Positive electrode material, secondary battery and electric device

Abstract

The invention provides a positive electrode material, a secondary battery and an electric device. By controlling the average particle size of the primary particles in the secondary particles of the positive electrode material and the porosity of the cross section obtained by the CP-SEM test of the secondary particles in a certain range, the low-temperature high-power discharge performance and the high-temperature cycle performance of the secondary battery adopting the positive electrode material are effectively improved.

Description

Technical Field

[0001] The present application relates to the technical field of positive electrode materials, and in particular to a positive electrode material, a secondary battery, and an electrical device. Background Art

[0002] Lead-acid batteries have been used in automotive start-stop systems for over a century, thanks to their low cost and mature technology. However, they suffer from low energy density, short cycle life, and significant environmental pollution, causing significant environmental pollution from production to disposal. As vehicles become more intelligent and connected, the power consumption of electrical accessories is gradually increasing, and more and more intelligent devices require more power. Lithium iron phosphate batteries offer high specific energy, long cycle life at room temperature, excellent safety, low cost, and environmental friendliness, making them more suitable for energy conservation and emission reduction. The use of lithium-ion batteries in start-stop power systems is an inevitable trend. However, current lithium iron phosphate batteries for start-stop power systems struggle to balance low-temperature high-power discharge performance with high-temperature cycle performance. Summary of the Invention

[0003] The purpose of this application is to provide a positive electrode material, a secondary battery and an electrical device to improve the low-temperature high-power discharge performance and high-temperature cycle performance of the secondary battery.

[0004] To achieve the above-mentioned objectives, the first aspect of the present application provides a positive electrode material, which includes a lithium iron phosphate positive electrode material, and the lithium iron phosphate positive electrode material includes secondary particles, and the secondary particles are formed by aggregation of primary particles. The average particle size of the primary particles is 80nm~120nm, and the secondary particles have pores. According to CP-SEM testing, the porosity of the cross-section of the secondary particles is 20%~60%.

[0005] As an embodiment of the present application, among the primary particles, the number distribution of particles with a size less than 50 nm accounts for less than 20%, and the number distribution of particles with a size greater than 200 nm accounts for less than 10%.

[0006] As an embodiment of the present application, the primary particles include a lithium iron phosphate core and a carbon coating layer disposed on the surface of the lithium iron phosphate core, and the carbon content in the positive electrode material is 1.0 wt% to 1.8 wt%.

[0007] As an embodiment of the present application, the D of the lithium iron phosphate positive electrode material v 50 is D1μm, the lithium iron phosphate positive electrode material is subjected to 30KN / cm 2 D after the pressure v 50 is D2μm, satisfying: 1.05≤D1 / D2≤1.50.

[0008] As an implementation scheme of the present application, 5≤D1≤12.

[0009] As an embodiment of the present application, the lithium iron phosphate positive electrode material contains M elements, and the M elements include at least one of Mg, Ti, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0010] As an embodiment of the present application, the lithium iron phosphate positive electrode material contains M elements, and the M elements include Ti elements.

[0011] As an embodiment of the present application, the lithium iron phosphate positive electrode material contains M elements, and the M elements include Ti elements and at least one of Mg, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0012] As an embodiment of the present application, the lithium iron phosphate positive electrode material contains an M element, and the mass fraction of the M element is 1200ppm to 4000ppm.

[0013] In a second aspect of the present application, a secondary battery is provided, comprising a positive electrode plate, wherein the positive electrode plate comprises a positive electrode collector and a positive electrode active material layer arranged on at least one surface of the positive electrode collector, wherein the positive electrode active material layer comprises the lithium iron phosphate positive electrode material described in the first aspect of the present application.

[0014] As an embodiment of the present application, the surface density of the positive electrode sheet is 7.5 mg / cm 2 ~10mg / cm 2 .

[0015] As an embodiment of the present application, the BET of the positive electrode sheet is 5m 2 / g~10m 2 / g.

[0016] In a third aspect of the present application, an electrical device is provided, wherein the electrical device includes the secondary battery described in the second aspect of the present application.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] The present application effectively improves the low-temperature, high-power discharge performance and high-temperature cycle performance of secondary batteries using the positive electrode material by controlling the average particle size of the primary particles in the secondary particles of the positive electrode material and the porosity of the cross section obtained by the CP-SEM test of the secondary particles within a certain range. DETAILED DESCRIPTION

[0019] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0020] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0021] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0022] The reagents and instruments used in this application without manufacturer indication are all conventional products that can be purchased commercially.

[0023] An embodiment of the present application provides a positive electrode material, which includes a lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material includes secondary particles, which are formed by aggregation of primary particles. The average particle size of the primary particles is 80 to 120 nm, and the secondary particles have pores. According to CP-SEM (cross-section polishing-scanning electron microscopy) testing, the porosity of the cross section of the secondary particles is 20% to 60%.

[0024] The kinetic performance of lithium iron phosphate secondary batteries is usually directly related to the size of their primary particles. Small particles mean a short lithium ion diffusion path, good kinetics, and better rate performance and low-temperature performance of the secondary battery. However, blindly pursuing small particle size will also affect other properties, such as reduced tap density and compacted density, increased BET, which will reduce the volume energy density of the battery and may lead to increased Fe dissolution during cyclic storage, accelerating performance degradation; in addition, when preparing lithium-ion battery positive electrode material slurry, nano-scale lithium iron phosphate has a large surface area and is prone to agglomeration and difficult to disperse. It has the defects of difficulty in dispersing small particle size particles, long dispersion time, higher stirring speed requirements, greater heat generation, and severe slurry viscosity rebound. The inventors of this application have found that preparing secondary particles while keeping the size of lithium iron phosphate primary particles small can effectively solve the above problems, but at this time it is necessary to control the porosity of the secondary particles within a certain range to ensure their excellent dynamic performance. Furthermore, the inventors have found through research that when the average particle size of lithium iron phosphate primary particles is controlled at 80-120nm and the porosity of the cross section obtained by CP-SEM testing of the secondary particles is 20%-60%, it can ensure that the lithium iron phosphate secondary battery has a high low-temperature power discharge capability and maintain the battery's capacity and high-temperature cycle performance at a better level.

[0025] In this application, the specific method of testing the porosity of the cross section of secondary particles through CP-SEM is as follows: first, the cross section of the positive electrode sheet containing secondary particles is polished using an argon ion beam, and then the microscopic morphology of the material is photographed using a scanning electron microscope (SEM) and the SEM image is magnified at 30K. Since the contrast of the sample particles on the SEM image is obviously different from the pores between the particles, the visualization software AVIZO is used to identify the boundary between the sample particles and the pores on the image based on the image contrast difference, and then the porosity of the cross section is obtained.

[0026] In some embodiments, the average particle size of the primary particles is 80 nm to 120 nm. For example, the average particle size of the primary particles is any value selected from 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 105 nm, 110 nm, 115 nm, and 120 nm, or a range consisting of any two of the aforementioned values.

[0027] In this application, the average particle size is obtained by testing using the following method: a scanning electron microscope (SEM) is used to photograph the microscopic morphology of any position on the surface of the secondary particles and obtain an SEM image after magnification at 30K. Based on the contrast difference between the sample particles on the SEM image, the image contrast difference is used to identify the boundary of the sample particles on the image by the visualization software AVIZO, and then the area occupied by a single particle is obtained. The particles are equivalent to circles of the same area to obtain the equivalent diameter of the particles, that is, the particle size, and the average value of all particle sizes at 30K is the average particle size.

[0028] In some embodiments, the porosity of the cross-section of the secondary particles is 20% to 60% as measured by CP-SEM. For example, the porosity of the cross-section of the secondary particles is any value selected from 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, and 60%, or a range consisting of any two of the foregoing values.

[0029] In some embodiments, the primary particles have a particle size distribution of less than 50 nm, accounting for less than 20%, and a particle size distribution of greater than 200 nm, accounting for less than 10%. When the particle size distribution of the lithium iron phosphate primary particles meets the above conditions, lithium ions have a suitable solid-phase diffusion path, which is conducive to improving kinetics. In addition, the appropriate number distribution reduces side reactions between the positive electrode and the electrolyte, which is conducive to improving long-term cycle performance.

[0030] In some embodiments, the primary particles include a lithium iron phosphate core and a carbon coating disposed on the surface of the lithium iron phosphate core, and the carbon content in the positive electrode material is 1.0 wt% to 1.8 wt%. Exemplarily, the carbon content in the positive electrode material is any value selected from 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, and 1.8 wt%, or a range consisting of any two of the foregoing values. When the carbon content in the positive electrode material is within the above range, the overall electronic conductivity and powder compaction density of the positive electrode material can be significantly improved.

[0031] In some embodiments, the lithium iron phosphate core comprises a chemical formula of Li a Fe (1-y) A y A compound of PO4, wherein 0.95≤a≤1.1, 0≤y≤0.05, and A includes at least one of Mg, Ti, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0032] In some embodiments, the carbon coating layer has a thickness of 2 to 10 nm.

[0033] In some embodiments, the D of the lithium iron phosphate positive electrode material v 50 is D1μm, the lithium iron phosphate positive electrode material is subjected to 30KN / cm 2 D after the pressure v50 is D2μm, satisfying: 1.05≤D1 / D2≤1.50. For example, D1 / D2 is any value among 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50 or a range consisting of any two of the above values. D v The ratio of D1 / D2 reflects the degree of breakage of the positive electrode material particles. When D1 / D2 is within the above range, the positive electrode material can maintain a high mechanical strength, which can alleviate the phenomenon of secondary particle breakage, thereby effectively avoiding the generation of more interfaces due to the breakage of secondary particles, avoiding the intensification of side reactions between the positive electrode material and the electrolyte, and improving the high-temperature cycle performance of the secondary battery.

[0034] In some embodiments, the D of the lithium iron phosphate positive electrode material v 50 is D1 μm, satisfying: 5≤D1≤12. For example, the D v 50 is any value among 5, 6, 7, 8, 9, 10, 11, 12, or a range consisting of any two of the above values.

[0035] In this application, D v 50 represents the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%. It can be conveniently measured by referring to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method using a laser particle size analyzer, such as the Mastersizer 2000E Laser Particle Size Analyzer and the LS-909 Laser Particle Size Analyzer (OMEC) from Malvern Instruments Ltd., UK.

[0036] In some embodiments, the lithium iron phosphate positive electrode material contains an M element, and the M element includes at least one of Mg, Ti, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb, and the mass fraction of the M element is 1200ppm to 4000ppm. Exemplarily, the mass fraction of the M element is any value of 1200ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, or a range consisting of any two of the above values. When the lithium iron phosphate positive electrode material contains the above elements and the addition amount is controlled within the above range, it is beneficial to improve the kinetic performance of the secondary battery and ensure better high-temperature cycle performance.

[0037] In some embodiments, the lithium iron phosphate cathode material comprises an M element, wherein the M element comprises a Ti element. In some embodiments, the lithium iron phosphate cathode material comprises an M element, wherein the M element comprises a Ti element and at least one of Mg, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0038] In a second aspect of the present application, a secondary battery is provided, comprising a positive electrode plate, the positive electrode plate comprising a positive electrode collector and a positive electrode active material layer arranged on at least one surface of the positive electrode collector, the positive electrode active material layer comprising the lithium iron phosphate positive electrode material described in the present application.

[0039] In some embodiments, the surface density of the positive electrode sheet is 7.5 mg / cm 2 ~10mg / cm 2 For example, the surface density of the positive electrode sheet is 7.5 mg / cm 2 , 8mg / cm 2 , 8.5mg / cm 2 , 9mg / cm 2 , 9.5mg / cm 2 、10mg / cm 2 When the surface density of the positive electrode sheet is within the above range, the active ions have a suitable liquid phase transmission path, which is beneficial to improving the kinetic performance of the secondary battery and ensuring a higher energy density.

[0040] In some embodiments, the BET of the positive electrode is 5m 2 / g~10m 2 / g. For example, the positive electrode BET is 5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g、10m 2 By controlling the BET of the positive electrode within the above range, the capacity and low-temperature power of the secondary battery can be guaranteed while avoiding excessive side reactions in the secondary battery, thereby achieving better cycle performance of the secondary battery.

[0041] In some embodiments, the secondary battery further includes a negative electrode sheet, wherein the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0042] The type of negative electrode active material is not specifically limited and can be selected according to actual needs. For example, the negative electrode active material can be artificial graphite, natural graphite, silicon-carbon composite material, silicon oxide, hard carbon, lithium metal and lithium titanate.

[0043] The type of negative electrode current collector is not particularly limited and can be selected according to actual needs. Preferably, copper foil or carbon-coated copper foil can be used.

[0044] In some embodiments, the secondary battery further comprises a separator. The type of separator is not particularly limited and can be selected based on actual needs. The separator can be a polypropylene film, a polyethylene film, a polyvinylidene fluoride film, a spandex film, an aramid film, or a multilayer composite film modified by a coating.

[0045] In some embodiments, the secondary battery further comprises an electrolyte comprising an organic solvent and a lithium salt. In some embodiments, the organic solvent comprises a linear carbonate and a cyclic carbonate, the linear carbonate comprising at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and the cyclic carbonate comprising at least one of ethylene carbonate and propylene carbonate.

[0046] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0047] In some embodiments, the preparation of a secondary battery includes: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, so that the separator is located between the positive and negative electrode sheets to play an isolating role, and then winding them into a square bare battery cell, placing them in a battery shell, and then baking them at 65-95°C to remove water, injecting electrolyte, sealing, and obtaining a secondary battery after standing, hot and cold pressing, formation, clamping, capacity division and other processes.

[0048] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package, and the material of the soft package may be plastic, such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0049] In some embodiments, the shape of the secondary battery is not particularly limited and can be cylindrical, square, or any other shape.

[0050] The third aspect of the present application provides an electric device, which includes the secondary battery described in the second aspect of the present application. The electric device can be an application device such as a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, and an electric tool. The vehicle can be a new energy vehicle, which can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle; the spacecraft includes an airplane, a rocket, a space shuttle, and a spacecraft; the electric toys include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; the electric tools include metal cutting electric tools, grinding electric tools, assembly electric tools, and railway electric tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. The embodiments of the present application do not impose any special restrictions on the above-mentioned devices.

[0051] The following are specific examples of the present application, and the technical solutions of the present application are further described in conjunction with the examples, but the present application is not limited to these examples. The reagents, methods and equipment used in this application, unless otherwise specified, are conventional reagents, methods and equipment in the art.

[0052] Example 1

[0053] Example 1 provides a secondary battery, and the specific preparation process of the secondary battery includes:

[0054] (1) Preparation of lithium iron phosphate positive electrode materials

[0055] Anhydrous iron phosphate and lithium carbonate were weighed in a molar ratio of 1:1.05, and then 0.3wt% titanium dioxide was mixed as an ion doping additive, 5wt% glucose and 1wt% polyethylene glycol were added as organic carbon sources, and 1wt% polyurethane was added as a binder and carbon source. Then, pure water was added to prepare a slurry with a solid content of 40%, and sand milling was performed; the particle size D of the sand milled product was controlled. v 50 is 0.28 μm; the temperature of the slurry is maintained at 80°C ~ 85°C, and after slow stirring for 2 hours, it is spray-dried (the spray pressure is adjusted to 0.9 MPa, and the inner diameter of the nozzle is 0.75 mm) to obtain a carbon-coated lithium iron phosphate precursor powder; the precursor powder is transferred into a sintering furnace, and under a nitrogen protective atmosphere, the temperature is increased from room temperature to 380°C at a heating rate of 10°C / min, and kept warm for 4 hours; then the temperature is increased to 750°C at a heating rate of 10°C / min and sintered for 10 hours, and then naturally cooled to obtain a sintered powder; the sintered material is sieved and iron-removed, and then the iron-removed material is introduced into a constant temperature and humidity packaging room, and vacuum-packed in an environment of humidity ≤10% and temperature of 25°C to obtain a lithium iron phosphate positive electrode material with an average primary particle size of 98 nm. Other parameters of the obtained lithium iron phosphate positive electrode material are shown in Table 1.

[0056] (2) Preparation of positive electrode sheet

[0057] Lithium iron phosphate positive electrode material with an average primary particle size of 98 nm, binder PVDF, conductive agent carbon black, and dispersant PVP (polyvinyl pyrrolidone) were mixed in a mass ratio of 93:4.55:2.2:0.25, and solvent NMP was added; then the mixture was transferred to a vacuum mixer for high-speed dispersion (speed of 2500 r / min, dispersion time of 5 h). After the dispersion was completed, vacuum inversion defoaming was performed to finally obtain a positive electrode slurry with a viscosity of 3500-5000 mPa·s. The positive electrode slurry was evenly coated on both sides of a 14 μm carbon-coated aluminum foil (including a double-sided carbon coating layer with a thickness of 1 μm each), and the coating surface density was controlled to 8 mg / cm 2 The coated electrode is dried in an oven at 100-120°C, cold pressed and cut to obtain the positive electrode.

[0058] (3) Preparation of negative electrode sheet

[0059] The negative electrode active material graphite, conductive agent carbon black, thickener CMC, and binder SBR were mixed in a mass ratio of 94.3:1.5:1.2:3, and deionized water was added as a solvent. The mixture was stirred in a stirrer until the system became uniform to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on the negative electrode current collector copper foil, placed in an oven for drying, and then rolled and cut to obtain negative electrode sheets.

[0060] (4) Preparation of electrolyte

[0061] Organic solvents ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1. In an argon atmosphere glove box with a water content of <10 ppm, fully dried lithium hexafluorophosphate was dissolved in the above organic solvents to a content of 1 mol / L. The mixture was mixed evenly to obtain an electrolyte.

[0062] (5) Preparation of secondary batteries

[0063] The positive electrode sheet, separator (polyethylene), and negative electrode sheet are wound in order to obtain a bare cell; the bare cell is placed in an outer packaging shell, vacuum-dried, and then injected with electrolyte, allowed to stand, formed, shaped, and capacity divided to obtain a secondary battery.

[0064] Example 2

[0065] Example 2 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the particle size D of the sand-milled product during the preparation of the lithium iron phosphate positive electrode material is controlled. v 50 is 0.22μm.

[0066] Example 3

[0067] Example 3 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the particle size D of the sand-milled product during the preparation of the lithium iron phosphate positive electrode material is controlled. v 50 is 0.36μm.

[0068] Example 4

[0069] Example 4 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the solid content of the slurry during the preparation of the lithium iron phosphate positive electrode material is controlled to be 30%.

[0070] Example 5

[0071] Example 5 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the solid content of the slurry during the preparation of the lithium iron phosphate positive electrode material is controlled to be 50%.

[0072] Example 6

[0073] Example 6 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that 3wt% glucose and 0.5wt% polyethylene glycol are added as organic carbon sources and 1wt% polyurethane is used as a binder and carbon source during the preparation of the lithium iron phosphate positive electrode material.

[0074] Example 7

[0075] Example 7 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that 7wt% glucose and 2wt% polyethylene glycol are added as organic carbon sources and 1wt% polyurethane is used as a binder and carbon source during the preparation of the lithium iron phosphate positive electrode material.

[0076] Example 8

[0077] Example 8 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that 0.18 wt % of titanium dioxide is mixed into the lithium iron phosphate positive electrode material as an ion doping additive during the preparation process.

[0078] Example 9

[0079] Example 9 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that 0.5 wt % of titanium dioxide is mixed into the lithium iron phosphate positive electrode material as an ion doping additive during the preparation process.

[0080] Example 10

[0081] Example 10 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that 0.3 wt % titanium dioxide and 0.3 wt % vanadium pentoxide are mixed into the lithium iron phosphate positive electrode material as ion doping additives during the preparation process.

[0082] Example 11

[0083] Example 11 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 of the lithium iron phosphate positive electrode sheet is controlled to be 7.6 mg / cm 2 .

[0084] Example 12

[0085] Example 12 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 of the lithium iron phosphate positive electrode sheet is controlled to be 9.8 mg / cm 2 .

[0086] Example 13

[0087] Example 13 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that, during the preparation process of the lithium iron phosphate positive electrode material, the spray pressure is adjusted to 1.2 MPa and the inner diameter of the nozzle is 0.5 mm.

[0088] Example 14

[0089] Example 14 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that, during the preparation process of the lithium iron phosphate positive electrode material, the spray pressure is adjusted to 0.7 MPa and the inner diameter of the nozzle is 1 mm.

[0090] Comparative Example 1

[0091] Comparative Example 1 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the particle size D of the sand-milled product during the preparation of the lithium iron phosphate positive electrode material is controlled. v 50 is 0.18μm.

[0092] Comparative Example 2

[0093] Comparative Example 2 provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the particle size D of the sand-milled product during the preparation of the lithium iron phosphate positive electrode material is controlled. v 50 is 0.5 μm, and during the preparation process of the lithium iron phosphate positive electrode material, 3 wt% glucose and 0.5 wt% polyethylene glycol are added as organic carbon sources, and 1 wt% polyurethane is used as a binder and carbon source.

[0094] The physical properties of the lithium iron phosphate positive electrode materials in Examples 1 to 14 and Comparative Examples 1 to 2 are shown in Table 1, and the physical properties of the positive electrode sheets are shown in Table 2.

[0095] Table 1

[0096]

[0097]

[0098] Table 2

[0099]

[0100]

[0101] The electrochemical performance of the secondary batteries prepared in the examples and comparative examples was tested. The specific testing methods are as follows. The test results are shown in Table 3:

[0102] Low temperature cold start LTP voltage and times:

[0103] 1. Battery pre-treatment: Wrap the battery with thermal insulation cotton first, then perform the clamp test, clamp force: 130N;

[0104] 2. Volume separation and activation at 25±2℃:

[0105] (1) Let it stand for 30 minutes;

[0106] (2) 1C constant current discharge to 2.0V;

[0107] (3) Let it stand for 30 minutes;

[0108] (4) 1C constant current charging to 3.6V, followed by constant voltage charging, with a constant voltage cutoff current of 0.05C;

[0109] (5) Let stand for 30 minutes;

[0110] (6) 1C constant current discharge to 2.0V (obtain the actual battery capacity C0);

[0111] (7) Let stand for 30 minutes;

[0112] (8) 1C0 constant current charging to 3.6V, then constant voltage charging, constant voltage cut-off current is 0.05C0;

[0113] (9) Let stand for 30 minutes;

[0114] (10) 0.5C0 constant current discharge to 2.0V;

[0115] (11) Let stand for 5 minutes;

[0116] (12) 0.5C0 constant current charging to 3.6V, followed by constant voltage charging, with the constant voltage cutoff current being 0.05C0;

[0117] (13) Let stand for 5 minutes;

[0118] (14) Repeat (10) to (13) twice (make sure the last charge is full);

[0119] 3. LTP test:

[0120] (1) Adjust the temperature of the test equipment and place the battery in a -20±2℃ environment for 4 hours;

[0121] (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 process until the voltage V is less than 2.0V. Record the lower limit of the second (55*C0)W discharge pulse voltage as the low-temperature cold start LTP voltage; record the number of low-temperature cold start LTPs with multiple complete LTP pulses with V less than 2.0V.

[0122] Low temperature 25C discharge 1s DCR:

[0123] 1. Adjust the temperature to 25±2℃ and perform capacity division to obtain the actual battery capacity C0:

[0124] (1) Let it stand for 30 minutes;

[0125] (2) 1C constant current discharge to 2.0V;

[0126] (3) Let it stand for 30 minutes;

[0127] (4) 1C constant current charging to 3.6V, followed by constant voltage charging, with a constant voltage cutoff current of 0.05C;

[0128] (5) Let stand for 30 minutes;

[0129] (6) 1C constant current discharge to 2.0V (obtain the actual battery capacity C0);

[0130] (7) Let stand for 30 minutes;

[0131] (8) 1C0 constant current charging to 3.6V, followed by constant voltage charging, with the constant voltage cutoff current being 0.05C0 (100% SOC state);

[0132] (9) Let stand for 30 minutes;

[0133] 2. Adjust the temperature to -20±2℃ for low temperature DCR test:

[0134] (10) Let stand for 4 hours;

[0135] (11) 25C0 constant current discharge for 1s (DCR can be calculated);

[0136] (12) Let stand for 30 minutes.

[0137] 45℃ cycle performance:

[0138] (1) 3C constant current charging to 3.6V, followed by constant voltage charging, with a constant voltage cutoff current of 0.05C;

[0139] (2) Let stand for 30 minutes;

[0140] (3) 3C constant current discharge to 2.0V;

[0141] (4) Let stand for 30 minutes;

[0142] (5) Repeat steps (1) to (4). When the cycle capacity retention rate reaches 90%, record the number of cycles at this time.

[0143] Table 3

[0144]

[0145]

[0146] As can be seen from Table 3, the present application effectively improves the low-temperature discharge performance and high-temperature cycle performance of the secondary battery using the positive electrode material by controlling the average particle size of the primary particles in the secondary particles of the positive electrode material and the porosity of the cross section obtained by the CP-SEM test of the secondary particles within a certain range.

[0147] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A positive electrode material, characterized in that The positive electrode material includes a lithium iron phosphate positive electrode material, and the lithium iron phosphate positive electrode material includes secondary particles. The secondary particles are formed by aggregation of primary particles. The average particle size of the primary particles is 80nm to 120nm. The secondary particles have pores. According to CP-SEM testing, the porosity of the cross section of the secondary particles is 20% to 60%.

2. The positive electrode material according to claim 1, characterized in that Among the primary particles, the number distribution of particles with a size less than 50 nm accounts for less than 20%, and the number distribution of particles with a size greater than 200 nm accounts for less than 10%.

3. The positive electrode material according to claim 1, characterized in that The primary particles include a lithium iron phosphate core and a carbon coating layer disposed on the surface of the lithium iron phosphate core. The carbon content in the positive electrode material is 1.0 wt% to 1.8 wt%.

4. The positive electrode material according to claim 1, characterized in that The D of the lithium iron phosphate positive electrode material v 50 is D1μm, the lithium iron phosphate positive electrode material is subjected to 30KN / cm 2 D after the pressure v 50 is D2μm, satisfying: 1.05≤D1 / D2≤1.

50.

5. The positive electrode material according to claim 4, characterized in that 5≤D1≤12。 6. The positive electrode material according to claim 1, characterized in that The lithium iron phosphate positive electrode material contains M elements, and the M elements include 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 positive electrode material according to claim 1, characterized in that The lithium iron phosphate positive electrode material contains M elements, and the M elements include Ti elements.

8. The positive electrode material according to claim 1, characterized in that The lithium iron phosphate positive electrode material contains M element, and M element includes Ti element and at least one of Mg, V, Zn, Al, Ni, Co, Mn, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

9. The positive electrode material according to claim 6, characterized in that The lithium iron phosphate positive electrode material contains an M element, and the mass fraction of the M element is 1200ppm to 4000ppm.

10. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, which includes a positive electrode collector and a positive electrode active material layer disposed on at least one surface of the positive electrode collector, wherein the positive electrode active material layer includes the positive electrode material according to any one of claims 1 to 9.

11. The secondary battery according to claim 10, wherein The surface density of the positive electrode sheet is 7.5 mg / cm 2 ~10mg / cm 2 .

12. The secondary battery according to claim 11, wherein The BET of the positive electrode is 5m 2 / g~10m 2 / g.

13. An electrical device, characterized in that: The electric device comprises the secondary battery according to any one of claims 10 to 12.

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