positive electrode active material layer, prevent particle aggregation, and improve the dispersion uniformity and stability of particles in the positive electrode active material layer, such as positive electrode active material, positive electrode conductive agent and positive electrode binder, thereby improving the low-temperature high-power discharge performance and high-temperature cycle performance of the secondary battery; in particular, when the absolute value of the Zeta potential of the positive electrode active material layer is further selected within the further preferred range of this application, the overall performance of the obtained secondary battery is even better.

[0031] In one embodiment, the roundness R of the primary particles of the positive electrode active material is R > 0.6.

[0032] In one embodiment, the roundness R of the primary particles of the positive electrode active material is 0.62~0.85.

[0033] For example, the roundness R of the primary particles of the positive electrode active material can be any point value or a range between any two points between 0.62 and 0.85, such as 0.62, 0.64, 0.66, 0.68, 0.7, 0.72, 0.74, 0.76, 0.78, 0.8, 0.82, 0.84, 0.85, etc.

[0034] It should be noted that the test method for the roundness of the primary particles of the positive electrode active material is as follows: A secondary battery is disassembled to obtain the positive electrode sheet. A 30K magnified SEM image of the microstructure at any position on the cross-section of the positive electrode sheet is taken using a scanning electron microscope (SEM). Since there are contrast differences between sample particles in the SEM image, the boundaries of the sample particles in the image can be identified using the visualization software AVIZO. This allows us to obtain the area occupied by a single particle and the particle perimeter. The particle roundness R can then be calculated using the formula: R = 4r × r_s ... 2 .

[0035] In one embodiment, the roundness R of the primary particles of the positive electrode active material is 0.7 to 0.8. For example, it can be 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, etc.

[0036] This application research found that limiting the roundness of the primary particles of the positive electrode active material affects the packing density between the primary particles and the uniformity of the contact points, thereby affecting the migration path of lithium ions. At the same time, roundness also affects the surface smoothness and specific surface area of ​​the positive electrode active material, thereby affecting the wetting ability of the electrolyte to the positive electrode active material. When the roundness of the primary particles of the positive electrode active material is further selected within the range given in this application, the resulting secondary battery has better low-temperature high-power discharge capability and better high-temperature cycling capability.

[0037] For example, the average particle size D of the primary particles of the positive electrode active material can be any point value between 50 nm and 200 nm or a range value between any two points, such as 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, etc.

[0038] It should be noted that the test method for the average particle size of the primary particles of the positive electrode active material is as follows: the secondary battery is disassembled to obtain the positive electrode sheet, and then the microscopic morphology of any position in the positive electrode sheet area is photographed by scanning electron microscopy (SEM) and magnified to 30K. Since there is a contrast difference between sample particles in the SEM image, the boundary of the sample particles in the image can be identified by the contrast difference in the image using the visualization software AVIZO, and then the area occupied by a single particle can be obtained. The particle is equivalent to a circle of the same area to obtain the equivalent diameter of the particle, i.e., the particle size. The average size of all particles at 30K is the average particle size.

[0039] In one embodiment, the average particle size D of the primary particles of the positive electrode active material is 90 nm to 100 nm. For example, it can be 90 nm, 92 nm, 94 nm, 96 nm, 98 nm, 100 nm, etc.

[0040] This application research found that the average particle size of the primary particles of the positive electrode active material affects the length of the lithium ion diffusion path and the size of the reaction surface area with the electrolyte. When the average particle size of the primary particles of the positive electrode active material is selected within the range given in this application, especially within the further preferred range, the resulting secondary battery has better low-temperature high-power discharge capability and better high-temperature cycling capability.

[0041] For example, the mass percentage C of carbon element in the positive electrode active material can be any point value or a range between any two points between 1.2% and 1.8%, such as 1.2%, 1.25%, 1.3%, 1.35%, 1.4%, 1.45%, 1.5%, 1.55%, 1.6%, 1.65%, 1.7%, 1.75%, 1.8%, etc.

[0042] It should be noted that the method for testing the mass percentage of carbon in the positive electrode active material is as follows: a fully discharged, clean secondary battery is disassembled to obtain the positive electrode sheet. The positive electrode active layer on the positive electrode sheet is scraped off and ultrasonically dispersed in NMP for 30 min. After standing for ≥12 h, the upper suspension is discarded, NMP is added again and ultrasonically for 30 min, and it is left to stand for ≥12 h. The upper suspension is discarded, deionized water is added, ultrasonically for 30 min, and it is left to stand for ≥12 h. The upper suspension is then discarded and dried in an oven at 120℃ for 24 h to obtain the positive electrode active material. The carbon content of the obtained lithium iron phosphate powder is then tested using a carbon-sulfur analyzer, referring to GB / T20123-2006.

[0043] In one embodiment, the mass percentage C of carbon element in the positive electrode active material is 1.50% to 1.62%. For example, it can be 1.5%, 1.52%, 1.54%, 1.56%, 1.58%, 1.6%, 1.62%, etc.

[0044] This application research found that the mass percentage of carbon in the positive electrode active material affects the formation of the electronic conductive network, thereby affecting the overall conductivity of the material; it also affects the specific capacity of the positive electrode active material, thereby affecting the energy density of the battery; in addition, it also affects the degree of decomposition reaction of the electrolyte; when the mass percentage of carbon in the positive electrode active material is selected within the range given in this application, especially within the further preferred range, the overall performance of the secondary battery is better.

[0045] In one embodiment, the secondary battery satisfies 1 ≤ μ × υ × U ≤ 2; μ is the ratio of the 3C first-cycle rate discharge capacity of the positive electrode active material under the conditions of voltage 2.0 V~3.7V and temperature 25℃ to the 0.1C first-cycle rate discharge capacity of the positive electrode active material under the conditions of voltage 2.0 V~3.7V and temperature 25℃. υ is the ratio of the discharge capacity of the positive electrode active material at 0.33C and -20℃ to the discharge capacity of the positive electrode active material at 0.1C under the condition of 25℃; U is the average voltage of the positive electrode active material under 3C first-cycle rate discharge conditions of 2.0 V to 3.7 V and 25 °C, in V.

[0046] For example, μ×υ×U can be any point value between 1 and 2 or a range value between any two points, such as 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, etc.

[0047] It should be noted that the testing methods for μ, υ, and U of the positive electrode active material are as follows: Coin charge rate test: Voltage range: 2.0V~3.7V; Nominal capacity: 154mAh / g; Test environment: 25℃ constant temperature chamber; Test procedure: Perform 1 cycle of 0.1C charge / discharge, 2 cycles of 0.33C charge / discharge, 3 cycles of 1C charge / discharge, 3 cycles of 2C charge / discharge, and 3 cycles of 3C charge / discharge. Details are as follows: 1) Let it rest for 5 minutes, then charge it at a constant current of 0.1C to 3.7V, and then maintain a constant voltage of 50uA (approximately 0.02C); 2) Let stand for 5 minutes, then discharge at 0.1C to 2.0V.

[0048] 3) Let it rest for 5 minutes, charge it to 3.7V with a constant current of 0.33C, and then charge it to 50uA (about 0.02C) with a constant voltage. 4) Let stand for 5 minutes, then discharge at 0.33C to 2.0V.

[0049] 5) Let it rest for 5 minutes, charge it to 3.7V at a constant current of 0.33C, and then charge it to 50uA (about 0.02C) at a constant voltage. 6) Let stand for 5 minutes, then discharge at 0.33C to 2.0V.

[0050] 7) Let stand for 5 minutes, then charge at a constant current of 1C to 3.7V, and then maintain a constant voltage of 50uA (approximately 0.02C); 8) Let stand for 5 minutes, then discharge at 1C to 2.0V.

[0051] 9) Let stand for 5 minutes, then charge at a constant current of 1C to 3.7V, and then maintain a constant voltage of 50uA (approximately 0.02C); 10) Let stand for 5 minutes, then discharge at 1C to 2.0V.

[0052] 11) Let stand for 5 minutes, then charge at a constant current of 1C to 3.7V, and then maintain a constant voltage of 50uA (approximately 0.02C); 12) Let stand for 5 minutes, then discharge at 1C to 2.0V.

[0053] 13) Let stand for 30 minutes, charge at a constant current of 2C to 3.7V, then maintain a constant voltage of 50uA (approximately 0.02C); 14) Let stand for 30 minutes, then discharge at 2C to 2.0V.

[0054] 15) Let stand for 30 minutes, charge at a constant current of 2C to 3.7V, then maintain a constant voltage of 50uA (approximately 0.02C); 16) Let stand for 30 minutes, then discharge at 2C to 2.0V.

[0055] 17) Let stand for 30 minutes, charge at a constant current of 2C to 3.7V, then maintain a constant voltage of 50uA (approximately 0.02C); 18) Let stand for 30 minutes, then discharge at 2C to 2.0V.

[0056] 19) Let stand for 30 minutes, charge at a constant current of 3C to 3.7V, then maintain a constant voltage to 50uA (approximately 0.02C); 20) Let stand for 30 minutes, then discharge at 3C to 2.0V.

[0057] 21) Let stand for 30 minutes, charge at a constant current of 3C to 3.7V, then maintain a constant voltage to 50uA (approximately 0.02C); 22) Let stand for 30 minutes, then discharge at 3C to 2.0V. (Obtain the first-cycle 3C discharge capacity and average voltage). 23) Let stand for 30 minutes, charge at a constant current of 3C to 3.7V, then maintain a constant voltage of 50uA (approximately 0.02C); 24) Let stand for 30 minutes, then discharge at 3C to 2.0V.

[0058] Low-temperature test of coin cell: Voltage range: 2.0V~3.7V; Nominal capacity: 154mAh / g; Test environment: High and low temperature chamber; Test procedure: At 25℃, perform one cycle of 0.1C charge-discharge, followed by charging at 0.33C. Then, adjust the chamber temperature to -20℃, let it stand for 3 hours, and then discharge at 0.33C. Details are as follows: 1) Let it rest for 5 minutes, then charge it at a constant current of 0.1C to 3.7V, and then maintain a constant voltage of 50uA (approximately 0.02C); 2) Let stand for 5 minutes, then discharge at 0.1C to 2.0V.

[0059] 3) Let it rest for 5 minutes, charge it to 3.7V with a constant current of 0.33C, and then charge it to 50uA (about 0.02C) with a constant voltage. 4) Adjust the temperature of the incubator to -20℃ and let it stand for 3 hours; 5) Discharge at 0.33C to 2.0V. (To obtain discharge capacity at -20℃).

[0060] This application research found that the μ×υ×U value of the material can be used to preliminarily judge the low-temperature power performance of the secondary battery at the coin cell level, avoiding the waste of resources caused by the performance of the finished secondary battery not meeting the requirements. Therefore, by limiting the μ×υ×U value to 1~2, the secondary battery can have good low-temperature power discharge capability and can better maintain the capacity of the secondary battery, that is, the secondary battery has excellent cycle performance.

[0061] In one embodiment, 0.75 ≤ μ ≤ 0.9.

[0062] For example, μ can be any point value between 0.75 and 0.9 or a range value between any two points, such as 0.75, 0.78, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, etc.

[0063] This study found that the value of μ affects the duration of low-temperature power discharge of the secondary battery, i.e., the number of LTP discharges; when the value of μ is further selected within the above range, the secondary battery can have good low-temperature power discharge capability.

[0064] In one embodiment, 0.4 ≤ υ ≤ 0.7.

[0065] For example, υ can be any point value between 0.4 and 0.7 or a range value between any two points, such as 0.4, 0.42, 0.44, 0.46, 0.48, 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, etc.

[0066] This study found that the value of υ affects the capacity performance of secondary batteries at low temperatures; when the value of υ is further selected within the above range, the secondary battery can have excellent low-temperature discharge capacity while maintaining good cycle stability.

[0067] In one embodiment, U>3.10.

[0068] In one embodiment, U is 3.12~3.26.

[0069] For example, U can be any point value between 3.12 and 3.26 or a range value between any two points, such as 3.12, 3.13, 3.14, 3.15, 3.16, 3.17, 3.18, 3.19, 3.2, 3.21, 3.22, 3.23, 3.24, 3.25, 3.26, etc.

[0070] This study found that the value of U affects the voltage of low-temperature power discharge; when the value of U is further selected within the above range, the secondary battery can maintain a high voltage under high-current and high-power discharge conditions, thus avoiding directly reaching the cutoff condition.

[0071] In one embodiment, the oil absorption value W of the positive electrode active material satisfies: 20mL / 100g < W < 50mL / 100g.

[0072] For example, the oil absorption value W of the positive electrode active material can be any point value or a range between any two points between 20mL / 100g (excluding 20) and 50mL / 100g (excluding 50), such as 21mL / 100g, 22mL / 100g, 24mL / 100g, 26mL / 100g, 28mL / 100g, 30mL / 100g, 32mL / 100g, 34mL / 100g, 36mL / 100g, 38mL / 100g, 40mL / 100g, 42mL / 100g, 44mL / 100g, 46mL / 100g, 48mL / 100g, 49mL / 100g, etc.

[0073] It should be noted that the test method for the oil absorption value of the positive electrode active material is as follows: Referring to GB / T 3780.2-2017 Carbon Black Part 2, Optimized Method for Determination of Oil Absorption Value, the following steps were performed: A secondary battery was disassembled to obtain the positive electrode sheet. The positive electrode active material layer was then scraped off and ultrasonically dispersed in NMP for 30 min. After standing for ≥12 h, the upper suspension was discarded. NMP was added again, and the mixture was ultrasonically dispersed for 30 min, then stood for ≥12 h. The upper suspension was discarded, deionized water was added, and the mixture was ultrasonically dispersed for 30 min, then stood for ≥12 h. The upper suspension was discarded, and the mixture was dried in a 120℃ oven for 24 h to obtain the positive electrode active material sample. The oil absorption value was then tested. The sample was first baked in a 125℃ oven for 2 hours. Oil droplets (dibutyl phthalate, DBP) were then added to the powder sample in the sample chamber of the oil absorption value tester at a rate of 4 mL / min (60 mL sample volume). The rotor in the sample chamber began stirring. As oil was continuously added, the sample gradually formed semi-plastic agglomerates, and the rotor torque continuously increased. When the preset torque value was reached (reaching its maximum and then decreasing by 0.3 Nm), oil dripping and stirring were stopped. The value corresponding to 70% of the maximum torque on the fitted curve was recorded as the oil absorption volume of the sample. The oil absorption value (mL / 100g) was obtained by converting the endpoint oil absorption volume to the mass of the sample to be tested.

[0074] This study found that the oil absorption value of the positive electrode active material affects the slurry processing performance of the material. When the oil absorption value is further selected within the above range, the secondary battery has normal processing performance, good slurry dispersion, and easy drying of moisture in the secondary battery, thus enabling the secondary battery to have both excellent power performance and high-temperature cycling stability.

[0075] In one embodiment, the iron dissolution rate P of the positive electrode active material satisfies: P < 200 ppm.

[0076] In one embodiment, the iron dissolution rate P of the positive electrode active material is 10ppm to 190ppm.

[0077] For example, the iron dissolution rate P of the positive electrode active material can be any point value between 10ppm and 190ppm or a range between any two points, such as 10ppm, 20ppm, 40ppm, 60ppm, 80ppm, 100ppm, 120ppm, 140ppm, 160ppm, 180ppm, etc.

[0078] It should be noted that the test method for the iron dissolution rate of the positive electrode active material is as follows: A positive electrode sheet is obtained by referring to the oil absorption value test of the positive electrode active material. 10g of the positive electrode sheet is placed in hydrochloric acid with a concentration of 0.008mol / L~0.01mol / L, and the mass ratio of the positive electrode sheet to the hydrochloric acid is 1:(20~25). The sheet is soaked at a temperature of 25±5℃ for 2 h~3 h, then filtered to obtain the filtrate. 1mL of the filtrate is transferred and diluted to 50mL. Subsequently, the Fe content is tested by inductively coupled plasma atomic emission spectrometry (ICP-OES) according to EPA 6010D-2018.

[0079] This study found that the iron dissolution rate of the positive electrode active material affects the high-temperature cycle storage performance of the secondary battery. When the iron dissolution rate is reduced within a certain range, the amount of iron dissolved from the positive electrode migrates to the negative electrode during the secondary battery cycle, thereby slowing down the continuous damage to the SEI film of the negative electrode and the large amount of active lithium consumed in repair, thus slowing down the capacity decay of the secondary battery. When the iron dissolution rate is further selected within the above range, the high-temperature cycle performance of the obtained secondary battery is even better.

[0080] In one embodiment, the proportion of particles with a primary particle size of less than 50 nm in the positive electrode active material is less than 20%, and the proportion of particles with a primary particle size of greater than 200 nm in the positive electrode active material is less than 10%.

[0081] In one embodiment, the number of primary particles with a diameter less than 50 nm in the positive electrode active material accounts for 5% to 12%, and the number of primary particles with a diameter greater than 200 nm in the positive electrode active material accounts for 1% to 8%.

[0082] For example, the proportion of primary particles with a diameter less than 50 nm in the positive electrode active material can be any value between 5% and 12%, or a range between any two values, such as 5%, 6%, 8%, 10%, 12%, etc. The proportion of primary particles with a diameter greater than 200 nm in the positive electrode active material can be any value between 1% and 8%, or a range between any two values, such as 1%, 3%, 5%, 8%, etc.

[0083] It should be noted that the test method for the proportion of primary particles with a diameter less than 50 nm and the proportion of primary particles with a diameter greater than 200 nm in the positive electrode active material is as follows: The secondary battery is disassembled to obtain the positive electrode sheet. The microscopic morphology of any position on the cross-section of the electrode sheet is photographed by scanning electron microscopy (SEM) and magnified to 30K. Since there is a contrast difference between sample particles in the SEM image, the boundary of the sample particles in the image can be identified by the contrast difference in the image using the visualization software AVIZO. Then, the area occupied by a single particle can be obtained. The particle is equivalent to a circle of the same area to obtain the equivalent diameter of the particle, i.e., the particle size, as well as the proportion of particle size distribution to specific particle size data.

[0084] This study found that the proportion of primary particles with a diameter less than 50 nm and the proportion of primary particles with a diameter greater than 200 nm in the positive electrode active material affect the length of the lithium-ion solid-phase diffusion path, as well as the number of contact points between particles, thus affecting electron transport. In addition, it also affects the degree of side reactions with the electrolyte. When the proportions of both are further selected within the above-mentioned ranges, the overall performance of the secondary battery is better.

[0085] In one embodiment, the positive electrode active material further includes element M, wherein the mass content of element M is 1000ppm to 6000ppm; The element M includes at least one of Mg, Ti, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0086] For example, the mass content of element M can be any point value between 1000ppm and 6000ppm or a range between any two points, such as 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, etc.

[0087] It should be noted that the test method for the type of element M and the mass content of element M is as follows: a fully discharged, clean secondary battery is disassembled to obtain the positive electrode sheet. After scraping off the positive electrode active layer on the positive electrode sheet, it is placed in NMP and ultrasonically dispersed for 30 min, allowed to stand for ≥12 h, the upper suspension is poured off, NMP is added again, ultrasonication is performed for 30 min, and it is allowed to stand for ≥12 h. The upper suspension is poured off, deionized water is added, ultrasonication is performed for 30 min, and it is allowed to stand for ≥12 h. The upper suspension is poured off, and it is dried in an oven at 120℃ for 24 h to obtain the positive electrode active material. Subsequently, the element content in the positive electrode active material is tested by inductively coupled plasma atomic emission spectrometry (ICP-OES) according to EPA 6010D-2018.

[0088] This study found that introducing element M into the positive electrode active material, the type and mass content of element M affect the diffusion rate and stability of lithium ions. When element M is selected as the above type and the mass content is within the above range, the cycle performance of the secondary battery is better.

[0089] In one embodiment, the specific surface area of ​​the positive electrode active material is 7 m². 2 / g ~12 m 2 / g.

[0090] For example, the specific surface area of ​​the positive electrode active material can be 7m². 2 / g~12 m 2 Any point value between / g or a range of values ​​between any two points, for example, 7 m 2 / g、8 m 2 / g、9 m 2 / g、10 m 2 / g、11 m 2 / g、12 m 2 / g etc.

[0091] It should be noted that the test method for the specific surface area of ​​the positive electrode active material is as follows: a fully discharged, clean secondary battery is disassembled to obtain the positive electrode sheet. The positive electrode active layer on the positive electrode sheet is scraped off and placed in NMP for ultrasonic dispersion for 30 min. After standing for ≥12 h, the upper suspension is discarded. NMP is added again, followed by ultrasonication for 30 min, standing for ≥12 h, discarding the upper suspension, adding deionized water, ultrasonication for 30 min, standing for ≥12 h, discarding the upper suspension, and drying in a 120℃ oven for 24 h to obtain the positive electrode active material. Subsequently, the specific surface area of ​​the positive electrode active material is determined according to GB / T 19587-2017 Gas Adsorption BET Method.

[0092] This study found that the specific surface area of ​​the positive electrode active material has a significant impact on the material's processing performance and electrochemical performance. Reducing the specific surface area to a certain extent can reduce surface energy, improve the dispersion ability and uniformity of the slurry, thereby improving the consistency of the battery, alleviating the problem of local delithiation difficulties, and reducing the risk of lithium plating caused by overcharging and over-discharging. Increasing the specific surface area to a certain extent can effectively improve its kinetic and power performance, thereby improving the overall performance of the secondary battery.

[0093] In one embodiment, the positive electrode active material includes lithium iron phosphate.

[0094] In one embodiment, the method for preparing the positive electrode sheet includes the following steps: (1) Mix the iron source, lithium source and M source according to the dosage ratio, then add the carbon source and water to mix and obtain a slurry; (2) The slurry is ball-milled and sand-milled in sequence. After sand milling, it is stirred and reacted, and then spray-dried to obtain precursor powder. (3) Sinter the precursor powder, and after sintering, allow it to cool naturally. Then crush, sieve, and remove iron to obtain the positive electrode active material. (4) Mix the positive electrode active material, positive electrode binder, positive electrode conductive agent and positive electrode dispersant and then add them to an organic solvent for dispersion. After dispersion, defoam and adjust the pH value with a pH adjuster to obtain positive electrode slurry. (5) The positive electrode slurry is coated on at least one surface of the positive electrode current collector, then dried, cold-pressed and cut to obtain a positive electrode sheet.

[0095] In some embodiments, in step (1), the iron source includes at least one of iron phosphate and iron oxide, the lithium source includes at least one of lithium carbonate and lithium dihydrogen phosphate, and the M source includes at least one of titanium or vanadium oxide.

[0096] In some embodiments, in step (1), the carbon source includes at least one of glucose, sucrose, polyethylene glycol, citric acid, polytetrafluoroethylene, and graphene.

[0097] In some embodiments, the solid content of the slurry in step (1) is 30% to 45%.

[0098] In some embodiments, in step (2), the Dv50 particle size of the solid particles in the slurry after sand milling is 0.2μm~0.3μm.

[0099] In some embodiments, in step (2), the temperature of the stirring reaction is 80°C to 85°C, and the stirring reaction time is 1.5h to 2.5h.

[0100] In some embodiments, the sintering process in step (3) is as follows: first, the temperature is raised to 360℃~400℃ at a heating rate of 8℃ / min~12℃ / min and then held for 3h~5h, and then the temperature is raised to 700℃~800℃ at a heating rate of 8℃ / min~12℃ / min and then held for 9h~11h.

[0101] In some embodiments, in step (4), the positive electrode binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and polyimide; the positive electrode conductive agent includes at least one of conductive carbon black, carbon nanotubes, and graphene; the positive electrode dispersant includes at least one of polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS), polyethylene glycol (PEG), and polyphosphate-polyester composite system; and the organic solvent includes N-methylpyrrolidone.

[0102] In some embodiments, in step (4), the mass ratio of the positive electrode active material, positive electrode binder, positive electrode conductive agent and positive electrode dispersant is positive electrode active material: positive electrode binder: positive electrode conductive agent: positive electrode dispersant = (90~95): (2~4): (2~4): (0.1~0.5).

[0103] In some embodiments, in step (4), the pH adjuster includes at least one of citric acid, acetic acid, and triethanolamine.

[0104] In some embodiments, in step (4), the pH value of the positive electrode slurry is 7 to 10.

[0105] It should be noted that the absolute value Z of the Zeta potential of the positive electrode active material layer can be controlled by the following methods: a. The average particle size of the positive electrode active material can be controlled by the grinding particle size of the precursor, the sintering temperature and the holding time. If the particles of the positive electrode active material are too small, the specific surface area is larger and the surface energy is larger. The carbon coating effect is worse and the material tends to agglomerate, and the absolute value of the Zeta potential is reduced. b. Adjust the carbon coating effect and functional group properties on the surface of the cathode material by selecting the carbon source (e.g., whether to introduce more oxygen-containing functional groups) and the sintering process (e.g., microwave sintering, which results in thinner, more uniform, and denser carbon coating); the more oxygen-containing functional groups on the surface, the larger the absolute value of the Zeta potential. c. By controlling the particle size and roundness of the iron source and the preparation of the slurry, the roundness and specific surface area of ​​the cathode material particles can be controlled. Under the same particle size, the rounder the particles, the smaller the specific surface area, the better the carbon coating effect, and the larger the absolute value of the Zeta potential. d. By controlling the selection and addition amount of dispersant during the preparation of positive electrode slurry, the zeta potential of the slurry can also be adjusted, thereby controlling the absolute value of the zeta potential of the positive electrode active material. e. By controlling the pH adjuster and its addition amount during the preparation of the positive electrode slurry, the zeta potential of the slurry can be adjusted, thereby achieving the regulation of the absolute value of the zeta potential of the positive electrode active material.

[0106] It should be noted that the roundness of the primary particles of the positive electrode active material can be changed by controlling the preparation of the iron phosphate precursor, the grinding time, and the grinding medium.

[0107] It should be noted that the average particle size of the primary particles of the positive electrode active material can be changed by adjusting the precursor grinding particle size, sintering temperature and holding time.

[0108] It should be noted that the mass percentage of carbon in the positive electrode active material can be changed by adjusting the amount of carbon source added, sintering temperature, and time.

[0109] It should be noted that the oil absorption value of the positive electrode active material can be changed by adjusting the particle size, spraying process, porosity, carbon content, etc.

[0110] It should be noted that the iron dissolution rate of the positive electrode active material can be changed by adjusting the type and content of the carbon source.

[0111] It should be noted that the mass content of the M element in the positive electrode active material can be changed by adding more M dopant source.

[0112] In one embodiment, the areal density of the positive electrode sheet is 7.5 mg / cm³. 2 ~10mg / cm 2 .

[0113] For example, the areal density of the positive electrode sheet may be 7.5 mg / cm³. 2 ~10mg / cm 2 The value at any point between or between any two points, for example, 7.5 mg / cm². 2 7.8 mg / cm 2 8 mg / cm 2 8.2 mg / cm 2 8.5 mg / cm 2 8.8 mg / cm 2 9 mg / cm 2 9.2 mg / cm 2 9.5 mg / cm 2 9.8 mg / cm 2 10 mg / cm2 wait.

[0114] It should be noted that the method for testing the areal density of the positive electrode sheet is as follows: A fully discharged, clean secondary battery is disassembled to obtain the positive electrode sheet and a cleaned foil. The positive electrode sheet and the corresponding foil are then cut into 1540.25mm pieces using a standard mold. 2 The positive electrode sheet and foil are weighed separately, and the surface density of the positive electrode sheet can be calculated.

[0115] This study found that the areal density of the positive electrode sheet affects the kinetic performance and energy density of the secondary battery; when the areal density of the positive electrode sheet is selected within the above range, the overall performance of the secondary battery is better.

[0116] It should be noted that the areal density of the positive electrode sheet can be changed through a coating process.

[0117] In one embodiment, the film resistance of the positive electrode is 100mΩ to 500mΩ.

[0118] For example, the film resistance of the positive electrode can be any point value or a range between any two points between 100mΩ and 500mΩ, such as 100 mΩ, 150 mΩ, 200 mΩ, 250 mΩ, 300 mΩ, 350 mΩ, 400 mΩ, 450 mΩ, 500 mΩ, etc.

[0119] It should be noted that the test method for the film resistance of the positive electrode is as follows: a fully discharged and clean secondary battery is disassembled to obtain the positive electrode, and the film resistance of the positive electrode is tested using a film resistance meter.

[0120] This application research found that when the film resistance of the positive electrode is selected within the above range, it can ensure that the secondary battery has excellent low-temperature high-power discharge capability as well as excellent high-temperature sequential storage capability.

[0121] It should be noted that the film resistance of the positive electrode can be changed by adjusting the content of the positive electrode conductive agent.

[0122] In one embodiment, the specific surface area of ​​the positive electrode is 5m². 2 / g~10m 2 / g.

[0123] For example, the specific surface area of ​​the positive electrode sheet can be 5m². 2 / g~10m 2 Any point value between / g or a range of values ​​between any two points, for example, 5 m 2 / g, 5.5 m2 / g、6 m 2 / g, 6.5 m 2 / g、7 m 2 / g, 7.5 m 2 / g、8 m 2 / g, 8.5 m 2 / g、9 m 2 / g, 9.5 m 2 / g、10 m 2 / g etc.

[0124] It should be noted that the test method for the specific surface area of ​​the positive electrode is as follows: a fully discharged, clean secondary battery is disassembled to obtain the positive electrode, and then the specific surface area of ​​the positive electrode is determined by the gas adsorption BET method according to GB / T 19587-2017.

[0125] This study found that selecting a positive electrode with a specific surface area within the above-mentioned range can ensure the capacity utilization and low-temperature power of the secondary battery, while reducing the occurrence of side reactions within the secondary battery and improving its cycle performance.

[0126] It should be noted that the specific surface area of ​​the positive electrode sheet can be changed by material selection, formula design, electrode coating weight, and electrode compaction size.

[0127] In one embodiment, the positive electrode active material layer further includes a positive electrode conductive agent, a positive electrode binder, and a positive electrode additive.

[0128] In one embodiment, the mass ratio of the positive electrode active material, the positive electrode conductive agent, the positive electrode binder and the positive electrode additive in the positive electrode active material layer is (90~95):(2~4):(2~4):(0.1~0.5).

[0129] This application does not have any special requirements for the selection of the positive electrode conductive agent; any positive electrode conductive agent conventionally available in the art can be used. For example, the positive electrode conductive agent may be at least one of conductive carbon black, carbon nanotubes, and conductive graphite.

[0130] This application does not have any special requirements for the selection of the positive electrode binder; any positive electrode binder conventionally available in the art can be used. For example, the positive electrode binder may be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyimide (PI).

[0131] This application does not have any special requirements for the selection of cathode additives; conventional cathode additives available in the art can be used. For example, the cathode additive may be at least one of polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS), polyethylene glycol (PEG), or a polyphosphate-polyester composite system.

[0132] In one embodiment, the secondary battery further includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including a negative active material, a negative binder, a negative conductive agent and a negative thickener.

[0133] 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 may be at least one of artificial graphite, natural graphite, silicon-carbon composite material, silicon suboxide, hard carbon, lithium metal, and lithium titanate.

[0134] 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 may be at least one of carbon black and CNTs.

[0135] This application does not have any special requirements for the selection of the negative electrode binder; any negative electrode binder conventionally available in the art can be used. For example, the negative electrode binder may be at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and sodium alginate (SA).

[0136] 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 at least one of carboxymethyl cellulose (CMC) and polyacrylic acid (PAA).

[0137] It should be noted that the negative electrode can be a single-sided or double-sided electrode. When the negative electrode is a single-sided electrode, the negative active material layer is disposed on one surface of the negative current collector; when the negative electrode is a double-sided electrode, the negative active material layer is disposed on both surfaces of the negative current collector. A single-sided negative electrode region and a double-sided negative electrode region can also coexist on the negative electrode. When both single-sided and double-sided negative electrode regions exist, the thickness of the negative electrode is the same as the thickness of the double-sided negative electrode region.

[0138] This application does not have any special requirements for the selection of the negative electrode current collector; any negative electrode current collector conventionally available in the art can be used. For example, the negative electrode current collector can be copper foil or carbon-coated copper foil.

[0139] In one embodiment, the secondary battery further includes a separator.

[0140] This application does not have any special requirements for the selection of the diaphragm; any diaphragm conventionally available in the art can be used. For example, the diaphragm may be a polypropylene membrane, a polyethylene membrane, a polyvinylidene fluoride membrane, a spandex membrane, an aramid membrane, or a multilayer composite membrane modified with a coating, etc.

[0141] In one embodiment, the secondary battery further includes an electrolyte; the electrolyte includes an organic solvent and a lithium salt.

[0142] This application does not impose any particular restrictions on the selection of organic solvents; conventional organic solvents in the art can be used. For example, the organic solvent may be at least one of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate.

[0143] This application does not impose any particular restrictions on the selection of lithium salts, and conventional lithium salts in the art can be used. For example, the lithium salt may be at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium imine difluorosulfonate, lithium difluorooxalate borate, and lithium trifluoromethanesulfonate.

[0144] In one embodiment, the method for preparing the secondary battery includes the following steps: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. After being wound into a square bare cell, it is installed in the battery casing. Then, it is baked at 95~105℃ to remove water, followed by the injection of electrolyte and sealing. After processes such as standing, hot and cold pressing, formation, and capacity testing, a secondary battery is obtained.

[0145] In one embodiment, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging 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 at least one of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0146] This application does not have any special requirements regarding the shape of the secondary battery. For example, it can be cylindrical, square, or any other arbitrary shape.

[0147] A second aspect of this application provides an electrical device comprising the secondary battery described in this application.

[0148] Exemplary examples show that the electrical device can be a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, or power tool. Specifically, vehicles can be gasoline-powered cars or new energy vehicles; new energy vehicles can be 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 any special limitations on the above-mentioned devices.

[0149] Example 1 This application provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of positive electrode active material S1. Weigh out the iron source (anhydrous iron phosphate, roundness 0.72) and lithium source (lithium carbonate) according to the dosage ratio (molar ratio of 1:1.05). Then add the M source (titanium dioxide, the mass percentage of titanium dioxide is 0.3% based on the total mass of iron and lithium sources). Then add the carbon source (glucose and polyethylene glycol, the mass percentage of glucose is 5% and the mass percentage of polyethylene glycol is 1.5% based on the total mass of iron and lithium sources). Then add pure water to prepare a slurry with a solid content of 42%. S2. The slurry is ball-milled, and the ball-milled slurry is then transferred to a sand mill for sand milling. The particle size Dv50 of the solid particles in the sand milling product is controlled to be in the range of 0.2μm~0.3μm. The temperature of the slurry is kept at 82℃, and after slow stirring for 2 hours, it is spray-dried to obtain the precursor powder. S3. The precursor powder is transferred into a sintering furnace and heated to 380°C at a rate of 10°C / min under a nitrogen protective atmosphere, and held for 4 hours. Then, the temperature is increased to 750°C at a rate of 10°C / min and sintered for 10 hours. The sintered powder is then naturally cooled to obtain sintered powder. The sintered material is then pulverized, sieved, and iron removed by an air jet mill. The iron-removed material is then introduced into a constant temperature and humidity packaging room and vacuum-packed at a humidity ≤10% and a temperature of 25°C to obtain a primary positive electrode active material with an average particle size of 100nm. (2) Preparation of positive electrode sheet S4. The positive electrode active material, positive electrode binder (PVDF), positive electrode conductive agent (SP), and positive electrode additive (PVP) prepared in S3 are mixed at a mass ratio of 93:4.55:2.2:0.25, and then N-methylpyrrolidone solvent is added. Then, the mixture is transferred to a vacuum mixer for high-speed dispersion (2500 r / min, dispersion time 5 h). After dispersion, vacuum reversal defoaming is performed, and the pH of the system is adjusted to 9.5 using acetic acid to obtain a positive electrode slurry with a viscosity of 4000 mPa·s. S5. The positive electrode slurry is uniformly coated on both sides onto a 14μm carbon-coated aluminum foil (including a 1μm thickness for each side of the carbon coating), and the coating density is controlled to be 8mg / cm³. 2 The coated electrode sheets are dried in an oven at 110℃, cold-pressed (pressure of 30 tons, temperature of 25℃), and then slit to obtain positive electrode sheets. (3) Preparation of negative electrode sheet The negative electrode active material (graphite), negative electrode conductive agent (carbon black), negative electrode thickener (CMC), and negative electrode binder (SBR) are mixed in a mass ratio of 94.3:1.5:1.2:3. Deionized water is added as a solvent, and the mixture is stirred until the system is homogeneous to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, dried in an oven, rolled, and slit to obtain the negative electrode sheet. (4) Preparation of electrolyte Ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a 1:1:1 volume ratio. In an argon-atmosphere glove box with a water content of <10 ppm, thoroughly dried lithium hexafluorophosphate was dissolved in the above organic solvent at a concentration of 1 mol / L and mixed thoroughly to obtain the electrolyte. (5) Preparation of secondary batteries The above-mentioned positive electrode sheet, separator (polyethylene) and negative electrode sheet are wound in sequence to obtain a bare cell; the bare cell is placed in an outer packaging, vacuum dried, and then injected with electrolyte; it is then allowed to stand, form, shape, and be capacity tested to obtain a secondary battery. In the example, the secondary battery prepared was disassembled to obtain the positive electrode sheet. The positive electrode active material layer was then scraped off, ultrasonically dispersed in NMP for 30 min, allowed to stand for ≥12 h, the upper suspension was discarded, NMP was added again and ultrasonically for 30 min, allowed to stand for ≥12 h, the upper suspension was discarded, deionized water was added, ultrasonically for 30 min, allowed to stand for ≥12 h, the upper suspension was discarded, and the sample was dried in a 120℃ oven for 24 h to obtain the positive electrode active material sample. The SEM image of the positive electrode active material sample is shown below. Figure 1 As shown, the SEM chart is analyzed using visualization software. Figure 2 As shown, the size distribution diagram of the primary particles is as follows: Figure 3As shown (the horizontal axis represents the particle size range in nm, and the vertical axis represents the number of particles in the corresponding particle size range on the horizontal axis).

[0150] Examples 2-4 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the average particle size of the primary particles of the positive electrode active material is changed by adjusting the particle size of the milling product in step S2 and the sintering temperature in step S3, while achieving the parameters in Tables 1-2.

[0151] Examples 5-7 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of carbon source added in step S1 and the sintering temperature in step S3 are adjusted to change the mass percentage of carbon element in the positive electrode active material, and at the same time achieve the parameters in Tables 1 to 2.

[0152] Examples 8-10 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the roundness of the iron source in step S1 is adjusted to change the roundness of the primary particles of the positive electrode active material, and at the same time achieves the parameters in Tables 1-2.

[0153] Examples 11-12 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of carbon source added in step S1, the particle size of the milling product in step S2, and the sintering temperature in step S3 are adjusted to change the oil absorption value of the positive electrode active material, and at the same time achieve the parameters in Tables 1 to 2.

[0154] Examples 13-14 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of carbon source added in step S1 and the particle size of the milling product in step S2 are adjusted to change the iron dissolution rate of the positive electrode active material, and at the same time achieve the parameters in Tables 1 to 2.

[0155] Examples 15-17 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of M source added in step S1 is adjusted to change the mass content of the M element, while achieving the parameters in Tables 1-2.

[0156] Examples 18-19 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 M source in step S1 is adjusted to change the type of M element, while achieving the parameters in Tables 1-2.

[0157] Examples 20-21 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the coating surface density in step S5 is adjusted to change the surface density of the positive electrode sheet, and at the same time achieves the parameters in Tables 1-2.

[0158] Examples 22-23 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the proportions of the positive electrode conductive agent, positive electrode additive, positive electrode binder and positive electrode active material in step S4 are adjusted to change the film resistance of the positive electrode sheet, and at the same time achieve the parameters in Tables 1 to 2.

[0159] Examples 24-25 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the cold pressing pressure in step S5 is adjusted to change the specific surface area of ​​the positive electrode sheet, and at the same time achieves the parameters in Tables 1-2.

[0160] Comparative Examples 1-2 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 the average particle size of the primary particles of the positive electrode active material is changed by adjusting the particle size of the milling product in step S2 and the sintering temperature in step S3, while achieving the parameters in Tables 1-2.

[0161] Comparative Examples 3-4 This application provides a secondary battery in comparison. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of carbon source added in step S1 and the sintering temperature in step S3 are adjusted to change the mass percentage of carbon element in the positive electrode active material, and at the same time achieve the parameters in Tables 1 to 2.

[0162] 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 roundness of the iron source in step S1 is adjusted to change the roundness of the primary particles of the positive electrode active material, and at the same time achieves the parameters in Tables 1-2.

[0163] Comparative Example 6 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 absolute value of the Zeta potential of the positive electrode active material layer is changed by adjusting the particle size, carbon content and particle roundness of the material, while achieving the parameters in Tables 1-2.

[0164] The following parameters are given in the examples and comparative examples: Z, R, D, C, μ, υ, U, μ×υ×U, W, P, percentage of primary particles with a diameter less than 50 nm (w1), percentage of primary particles with a diameter greater than 200 nm (w2), type of element M, mass content of element M (Q), type of positive electrode active material, areal density of positive electrode sheet (CW), film resistance of positive electrode sheet (R), and specific surface area of ​​positive electrode sheet (S). Table 1 Table 2 The performance of the secondary batteries prepared in the examples and comparative examples was tested, including the following aspects: 1. Low-temperature cold start LTP voltage: 1) Pretreatment of secondary batteries: First, wrap the battery with heat insulation cotton, and then perform the clamping test. Clamping force: 130N; 2) Volume separation and activation at 25±2℃: (1) Let stand for 30 minutes; (2) Discharge at a constant current of 1C to 2.0V; (3) Let stand for 30 minutes; (4) Charge to 3.6V with 1C constant current, then perform constant voltage, and the constant voltage cutoff current is 0.05C; (5) Let stand for 30 minutes; (6) Discharge at 1C constant current to 2.0V (obtain the actual battery capacity C0); (7) Let stand for 30 minutes; (8) Charge 1C0 at a constant current to 3.6V, then perform constant voltage charging. The constant voltage cutoff current is 0.05C0. (9) Let stand for 30 minutes; (10) Discharge at a constant current of 0.5C0 to 2.0V; (11) Let stand for 5 minutes; (12) Charge to 3.6V with a constant current of 0.5C0, then perform constant voltage charging. The constant voltage cutoff current is 0.05C0. (13) Let stand for 5 minutes; (14) Repeat (10) to (13) twice (to ensure the last charge is full); 3) LTP test: (1) Adjust the temperature of the test equipment, place the battery in an environment of -20±2℃, and let it stand for 4 hours; (2) First, perform a high-power (55×C0)W constant power discharge pulse for 0.2s, followed by a low-power (10×C0)W constant power discharge pulse for 59.8s. This constitutes one complete LTP pulse. Repeat this step until the voltage V < 2.0V. Record the lower limit of the (55×C0)W second discharge pulse voltage as the low-temperature cold start LTP voltage; record the number of complete LTP pulses with V < 2.0V for low-temperature cold start LTP.

[0165] 2. Impedance test of 1 second discharge at 25°C (low temperature 25°C DC 1s DCR): 1) Adjust the temperature to 25±2℃ and perform capacity testing to obtain the actual battery capacity C0: (1) Let stand for 30 minutes; (2) Discharge at a constant current of 1C to 2.0V; (3) Let stand for 30 minutes; (4) Charge to 3.6V with 1C constant current, then perform constant voltage, and the constant voltage cutoff current is 0.05C; (5) Let stand for 30 minutes; (6) Discharge at 1C constant current to 2.0V (obtain the actual battery capacity C0); (7) Let stand for 30 minutes; (8) Charge 1C0 at constant current to 3.6V, then perform constant voltage charging. The constant voltage cutoff current is 0.05C0 (100% SOC state). (9) Let stand for 30 minutes; 2) Adjust the temperature to -20±2℃ to conduct a low-temperature DCR test: (10) Let stand for 4 hours; (11) 25C0 constant current discharge for 1s (DCR can be obtained by calculation); (12) Let stand for 30 minutes.

[0166] 3. 45℃ 3C / 3C cycle performance @ 90% / cycle: (1) Charge to 3.6V with 3C constant current, then perform constant voltage, and the constant voltage cutoff current is 0.05C; (2) Let stand for 30 minutes; (3) 3C constant current discharge to 2.0V; (4) Let stand for 30 minutes; (5) Repeat steps (1) to (4). When the loop capacity retention rate reaches 90%, record the number of loops at this time.

[0167] The results are shown in Table 3. Table 3 As can be seen from Table 3, when the technical solution provided in this application is adopted, the obtained secondary battery has excellent low-temperature high-power discharge performance and high-temperature cycling performance; specifically, the obtained secondary battery has a low-temperature cold start LTP voltage of more than 2.21V, a low-temperature cold start LTP number of more than 6, a low-temperature 25C DC 1s DCR of less than 23.75 mΩ, and a 45℃ 3C / 3C cycle performance @90% / cycle of more than 1600 cycles; As can be seen from Examples 1-25 and Comparative Examples 1-2, when the average particle size of the primary particles of the positive electrode active material is within the range given in this application, the resulting secondary battery exhibits excellent overall performance. As can be seen from Examples 1-25 and Comparative Examples 3-4, when the mass percentage of carbon in the positive electrode active material is within the range given in this application, the resulting secondary battery exhibits excellent low-temperature high-power discharge performance and high-temperature cycling performance. As can be seen from Examples 1-25 and Comparative Example 5, when the roundness of the primary particles of the positive electrode active material is within the range given in this application, the desired effect of this application can be achieved. As can be seen from Examples 1-25 and Comparative Example 6, when the absolute value of the Zeta potential of the positive electrode active material layer is within the range given in this application, the desired effect of this application can be achieved.

[0168] 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, 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 active material layer comprising a positive active material, the positive active material comprising carbon, characterized in that... ； The absolute value of the Zeta potential Z of the positive electrode active material layer satisfies: Z > 10mV; The roundness R of the primary particles of the positive electrode active material satisfies: R > 0.6; The average particle size D of the primary particles of the positive electrode active material satisfies: 50nm≤D≤200nm; The mass percentage C of carbon element in the positive electrode active material satisfies: 1.2% ≤ C ≤ 1.8%.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies 1≤μ×υ×U≤2; μ is the ratio of the 3C first-cycle rate discharge capacity of the positive electrode active material under the conditions of voltage 2.0V~3.7V and temperature 25℃ to the 0.1C first-cycle rate discharge capacity of the positive electrode active material under the conditions of voltage 2.0V~3.7V and temperature 25℃. υ is the ratio of the discharge capacity of the positive electrode active material at 0.33C and -20℃ to the discharge capacity of the positive electrode active material at 0.1C under the condition of 25℃; U is the average voltage of the positive electrode active material under 3C first-cycle rate discharge conditions of 2.0V~3.7V and 25℃, in V.

3. The secondary battery according to claim 2, characterized in that, The positive electrode active material satisfies any one of the following: a. 0.75 ≤ μ ≤ 0.9; b. 0.4 ≤ υ ≤ 0.7; c, U>3.

10.

4. The secondary battery according to claim 1, characterized in that, The positive electrode active material satisfies any one of the following: d. The oil absorption value W of the positive electrode active material satisfies: 20mL / 100g < W < 50mL / 100g; e. The iron dissolution rate P of the positive electrode active material satisfies: P < 200 ppm.

5. The secondary battery according to claim 1, characterized in that, The proportion of particles with a primary particle size of less than 50 nm in the positive electrode active material is less than 20%, and the proportion of particles with a primary particle size of greater than 200 nm in the positive electrode active material is less than 10%.

6. The secondary battery according to claim 1, characterized in that, The positive electrode active material also includes element M, and the mass content of element M is 1000ppm~6000ppm; The element M includes at least one of Mg, Ti, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

7. The secondary battery according to claim 6, characterized in that, The positive electrode active material includes lithium iron phosphate.

8. The secondary battery according to claim 7, characterized in that, The method for preparing the positive electrode sheet includes the following steps: (1) Mix the iron source, lithium source and M source according to the dosage ratio, then add the carbon source and water to mix and obtain a slurry; (2) The slurry is ball-milled and sand-milled in sequence. After sand milling, it is stirred and reacted, and then spray-dried to obtain precursor powder. (3) Sinter the precursor powder, and after sintering, allow it to cool naturally. Then crush, sieve, and remove iron to obtain the positive electrode active material. (4) Mix the positive electrode active material, positive electrode binder, positive electrode conductive agent and positive electrode dispersant and then add them to an organic solvent for dispersion. After dispersion, defoam and adjust the pH value with a pH adjuster to obtain positive electrode slurry. (5) The positive electrode slurry is coated on at least one surface of the positive electrode current collector, then dried, cold-pressed and cut to obtain a positive electrode sheet.

9. The secondary battery according to claim 1, characterized in that, The positive electrode sheet satisfies any one of the following: f. The areal density of the positive electrode sheet is 7.5 mg / cm³. 2 ~10mg / cm 2 ; g. The film resistance of the positive electrode is 100mΩ~500mΩ; h. The specific surface area of ​​the positive electrode is 5m². 2 / g~10m 2 / g.

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

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