Secondary battery, method for preparing same, battery pack, energy storage system, and electrical equipment

Abstract

The present application relates to the field of energy storage technologies, and particularly to a secondary battery, a preparation method thereof, a battery pack, an energy storage system, and an electrical equipment. The secondary battery includes positive active particles, and the positive active particles include a core and a carbon layer coating the core. The core includes one or more of lithium transition metal phosphates and lithium transition metal oxides; a part of the positive active particles are cracked, and the width of the crack is 10 nm to 30 nm, and the length is 300 nm to 800 nm. The secondary battery provided by the present application has good cycle performance while improving the discharge specific capacity, which is beneficial to wide application in the field of energy storage.

Description

Technical Field

[0001] The present application relates to the technical field of energy storage, and particularly to a secondary battery, a preparation method thereof, a battery pack, an energy storage system, and an electrical device. Background Art

[0002] The large-scale use of traditional fossil fuels has led to increasingly serious problems such as resource shortages and environmental pollution. It is of great significance to accelerate the transformation of the energy structure and develop clean and renewable new energy. Among them, secondary batteries such as lithium-ion batteries are widely used in the fields of 3C products, energy storage systems, and power batteries due to their advantages such as high energy density, long cycle life, good rate performance, and low manufacturing cost. How to further improve the energy density, cycle life, and safety performance of secondary batteries has also become a research hotspot in the field of secondary batteries. At present, secondary batteries are difficult to balance high capacity and long cycle performance, which limits their application in the energy storage field. Summary of the Invention

[0003] Based on this, it is necessary to provide a secondary battery, a preparation method thereof, a battery pack, an energy storage system, and an electrical device to solve the problem that secondary batteries are difficult to balance high capacity and long cycle performance.

[0004] The above object of the present application is achieved by the following technical solutions:

[0005] In the first aspect of the present application, a secondary battery is provided. The secondary battery includes positive electrode active particles, and the positive electrode active particles include a core and a carbon layer coating the core. The core includes one or more of lithium transition metal phosphate and lithium transition metal oxide;

[0006] Some of the positive electrode active particles are cracked, and the width of the crack is 10 nm to 30 nm, and the length is 300 nm to 800 nm.

[0007] In some embodiments, 2.5% to 8% of the positive electrode active particles are cracked.

[0008] In some embodiments, the tap density of the positive electrode active particles is 2.4 g / cm 3 ~2.8 g / cm 3 .

[0009] In some embodiments, the D10 particle size of the positive electrode active particles is 0.3 μm to 0.6 μm, the D50 particle size is 0.6 μm to 1.2 μm, and the D90 particle size is 2 μm to 4 μm.

[0010] In some embodiments, the specific surface area of the positive electrode active particles is ≥10.63 m 2 / g, and the pore volume is ≥0.0052 cm 3 / g.

[0011] In a second aspect of the present application, a method for preparing a secondary battery is provided, including the following steps:

[0012] Providing a precursor, the precursor includes a core and a carbon layer coating the core, and the core includes one or more of lithium transition metal phosphate and lithium transition metal oxide;

[0013] Performing segmented compaction treatment on the precursor to obtain positive electrode active particles;

[0014] Assembling the positive electrode active particles to obtain the secondary battery;

[0015] Part of the positive electrode active particles are cracked, and the width of the crack is 10 nm to 30 nm, and the length is 300 nm to 800 nm.

[0016] In some embodiments, the segmented compaction treatment is carried out under a pressure of 360 MPa to 372 MPa, and the total time is 300 s to 750 s.

[0017] In some embodiments, the segmented compaction treatment includes at least three stages of compaction treatment, and the pressure of each stage of compaction treatment is not lower than the pressure of the previous stage of compaction treatment.

[0018] In some embodiments, the segmented compaction treatment is a five-stage compaction treatment, including the following steps:

[0019] Under a pressure of 360 MPa to 362 MPa, maintaining the pressure for 100 s to 150 s;

[0020] Under a pressure of 362 MPa to 364 MPa, maintaining the pressure for 100 s to 150 s;

[0021] Under a pressure of 364 MPa to 366 MPa, maintaining the pressure for 100 s to 150 s;

[0022] Under a pressure of 366 MPa to 368 MPa, maintaining the pressure for 100 s to 150 s;

[0023] Under a pressure of 368 MPa to 372 MPa, maintaining the pressure for 100 s to 150 s.

[0024] In some embodiments, the core includes a lithium iron phosphate material, and the method for preparing the precursor includes the following steps:

[0025] Mixing lithium phosphate, ferrous salt, surfactant and solvent to obtain a mixed solution;

[0026] Performing heat treatment on the mixed solution to obtain a reaction slurry;

[0027] Cool, wash, filter, and dry the reaction slurry to obtain a powder.

[0028] Mix the powder and a carbon source, and perform a calcination treatment to obtain the precursor.

[0029] In some embodiments, the lithium phosphate includes one or more of lithium phosphate, dilithium hydrogen phosphate, and lithium dihydrogen phosphate.

[0030] In some embodiments, the ferrous salt includes one or more of ferrous sulfate, ferrous chloride, ferrous acetate, and ferrous oxalate.

[0031] In some embodiments, the molar ratio of phosphorus element in the lithium phosphate to iron element in the ferrous salt is (1 - 1.2):2.

[0032] In some embodiments, the surfactant includes one or more of alkylbenzene sulfonate, alkyl sulfonate, α-olefin sulfonate, alkyl naphthalene sulfonate, lignin sulfonate, succinate sulfonate, fatty alcohol sulfate, and fatty alcohol polyoxyethylene ether sulfate.

[0033] In some embodiments, the mass fraction of the surfactant in the mixed solution is 1% - 2%.

[0034] In some embodiments, the carbon source includes one or more of glucose, sucrose, fructose, ascorbic acid, polyethylene glycol, and polyvinyl alcohol.

[0035] In some embodiments, the molar ratio of the powder to the carbon source is 1:(1.5 - 2.5).

[0036] In some embodiments, the heating treatment includes the following steps: perform a water bath heating at 120°C - 180°C for 12 h - 18 h.

[0037] In some embodiments, the drying treatment includes the following steps: perform a vacuum drying at 50°C - 100°C for 10 h - 24 h.

[0038] In some embodiments, the calcination treatment includes the following steps: perform a calcination at 500°C - 900°C for 8 h - 12 h in a protective atmosphere.

[0039] In the third aspect of the present application, there is provided a battery pack, including a battery box and a plurality of secondary batteries disposed in the battery box, where the secondary battery includes the secondary battery as described above, or includes a secondary battery prepared by the preparation method of the secondary battery as described above.

[0040] In the fourth aspect of the present application, there is provided an energy storage system, including the battery pack as described above.

[0041] In the fifth aspect of the present application, an electrical device is provided, including the energy storage system as described above.

[0042] The present application has at least the following beneficial effects:

[0043] In traditional technology, compaction treatment is usually adopted to obtain a positive electrode active material with a high tap density to improve the battery capacity. Although compaction treatment has been carried out, the tap density of the current positive electrode active material is still low, which reduces the conductivity of the material, affects the consistency and uniformity of the positive electrode active layer, and limits the energy density of the battery. This is because in traditional compaction treatment, cracking of the positive electrode active particles needs to be avoided as much as possible. On the one hand, the cracks on the particle surface will affect the coating effect of the carbon layer, resulting in poor conductivity of the material and being unfavorable for the improvement of the discharge specific capacity. On the other hand, the electrolyte will enter the particle interior along the cracks, increasing the interfacial side reactions and causing the simultaneous reduction of the structural stability, thermal stability and cycle stability of the material, ultimately leading to the deterioration of the battery cycle performance.

[0044] However, through research, the applicant found that by inducing cracking of some of the positive electrode active particles, cracks with a width of 10 nm to 30 nm and a length of 300 nm to 800 nm are formed on the surface of the cracked particles, which can expose the electrochemically reactive sites of the inner core, promote the insertion and extraction of lithium ions during charge and discharge, and enhance the kinetic performance of the material. While cracks of appropriate sizes are formed in some particles, the positive electrode active particles exhibit a higher tap density, which is beneficial for improving the discharge specific capacity of the battery, and the structural stability of the particles remains basically the same. The improvement of the kinetic performance and the increase of the active sites are sufficient to make up for the difference in conductivity caused by the cracking of the carbon layer. Generally speaking, it can effectively avoid the reduction of the battery cycle performance. Therefore, the secondary battery provided by the present application has good cycle performance while improving the discharge specific capacity, which is beneficial for wide applications in the energy storage field. Description of the Drawings

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the present application and understand the present application and its beneficial effects more completely, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can obtain other drawings based on these drawings without creative efforts.

[0046] Figure 1 It is a schematic flow chart of the preparation method of the secondary battery in an embodiment;

[0047] Figure 2 It is a schematic flow chart of the preparation method of the precursor in an embodiment;

[0048] Figure 3XRD comparison chart of the positive electrode active particles of Example 1 and Comparative Example 1;

[0049] Figure 4 SEM image of the positive electrode active particles of Example 1;

[0050] Figure 5 SEM image of the positive electrode active particles of Comparative Example 1. Detailed implementation manners

[0051] To facilitate the understanding of this application, the following further elaborates on this application in conjunction with specific examples. However, this application can be implemented in many different forms and is not limited to the examples described herein. On the contrary, the purpose of providing these examples is to make the understanding of the disclosed content of this application more thorough and comprehensive.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs. The terms used in the description of this application herein are only for the purpose of describing specific examples and are not intended to limit this application.

[0053] In this application, the meaning of "and / or" includes any and all combinations of one or more of the related listed items. The meaning of "at least one" is more than one, such as one, two or more. The meaning of "multiple" or "several" is at least two, such as two, three, etc. The meaning of "multiple layers" is at least two layers, such as two layers, three layers, etc., unless otherwise specifically defined. In the description of this application, the meaning of "several" is at least one, such as one, two, etc., unless otherwise specifically defined.

[0054] When a numerical range is disclosed in this application, the above range is considered continuous and includes the minimum and maximum values of the range, as well as each value between such minimum and maximum values. Further, when the range refers to an integer, it includes each integer between the minimum and maximum values of the range. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed in this application should be understood to include any and all sub-ranges subsumed therein.

[0055] Unless otherwise specified, all steps of this application can be carried out sequentially or randomly. For example, the method includes steps (a) and (b), indicating that the method can include steps (a) and (b) carried out sequentially, or can also include steps (b) and (a) carried out sequentially. For example, when it is mentioned that the method may further include step (c), it means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c) in sequence, or can also include steps (a), (c), and (b), or can include steps (c), (a), and (b), etc.

[0056] In this application, "above" or "below" both include the corresponding number. For example, below 1 includes 1.

[0057] The temperature parameter in this application, unless otherwise specified, allows both constant temperature treatment and variation within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument, and allows fluctuations within ranges such as ±5°C, ±4°C, ±3°C, ±2°C, ±1°C.

[0058] In this application, room temperature refers to indoor temperature, normal temperature or general temperature. Generally, the range of room temperature can be any one of the following temperature ranges: 23°C ± 2°C, 25°C ± 5°C or 20°C ± 5°C.

[0059] Term

[0060] Unless otherwise stated or there is a contradiction, the terms or phrases used herein have the following meanings:

[0061] Compaction density: It refers to the ratio of the surface density to the thickness of the material, with the unit of g / cm 3 . Generally, the greater the compaction density, the higher the capacity of the battery. Therefore, the compaction density is also regarded as one of the reference indicators of the energy density.

[0062] Particle size: For spherical particles, the particle size refers to the diameter of the spherical particle. For non-spherical particles, the particle size generally refers to the equivalent particle size of the non-spherical particle (generally simply referred to as particle size), and the equivalent particle size of the particle can be obtained by using a scanning electron microscope (SEM) or a laser particle size analyzer. Among them, the equivalent particle size means that when a certain physical property of a particle is the same or similar to that of a homogeneous spherical particle, the diameter of the spherical particle is used to represent the diameter of this actual particle. Unless otherwise stated or there is a contradiction, the particle size in this application all represents the equivalent particle size.

[0063] Particle size distribution parameter: In the particle size distribution curve of particles, the particle size corresponding to the cumulative particle size distribution percentage reaching N% is called the DN particle size, indicating that the proportion of particles smaller than this particle size in all particles reaches N%, where N = 0 to 100. When N = 100, the D100 particle size represents the particle size corresponding to the cumulative particle size distribution percentage reaching 100%. When N = 50, the D50 particle size is the particle size corresponding to the cumulative particle size distribution percentage reaching 50%, representing the median particle size or median diameter, indicating that the particles smaller and larger than this particle size each account for 50%. For example, D50 particle size = 1 mm means that the particles with a particle size smaller than 1 mm and the particles with a particle size larger than 1 mm each account for 50% in all particles. The DN particle size can be measured by a laser particle size analyzer.

[0064] Span: Also known as the span, its calculation formula is: Span = (D90 - D10) ÷ D50. The smaller the span, the more concentrated the particle size distribution; the larger the span, the greater the particle size difference and the more dispersed the distribution.

[0065] Specific Surface Area: It refers to the total area per unit mass of solid materials, with the unit of m 2 / g, and it can be characterized with reference to GB / T 19587 - 2004.

[0066] Pore volume: Also known as pore capacity (Pore Volume, Vg), it refers to the total volume of fine pores per unit mass of porous materials, with the unit of cm 3 / g. The pore volume can be measured by the carbon tetrachloride method, that is, under a certain carbon tetrachloride vapor pressure, carbon tetrachloride condenses and fills the fine pores of the porous material, and the volume of the condensed carbon tetrachloride at this time is the pore volume of the porous material.

[0067] With the rapid development of new energy technologies, due to the increasing requirements for the energy density, cycle life, and safety performance of secondary batteries, the improvement of the performance of cathode active materials has become a research hotspot in the battery field.

[0068] The tap density of the powder is of great significance in the application of positive and negative active materials, which is mainly manifested in the following aspects: (1) A high tap density increases the density of the active material. After compaction treatment, the voids between the powder particles are reduced, and the content of the active substance per unit volume increases, which helps to improve the capacity and energy density of the battery; (2) The reduction of the voids between the powder particles is also beneficial to reducing the contact resistance between the particles, improving the electron transmission path, and enhancing the conductivity of the material; (3) A high tap density can optimize the powder particle distribution, promote uniform particle distribution, reduce the slurry agglomeration problem during the subsequent electrode coating and rolling process, optimize the processing performance, and thus improve the consistency and uniformity of the electrode; (4) During the homogenization coating process, the active material with a high tap density is more easily mixed uniformly with the conductive agent and the binder, improving the uniformity and consistency of the slurry, and thus reducing the defects of the active layer. However, traditional secondary batteries are difficult to balance high capacity and long cycle performance, which limits their application in the energy storage field.

[0069] Based on this, in the first aspect of the present application, a secondary battery is provided, aiming to solve the problem that traditional secondary batteries are difficult to balance high capacity and long cycle performance, so as to promote their wide application in the energy storage field.

[0070] In some embodiments, the secondary battery includes positive active particles, and the positive active particles include a core and a carbon layer coating the core, and the core includes one or more of lithium transition metal phosphates and lithium transition metal oxides;

[0071] Some of the positive active particles are cracked, and the width of the crack is 10 nm to 30 nm, and the length is 300 nm to 800 nm.

[0072] Traditional technologies usually adopt compaction treatment to obtain positive active materials with a high tap density to improve the battery capacity. Although compaction treatment has been carried out, the current tap density of the positive active material is still low, which reduces the conductivity of the material, affects the consistency and uniformity of the positive active layer, and limits the energy density of the battery. This is because traditional compaction treatment needs to avoid cracking of the positive active particles as much as possible. On the one hand, the cracks on the particle surface will affect the coating effect of the carbon layer, resulting in poor conductivity of the material and being unfavorable for the improvement of the discharge specific capacity. On the other hand, the electrolyte will enter the particle interior along the cracks, increasing the interfacial side reactions and causing the simultaneous reduction of the structural stability, thermal stability and cycle stability of the material, and ultimately leading to the deterioration of the battery cycle performance.

[0073] However, through research, the applicant has found that by causing some of the positive electrode active particles to crack, cracks with a width of 10 nm to 30 nm and a length of 300 nm to 800 nm are formed on the surface of the cracked particles, which can expose the electrochemically reactive sites of the inner core, promote the insertion and extraction of lithium ions during the charge and discharge process, and enhance the kinetic performance of the material. While cracks of appropriate size are formed in some of the particles, the positive electrode active particles exhibit a higher tap density, which is beneficial to improving the discharge specific capacity of the battery, and the structural stability of the particles remains basically the same. The improvement of the kinetic performance and the increase in the active sites are sufficient to compensate for the difference in conductivity caused by the cracking of the carbon layer. Generally speaking, it can effectively avoid the reduction of the battery cycle performance. Therefore, the secondary battery provided in this application has good cycle performance while improving the discharge specific capacity, which is beneficial to its wide application in the energy storage field.

[0074] In this application, the secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charge and discharge process of the battery, active ions are inserted and extracted back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, mainly to prevent short circuit between the positive and negative electrodes, and at the same time allows ions to pass through.

[0075] The positive electrode sheet will be described in detail below.

[0076] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector. It can be understood that the positive electrode current collector has two opposite surfaces in the thickness direction, and the positive electrode active layer is disposed on one or both of the two opposite surfaces of the positive electrode current collector.

[0077] In some embodiments, the positive electrode current collector includes a metal current collector or a composite current collector. As an example, the metal current collector can be made of aluminum foil. The composite current collector includes a polymer layer and a metal layer disposed on at least one surface of the polymer layer; as an example, the material of the polymer layer can be selected from one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE); the material of the metal layer can be selected from one or more of aluminum, nickel, titanium, silver, and alloys of the above metals.

[0078] In some embodiments, the positive electrode active layer at least includes positive electrode active particles. The positive electrode active particles include a core and a carbon layer coating the core. The core includes one or more of lithium transition metal phosphates and lithium transition metal oxides. Among them, the lithium transition metal phosphates include one or more of lithium iron phosphate (LiFePO4, LFP), lithium manganese phosphate (LiMnPO4, LMP), and lithium iron manganese phosphate (LFMP). The lithium transition metal oxides include one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. As an example, the lithium cobalt oxide is selected from LiCoO2; the lithium nickel oxide is selected from LiNiO2; the lithium manganese oxide is selected from LiMnO2, LiMn2O4, etc.; the lithium nickel cobalt oxide is selected from LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM 811 ), etc.; the lithium nickel cobalt aluminum oxide is selected from LiNi 0.8 Co 0.15 Al 0.05 O2. Further, the core includes one or more of lithium iron phosphate (LFP) and lithium nickel cobalt oxide (i.e., ternary positive electrode material). Still further, the core is lithium iron phosphate (LFP).

[0079] In the present application, the carbon layer can completely coat all surfaces of the core to form a continuous film structure, or can partially coat at least a part of the surface of the core to form a dispersed island structure. Further, the carbon layer completely coats all surfaces of the core to form a continuous film structure, thereby improving the conductivity of the positive electrode active particles.

[0080] In some embodiments, in the positive electrode active particles, the thickness of the carbon layer is 2 nm to 10 nm, including but not limited to 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, or 10 nm.

[0081] In the core material of the positive active particles, LiFePO₄ (LFP) is widely used in the fields of energy storage systems and power batteries due to its high safety, long cycle life, relatively low cost, and good thermal stability. However, the current LFP cathode materials have a low tap density, which limits the improvement of their energy density and affects the consistency and uniformity of the positive active layer. In addition, in order to improve the conductivity, the surface of the LFP cathode material is usually coated with a carbon layer, but this makes the insertion and extraction of lithium ions relatively hindered, which is not conducive to the improvement of the cycle performance.

[0082] To address the above problems, in this application, by causing the surface of some positive active particles to crack and form cracks with appropriate sizes, the tap density, specific surface area, and pore volume of the positive active particles can be increased, thereby improving the kinetic performance of the material while increasing the discharge specific capacity to compensate for the negative effects of particle cracking and effectively avoiding the reduction of the cycle performance, enabling the secondary battery to have both high capacity and long cycle performance.

[0083] It can be understood that the method of improving both high capacity and cycle performance by causing some particles to crack is not only applicable to lithium iron phosphate (LFP), but also applicable to lithium transition metal phosphates such as lithium manganese phosphate (LMP) and lithium manganese iron phosphate (LFMP), as well as lithium transition metal oxides such as ternary cathode materials. This application does not make special limitations on the core material of the positive active particles.

[0084] In some embodiments, the surface of the cracked particles has cracks. Among them, the width of the cracks is 10 nm to 30 nm, including but not limited to 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, 22 nm, 25 nm, 28 nm, or 30 nm; the length of the cracks is 250 nm to 800 nm, including but not limited to 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, or 800 nm. Further, the width of the cracks is 10 nm to 20 nm, and the length is 280 nm to 500 nm.

[0085] Understandably, the width and length of the cracks can be characterized by a Scanning Electron Microscope (SEM). The depth of the cracks can be characterized by a Transmission Electron Microscope (TEM), and the range is from a few nanometers to several hundred nanometers, which is not particularly limited in this application. In the cracked cathode active particles, the cracks start from the carbon layer on the particle surface and gradually extend inward. Some cracks can extend to the core, causing the surface layer of the core close to the carbon layer to also crack, thereby exposing more electrochemically active sites, promoting the migration and transmission rate of lithium ions, and thus improving the kinetic performance of the material. However, this application is not limited thereto. Some cracks may not extend to the core, that is, only the carbon layer cracks and exposes the electrochemically active sites on the surface of the core. This is also beneficial to improving the kinetic performance of the material.

[0086] In some embodiments, in the cathode active particles, the proportion of the cracked particles is 2.5% - 8%, including but not limited to 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5% or 8%. Further, in the cathode active particles, the proportion of the cracked particles is 3% - 4%.

[0087] Thus, 2.5% - 8% of the cathode active particles are cracked. The formed cracks can expose more active sites, and at the same time increase the specific surface area and pore volume of the particles, making it easier for lithium ions to embed and extract, and the kinetic performance is significantly improved. At the same time, a small number of particles are cracked, so that the overall structural stability of the cathode active particles is basically not affected. The improvement of the kinetic performance and the increase of the active sites are sufficient to make up for the difference in conductivity caused by the cracking of the carbon layer. Therefore, it will not have a negative impact on the cycling performance.

[0088] In some embodiments, the tap density of the cathode active particles is 2.4 g / cm 3 ~2.8 g / cm 3 , including but not limited to 2.4 g / cm 3 , 2.45 g / cm 3 , 2.5 g / cm 3 , 2.55 g / cm 3 , 2.6 g / cm 3 , 2.65 g / cm 3 , 2.7 g / cm 3 , 2.75 g / cm 3 or 2.8 g / cm 3 , and further preferably 2.5 g / cm 3 ~2.7 g / cm 3 .

[0089] Therefore, the tap density of the positive electrode active particles is high, which can enhance the conductivity of the positive electrode active particles, improve the consistency and uniformity of the positive electrode active layer, and is of great significance for the preparation of secondary batteries with large capacity and high energy density.

[0090] In some embodiments, the D10 particle size of the positive electrode active particles is 0.3 μm to 0.6 μm, the D50 particle size is 0.6 μm to 1.2 μm, and the D90 particle size is 2 μm to 4 μm. As an example, the D10 particle size of the positive electrode active particles can be 0.3 μm, 0.4 μm, 0.5 μm or 0.6 μm, the D50 particle size can be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm or 1.2 μm, and the D90 particle size can be 2 μm, 2.2 μm, 2.5 μm, 2.8 μm, 3 μm, 3.2 μm, 3.5 μm, 3.8 μm or 4 μm. Further, the D10 particle size of the positive electrode active particles is 0.35 μm to 0.45 μm, the D50 particle size is 0.7 μm to 0.9 μm, and the D90 particle size is 2.2 μm to 2.5 μm.

[0091] Therefore, the particle size distribution range of the positive electrode active particles is narrow, and the particle size uniformity is good, which is beneficial to promoting capacity utilization and extending the cycle life. If the particle size of the positive electrode active particles varies greatly, the following phenomena will occur: during the charging process, the large-size and large-volume particles are not completely de-lithiated inside, resulting in low capacity utilization; during the discharging process, the small-size and small-volume particles are prone to over-discharge and damage.

[0092] In some embodiments, the span of the positive electrode active particles is 2.2 to 3.2, including but not limited to 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, and further preferably 2.3 to 2.6.

[0093] In some embodiments, the specific surface area of the positive electrode active particles ≥ 10.63 m 2 / g, including but not limited to 10.63 m 2 / g, 10.65 m 2 / g, 10.68 m 2 / g, 10.7 m 2 / g, 10.72 m 2 / g, 10.75 m 2 / g, 10.78 m 2 / g or 10.8 m 2 / g, and further preferably 10.63 m 2 / g to 10.78 m 2 / g.

[0094] In some embodiments, the pore volume of the positive electrode active particles is ≥ 0.0052 cm 3 / g, including but not limited to 0.0052 cm 3 / g, 0.0053 cm 3 / g, 0.0054 cm 3 / g, 0.0055 cm 3 / g, 0.0056 cm 3 / g, 0.0057 cm 3 / g, 0.0058 cm 3 / g or 0.0060 cm 3 / g, and further optionally 0.0052 cm 3 / g to 0.0058 cm 3 / g.

[0095] Due to the cracking of some particles, the specific surface area and pore volume of the overall positive electrode active particles are improved, which is beneficial to accelerating the migration and transmission rate of lithium ions, thereby improving the kinetic performance of the material.

[0096] In some embodiments, the positive electrode active layer further includes one or more of a positive electrode conductive agent and a positive electrode binder. Among them, the positive electrode conductive agent includes one or more of conductive carbon black, conductive graphite, acetylene black, Ketjen black, carbon quantum dots, carbon nanotubes, graphene, and carbon nanofibers. The positive electrode binder includes one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0097] In some embodiments, the method for preparing the positive electrode sheet includes the following steps: dispersing the positive electrode active particles, the positive electrode binder, and the positive electrode conductive agent in a solvent to obtain a positive electrode slurry; coating the positive electrode slurry on at least one surface of the positive electrode current collector, and after processes such as drying and cold pressing, obtaining the positive electrode sheet. Among them, the solvent of the positive electrode slurry can be N-methylpyrrolidone (NMP).

[0098] The following gives a detailed description of the negative electrode sheet.

[0099] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector. It can be understood that the negative electrode current collector has two opposite surfaces in the thickness direction, and the negative electrode active layer is provided on one or both of the two opposite surfaces of the negative electrode current collector.

[0100] In some embodiments, the negative electrode current collector includes a metal current collector or a composite current collector. As an example, the metal current collector can be a copper foil. The composite current collector includes a polymer layer and a metal layer disposed on at least one surface of the polymer layer; as an example, the material of the polymer layer can be selected from one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE); the material of the metal layer can be selected from one or more of copper, nickel, titanium, silver, and alloys of the above metals.

[0101] In some embodiments, the negative electrode active layer at least includes negative electrode active particles, and the negative electrode active particles include one or more of carbon materials, silicon-based materials, tin-based materials, and lithium titanate. Among them, the carbon materials include one or more of natural graphite, artificial graphite, soft carbon, and hard carbon. The silicon-based materials include one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based materials include one or more of elemental tin, tin oxides, and tin alloys.

[0102] In some embodiments, the negative electrode active layer further includes one or more of a negative electrode conductive agent, a negative electrode binder, and a functional additive. Among them, the negative electrode conductive agent includes one or more of conductive carbon black, conductive graphite, acetylene black, Ketjen black, carbon quantum dots, carbon nanotubes, graphene, and carbon nanofibers. The negative electrode binder includes one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The functional additive can be a thickener such as sodium carboxymethyl cellulose (CMC-Na).

[0103] In some embodiments, the method for preparing the negative electrode sheet includes the following steps: dispersing negative electrode active particles, a negative electrode conductive agent, a negative electrode binder, and a functional additive in a solvent to obtain a negative electrode slurry; coating the negative electrode slurry on at least one surface of the negative electrode current collector, and after processes such as drying and cold pressing, a negative electrode sheet is obtained. Among them, the solvent of the negative electrode slurry can be deionized water.

[0104] The electrolyte, separator, and outer package of the secondary battery are described in detail below.

[0105] In some embodiments, the electrolyte can be liquid, gel-like, or all-solid-state. Further, an electrolytic solution is used as the electrolyte, and the electrolytic solution includes an electrolyte salt and a solvent.

[0106] In some embodiments, the electrolyte salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluoro bis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0107] In some embodiments, the solvent includes one or more of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butenyl carbonate, fluorinated ethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0108] In some embodiments, the electrolyte solution further includes additives, such as one or more of fluorinated ethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0109] In some embodiments, the material of the separator includes one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0110] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator are made into an electrode assembly by a winding process or a stacking process.

[0111] In some embodiments, the secondary battery includes an outer package for encapsulating the above-mentioned electrode assembly and electrolyte. The outer package of the secondary battery includes one or more of a hard shell and a soft package. Among them, the hard shell can be a hard plastic shell, an aluminum shell, a steel shell, etc., and the soft package can be a pouch soft package or a plastic soft package.

[0112] In the second aspect of the present application, a method for preparing a secondary battery is provided for preparing the above-mentioned secondary battery.

[0113] In some embodiments, as Figure 1 shown, the method for preparing a secondary battery includes the following steps:

[0114] S100: Provide a precursor, the precursor includes a core and a carbon layer coating the core, and the core includes one or more of lithium transition metal phosphate and lithium transition metal oxide;

[0115] S200: Perform segmented compaction treatment on the precursor to obtain positive electrode active particles;

[0116] S300: Assemble the positive electrode active particles to obtain a secondary battery;

[0117] Some of the positive electrode active particles are cracked, and the width of the cracks is 10 nm to 30 nm, and the length is 300 nm to 800 nm.

[0118] In this application, by performing segmented compaction treatment on the precursor coated with a carbon layer, some of the positive electrode active particles are caused to crack, and cracks with a width of 10 nm to 30 nm and a length of 300 nm to 800 nm are formed on the surface of the cracked particles, which can expose the electrochemically reactive sites of the core, promote the insertion and extraction of lithium ions during charge and discharge, and enhance the kinetic performance of the material. While cracks of appropriate size are formed in some of the particles, the positive electrode active particles exhibit a higher tap density, which is beneficial to improving the discharge capacity per gram of the battery, and the structural stability of the particles remains basically the same. The improvement of the kinetic performance and the increase in the active sites are sufficient to make up for the difference in conductivity caused by the cracking of the carbon layer. Generally speaking, it can effectively avoid the reduction of the battery cycle performance. Therefore, the secondary battery prepared in this application takes into account both high capacity and long cycle performance, which is beneficial to its wide application in the energy storage field.

[0119] The preparation method of the secondary battery is described in detail below in a step-by-step manner.

[0120] S100: Provide a precursor, which includes a core and a carbon layer coating the core. The core includes one or more of lithium transition metal phosphates and lithium transition metal oxides.

[0121] It can be understood that the precursor in step S100 can be positive electrode active particles that are completely free of particle cracking, or positive electrode active particles with very few cracked particles and small crack sizes. Therefore, segmented compaction treatment is required to make the crack sizes of the cracked particles meet the requirements described above. In the precursor, the lithium transition metal phosphates and lithium transition metal oxides of the core and the carbon layer are as described above, and will not be elaborated in this application.

[0122] In some embodiments, the core of the precursor includes lithium iron phosphate (LFP) material. As Figure 2 shown, the preparation method of the precursor includes the following steps:

[0123] S110: Mix lithium phosphate, ferrous salt, surfactant and solvent to obtain a mixed solution;

[0124] S120: Heat-treat the mixed solution to obtain a reaction slurry;

[0125] S130: Cool, wash, filter and dry-treat the reaction slurry to obtain a powder;

[0126] S140: Mix the powder and a carbon source, and perform a calcination treatment to obtain a precursor.

[0127] In some embodiments, the lithium phosphate includes one or more of lithium phosphate (Li3PO4), lithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4), and is further optionally lithium phosphate (Li3PO4).

[0128] Compared with the traditional technology of using three raw materials, namely lithium salt, phosphate, and ferrous salt, to prepare LFP materials, the precursor prepared from two raw materials, lithium phosphate and ferrous salt, has higher purity and crystallinity, and the particle size is more uniform and the morphology is more regular. Thus, in the segmented compaction, the acting force on the precursor is more uniform, which prompts some particles to crack and form the positive electrode active particles designed in the present application.

[0129] In some embodiments, the ferrous salt includes one or more of ferrous sulfate (FeSO4), ferrous chloride (FeCl2), ferrous acetate (Fe(CH3COO)2), and ferrous oxalate (FeC2O4), and is further optionally ferrous sulfate (FeSO4). Among them, the raw material of the ferrous salt may not contain crystal water or may contain crystal water, such as FeSO4·7H2O, FeCl2·4H2O, or FeC2O4·2H2O.

[0130] In some embodiments, the molar ratio of lithium element in the lithium phosphate to iron element in the ferrous salt is (3 - 3.6):2, including but not limited to 3:2, 3.1:2, 3.2:2, 3.3:2, 3.4:2, 3.5:2, or 3.6:2.

[0131] In some embodiments, the surfactant includes one or more of alkylbenzene sulfonates, alkyl sulfonates, α-olefin sulfonates, alkyl naphthalene sulfonates, lignin sulfonates, succinate sulfonates, fatty alcohol sulfates, and fatty alcohol polyoxyethylene ether sulfates. As an example, suitable surfactants include but are not limited to sodium dodecylbenzenesulfonate (LAS), sodium dodecyl sulfonate (i.e., sodium lauryl sulfonate), sodium dibutylnaphthalene sulfonate, sodium lignin sulfonate, ammonium lignin sulfonate, dioctyl sulfosuccinate sodium salt, sodium dodecyl sulfate (SDS), and sodium lauryl polyether sulfate (SLES), etc. Further, the surfactant is optionally sodium dodecylbenzenesulfonate (LAS).

[0132] Sodium dodecylbenzenesulfonate (LAS) and the like, as anionic surfactants, can be ionized in the mixed solution, and the organic chain part with surface activity shows a hydrophobic anionic effect, which can promote the uniform dispersion of raw materials, avoid excessive local concentration of raw materials, and regulate the size, morphology, and crystal form of the product, so as to form a precursor with high purity, good crystallinity, uniform size, and regular morphology.

[0133] In some embodiments, the mass fraction of the surfactant in the mixed solution is 1% - 2%, including but not limited to 1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%.

[0134] In some embodiments, in step S110, the solvent includes water, such as deionized water, pure water, ultrapure water, etc. In other examples, the solvent can also be a mixture of water and an organic solvent, such as water and ethanol.

[0135] In some embodiments, in step S110, mixing lithium phosphate, ferrous salt, surfactant and solvent includes the following steps: Weigh appropriate amounts of lithium phosphate and ferrous salt, disperse them in the solvent respectively, and stir for 10 min - 30 min to obtain a lithium phosphate solution and a ferrous salt solution; Mix the two solutions, add the surfactant, and ultrasonically disperse for 30 min - 60 min to obtain a mixed solution.

[0136] In some embodiments, in step S120, the heat treatment includes the following steps: Water bath heat at 120°C - 180°C for 12 h - 18 h. As an example, the temperature of the heat treatment can be 120°C, 130°C, 140°C, 150°C, 160°C, 170°C or 180°C, and the time of the heat treatment can be 12 h, 13 h, 14 h, 15 h, 16 h, 17 h or 18 h.

[0137] In some embodiments, in step S130, cooling, washing, filtering and drying the reaction slurry includes the following steps: Cool the reaction slurry to room temperature, wash it alternately with absolute ethanol and deionized water for 1 - 5 times, perform vacuum filtration after each washing, and dry the obtained filter cake.

[0138] In some embodiments, in step S130, the drying treatment includes the following steps: Vacuum dry at 50°C - 100°C for 10 h - 24 h. As an example, the temperature of the drying treatment can be 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, and the time of the drying treatment can be 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h or 24 h.

[0139] In some embodiments, the carbon source includes one or more of glucose, sucrose, fructose, ascorbic acid, polyethylene glycol (PEG) and polyvinyl alcohol (PVA). Among them, the molecular weight of polyethylene glycol can be 200 - 10000, including but not limited to PEG - 200, PEG - 400, PEG - 600, PEG - 1000, PEG - 2000, PEG - 4000, PEG - 6000, PEG - 8000 or PEG - 10000.

[0140] In some embodiments, the molar ratio of the powder material to the carbon source is 1:(1.5 - 2.5), including but not limited to 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4 or 1:2.5, and further optionally 1:(1.8 - 2.2).

[0141] In some embodiments, in step S140, the calcination treatment includes the following steps: calcining at 500°C - 900°C for 8h - 12h in a protective atmosphere. As an example, the protective atmosphere includes one or more of nitrogen, helium, neon, argon, krypton and xenon. The temperature of the calcination treatment can be 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C or 900°C, and the time of the calcination treatment can be 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h or 12h.

[0142] In some embodiments, in step S140, after the calcination treatment, it further includes a pulverization treatment step. The precursor after the pulverization treatment satisfies: the D10 particle size is 0.3μm - 0.6μm, the D50 particle size is 0.6μm - 1.2μm, the D90 particle size is 2μm - 4μm, and the span is 2.2 - 3.2.

[0143] S200: Perform a segmented compaction treatment on the precursor to obtain the cathode active particles.

[0144] In some embodiments, the segmented compaction treatment is carried out under a pressure of 360MPa - 372MPa, and the total time is 300s - 750s. As an example, the pressure of the segmented compaction treatment can be 360MPa, 361MPa, 362MPa, 363MPa, 364MPa, 365MPa, 366MPa, 367MPa, 368MPa, 369MPa, 370MPa, 371MPa or 372MPa, and the total time of the segmented compaction treatment can be 300s, 350s, 400s, 450s, 500s, 550s, 600s, 650s, 700s or 750s.

[0145] In some embodiments, the segmented compaction treatment includes at least three compaction treatments, and the pressure of each compaction treatment is not lower than that of the previous compaction treatment. In addition, the time of each compaction treatment can be the same or different, and the present application does not make a special limitation here.

[0146] Through a single compaction treatment, such as compaction at 372 MPa for 750 s, generally only fluffy granular materials can be pressed into compact and orderly laminated materials, and the resulting materials will basically not have particle cracking, making it difficult to achieve an ideal cracking effect. In contrast, by using at least three-stage compaction treatment and gradually increasing the pressure, on the basis of obtaining laminated materials, some particles are more likely to crack under the action of stepwise increasing external forces, and the proportion of the number of cracked particles and the size of the cracks are more appropriate.

[0147] In some embodiments, the segmented compaction treatment is a five-stage compaction treatment, including the following steps:

[0148] Under a pressure of 360 MPa to 362 MPa, hold the pressure for 100 s to 150 s;

[0149] Under a pressure of 362 MPa to 364 MPa, hold the pressure for 100 s to 150 s;

[0150] Under a pressure of 364 MPa to 366 MPa, hold the pressure for 100 s to 150 s;

[0151] Under a pressure of 366 MPa to 368 MPa, hold the pressure for 100 s to 150 s;

[0152] Under a pressure of 368 MPa to 372 MPa, hold the pressure for 100 s to 150 s.

[0153] S300: Assemble the positive active particles to obtain a secondary battery.

[0154] In some embodiments, assembling the positive active particles to obtain a secondary battery includes the following steps: making the positive active particles into a positive electrode plate; making the positive electrode plate, negative electrode plate and separator into an electrode assembly through a winding process or a stacking process; installing the electrode assembly into an outer package, injecting electrolyte after drying and removing water, then sealing, and obtaining a secondary battery after processes such as standing, hot and cold pressing, formation, jigging, and grading.

[0155] In the third aspect of the present application, a battery pack is provided, which includes a battery box and a plurality of secondary batteries arranged in the battery box. The secondary battery includes the secondary battery as described above, or includes the secondary battery prepared by the preparation method of the secondary battery as described above. Thus, the battery pack has both high capacity and long cycle performance.

[0156] In the fourth aspect of the present application, an energy storage system is provided, which includes the battery pack as described above. Thus, the energy storage system has both high capacity and long cycle performance.

[0157] In the present application, the energy storage system can be a cabinet-type energy storage system, usually called an energy storage cabinet. The energy storage cabinet includes a cabinet body and a plurality of battery packs arranged in the cabinet body.

[0158] In the fifth aspect of the present application, an electrical device is provided, which includes the energy storage system as described above.

[0159] In the present application, the energy storage system can be used as the power source or energy storage unit of the electrical device. The electrical device includes mobile devices, electric vehicles, electric trains, ships, satellites, energy storage systems, etc. Mobile devices can be mobile phones, tablet computers, laptop computers, etc.; electric vehicles can be pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.

[0160] The following will be further described in combination with specific examples and comparative examples. The raw materials involved in the following specific examples and comparative examples, unless otherwise specified, can all be obtained commercially. The instruments used, unless otherwise specified, can all be obtained commercially. The processes involved, unless otherwise specified, are all conventional selections of those skilled in the art.

[0161] Example 1

[0162] The preparation method of the secondary battery in this example is as follows:

[0163] (1) Preparation of the precursor:

[0164] Weigh appropriate amounts of lithium phosphate and ferrous sulfate according to the molar ratio of lithium element to iron element of 3:2, disperse them in deionized water respectively, stir for 20 min to obtain a lithium phosphate solution and a ferrous sulfate solution; mix the two solutions, add sodium dodecylbenzenesulfonate LAS with a mass fraction of 1.5%, and ultrasonically disperse for 40 min to obtain a mixed solution.

[0165] Heat the mixed solution in a water bath at 150 °C for 15 h to obtain a reaction slurry; cool the reaction slurry to room temperature, wash it alternately 3 times with absolute ethanol and deionized water during vacuum filtration, and vacuum dry the obtained filter cake at 80 °C for 16 h to obtain a powder.

[0166] Mix the powder and glucose evenly according to the molar ratio of 1:2, and calcine them in a tube furnace at 700 °C for 10 h under nitrogen protection to obtain the precursor.

[0167] (2) Preparation of the positive active particles: Place the precursor in a compaction density meter for segmented compaction:

[0168] First stage: Under a pressure of 362 MPa, keep the pressure for 150 s;

[0169] Second stage: Under a pressure of 364 MPa, keep the pressure for 150 s;

[0170] Third stage: Under a pressure of 366 MPa, keep the pressure for 150 s;

[0171] Fourth stage: Keep the pressure at 368 MPa for 150 s;

[0172] Fifth stage: Keep the pressure at 372 MPa for 150 s.

[0173] (3) Preparation of the positive electrode sheet: Dispersed the above-mentioned positive electrode active particles, conductive carbon black SP, and binder PVDF in a weight ratio of 98:1:1 into NMP and mixed evenly to obtain a positive electrode slurry; Coated the positive electrode slurry evenly on both surfaces of the aluminum foil, and after drying and cold pressing, obtained the positive electrode sheet.

[0174] (4) Preparation of the negative electrode sheet: Mixed negative electrode active particles graphite, thickener sodium carboxymethyl cellulose CMC, binder styrene-butadiene rubber SBR, and conductive agent acetylene black in a mass ratio of 97:1:1:1, added deionized water, and obtained a negative electrode slurry under the action of a vacuum mixer; Coated the negative electrode slurry evenly on both surfaces of the copper foil, and after drying and cold pressing, obtained the negative electrode sheet.

[0175] (5) Separator: Select a 12-μm-thick polypropylene separator.

[0176] (6) Preparation of the electrolyte: Select ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1 as the organic solvent, dissolved LiPF6 in the organic solvent, and mixed evenly to obtain an electrolyte with a LiPF6 concentration of 1 mol / L.

[0177] (7) Preparation of the secondary battery: Stacked the positive electrode sheet, separator, and negative electrode sheet in sequence, wound them into a square bare battery core, and then placed them in an aluminum-plastic film; After drying to remove water, injected the electrolyte, then sealed it, and after processes such as standing, hot and cold pressing, formation, clamping, and grading, obtained the secondary battery.

[0178] Example 2

[0179] The preparation method of the secondary battery in Example 2 is basically the same as that in Example 1, except that: the pressure and pressure holding time of segmented compaction are different, specifically as follows:

[0180] First stage: Keep the pressure at 360 MPa for 100 s;

[0181] Second stage: Keep the pressure at 362 MPa for 100 s;

[0182] Third stage: Keep the pressure at 364 MPa for 100 s;

[0183] Fourth stage: Keep the pressure at 366 MPa for 100 s;

[0184] The fifth stage: Keep the pressure at 370 MPa for 100 s.

[0185] Example 3

[0186] The preparation method of the secondary battery in Example 3 is basically the same as that of the secondary battery in Example 1, except that: the segmented compaction is four-stage compaction, specifically as follows:

[0187] The first stage: Keep the pressure at 362 MPa for 150 s;

[0188] The second stage: Keep the pressure at 365 MPa for 150 s;

[0189] The third stage: Keep the pressure at 368 MPa for 150 s;

[0190] The fourth stage: Keep the pressure at 372 MPa for 150 s.

[0191] Example 4

[0192] The preparation method of the secondary battery in Example 4 is basically the same as that of the secondary battery in Example 1, except that: the segmented compaction is three-stage compaction, specifically as follows:

[0193] The first stage: Keep the pressure at 362 MPa for 150 s;

[0194] The second stage: Keep the pressure at 367 MPa for 150 s;

[0195] The third stage: Keep the pressure at 372 MPa for 150 s.

[0196] Example 5

[0197] The preparation method of the secondary battery in Example 5 is basically the same as that of the secondary battery in Example 1, except that: the preparation conditions of the precursor are different, specifically as follows:

[0198] Weigh appropriate amounts of lithium phosphate and ferrous sulfate according to the molar ratio of lithium element to iron element of 3.6:2, disperse them in deionized water respectively, stir for 30 min to obtain a lithium phosphate solution and a ferrous sulfate solution; mix the two solutions, add 1% (by mass) of the surfactant sodium dodecylbenzenesulfonate LAS, and ultrasonically disperse for 60 min to obtain a mixed solution.

[0199] Heat the mixed solution in a water bath at 180 °C for 18 h to obtain a reaction slurry; cool the reaction slurry to room temperature, wash it alternately 3 times with absolute ethanol and deionized water during vacuum filtration, and vacuum-dry the obtained filter cake at 100 °C for 12 h to obtain a powder.

[0200] Mix the powder and glucose evenly according to a molar ratio of 1:2, and calcine them in a tube furnace at 900 °C for 12 h under nitrogen protection to obtain the precursor.

[0201] Example 6

[0202] The preparation method of the secondary battery in Example 6 is basically the same as that of the secondary battery in Example 1, except that: the raw materials for preparing the precursor are different, specifically as follows:

[0203] Weigh appropriate amounts of lithium carbonate, ammonium phosphate, and ferrous sulfate according to a molar ratio of lithium, phosphorus, and iron elements of 3:1:2, disperse them in deionized water respectively, and stir for 30 min to obtain a lithium phosphate solution and a ferrous sulfate solution; mix the two solutions, add a surfactant with a mass fraction of 1.5%, and ultrasonically disperse for 60 min to obtain a mixed solution.

[0204] Heat the mixed solution in a water bath at 150 °C for 15 h to obtain a reaction slurry; cool the reaction slurry to room temperature, wash it alternately 3 times with absolute ethanol and deionized water during vacuum filtration, and vacuum-dry the obtained filter cake at 80 °C for 16 h to obtain the powder.

[0205] Mix the powder and glucose evenly according to a molar ratio of 1:2, and calcine them in a tube furnace at 700 °C for 10 h under nitrogen protection to obtain the precursor.

[0206] Example 7

[0207] The preparation method of the secondary battery in Example 7 is basically the same as that of the secondary battery in Example 1, except that: the raw materials for preparing the precursor are different, specifically as follows:

[0208] Weigh appropriate amounts of lithium phosphate and ferrous sulfate according to a molar ratio of lithium and iron elements of 3:2, disperse them in deionized water respectively, and stir for 20 min to obtain a lithium phosphate solution and a ferrous sulfate solution; mix the two solutions and ultrasonically disperse for 40 min to obtain a mixed solution.

[0209] Heat the mixed solution in a water bath at 150 °C for 15 h to obtain a reaction slurry; cool the reaction slurry to room temperature, wash it alternately 3 times with absolute ethanol and deionized water during vacuum filtration, and vacuum-dry the obtained filter cake at 80 °C for 16 h to obtain the powder.

[0210] Mix the powder and glucose evenly according to a molar ratio of 1:2, and calcine them in a tube furnace at 700 °C for 10 h under nitrogen protection to obtain the precursor.

[0211] Example 8

[0212] The preparation method of the secondary battery of Example 8 is basically the same as that of the secondary battery of Example 1, except that the precursor is a ternary material NCM 811 , specifically, it is NCM 811 series S85E of Ningbo Ronbay New Energy Technology Co., Ltd.

[0213] Comparative Example 1

[0214] The preparation method of the secondary battery of Comparative Example 1 is basically the same as that of the secondary battery of Example 1, except that the precursor of Comparative Example 1 is not subjected to segmented compaction treatment and is directly used as the positive electrode active particles for preparing the positive electrode sheet.

[0215] Comparative Example 2

[0216] The preparation method of the secondary battery of Comparative Example 2 is basically the same as that of the secondary battery of Example 1, except that the precursor is subjected to a single compaction treatment, that is, under 372 MPa, the pressure is maintained for 750 s.

[0217] Test Example

[0218] 1. Test items:

[0219] (1) Morphology characterization: Use SEM to characterize the microscopic morphology of the positive electrode active particles; use particle size analysis software to measure the D10 particle size, D50 particle size, and D90 particle size, and calculate the diameter distance according to (D90 - D10) ÷ D50; calculate the proportion of the number of cracked particles in the positive electrode active particles, and observe the length and width of the cracks.

[0220] (2) Phase characterization: Use X-ray diffraction analysis (XRD) to characterize the phase of the positive electrode active particles.

[0221] (3) Specific surface area: Measured by the gas adsorption BET method.

[0222] (4) Pore volume: Measured by the carbon tetrachloride method.

[0223] (5) Compaction density: Measured by a compaction density meter.

[0224] (6) Electrical performance: Measure the discharge capacity in grams of the secondary battery at 0.1C, 0.5C, and 1C rates, and at 0.3C rate, test the number of cycles when the capacity drops to 80% and the number of cycles when the energy efficiency drops to 80%. The test temperature is 25 °C, and the voltage range is 2.0 - 3.75 V.

[0225] 2. Analysis of test results:

[0226] The results measured according to the above test methods are shown in Tables 1 to 3 and Figures 3 to 5 as shown.

[0227] Table 1. Particle Size of Cathode Active Particles

[0228]

[0229] Table 2. Parameters of Cathode Active Particles

[0230]

[0231] Table 3. Performance of Secondary Battery

[0232]

[0233] As can be seen from Table 1, for the cathode active particles prepared in Examples 1 to 5, the D10 particle size is 0.367 μm to 0.426 μm, the D50 particle size is 0.783 μm to 0.893 μm, the D90 particle size is 2.295 μm to 2.497 μm, and the diameter distance is 2.32 to 2.55, showing the characteristics of a small diameter distance, a narrow particle size distribution, and good particle size uniformity. At the same time, as Figure 3 can be seen, the cathode active particles prepared in Example 1 have high purity and good crystallinity. The increase in the diameter distance or the overall particle size of the cathode active particles prepared in Examples 6 to 7 indicates that in Examples 1 to 5, through the synergistic cooperation of lithium phosphate and the surfactant, the particle size and morphology of the product are effectively regulated, so as to obtain a precursor with uniform particle size, regular morphology, high purity, and good crystallinity, which is beneficial to the uniform stress during the segmented compaction process and to obtain cathode active particles with appropriate cracking conditions.

[0234] As can be seen from Table 2, compared with Comparative Example 1 without compaction treatment, after the segmented compaction treatment in Examples 1 to 8, the specific surface area, pore volume, and compaction density of the cathode active particles have all been improved, which is beneficial to improving the discharge specific capacity and kinetic performance. It can be observed by SEM that some particles of the cathode active particles in Examples 1 to 8 are cracked, Figure 4 and the cracked particles are circled by the dotted line frame in. Among the cathode active particles in Examples 1 to 8, the proportion of the number of cracked particles is 2.8% to 7%, the width of the cracked cracks is 10 nm to 30 nm, and the length is 280 nm to 600 nm. As Figure 5 can be seen, the cathode active particles in Comparative Example 1 have not been subjected to compaction treatment, and no particle cracking phenomenon has been observed by SEM. Although Comparative Example 2 has undergone a single compaction treatment, the proportion of the number of cracked particles is only 1%, the width and length of the cracks are relatively low, the specific surface area has slightly increased compared with Comparative Example 1, but the pore volume has not changed significantly, indicating that the segmented compaction treatment is the key to causing some particles in the cathode active particles to crack and form cracks with specific sizes.

[0235] As can be seen from Table 3, compared with Comparative Examples 1-2, after the positive active particles of Examples 1-7 are assembled into secondary batteries, their discharge specific capacities at 0.1C, 0.5C, and 1C rates are significantly improved. At the same time, when the capacity and energy efficiency of the secondary batteries of Examples 1-8 are reduced to 80%, the number of cycles is basically the same as or even improved compared with Comparative Examples 1-2, indicating that the use of the positive active particles of Examples 1-7 endows the secondary batteries with the characteristics of both high capacity and long cycle performance.

[0236] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0237] The above-described embodiments merely represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the protection scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the appended claims.

Claims

1. A secondary battery, characterized in that: The secondary battery comprises positive electrode active particles, wherein the positive electrode active particles comprise a core and a carbon layer covering the core, wherein the core comprises one or more of a lithium transition metal phosphate and a lithium transition metal oxide; 2.5% to 8% of the positive electrode active particles are cracked, and the width of the cracks is 10nm to 30nm, and the length is 300nm to 800nm.

2. The secondary battery according to claim 1, wherein: 3% to 4% of the positive electrode active particles are cracked.

3. The secondary battery according to claim 1 or 2, characterized in that: The positive electrode active particles meet one or more of the following conditions: (1) The compacted density of the positive electrode active particles is 2.4 g / cm 3 ~2.8g / cm 3 ; (2) The D10 particle size of the positive electrode active particles is 0.3 μm to 0.6 μm, the D50 particle size is 0.6 μm to 1.2 μm, and the D90 particle size is 2 μm to 4 μm; (3) The specific surface area of ​​the positive electrode active particles is ≥ 10.63 m 2 / g, pore volume ≥0.0052cm 3 / g.

4. A method for preparing a secondary battery, characterized in that: The following steps are involved: Providing a precursor, the precursor comprising a core and a carbon layer covering the core, the core comprising one or more of a lithium transition metal phosphate and a lithium transition metal oxide; The precursor is subjected to segmented compaction treatment to obtain positive electrode active particles; Assembling the positive electrode active particles to obtain the secondary battery; 2.5% to 8% of the positive electrode active particles are cracked, and the width of the cracks is 10nm to 30nm, and the length is 300nm to 800nm.

5. The method for preparing a secondary battery according to claim 4, characterized in that: The segmented compaction process is carried out at a pressure of 360 MPa to 372 MPa, and the total time is 300 s to 750 s.

6. The method for preparing a secondary battery according to claim 5, characterized in that: The segmented compaction process includes at least three stages of compaction processes, and the pressure of each stage of compaction process is not lower than the pressure of the previous stage of compaction process.

7. The method for preparing a secondary battery according to claim 6, characterized in that: The segmented compaction process is a five-stage compaction process, comprising the following steps: Maintain pressure for 100s~150s at a pressure of 360MPa~362MPa; Maintain pressure for 100s~150s at a pressure of 362MPa~364MPa; Maintain pressure for 100s~150s at a pressure of 364MPa~366MPa; Maintain pressure for 100s~150s at a pressure of 366MPa~368MPa; Maintain the pressure for 100s~150s at a pressure of 368MPa~372MPa.

8. The method for preparing a secondary battery according to any one of claims 4 to 7, characterized in that: The core includes lithium iron phosphate material, and the preparation method of the precursor includes the following steps: mixing lithium phosphate, ferrous salt, surfactant and solvent to obtain a mixed solution; heating the mixed solution to obtain a reaction slurry; Cooling, washing, filtering and drying the reaction slurry to obtain a powder; The powder and the carbon source are mixed and calcined to obtain the precursor.

9. The method for preparing a secondary battery according to claim 8, characterized in that: One or more of the following conditions are met: (1) The lithium phosphate includes one or more of lithium phosphate, dilithium hydrogen phosphate and lithium dihydrogen phosphate; (2) The ferrous salt includes one or more of ferrous sulfate, ferrous chloride, ferrous acetate and ferrous oxalate; (3) The molar ratio of phosphorus in the lithium phosphate to iron in the ferrous salt is (1-1.2):2; (4) The surfactant includes one or more of alkylbenzene sulfonate, alkyl sulfonate, α-olefin sulfonate, alkylnaphthalene sulfonate, lignin sulfonate, succinate sulfonate, fatty alcohol sulfate and fatty alcohol polyoxyethylene ether sulfate; (5) The mass fraction of the surfactant in the mixed solution is 1% to 2%; (6) The carbon source includes one or more of glucose, sucrose, fructose, ascorbic acid, polyethylene glycol and polyvinyl alcohol; (7) The molar ratio of the powder to the carbon source is 1:(1.5-2.5).

10. The method for preparing a secondary battery according to claim 9, characterized in that: One or more of the following conditions are met: (1) The heating treatment comprises the following steps: heating in a water bath at 120°C to 180°C for 12h to 18h; (2) The drying process comprises the following steps: vacuum drying at 50°C to 100°C for 10 h to 24 h; (3) The calcination treatment comprises the following steps: calcining at 500°C to 900°C for 8h to 12h in a protective atmosphere.

11. A battery pack, characterized in that: The invention comprises a battery box and a plurality of secondary batteries arranged in the battery box, wherein the secondary battery comprises the secondary battery according to any one of claims 1 to 3, or comprises a secondary battery prepared by the method for preparing a secondary battery according to any one of claims 4 to 10.

12. An energy storage system, characterized in that: Comprising the battery pack as claimed in claim 11.

13. An electrical equipment, characterized in that: Comprising the energy storage system as claimed in claim 12.

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