Secondary battery and preparation method thereof, battery pack, energy storage system and electric equipment

Abstract

The invention relates to the technical field of energy storage, in particular to a secondary battery and a preparation method thereof, a battery pack, an energy storage system and electric equipment. The secondary battery comprises a positive electrode active particle, the positive electrode active particle comprises an inner core and a carbon layer coating the inner core, and the inner core comprises one or more of lithium transition metal phosphate and lithium transition metal oxide; part of the positive electrode active particles are cracked, the width of the cracked crack is 10 nm to 30 nm, and the length of the cracked crack is 300 nm to 800 nm. The secondary battery provided by the invention has good cycle performance while the discharge gram capacity is improved, and is beneficial to wide application in the field of energy storage.

Description

Technical Field

[0001] The present application relates to the field of energy storage technology, and in particular to a secondary battery and a preparation method thereof, a battery pack, an energy storage system and electrical equipment. 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 energy structure and develop clean renewable new energy. Among them, secondary batteries such as lithium-ion batteries are widely used in 3C products, energy storage systems and power batteries due to their high energy density, long cycle life, good rate performance and low 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, it is difficult for secondary batteries to have both high capacity and long cycle performance, which limits their application in the field of energy storage. Summary of the invention

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

[0004] The above-mentioned purpose of the present application is achieved through the following technical solutions:

[0005] In a first aspect of the present application, a secondary battery is provided, the secondary battery comprising positive electrode active particles, the positive electrode active particles 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;

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

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

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

[0009] In some embodiments, the positive electrode active particles have a D10 particle size of 0.3 μm to 0.6 μm, a D50 particle size of 0.6 μm to 1.2 μm, and a D90 particle size of 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, pore volume ≥0.0052cm 3 / g.

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

[0012] 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;

[0013] The precursor is subjected to segmented compaction treatment to obtain positive electrode active particles;

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

[0015] Some of the positive electrode active particles are cracked, and the width of the cracks is 10nm-30nm, and the length is 300nm-800nm.

[0016] In some embodiments, the staged compaction process is performed at a pressure of 360 MPa to 372 MPa, and the total time is 300 s to 750 s.

[0017] In some embodiments, the staged 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.

[0018] In some embodiments, the staged compaction process is a five-stage compaction process, comprising the following steps:

[0019] Maintain pressure for 100s~150s at a pressure of 360MPa~362MPa;

[0020] Maintain pressure for 100s~150s at a pressure of 362MPa~364MPa;

[0021] Maintain pressure for 100s~150s at a pressure of 364MPa~366MPa;

[0022] Maintain pressure for 100s~150s at a pressure of 366MPa~368MPa;

[0023] Maintain the pressure for 100s~150s at a pressure of 368MPa~372MPa.

[0024] In some embodiments, the core includes 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] heating the mixed solution to obtain a reaction slurry;

[0027] Cooling, washing, filtering and drying the reaction slurry to obtain a powder;

[0028] The powder and the carbon source are mixed and calcined 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 the phosphorus element in the lithium phosphate to the iron element in the ferrous salt is (1-1.2):2.

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

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

[0034] In some embodiments, the carbon source comprises 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 heat treatment comprises the following steps: heating in a water bath at 120° C. to 180° C. for 12 h to 18 h.

[0037] In some embodiments, the drying process comprises the following steps: vacuum drying at 50°C to 100°C for 10 hours to 24 hours.

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

[0039] In a third aspect of the present application, a battery pack is provided, comprising a battery box and a plurality of secondary batteries arranged in the battery box, wherein the secondary battery comprises the secondary battery as described above, or comprises a secondary battery manufactured by the secondary battery manufacturing method as described above.

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

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

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

[0043] Traditional technology usually uses compaction treatment to obtain positive electrode active materials with high compaction density to increase battery capacity. Despite the compaction treatment, the compaction density of the current positive electrode active materials 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 the traditional compaction treatment needs to avoid cracking of the positive electrode active particles as much as possible. On the one hand, the cracks on the surface of the particles will affect the coating effect of the carbon layer, resulting in poor conductivity of the material, which is not conducive to the improvement of the discharge capacity. On the other hand, the electrolyte will enter the interior of the particles along the cracks, which will increase the interface side reactions and cause the structural stability, thermal stability and cycle stability of the material to decrease at the same time, ultimately leading to the degradation of the battery cycle performance.

[0044] However, the applicant has found through research that by causing some of the positive active particles to crack, cracks with a width of 10nm~30nm and a length of 300nm~800nm ​​are formed on the surface of the cracked particles, which can expose the electrochemical reaction active sites of the inner core, promote the insertion and extraction of lithium ions during the charge and discharge process, and enhance the kinetic properties of the material. While some particles form cracks of appropriate size, the positive active particles show a higher compaction density, which is beneficial to improve the discharge gram capacity of the battery, and the structural stability of the particles remains basically consistent. The improvement in kinetic performance and the increase in active sites are sufficient to make up for the difference in conductivity caused by the cracking of the carbon layer. In general, it can effectively avoid the reduction of battery cycle performance. Therefore, the secondary battery provided by the present application has good cycle performance while improving the discharge gram capacity, which is conducive to its wide application in the field of energy storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the present application and to more completely understand the present application and its beneficial effects, the following is a brief introduction to the drawings required for the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application, and those skilled in the art can obtain other drawings based on these drawings without creative work.

[0046] Figure 1 is a schematic flow chart of a method for preparing a secondary battery in one embodiment;

[0047] Figure 2 is a schematic flow chart of a method for preparing a precursor in one embodiment;

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

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

[0050] Figure 5 This is the SEM image of the positive electrode active particles of Comparative Example 1. DETAILED DESCRIPTION

[0051] In order to facilitate the understanding of the present application, the present application is further described in detail below in conjunction with specific embodiments. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

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

[0053] In the present application, the meaning of "and / or" includes any and all combinations of one or more related listed items. "At least one" means more than one, such as one, two and more than two. "Multiple" or "several" means at least two, such as two, three, etc., and "multi-layer" means at least two layers, such as two layers, three layers, etc., unless otherwise clearly and specifically defined. In the description of the present application, "several" means at least one, such as one, two, etc., unless otherwise clearly and specifically defined.

[0054] When a numerical range is disclosed in this application, the above range is considered to be continuous and includes 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 features or characteristics, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed in this application should be understood to include any and all subranges included therein.

[0055] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

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

[0057] The temperature parameters in this application, unless otherwise specified, are allowed to be either constant temperature treatment or to vary 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. Fluctuations within the range of ±5°C, ±4°C, ±3°C, ±2°C, and ±1°C are allowed.

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

[0059] the term

[0060] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0061] Compaction density: refers to the ratio of the surface density and thickness of the material, in g / cm 3 Generally, the greater the compaction density, the higher the battery capacity, so compaction density is also considered as one of the reference indicators of energy density.

[0062] Particle size: For spherical particles, the particle size refers to the diameter of the spherical particles. For non-spherical particles, the particle size usually refers to the equivalent particle size of the non-spherical particles (generally referred to as the particle size), which 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 the actual particle. Unless otherwise specified or there is a contradiction, the particle size in this application represents the equivalent particle size.

[0063] Particle size distribution parameters: 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, which means that the particles smaller than this particle size account for N% of all particles, where N=0~100. When N=100, the D100 particle size indicates 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%, indicating the median particle size or median diameter, indicating that particles smaller than and larger than this particle size each account for 50%. For example, D50 particle size = 1mm, which means that particles with a particle size smaller than 1mm and particles with a particle size larger than 1mm each account for 50% of all particles. The DN particle size can be measured by a laser particle size analyzer.

[0064] Span: Also known as span, the 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 difference in particle size and the more dispersed the distribution.

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

[0066] Pore ​​volume: also known as pore volume (Vg), refers to the total volume of pores per unit mass of porous materials, in cm 3 / g. The pore volume can be determined by the carbon tetrachloride method, that is, under a certain carbon tetrachloride vapor pressure, carbon tetrachloride condenses and fills the pores of the porous material. At this time, the volume of the condensed carbon tetrachloride is the pore volume of the porous material.

[0067] With the rapid development of new energy technologies, based on the increasing demands for energy density, cycle life and safety performance of secondary batteries, the improvement of the performance of positive electrode active materials has become a research hotspot in the battery field.

[0068] The compaction density of powder is of great significance in the application of positive and negative electrode active materials, mainly in the following aspects: (1) High compaction density increases the density of active materials, the gaps between powder particles after compaction are reduced, and the content of active materials per unit volume is increased, which helps to improve the capacity and energy density of the battery; (2) The reduction of gaps between powder particles is also conducive to reducing the contact resistance between particles, improving the electron transmission path, and improving the conductivity of the material; (3) High compaction density can optimize the distribution of powder particles, promote uniform distribution of particles, reduce the problem of slurry agglomeration in the subsequent electrode coating roller pressing process, optimize processing performance, and thus improve the consistency and uniformity of the electrode; (4) In the process of slurry coating, active materials with high compaction density are easier to mix evenly with conductive agents and binders, improve the uniformity and consistency of slurry, and thus reduce the defects of active layer. However, it is difficult for traditional secondary batteries to take into account both high capacity and long cycle performance, which limits their application in the field of energy storage.

[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 achieve both high capacity and long cycle performance, so as to promote their widespread application in the field of energy storage.

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

[0071] Some of the positive electrode active particles cracked, and the width of the cracks was 10nm~30nm and the length was 300nm~800nm.

[0072] Traditional technology usually uses compaction treatment to obtain positive electrode active materials with high compaction density to increase battery capacity. Despite the compaction treatment, the compaction density of the current positive electrode active materials 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 the traditional compaction treatment needs to avoid cracking of the positive electrode active particles as much as possible. On the one hand, the cracks on the surface of the particles will affect the coating effect of the carbon layer, resulting in poor conductivity of the material, which is not conducive to the improvement of the discharge capacity. On the other hand, the electrolyte will enter the interior of the particles along the cracks, which will increase the interface side reactions and cause the structural stability, thermal stability and cycle stability of the material to decrease at the same time, ultimately leading to the degradation of the battery cycle performance.

[0073] However, the applicant has found through research that by causing some of the positive active particles to crack, cracks with a width of 10nm~30nm and a length of 300nm~800nm ​​are formed on the surface of the cracked particles, which can expose the electrochemical reaction active sites of the inner core, promote the insertion and extraction of lithium ions during the charge and discharge process, and enhance the kinetic properties of the material. While some particles form cracks of appropriate size, the positive active particles show a higher compaction density, which is beneficial to improve the discharge gram capacity of the battery, and the structural stability of the particles remains basically consistent. The improvement in kinetic performance and the increase in active sites are sufficient to make up for the difference in conductivity caused by the cracking of the carbon layer. In general, it can effectively avoid the reduction of battery cycle performance. Therefore, the secondary battery provided by the present application has good cycle performance while improving the discharge gram capacity, which is conducive to its wide application in the field of energy storage.

[0074] In the present application, the secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the battery charging and discharging process, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0075] The positive electrode plate is described in detail below.

[0076] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector. It can be understood that the positive current collector has two opposite surfaces along the thickness direction, and the positive active layer is disposed on one or both of the two opposite surfaces of the positive 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 may be 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 may 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 may 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 includes at least positive electrode active particles, the positive electrode active particles include an inner core and a carbon layer coating the inner core, and the inner core includes one or more of lithium transition metal phosphates and lithium transition metal oxides. Among them, the lithium transition metal phosphate includes one or more of lithium iron phosphate (LiFePO4, LFP), lithium manganese phosphate (LiMnPO4, LMP) and lithium iron manganese phosphate (LFMP). The lithium transition metal oxide includes 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, lithium cobalt oxide is selected from LiCoO2; lithium nickel oxide is selected from LiNiO2; lithium manganese oxide is selected from LiMnO2, LiMn2O4, etc.; 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.; 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 (ie, ternary positive electrode material). Further, the core is lithium iron phosphate (LFP).

[0079] In the present application, the carbon layer may completely cover all surfaces of the core to form a continuous film structure, or may partially cover at least a portion of the surface of the core to form a dispersed island structure. Further, the carbon layer completely covers 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] Among the core materials of positive electrode active particles, LFP is widely used in energy storage systems and power batteries due to its high safety, long cycle life, relatively low cost and good thermal stability. However, the compaction density of the current LEP positive electrode material is relatively low, which limits the improvement of its energy density and affects the consistency and uniformity of the positive electrode active layer. In addition, in order to improve conductivity, the surface of LFP positive electrode materials 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 cycle performance.

[0082] In response to the above problems, the present application increases the compaction density, specific surface area and pore volume of the positive electrode active particles by causing the surface of some positive electrode active particles to crack and form cracks of suitable size, thereby increasing the discharge capacity and improving the kinetic properties of the material to compensate for the negative effects of particle cracking, effectively avoiding the reduction of cycle performance, and allowing the secondary battery to have both high capacity and long cycle performance.

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

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

[0085] It is understandable that the width and length of the crack can be characterized by a scanning electron microscope (SEM). The depth of the crack can be characterized by a transmission electron microscope (TEM), which ranges from a few nanometers to hundreds of nanometers, and the present application does not specifically limit this. In the cracked positive electrode active particles, the cracks start from the carbon layer on the surface of the particles and gradually extend inward. Some cracks can extend to the inner core, so that the surface layer of the inner core close to the carbon layer also cracks, thereby exposing more electrochemically active sites, promoting the migration and transmission rate of lithium ions, and thus improving the kinetic properties of the material. However, the present application is not limited to this, and some cracks may not extend to the inner core, that is, only the carbon layer cracks and exposes the electrochemically active sites on the surface of the inner core, which is also beneficial to improving the kinetic properties of the material.

[0086] In some embodiments, among the positive electrode active particles, the number of cracked particles accounts for 2.5% to 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, among the positive electrode active particles, the number of cracked particles accounts for 3% to 4%.

[0087] As a result, 2.5% to 8% of the positive electrode active particles crack, and the cracks formed can expose more active sites, while increasing the specific surface area and pore volume of the particles, making it easier for lithium ions to embed and escape, and significantly improving the kinetic performance. At the same time, the cracking of a small number of particles basically does not affect the overall structural stability of the positive electrode active particles. The improvement in kinetic performance and the increase in active sites are sufficient to make up for the difference in conductivity caused by the cracking of the carbon layer, so there will be no negative impact on the cycle performance.

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

[0089] Therefore, the compaction density of the positive electrode active particles is high, which can enhance the conductivity of the positive electrode active particles and improve the consistency and uniformity of the positive electrode active layer, which 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. Furthermore, 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] As a result, the particle size distribution range of the positive electrode active particles is narrow and the particle size uniformity is good, which is conducive to promoting capacity and extending cycle life. If the particle size of the positive electrode active particles varies greatly, the following phenomena will occur: during the charging process, the lithium removal of large-sized and large-volume particles is not complete, resulting in low capacity; during the discharge process, small-sized and small-volume particles are prone to over-discharge and damage.

[0092] In some embodiments, the span of the positive electrode active particles is 2.2-3.2, 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 can further be 2.3-2.6.

[0093] In some embodiments, the specific surface area of ​​the positive electrode active particles is ≥ 10.63 m 2 / g, including but not limited to 10.63m 2 / g, 10.65m 2 / g, 10.68m 2 / g, 10.7m 2 / g, 10.72m 2 / g, 10.75m 2 / g, 10.78m 2 / g or 10.8m 2 / g, further optional to 10.63m 2 / g~10.78m 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 0.0052cm 3 / g, 0.0053cm 3 / g, 0.0054cm 3 / g, 0.0055cm 3 / g, 0.0056cm 3 / g, 0.0057cm 3 / g, 0.0058cm 3 / g or 0.0060cm 3 / g, further optional 0.0052cm 3 / g~0.0058cm 3 / g.

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

[0096] In some embodiments, the positive electrode active layer further includes one or more of a positive electrode conductor and a positive electrode binder. Among them, the positive electrode conductor 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 fluorine-containing acrylate resin.

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

[0098] The negative electrode plate is described in detail below.

[0099] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed 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 along the thickness direction, and the negative electrode active layer is disposed 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 may be 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 may 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 may 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 includes at least 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 material includes one or more of natural graphite, artificial graphite, soft carbon and hard carbon. The silicon-based material includes one or more of elemental silicon, silicon oxide compounds, silicon carbon composites, silicon nitrogen composites and silicon alloys. The tin-based material includes one or more of elemental tin, tin oxide compounds 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 preparation method of the negative electrode sheet includes the following steps: dispersing negative electrode active particles, negative electrode conductive agent, negative electrode binder and functional additive in a solvent to obtain negative electrode slurry; coating the negative electrode slurry on at least one surface of the negative electrode current collector, and obtaining the negative electrode sheet after drying, cold pressing and other processes. 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 may be in a liquid state, a gel state or a fully solid state. Further, the electrolyte is an electrolyte solution, which 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 bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[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, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0108] In some embodiments, the electrolyte further includes additives, such as one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethylethylene carbonate (TFPC), and the like.

[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 formed into an electrode assembly through a winding process or a lamination process.

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

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

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

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

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

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

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

[0118] The present application performs a segmented compaction treatment on the precursor coated with a carbon layer, which causes some particles in the positive electrode active particles to crack. The cracked particle surface forms cracks with a width of 10nm~30nm and a length of 300nm~800nm, which can expose the electrochemical reaction active sites of the kernel, promote the insertion and extraction of lithium ions during the charge and discharge process, and enhance the kinetic properties of the material. While some particles form cracks of appropriate size, the positive electrode active particles show a higher compaction density, which is conducive to improving the discharge gram capacity of the battery, and the structural stability of the particles remains basically the same. The improvement of kinetic performance and the increase of active sites are sufficient to make up for the difference in conductivity caused by the cracking of the carbon layer. In general, it can effectively avoid the reduction of battery cycle performance. Therefore, the secondary battery prepared by the present application takes into account both high capacity and long cycle performance, which is conducive to wide application in the field of energy storage.

[0119] The following is a detailed description of the method for preparing a secondary battery in a step-by-step manner.

[0120] S100: providing a precursor, wherein the precursor comprises 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.

[0121] It is understandable that the precursor in step S100 can be a positive electrode active particle with no particle cracking at all, or a positive electrode active particle with very few particles cracking and the crack size is small, so it is necessary to make the crack size of the cracked particles meet the requirements described above through segmented compaction. In the precursor, the lithium transition metal phosphate and lithium transition metal oxide of the inner core and the carbon layer are as described above, and this application will not be repeated here.

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

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

[0124] S120: heating the mixed solution to obtain a reaction slurry;

[0125] S130: Cooling, washing, filtering and drying the reaction slurry to obtain a powder;

[0126] S140: mixing powder and carbon source, and performing calcination treatment to obtain a precursor.

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

[0128] Compared with the traditional technology of preparing LFP materials using three raw materials of lithium salt, phosphate and ferrous salt, the precursor prepared using two raw materials of lithium phosphate and ferrous salt has higher purity and crystallinity, and the particle size is more uniform and the morphology is more regular. In this way, during the segmented compaction, the precursor is subjected to a more uniform force, which causes some particles to crack and form the positive active particles designed in this 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 can further be ferrous sulfate (FeSO4). The raw material of the ferrous salt may contain or contain water of crystallization, such as FeSO4·7H2O, FeCl2·4H2O or FeC2O4·2H2O.

[0130] In some embodiments, the molar ratio of the lithium element in the lithium phosphate to the 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 sulfonate, alkyl sulfonate, α-olefin sulfonate, alkylnaphthalene sulfonate, lignin sulfonate, succinate ester sulfonate, fatty alcohol sulfate and fatty alcohol polyoxyethylene ether sulfate. As an example, suitable surfactants include but are not limited to sodium dodecylbenzene sulfonate (LAS), sodium dodecyl sulfonate (i.e., sodium lauryl sulfonate), sodium dibutylnaphthalene sulfonate, sodium lignin sulfonate, ammonium lignin sulfonate, sodium dioctyl sulfosuccinate, sodium dodecyl sulfate (SDS) and sodium lauryl alcohol polyether sulfate (SLES). Further, the surfactant can be sodium dodecylbenzene sulfonate (LAS).

[0132] Sodium dodecylbenzene sulfonate (LAS) and other anionic surfactants can be ionized in the mixed solution. The surface-active organic chain part exhibits a hydrophobic anionic effect, which can promote uniform dispersion of raw materials, avoid excessive local concentration of raw materials, and adjust the size, morphology and crystal form of the product, thereby forming 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% to 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 may also be a mixture of water and an organic solvent, such as water and ethanol.

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

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

[0137] In some embodiments, in step S130, the reaction slurry is cooled, washed, filtered and dried, comprising the following steps: cooling the reaction slurry to room temperature, washing it alternately with anhydrous ethanol and deionized water for 1 to 5 times, performing vacuum filtration after each washing, and drying the obtained filter cake.

[0138] In some embodiments, in step S130, the drying process includes the following steps: vacuum drying at 50° C. to 100° C. for 10 h to 24 h. As an example, the drying process temperature may 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 drying process time may 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). The molecular weight of polyethylene glycol may be 200 to 10,000, 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 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 can further be 1:(1.8~2.2).

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

[0142] In some embodiments, in step S140, after the calcination treatment, a crushing treatment step is also included, and the precursor after the crushing treatment meets the following conditions: D10 particle size is 0.3μm~0.6μm, D50 particle size is 0.6μm~1.2μm, D90 particle size is 2μm~4μm, and the span is 2.2~3.2.

[0143] S200: The precursor is compacted in sections to obtain positive electrode active particles.

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

[0145] In some embodiments, the segmented compaction process includes at least three segments of compaction processes, and the pressure of each segment of compaction process is not less than the pressure of the previous segment of compaction process. In addition, the time of each segment of compaction process can be the same or different, and this application does not make any special limitation here.

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

[0147] In some embodiments, the staged compaction process is a five-stage compaction process, comprising the following steps:

[0148] Maintain pressure for 100s~150s at a pressure of 360MPa~362MPa;

[0149] Maintain pressure for 100s~150s at a pressure of 362MPa~364MPa;

[0150] Maintain pressure for 100s~150s at a pressure of 364MPa~366MPa;

[0151] Maintain pressure for 100s~150s at a pressure of 366MPa~368MPa;

[0152] Maintain the pressure for 100s~150s at a pressure of 368MPa~372MPa.

[0153] S300: Assembling positive electrode active particles to obtain a secondary battery.

[0154] In some embodiments, positive electrode active particles are assembled to obtain a secondary battery, comprising the following steps: the positive electrode active particles are made into positive electrode sheets; the positive electrode sheets, negative electrode sheets and separators are made into electrode assemblies through a winding process or a stacking process; the electrode assemblies are loaded into an outer package, dried and dehydrated, and then injected with electrolyte, and then sealed, and after standing, hot and cold pressing, formation, clamping, capacity division and other processes, a secondary battery is obtained.

[0155] In a 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, wherein the secondary batteries include the secondary batteries described above, or include secondary batteries prepared by the method for preparing the secondary batteries described above. Thus, the battery pack has both high capacity and long cycle performance.

[0156] In a 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 may be a cabinet-type energy storage system, generally referred to as an energy storage cabinet. The energy storage cabinet includes a cabinet and a plurality of battery packs arranged in the cabinet.

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

[0159] In this application, the energy storage system can be used as a power source or energy storage unit for electrical devices. Electrical devices include mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Mobile devices can be mobile phones, tablet computers, laptops, 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 is further described in conjunction with specific embodiments and comparative examples. The raw materials involved in the following specific embodiments and comparative examples, unless otherwise specified, can all be commercially available, the instruments used, unless otherwise specified, can all be commercially available, and the processes involved, unless otherwise specified, are all routinely selected by those skilled in the art.

[0161] Example 1

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

[0163] (1) Preparation of precursor:

[0164] According to the molar ratio of lithium element to iron element of 3:2, appropriate amounts of lithium phosphate and ferrous sulfate were weighed, dispersed in deionized water respectively, and stirred for 20 minutes to obtain lithium phosphate solution and ferrous sulfate solution; the two solutions were mixed, and 1.5% by mass fraction of surfactant sodium dodecylbenzene sulfonate LAS was added, and ultrasonic dispersion was carried out for 40 minutes to obtain a mixed solution.

[0165] The mixed solution was heated in a water bath at 150° C. for 15 h to obtain a reaction slurry; the reaction slurry was cooled to room temperature, and washed alternately with anhydrous ethanol and deionized water for 3 times during vacuum filtration, and the obtained filter cake was vacuum dried at 80° C. for 16 h to obtain a powder.

[0166] The powder and glucose were mixed evenly in a molar ratio of 1:2, and calcined at 700°C for 10 h in a tube furnace under nitrogen protection to obtain a precursor.

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

[0168] The first stage: maintain pressure for 150 seconds at a pressure of 362 MPa;

[0169] The second stage: maintain the pressure for 150 seconds at a pressure of 364 MPa;

[0170] The third stage: maintain the pressure for 150 seconds at a pressure of 366 MPa;

[0171] The fourth stage: maintain the pressure for 150 seconds at a pressure of 368 MPa;

[0172] The fifth stage: maintain the pressure for 150 seconds at a pressure of 372 MPa.

[0173] (3) Preparation of positive electrode sheet: The positive electrode active particles, conductive carbon black SP and binder PVDF are dispersed in NMP at a weight ratio of 98:1:1 and mixed evenly to obtain positive electrode slurry; the positive electrode slurry is evenly coated on both surfaces of aluminum foil, and after drying and cold pressing, a positive electrode sheet is obtained.

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

[0175] (5) Diaphragm: Use 12μm thick polypropylene diaphragm.

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

[0177] (7) Preparation of secondary batteries: The positive electrode, separator and negative electrode are stacked in order, and then wound into a square bare cell, which is then enclosed in an aluminum-plastic film. After drying and dehydration, the electrolyte is injected and then sealed. After standing, hot and cold pressing, formation, clamping, and capacity division, a secondary battery is obtained.

[0178] Example 2

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

[0180] The first stage: maintain pressure for 100s at a pressure of 360MPa;

[0181] The second stage: maintain the pressure for 100s at a pressure of 362MPa;

[0182] The third stage: maintain the pressure for 100s at a pressure of 364MPa;

[0183] The fourth stage: maintain the pressure for 100 seconds at a pressure of 366 MPa;

[0184] The fifth stage: maintain the pressure at 370 MPa for 100 seconds.

[0185] Example 3

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

[0187] The first stage: maintain pressure for 150 seconds at a pressure of 362 MPa;

[0188] The second stage: maintain the pressure for 150 seconds at a pressure of 365 MPa;

[0189] The third stage: maintain the pressure for 150 seconds at a pressure of 368 MPa;

[0190] The fourth stage: maintain the pressure for 150 seconds at a pressure of 372 MPa.

[0191] Example 4

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

[0193] The first stage: maintain pressure for 150 seconds at a pressure of 362 MPa;

[0194] The second stage: maintain the pressure for 150 seconds at a pressure of 367 MPa;

[0195] The third stage: maintain the pressure for 150 seconds at a pressure of 372 MPa.

[0196] Example 5

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

[0198] According to the molar ratio of lithium element to iron element of 3.6:2, appropriate amounts of lithium phosphate and ferrous sulfate were weighed, dispersed in deionized water respectively, and stirred for 30 minutes to obtain lithium phosphate solution and ferrous sulfate solution; the two solutions were mixed, and 1% by mass of surfactant sodium dodecylbenzene sulfonate LAS was added, and ultrasonic dispersion was carried out for 60 minutes to obtain a mixed solution.

[0199] The mixed solution was heated in a water bath at 180° C. for 18 h to obtain a reaction slurry; the reaction slurry was cooled to room temperature, and was alternately washed with anhydrous ethanol and deionized water for 3 times during vacuum filtration, and the obtained filter cake was vacuum dried at 100° C. for 12 h to obtain a powder.

[0200] The powder and glucose were mixed evenly in a molar ratio of 1:2, and calcined in a tube furnace at 900° C. for 12 h under nitrogen protection to obtain a precursor.

[0201] Example 6

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

[0203] According to the molar ratio of lithium element, phosphorus element and iron element of 3:1:2, appropriate amounts of lithium carbonate, ammonium phosphate and ferrous sulfate were weighed, dispersed in deionized water respectively, and stirred for 30 minutes to obtain lithium phosphate solution and ferrous sulfate solution; the two solutions were mixed, and a surfactant with a mass fraction of 1.5% was added, and ultrasonic dispersion was carried out for 60 minutes to obtain a mixed solution.

[0204] The mixed solution was heated in a water bath at 150° C. for 15 h to obtain a reaction slurry; the reaction slurry was cooled to room temperature, and washed alternately with anhydrous ethanol and deionized water for 3 times during vacuum filtration, and the obtained filter cake was vacuum dried at 80° C. for 16 h to obtain a powder.

[0205] The powder and glucose were uniformly mixed in a molar ratio of 1:2, and calcined at 700°C for 10 h in a tube furnace under nitrogen protection to obtain a precursor.

[0206] Example 7

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

[0208] According to the molar ratio of lithium element to iron element of 3:2, appropriate amounts of lithium phosphate and ferrous sulfate were weighed, dispersed in deionized water respectively, and stirred for 20 minutes to obtain lithium phosphate solution and ferrous sulfate solution; the two solutions were mixed and ultrasonically dispersed for 40 minutes to obtain a mixed solution.

[0209] The mixed solution was heated in a water bath at 150° C. for 15 h to obtain a reaction slurry; the reaction slurry was cooled to room temperature, and washed alternately with anhydrous ethanol and deionized water for 3 times during vacuum filtration, and the obtained filter cake was vacuum dried at 80° C. for 16 h to obtain a powder.

[0210] The powder and glucose were uniformly mixed in a molar ratio of 1:2, and calcined at 700°C for 10 h in a tube furnace under nitrogen protection to obtain a 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 the NCM 811 series S85E of Ningbo Rongbai 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 Example 1, except that the precursor of Comparative Example 1 is not subjected to segmented compaction treatment, but is directly used as positive electrode active particles and used to prepare positive electrode sheets.

[0215] Comparative Example 2

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

[0217] Test Case

[0218] 1. Test items:

[0219] (1) Morphological characterization: The microscopic morphology of the positive electrode active particles was characterized by SEM. The particle size analysis software was used to measure the D10 particle size, D50 particle size, and D90 particle size, and the diameter distance was calculated according to (D90-D10)÷D50. The number and percentage of cracked particles in the positive electrode active particles were calculated, and the length and width of the cracks were observed.

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

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

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

[0223] (5) Compacted density: measured using a compacted density meter.

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

[0225] 2. Test result analysis:

[0226] The results measured according to the above test method are shown in Tables 1 to 3 and Figure 3~Figure 5 shown.

[0227] Table 1. Particle size of positive electrode active particles

[0228]

[0229] Table 2. Parameters of positive electrode active particles

[0230]

[0231] Table 3. Performance of secondary batteries

[0232]

[0233] As shown in Table 1, the positive electrode active particles prepared in Examples 1 to 5 have a D10 particle size of 0.367 μm to 0.426 μm, a D50 particle size of 0.783 μm to 0.893 μm, a D90 particle size of 2.295 μm to 2.497 μm, and a diameter distance of 2.32 to 2.55, showing the characteristics of small diameter distance, narrow particle size distribution, and good particle size uniformity. Figure 3 It can be seen that the positive electrode active particles prepared in Example 1 have high purity and good crystallinity. The diameter distance or overall particle size of the positive electrode active particles prepared in Examples 6-7 increases, indicating that Examples 1-5 effectively control the particle size and morphology of the product through the synergistic combination of lithium phosphate and surfactant, thereby obtaining a precursor with uniform particle size, regular morphology, high purity and good crystallinity, which is conducive to uniform force during segmented compaction treatment and obtaining positive electrode active particles with suitable cracking conditions.

[0234] As shown in Table 2, compared with Comparative Example 1 without compaction treatment, the specific surface area, pore volume and compaction density of the positive electrode active particles of Examples 1 to 8 are improved after the segmented compaction treatment, which is beneficial to improving the discharge capacity and kinetic performance. SEM observation shows that some particles of the positive electrode active particles of Examples 1 to 8 are cracked. Figure 4 The dotted box in the figure is the cracked particle. Among the positive active particles of Examples 1 to 8, the number of cracked particles accounts for 2.8% to 7%, and the width of the cracks is 10nm to 30nm, and the length is 280nm to 600nm. Figure 5 It can be seen that the positive active particles of Comparative Example 1 were not compacted, and no particle cracking was observed by SEM. Although Comparative Example 2 was compacted once, the number of cracked particles accounted for only 1%, the width and length of the cracks were low, and the specific surface area was slightly increased compared with Comparative Example 1, but the pore volume did not change significantly, indicating that the segmented compaction treatment is the key to causing some particles in the positive active particles to crack and form cracks of a specific size.

[0235] As shown in Table 3, compared with Comparative Examples 1-2, after the positive electrode active particles of Examples 1-7 are assembled into secondary batteries, their discharge gram 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 that of Comparative Examples 1-2, or even improved, which shows that the use of the positive electrode active particles of Examples 1-7 enables the secondary batteries to have both high capacity and long cycle performance.

[0236] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0237] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of protection of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the scope of protection of the present application. Therefore, the scope of protection of the present application shall be subject to the attached 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; Some of the positive electrode active particles are cracked, and the width of the cracks is 10nm-30nm, and the length is 300nm-800nm.

2. The secondary battery according to claim 1, wherein: 2.5% to 8% 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; Some of the positive electrode active particles are cracked, and the width of the cracks is 10nm-30nm, and the length is 300nm-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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