Positive electrode active material, method of preparation, positive electrode sheet, battery, and electric device

Abstract

The application relates to the technical field of batteries, in particular to a positive electrode active material, a preparation method, a positive electrode sheet, a battery and an electric device, the positive electrode active material comprises secondary particles containing primary particles and a binder, the primary particles comprise a lithium iron phosphate material, and at least part of the first binder is located between adjacent primary particles. In the application, the positive electrode active material adopts the secondary particles, the particle size of the positive electrode active material can be increased, the first charge-discharge efficiency of the positive electrode active material is relatively low, and part of the active substances cannot be used to generate capacity in the initial stage of the cycle. With the cycle, the binder between the primary particles is fully infiltrated by the electrolyte, ion channels are gradually formed, part of the active substances that cannot be used to generate capacity are gradually activated, the attenuation of the lithium iron phosphate in the early stage is compensated, and the battery has smaller attenuation in the early stage of the cycle, has no attenuation or even capacity increase.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode active material, a preparation method, a positive electrode sheet, a battery and an electric equipment. BACKGROUND

[0002] With the development of modern science and technology, lithium ion batteries are considered to be the first choice of green and environmentally friendly batteries because of their high energy density, long cycle life and good environmental protection. Lithium ion batteries can be widely used in energy storage power supply systems such as hydraulic, thermal, wind and solar power stations, and in many fields such as electric vehicles, electric tools, military equipment and aerospace.

[0003] Lithium iron phosphate as a positive electrode material of lithium ion battery has high stability and is widely used in lithium ion batteries. However, the capacity of lithium iron phosphate battery decays rapidly in the early stage of cycling, which brings a bad experience to the user. SUMMARY

[0004] Therefore, the present application mainly solves the technical problem of the rapid capacity decay of lithium iron phosphate battery in the early stage of cycling, thereby providing a positive electrode active material, a preparation method, a positive electrode sheet, a battery and an electric equipment, which can improve the capacity decay of lithium iron phosphate as a positive electrode active material in the early stage of cycling.

[0005] To solve the above technical problems, one technical scheme adopted by the present application is to provide a positive electrode active material, which comprises secondary particles, the secondary particles comprise primary particles and a binder, and the primary particles comprise a lithium iron phosphate material; and at least part of the binder is located between adjacent primary particles.

[0006] In the technical scheme of the present application, the lithium iron phosphate material primary particles of the positive electrode active material are bonded by the binder to form secondary particles, the particle size of the lithium iron phosphate secondary particles is increased relative to the particle size of the primary particles, which leads to relatively large polarization of the positive electrode active material, and the first charge-discharge efficiency of the positive electrode active material is low, and part of the active material cannot develop capacity in the early stage of cycling. With the progress of cycling, the binder between the primary particles is fully infiltrated by the electrolyte, and ion channels are gradually formed, plus the expansion and contraction of the lithium iron phosphate material in the process of deintercalating lithium, which increases the gap between the originally closely bonded primary particles, and part of the active material that cannot develop capacity is gradually activated, thereby making up for the decay of the lithium iron phosphate positive electrode active material in the early stage of cycling. On the other hand, since the overall polarization of the lithium iron phosphate positive electrode active material is reduced, the impedance of the battery formed by the positive electrode active material is relatively reduced, thereby showing further improved capacity, and the capacity decay in the early stage of cycling is small, no decay or even capacity increase.

[0007] In the embodiments of the present application, the cycle number in the initial cycle is less than the cycle number in the early cycle. The initial cycle refers to about 200 cycles before the cycle, and the early cycle refers to about 500 cycles before the cycle. Of course, the cycle number in the initial cycle and the cycle number in the early cycle also depend on the specific material of the positive active material, and the initial cycle can also be 200 cycles before the cycle, or 300 cycles before the cycle, or 150 cycles before the cycle, etc. The early cycle can also be 400 cycles before the cycle, or 600 cycles before the cycle, etc.

[0008] In any embodiment, the volume average particle size Dv50 of the secondary particles is in the range of 2 μm-50 μm. In the embodiments of the present application, by controlling the volume average particle size Dv50 of the secondary particles of the positive active material in the range, the polarization degree of the positive active material is better, and the capacity of the battery in the early stage is also maintained in a better condition.

[0009] wherein the volume average particle size Dv50 represents that the particle diameter of 50% of the total volume is greater than this value, and the particle diameter of the other 50% of the total volume is less than this value, and Dv50 represents the median particle size of the powder.

[0010] In any embodiment, the volume average particle size Dv50 of the secondary particles is in the range of 10 μm-30 μm. In the embodiments of the present application, the volume average particle size Dv50 of the secondary particles of the positive active material is in the range, so that the polarization degree of the positive active material is better, and the capacity of the battery in the early stage is also maintained in a better condition.

[0011] In any embodiment, the volume average particle size Dv50 of the primary particles is in the range of 0.1 μm-2 μm. In the embodiments of the present application, the volume average particle size Dv50 of the primary particles of the positive active material is in the range, so that the polarization degree of the positive active material is reduced during the later cycle, and the capacity of the battery in the later stage is also maintained without attenuation or with smaller attenuation.

[0012] In any embodiment, the volume average particle size Dv50 of the primary particles is in the range of 0.2 μm-1.5 μm. In the embodiments of the present application, the volume average particle size Dv50 of the primary particles of the positive active material is in the range, so that the polarization degree of the positive active material is smaller during the later cycle, and the capacity of the battery in the later stage is also maintained without attenuation or with an increase in capacity.

[0013] In any embodiment, the mass fraction of the binder is 0.1%-2% based on the total mass of the positive electrode active material. In the embodiments of the present application, the capacity development and long-term performance of the positive electrode active material can be controlled by controlling the proportion of the binder: when the proportion of the binder is high, the prepared secondary particle material has large polarization, low initial capacity, but the capacity rises more significantly after the initial stage; when the proportion of the binder is low, the final prepared lithium iron phosphate material has high initial capacity, but the capacity decays quickly in the early stage of cycling, and the improvement of the capacity decay in the early stage of cycling is small.

[0014] In any embodiment, the thickness of the binder between adjacent primary particles ranges from 0.1 μm to 1 μm. In the embodiments of the present application, by controlling the thickness of the binder, the cross-sectional area of the ion channel formed when the electrolyte infiltrates the binder of the positive electrode active material can be controlled, which can preferably improve the capacity decay of the battery in the early stage.

[0015] In any embodiment, the binder comprises:

[0016]

[0017] wherein R1, R2, R3 each independently comprises hydrogen, unsubstituted straight-chain or branched alkyl having 1-6 carbon atoms; R4 comprises unsubstituted straight-chain or branched alkyl having 1-6 carbon atoms; the value of m ranges from 100 to 3000; the value of n ranges from 100 to 3000. In the embodiments of the present application, the binder comprises a carbonic acid group polymerization unit, which can increase the molecular weight of the binder, improve the binding force of the binder, and preferably bind the primary particles to form secondary particles.

[0018] In any embodiment, the unsubstituted straight-chain or branched alkyl having 1-6 carbon atoms comprises methyl, ethyl, n-butyl or t-butyl. In the embodiments of the present application, the binding force of the binder is further improved, which can better bind the primary particles to form secondary particles.

[0019] In any embodiment, the binder comprises:

[0020]

[0021] any one or several of the following:

[0022] In any embodiment, the lithium iron phosphate material comprises Li x Fe y M zPO4, wherein M comprises any one or several of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb, 0.05≦x≦1.2, 0.2≦y≦1, and 0≦z≦0.8. In the embodiment, the lithium iron phosphate material comprises lithium iron phosphate or lithium iron phosphate doped with metal elements. The lithium iron phosphate has strong stability during charging and discharging. The lithium iron phosphate doped with the metal elements can improve the internal conductivity of the lithium iron phosphate primary particles. After the battery cell is formed or cycled, the atomic ratio of P in the atomic ratio of the constituent elements of the lithium iron phosphate material can be greater than 1 or less than 1 and greater than 0, and the atomic ratio of O can be greater than 4 or less than 4 and greater than 0.

[0023] In any embodiment, the lithium iron phosphate material further comprises a coating layer, and the coating layer comprises a carbon layer. In the embodiment, the lithium iron phosphate has a coating layer such as a carbon layer on the surface, which can improve the conductivity of the lithium iron phosphate.

[0024] In any embodiment, the thickness of the coating layer is greater than 0 and less than or equal to 20 nm. In the embodiment, the stability and conductivity of the lithium iron phosphate material are controlled in a better range by controlling the thickness of the coating layer.

[0025] The second aspect of the application further provides a preparation method of a positive electrode active material, comprising: dispersing primary particles comprising a lithium iron phosphate material and a binder in a solvent to form secondary particles by spray drying, wherein at least part of the binder is located between adjacent primary particles. In the embodiment, the primary particles and the binder are made into secondary particles by spray drying, so that the uniformity of the secondary particles is good, the size of the secondary particles is easy to control, and the uniformity of the adhesion of the binder on the surface of the primary particles is easy to control. The primary particles in the secondary particles are in close contact with each other and with the binder. The obtained lithium iron phosphate secondary particles have relatively large polarization, the first charge-discharge efficiency of the lithium iron phosphate secondary particles is low, and part of the active material cannot develop capacity in the early stage of cycling; as the cycle progresses, the binder between the primary particles is fully infiltrated by the electrolyte, ion channels are gradually formed, and part of the active material that cannot develop capacity is gradually activated, thereby compensating for the attenuation of the lithium iron phosphate in the early stage, so that the attenuation in the early stage of cycling is small, there is no attenuation, or the capacity increases.

[0026] In any embodiment, the temperature range of the spray drying is 150-350°C. By controlling the temperature range of the spray drying, the particle size of the secondary particles and the thickness of the binder formed on the surface of the primary particles can be controlled.

[0027] The third aspect of the present application also provides a positive electrode tab, which comprises the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the preparation method of the second aspect of the present application. Since the positive electrode tab of the present application comprises the positive electrode active material provided by the present application and / or the positive electrode active material prepared by the preparation method of the positive electrode active material, at least the same advantages as the battery cell are achieved. On the other hand, the positive electrode tab of the embodiment of the present application adopts the positive electrode active material comprising lithium iron phosphate secondary particles and the binder between the primary particles, so that the capacity of the positive electrode active material in the embodiment of the present application to store electrolyte is enhanced, which is beneficial to improve the cycle performance of the battery.

[0028] The fourth aspect of the present application also provides a battery comprising the positive electrode tab of the third aspect of the present application. Since the battery of the present application comprises the positive electrode tab provided by the present application, at least the same advantages as the positive electrode tab are achieved.

[0029] The fifth aspect of the present application also provides an electric device comprising the battery of the fourth aspect of the present application. Since the electric device of the present application comprises the battery provided by the present application, at least the same advantages as the battery are achieved.

[0030] The beneficial effects of the present application are as follows: Different from the prior art, in the technical solution of the embodiment of the present application, the lithium iron phosphate material primary particles of the positive electrode active material are bonded by the binder to form secondary particles, the particle size of the lithium iron phosphate secondary particles is increased relative to the particle size of the primary particles, which leads to relatively large polarization of the positive electrode active material, low first charge-discharge efficiency of the positive electrode active material, and part of the active material cannot develop capacity in the early stage of the cycle. With the progress of the cycle, the binder between the primary particles is fully infiltrated by the electrolyte, the ion channel is gradually formed, and the gap between the originally closely bonded primary particles is increased due to the expansion and contraction of the lithium iron phosphate material in the process of deintercalation of lithium, so that part of the active material which cannot develop capacity is gradually activated, thereby making up for the attenuation of the lithium iron phosphate positive electrode active material in the early stage of the cycle. On the other hand, since the overall polarization of the lithium iron phosphate positive electrode active material is reduced, the impedance of the battery formed by the positive electrode active material is relatively reduced, thereby showing further improved capacity, less early stage attenuation, no attenuation or even capacity increase. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a structural schematic diagram of a vehicle of an embodiment of the present application.

[0032] Figure 2 is an exploded structural schematic diagram of a battery of an embodiment of the present application.

[0033] Figure 3 is an exploded structural schematic diagram of a battery cell of an embodiment of the present application.

[0034] Figure 4 is a schematic view of a structure of a secondary particle according to an embodiment of the present application.

[0035] Figure 5a is a scanning electron microscope image of a secondary particle according to an embodiment of the present application.

[0036] Figure 5b is another scanning electron microscope image of a secondary particle according to an embodiment of the present application.

[0037] Figure 5c is yet another scanning electron microscope image of a secondary particle according to an embodiment of the present application.

[0038] Figure 5d is still another scanning electron microscope image of a secondary particle according to an embodiment of the present application.

[0039] Figure 6 is a scanning electron microscope image of a cross section of a positive electrode tab according to an embodiment of the present application.

[0040] Figure 7 is a scanning electron microscope image of a cross section of a positive electrode tab of Comparative Example 1.

[0041] Figure 8 is an infrared spectrogram of an embodiment of a binder according to an embodiment of the present application. DETAILED DESCRIPTION

[0042] Hereinafter, embodiments of a battery cell, a battery, and an electric device according to the present application are specifically disclosed with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known to those skilled in the art, repeated description of substantially identical structures, are omitted. This is to avoid the following description becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0043] The ranges disclosed herein are meant to be inclusive of the endpoints and include the end values in the range. Ranges can be combined to form new ranges, e.g., a range of "60-120 and 80-110" is understood to include 60-110 and 80-120. Further, if a minimum range value is listed as 1 and a maximum range value is listed as 3, 4, and 5, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise indicated, a numerical range "a-b" means a range of any combination of the numbers a and b, wherein both a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between 0 and 5 have been listed herein, and "0-5" is merely a shorthand for listing all of those numbers. Also, when a parameter is stated to be an integer > 2, it is equivalent to state that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0044] All embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0045] All technical features and optional technical features of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0046] All steps of the present application can be performed in sequence or randomly, preferably in sequence, unless otherwise specified. For example, a method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, it is mentioned that the method can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0047] Unless otherwise specified, "including" and "comprising" mentioned in the present application means open-ended, and can also be closed-ended. For example, "including" and "comprising" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0048] If not specifically stated, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following satisfy the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0049] The lithium iron phosphate material structure is very stable, and there is almost no structural change in the first charge and discharge process, so the first charge and discharge efficiency is high, and the active lithium is basically returned to the positive electrode after the first discharge. However, the battery is still consuming active lithium during the early use process, forming a relatively stable SEI film, which leads to faster capacity attenuation of the lithium iron phosphate battery in the early cycle.

[0050] The battery disclosed in the embodiments of the present application can be used in a power consumption device using the battery as a power source or a variety of energy storage systems using the battery as an energy storage element. The power consumption device can be, but is not limited to, a mobile phone, a tablet, a notebook computer, an electric toy, an electric tool, an electric vehicle, an electric car, a ship, a spacecraft, etc. Among them, the electric toy can include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric plane toy, etc. The spacecraft can include an airplane, a rocket, a space shuttle, a spacecraft, etc.

[0051] The following embodiments are described by taking a power consumption device in an embodiment of the present application, i.e., a vehicle 1000, as an example for convenience of description.

[0052] Please refer to Figure 1 , Figure 1 The vehicle 1000 provided in some embodiments of the present application is a structural schematic diagram. The vehicle 1000 can be a fuel car, a gas car, or a new energy car, which can be a pure electric car, a hybrid car, or an extended range car, etc. The vehicle 1000 is internally provided with a battery 100, which can be arranged at the bottom, the head, or the tail of the vehicle 1000. The battery 100 can be used for power supply of the vehicle 1000, for example, the battery 100 can be used as an operating power source of the vehicle 1000. The vehicle 1000 can further include a controller 200 and a motor 300, and the controller 200 is used to control the battery 100 to supply power to the motor 300, for example, to meet the working power demand of the vehicle 1000 during starting, navigation, and driving.

[0053] In some embodiments of the present application, the battery 100 can not only be used as an operating power source of the vehicle 1000, but also be used as a driving power source of the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0054] Please refer to Figure 2 , Figure 2A schematic diagram of a battery 100 is provided in some embodiments of the present application. The battery 100 includes a box 10 and a battery cell 20, which is accommodated in the box 10. The box 10 is used to provide an accommodation space for the battery cell 20, and the box 10 can adopt various structures. In some embodiments, the box 10 can include a first part 11 and a second part 12, the first part 11 and the second part 12 are overlapped with each other, and the first part 11 and the second part 12 jointly define an accommodation space for accommodating the battery cell 20. The second part 12 can be a hollow structure with one end open, and the first part 11 can be a plate structure, which is overlapped with the open side of the second part 12 to jointly define the accommodation space with the second part 12. The first part 11 and the second part 12 can also be hollow structures with one side open, and the open side of the first part 11 is overlapped with the open side of the second part 12. Of course, the box 10 formed by the first part 11 and the second part 12 can have various shapes, such as a cylinder, a cuboid, etc.

[0055] In the battery 100, the battery cell 20 can be multiple, and the multiple battery cells 20 can be connected in series, in parallel, or in a mixed connection. The mixed connection means that there are both series connection and parallel connection among the multiple battery cells 20. The multiple battery cells 20 can be directly connected in series, in parallel, or in a mixed connection, and then the whole of the multiple battery cells 20 is accommodated in the box 10. Of course, the battery 100 can also be that the multiple battery cells 20 are first connected in series, in parallel, or in a mixed connection to form a battery module, and then multiple battery modules are connected in series, in parallel, or in a mixed connection to form a whole, which is accommodated in the box 10. The battery 100 can also include other structures, for example, the battery 100 can also include a busbar component for realizing the electrical connection between the multiple battery cells 20.

[0056] Each battery cell 20 can be a secondary battery or a primary battery, and can also be a lithium-sulfur battery, a sodium-ion battery, or a magnesium-ion battery, but is not limited thereto. The battery cell 20 can be in the shape of a cylinder, a flat body, a cuboid, or other shapes, etc.

[0057] Please refer to Figure 3 , Figure 3 A schematic diagram of a battery cell 20 is provided in some embodiments of the present application. The battery cell 20 refers to the smallest unit that constitutes a battery. As Figure 3 , the battery cell 20 includes an end cover 21, a shell 22, an electrode assembly 23, and other functional components.

[0058] The end cover 21 refers to a component that covers the opening of the shell 22 to isolate the internal environment of the battery monomer 20 from the external environment. Without limitation, the shape of the end cover 21 can be adapted to the shape of the shell 22 to fit the shell 22. Optionally, the end cover 21 can be made of a material with certain hardness and strength, such as aluminum alloy, so that the end cover 21 is not easily deformed when subjected to extrusion collision, so that the battery monomer 20 can have higher structural strength, and the safety performance can also be improved. The end cover 21 can be provided with functional components such as electrode terminals 21a. The electrode terminals 21a can be used to electrically connect with the cell assembly 23 for output or input of the electrical energy of the battery monomer 20. In some embodiments, the end cover 21 can also be provided with a pressure relief mechanism for relieving the internal pressure of the battery monomer 20 when the internal pressure or temperature of the battery monomer 20 reaches a threshold value. The material of the end cover 21 can also be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and the embodiments of the present application do not make special limitations. In some embodiments, an insulating member can also be provided on the inner side of the end cover 21, which can be used to isolate the electrical connection components in the shell 22 from the end cover 21 to reduce the risk of short circuit. For example, the insulating member can be plastic, rubber, etc.

[0059] The shell 22 is a component for fitting the end cover 21 to form the internal environment of the battery monomer 20, wherein the formed internal environment can be used to accommodate the cell assembly 23, electrolyte and other components. The shell 22 and the end cover 21 can be independent components, and an opening can be provided on the shell 22, and the end cover 21 is covered on the opening to form the internal environment of the battery monomer 20. Without limitation, the end cover 21 and the shell 22 can also be integrated, specifically, the end cover 21 and the shell 22 can form a common connecting surface before other components enter the shell, and when it is necessary to encapsulate the internal environment of the shell 22, the end cover 21 is covered on the shell 22. The shell 22 can be various shapes and various sizes, such as rectangular parallelepiped, cylindrical, hexagonal prism, etc. Specifically, the shape of the shell 22 can be determined according to the specific shape and size of the cell assembly 23. The material of the shell 22 can be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and the embodiments of the present application do not make special limitations.

[0060] The electrode assembly 23 is a component in which electrochemical reactions occur in the battery cell 100. One or more electrode assemblies 23 can be included in the casing 22. The electrode assembly 23 is mainly formed by winding or stacking a positive electrode tab and a negative electrode tab, and a separator is generally provided between the positive electrode tab and the negative electrode tab. The positive electrode tab and the negative electrode tab have portions with active materials that constitute a main body of the electrode assembly, and portions without active materials that each constitute a tab 23a. The positive electrode tab and the negative electrode tab can be located together at one end of the main body or at opposite ends of the main body. During charging and discharging of the battery, the positive active material and the negative active material react with the electrolyte, and the tab 23a connects the electrode terminal to form a current loop.

[0061] Some embodiments of the present application provide a positive active material, such as Figure 4 As shown, the positive active material includes secondary particles 30, the secondary particles 30 including primary particles 31 and a binder 32, the primary particles 31 including a lithium iron phosphate material; and at least part of the binder 32 is located between adjacent primary particles 31.

[0062] In the technical solution of the embodiments of the present application, the lithium iron phosphate material primary particles 31 of the positive active material are bonded to form secondary particles 30 through the binder 32, the particle size of the lithium iron phosphate secondary particles 30 is increased relative to the particle size of the primary particles 31, which results in relatively large polarization of the positive active material, and the first charge-discharge efficiency of the positive active material is relatively low, and part of the active material cannot develop capacity in the early stage of cycling. With the progress of the cycle, the binder 32 between the primary particles 31 is fully infiltrated by the electrolyte, and ion channels are gradually formed, plus the expansion and contraction of the lithium iron phosphate material during lithium extraction, which increases the gap between the originally closely bonded primary particles 31, and part of the active material that cannot develop capacity is gradually activated, thereby compensating for the attenuation of the lithium iron phosphate positive active material in the early stage of cycling. On the other hand, since the overall polarization of the lithium iron phosphate positive active material is reduced, the impedance of the battery cell formed by the positive active material is relatively reduced, thereby showing further capacity improvement, and less early stage attenuation, no attenuation, or even capacity increase.

[0063] In any embodiment, the volume average particle size DV50 of the secondary particles 30 of the positive electrode active material is in the range of 2 μm-50 μm. In the embodiments of the present application, by controlling the volume average particle size DV50 of the secondary particles 30 of the positive electrode active material in the range of 2 μm-50 μm, the polarization degree of the positive electrode active material is better, and the capacity of the battery in the early stage is also kept in a better condition. The volume average particle size DV50 of the secondary particles 30 can be 2 μm, 3 μm, 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 20 μm, 22 μm, 26 μm, 28 μm, 30 μm, 35 μm, 38 μm, 40 μm, 42 μm, 45 μm, 48 μm, 50 μm, or a range formed by any two of the above values, for example, 2 μm-10 μm, 10 μm-22 μm, 22 μm-40 μm, or 40 μm-50 μm, etc.

[0064] In any embodiment, the volume average particle size DV50 of the secondary particles 30 of the positive electrode active material is in the range of 10 μm-30 μm. In the embodiments of the present application, by controlling the volume average particle size DV50 of the secondary particles 30 of the positive electrode active material in the range of 10 μm-30 μm, the polarization degree of the positive electrode active material is better, and the capacity of the battery in the early stage is also kept in a better condition. The volume average particle size DV50 of the secondary particles 30 can be 10 μm, 13 μm, 15 μm, 16 μm, 18 μm, 20 μm, 22 μm, 25 μm, 26 μm, 28 μm, 30 μm, or a range formed by any two of the above values, for example, 10 μm-18 μm, 16 μm-30 μm, or 25 μm-30 μm, etc.

[0065] In any embodiment, the volume average particle size DV50 of the primary particles 31 of the positive electrode active material is in the range of 0.1 μm-2 μm. In the embodiments of the present application, by controlling the volume average particle size DV50 of the primary particles 31 of the positive electrode active material in the range of 0.1 μm-2 μm, the polarization degree of the positive electrode active material is reduced in the later cycle process, and the capacity of the battery in the later stage is also kept without attenuation or with less attenuation. The volume average particle size DV50 of the primary particles 31 can be 0.1 μm, 0.2 μm, 0.3 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, or 2 μm, or a range formed by any two of the above values, for example, 0.1 μm-0.8 μm, 0.8 μm-1.5 μm, or 1.5 μm-2 μm, etc.

[0066] In any embodiment, the volume average particle size DV50 of the primary particles 31 of the positive electrode active material body ranges from 0.2 μm to 1.5 μm. In the embodiment of the present application, the volume average particle size DV50 of the primary particles 31 of the positive electrode active material ranges from 0.2 μm to 1.5 μm, so that the positive electrode active material has a small degree of polarization during later cycles, and the capacity of the battery in the later stage remains unchanged or increases. The volume average particle size DV50 of the primary particles 31 can be 0.2 μm, 0.3 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, or 1.5 μm, or a range formed by any two of the above values, for example, 0.2 μm to 0.6 μm, 0.6 μm to 1.1 μm, or 1.1 μm to 1.5 μm.

[0067] In any embodiment, the mass fraction of the binder 32 is 0.1% to 2% based on the total mass of the positive electrode active material. In the embodiment of the present application, the capacity of the positive electrode active material and the long-term performance can be controlled by controlling the proportion of the binder 32: when the proportion of the binder 32 is high, the prepared secondary particles 30 have a large polarization, a low initial capacity, and a more significant capacity increase after the initial stage; when the proportion of the binder 32 is low, the finally prepared lithium iron phosphate material has a high initial capacity, but the capacity decreases rapidly in the early stage of the cycle, and the improvement of the capacity decrease in the early stage of the cycle is small. The mass fraction of the binder 32 can be 0.1%, 0.2%, 0.3%, 0.5%, 0.6%, 0.7%, 0.8%, 1%, 1.1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2% based on the total mass of the positive electrode active material, or a range formed by any two of the above values, for example, 0.1% to 0.6%, 0.6% to 1.2%, or 1.2% to 2%.

[0068] In any embodiment, the thickness of the binder 32 between adjacent primary particles 31 ranges from 0.1 μm to 1 μm. In the embodiment of the present application, by controlling the thickness of the binder 32, the ion channel cross-sectional area formed when the electrolyte infiltrates the binder 32 of the positive electrode active material can be controlled, and the initial capacity decrease of the battery can be improved. The thickness of the binder 32 between the primary particles 31 can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1 μm, or a range formed by any two of the above values, for example, 0.1 μm to 0.4 μm, 0.4 μm to 0.7 μm, or 0.7 μm to 1 μm.

[0069] In any embodiment, the binder 32 comprises:

[0070]

[0071] wherein R1, R2, R3 each independently includes hydrogen, unsubstituted straight-chain or branched alkyl having 1-6 carbon atoms; R4 includes unsubstituted straight-chain or branched alkyl having 1-6 carbon atoms; m ranges from 100 to 3000; n ranges from 100 to 3000. In the embodiment, the binder 32 includes a carbonate group, which can increase the molecular weight of the binder 32, improve the binding force of the binder 32, and better bind the primary particles 31 to form the secondary particles 30. The value of m can be 100, 200, 500, 1000, 1500, 1800, 2000, 2500, or 3000, or a range between any two of the above values, such as 100-1000, 1000-2000, or 2000-3000. The value of n can be 100, 200, 500, 1000, 1500, 1800, 2000, 2500, or 3000, or a range between any two of the above values, such as 100-1000, 1000-2000, or 2000-3000.

[0072] In any embodiment, the unsubstituted straight-chain or branched alkyl having 1-6 carbon atoms includes methyl, ethyl, n-butyl, or t-butyl. In the embodiment, the binder 32 has a further improved binding force, which can better bind the primary particles 31 to form the secondary particles 30.

[0073] In any embodiment, the binder 32 includes:

[0074]

[0075] any one or more of the above.

[0076] In any embodiment, the lithium iron phosphate material includes Li x Fe y M z PO4, wherein M includes any one or more of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb, 0.05≦x≦1.2, 0.2≦y≦1, and 0≦z≦0.8. In the embodiment, the lithium iron phosphate material includes lithium iron phosphate or lithium iron phosphate doped with a metal element. The lithium iron phosphate has strong stability during charging and discharging. The lithium iron phosphate doped with the metal element can improve the internal conductivity of the primary particles 31 of the lithium iron phosphate material.

[0077] In any embodiment, the lithium iron phosphate material further comprises a coating layer, and the coating layer comprises a carbon layer. In the embodiments of the present application, the lithium iron phosphate material has a coating layer, such as a carbon layer, on the surface of the lithium iron phosphate material, which can improve the electrical conductivity of the lithium iron phosphate material.

[0078] In any embodiment, the thickness of the coating layer is greater than 0 and less than or equal to 20 nm. In the embodiments of the present application, the stability and electrical conductivity of the lithium iron phosphate material are controlled within a preferred range by controlling the thickness of the coating layer. The thickness of the coating layer can be 1 nm, 2 nm, 3 nm, 5 nm, 8 nm, 10 nm, 11 nm, 12 nm, 13 nm, 15 nm, 16 nm, 18 nm, 19 nm, or 20 nm, or a range between any two of the above values, such as 1-10, 10-15, or 15-20.

[0079] The second aspect of the present application also provides a method for preparing a positive electrode active material, which comprises: dispersing primary particles 31 comprising a lithium iron phosphate material and a binder 32 in a solvent, and forming secondary particles 30 by spray drying, wherein at least part of the binder 32 is located between adjacent primary particles 31. In the embodiments of the present application, the primary particles 31 and the binder 32 are made into secondary particles 30 by spray drying, so that the secondary particles 30 have good uniformity, the size of the secondary particles 30 is easy to control, and the uniformity of the binder 32 on the surface of the primary particles 31 is easy to control. The primary particles 31 in the secondary particles 30 are in close contact with each other and with the binder. The obtained lithium iron phosphate secondary particles 30 have a relatively large polarization, a low first charge-discharge efficiency, and part of the active material cannot develop capacity in the early stage of cycling. With the progress of the cycle, the binder 32 between the primary particles 31 is fully infiltrated by the electrolyte, ion channels are gradually formed, and part of the active material that cannot develop capacity is gradually activated, thereby compensating for the attenuation of the lithium iron phosphate in the early stage, and the attenuation in the early stage is small, there is no attenuation, or the capacity even increases.

[0080] In any embodiment, the temperature range for spray drying is 150-350°C. By controlling the temperature range for spray drying, the particle size of the secondary particles 30 can be controlled. The temperature for spray drying can be 150°C, 160°C, 180°C, 200°C, 210°C, 220°C, 240°C, 260°C, 300°C, 320°C, 340°C, 350°C, or a range between any two of the above values, such as 150-200°C, 200-260°C, or 260-350°C.

[0081] The third aspect of the present application also provides a positive electrode tab, which comprises the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the preparation method of the second aspect of the present application. Since the positive electrode tab of the present application comprises the positive electrode active material provided by the present application and / or the positive electrode active material prepared by the preparation method of the positive electrode active material, it at least has the same advantages as the battery cell.

[0082] The fourth aspect of the present application also provides a battery comprising the positive electrode tab of the third aspect of the present application. Since the battery of the present application comprises the positive electrode tab provided by the present application, it at least has the same advantages as the positive electrode tab.

[0083] The fifth aspect of the present application also provides an electrical equipment comprising the battery of the fourth aspect of the present application. Since the electrical equipment of the present application comprises the battery provided by the present application, it at least has the same advantages as the battery.

[0084] The solubility of the additive 222 in the electrolyte at 25°C can be 0.01 mmol / L, 0.05 mmol / L, 0.08 mmol / L, 0.1 mmol / L, 0.15 mmol / L, 0.2 mmol / L, 0.3 mmol / L, 0.5 mmol / L, 1 mmol / L, 2 mmol / L, 3 mmol / L, 5 mmol / L, 8 mmol / L, 10 mmol / L, 12 mmol / L, 15 mmol / L, 18 mmol / L, 20 mmol / L, 30 mmol / L, 35 mmol / L, 40 mmol / L, 46 mmol / L, 50 mmol / L, 54 mmol / L, 60 mmol / L, 65 mmol / L, 70 mmol / L, 75 mmol / L, 80 mmol / L, 87 mmol / L, 90 mmol / L, 95 mmol / L, 98 mmol / L, or 100 mmol / L, or a range between any two of the above values, for example, 0.1 mmol / L-80 mmol / L, 15 mmol / L-70 mmol / L, 35 mmol / L-60 mmol / L, 46 mmol / L-54 mmol / L, etc.

[0085] In any embodiment, the solubility of the additive 222 in the electrolyte at 25°C can be 100 mmol / L, 105 mmol / L, 110 mmol / L, 120 mmol / L, 125 mmol / L, 130 mmol / L, 140 mmol / L, etc., or a range between any two of the above values, for example, 100 mmol / L-140 mmol / L, 140 mmol / L or more, 105 mmol / L-130 mmol / L, etc.

[0086] In some embodiments, the electrolyte includes an electrolyte salt and a solvent, and the electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet.

[0087] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorobisoxalate phosphate, and lithium tetrafluorobisoxalate phosphate.

[0088] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl 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, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0089] [Positive electrode sheet]

[0090] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer includes the positive electrode active material of the first aspect of the present application.

[0091] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector.

[0092] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0093] In some embodiments, the positive electrode film layer further optionally includes a binder. For ease of distinction, the binder in the positive electrode active material can be referred to as a first binder, which is at least partially located between adjacent primary particles. The binder used in the positive electrode film layer can be referred to as a second binder. As preferred, the binder in the embodiments of the present application is different from the binder in the positive electrode active material. As an example, the second binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin. The first binder has a higher viscosity than the second binder.

[0094] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0095] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the second binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and then performing processes such as drying, cold pressing, and the like to obtain the positive electrode tab.

[0096] [Negative electrode tab]

[0097] The negative electrode tab includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0098] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0099] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. As a metal foil, for example, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0100] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0101] In some embodiments, the negative film layer can further optionally include a third binder. The third binder can be selected from at least one 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).

[0102] In some embodiments, the negative film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0103] In some embodiments, the negative film layer can further optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0104] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative current collector, and then drying, cold-pressing, and the like to obtain the negative electrode sheet.

[0105] [Separator]

[0106] In some embodiments, the battery further includes a separator. The present application does not have a particular limitation on the type of the separator, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0107] In some embodiments, the material of the separator can be selected from at least one 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, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0108] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be made into an electric cell assembly through a winding process or a stacking process.

[0109] In some embodiments, the shell 22 can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like.

[0110] The shape of the battery cell 20 is not particularly limited in the present application, and can be cylindrical, square, or any other arbitrary shape.

[0111] The beneficial effects of the present application will be further illustrated below in conjunction with examples.

[0112] In order to make the technical problems, technical solutions and beneficial effects solved by the embodiments of the present application more clear, the following will be further described in detail in conjunction with examples and drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The description of the at least one exemplary embodiment below is actually only illustrative, but not as any limitation on the present application and its application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0113] Embodiment 1

[0114] Preparation of the binder

[0115] Dissolve 1 mol of allyl methyl carbonate in 200 mL of tetrahydrofuran, vacuumize, continuously introduce N2 in a three-necked flask, add 0.05 g of azobisisobutyronitrile initiator, heat to 70℃, stir for 12 h, then pour the obtained crude product into ice ethyl ether at 0℃ for sedimentation, and the propylene carbonate polymer A is obtained.

[0116] Dissolve 0.1 mol of the propylene carbonate polymer A and 1 mol of vinylidene fluoride in 200 mL of tetrahydrofuran, vacuumize, continuously introduce N2 in a three-necked flask, add 0.05 g of azobisisobutyronitrile initiator, heat to 70℃, stir for 12 h, then pour the obtained crude product into ice ethyl ether at 0℃ for sedimentation, and the binder is obtained, which is the binder of formula (I) wherein R1, R2, and R3 are hydrogen, and R4 is methyl.

[0117] Specifically, in the embodiments of the present application, the binder is wherein m is between 100-3000, and n is between 100-3000.

[0118] Preparation of the positive active material

[0119] The lithium iron phosphate material primary particles with a volume average particle size Dv50 of 0.6 μm are mixed with the binder prepared above according to a mass ratio of 99:1, dispersed into an organic solvent N-methyl pyrrolidone, and spray dried at 200°C to form secondary particles, i.e. to obtain the positive electrode active material, the mass fraction of the binder being 1%, and the volume average particle size Dv50 of the secondary particles being 5.4 μm. Specifically, as shown in Figures 5a-5d the particle size distribution of the secondary particles is 2 μm-20 μm, the volume average particle size Dv50 of the secondary particles is 5.4 μm, and the primary particles are bonded by the binder. In the present application, the lithium iron phosphate material is LiFeMPO4. In the present application, the lithium iron phosphate material is lithium iron phosphate, and in other embodiments, the lithium iron phosphate material can also be a lithium iron phosphate doped material LixFe1-yMyPO4, wherein M includes any one or several of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, and wherein the trace element doping has little effect on the material performance. Or the lithium iron phosphate material can also be a lithium iron phosphate material coated with a carbon coating layer.

[0120]

Preparation of the positive electrode sheet

[0121] The positive electrode active material prepared above, conductive carbon and polyvinylidene fluoride (PVDF) are mixed uniformly according to a mass ratio of 96:2.5:1.5, a solvent N-methyl pyrrolidone (NMP) is added, the solid content is adjusted to 70%-80%, and the positive electrode slurry is obtained after uniform stirring, and then the positive electrode sheet is prepared through coating, drying, cold pressing and slitting.

[0122]

Preparation of the negative electrode sheet

[0123] The graphite, conductive carbon SP and binder SBR are dry mixed according to a ratio of 97:1:2, deionized water is added, the solid content is adjusted to 45%-55%, and the negative electrode slurry is obtained after uniform stirring, and then the negative electrode sheet is prepared through coating, drying, cold pressing and slitting.

[0124]

Preparation of the electrolyte

[0125] In an argon atmosphere glove box, ethylene carbonate (EC) / methyl ethyl carbonate (EMC) are mixed uniformly according to a volume ratio of 3 / 7 to obtain a solvent, LiPF6 is added to the above solvent to a mass percentage of 12.5% to be dissolved, and the electrolyte is obtained after uniform stirring.

[0126]

Separator

[0127] A polypropylene film is used as the isolation film.

[0128]

Preparation of the battery

[0129] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, the separator is arranged between the positive electrode and the negative electrode to play a role of separation, and a bare battery cell is obtained by winding. The bare battery cell is placed in an outer package, electrolyte solution is injected, and processes such as packaging, electrolyte injection, formation, and degassing are performed to obtain a lithium ion battery.

[0130] Example 2

[0131] The difference from Example 1 is that in the preparation of the positive active material, the mass ratio of lithium iron phosphate material primary particles to binder is 99.9:0.1, and the mass fraction of the binder is 0.1%. The other processes are the same as those in Example 1, and are not described here again.

[0132] Example 3

[0133] The difference from Example 1 is that in the preparation of the positive active material, the mass ratio of lithium iron phosphate material primary particles to binder is 99.5:0.5, and the mass fraction of the binder is 0.5%. The other processes are the same as those in Example 1, and are not described here again.

[0134] Example 4

[0135] The difference from Example 1 is that in the preparation of the positive active material, the mass ratio of lithium iron phosphate material primary particles to binder is 98.5:1.5, and the mass fraction of the binder is 1.5%. The other processes are the same as those in Example 1, and are not described here again.

[0136] Example 5

[0137] The difference from Example 1 is that in the preparation of the positive active material, the mass ratio of lithium iron phosphate material primary particles to binder is 98:2, and the mass fraction of the binder is 2%. The other processes are the same as those in Example 1, and are not described here again.

[0138] Example 6

[0139] The difference from Example 1 is that in the preparation of the binder, the reaction raw material is allyl ethyl carbonate, and the obtained binder of Formula (I) is wherein R1 is hydrogen, R2 is methyl, R3 is hydrogen, and R4 is methyl. The other processes are the same as those in Example 1, and are not described here again.

[0140] Example 7

[0141] The difference from Example 1 is that in the preparation of the binder, the reaction raw material is allyl ethyl carbonate, and the obtained binder is wherein R1 is hydrogen, R2 is methyl, R3 is tert-butyl, and R4 is methyl. The other processes are the same as those in Example 1, and are not described here again.

[0142] Example 8

[0143] The difference from Example 1 is that in the preparation of the binder, 1 mol of allyl ethyl carbonate and 1 mol of vinylidene fluoride are dissolved in 200 mL of tetrahydrofuran, vacuumized, 0.05 g of azobisisobutyronitrile initiator is added under N2 atmosphere, heated to 70°C, and after stirring for 12 h, the obtained crude product is poured into ice ethyl ether at 0°C to precipitate, to obtain the desired binder. The binder is wherein R1 is hydrogen, R2 is methyl, R3 is hydrogen, and R4 is methyl. The obtained binder is subjected to infrared spectrum analysis, and the test results are as shown in Figure 8 From the infrared spectrum of Figure 8 It can be seen from the infrared spectrum of -1 that a characteristic absorption peak of -CF2- appears at 1183 cm-1, a characteristic absorption peak of -CH2- appears at 1404 cm-1, and a characteristic absorption peak of -CO3- appears near 761 cm-1, indicating that the prepared binder contains functional groups -CF2- and -CO3-.

[0144] The other parts are the same as those in Example 1, which are not described here.

[0145] Example 9

[0146] The difference from Example 1 is that the preparation of the binder is not included. In the preparation of the positive active material, the binder used is polyvinylidene fluoride (PVDF).

[0147] Example 10

[0148] The difference from Example 1 is that in the preparation of the positive active material, the volume average particle size Dv50 of the lithium iron phosphate material primary particles is 0.1 μm; the temperature of the spray drying is 350°C; and the volume average particle size Dv50 of the secondary particles is 2 μm. The other parts are the same as those in Example 1, which are not described here.

[0149] Example 11

[0150] The difference from Example 1 is that in the preparation of the positive active material, the volume average particle size Dv50 of the lithium iron phosphate material primary particles is 0.2 μm; the temperature of the spray drying is 250°C; and the volume average particle size Dv50 of the secondary particles is 5 μm. The other parts are the same as those in Example 1, which are not described here.

[0151] Example 12

[0152] The difference from Example 1 is that in the preparation of the positive active material, the volume average particle size Dv50 of the lithium iron phosphate material primary particles is 1.5 μm; the temperature of the spray drying is 200°C; and the volume average particle size Dv50 of the secondary particles is 20 μm. The other parts are the same as those in Example 1, which are not described here.

[0153] Example 13

[0154] The difference from Example 1 is that in the preparation of the positive active material, the volume average particle size Dv50 of the lithium iron phosphate material primary particles is 2 μm; the temperature of the spray drying is 150°C; and the volume average particle size Dv50 of the secondary particles is 50 μm. The other aspects are the same as those of Example 1, and are not described here again.

[0155] Comparative Example 1

[0156] The difference from Example 1 is that the preparation of the binder and the preparation of the positive active material are not included. In the preparation of the positive electrode sheet, the positive active material used is lithium iron phosphate material primary particles, and the volume average particle size Dv50 of the lithium iron phosphate material primary particles is 0.6 μm.

[0157] The test methods of the relevant parameters in the above examples and comparative examples are as follows:

[0158] 2.1) Volume average particle size Dv50 test

[0159] Device model: Malvern 2000 (MasterSizer 2000) laser particle size analyzer, reference standard process: GB / T19077-2016 / ISO 13320:2009, specific test process: take an appropriate amount of sample to be tested (the sample concentration can ensure 8%-12% obscuration), add 20 ml of deionized water, and simultaneously super 5 min (53 KHz / 120 W) to ensure that the sample is completely dispersed, and then determine the sample according to the GB / T19077-2016 / ISO 13320:2009 standard.

[0160] 2.2 Battery performance test

[0161] 2.21) DC resistance DCR test

[0162] The battery is placed at 25°C for 30 min, charged at 0.33C to a voltage of 3.65V, and then charged at a constant voltage to a current of 0.05C. The battery is placed at 25°C for 5 min, discharged at 0.33C for 90 min, and the current at a cutoff current of 0.5C; then the battery is placed at 25°C for 1 h, discharged at 2C for 30 s, and then placed at 25°C for 5 min, and the DC resistance DCR is measured.

[0163] 2.22) Capacity test

[0164] The battery was rested at 25°C for 30 min, discharged at 0.33C to 2.5V, rested at 25°C for 30 min; charged at 0.33C constant current to 3.65V, constant voltage charging, cutoff current 0.05C; rested at 25°C for 30 min, then discharged at 0.33C to 2.5V, rested at 25°C for 30 min, and the charge-discharge capacity of the Nth cycle was recorded.

[0165] 2.23) Cycle test

[0166] Step 1, the battery was rested at 25°C for 30 min, discharged at 0.33C to 2.5V, rested at 25°C for 30 min.

[0167] Step 2, the battery was charged at 0.33C constant current to 3.65V, constant voltage charging, rested at 25°C for 30 min, discharged at 0.33C to 2.5V, rested at 25°C for 30 min. Step 2 was repeated for 1000 cycles, and the capacity retention rate of the battery after 1000 cycles was recorded.

[0168] The capacity retention rate CR(%) of the battery after n cycles = discharge capacity of the nth cycle / discharge capacity of the 1st cycle x 100%.

[0169] Table 1 Process and cycle performance parameters of positive electrode active materials of each example and comparative example

[0170]

[0171] Note: formula II is a binder of formula (I), and R1, R2, R3 are all hydrogen, and R4 is methyl; formula III is a binder of formula (I), and R1 is hydrogen, R2 is methyl, R3 is hydrogen, and R4 is methyl; formula IV is a binder of formula (I), and R1 is hydrogen, R2 is methyl, R3 is tert-butyl, and R4 is methyl; formula V is a binder of formula (I), R1 is hydrogen, R2 is methyl, R3 is hydrogen, and R4 is methyl; temperature represents the temperature of spray drying; primary particles represent the volume average particle size Dv50 of primary particles; secondary particles represent the volume average particle size Dv50 of secondary particles; thickness represents the thickness of the binder between adjacent primary particles.

[0172] Table 2 Performance parameters of positive electrode active materials of each example

[0173]

[0174] Note: initial DCR represents the first time direct current impedance DCR; 200 cycles DCR represents the direct current impedance DCR at 200 cycles; 500 cycles DCR represents the direct current impedance DCR at 500 cycles.

[0175] As can be seen from the data in Table 1, based on Comparative Example 1 and Examples 1-5, the volume average particle size Dv50 of the positive electrode active material of Comparative Example 1 is 0.6 μm, and the volume average particle size Dv50 of the secondary particles of Examples 1-5 is in the range of 2.3 μm-11.2 μm. Based on Comparative Example 1, the cycle capacity retention rate of Examples 1-5 is significantly improved to 83.2%-93.7% after 1000 cycles, which is higher than 80% of Comparative Example 1. Specifically, due to the fact that the positive electrode active material of Examples 1-5 is formed by bonding the primary particles by the binder to form the secondary particles, the particle size of the secondary particles is increased, the initial polarization of the material is increased, the initial direct current resistance DCR is larger, which leads to a slight decrease in the initial specific capacity; as shown in Table 2, the polarization gradually decreases during the cycle process, and the direct current resistance DCR at 200 cycles is compared with the initial direct current resistance DCR, it can be seen that the direct current resistance DCR of Examples 1-5 all decreases, which makes the active lithium component contained in the lithium iron phosphate material of Examples 1-5 slowly release. Subsequently, ion channels are formed in the secondary particles, and with the cycle process, the cathode polarization no longer decreases, and the direct current resistance DCR starts to increase at 500 cycles, wherein the growth rate of the direct current resistance DCR of Examples 1-5 is smaller, which indicates that the overall growth rate of the direct current resistance DCR of Examples 1-5 after 1000 cycles is also smaller.

[0176] As shown in Table 1 and Table 2, compared with Example 1, Examples 2-3 reduce the binder addition amount in the secondary granulation process of the lithium iron phosphate particles, the volume average particle size D50 of the formed secondary particles is relatively small, and the cycle capacity retention rate improvement effect is poor; and Examples 4-5 increase the binder addition amount, thereby increasing the D50 of the secondary particles, further increasing the polarization, although the cycle capacity retention rate is correspondingly improved, but the initial capacity is obviously decreased. Therefore, by adjusting the addition amount of the binder, the size of the secondary particles can be controlled, thereby controlling the performance of the final battery, and improving the cycle capacity retention rate effect.

[0177] Based on the adjustment of the synthetic raw materials in Example 1 and Examples 6-7 to regulate the structure of the binder, the structure of the binder in Example 8 is regulated by adjusting the synthetic raw materials and the synthetic method, so that the structure of the binder in Examples 6-8 conforms to Formula (I), and the binder obtained can also improve the cycle capacity retention rate to 91.3%-92.3% relative to Comparative Example 1. The binder in Examples 6-8 used to bond primary particles to form secondary particles can achieve the effects of increasing the initial polarization, increasing the initial direct current resistance DCR, and improving the cycle capacity retention rate; the cycle capacity retention rate of Example 9 also increases relative to Comparative Example 1. Example 9 uses PVDF to bond primary particles to form secondary particles, and Example 9 can also achieve the effect that part of the active material cannot develop capacity in the early stage of the cycle. As the cycle progresses, the binder PVDF between the primary particles is fully infiltrated by the electrolyte, and ion channels are gradually formed. In addition, the expansion and contraction of the lithium iron phosphate material during the deintercalation of lithium causes the gap between the primary particles that were originally tightly bonded to increase, and part of the active material that could not develop capacity is gradually activated, thereby making up for the attenuation of the lithium iron phosphate positive active material in the early stage of the cycle.

[0178] The cycle capacity retention rate of Example 9 is less than that of Examples 1 and 6-8, indicating that the binder of Formula (I) in Examples 1 and 6-8 has a better improvement effect on the cycle capacity retention rate of the positive active material relative to the PVDF binder in Example 9. The main reason is that the binder in Examples 1 and 6-8 increases the carbonic acid group polymerization unit, increases the molecular weight of the binder, and improves the adhesion of the binder, which can better bond the primary particles to form secondary particles. In addition, the carbonic acid group has better affinity with the electrolyte solvent in the battery due to the similar solubility characteristics, and the swelling is more obvious. It can absorb the electrolyte during the cycle process, allowing the electrolyte to constantly infiltrate into the interior of the secondary particles, build ion channels, reduce polarization, and achieve the purpose of capacity release, thereby improving the cycle capacity retention rate to a greater extent.

[0179] As shown in Figure 6 the cross-sectional scanning electron microscope image of the positive electrode sheet of Example 1 of the present application, Figure 7 the cross-sectional scanning electron microscope image of the positive electrode sheet of Comparative Example 1, and Figure 6As can be seen from the above, the lithium iron phosphate material primary particles in the secondary particles of the positive electrode active material of Embodiment 1 are bonded by the binder (the first binder), and the primary particles are closely bonded by the binder (the first binder), while the positive electrode active material in the positive electrode sheet formed by the positive electrode active material, the conductive carbon and the polyvinylidene fluoride binder (the second binder) is loose, and there is a gap between the positive electrode active material, which indicates that the first binder and the second binder in the positive electrode sheet have different functions and different forms, the first binder can increase the initial polarization of the formed lithium iron phosphate secondary particles, and the second binder is used to bond the positive electrode active material to the current collector. From the above, it can be seen that the lithium iron phosphate primary particles in the secondary particles of the positive electrode active material of Embodiment 1 are bonded by the binder (the first binder), and the primary particles are closely bonded by the binder (the first binder), while the positive electrode active material in the positive electrode sheet formed by the positive electrode active material, the conductive carbon and the polyvinylidene fluoride binder (the second binder) is loose, and there is a gap between the positive electrode active material, which indicates that the first binder and the second binder in the positive electrode sheet have different functions and different forms, the first binder can increase the initial polarization of the formed lithium iron phosphate secondary particles, and the second binder is used to bond the positive electrode active material to the current collector. Figure 7 As can be seen from the above, the lithium iron phosphate material primary particles in the secondary particles of the positive electrode active material of Embodiment 1 are bonded by the binder (the first binder), and the primary particles are closely bonded by the binder (the first binder), while the positive electrode active material in the positive electrode sheet formed by the positive electrode active material, the conductive carbon and the polyvinylidene fluoride binder (the second binder) is loose, and there is a gap between the positive electrode active material, which indicates that the first binder and the second binder in the positive electrode sheet have different functions and different forms, the first binder can increase the initial polarization of the formed lithium iron phosphate secondary particles, and the second binder is used to bond the positive electrode active material to the current collector.

[0180] In summary, the cycle capacity retention rates of Embodiments 1-13 are all greater than that of Comparative Example 1, which indicates that in the embodiments of the present application, the lithium iron phosphate positive electrode active material is bonded by the binder to form secondary particles, which increases the particle size of the secondary particles, increases the initial polarization of the material, increases the initial direct current resistance DCR, and part of the positive electrode active material cannot develop capacity, resulting in a slight decrease in the initial gram capacity. However, during the cycle process, with the increase of the cycle number, the direct current resistance DCR appears a trend of first decreasing and then increasing, and the polarization of the positive electrode active material appears a trend of first decreasing and then increasing. The polarization of the positive electrode active material decreases, and with the progress of the cycle, the binder between the primary particles is fully infiltrated by the electrolyte, and ion channels are gradually formed, plus the expansion and contraction of the lithium iron phosphate material during the deintercalation of lithium, which increases the gap between the originally closely bonded primary particles, and part of the active material that cannot develop capacity is gradually activated, thereby compensating for the attenuation of the lithium iron phosphate positive electrode active material in the early stage of the cycle. With the progress of the cycle, ion channels are formed in the secondary particles, and the polarization of the positive electrode no longer decreases, but appears a rising trend. At 500 cycles, the direct current resistance DCR appears the same or increases relative to the initial DCR.

[0181] The above is only an embodiment of the present application, and does not limit the patent scope of the present application, and any equivalent structure or equivalent process transformation using the content of the specification and drawings, or direct or indirect application in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A positive electrode active material, characterized by, The positive electrode active material comprises secondary particles, the secondary particles comprising: primary particles, the primary particles comprising a lithium iron phosphate material; a binder, at least part of the binder being located between adjacent ones of the primary particles.

2. The positive electrode active material according to claim 1, characterized by The volume average particle size Dv50 of the secondary particles is in the range of 2 μm-50 μm.

3. The positive electrode active material according to claim 1, characterized by The volume average particle size Dv50 of the secondary particles is in the range of 10 μm-30 μm.

4. The positive electrode active material according to any one of claims 1 to 3, characterized by, The volume average particle size Dv50 of the primary particles is in the range of 0.1 μm-2 μm.

5. The positive electrode active material according to any one of claims 1 to 4, characterized by, The volume average particle size Dv50 of the primary particles is in the range of 0.2 μm-1.5 μm.

6. The positive electrode active material according to any one of claims 1 to 5, characterized by, The mass fraction of the binder is in the range of 0.1 %-2 % based on the total mass of the positive electrode active material.

7. The positive electrode active material according to any one of claims 1-6, characterized in that, The thickness of the binder between adjacent ones of the primary particles is in the range of 0.1 μm-1 μm.

8. The positive electrode active material according to any one of claims 1 to 7, characterized by, The binder comprises: ， wherein R1, R2, R3 each independently comprises hydrogen, an unsubstituted straight-chain or branched alkyl group having 1-6 carbon atoms; R4 comprises an unsubstituted straight-chain or branched alkyl group having 1-6 carbon atoms; m is in the range of 100-3000; n is in the range of 100-3000.

9. The positive electrode active material according to claim 8, characterized in that, The unsubstituted straight-chain or branched alkyl group having 1-6 carbon atoms comprises a methyl group, an ethyl group, a n-butyl group or a t-butyl group.

10. The positive electrode active material according to any one of claims 1-9, characterized in that, The binder comprises: , , , and .

11. The positive electrode active material according to any one of claims 1-10, characterized in that, The lithium iron phosphate material includes Li x Fe y M z PO4, wherein M includes any one or several of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb, 0.05 ≦ x ≦ 1.2, 0.2 ≦ y ≦ 1, 0 ≦ z ≦ 0.

8.

12. The positive electrode active material according to any one of claims 1 to 11, characterized by, The lithium iron phosphate material further comprises a coating layer, the coating layer comprising carbon.

13. The positive electrode active material according to claim 12, characterized by The thickness of the coating layer is greater than 0 and less than or equal to 20 nm.

14. A method for producing a positive electrode active material, characterized by, comprising: dispersing primary particles comprising a lithium iron phosphate material and a binder in a solvent, forming secondary particles by spray drying, wherein at least part of the binder is located between adjacent ones of the primary particles.

15. The method of producing the positive electrode active material according to claim 14, characterized by, The temperature of the spray drying is in the range of 150 °C-350 °C.

16. A positive electrode sheet characterized by comprising: A positive electrode sheet comprising the positive electrode active material according to any one of claims 1-13 or / and the positive electrode active material prepared by the method according to claim 14 or 15.

17. A battery, characterized by comprising the positive electrode sheet according to claim 16.

18. An electrical device, comprising: comprising the battery according to claim 17.

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