CN115939305B - Positive plate and preparation method thereof, electrode assembly, battery monomer, battery and electric equipment - Google Patents

Abstract

The application relates to a positive plate and a preparation method thereof, an electrode assembly, a battery monomer, a battery and electric equipment, and belongs to the technical field of secondary batteries. The positive electrode active material layer of the positive electrode plate comprises a first positive electrode active material layer and a second positive electrode active material layer which are sequentially arranged outside and cover the surface of a positive electrode current collector, wherein the first positive electrode active material in the first positive electrode active material layer comprises a cobalt-free material with a chemical formula of LiNi _x1Mn_(1‑x1)O₂, and x1 is more than 0 and less than 1; the second positive electrode active material in the second positive electrode active material layer comprises a lithium manganese iron phosphate material with a chemical formula of LiFe _x2Mn_(1‑x2)PO₄, wherein x2 is more than 0 and less than 1; wherein the Dv50 of the cobalt-free material is nano-scale, and the Dv50 of the lithium iron manganese phosphate material is micro-scale. The capacity of the positive plate can be ensured, the capacity retention rate can be improved, the gas production can be reduced, the service life of the battery can be prolonged, and the cost can be reduced.

Description

Positive electrode sheet and preparation method thereof, electrode assembly, battery cell, battery and electrical equipment

Technical Field

The present application relates to the technical field of secondary batteries, and in particular to a positive electrode sheet and a preparation method thereof, an electrode assembly, a battery cell, a battery and an electrical device.

Background Art

In existing lithium-ion batteries, the positive electrode material is usually a ternary material, such as nickel cobalt manganese oxide Li(NiCoMn) _O2 material, which contains cobalt, which plays a key role in the kinetics and stability of the positive electrode material. However, cobalt only partially participates in the electrochemical reaction, and the presence of cobalt will reduce the nickel content and energy density, and the cost of cobalt is also high.

Summary of the invention

In view of the deficiencies in the prior art, the present application provides a positive electrode sheet and a preparation method thereof, an electrode assembly, a battery cell, a battery and an electrical device to reduce the use of cobalt.

In a first aspect, an embodiment of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer comprising a first positive electrode active material layer and a second positive electrode active material layer which are sequentially arranged outward from the surface of the positive electrode current collector and cover the first positive electrode active material layer, the first positive electrode active material in the first positive electrode active material layer comprises a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1- x1) O ₂ , 0＜x1＜1; the second positive electrode active material in the second positive electrode active material layer comprises a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ , 0＜x2＜1; wherein the Dv50 of the cobalt-free material is at the nanometer level, and the Dv50 of the lithium iron manganese phosphate material is at the micrometer level.

In the technical solution of the embodiment of the present application, a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ is used as the positive electrode active material, which can reduce the use of cobalt and reduce the cost of the positive electrode active material; at the same time, the Dv50 of the cobalt-free material is nanometer-level, which improves its fast charging capability compared with conventional cobalt-free materials; at the same time, a second positive electrode active material layer formed by a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ is arranged on the upper layer of the cobalt-free material, and the Dv50 of the lithium iron manganese phosphate material is micrometer-level, which can reduce the probability of direct contact between the cobalt-free material and the electrolyte to a certain extent, reduce gas production, and improve the stability of the positive electrode sheet. While ensuring the capacity retention rate of the positive electrode sheet, it can reduce gas production and reduce costs.

In some embodiments, the Dv50 of the cobalt-free material is 100nm to 300nm, and the Dv50 of the manganese iron phosphate lithium material is 1μm to 3μm. The cobalt-free material of the above particle size has a higher capacity, but poorer stability, low capacity retention rate, and more obvious gas production; its use in combination with the manganese iron phosphate lithium material of the above particle size can well improve the gas production problem, further improve the stability and safety of the positive electrode sheet, and at the same time, make its capacity retention rate higher.

In some embodiments, the mass proportion of the cobalt-free material in the first positive electrode active material and the second positive electrode active material is 20% to 80%. When the amount of the cobalt-free material added is within the above range, the capacity of the positive electrode sheet can be increased while further improving the capacity retention rate, and the gas generation problem can be well improved.

In some embodiments, the first positive electrode active material further includes a low-cobalt material having a chemical formula of LiNi _x3 Co _y3 Mn _1-x3-y3 O ₂ , wherein 0.5≤x3＜1, 0＜y3≤0.05, and the mass proportion of the low-cobalt material in the first positive electrode active material is not higher than 30%. A small amount of low-cobalt material can also be added to the first active material, which can improve the stability of the first positive electrode active material layer and further improve the capacity retention rate of the positive electrode sheet.

In some embodiments, the absolute value of the difference between the voltage platform of the first positive electrode active material and the voltage platform of the second positive electrode active material is ≤ 0.3 V. The smaller the difference between the voltage platforms, the higher the stability and energy density of the positive electrode sheet.

In some embodiments, 0.2≤x2≤0.8. When x2 in the lithium manganese iron phosphate material satisfies the above conditions, the stability and energy density of the positive electrode sheet can be high.

In some embodiments, the second positive electrode active material includes a lithium manganese iron phosphate material with a chemical formula of LiFe _0.5 Mn _0.5 PO ₄ and a chemical formula of LiFe _0.3 Mn _0.7 PO ₄ , and the mass proportion of the LiFe _0.3 Mn _0.7 PO ₄ material in the second positive electrode active material is not higher than 30%. When the Dv50 of the cobalt-free material is within the range of 100nm to 300nm, the lithium manganese iron phosphate material is combined with the materials of the above two chemical formulas, and its content meets the above range, which can make the stability of the electrode assembly containing the positive electrode sheet higher and improve its energy density to a certain extent.

In some embodiments, at least a portion of the surface of the first positive electrode active material is coated with a first structural conductive polymer layer, and the mass of the first structural conductive polymer layer accounts for 0.5% to 3% of the total mass of the first positive electrode active material and the first structural conductive polymer layer. The first structural conductive polymer is coated on the surface of the first positive electrode active material, which can improve the ionic conductivity and/or electronic conductivity of the first positive electrode active material, and enhance the ionic conductivity and/or electronic conductivity of the first positive electrode active material layer.

In some embodiments, at least part of the surface of the second positive electrode active material is coated with a second structural conductive polymer layer, and the mass of the second structural conductive polymer layer accounts for 0.5% to 3% of the total mass of the second positive electrode active material and the second structural conductive polymer layer. The second structural conductive polymer is coated on the surface of the second positive electrode active material, which can improve the ionic conductivity and/or electronic conductivity of the second positive electrode active material, and enhance the ionic conductivity and/or electronic conductivity of the second positive electrode active material layer.

In some embodiments, the material of the first structural conductive polymer layer and/or the material of the second structural conductive polymer layer includes an ionic polymer and an electronic polymer, which can enhance both the ionic and electronic conductive properties of the positive electrode active material and improve its rate performance.

In some embodiments, the ionic polymer includes at least one of polyethylene glycol, polyethylene oxide, polyethylene glycol succinate, and polyethylene glycol imine.

In some embodiments, the electronic polymer includes at least one of polyaniline, polythiophene, polypyrrole, and polyquinoline.

In some embodiments, the material of the first structural conductive polymer layer and/or the material of the second structural conductive polymer layer is a mixture of polyaniline and polyethylene glycol, and the mass proportion of polyaniline in the mixture is 30% to 70%. The ion conduction and electronic conduction properties of the positive electrode active material can be well enhanced to improve its rate performance.

In some embodiments, the weight average molecular weight of polyaniline is 20,000 to 80,000, and the structural formula of polyaniline is:

Wherein, y = 0.5. Polyaniline is an intermediate oxidation state in which the number of oxidation units and the number of reduction units are equal, and the effect of electron conduction is better.

In some embodiments, the weight average molecular weight of polyethylene glycol is 800-4000.

In a second aspect, the present application provides an electrode assembly, comprising a negative electrode sheet, a separator and any positive electrode sheet provided in the first aspect, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet.

In the technical solution of the embodiment of the present application, the electrode assembly formed using the above-mentioned positive electrode sheet has a higher capacity retention rate, lower gas production and lower cost.

In a third aspect, the present application provides a battery cell, comprising the electrode assembly provided in the second aspect.

In a fourth aspect, the present application provides a battery, comprising the battery cell provided in the third aspect.

In a fifth aspect, the present application provides an electrical device, comprising the battery provided in the fourth aspect.

In a sixth aspect, the present application provides a method for preparing a positive electrode sheet, comprising the following steps: coating a first positive electrode active slurry on the surface of a positive electrode current collector to form a first positive electrode active material layer, wherein the first positive electrode active material in the first positive electrode active slurry comprises a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ , and 0＜x1＜1. Coating a second positive electrode active slurry on the surface of the first positive electrode active material layer to form a second positive electrode active material layer, wherein the second positive electrode active material in the second positive electrode active slurry comprises a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ , and 0＜x2＜1. The Dv50 of the cobalt-free material is nanometer-level, and the Dv50 of the lithium iron manganese phosphate material is micrometer-level.

In the technical solution of the embodiment of the present application, a first positive electrode active material layer containing cobalt-free material near the current collector and a second positive electrode active material layer containing lithium manganese iron phosphate material are formed in the positive electrode active material by double-layer coating, and the second positive electrode active material layer covers the first positive electrode active material layer. While ensuring the capacity retention rate of the positive electrode sheet, gas generation can be reduced and costs can be reduced.

In some embodiments, the ratio of the coating weight of the first positive electrode active material to the coating weight of the second positive electrode active material is (2-8):(8-2). The coating weights of the first positive electrode active material and the second positive electrode active material satisfy the above ratio, which can further improve the comprehensive performance of the positive electrode sheet in terms of improving capacity retention, reducing gas production and reducing costs.

In some embodiments, before coating the first positive electrode active slurry, the step of coating at least part of the surface of the first positive electrode active material with a first structural conductive polymer layer is also included. Coating the surface of the first positive electrode active material with the first structural conductive polymer layer can enhance the ionic conductivity and/or electronic conductivity of the first positive electrode active material and the ionic conductivity and/or electronic conductivity of the first positive electrode active material layer.

In some embodiments, before coating the second positive electrode active slurry, the step of coating at least part of the surface of the second positive electrode active material with a second structural conductive polymer layer is also included. Coating the surface of the second positive electrode active material with the second structural conductive polymer layer can enhance the ionic conductivity and/or electronic conductivity of the second positive electrode active material and the ionic conductivity and/or electronic conductivity of the second positive electrode active material layer.

In some embodiments, the method of coating the first structural conductive polymer layer includes: uniformly mixing the first positive electrode active material, the first structural conductive polymer and the solvent, and then drying. The coating is relatively simple and easy to implement by the mixed coating method.

In some embodiments, the method of coating the second structural conductive polymer layer includes: uniformly mixing the second positive electrode active material, the second structural conductive polymer and the solvent, and then drying. The coating is relatively simple and easy to implement by the mixed coating method.

In some embodiments, the method of coating the first structural conductive polymer layer includes: uniformly mixing the first positive electrode active material, the first structural conductive polymer monomer, the initiator and the solvent, cross-linking the first structural conductive polymer monomer on at least part of the surface of the first positive electrode active material to form the first structural conductive polymer, and then drying. The structural conductive polymer layer is formed by coating and polymerizing at the same time, and its layer structure is relatively dense, and the performance of ionic conductivity and/or electronic conductivity is better.

In some embodiments, the method of coating the second structural conductive polymer layer includes: uniformly mixing the second positive electrode active material, the second structural conductive polymer monomer, the initiator and the solvent, cross-linking the second structural conductive polymer monomer on at least part of the surface of the second positive electrode active material to form the second structural conductive polymer, and then drying. The structural conductive polymer layer is formed by coating and polymerizing at the same time, and its layer structure is relatively dense, and the performance of ionic conductivity and/or electronic conductivity is better.

The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

FIG1 is a schematic diagram of the structure of a vehicle provided in some embodiments of the present application;

FIG2 is a schematic diagram of an exploded structure of a battery provided in some embodiments of the present application;

FIG3 is a schematic diagram of the structure of a battery cell provided in some embodiments of the present application;

FIG4 is an exploded view of a battery cell provided in some embodiments of the present application;

FIG5 is a schematic diagram of the first layer structure of a positive electrode sheet provided in some embodiments of the present application;

FIG6 is a schematic diagram of the second layer structure of the positive electrode sheet provided in some embodiments of the present application;

FIG. 7 is a flow chart of a process for preparing a positive electrode sheet provided in some embodiments of the present application.

Icons: 1000-vehicle; 100-battery; 10-casing; 11-accommodating space; 12-first part; 13-second part; 20-battery cell; 21-casing; 211-opening; 22-end cover assembly; 221-end cover; 222-electrode terminal; 23-electrode assembly; 231-positive electrode sheet; 2311-positive current collector; 2312-positive active material layer; 2312a-first positive active material layer; 2312b-second positive active material layer; 24-current collecting member; 25-insulating protection member; 200-controller; 300-motor.

DETAILED DESCRIPTION

The following embodiments of the technical solution of the present application will be described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

In the description of the embodiments of the present application, the term "and/or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and/or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character "/" in this article generally indicates that the associated objects before and after are in an "or" relationship.

In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the embodiments of the present application.

In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

At present, from the perspective of market development, the application of power batteries is becoming more and more extensive. Power batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, electric cars, as well as military equipment and aerospace and other fields. With the continuous expansion of the application field of power batteries, the market demand is also constantly expanding.

The power battery can be a lithium-ion battery. During the charging process of the lithium-ion battery, lithium ions are released from the positive electrode active material, transmitted through the electrolyte, passed through the isolation membrane, and embedded in the negative electrode active material. At present, the positive electrode active material is usually a ternary material, such as nickel cobalt manganese oxide Li (NiCoMn) O ₂ material, in which Ni is mainly used to increase the energy density of the battery, Mn is mainly used to improve the stability of the battery, and Co is mainly used to improve the kinetics and stability of the battery. However, cobalt only partially participates in the electrochemical reaction, and the presence of cobalt will reduce the nickel content and reduce the energy density, and the cost of cobalt is also relatively high.

Therefore, the present application intends to use a positive electrode active material (a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ ) to replace the existing nickel cobalt manganese oxide Li(NiCoMn)O ₂ material. The inventors have found that the kinetics of the cobalt-free material (Li(NiCoMn)O ₂ ) is poor. If it is prepared into small particles, the kinetics of the cobalt-free material can be improved. However, when the small particles of the cobalt-free material are in direct contact with the electrolyte, the gas production will deteriorate.

Based on the above considerations, in order to use a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ as a positive electrode material and improve the problem of poor cycle stability, the inventors have designed a positive electrode sheet after in-depth research, including a positive electrode collector and a positive electrode active material layer, the positive electrode active material layer including a first positive electrode active material layer 2312a and a second positive electrode active material layer which are sequentially arranged outwardly and cover the surface of the positive electrode collector, the first positive electrode active material in the first positive electrode active material layer includes a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ , 0＜x1＜1; the second positive electrode active material in the second positive electrode active material layer includes a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ , 0＜x2＜1; wherein the Dv50 of the cobalt-free material is nanometer-level, and the Dv50 of the lithium iron manganese phosphate material is micrometer-level.

In such a positive electrode sheet, a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ is used as the positive electrode active material, which can reduce the use of cobalt and reduce the cost of the positive electrode active material; at the same time, the Dv50 of the cobalt-free material is nanometer-level, which improves its fast charging capability compared with conventional cobalt-free materials; at the same time, a second positive electrode active material layer formed by a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ is arranged on the upper layer of the cobalt-free material, and the Dv50 of the lithium iron manganese phosphate material is micrometer-level, which can reduce the probability of direct contact between the cobalt-free material and the electrolyte to a certain extent, reduce gas production, and improve the stability of the positive electrode sheet. It can reduce gas production while ensuring the capacity retention rate of the positive electrode sheet and can reduce costs.

The positive electrode sheet can be used to prepare an electrode assembly, which can be used in, but not limited to, electrical equipment such as vehicles, ships or aircraft. The power supply system of the electrical equipment can be composed of the battery monomers and batteries disclosed in this application, which is conducive to increasing capacity, improving gas production problems, and reducing costs.

The embodiment of the present application provides an electric device that uses a battery as a power source. The electric device can be a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, an electric tool, and the like. The vehicle can be a fuel vehicle, a gas vehicle, or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle, and the like; the spacecraft includes an airplane, a rocket, a space shuttle, and a spacecraft, and the like; the electric toy includes a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy, and the like; the electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and an electric tool for railways, such as an electric drill, an electric grinder, an electric wrench, an electric screwdriver, an electric hammer, an impact drill, a concrete vibrator, and an electric planer, and the like. The embodiment of the present application does not impose any special restrictions on the above-mentioned electric devices.

For the convenience of description, the following embodiments are described by taking the electric device as a vehicle as an example.

Please refer to FIG. 1 , which is a schematic diagram of the structure of a vehicle 1000 provided in some embodiments of the present application. A battery 100 is disposed inside the vehicle 1000, and the battery 100 can be disposed at the bottom, head, or tail of the vehicle 1000. The battery 100 can be used to power the vehicle 1000, for example, the battery 100 can be used as an operating power source for the vehicle 1000.

The vehicle 1000 may further include a controller 200 and a motor 300 , wherein the controller 200 is used to control the battery 100 to supply power to the motor 300 , for example, to meet the power requirements of starting, navigating, and driving the vehicle 1000 .

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

Fig. 2 is a schematic diagram of the exploded structure of a battery 100 provided in some embodiments of the present application. Referring to Fig. 2 , the battery 100 includes a box body 10 and a battery cell 20 , wherein the battery cell 20 is accommodated in the box body 10 .

The box body 10 is used to provide a storage space 11 for the battery cell 20. In some embodiments, the box body 10 may include a first portion 12 and a second portion 13, and the first portion 12 and the second portion 13 cover each other to define the storage space 11 for accommodating the battery cell 20. Of course, the connection between the first portion 12 and the second portion 13 can be sealed by a sealing member (not shown in the figure), and the sealing member can be a sealing ring, a sealant, etc.

The first part 12 and the second part 13 can be in various shapes, such as a cuboid, a cylinder, etc. The first part 12 can be a hollow structure with one side open to form a receiving cavity for accommodating the battery cell 20, and the second part 13 can also be a hollow structure with one side open to form a receiving cavity for accommodating the battery cell 20, and the open side of the second part 13 covers the open side of the first part 12, thereby forming a box body 10 with a receiving space 11. Of course, as shown in FIG. 2, the first part 12 can also be a hollow structure with one side open, and the second part 13 can be a plate-like structure, and the second part 13 covers the open side of the first part 12, thereby forming a box body 10 with a receiving space 11.

In the battery 100, there can be one or more battery cells 20. If there are more than one battery cell 20, the multiple battery cells 20 can be connected in series, in parallel, or in mixed connection. Mixed connection means that the multiple battery cells 20 are both connected in series and in parallel. Multiple battery cells 20 can be directly connected in series, in parallel, or in mixed connection, and then the whole formed by the multiple battery cells 20 is accommodated in the box 10; of course, multiple battery cells 20 can also be connected in series, in parallel, or in mixed connection to form a battery module, and then multiple battery modules are connected in series, in parallel, or in mixed connection to form a whole, and accommodated in the box 10. The battery cell 20 can be cylindrical, flat, rectangular, or in other shapes. Figure 2 exemplarily shows the case where the battery cell 20 is square.

In some embodiments, the battery 100 may further include a busbar component (not shown), and the multiple battery cells 20 may be electrically connected via the busbar component to achieve series connection, parallel connection, or mixed connection of the multiple battery cells 20 .

FIG3 is a schematic diagram of the structure of a battery cell 20 provided in some embodiments of the present application, and FIG4 is an exploded view of a battery cell 20 provided in some embodiments of the present application. Referring to FIG3 and FIG4, the battery cell 20 may include a housing 21, an end cap assembly 22, and an electrode assembly 23. The housing 21 has an opening 211, the electrode assembly 23 is accommodated in the housing 21, and the end cap assembly 22 is used to cover the opening 211.

The shape of the housing 21 can be determined according to the specific shape of the electrode assembly 23. For example, if the electrode assembly 23 is a rectangular parallelepiped structure, the housing 21 can be a rectangular parallelepiped structure. FIG. 3 and FIG. 4 exemplarily show the case where the housing 21 and the electrode assembly 23 are square.

The shell 21 may be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, etc., and the embodiment of the present application does not impose any special restrictions on this.

The end cap assembly 22 includes an end cap 221 and an electrode terminal 222. The end cap assembly 22 is used to seal the opening 211 of the housing 21 to form a closed installation space (not shown), and the installation space is used to accommodate the electrode assembly 23. The installation space is also used to accommodate an electrolyte, such as an electrolyte. The end cap assembly 22 is a component for outputting the electrical energy of the electrode assembly 23. The electrode terminal 222 in the end cap assembly 22 is used to be electrically connected to the electrode assembly 23, that is, the electrode terminal 222 is electrically connected to the tab of the electrode assembly 23. For example, the electrode terminal 222 is connected to the tab through the current collecting member 24 to achieve electrical connection between the electrode terminal 222 and the tab.

It should be noted that the opening 211 of the shell 21 can be one or two. If the opening 211 of the shell 21 is one, the end cap assembly 22 can also be one, and two electrode terminals 222 can be provided in the end cap assembly 22, and the two electrode terminals 222 are respectively used to electrically connect to the positive pole ear and the negative pole ear of the electrode assembly 23. If the opening 211 of the shell 21 is two, for example, the two openings 211 are provided on opposite sides of the shell 21, and the end cap assembly 22 can also be two, and the two end cap assemblies 22 are respectively covered at the two openings 211 of the shell 21. In this case, the electrode terminal 222 in one end cap assembly 22 can be a positive electrode terminal, which is used to electrically connect to the positive pole ear of the electrode assembly 23; the electrode terminal 222 in the other end cap assembly 22 can be a negative electrode terminal, which is used to electrically connect to the negative electrode sheet of the electrode assembly 23.

In some embodiments, as shown in FIG4 , the battery cell 20 may further include an insulating protective member 25 fixed to the periphery of the electrode assembly 23, and the insulating protective member 25 is used to insulate and isolate the electrode assembly 23 from the housing 21. Exemplarily, the insulating protective member 25 is a tape bonded to the periphery of the electrode assembly 23. In some embodiments, there are multiple electrode assemblies 23, and the insulating protective member 25 is disposed around the periphery of the multiple electrode assemblies 23, and the multiple electrode assemblies 23 are formed into an integral structure to maintain the structural stability of the electrode assembly 23.

The electrode assembly 23 includes a positive electrode sheet, a negative electrode sheet and a separator. The positive electrode sheet includes a positive electrode collector and a positive electrode active material layer, the positive electrode active material layer is coated on the surface of the positive electrode collector, the positive electrode collector not coated with the positive electrode active material layer protrudes from the positive electrode collector coated with the positive electrode active material layer, and the positive electrode collector not coated with the positive electrode active material layer serves as a positive electrode tab.

The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer is coated on the surface of the negative electrode current collector, the negative electrode current collector not coated with the negative electrode active material layer protrudes from the negative electrode current collector coated with the negative electrode active material layer, and the negative electrode current collector not coated with the negative electrode active material layer serves as a negative electrode tab. The material of the negative electrode current collector may be copper, and the negative electrode active material may be carbon or silicon, etc. In order to ensure that a large current passes without melting, the number of positive electrode tabs is multiple and stacked together, and the number of negative electrode tabs is multiple and stacked together. The material of the isolation film may be PP (polypropylene) or PE (polyethylene), etc. In addition, the electrode assembly 23 may be a wound electrode assembly or a laminated electrode assembly, and the embodiments of the present application are not limited thereto.

Figure 5 is a schematic diagram of the first layer structure of the positive electrode sheet 231 provided in some embodiments of the present application, and Figure 6 is a schematic diagram of the second layer structure of the positive electrode sheet 231 provided in some embodiments of the present application; please refer to Figures 5 and 6, the positive electrode sheet 231 includes a positive electrode collector 2311 and a positive electrode active material layer 2312, the positive electrode active material layer 2312 includes a first positive electrode active material layer 2312a and a second positive electrode active material layer 2312b which are sequentially arranged outwardly and cover the surface of the positive electrode collector 2311, the first positive electrode active material in the first positive electrode active material layer 2312a includes a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ , 0＜x1＜1; the second positive electrode active material in the second positive electrode active material layer 2312b includes a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ , 0＜x2＜1; wherein, the Dv50 of the cobalt-free material is nanometer level, and the Dv50 of the lithium iron manganese phosphate material is micrometer level.

The material of the positive electrode current collector 2311 may be one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. Please continue to refer to FIG5. In one embodiment, a first positive electrode active material layer 2312a and a second positive electrode active material layer 2312b are sequentially disposed on one surface of the positive electrode current collector 2311. Please continue to refer to FIG6. In another embodiment, a first positive electrode active material layer 2312a and a second positive electrode active material layer 2312b are sequentially disposed on both surfaces of the positive electrode current collector 2311.

The first positive electrode active material layer 2312a refers to a layer in contact with the positive electrode current collector 2311; the second positive electrode active material layer 2312b refers to a layer in contact with the surface of the first positive electrode active material layer 2312a that is away from the positive electrode current collector 2311. The "active material" in the first positive electrode active material and the second positive electrode active material refers to a substance that can release or absorb lithium ions.

Nano-scale refers to particles with a particle size between 1nm and 500nm; micro-scale refers to particles with a particle size between 0.5μm and 10μm. The Dv50 of cobalt-free materials is nano-scale, which means that when the particles of cobalt-free materials are added up from small to large, and the cumulative amount accounts for 50% of the total volume, the particle size of the cobalt-free material particles is the value of Dv50, and the value of Dv50 is between 1nm and 500nm. The Dv50 of lithium iron manganese phosphate materials is micro-scale, which means that when the particles of lithium iron manganese phosphate are added up from small to large, and the cumulative amount accounts for 50% of the total volume, the particle size of the lithium iron manganese phosphate particles is the value of Dv50, and the value of Dv50 is between 0.5μm and 10μm.

In the technical solution of the embodiment of the present application, a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ is used as the positive electrode active material, which can reduce the use of cobalt and reduce the cost of the positive electrode active material; at the same time, the Dv50 of the cobalt-free material is nanometer-level, which improves its fast charging capability compared with conventional cobalt-free materials; at the same time, a second positive electrode active material layer formed by a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ is arranged on the upper layer of the cobalt-free material, and the Dv50 of the lithium iron manganese phosphate material is micrometer-level, which can reduce the probability of direct contact between the cobalt-free material and the electrolyte to a certain extent, reduce gas production, and improve the stability of the positive electrode sheet 231. While ensuring the capacity retention rate of the positive electrode sheet 231, it can reduce gas production and reduce costs.

Optionally, the cobalt-free material includes one or more of LiNi _0.1 Mn _0.9 O ₂ , LiNi _0.2 Mn _0.8 O ₂ , LiNi _0.3 Mn _0.7 O ₂ , LiNi _0.4 Mn _0.6 O ₂ , LiNi _0.5 Mn _0.5 O ₂ , LiNi _0.6 Mn _0.4 O ₂ , LiNi _0.7 Mn _0.3 O ₂ , _{LiNi 0.8} Mn _0.2 O ₂ , and LiNi _0.9 Mn _0.1 O 2 , and x1 in LiNi _x1 Mn _(1-x1) O ₂ may be any one or more values in the range of 0 to 1 (except 0 and 1).

Optionally, the lithium manganese iron phosphate material includes one or more of LiFe _0.1 Mn _0.9 PO ₄ , LiFe _0.2 Mn _0.8 PO ₄ , LiFe _0.3 Mn _0.7 PO ₄ , LiFe _0.4 Mn _0.6 PO ₄ , LiFe _0.5 Mn _0.5 PO ₄ , LiFe _0.6 Mn _0.4 PO ₄ , LiFe _0.7 Mn _0.3 PO ₄ , LiFe _0.8 Mn _0.2 PO ₄ , and LiFe _0.9 Mn _0.1 PO ₄ , and x2 in LiFe _x2 Mn _(1-x2) PO ₄ can be any one or more values in the range of 0 to 1 (except 0 and 1).

In some embodiments, the Dv50 of the cobalt-free material is 100nm to 300nm, and the Dv50 of the manganese iron phosphate lithium material is 1μm to 3μm. The cobalt-free material of the above particle size has a higher capacity, but poorer stability, lower capacity retention rate, and more obvious gas production; it can be used in conjunction with the manganese iron phosphate lithium material of the above particle size to improve the gas production problem, so that the positive electrode sheet 231 has a higher capacity and further improved stability and safety.

As an example, the Dv50 of the cobalt-free material is 100nm, 120nm, 140nm, 160nm, 180nm, 200nm, 220nm, 240nm, 260nm, 280nm or 300nm, which can also be any value in the range of 100nm to 300nm. The Dv50 of the lithium manganese iron phosphate material is 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm or 3μm, which can also be any value in the range of 1μm to 3μm.

In some embodiments, the mass proportion of the cobalt-free material in the first positive electrode active material and the second positive electrode active material is 20% to 80%. Among them, the mass of the cobalt-free material/(the mass of the first positive electrode active material+the mass of the second positive electrode active material)×100%=20% to 80%. The first positive electrode active material includes a cobalt-free material and other optional active materials; the second positive electrode active material includes a lithium iron manganese phosphate material and other optional active materials. The addition amount of the cobalt-free material is within the above range, which can make the capacity of the positive electrode sheet 231 higher while further improving the capacity retention rate, and the gas production problem is well improved.

As an example, the mass proportion of the cobalt-free material in the first positive electrode active material and the second positive electrode active material is 20%, 30%, 40%, 50%, 60%, 70% or 80%, which can also be any value within the range of 20% to 80%.

In the present application, the first positive electrode active material in the first positive electrode active material layer 2312a is not limited to a cobalt-free material, and some low-cobalt materials or other positive electrode active materials may be added, which is not limited in the present application.

In some embodiments, the first positive electrode active material further includes a low-cobalt material having a chemical formula of LiNi _x3 Co _y3 Mn _1-x3-y3 O ₂ , wherein 0.5≤x3＜1, 0＜y3≤0.05, and the mass proportion of the low-cobalt material in the first positive electrode active material is not higher than 30%. A small amount of low-cobalt material may also be added to the first active material, which may improve the stability of the first positive electrode active material layer and further improve the capacity retention rate of the positive electrode sheet 231.

Optionally, the low cobalt material includes LiNi _0.5 Co _0.01 Mn _0.49 O ₂ , LiNi _0.6 Co _0.02 Mn _0.38 O ₂ , LiNi _0.7 Co _0.03 Mn _0.27 O ₂ , LiNi _0.8 Co _0.04 Mn _0.16 O ₂ , LiNi _0.9 Co _0.05 Mn _0.05 O ₂ , LiNi _0.5 Co _0.05 Mn _0.45 O ₂ , LiNi _0.6 Co _0.04 Mn _0.36 O ₂ , LiNi _0.7 Co _0.02 Mn _0.28 O ₂ , LiNi _0.8 Co _0.02 Mn _0.18 O ₂ , LiNi _0.9 Co _0.01 Mn 0.09 O 2 , and LiNi 0.5 Co 0.05 Mn _0.45 O 2 . ₂ , x3 in LiNi _x3 Co _y3 Mn _1-x3-y3 O ₂ can be any one or more values in the range of 0.5 to 1 (except 1), and y3 can be any one or more values in the range of 0 to 0.05 (except 0).

As an example, the mass proportion of the low-cobalt material in the first positive electrode active material is 0.5%, 1%, 2%, 4%, 8%, 12%, 16%, 20%, 24%, 28% or 30%, and it can also be any value in the range of 0 to 30% (when the addition is 0%, the first positive electrode active material does not contain the low-cobalt material).

In some embodiments, the absolute value of the difference between the voltage platform of the first positive electrode active material and the voltage platform of the second positive electrode active material is ≤ 0.3 V. The smaller the difference between the voltage platforms, the higher the stability and energy density of the positive electrode sheet 231 can be.

Optionally, the voltage platform of the first positive electrode active material minus the voltage platform of the second positive electrode active material=-0.3, -0.2, -0.1, 0, 0.1, 0.2 or 0.3, which can also be any value between -0.3 and 0.3.

In some embodiments, 0.2≤x2≤0.8. When x2 in the lithium manganese iron phosphate material satisfies the above conditions, the stability and energy density of the positive electrode sheet 231 can be high. By way of example, the lithium manganese iron phosphate material includes one or more of LiFe _0.2 Mn _0.8 PO ₄ , LiFe _0.3 Mn _0.7 PO ₄ , LiFe _0.4 Mn _0.6 PO ₄ , LiFe _0.5 Mn _0.5 PO ₄ , LiFe _0.6 Mn _0.4 PO ₄ , LiFe _0.7 Mn _0.3 PO ₄ , and LiFe _0.8 Mn _0.2 PO ₄ , and x2 in LiFe _x2 Mn _(1-x2) PO ₄ can be any one or more values in the range of 0.2 to 0.8.

In some embodiments, the second positive electrode active material includes a lithium manganese iron phosphate material with a chemical formula of LiFe _0.5 Mn _0.5 PO ₄ and a chemical formula of LiFe _0.3 Mn _0.7 PO ₄ , and the mass proportion of the LiFe _0.3 Mn _0.7 PO ₄ material in the second positive electrode active material is not higher than 30%. When the Dv50 of the cobalt-free material is within the range of 100nm to 300nm, the lithium manganese iron phosphate material is combined with the materials of the above two chemical formulas, and its content meets the above range, which can make the stability of the electrode assembly 23 containing the positive electrode sheet 231 higher and improve its energy density to a certain extent.

As an example, the mass proportion of LiFe _0.3 Mn _0.7 PO ₄ material in the second positive electrode active material is 0.5%, 1%, 2%, 4%, 8%, 12%, 16%, 20%, 24%, 28% or 30%, and it can also be any value in the range of 0 to 30% (when the addition is 0%, the second positive electrode active material does not contain LiFe _0.3 Mn _0.7 PO ₄ ).

In other embodiments, the second positive electrode active material may further contain a small amount of lithium iron phosphate, and the mass proportion of lithium iron phosphate in the second positive electrode active material is no more than 10%.

It should be noted that a small amount of other positive electrode active materials may be added to the first positive electrode active material and/or the second positive electrode active material in the present application, for example, other positive electrode active materials accounting for no more than 5% by mass, which is not limited in the present application.

In some embodiments, at least a portion of the surface of the first positive electrode active material is coated with a first structural conductive polymer layer, and the mass of the first structural conductive polymer layer accounts for 0.5% to 3% of the total mass of the first positive electrode active material and the first structural conductive polymer layer. Or/and, at least a portion of the surface of the second positive electrode active material is coated with a second structural conductive polymer layer, and the mass of the second structural conductive polymer layer accounts for 0.5% to 3% of the total mass of the second positive electrode active material and the second structural conductive polymer layer.

The first structural conductive polymer and the second structural conductive polymer refer to substances that can play the role of ion conductivity or electronic conductivity. The structural conductive polymer is coated on the surface of the positive electrode active material to improve the ion conductivity and/or electronic conductivity of the positive electrode active material, thereby enhancing the ion conductivity and/or electronic conductivity of the positive electrode active material layer 2312.

Optionally, in one embodiment, at least a portion of the surface of the first positive electrode active material is coated with a first structural conductive polymer layer; in another embodiment, at least a portion of the surface of the second positive electrode active material is coated with a second structural conductive polymer layer; in a third embodiment, at least a portion of the surface of the first positive electrode active material is coated with a first structural conductive polymer layer, and at least a portion of the surface of the second positive electrode active material is coated with a second structural conductive polymer layer. Wherein, "at least a portion of the surface" refers to a portion of the surface or the entire surface.

The mass of the first structural conductive polymer layer/(the mass sum of the first positive electrode active material and the first structural conductive polymer layer)=(0.5% to 3%). As an example, the mass proportion of the first structural conductive polymer layer to the sum of the first positive electrode active material and the first structural conductive polymer layer is 0.5%, 1%, 1.5%, 2%, 2.5% or 3%, which can also be any value within the range of 0.5% to 3%.

The mass of the second structural conductive polymer layer/(the mass sum of the second positive electrode active material and the second structural conductive polymer layer)=(0.5% to 3%). As an example, the mass proportion of the second structural conductive polymer layer to the sum of the second positive electrode active material and the second structural conductive polymer layer is 0.5%, 1%, 1.5%, 2%, 2.5% or 3%, which can also be any value in the range of 0.5% to 3%.

In some embodiments, the material of the first structural conductive polymer layer and/or the material of the second structural conductive polymer layer include ionic polymers and electronic polymers. The ionic and electronic conductivity properties of the positive electrode active material can be enhanced, thereby improving its rate performance. In other embodiments, the material of the structural conductive polymer layer is an ionic polymer or an electronic polymer.

The material of the first structural type conductive polymer layer and the material of the second structural type conductive polymer layer may be the same or different, which is not limited here.

Optionally, the ionic polymer includes at least one of polyethylene glycol, polyethylene oxide, polyethylene glycol succinate, and polyethylene glycol imine.

Optionally, the electronic polymer includes at least one of polyaniline, polythiophene, polypyrrole and polyquinoline.

In some embodiments, the material of the first structural conductive polymer layer and/or the material of the second structural conductive polymer layer is a mixture of polyaniline and polyethylene glycol, and the mass proportion of polyaniline in the mixture is 30% to 70%. The ion conduction and electronic conduction properties of the positive electrode active material can be well enhanced to improve its rate performance.

As an example, the mass proportion of polyaniline in the mixture of polyaniline and polyethylene glycol is 30%, 40%, 50%, 60% or 70%, which can also be any value within the range of 30% to 70%.

In some embodiments, the weight average molecular weight of polyaniline is 20,000 to 80,000, and the structural formula of polyaniline is:

Wherein, y = 0.5. Polyaniline is an intermediate oxidation state in which the number of oxidation units and the number of reduction units are equal, and the effect of electron conduction is better.

As an example, the weight average molecular weight of polyaniline is 20,000, 30,000, 40,000, 50,000, 60,000, 70,000 or 80,000, and it may be any value within the range of 20,000 to 80,000.

In some embodiments, the weight average molecular weight of polyethylene glycol is 800 to 4000. As an example, the weight average molecular weight of polyethylene glycol is 800, 1000, 1500, 2000, 2500, 3000, 3500 or 4000, and it can also be any value within the range of 800 to 4000.

After the above introduction to the material and structure of the positive electrode sheet 231 , the preparation method of the positive electrode sheet 231 will be specifically introduced below.

The preparation method of the positive electrode sheet 231 includes the following steps: coating a first positive electrode active slurry on the surface of the positive electrode current collector 2311 to form a first positive electrode active material layer 2312a, wherein the first positive electrode active material in the first positive electrode active slurry includes a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ , 0＜x1＜1. Coating a second positive electrode active slurry on the surface of the first positive electrode active material layer 2312a to form a second positive electrode active material layer 2312b, wherein the second positive electrode active material in the second positive electrode active slurry includes a manganese iron phosphate lithium material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ , 0＜x2＜1. The Dv50 of the cobalt-free material is nanometer-level, and the Dv50 of the manganese iron phosphate lithium material is micrometer-level.

In the technical solution of the embodiment of the present application, a first positive electrode active material layer 2312a containing cobalt-free material near the current collector and a second positive electrode active material layer 2312b containing lithium iron manganese phosphate material are formed in the positive electrode active material by double-layer coating, and the second positive electrode active material layer 2312b covers the first positive electrode active material layer 2312a. While ensuring the capacity retention rate of the positive electrode sheet 231, gas generation can be reduced and costs can be reduced.

FIG. 7 is a flow chart of a process for preparing a positive electrode sheet 231 provided in some embodiments of the present application. Referring to FIG. 7 , a method for preparing a positive electrode sheet 231 provided in an embodiment of the present application includes the following steps:

S110, preparing a first positive electrode active slurry: dispersing a first positive electrode active material, a binder and a conductive agent in a solvent to form a first positive electrode active slurry. The first positive electrode active material may be the first positive electrode active material described above, for example, a cobalt-free material having a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ (0＜x1＜1), the Dv50 of the cobalt-free material being nanometer-level, and a low-cobalt material having a chemical formula of LiNi _x3 Co _y3 Mn _1-x3-y3 O ₂ (0.5≤x3＜1, 0＜y3≤0.05) may be optionally added, and a small amount of other positive electrode active materials may also be optionally added.

For the specific selection of the cobalt-free material and the low-cobalt material, reference may be made to the selection of the cobalt-free material and the low-cobalt material in the first positive electrode active material layer 2312a in the aforementioned positive electrode sheet, which will not be described in detail here.

The binder may be one or more of styrene-butadiene rubber, water-based acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, polyvinyl alcohol and polyvinyl butyral. The conductive agent may be at least one of conductive carbon black, carbon fiber, carbon nanotube, Ketjen black, graphene or acetylene black. The solvent may be one or more of dimethyl glutarate, N-methylpyrrolidone and deionized water. A leveling agent, a dispersant and the like may also be added to the first positive electrode active slurry, which is not limited in this application.

S120, preparing a second positive electrode active slurry: dispersing a second positive electrode active material, a binder and a conductive agent in a solvent to form a second positive electrode active slurry. The second positive electrode active material may be the second positive electrode active material described above, for example, a lithium iron manganese phosphate material having a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ (0＜x2＜1), wherein the Dv50 of the lithium iron manganese phosphate material is micron-level, lithium iron phosphate may be optionally added, and a small amount of other positive electrode active materials may also be optionally added.

For the specific selection of lithium iron manganese phosphate material and lithium iron phosphate material, reference may be made to the selection of lithium iron manganese phosphate material and lithium iron phosphate material in the second positive electrode active material layer 2312b in the aforementioned positive electrode sheet, which will not be repeated here.

The binder, the conductive agent and the solvent may be the binder, the conductive agent and the solvent in the first positive electrode active slurry, etc. The binder in the first positive electrode active slurry may be the same as or different from the binder in the second positive electrode active material; the conductive agent in the first positive electrode active slurry may be the same as or different from the conductive agent in the second positive electrode active material; the solvent in the first positive electrode active slurry may be the same as or different from the solvent in the second positive electrode active material. At the same time, a leveling agent, a dispersant, etc. may also be added to the second positive electrode active slurry, which is not limited in this application.

The first positive electrode active material may also be coated with a first structural conductive polymer layer on at least a portion of its surface. And/or, the second positive electrode active material may also be coated with a second structural conductive polymer layer on at least a portion of its surface. The materials of the first structural conductive polymer layer and the second structural conductive polymer layer have been described above and will not be described again here.

The method of coating the first structural conductive polymer layer is related to the material of the first structural conductive polymer layer. In one embodiment, the method of coating the first structural conductive polymer layer can be: uniformly mixing the first positive electrode active material, the first structural conductive polymer and the solvent, and then drying. Optionally, the material of the first structural conductive polymer layer can be a single conductive polymer material or a composite conductive polymer material (a material mixed with two or more conductive polymers). (The mass of the first structural conductive polymer)/(the mass of the first structural conductive polymer+the mass of the first positive electrode active material)×100%=0.5%~3%.

For example, the material of the first structural conductive polymer layer is a mixture of polyaniline and polyethylene glycol. The method for coating the first structural conductive polymer layer can be: stirring the first positive electrode active material, polyaniline, polyethylene glycol and a solvent (for example, ethanol) at a speed of 200 r/min to 500 r/min for 4 h to 8 h, and then drying at 100° C. to 150° C. for 10 h to 15 h to obtain the coated first positive electrode active material; (mass of polyaniline + mass of polyethylene glycol)/(mass of polyaniline + mass of polyethylene glycol + mass of positive electrode active material)×100%=0.5% to 3%.

In another embodiment, the method of coating the first structural conductive polymer layer can be: uniformly mixing the first positive electrode active material, the first structural conductive polymer monomer, the initiator and the solvent, so that the first structural conductive polymer monomer is cross-linked on at least part of the surface of the first positive electrode active material to form the first structural conductive polymer, and then drying. Optionally, the material of the first structural conductive polymer layer can be a single conductive polymer material. (The mass of the first structural conductive polymer monomer)/(The mass of the first structural conductive polymer monomer+the mass of the first positive electrode active material)×100%=0.5%～3%.

For example, the material of the first structural conductive polymer layer is polyethylene glycol, and the method for coating the first structural conductive polymer layer can be: stirring the first positive electrode active material, ethylene oxide and water at a speed of 200 r/min to 500 r/min for 4 h to 8 h, so that the ethylene oxide is hydrolyzed on the surface of the positive electrode active material to form polyethylene glycol, and then drying at 100° C. to 150° C. for 10 h to 15 h to obtain the coated positive electrode active material. (mass of ethylene oxide)/(mass of ethylene oxide+mass of the first positive electrode active material)×100%=0.5% to 3%.

The coating method of the second structural conductive polymer layer can be the same as that of the first structural conductive polymer layer, except that the coated object is replaced by the second positive electrode active material instead of the first positive electrode active material, and the specific coating method is not described in detail here.

S130, preparing the first positive electrode active material layer 2312a: coating the first positive electrode active slurry on the surface of the positive electrode current collector 2311, and then drying to form the first positive electrode active material layer 2312a. The coating can be applied to one surface or both surfaces of the positive electrode current collector 2311 according to requirements.

The coating method may be: scraping, roller coating, slit coating, etc., which is not limited in this application. It should be noted that step S120 and step S130 can be interchanged or performed simultaneously, which is not limited in this application.

S140, preparing the second positive electrode active material layer 2312b: coating the second positive electrode active slurry on the surface of the first positive electrode active material layer 2312a, and then drying to form the second positive electrode active material layer 2312b. During coating, the second positive electrode active material layer 2312b can be formed on the surface of the first positive electrode active material layer 2312a according to the situation of the first positive electrode active material layer 2312a.

In the present application, the ratio of the coating weight of the first positive electrode active material to the coating weight of the second positive electrode active material is (2-8):(8-2). That is, the ratio of the coating weight of the first positive electrode active material in the first positive electrode active slurry to the coating weight of the second positive electrode active material in the second positive electrode active slurry is (2-8):(8-2). After the positive electrode sheet 231 is prepared, the weight ratio of the first positive electrode active material in the first positive electrode active material layer 2312a to the second positive electrode active material in the second positive electrode active material layer 2312b is (2-8):(8-2).

As an example, the weight ratio of the first positive electrode active material in the first positive electrode active material layer 2312a to the second positive electrode active material in the second positive electrode active material layer 2312b is 2:8, 3:8, 4:8, 5:8, 6:8, 7:8, 8:8, 8:7, 8:6, 8:5, 8:4, 8:3 or 8:2, which can also be any value in the above range.

S150, rolling the second positive electrode active material layer 2312b to obtain the positive electrode sheet 231. Optionally, the first positive electrode active material layer 2312a may be prepared and then rolled, and then the second positive electrode active material layer 2312b may be prepared and then rolled to obtain the positive electrode sheet 231; or the first positive electrode active material layer 2312a and the second positive electrode active material layer 2312b may be prepared in sequence and then rolled to obtain the positive electrode sheet 231, which is not limited in this application.

After the positive electrode sheet 231 is prepared, the first isolation film, the positive electrode sheet 231, the second isolation film and the negative electrode sheet are stacked in sequence, wound to form a wound flat structure, and then hot pressed to obtain a wound electrode assembly 23; or, after the positive electrode sheet 231 is prepared, the positive electrode sheet 231, the isolation film, the negative electrode sheet, the isolation film, and so on are stacked in sequence to form a stacked electrode assembly 23.

The electrode assembly 23 can be used to prepare a battery cell 20 , and the battery cell 20 can be used to prepare a battery 100 and provide electrical energy to electrical devices.

Next, one or more embodiments are described in more detail with reference to the following examples. Of course, these examples do not limit the scope of one or more embodiments.

Example 1

Preparation of negative electrode sheet:

95wt% of negative electrode active material particle graphite, 2wt% of conductive agent acetylene black, 2wt% of thickener sodium carboxymethyl cellulose and 1wt% of binder styrene butadiene rubber are mixed, and an appropriate amount of deionized water is added and stirred to obtain a negative electrode active slurry. The negative electrode active slurry is coated on both surfaces of a copper foil with a thickness of 6μm at a coating amount of 0.2g/ ^1540.25mm2 , and then dried, cold pressed and cut to obtain a negative electrode sheet.

Preparation of positive electrode sheet 231:

96 wt% of a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ , 2 wt% of a conductive agent acetylene black (Jinghong New Energy, ≥99%), and 2 wt% of a binder polyvinylidene fluoride (Shanghai Aladdin Biochemical Technology, ≥99.5%) were mixed, and N-methylpyrrolidone (Shanghai Aladdin Biochemical Technology, AR) was used as a solvent. The mixture was fully stirred and mixed to obtain a first positive electrode active slurry.

96 wt% of lithium manganese iron phosphate material with the chemical formula LiFe _x2 Mn _(1-x2) PO ₄ , 2 wt% of conductive agent acetylene black, and 2 wt% of binder polyvinylidene fluoride are mixed, and N-methylpyrrolidone is used as a solvent. The mixture is fully stirred to obtain a second positive electrode active slurry.

The first positive electrode active slurry is coated on both surfaces of an aluminum foil with a thickness of 9 μm at a coating amount of 0.15 g/1540.25 mm ² and then dried to obtain a first positive electrode active slurry layer; the second positive electrode active slurry is coated on the two first positive electrode active slurry layers at a coating amount of 0.15 g/1540.25 mm ² and then dried to obtain a second positive electrode active slurry layer. Then, cold pressing and cutting are performed to obtain a positive electrode sheet 231.

Preparation of electrode assembly 23:

The separator is made of PP (polypropylene) and has a thickness of 20 μm. The negative electrode sheet, the first separator, the positive electrode sheet 231 and the second separator are stacked and then wound to form the electrode assembly 23 .

Preparation of battery cell 20:

The electrode assembly 23 is placed in the shell, and the electrolyte is injected. Then, the battery cell 20 is obtained through vacuum packaging, standing, forming, shaping and other processes.

Among them, the parameters of the positive electrode sheet 231 are shown in Table 1:

Table 1 Parameters of positive electrode sheet 231

The performance of the battery cells 20 prepared in the examples and comparative examples was tested:

(1) Capacity test of battery 100

At 25° C., the lithium ion secondary battery 100 is first charged to 4.3 V at a constant current of 1 C, further charged to a current of 0.05 C at a constant voltage of 4.3 V, and then discharged to 2.0 V at 0.33 C. The discharge amount is recorded as the capacity of the electrode assembly 23 .

(2) 25℃ Cyclic Performance Test

At 25°C, the lithium-ion secondary battery 100 is first charged to 4.3V at a constant current of 1C, then further charged to a current of 0.05C at a constant voltage of 4.3V, and then discharged to 2.0V at a constant current of 1C. This is a charge-discharge cycle process, and the discharge capacity this time is the discharge capacity of the first cycle. The battery cell 20 is subjected to multiple cycle charge-discharge tests in the above manner, and the discharge capacity of the 200th cycle is detected, and the capacity retention rate of the battery 100 after the cycle is calculated by the following formula.

Capacity retention rate (%) of the battery after 100 cycles = [discharge capacity at the 200th cycle/discharge capacity at the 1st cycle] × 100%.

(3) Gas production test of electrode assembly 23

At 25°C, the lithium-ion secondary battery 100 is first charged to 4.3V at a constant current of 1C, and further charged to a current of 0.05C at a constant voltage of 4.3V. The temperature is raised to 70°C for storage, and the battery is taken out every 5 days. The volume of the electrode assembly 23 is tested using the drainage method, with the cutoff condition being 60 days of storage or when the volume of the electrode assembly 23 exceeds 30%.

The performance of the battery cell 20 is shown in Table 2:

Table 2 Performance of battery cell 20

Combining the contents recorded in Table 1 and Table 2, it can be seen that when the particle size of the cobalt-free material is small, the capacity of the obtained electrode assembly 23 is high, and when the particle size of the cobalt-free material is large, the capacity of the obtained electrode assembly 23 is low. If the lithium manganese iron phosphate material layer is not provided above the cobalt-free material, although the capacity of the electrode assembly 23 is high, its gas production is also large, and the capacity retention rate is low.

From the comparison of R1 to R3, it can be seen that the more the amount of cobalt-free material added, the higher the capacity of the electrode assembly 23; at the same time, the lower the capacity retention rate, the higher the gas production. Combining R1 to R3 and R12 and R13, it can be seen that when the amount of cobalt-free material added is 30% to 70%, the comprehensive performance of the electrode assembly 23 is better.

At the same time, by comparing R1 with R4~R11, it can be seen that when the Dv50 of the cobalt-free material is 100nm~300nm and the Dv50 of the lithium manganese iron phosphate material is 1μm~3μm, the comprehensive performance of the electrode assembly 23 is better.

Example 2

R14 to R22 are basically the same as R1, except that the selection of positive electrode active materials is different. The specific material selection and the corresponding performance of the electrode assembly 23 are shown in Table 3 (the detection method is the same as in Example 1):

Table 3 Performance of positive electrode active materials and electrode assembly 23

It can be seen from Table 3 that by comparing R1, R14 and R15, the more Ni is added in the cobalt-free material, the higher the capacity of the electrode assembly 23, the more obvious the gas production, and the lower the capacity retention rate; therefore, in the cobalt-free material of LiNi _x1 Mn _(1-x1) O ₂ , 0.4≤x1≤0.6, the performance of the electrode assembly 23 is better.

From the comparison of R1 and R16~R19, it can be seen that the amount of Fe added in the lithium manganese iron phosphate material basically does not affect the capacity of the electrode assembly 23; the more Fe is added, the higher the capacity retention rate and the less gas production, but its voltage platform will be lower, resulting in lower energy density; therefore, in the lithium manganese iron phosphate material of LiFe _x2 Mn _(1-x2) PO ₄ , 0.2≤x2≤0.8, and the performance of the electrode assembly 23 is better.

Comparing R1 and R20, it can be seen that adding a small amount of low-cobalt material to the cobalt-free material basically does not affect the capacity of the electrode assembly 23, but its capacity retention rate is significantly improved and the gas production is significantly reduced.

Comparison between R1 and R21 shows that adding a small amount of LiFe _0.6 Mn _0.4 PO ₄ to the LiFe _0.5 Mn _0.5 PO ₄ material can maintain the energy density of the electrode assembly 23 while improving its capacity retention rate and reducing gas production.

Comparing R1 with R22, it can be seen that adding a small amount of LiFe _0.3 Mn _0.7 PO ₄ to the LiFe _0.5 Mn _0.5 PO ₄ material can ensure its capacity retention rate, reduce the gas production, and improve its energy density.

Example 3

R23~R33 are basically the same as R1, except that the materials of the structural conductive polymer layer coated on the surface of the positive electrode active material are different. The preparation method of the positive electrode active material is as follows: the positive electrode active material and a certain proportion of the conductive polymer are dissolved in ethanol, stirred at a low speed (400r/min) with a magnetic stirrer for 6 hours, and then vacuum dried at 120°C for 12 hours to obtain the coated positive electrode active material.

The properties of the conductive polymer and the corresponding electrode assembly 23 are shown in Table 4 (the weight average molecular weight of isoaniline in Table 4 is 40,000, the weight average molecular weight of polyethylene glycol is 2,000, and the detection method is the same as in Example 1):

Table 4 Performance of the conductive layer on the surface of the positive electrode active material and the electrode assembly 23

It can be seen from Table 4 that after the surface of the positive electrode active material layer 2312 is coated with a conductive polymer layer, the capacity of the electrode assembly 23 increases, the capacity cycle retention rate also increases, and the gas generation decreases.

From the comparison of R23 to R25, it can be seen that the amount of conductive polymer layer added gradually increases, which will reduce the capacity of the electrode assembly 23 to a certain extent, but its cycle capacity retention rate is greatly improved and the gas production is greatly reduced. And compared with R1, it can be seen that compared with no conductive polymer layer, adding a small amount of conductive polymer layer (1% to 3%) can not only increase the capacity of the electrode assembly 23, but also increase its capacity retention rate and reduce gas production.

By comparing R24 and R26, it can be seen that after the surface of the positive electrode active material layer 2312 is coated with a conductive polymer layer, the amount of cobalt-free material added is increased, which can not only increase the capacity of the electrode assembly 23, but also increase the capacity retention rate of the electrode assembly 23, while reducing the gas production of the electrode assembly 23.

Comparison of R24 with R27, R28, R31 to R33 shows that, compared with the surface of the cobalt-free material only coated with isoaniline or polyethylene glycol, the surface of the cobalt-free material is coated with a mixture of isoaniline and polyethylene glycol, which can make the capacity retention rate of the electrode assembly 23 higher and the gas production less. Comparison of R24 with R29 and R30 shows that the surface of the lithium iron manganese phosphate material is only coated with isoaniline or polyethylene glycol, or the surface of the lithium iron manganese phosphate material is coated with a mixture of isoaniline and polyethylene glycol, which has little effect on the performance of the electrode assembly 23.

The embodiments described above are part of the embodiments of the present application, rather than all of the embodiments. The detailed description of the embodiments of the present application is not intended to limit the scope of the present application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

Claims (22)

1. A positive electrode sheet, characterized in that it includes a positive electrode collector and a positive electrode active material layer, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer which are sequentially arranged outward from the surface of the positive electrode collector and cover the surface of the positive electrode collector, the first positive electrode active material in the first positive electrode active material layer includes a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ , 0＜x1＜1; the second positive electrode active material in the second positive electrode active material layer includes a lithium iron manganese phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ , 0＜x2＜1; wherein the Dv50 of the cobalt-free material is nanometer-level, and the Dv50 of the lithium iron manganese phosphate material is micrometer-level. 2. The positive electrode sheet according to claim 1 is characterized in that the Dv50 of the cobalt-free material is 100nm~300nm, and the Dv50 of the lithium manganese iron phosphate material is 1μm~3μm. 3 . The positive electrode sheet according to claim 1 , wherein the mass proportion of the cobalt-free material in the first positive electrode active material and the second positive electrode active material is 20% to 80%. 4. The positive electrode sheet according to claim 1 is characterized in that the first positive electrode active material also includes a low-cobalt material with a chemical formula of LiNi _x3 Co _y3 Mn _1-x3-y3 O ₂ , wherein 0.5≤x3＜1, 0＜y3≤0.05, and the mass proportion of the low-cobalt material in the first positive electrode active material is not higher than 30%. 5 . The positive electrode sheet according to claim 2 , wherein the absolute value of the difference between the voltage platform of the first positive electrode active material and the voltage platform of the second positive electrode active material is ≤0.3V. 6 . The positive electrode sheet according to claim 5 , wherein 0.2≤x2≤0.8. 7. The positive electrode sheet according to claim 6, characterized in that the second positive electrode active material comprises lithium manganese iron phosphate materials with a chemical formula of LiFe _0.5 Mn _0.5 PO ₄ and a chemical formula of LiFe _0.3 Mn _0.7 PO ₄ , and the mass proportion of the LiFe _0.3 Mn _0.7 PO ₄ material in the second positive electrode active material is not higher than 30%. 8. The positive electrode sheet according to any one of claims 1 to 7, characterized in that at least part of the surface of the first positive electrode active material is coated with a first structural conductive polymer layer, and the mass of the first structural conductive polymer layer accounts for 0.5% to 3% of the total mass of the first positive electrode active material and the first structural conductive polymer layer; And/or, at least part of the surface of the second positive electrode active material is coated with a second structural conductive polymer layer, and the mass of the second structural conductive polymer layer accounts for 0.5% to 3% of the total mass of the second positive electrode active material and the second structural conductive polymer layer. 9 . The positive electrode sheet according to claim 8 , wherein the material of the first structural conductive polymer layer and/or the material of the second structural conductive polymer layer comprises an ionic polymer and an electronic polymer. 10. The positive electrode sheet according to claim 9, characterized in that the ionic polymer comprises at least one of polyethylene glycol, polyethylene oxide, polyethylene glycol succinate, and polyethylene glycol imine; The electronic polymer includes at least one of polyaniline, polythiophene, polypyrrole and polyquinoline. 11 . The positive electrode sheet according to claim 9 , wherein the material of the first structural conductive polymer layer and/or the material of the second structural conductive polymer layer is a mixture of polyaniline and polyethylene glycol, and the mass proportion of the polyaniline in the mixture is 30% to 70%. 12. The positive electrode sheet according to claim 11, characterized in that the weight average molecular weight of the polyaniline is 20,000 to 80,000, and the structural formula of the polyaniline is: Among them, y=0.5. 13 . The positive electrode sheet according to claim 11 , wherein the weight average molecular weight of the polyethylene glycol is 800 to 4000. 14 . An electrode assembly, comprising a negative electrode sheet, a separator and the positive electrode sheet according to claim 1 , wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet. 15 . A battery cell, comprising the electrode assembly according to claim 14 . 16. A battery, comprising the battery cell according to claim 15. 17. An electrical device, characterized by comprising the battery according to claim 16. 18. A method for preparing a positive electrode sheet, characterized in that it comprises the following steps: Coating a first positive electrode active slurry on the surface of the positive electrode current collector to form a first positive electrode active material layer, wherein the first positive electrode active material in the first positive electrode active slurry comprises a cobalt-free material with a chemical formula of LiNi _x1 Mn _(1-x1) O ₂ , 0＜x1＜1; Coating a second positive electrode active slurry on the surface of the first positive electrode active material layer to form a second positive electrode active material layer, wherein the second positive electrode active material in the second positive electrode active slurry includes a lithium manganese iron phosphate material with a chemical formula of LiFe _x2 Mn _(1-x2) PO ₄ , 0＜x2＜1; Among them, the Dv50 of the cobalt-free material is nanometer-level, and the Dv50 of the lithium iron manganese phosphate material is micrometer-level. 19. The preparation method according to claim 18, characterized in that the ratio of the coating weight of the first positive electrode active material to the coating weight of the second positive electrode active material is (2-8):(8-2). 20. The preparation method according to claim 18, characterized in that before coating the first positive electrode active slurry, it also includes the step of coating at least part of the surface of the first positive electrode active material with a first structural conductive polymer layer; Or/and, before coating the second positive electrode active slurry, the method further comprises the step of coating at least a portion of the surface of the second positive electrode active material with a second structural conductive polymer layer. 21. The preparation method according to claim 20, characterized in that the method of coating the first structural conductive polymer layer comprises: uniformly mixing the first positive electrode active material, the first structural conductive polymer and a solvent, and then drying; Or/and, a method for coating the second structural conductive polymer layer comprises: uniformly mixing the second positive electrode active material, the second structural conductive polymer and a solvent, and then drying. 22. The preparation method according to claim 20, characterized in that the method of coating the first structural conductive polymer layer comprises: uniformly mixing the first positive electrode active material, the first structural conductive polymer monomer, an initiator and a solvent, so that the first structural conductive polymer monomer is cross-linked on at least part of the surface of the first positive electrode active material to form the first structural conductive polymer, and then drying; Or/and, a method for coating the second structural conductive polymer layer, comprising: uniformly mixing the second positive electrode active material, a second structural conductive polymer monomer, an initiator and a solvent, allowing the second structural conductive polymer monomer to cross-link on at least part of the surface of the second positive electrode active material to form a second structural conductive polymer, and then drying.