Composite material and preparation method thereof, positive electrode material, positive electrode sheet, electrode assembly, battery cell and preparation method thereof

Abstract

The application provides a composite material and a preparation method thereof, a positive electrode material, a positive electrode sheet, an electrode assembly, a battery cell and a preparation method thereof, and belongs to the technical field of batteries. The composite material comprises lithium iron phosphate and carbon fluoride coating the lithium iron phosphate, and the atomic ratio of fluorine atoms to carbon atoms in the carbon fluoride is 0.1-1.2:1. The lithium iron phosphate particles are coated with the carbon fluoride. In the discharging process, the carbon fluoride can react irreversibly with lithium ions to generate lithium fluoride and carbon. The lithium fluoride and the carbon form a mixed coating layer to coat the surface of the lithium iron phosphate particles. The lithium fluoride can serve as an ion conductor to improve the ion conductivity of the electrode, and the carbon can serve as an electron conductor to improve the electron conductivity of the electrode, thereby improving the poor electrical conductivity of the lithium iron phosphate. The mixed coating layer formed by the lithium fluoride and the carbon can prevent the lithium iron phosphate particles from directly contacting the electrolyte, and avoid the corrosion of the lithium iron phosphate particles by hydrofluoric acid in the electrolyte to cause the dissolution of transition metal iron ions.

Description

[0001] This application is a divisional application of Chinese patent application (2022109453251) filed on August 8, 2022, the description of which is incorporated herein by reference in its entirety. Technical Field

[0002] This application relates to the field of battery technology, and more specifically, to a composite material and its preparation method, a positive electrode material, a positive electrode sheet, an electrode assembly, a battery cell and its preparation method. Background Technology

[0003] Lithium iron phosphate (LFP) is a cathode material for lithium-ion batteries. It boasts advantages such as high reversible charge-discharge capacity, wide availability of raw materials, low pollution, good safety, and long cycle life, making it a relatively ideal cathode material for lithium-ion batteries. However, its inherent structure results in low conductivity, preventing high-rate charge-discharge operation. Summary of the Invention

[0004] In view of the above problems, this application provides a composite material and its preparation method, a positive electrode material, a positive electrode sheet, an electrode assembly, a battery cell and its preparation method, which can improve the problem of low conductivity of lithium iron phosphate itself.

[0005] In a first aspect, this application provides a composite material comprising lithium iron phosphate and fluorinated carbon coating the lithium iron phosphate.

[0006] In the technical solution of this application embodiment, lithium iron phosphate particles are coated with fluorinated carbon. During discharge, the fluorinated carbon can undergo an irreversible reaction with lithium ions to generate lithium fluoride and carbon. The lithium fluoride and carbon form a mixed coating layer on the surface of the lithium iron phosphate particles. Lithium fluoride can act as an ion conductor, improving the ionic conductivity of the electrode, while carbon can act as an electronic conductor, improving the electronic conductivity of the electrode, thereby improving the poor conductivity of lithium iron phosphate itself. Furthermore, the mixed coating layer formed by lithium fluoride and carbon can prevent the lithium iron phosphate particles from directly contacting the electrolyte, thus avoiding the corrosion of the lithium iron phosphate particles by hydrofluoric acid in the electrolyte, which would lead to the dissolution of transition metal iron ions. This keeps the iron ion content in the electrolyte at a low level, improving the stability of the prepared electrode material.

[0007] In some embodiments, the atomic ratio of fluorine atoms to carbon atoms in the fluorinated carbon is 0.1–1.2:1. Optionally, the atomic ratio of fluorine atoms to carbon atoms in the fluorinated carbon is 0.3–0.4:1. Fluorine atoms can combine with lithium ions to form lithium fluoride, increasing the ionic conductivity of the electrode. Increasing the fluorine ion content is beneficial for improving the ionic conductivity of the activated electrode. Carbon atoms can form a conductive carbon layer after the reaction, increasing the electronic conductivity of the electrode. Increasing the carbon atom content is beneficial for improving the electronic conductivity of the activated electrode. Choosing an appropriate atomic ratio of fluorine atoms to carbon atoms in the fluorinated carbon is beneficial for simultaneously improving the ionic and electronic conductivity of the lithium iron phosphate electrode.

[0008] In some embodiments, the mass ratio of fluorinated carbon to lithium iron phosphate is 0.1–20:100. Optionally, the mass ratio of fluorinated carbon to lithium iron phosphate is 0.1–0.2:1. When the mass ratio of fluorinated carbon to lithium iron phosphate is low, the thickness of the mixed coating layer formed by lithium fluoride and carbon covering the lithium iron phosphate particles is low, and the coating layer cannot provide sufficient conductive agent, resulting in less improvement in the electrical performance of the prepared battery. When the mass ratio of fluorinated carbon to lithium iron phosphate is high, since the coating layer not only does not contain active lithium ions but also consumes lithium ions during the initial reaction to form lithium fluoride and carbon, the specific capacity of the lithium iron phosphate cathode active material is reduced. That is, for the same mass of lithium iron phosphate cathode active material, the effective lithium iron phosphate mass ratio is reduced, thereby reducing the energy density of the prepared battery. In other words, choosing an appropriate mass ratio of fluorinated carbon to lithium iron phosphate is beneficial for generating sufficient carbon and lithium fluoride to coat the surface of lithium iron phosphate particles, thereby reducing the damage of hydrofluoric acid in the electrolyte to lithium iron phosphate particles, improving the battery capacity retention rate, and reducing battery impedance.

[0009] In some embodiments, the D of the composite material v50 The particle size ranges from 1 to 50 μm. Selecting a suitable particle size for the composite material is beneficial for the compaction density during application, thereby improving the electrical contact of the active particles and thus increasing the conductivity of the electrode.

[0010] Secondly, this application provides a method for preparing the composite material in the above embodiments, which includes mixing fluorinated carbon powder and lithium iron phosphate powder to obtain a mixed powder, and placing the mixed powder at 400℃~800℃ for 3h~8h.

[0011] In the technical solutions of this application, the preparation method of the composite material is simple and the obtained composite material is stable.

[0012] Thirdly, this application provides a cathode material, which includes the composite material in the above embodiments.

[0013] Fourthly, this application provides a positive electrode sheet, which includes a positive current collector and the positive electrode material in the above embodiments, wherein the positive electrode material is disposed on at least one side of the positive current collector along the thickness direction.

[0014] Fifthly, this application provides an electrode assembly including a negative electrode, a separator, and the positive electrode as described in the above embodiments.

[0015] Sixthly, this application provides a method for preparing a battery cell, comprising: activating a semi-finished product. The activation treatment includes a discharge step, the discharge rate of which is 0.01C to 0.1C, and the activation treatment is performed 2 to 15 times. The semi-finished product is assembled from an electrolyte assembly and the electrode assembly described in the above embodiments.

[0016] In the technical solution of this application embodiment, fluorinated carbon can generate lithium fluoride and carbon after discharge. When the discharge rate is low, the fluorinated carbon can react fully, improving its conversion efficiency into lithium fluoride and carbon. However, too low a rate will lead to an excessively long activation time in the early stages. When the discharge rate is too high, it will affect the degree of lithium fluoride reaction, thereby affecting the activation effect. This application uses a discharge rate of 0.01C to 0.1C for multiple discharges, which allows the fluorinated carbon to react almost completely to generate lithium fluoride and carbon, ensuring the subsequent normal electrochemical reaction, improving the rate performance and cycle stability of the battery cell, and the activation time is not too long.

[0017] Seventhly, this application provides a battery cell prepared according to the preparation method of the battery cell in the above embodiments.

[0018] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0019] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0020] Figure 1 This is a schematic diagram of the vehicle structure according to some embodiments of this application;

[0021] Figure 2 This is an exploded structural diagram of a battery according to some embodiments of this application;

[0022] Figure 3This is an exploded structural diagram of a battery cell according to some embodiments of this application.

[0023] 1000 - Vehicles;

[0024] 100 - Battery; 200 - Controller; 300 - Motor;

[0025] 10-Box body; 11-First part; 12-Second part;

[0026] 20-Battery cell; 21-End cap; 22-Housing casing; 23-Electrode assembly. Detailed Implementation

[0027] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0029] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0030] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0031] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0032] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0033] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0034] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0035] Currently, judging from market trends, the application of power batteries is becoming increasingly widespread. Power batteries are not only used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, but also extensively used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With the continuous expansion of power battery applications, market demand is also constantly increasing.

[0036] The inventors noted that lithium iron phosphate, as the most common cathode material for lithium-ion batteries, has low conductivity due to its structure, making it impossible to achieve high-rate charge and discharge.

[0037] To address the issue of low conductivity in lithium iron phosphate, the applicant discovered that coating the surface of lithium iron phosphate particles with a highly conductive material can improve their conductivity.

[0038] Based on the above considerations, to address the issue of low conductivity in lithium iron phosphate (LFP) batteries, the inventors, through in-depth research, designed a composite material that coats LFP particles with fluorinated carbon. During discharge, the fluorinated carbon undergoes an irreversible reaction with lithium ions to generate lithium fluoride and carbon. This lithium fluoride and carbon form a mixed coating layer on the surface of the LFP particles. Lithium fluoride acts as an ionic conductor, improving the ionic conductivity of the electrode, while carbon acts as an electronic conductor, improving the electronic conductivity of the electrode. This addresses the inherently poor conductivity of LFP. Furthermore, the mixed coating layer formed by lithium fluoride and carbon prevents LFP particles from directly contacting the electrolyte, thus avoiding the corrosion of LFP particles by hydrofluoric acid in the electrolyte, which could lead to the dissolution of transition metal iron ions. This keeps the iron ion content in the electrolyte at a low level, improving the stability of the resulting electrode material.

[0039] The battery mentioned in the embodiments of this application refers to a single physical module comprising one or more battery cells to provide higher voltage and capacity. A battery generally includes a battery housing for encapsulating one or more battery cells, which prevents liquids or other foreign matter from affecting the charging or discharging of the battery cells.

[0040] Battery cells may include lithium-ion rechargeable battery cells, but this application embodiment is not limited to this. Battery cells may be cylindrical, flat, cuboid, or other shapes, and this application embodiment is not limited to this either. Battery cells are generally classified into three types according to their packaging method: cylindrical battery cells, prismatic battery cells, and pouch battery cells.

[0041] A single battery cell includes an electrode assembly and an electrolyte. The electrode assembly consists of a positive electrode, a negative electrode, and a separator. The battery cell primarily functions by the movement of metal ions between the positive and negative electrodes. The positive electrode includes a positive current collector and a positive active material layer. The positive active material layer is coated on the surface of the positive current collector, and the uncoated positive current collector protrudes beyond the coated one, serving as the positive electrode tab. Taking a lithium-ion battery as an example, the positive current collector can be made of aluminum, and the positive active material can be lithium cobalt oxide, lithium iron phosphate, ternary lithium, or lithium manganese oxide, etc. The negative electrode includes a negative current collector and a negative active material layer. The negative active material layer is coated on the surface of the negative current collector, and the uncoated negative current collector protrudes beyond the coated one, serving as the negative electrode tab. The negative electrode current collector can be made of copper, and the negative electrode active material can be carbon or silicon, etc. To ensure that a large current can pass through without melting, there are multiple positive electrode tabs stacked together, and there are multiple negative electrode tabs stacked together. The separator can be made of polypropylene (PP) or polyethylene (PE), etc. Furthermore, the electrode assembly can be a wound structure or a stacked structure, and the embodiments of this application are not limited to these.

[0042] The battery cell also includes a current collector, which is used to electrically connect the tabs and electrode terminals of the battery cell to deliver electrical energy from the electrode assembly to the electrode terminals, and then to the outside of the battery cell via the electrode terminals. Multiple battery cells are electrically connected through a current collector to realize series, parallel or mixed connection of multiple battery cells.

[0043] The battery also includes sampling terminals and a battery management system. The sampling terminals are connected to the busbar and are used to collect information from individual battery cells, such as voltage or temperature. The sampling terminals transmit the collected information from individual battery cells to the battery management system. When the battery management system detects that the information from a battery cell exceeds the normal range, it limits the battery's output power to achieve safety protection.

[0044] It is understood that the power devices applicable to the use of batteries described in the embodiments of this application can take many forms, such as mobile phones, portable devices, laptops, electric vehicles, electric cars, ships, spacecraft, electric toys, and power tools, etc. For example, spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers.

[0045] The battery cells and batteries described in the embodiments of this application are not limited to the electrical devices described above, but can also be applied to all electrical devices that use battery cells and batteries. However, for the sake of brevity, the following embodiments are all illustrated using electric vehicles as an example.

[0046] Please see Figure 1 , Figure 1 This is a schematic diagram of the structure of a vehicle provided in some embodiments of this application. The vehicle 1000 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid electric vehicle, or a range-extended electric vehicle, etc. A battery 100 is disposed inside the vehicle 1000, and the battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The battery 100 can be used to power the vehicle 1000; for example, the battery 100 can serve as the operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 200 and a motor 300. The controller 200 is used to control the battery 100 to supply power to the motor 300, for example, to meet the power needs of the vehicle 1000 during startup, navigation, and driving.

[0047] In some embodiments of this application, the battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0048] Please refer to Figure 2 , Figure 2This is an exploded view of a battery provided in some embodiments of this application. The battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10. The housing 10 provides a space for the battery cell 20 and can have various structures. In some embodiments, the housing 10 may include a first portion 11 and a second portion 12, which overlap each other, jointly defining a space for accommodating the battery cell 20. The second portion 12 may be a hollow structure with one open end, and the first portion 11 may be a plate-like structure, covering the open side of the second portion 12 so that the first portion 11 and the second portion 12 jointly define the space; alternatively, the first portion 11 and the second portion 12 may both be hollow structures with one open side, with the open side of the first portion 11 covering the open side of the second portion 12. Of course, the housing 10 formed by the first portion 11 and the second portion 12 can have various shapes, such as a cylinder, a cuboid, etc.

[0049] In battery 100, there can be multiple battery cells 20, which can be connected in series, parallel, or in a mixed manner. A mixed connection means that multiple battery cells 20 are connected in both series and parallel configurations. Multiple battery cells 20 can be directly connected in series, parallel, or in a mixed manner, and then the entire assembly of the multiple battery cells 20 is housed within the housing 10. Alternatively, battery 100 can also be composed of multiple battery cells 20 first connected in series, parallel, or in a mixed manner to form a battery module, and then multiple battery modules are connected in series, parallel, or in a mixed manner to form a whole, which is also housed within the housing 10. Battery 100 may also include other structures; for example, it may include a busbar component for electrical connection between the multiple battery cells 20.

[0050] Each battery cell 20 can be a secondary battery or a primary battery; it can also be a lithium-sulfur battery, a sodium-ion battery, or a magnesium-ion battery, but is not limited to these. The battery cell 20 can be cylindrical, flat, cuboid, or other shapes.

[0051] Please refer to Figure 3 , Figure 3 This is an exploded structural diagram of a battery cell provided in some embodiments of this application. The battery cell 20 refers to the smallest unit that makes up the battery. For example... Figure 3 The battery cell 20 includes an end cap 21, a housing 22, an electrode assembly 23, and other functional components.

[0052] End cap 21 refers to a component that covers the opening of housing 22 to isolate the internal environment of battery cell 20 from the external environment. The shape of end cap 21 can be adapted to the shape of housing 22 to fit it. Optionally, end cap 21 can be made of a material with certain hardness and strength (such as aluminum alloy), so that end cap 21 is not easily deformed under pressure and impact, giving battery cell 20 higher structural strength and improved safety performance. Functional components such as electrode terminals can be provided on end cap 21. Electrode terminals can be used for electrical connection with electrode assembly 23 for outputting or inputting electrical energy into battery cell 20. In some embodiments, end cap 21 can also be provided with a pressure relief mechanism for releasing internal pressure when the internal pressure or temperature of battery cell 20 reaches a threshold. The material of end cap 21 can also be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and this application embodiment does not impose special limitations on this. In some embodiments, an insulating element may be provided on the inner side of the end cap 21. The insulating element can be used to isolate the electrical connection components within the housing 22 from the end cap 21 to reduce the risk of short circuits. For example, the insulating element may be made of plastic, rubber, etc.

[0053] The housing 22 is a component used to cooperate with the end cap 21 to form the internal environment of the battery cell 20. This internal environment can accommodate the electrode assembly 23, electrolyte, and other components. The housing 22 and the end cap 21 can be independent components. An opening can be provided on the housing 22, and the end cap 21 can be used to close the opening to form the internal environment of the battery cell 20. Alternatively, the end cap 21 and the housing 22 can be integrated. Specifically, the end cap 21 and the housing 22 can form a common connecting surface before other components are inserted into the housing. When it is necessary to encapsulate the interior of the housing 22, the end cap 21 closes the housing 22. The housing 22 can be of various shapes and sizes, such as cuboid, cylindrical, hexagonal prism, etc. Specifically, the shape of the housing 22 can be determined according to the specific shape and size of the electrode assembly 23. The material of the housing 22 can be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc. This application embodiment does not impose any special limitations on this.

[0054] Electrode assembly 23 is the component in the battery cell 20 where electrochemical reactions occur. The casing 22 may contain one or more electrode assemblies 23. The electrode assembly 23 is mainly formed by winding or stacking positive and negative electrode sheets, and typically a separator is provided between the positive and negative electrode sheets. The portions of the positive and negative electrode sheets containing active material constitute the main body of the electrode assembly, while the portions without active material each constitute a tab. The positive and negative tabs may be located together at one end of the main body or separately at both ends of the main body. During the charging and discharging process of the battery, the positive and negative active materials react with the electrolyte, and the tabs connect to the electrode terminals to form a current loop.

[0055] According to some embodiments of this application, this application provides a composite material including lithium iron phosphate and fluorinated carbon coated with lithium iron phosphate.

[0056] Lithium iron phosphate is formed in particulate form, with fluorinated carbon coating part or all of its surface.

[0057] By coating lithium iron phosphate (LFP) particles with fluorinated carbon, an irreversible reaction occurs between the fluorinated carbon and lithium ions during discharge, producing lithium fluoride and carbon. This lithium fluoride and carbon form a mixed coating layer on the surface of the LFP particles. Lithium fluoride acts as an ionic conductor, improving the ionic conductivity of the electrode, while carbon acts as an electronic conductor, improving the electronic conductivity. This addresses the issue of the inherently poor conductivity of lithium iron phosphate. Furthermore, the mixed coating layer prevents the LFP particles from directly contacting the electrolyte, thus avoiding the corrosion of the LFP particles by hydrofluoric acid in the electrolyte, which could lead to the dissolution of transition metal iron ions. This keeps the iron ion content in the electrolyte at a low level, improving the stability of the resulting electrode material.

[0058] It should be noted that compared to directly coating lithium iron phosphate with a mixture of lithium fluoride and carbon, the process of coating lithium iron phosphate with fluorinated carbon is simpler. Moreover, in the mixed coating layer formed by subsequent discharge, the distribution of lithium fluoride and carbon is more uniform, which is beneficial to improving the conductivity of lithium iron phosphate particles.

[0059] According to some embodiments of this application, optionally, the atomic ratio of fluorine atoms to carbon atoms in fluorinated carbon is 0.1 to 1.2:1.

[0060] As an example, the atomic ratio of fluorine atoms to carbon atoms in fluorinated carbon can be 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, or 1.2:1.

[0061] Fluorine atoms can combine with lithium ions to form lithium fluoride, increasing the ionic conductivity of the electrode. Increasing the fluorine ion content is beneficial for improving the ionic conductivity of the activated electrode. Carbon atoms can form a conductive carbon layer after the reaction, increasing the electronic conductivity of the electrode. Increasing the carbon atom content is beneficial for improving the electronic conductivity of the activated electrode. Choosing an appropriate atomic ratio of fluorine to carbon in the fluorinated carbon is beneficial for simultaneously improving the ionic and electronic conductivity of the lithium iron phosphate electrode.

[0062] Optionally, the atomic ratio of fluorine atoms to carbon atoms in fluorinated carbon is 0.3 to 0.4:1.

[0063] According to some embodiments of this application, optionally, the mass ratio of fluorinated carbon to lithium iron phosphate is 0.1 to 20:100.

[0064] As an example, the mass ratio of carbon fluoride to lithium iron phosphate can be 0.1:100, 0.5:100, 1:100, 2:100, 5:100, 8:100, 10:100, 12:100, 15:100, 18:100 or 20:100.

[0065] When the mass ratio of fluorinated carbon to lithium iron phosphate is low, the thickness of the mixed coating layer formed by lithium fluoride and carbon covering the lithium iron phosphate particles is relatively small. This coating layer cannot provide sufficient conductive agent, resulting in minimal improvement in the battery's electrical performance. Conversely, when the mass ratio of fluorinated carbon to lithium iron phosphate is high, the coating layer not only lacks active lithium ions but also consumes lithium ions during the initial reaction to form lithium fluoride and carbon. This leads to a decrease in the specific capacity of the lithium iron phosphate cathode active material; that is, for the same mass of lithium iron phosphate cathode active material, the effective lithium iron phosphate mass ratio is lower, thus reducing the energy density of the prepared battery. Therefore, selecting an appropriate mass ratio of fluorinated carbon to lithium iron phosphate is beneficial for generating sufficient carbon and lithium fluoride to coat the surface of the lithium iron phosphate particles. This reduces the damage to the lithium iron phosphate particles caused by hydrofluoric acid in the electrolyte, improves battery capacity retention, and reduces battery impedance.

[0066] Optionally, the mass ratio of fluorinated carbon to lithium iron phosphate is 0.1 to 0.2:1.

[0067] According to some embodiments of this application, optionally, the D of the composite material v50 The range is 1–50 μm.

[0068] As an example, the D of composite materials v50 It can be 1μm, 2μm, 5μm, 8μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm or 50μm.

[0069] Choosing a composite material with an appropriate particle size is beneficial for the compaction density during composite material application, thereby improving the electrical contact of the active particles and thus increasing the conductivity of the electrode.

[0070] Optionally, according to some embodiments of this application, this application provides a method for preparing the composite material in the above embodiments, which includes mixing fluorinated carbon powder and lithium iron phosphate powder to obtain a mixed powder, and placing the mixed powder at 400℃~800℃ for 3h~8h.

[0071] As an example, the insulation temperature of the mixed powder can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃.

[0072] As an example, the heat preservation time for the mixed powder can be 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h or 8h.

[0073] The method for preparing the composite material of this application is simple, and the resulting composite material is stable.

[0074] One method for mixing fluorocarbon powder and lithium iron phosphate powder is ball milling.

[0075] Optionally, the ball milling mixing includes placing fluorinated carbon powder and lithium iron phosphate powder in a planetary ball mill according to a certain ratio and ball milling at a speed of 300 r / min to 400 r / min for 6 to 8 hours.

[0076] Optionally, before placing the mixed powder at 400℃~800℃ for 3h~8h, the mixed powder is first dried at 60℃~80℃ for 10h~12h.

[0077] Optionally, the heat treatment process of the mixed powder at 400℃ to 800℃ is carried out under the protection of an inert gas.

[0078] Optionally, according to some embodiments of this application, this application also provides a method for preparing the battery cell in the above embodiments, which includes: activating the semi-finished product. The activation treatment includes a discharge step, the discharge rate of the discharge step is 0.01C to 0.1C, and the number of activation cycles is 2 to 15. The semi-finished product is assembled from a positive electrode, a negative electrode, a separator, and an electrolyte.

[0079] Because fluorinated carbon itself has poor conductivity, the activation process is the process of reacting fluorinated carbon to generate lithium fluoride and carbon, and forming a mixed coating layer on the surface of lithium iron phosphate particles.

[0080] As an example, the discharge rate of the discharge step can be 0.01C, 0.02C, 0.03C, 0.04C, 0.05C, 0.06C, 0.07C, 0.08C, 0.09C, or 0.1C.

[0081] As an example, the number of activation cycles can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15.

[0082] Fluorinated carbon can be converted into lithium fluoride and carbon upon discharge. A lower discharge rate allows for a more complete reaction of the fluorinated carbon, increasing its conversion efficiency into lithium fluoride and carbon. However, a too-low discharge rate leads to excessively long activation times in the initial stages. Conversely, a too-high discharge rate affects the extent of the lithium fluoride reaction, thus impacting the activation effect. This application utilizes multiple discharges at a rate of 0.01C to 0.1C, enabling almost complete reaction of the fluorinated carbon into lithium fluoride and carbon. This ensures the subsequent normal electrochemical reactions, improves the rate performance and cycle stability of the battery cells, and minimizes the activation time.

[0083] Optionally, the discharge step includes discharging at 0.01C to 0.1C to 2.0V to 2.8V.

[0084] As an example, the discharge step can discharge to 2.0V, 2.1V, 2.2V, 2.3V, 2.4V, 2.5V, 2.6V, 2.7V, or 2.8V.

[0085] Discharging at the above discharge voltage can maintain the stability of lithium iron phosphate materials.

[0086] Optionally, the discharge step includes discharging to 2.5V at 0.01C to 0.1C.

[0087] Optionally, the activation process includes a charging step, which includes first charging at a constant current of 0.1C to 1C to 3.5V to 3.8V, and then charging at a constant voltage to a current of 0.05C.

[0088] As an example, the charging rate of the charging step can be 0.1C, 0.2C, 0.3C, 0.4C, 0.5C, 0.6C, 0.7C, 0.8C, 0.9C, or 1C.

[0089] As an example, the charging process can charge to 3.5V, 3.6V, 3.7V, or 3.8V.

[0090] Charging with the above-mentioned charging current and charging voltage can maintain the stability of lithium iron phosphate materials.

[0091] Optionally, the number of cycles is determined by the following method: the activation treatment includes a charging step, a repeated charging step, and a discharging step, until the discharge capacity change of the battery cell is ≤99.8%, thus obtaining the minimum number of cycles.

[0092] The change in discharge capacity is calculated as the discharge capacity of the battery cell after this activation treatment / the discharge capacity after the previous activation treatment * 100%.

[0093] The minimum number of cycles is the number of cycles required for the previous activation treatment when the discharge capacity change of a single battery cell is ≤99.8%.

[0094] For example, if the discharge capacity of a battery cell after the second activation treatment / the discharge capacity after the first activation treatment * 100% is greater than 99.8%, and the discharge capacity of a battery cell after the third activation treatment / the discharge capacity after the second activation treatment * 100% is ≤ 99.8%, then the minimum number of cycles is 2.

[0095] During the initial discharge, fluorinated carbon does not completely react to form lithium fluoride and carbon. During charging, lithium fluoride and carbon do not react to form fluorinated carbon. Therefore, in the second cycle, the remaining fluorinated carbon continues to be consumed, and lithium fluoride is generated again, improving the conversion efficiency from fluorinated carbon to lithium fluoride and carbon. Once all the fluorinated carbon has been converted, continuing the activation cycle becomes meaningless. Therefore, a suitable number of activation cycles, i.e., the minimum number of cycles, is needed to ensure that the fluorinated carbon reacts completely without creating redundancy in the process.

[0096] Optionally, the discharge rate of the discharge step is 0.01C, and the activation treatment is repeated at least twice.

[0097] When the discharge rate of the discharge step is 0.01C, after the activation treatment cycle reaches 2 times, the change in discharge capacity of the battery cell is ≤99.8%.

[0098] Optionally, the discharge rate of the discharge step is 0.02C, and the activation treatment is repeated at least 5 times.

[0099] When the discharge rate of the discharge step is 0.02C, after 5 cycles of activation treatment, the change in discharge capacity of the battery cell is ≤99.8%.

[0100] Optionally, the discharge rate of the discharge step is 0.04C, and the activation treatment is repeated at least 7 times.

[0101] When the discharge rate of the discharge step is 0.04C, after 7 cycles of activation treatment, the change in discharge capacity of the battery cell is ≤99.8%.

[0102] Optionally, the discharge rate of the discharge step is 0.06C, and the activation treatment is repeated at least 9 times.

[0103] When the discharge rate of the discharge step is 0.06C, after 9 cycles of activation treatment, the change in discharge capacity of the battery cell is ≤99.8%.

[0104] Optionally, the discharge rate of the discharge step is 0.08C, and the number of activation treatment cycles is at least 11.

[0105] When the discharge rate of the discharge step is 0.08C, after 11 cycles of activation treatment, the change in discharge capacity of the battery cell is ≤99.8%.

[0106] Optionally, the discharge rate of the discharge step is 0.1C, and the number of activation treatment cycles is at least 13.

[0107] When the discharge rate of the discharge step is 0.01C, after 13 cycles of activation treatment, the change in discharge capacity of the battery cell is ≤99.8%.

[0108] The following describes in further detail, with reference to embodiments, a composite material and its preparation method, a positive electrode material, a positive electrode sheet, an electrode assembly, a battery cell and its preparation method.

[0109] Example 1

[0110] This application provides a composite material and its preparation method, a positive electrode material, a positive electrode sheet, an electrode assembly, a battery cell and its preparation method.

[0111] 1. Composite materials and their preparation methods

[0112] Take 3 parts by weight of fluorinated carbon and 100 parts by weight of lithium iron phosphate and place them in a planetary ball mill. Ball mill at 350 r / min for 7 h to obtain a mixed powder. Place the obtained mixed powder in a vacuum drying oven and dry it at 70℃ for 11 h. Then heat the dried mixed powder to 600℃ in a nitrogen atmosphere and keep it at 600℃ for 5 h to obtain a composite material.

[0113] 2. Positive electrode sheet and its preparation method

[0114] The prepared composite material, carbon black, polyvinylidene fluoride and N-methylpyrrolidone were mixed evenly in a mass ratio of 90:5:5:100 to prepare a positive electrode slurry. The positive electrode slurry was then evenly coated on the surface of aluminum foil, and then dried, cold-pressed and slit to obtain the positive electrode sheet.

[0115] 3. Battery cells and their preparation methods

[0116] Preparation of negative electrode sheet: Artificial graphite, carbon black, styrene-butadiene rubber, and sodium hydroxymethyl cellulose are dissolved in deionized water at a weight ratio of 96.2:0.8:0.8:1.2 and stirred evenly to obtain a negative electrode slurry. The negative electrode slurry is then evenly coated on the surface of copper foil, and then dried, cold-pressed, and slit to obtain the negative electrode sheet.

[0117] Electrolyte preparation: In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), ethylene carbonate / ethyl methyl carbonate were mixed evenly at a volume ratio of 3:7 to obtain a first mixture. LiPF6 with a concentration of 12.5wt% was then added to the first mixture and stirred evenly to obtain the electrolyte.

[0118] The release liner is a polypropylene film.

[0119] The prepared positive electrode sheet, negative electrode sheet, and separator are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, the electrode assembly is wound to obtain an electrode assembly. The electrode assembly is then welded with tabs and installed in an aluminum shell. It is then baked at 80°C to remove water. Electrolyte is then injected and the shell is sealed to obtain a non-charged battery cell. The non-charged battery cell is then subjected to a series of processes, including standing, hot and cold pressing, formation, shaping, and capacity testing, to obtain the battery cell.

[0120] 4. Activation of individual battery cells

[0121] At 25℃, the battery cell is charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C, left to stand for 5 minutes, and then discharged at 0.01C to 2.5V. This cycle is repeated 3 times to obtain a minimum number of cycles of 2.

[0122] The parameters of the battery cells and their preparation methods in Examples 1-24 and Comparative Examples 1-2 are shown in Table 1.

[0123] Table 1. Battery cells and their preparation methods in Examples 1-24 and Comparative Examples 1-2.

[0124]

[0125]

[0126] Experimental Example 1

[0127] The 0.1C discharge capacity, 0.5C discharge capacity, 1C discharge capacity, 0.1C cycle capacity after 20 cycles and capacity retention of the battery cells prepared in Examples 1-24 and Comparative Examples 1-2 were tested respectively, as shown in Table 2.

[0128] The method for testing 0.1C discharge capacity is as follows: charge to 3.65V at 0.1C and then charge at a constant voltage to 0.05C; discharge to 2.5V at 0.1C (take the capacity after discharge as the capacity evaluation standard).

[0129] The method for testing 0.5C discharge capacity is as follows: charge to 3.65V at 0.5C and then charge at a constant voltage to 0.05C; discharge to 2.5V at 0.5C (take the capacity after discharge as the capacity evaluation standard).

[0130] The method for testing 1C discharge capacity is as follows: charge to 3.65V with 1C and then charge at a constant voltage to 0.05C; discharge to 2.5V with 1C (take the capacity after discharge as the capacity evaluation standard).

[0131] The method for testing the capacity after 20 cycles at 0.1C is as follows: charge to 3.65V at 0.1C, then charge at a constant voltage to 0.05C; discharge to 2.5V at 0.1C, and repeat the above steps 20 times (the capacity after discharge is taken as the capacity evaluation standard).

[0132] The method for detecting capacity retention is to divide the discharge capacity of the 20th cycle at 0.1C by the discharge capacity of the first cycle.

[0133] Table 2 shows the discharge capacity and capacity retention of the battery cells in Examples 1-24 and Comparative Examples 1-2.

[0134]

[0135] A comparison of Example 1 and Comparative Example 1 shows that when the lithium iron phosphate particles have no coating layer, the 0.1C discharge capacity of the battery cell is 2.787 Ah, the 0.5C discharge capacity is 2.689 Ah, the 1C discharge capacity is 2.113 Ah, the 0.1C cycle capacity after 20 cycles is 2.758 Ah, and the capacity retention rate is 98.96%. That is, the 0.1C discharge capacity, 0.5C discharge capacity, 1C discharge capacity, 0.1C cycle capacity after 20 cycles, and capacity retention rate of the battery cell in Comparative Example 1 are lower than those of the battery cell in Example 1.

[0136] As shown in Examples 1 and 2-4, when the atomic ratio of fluorine atoms to carbon atoms in the fluorinated carbon is 0.1-1.2:1, the 0.1C discharge capacity of the battery cell is 2.569 Ah-2.895 Ah, the 0.5C discharge capacity is 2.547 Ah-2.869 Ah, the 1C discharge capacity is 2.520 Ah-2.816 Ah, the 0.1C cycle capacity after 20 cycles is 2.551 Ah-2.885 Ah, and the capacity retention rate is 99.29%-99.94%. Furthermore, when the atomic ratio of fluorine atoms to carbon atoms in the fluorinated carbon is 0.3-0.4:1, the 0.1C discharge capacity, 0.5C discharge capacity, 1C discharge capacity, 0.1C cycle capacity after 20 cycles, and capacity retention rate of the battery cell all remain at a high level.

[0137] As shown in Examples 1 and 5-7, when the mass ratio of fluorinated carbon to lithium iron phosphate is 0.1-20:100, the 0.1C discharge capacity of the battery cell is 1.860 Ah-2.887 Ah, the 0.5C discharge capacity is 1.853 Ah-2.860 Ah, the 1C discharge capacity is 1.840 Ah-2.816 Ah, the 0.1C cycle capacity after 20 cycles is 1.859 Ah-2.885 Ah, and the capacity retention rate is 99.87%-99.94%. Furthermore, when the mass ratio of fluorinated carbon to lithium iron phosphate is 0.1-0.2:1, the 0.1C discharge capacity, 0.5C discharge capacity, 1C discharge capacity, 0.1C cycle capacity after 20 cycles, and capacity retention rate of the battery cell all remain at a high level.

[0138] As shown in Examples 1 and 8-9, when the constant current charging current of the activation treatment is 0.1C to 1C, the 0.1C discharge capacity of the battery cell is 2.869Ah to 2.904Ah, the 0.5C discharge capacity is 2.835Ah to 2.875Ah, the 1C discharge capacity is 2.802Ah to 2.831Ah, the capacity after 20 cycles at 0.1C is 2.865Ah to 2.882Ah, and the capacity retention rate is 99.87% to 99.97%.

[0139] As shown in Examples 1 and 10-11, when the constant voltage charging voltage of the activation treatment is 2.5V to 3.8V, the 0.1C discharge capacity of the battery cell is 2.884Ah to 2.907Ah, the 0.5C discharge capacity is 2.858Ah to 2.884Ah, the 1C discharge capacity is 2.816Ah to 2.833Ah, the capacity after 20 cycles at 0.1C is 2.882Ah to 2.896Ah, and the capacity retention rate is 99.62% to 99.94%.

[0140] As shown in Examples 1 and 12, when the discharge current of the activation treatment is 0.01C, the number of cycles is 2 or 3, and the minimum number of cycles is 2. The discharge capacity of the battery cell at 0.1C is 2.884Ah to 2.887Ah, the discharge capacity of the battery cell at 0.5C is 2.860Ah to 2.865Ah, the discharge capacity of the battery cell at 1C is 2.815Ah to 2.816Ah, the capacity of the battery cell after 20 cycles at 0.1C is 2.878Ah to 2.885Ah, and the capacity retention rate is 99.81% to 99.94%.

[0141] As shown in Examples 13 and 14, when the discharge current of the activation treatment is 0.02C, the number of cycles is 5 or 6, and the minimum number of cycles is 5. The 0.1C discharge capacity of the battery cell is 2.884Ah to 2.885Ah, the 0.5C discharge capacity of the battery cell is 2.865Ah to 2.871Ah, the 1C discharge capacity of the battery cell is 2.816Ah to 2.824Ah, the capacity of the battery cell after 20 cycles at 0.1C is 2.878Ah to 2.884Ah, and the capacity retention rate is 99.81% to 99.94%.

[0142] As shown in Examples 15 and 16, when the discharge current of the activation treatment is 0.07C, the number of cycles is 7 or 8, and the minimum number of cycles is 7. The 0.1C discharge capacity of the battery cell is 2.871Ah to 2.880Ah, the 0.5C discharge capacity of the battery cell is 2.851Ah to 2.864Ah, the 1C discharge capacity of the battery cell is 2.796Ah to 2.805Ah, the capacity of the battery cell after 20 cycles at 0.1C is 2.867Ah to 2.879Ah, and the capacity retention rate is 99.87% to 99.97%.

[0143] As shown in Examples 17 and 18, when the discharge current of the activation treatment is 0.06C, the number of cycles is 9 or 10, and the minimum number of cycles is 9. The 0.1C discharge capacity of the battery cell is 2.875Ah to 2.882Ah, the 0.5C discharge capacity of the battery cell is 2.860Ah to 2.865Ah, the 1C discharge capacity of the battery cell is 2.798Ah to 2.809Ah, the capacity of the battery cell after 20 cycles at 0.1C is 2.871Ah to 2.873Ah, and the capacity retention rate is 99.62% to 99.94%.

[0144] As shown in Examples 19 and 20, when the discharge current of the activation treatment is 0.08C, the number of cycles is 11 or 12, and the minimum number of cycles is 11. The discharge capacity of the battery cell at 0.1C is 2.878Ah, the discharge capacity of the battery cell at 0.5C is 2.858Ah to 2.860Ah, the discharge capacity of the battery cell at 1C is 2.820Ah, the capacity of the battery cell after 20 cycles at 0.1C is 2.871Ah to 2.876Ah, and the capacity retention rate is 99.75% to 99.94%.

[0145] As shown in Examples 21 and 22, when the discharge current of the activation treatment is 0.1C, the number of cycles is 13 or 14, and the minimum number of cycles is 13. The 0.1C discharge capacity of the battery cell is 2.875Ah to 2.876Ah, the 0.5C discharge capacity is 2.856Ah to 2.864Ah, the 1C discharge capacity is 2.805Ah to 2.813Ah, and the capacity after 20 cycles at 0.1C is 2.865Ah to 2.873Ah, with a capacity retention rate of 99.68% to 99.87%.

[0146] As shown in Comparative Example 2, when the discharge current of the activation treatment is 0.12C and the number of cycles is 15, the 0.1C discharge capacity of the battery cell is 2.851Ah, the 0.5C discharge capacity is 2.712Ah, the 1C discharge capacity is 2.473Ah, and the capacity after 20 cycles at 0.1C is 2.848Ah, with a capacity retention rate of 99.89%. This indicates that the 0.5C and 1C discharge capacities of Comparative Example 2 are relatively low.

[0147] A comparison of Examples 1 and 23-24 shows that when the discharge voltage of the activation treatment is 2V-2.8V, the 0.1C discharge capacity of the battery cell is 2.884Ah-2.889Ah, the 0.5C discharge capacity is 2.855Ah-2.862Ah, the 1C discharge capacity is 2.802Ah-2.822Ah, the capacity after 20 cycles at 0.1C is 2.876Ah-2.885Ah, and the capacity retention rate is 99.75%-99.94%.

[0148] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A composite material, characterized in that, The composite material includes: lithium iron phosphate, and fluorinated carbon coating the lithium iron phosphate, wherein the atomic ratio of fluorine atoms to carbon atoms in the fluorinated carbon is 0.3~0.4:

1.

2. The composite material according to claim 1, characterized in that, The mass ratio of the fluorinated carbon to the lithium iron phosphate is 0.1~20:

100.

3. The composite material according to claim 1, characterized in that, The mass ratio of the fluorinated carbon to the lithium iron phosphate is 0.1~0.2:

1.

4. The composite material according to claim 1, characterized in that, The composite material's D v50 The range is 1~50μm.

5. A method for preparing the composite material according to any one of claims 1 to 4, characterized in that, The method for preparing the composite material includes mixing fluorinated carbon powder and lithium iron phosphate powder to obtain a mixed powder, and then placing the mixed powder at 400℃~800℃ for 3h~8h.

6. A positive electrode material, characterized in that, The positive electrode material includes the composite material described in any one of claims 1 to 4.

7. A positive electrode sheet, characterized in that, The positive electrode includes a positive current collector and the positive electrode material as described in claim 6, wherein the positive electrode material is disposed on at least one side of the positive current collector along the thickness direction.

8. An electrode assembly, characterized in that, The electrode assembly includes a negative electrode, a separator, and a positive electrode as described in claim 7.

9. A method for preparing a single battery cell, characterized in that, The method for preparing the battery cell includes: activating the semi-finished product; The activation treatment includes a discharge step, wherein the discharge rate of the discharge step is 0.01C to 0.1C, and the activation treatment is repeated 2 to 15 times. The semi-finished product is assembled from an electrolyte and the electrode assembly as described in claim 8.

10. A single battery cell, characterized in that, The battery cell is prepared according to the method for preparing a battery cell according to claim 9.

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