Positive electrode active material, method for producing same, electrode assembly, battery, and power storage device

Abstract

The application provides a positive electrode active material and a preparation method therefor, an electrode assembly, a battery and a power utilization device. The positive electrode active material comprises a core material and a coating layer, the coating layer being coated on at least part of the surface of the core material. The core material comprises lithium iron manganese phosphate particles, the coating layer comprises conductive layers and lithium iron manganese phosphate layers which are alternately arranged in layers, and the outermost side of the coating layer away from the core material is the conductive layer.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to a positive electrode active material and its preparation method, electrode assembly, battery and power device. Background Technology

[0002] Secondary batteries rely on the repeated insertion and extraction of active ions between the positive and negative electrodes for charging and discharging. They possess outstanding characteristics such as high energy density, long cycle life, and no pollution or memory effect. Therefore, as a clean energy source, secondary batteries have gradually expanded from electronic products to large-scale devices such as electric vehicles, in line with sustainable development strategies for the environment and energy. This also places higher demands on the cycle performance of secondary batteries. Summary of the Invention

[0003] To achieve the above objectives, this application provides a positive electrode active material and its preparation method, an electrode assembly, a battery, and an electrical device, which enable the battery and electrical device to have good cycle performance.

[0004] A first aspect of this application provides a positive electrode active material, comprising a core material and a coating layer, wherein the coating layer covers at least a portion of the surface of the core material. The core material comprises lithium manganese iron phosphate particles, and the coating layer comprises alternating layers of conductive layers and lithium manganese iron phosphate layers, wherein the outermost part of the coating layer away from the core material is a conductive layer.

[0005] Not intended to be limited by any theory or explanation, in the positive electrode active material of this application embodiment, the outermost layer of the coating layer, furthest from the core material, is a conductive layer. This outermost conductive layer reduces the contact between lithium manganese iron phosphate and air, lowering the water content of the positive electrode active material; it also reduces the direct contact between lithium manganese iron phosphate and the electrolyte, reducing manganese dissolution and improving the stability of the lithium manganese iron phosphate structure. Furthermore, in the coating layer of this application embodiment, the conductive layer and the lithium manganese iron phosphate layer are alternately arranged, giving the entire coating layer excellent conductivity. This allows the positive electrode active material to possess both a large particle size and good conductivity. When the particle size of the positive electrode active material is large, the specific surface area can be reduced, effectively reducing its hygroscopicity and water content. When the positive electrode active material has good conductivity, its impedance can be reduced, thereby improving its specific capacity.

[0006] Therefore, the positive electrode active material of this application embodiment, when applied to a secondary battery, can, on the one hand, reduce the difficulty and shorten the baking time of the battery to remove water, thus maintaining good mechanical properties of the negative electrode sheet; on the other hand, the positive electrode active material has low impedance and high structural stability, which can reduce the battery's internal resistance and capacity decay. This improves the cycle performance of the secondary battery and extends its cycle life.

[0007] In any embodiment of this application, the sum of the total number of conductive layers and the total number of lithium manganese iron phosphate layers is greater than or equal to 3, and can be selected as 3-15, or more preferably 5-11.

[0008] When the sum of the total number of conductive layers and the total number of lithium manganese iron phosphate layers is within the aforementioned suitable range, the positive electrode active material can possess both low water content and low impedance. Therefore, applying this positive electrode active material to secondary batteries can further improve the battery's cycle stability, reduce internal resistance and capacity decay, thereby further extending the battery's cycle life.

[0009] In any embodiment of this application, the volume distribution particle size Dv of the positive electrode active material 1 50 is greater than or equal to 2μm, and can be selected from 2μm to 5μm. This can reduce the hygroscopicity of the positive electrode active material, thereby effectively reducing the water content of the positive electrode active material.

[0010] In any embodiment of this application, the specific surface area of ​​the positive electrode active material is greater than or equal to 1 m². 2 / g, optional 1m 2 / g-3m 2 / g. This effectively reduces the hygroscopicity of the positive electrode active material, thereby reducing its water content.

[0011] In any embodiment of this application, the volume distribution particle size Dv of the core material 2 50 nm is less than or equal to 800 nm, and can be selected from 400 nm to 800 nm. This helps the positive electrode active material to have lower hygroscopicity, thereby reducing the water content of the positive electrode active material; on the other hand, it helps to reduce the impedance of the positive electrode active material and improve the specific capacity of the positive electrode active material.

[0012] In any embodiment of this application, the thickness of the single conductive layer is 10nm-300nm, and can be selected as 10nm-100nm. A thickness within the aforementioned suitable range can improve the conductivity of the coating layer, reduce the impedance of the positive electrode active material, and also increase the content of lithium manganese iron phosphate in the positive electrode active material, thereby increasing the specific capacity of the positive electrode active material.

[0013] In any embodiment of this application, based on the total mass of the positive electrode active material, the mass percentage of the conductive layer is 0.5%-7%, optionally 0.5%-4%. This can improve the specific capacity of the positive electrode active material, thereby improving the cycle performance of the battery and extending its cycle life.

[0014] In any embodiment of this application, the thickness of the monolayer lithium manganese iron phosphate layer is less than or equal to 500 nm, and can be selected as 300 nm-500 nm. This not only allows the coating layer to maintain good conductivity, but also increases the lithium manganese iron phosphate content in the positive electrode active material, thereby increasing the specific capacity of the positive electrode active material.

[0015] In any embodiment of this application, lithium manganese iron phosphate particles have the general formula LiMn x Fe 1-x PO4, 0.1≤x≤0.9.

[0016] In any embodiment of this application, the lithium manganese iron phosphate layer has the general formula LiMn y Fe 1-y PO4, 0.1≤y≤0.9.

[0017] Optionally, y < x. This can improve the stability of the positive electrode active material, thereby enhancing the cycle stability of the battery and extending its cycle life.

[0018] In any embodiment of this application, the conductive layer includes one or more of conductive carbon materials and metals.

[0019] Optionally, the conductive carbon material includes one or more of carbon nanotubes, graphene, carbon fiber, Ketjen black, acetylene black, porous carbon materials, and conductive graphite.

[0020] Optionally, the metal includes one or more of silver, platinum, gold, palladium, rhodium, iridium, osmium, and ruthenium.

[0021] This can improve the specific capacity of the positive electrode active material, reduce the internal resistance and capacity decay of the battery, thereby improving the cycle performance and extending the cycle life of the battery.

[0022] A second aspect of this application provides a method for preparing the positive electrode active material of the first aspect, comprising:

[0023] Provide core materials, including lithium iron phosphate particles.

[0024] A positive electrode active material is obtained by forming a coating layer on the surface of the core material through solid-state sintering. The coating layer includes an alternating conductive layer and a lithium manganese iron phosphate layer, and the outermost part of the coating layer away from the core material is the conductive layer.

[0025] In any embodiment of this application, a coating layer is formed on the surface of the core material by solid-state sintering, including:

[0026] The conductive layer and lithium manganese iron phosphate layer are alternately stacked on the surface of the core material by solid-state sintering.

[0027] The conductive layer is obtained by sintering a conductive material precursor, which includes one or more of carbon sources and metal sources.

[0028] The lithium manganese iron phosphate layer is obtained by sintering a lithium manganese iron phosphate precursor, which includes lithium manganese phosphate and a lithium source.

[0029] A third aspect of this application provides a positive electrode sheet, which includes a positive current collector and a positive electrode film layer located on at least one side of the positive current collector, wherein the positive electrode film layer includes the positive electrode active material of the first aspect of this application or the positive electrode active material prepared according to the method of the second aspect.

[0030] A fourth aspect of this application provides an electrode assembly that includes the positive electrode sheet of the third aspect of this application.

[0031] The fifth aspect of this application provides a battery that includes the electrode assembly of the fourth aspect of this application.

[0032] The sixth aspect of this application provides an electrical device that includes the battery of the fifth aspect of this application, said battery being used to provide electrical energy.

[0033] The electrical device of this application includes the battery provided in this application, and therefore has at least the same advantages as the battery. Attached Figure Description

[0034] Figure 1 This is a schematic diagram illustrating an embodiment of the positive electrode active material of this application.

[0035] Figure 2 This is a schematic diagram illustrating an embodiment of the battery cell of this application.

[0036] Figure 3 yes Figure 1 An exploded view of an embodiment of the battery cell of this application is shown.

[0037] Figure 4 This is a schematic diagram of one embodiment of the battery module of this application.

[0038] Figure 5 This is a schematic diagram of one embodiment of the battery pack of this application.

[0039] Figure 6 yes Figure 5 The diagram shown is an exploded view of an embodiment of the battery pack of this application.

[0040] Figure 7 This is a schematic diagram of an embodiment of the battery of this application used as a power source for an electrical device.

[0041] 100 Positive electrode active material; 110 Core material; 120 Coating layer; 121 Conductive layer; 122 Lithium manganese iron phosphate layer; 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0042] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its preparation method, electrode assembly, battery, and power device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0043] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0044] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0045] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0046] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0047] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

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

[0049] In this document, the term "cladding layer" refers to a layer of material covering the core, which may completely or partially cover the core. The use of "cladding layer" is for ease of description only and is not intended to limit the invention. Furthermore, each cladding layer may be a complete or partial covering.

[0050] In this document, the term "source" refers to a compound that is the source of a certain element. For example, the types of "sources" include, but are not limited to, carbonates, sulfates, nitrates, elements, halides, oxides, and hydroxides.

[0051] With the application and promotion of secondary batteries in various electronic products and new energy vehicles, higher requirements have been placed on the cycle performance of secondary batteries.

[0052] Lithium manganese iron phosphate has the advantages of high capacity, high safety, good cycle performance, and is safe and non-toxic, making it an important cathode material for lithium-ion batteries.

[0053] The lithium manganese iron phosphate material commonly used in cathodes is typically a primary particle with a small particle size and large surface area. This makes it prone to absorbing moisture from the external environment (such as air), resulting in a high water content in the cathode of lithium-ion batteries. When the cathode has a high water content, the battery requires prolonged baking. Furthermore, even after baking, a small amount of moisture may still remain in the cathode. Prolonged baking may affect the performance of the anode, causing risks such as wrinkling of the anode electrode sheet. Residual moisture in the cathode may lead to internal side reactions, resulting in the loss of active lithium ions. In addition, byproducts can damage the positive electrode active material and SEI film, accelerating battery capacity decay. Consequently, this leads to decreased cycle performance and reduced cycle life of the lithium-ion battery.

[0054] In view of this, this application provides a positive electrode active material and its preparation method, an electrode assembly, a battery, and an electrical device.

[0055] Positive electrode active material

[0056] The first aspect of this application discloses a positive electrode active material, including a core material and a coating layer, wherein the coating layer covers at least a portion of the surface of the core material. The core material includes lithium manganese iron phosphate particles, and the coating layer includes alternately stacked conductive layers and lithium manganese iron phosphate layers, with the outermost layer of the coating layer furthest from the core material being the conductive layer.

[0057] Figure 1 A schematic diagram of the positive electrode active material according to one embodiment of this application is shown. Figure 1 As shown, the positive electrode active material 100 may include a core material 110 and a coating layer 120 covering the surface of the core material 110. The coating layer 120 is composed of alternating layers of conductive layers 121 and lithium manganese iron phosphate layers 122, with the outermost layer of the coating layer 120 away from the core material 110 being the conductive layer 121. It should be noted that... Figure 1 This is just one example of a positive electrode active material, used only to explain the embodiments of this application, and does not constitute a limitation on the embodiments of this application.

[0058] It should be noted that the lithium manganese iron phosphate contained in any lithium manganese iron phosphate layer of the core material and the coating layer can be independently selected from lithium manganese iron phosphate materials and their modified materials known in the art. The lithium manganese iron phosphate contained in any lithium manganese iron phosphate layer of the core material and the coating layer can be the same or different, and no limitation is made here. In the conductive layers of the coating layer, the conductive materials contained in any two conductive layers can be the same or different, and can be independently selected from conductive materials known in the art, and no limitation is made here.

[0059] Not intended to be limited by any theory or explanation, in the positive electrode active material of this application embodiment, the outermost layer of the coating layer, furthest from the core material, is a conductive layer. This outermost conductive layer reduces the contact between lithium manganese iron phosphate and air, lowering the water content of the positive electrode active material; it also reduces the direct contact between lithium manganese iron phosphate and the electrolyte, reducing manganese dissolution and improving the stability of the lithium manganese iron phosphate structure. Furthermore, in the coating layer of this application embodiment, the conductive layer and the lithium manganese iron phosphate layer are alternately arranged, giving the entire coating layer excellent conductivity. This allows the positive electrode active material to possess both a large particle size and good conductivity. When the particle size of the positive electrode active material is large, the specific surface area can be reduced, effectively reducing its hygroscopicity and water content. When the positive electrode active material has good conductivity, its impedance can be reduced, thereby improving its specific capacity.

[0060] Therefore, the positive electrode active material of this application embodiment, when applied to a secondary battery, can, on the one hand, reduce the difficulty and shorten the baking time of the battery to remove water, thus maintaining good mechanical properties of the negative electrode sheet; on the other hand, the positive electrode active material has low impedance and high structural stability, which can reduce the battery's internal resistance and capacity decay. This improves the cycle performance of the secondary battery and extends its cycle life.

[0061] In some embodiments, the sum of the total number of conductive layers and the total number of lithium manganese iron phosphate layers can be greater than or equal to 3, for example, it can be 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, or a range of any two of the above values.

[0062] Optionally, in some embodiments, the sum of the total number of conductive layers and the total number of lithium manganese iron phosphate layers can be 3-15, 3-13, 3-11, 3-9, 3-7, 3-5, 5-15, 5-13, 5-11, 5-9, 5-7, 7-15, 7-13, 7-11, 7-9, etc.

[0063] Not intended to be limited by any theory or explanation, when the sum of the total number of conductive layers and the total number of lithium manganese iron phosphate layers is within the aforementioned suitable range, the positive electrode active material can possess both a small specific surface area and high conductivity, thereby achieving both low water content and low impedance. Therefore, when applied to secondary batteries, this positive electrode active material can further improve the battery's cycle stability, reduce internal resistance and capacity decay, and thus further extend the battery's cycle life. Furthermore, satisfying the given range for the sum of the total number of conductive layers and the total number of lithium manganese iron phosphate layers can also reduce the preparation difficulty of the positive electrode active material in this application embodiment, facilitating large-scale preparation of the positive electrode active material.

[0064] In some embodiments, the volume distribution particle size Dv of the positive electrode active material 1 50 is greater than or equal to 2μm, for example, it can be 2μm, 2.2μm, 2.5μm, 2.9μm, 3.1μm, 3.3μm, 3.6μm, 4μm, 4.2μm, 4.5μm, 4.9μm, 5.1μm, 5.4μm, 6μm, or any range of two of the above values.

[0065] Optionally, in some embodiments, the volume distribution particle size Dv of the positive electrode active material is... 1 50 can also be 2μm-5μm, 2μm-4.5μm, 2μm-4μm, 2μm-3.3μm, 2μm-2.9μmμm, 2.2μm-4.9μm, 2.2μm-4.2μm, 2μm-3.6μm, 2.5μm-3.3μm, 2.9μm-3.3μm, etc.

[0066] This is not intended to be limited by any theory or explanation, but rather to consider the volume distribution particle size Dv of the positive electrode active material. 1 When 50 meets the given range, the positive electrode active material can have a smaller specific surface area, thereby reducing its hygroscopicity and effectively lowering its water content. This, in turn, helps improve the battery's cycle performance and extend its cycle life.

[0067] Volume distribution particle size Dv of positive electrode active material 1 The value 50 has a well-known meaning in the art, representing the particle size corresponding to a cumulative particle size distribution percentage of 50% in a volumetric particle size distribution of the positive electrode active material. The volumetric particle size distribution (Dv50) can be determined using equipment and methods known in the art. For example, it can be determined using a laser particle size analyzer (e.g., Malvern Master Size 3000) in accordance with GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0068] In some embodiments, the specific surface area of ​​the positive electrode active material can be greater than or equal to 1 m². 2 / g, for example, can be 1m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2m 2 / g, 2.2m 2 / g, 2.5m 2 / g, 2.8m 2 / g, 3m 2 / g, 3.2m 2 / g, 3.5m 2 / g, 3.8m2 / g, or a range consisting of any two of the above values.

[0069] Optionally, in some embodiments, the specific surface area of ​​the positive electrode active material can also be 1m². 2 / g-3m 2 / g, 1m 2 / g-2.8m 2 / g, 1m 2 / g-2.5m 2 / g, 1m 2 / g-2.2m 2 / g, 1m 2 / g-2m 2 / g, 1m 2 / g-1.8m 2 / g, 1m 2 / g-1.5m 2 / g, 1.2m 2 / g-2.7m 2 / g, 1.2m 2 / g-2.3m 2 / g, 1.2m 2 / g-1.9m 2 / g, 1.5m 2 / g-2.5m 2 / g etc.

[0070] When the specific surface area of ​​the positive electrode active material meets the given range, its hygroscopicity can be effectively reduced, thereby lowering its water content. This, in turn, helps improve the battery's cycle performance and extend its cycle life.

[0071] The specific surface area of ​​a positive electrode active material has a meaning known in the art and can be determined using methods known in the art. For example, a specific surface area analyzer (e.g., Tristar II 3020M) can be used to measure the specific surface area of ​​a positive electrode active material by nitrogen adsorption / desorption.

[0072] In some implementations, the volumetric particle size Dv of the core material 2 50 can be less than or equal to 800nm, for example, it can be 800nm, 700nm, 600nm, 500nm, 400nm, 300nm, or any range of two of the above values.

[0073] Optionally, in some embodiments, the volume distribution particle size Dv of the core material 250 can also be 400nm-800nm, for example, it can be 400nm, 420nm, 450nm, 480nm, 500nm, 520nm, 550nm, 580nm, 600nm, 620nm, 650nm, 680nm, 700nm, 720nm, 750nm, 780nm, 800nm, or any range of any two of the above values.

[0074] This is not intended to be limited by any theory or explanation, when the volume distribution particle size Dv of the core material... 2 When 50 meets the given range, it is beneficial for the positive electrode active material to have a suitable particle size, thereby ensuring a suitable specific surface area. This contributes to the positive electrode active material having lower hygroscopicity, thus reducing its water content. Furthermore, when the core material has the aforementioned small volume distribution particle size Dv... 2 At 50°C, the core material can also have a lower impedance, which helps to reduce the impedance of the positive electrode active material and improve its specific capacity. Therefore, the positive electrode active material of this application, when applied to a secondary battery, can further improve the battery's cycle performance and extend its cycle life.

[0075] In some implementations, the thickness of a single conductive layer can be 10nm-300nm, for example, it can be 10nm, 20nm, 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, or any range of two of the above values.

[0076] Optionally, in some embodiments, the thickness of the single conductive layer can be 10nm-100nm, for example, it can be 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, or any range of two of the above values.

[0077] Not intended to be limited by any particular theory or explanation, the thickness of the single conductive layer within the aforementioned suitable range allows for higher coating uniformity, thereby improving the conductivity of the coating layer, reducing the impedance of the positive electrode active material, and ultimately enhancing the battery's cycle performance. Furthermore, a thickness within this suitable range also facilitates increasing the lithium manganese iron phosphate content in the positive electrode active material, thereby increasing its specific capacity. This, in turn, contributes to improving the battery's energy density.

[0078] In some implementations, the thickness of the single-layer lithium manganese iron phosphate layer can be less than or equal to 500 nm, for example, it can be 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, or any range of two of the above values.

[0079] Optionally, in some embodiments, the thickness of the single-layer lithium manganese iron phosphate layer can be 300nm-500nm, for example, it can be 300nm, 350nm, 400nm, 450nm, 500nm, or any range of two of the above values.

[0080] Not intended to be limited by any theory or explanation, the thickness of the monolayer lithium manganese iron phosphate layer within the aforementioned suitable range not only allows the coating layer to maintain good conductivity but also increases the lithium manganese iron phosphate content in the positive electrode active material, thereby increasing the specific capacity of the positive electrode active material. This, in turn, is beneficial for improving the energy density of the battery.

[0081] The volume distribution particle size Dv of the above-mentioned core material 2 50. The thickness of the single conductive layer and the thickness of the single lithium manganese iron phosphate layer can both have meanings known in the art and can be determined by equipment and methods known in the art. As an example, they can be obtained by the following method: the positive electrode active material is mixed uniformly with a binder and coated on the surface of the positive electrode current collector. The mixture is then dried and cold-pressed to prepare a positive electrode sheet (when sampling from the battery, the battery can be directly disassembled, the positive electrode sheet removed, and dried); a randomly selected area is subjected to argon-ion cross-section polishing, and an electron microscope image (SEM image) is taken at a magnification of 3000x or higher (e.g., 3000x or 5000x); the SEM image is processed, and the contrast is adjusted appropriately until the outline of the positive electrode active material is clearly visible; the particle size of each positive electrode active material in the cross-section, as well as the thickness of the single conductive layer and the thickness of the single lithium manganese iron phosphate layer, are statistically analyzed using software to obtain the volume distribution particle size Dv of the core material. 2 50. The thickness of the single conductive layer and the thickness of the single lithium manganese iron phosphate layer. As another example, the positive electrode active material powder can also be directly subjected to argon ion cross-section polishing and SEM images can be taken. The volume distribution particle size Dv of the core material can then be determined using the SEM images. 2 50. The thickness of the single conductive layer and the thickness of the single lithium manganese iron phosphate layer.

[0082] In some implementations, the mass percentage of the conductive layer is 0.5%-7% based on the total mass of the positive electrode active material. For example, it can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, or any combination of two of the above values.

[0083] Optionally, in some embodiments, the mass percentage of the conductive layer based on the total mass of the positive electrode active material can be 0.5%-4%, for example, it can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or any range of two of the above values.

[0084] It is not intended to be limited by any theory or explanation. In the positive electrode active material, if the mass ratio of the conductive layer is within the appropriate range mentioned above, the positive electrode active material can have both low impedance and high specific capacity. This can improve the specific capacity of the positive electrode active material, thereby improving the cycle performance of the battery and extending the cycle life of the battery.

[0085] The mass percentage of the conductive layer has a meaning known in the art and can be determined using equipment and methods known in the art. As an example, when the conductive layer is a conductive carbon layer, the mass percentage of the conductive layer can be determined by thermogravimetric analysis. Specifically, a certain mass of positive electrode active material can be heated to 600°C in air at a rate of 10°C / min; the weight loss is the carbon coating amount. The ratio of the carbon coating amount to the mass of the positive electrode active material is the mass percentage of the conductive layer. As another example, when the conductive layer is a conductive metal layer, a certain mass of positive electrode material can be dissolved in aqua regia, diluted a certain factor, and then tested by inductively coupled plasma atomic emission spectrometry (ICP-AES) to obtain the coating metal content. The ratio of the coating metal content to the mass of the positive electrode active material is the mass percentage of the conductive layer.

[0086] In some embodiments, the lithium manganese iron phosphate particles of the core material may have the general formula LiMn x Fe 1-x PO4, 0.1≤x≤0.9.

[0087] In some embodiments, the lithium manganese iron phosphate layer may have the general formula LiMn y Fe 1-y PO4, 0.1≤y≤0.9.

[0088] In some embodiments, the lithium manganese iron phosphate particles of the core material have the general formula LiMn x Fe 1-x PO4, 0.1 < x ≤ 0.9. The lithium manganese iron phosphate layer has the general formula LiMn. y Fe 1-y For PO4, 0.1 ≤ y < 0.9. The positive electrode active material can satisfy: y < x.

[0089] This is not intended to be limited to any particular theory or explanation. During the charge-discharge cycle of a battery, lithium iron phosphate (LFP) in the coating layer preferentially releases active lithium ions compared to LFP in the core material, and the amount of active lithium released is also greater, with a higher likelihood of manganese dissolution. When the LFP layer has a lower manganese content, manganese dissolution from the LFP layer can be reduced, thereby decreasing the amount of manganese dissolution from the positive electrode active material. This improves the stability of the positive electrode active material, thus enhancing the cycle stability of the battery and extending its cycle life.

[0090] In some embodiments, the conductive layer may include one or more of a conductive carbon material and a metal.

[0091] Optionally, in some embodiments, the conductive carbon material may include one or more of carbon nanotubes, graphene, carbon fiber, Ketjen black, acetylene black, porous carbon materials, and conductive graphite.

[0092] Optionally, in some embodiments, the metal may include one or more of silver, platinum, gold, palladium, rhodium, iridium, osmium, and ruthenium.

[0093] Not intended to be limited to any particular theory or explanation, the aforementioned types of conductive materials possess excellent conductivity. When applied to positive electrode active materials, they can effectively improve the conductivity and reduce the impedance of the positive electrode active materials. This, in turn, can enhance the specific capacity of the positive electrode active material, reduce the battery's internal resistance and capacity decay, thereby improving the battery's cycle performance and extending its cycle life.

[0094] Methods for preparing positive electrode active materials

[0095] The second aspect of this application provides a method for preparing the positive electrode active material of the first aspect, including the steps S10 and S20.

[0096] S10 provides core materials, including lithium manganese iron phosphate particles.

[0097] In step S10, the core material can be as described in any embodiment of the first aspect. The embodiments of the core material have been described and illustrated in detail above and will not be repeated here. The core material can be commercially available or prepared by methods known in the art.

[0098] S20, a coating layer is formed on the surface of the core material by solid-state sintering to obtain a positive electrode active material. The coating layer consists of an alternating layer of conductive layer and lithium manganese iron phosphate layer, with the outermost layer of the coating layer away from the core material being the conductive layer.

[0099] In step S20, the coating layer can be as described in any embodiment of the first aspect. The embodiments of the coating layer have been described in detail above and will not be repeated here. In step S20, the temperature and time of solid-state sintering can be those known in the art, and those skilled in the art can adjust them as needed, without limitation.

[0100] In some embodiments, step S20 may specifically include: forming an alternating layer of conductive layer and lithium manganese iron phosphate layer on the surface of the core material by solid-state sintering.

[0101] The conductive layer can be obtained by sintering a conductive material precursor, which may include one or more of carbon sources and metal sources.

[0102] The carbon source and metal source can be any carbon source and metal source known in the art. For example, the carbon source can be one or more of sucrose, glucose, citric acid, and polymers (such as polyethylene glycol), and the metal source can be one or more of metal nitrates, halides, sulfates, phosphates, acetates, and acetylacetone salts. Those skilled in the art can select the appropriate source as needed, and no limitation is made here.

[0103] The lithium manganese iron phosphate layer can be obtained by sintering a lithium manganese iron phosphate precursor, which may include lithium manganese phosphate and a lithium source.

[0104] The lithium source can be any lithium source known in the art, such as one or more of lithium carbonate, lithium hydroxide monohydrate, lithium nitrate, lithium acetate, and lithium chloride. Those skilled in the art can select the appropriate source as needed, and no limitation is made here.

[0105] As an example, the positive electrode active material can be prepared by the following steps: (1) After uniformly mixing manganese iron phosphate and lithium source, water is added for grinding, and after spray drying, sintering (the sintering temperature can be 400℃-700℃) and gas crushing are performed to obtain the lithium manganese iron phosphate core material. (2) After uniformly mixing the obtained core material with carbon source and / or metal source, it is sintered at high temperature (e.g., 400℃-1000℃) under a specific atmosphere to obtain the intermediate 1 of lithium manganese iron phosphate coated with conductive layer. When the raw material is carbon source, the above-mentioned specific atmosphere can be an inert atmosphere; when the raw material is metal source, the above-mentioned specific atmosphere can be a mixed atmosphere of inert atmosphere and reducing gas, for example, a mixed atmosphere of 95% Ar and 5% H2. (3) After uniformly mixing intermediate 1 with manganese iron phosphate and lithium source, water is added for grinding, and after spray drying, sintering and gas crushing are performed to obtain intermediate 2 of intermediate 1 coated with lithium manganese iron phosphate layer. (4) After uniformly mixing intermediate 2 with a carbon source and / or a metal source, the mixture is sintered at high temperature under an inert atmosphere to coat the surface of the lithium manganese iron phosphate layer with a conductive layer. This forms an alternating layer of conductive and lithium manganese iron phosphate layers on the surface of the core material. By repeating steps (3) and (4), the total number of conductive and lithium manganese iron phosphate layers can be adjusted, thereby adjusting the particle size of the positive electrode active material. Alternatively, the particle size of the positive electrode active material can be adjusted by changing the thickness of each layer.

[0106] Positive electrode sheet

[0107] A third aspect of this application provides a positive electrode sheet, which includes a positive current collector and a positive electrode film layer located on at least one side of the positive current collector, wherein the positive electrode film layer includes the positive electrode active material of the first aspect, or the positive electrode active material prepared according to the method of the second aspect.

[0108] The positive electrode sheet of this application embodiment includes the positive electrode active material of the first aspect, or the positive electrode active material prepared according to the method of the second aspect. When applied to a secondary battery, it can improve the cycle performance of the secondary battery and extend the cycle life of the secondary battery.

[0109] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0110] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0111] In some embodiments, the positive electrode film layer may further include a second positive electrode active material, which may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the second positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the embodiments of this application are not limited to these materials, and other conventional materials that can be used as positive electrode active materials for lithium-ion batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0112] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0113] In some embodiments, the positive electrode film may optionally include a conductive agent and an optional dispersant. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0114] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0115] Electrode assembly

[0116] A fourth aspect of this application provides an electrode assembly. Typically, the electrode assembly includes a positive electrode, a negative electrode, and a separator. In some embodiments, the positive electrode, negative electrode, and separator are fabricated using a winding process or a stacking process.

[0117] [Positive electrode plate]

[0118] The positive electrode of the electrode assembly in this application includes the positive electrode of the third aspect of this application. The embodiments of the positive electrode have been described and illustrated in detail above, and will not be repeated here. It is understood that the electrode assembly in this application can achieve the beneficial effects of any of the above embodiments of the positive electrode of this application.

[0119] [Negative electrode plate]

[0120] In the electrode assembly of this application embodiment, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector and comprising a negative electrode active material. For example, the negative current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0121] In the electrode assembly of this application embodiment, the negative electrode active material can be any negative electrode active material known in the art for use in secondary batteries. As an example, the negative electrode active material may include one or more of graphite, soft carbon, hard carbon, mesophase carbon microspheres, silicon-based materials, tin-based materials, and lithium titanate. Silicon-based materials may include one or more of elemental silicon, silicon oxide, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide, and tin alloys. This application embodiment is not limited to these materials; other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used individually or in combination of two or more.

[0122] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil or aluminum foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0123] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0124] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0125] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0126] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode film layer, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto one or both surfaces of the negative electrode current collector; and obtaining the negative electrode sheet of the present application embodiment after drying, cold pressing and other processes.

[0127] Furthermore, the negative electrode sheet of this application embodiment does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application embodiment may also include a conductive undercoat layer (e.g., composed of a conductive agent and an adhesive) disposed between the negative electrode current collector and the negative electrode film layer. In other embodiments, the negative electrode sheet of this application embodiment also includes a protective layer covering the surface of the negative electrode film layer.

[0128] [Isolation membrane]

[0129] A separator is disposed between the positive and negative electrode plates to provide isolation. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0130] In some embodiments, the material of the separator can be selected from one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0131] Battery

[0132] A fifth aspect of this application provides a battery that includes the electrode assembly of the fourth aspect of this application.

[0133] The battery mentioned in the embodiments of this application may be a single physical module comprising one or more battery cells to provide higher voltage and capacity. When there are multiple battery cells, the multiple battery cells are connected in series, parallel, or mixed via a busbar.

[0134] Generally, a battery cell also includes an electrolyte, which acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid-state.

[0135] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0136] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0137] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0138] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0139] In some embodiments, the casing of the battery cell can be a rigid casing, such as a hard plastic casing, an aluminum casing, or a steel casing. The casing of the battery cell can also be a pouch, such as a pouch-type pouch. The material of the pouch can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0140] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 The example shown is a square-structured battery cell 5.

[0141] In some implementations, refer to Figure 3The outer casing may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0142] The method for preparing the battery cell in this application is well known. In some embodiments, the electrode assembly can be placed in a housing, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, the battery cell is obtained.

[0143] In some implementations, the battery may be a battery module or a battery pack. A battery module or battery pack generally includes a housing for encapsulating one or more individual battery cells. The housing prevents liquids or other foreign matter from affecting the charging or discharging of the individual battery cells.

[0144] In some implementations, a battery module or battery pack may contain multiple battery cells, which can be connected in series, parallel, or a combination thereof. A combination thereof means that multiple battery cells are connected in both series and parallel configurations. Multiple battery cells can be directly connected in series, parallel, or a combination thereof, and then the entire assembly of these battery cells is housed within a housing. Alternatively, multiple battery cells can first be connected in series, parallel, or a combination thereof to form a battery module, and then these battery modules can be connected in series, parallel, or a combination thereof to form a whole, which is then housed within a housing.

[0145] Figure 4 This is a schematic diagram of battery module 4 as an example. Figure 4 As shown, there are multiple battery cells 5, which are connected in series, parallel, or a combination thereof to form a battery module 4. The multiple battery cells 5 in the battery module 4 can be electrically connected through a busbar to achieve the series, parallel, or combination connection. In the battery module 4, the multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other arbitrary manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0146] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0147] Figure 5 and Figure 6 This is a schematic diagram of battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a housing and multiple battery modules 4 disposed within the housing. The multiple battery modules 4 in the battery pack 1 can be electrically connected via a busbar component to achieve series, parallel, or mixed connection. The housing includes an upper housing 2 and a lower housing 3. The upper housing 2 covers the lower housing 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery housing.

[0148] Electrical appliances

[0149] This application also provides an electrical device, which includes a battery provided in this application embodiment. The battery is used to provide electrical energy. The battery can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0150] As the electrical device, a single battery cell, a battery module containing multiple battery cells, or a battery pack can be selected according to its usage requirements.

[0151] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0152] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0153] Example

[0154] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0155] Examples 1-20

[0156] Preparation of positive electrode sheet

[0157] The positive electrode active material, the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black were mixed in a weight ratio of 97:2:1. N-methylpyrrolidone (NMP) was added as a solvent, and the slurry was stirred under vacuum until uniform. The resulting slurry was coated on aluminum foil, and then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0158] The aforementioned positive electrode active material includes a core material and a coating layer, with the coating layer covering at least a portion of the surface of the core material. The core material is lithium manganese iron phosphate particles, and the coating layer includes alternately stacked conductive layers and lithium manganese iron phosphate layers, with the outermost layer of the coating layer furthest from the core material being the conductive layer. The volume distribution particle size Dv of the positive electrode active material is also specified. 1 50. Specific surface area of ​​positive electrode active material, volume distribution and particle size Dv of core material 2 50. The sum of the total number of conductive layers and the total number of lithium manganese iron phosphate layers, n, the average thickness of the conductive layer, d1, the average thickness of the lithium manganese iron phosphate layer, d2, and the mass percentage w of the conductive layer in the positive electrode active material are shown in Table 1.

[0159] The total number of conductive layers and the sum of the total number of lithium manganese iron phosphate layers, n, as well as the average thickness d1 and average thickness d2 of the conductive layers and lithium manganese iron phosphate layers, can be determined by the following steps: The positive electrode active material powder is directly subjected to argon ion cross-section polishing, and an SEM image is taken at a magnification of 5000x; the SEM image is processed, and the contrast is adjusted appropriately until the outline of the positive electrode active material is clearly visible; the cross-section of the positive electrode active material in the SEM image is observed to determine the total number of conductive layers and the sum of the total number of lithium manganese iron phosphate layers, n. The thickness of each conductive layer and the thickness of each lithium manganese iron phosphate layer are counted, and the average values ​​are calculated to obtain the average thickness d1 and average thickness d2 of the conductive layers and lithium manganese iron phosphate layers, respectively.

[0160] Preparation of negative electrode sheet

[0161] The negative electrode active material artificial graphite, conductive agent carbon black, and binder SBR are mixed in a mass ratio of 96:2:2 and dispersed in deionized water. After stirring evenly, a negative electrode slurry is obtained. The negative electrode slurry is evenly coated on the surface of the negative electrode current collector copper foil, dried, and then dried, cold-pressed, and slit to obtain the negative electrode sheet.

[0162] Separating membrane

[0163] Polypropylene film is used as the separator.

[0164] Preparation of electrolyte

[0165] LiPF6 was dissolved in a solvent prepared by mixing ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1 to obtain an electrolyte with a LiPF6 concentration of 1 mol / L.

[0166] Preparation of secondary batteries

[0167] The positive electrode, separator, and negative electrode are stacked in sequence to obtain an electrode assembly. The electrode assembly is placed in a packaging shell and dried for more than 12 hours. After the water content of the electrode is less than 200 ppm, the electrolyte is injected. After vacuum sealing, standing, formation, and shaping, a secondary battery is obtained.

[0168] Comparative Examples 1-2

[0169] Based on the preparation process of the positive electrode, negative electrode, separator, electrolyte and secondary battery in Examples 1 to 20, the positive electrode active materials of Comparative Examples 1 and 2 were adjusted according to Table 1 to prepare the secondary batteries of Comparative Examples 1 and 2.

[0170] The following tests were conducted on Examples 1 to 20 and Comparative Examples 1 to 2, and the test results are shown in Table 2.

[0171] (1) Moisture content test of positive electrode sheet

[0172] In the preparation process of the above-mentioned secondary battery, after the electrode assembly is placed in the packaging shell, the positive electrode sheet is vacuum dried at 100°C for 12 hours, and the moisture content of the positive electrode sheet is tested using a Karl Fischer moisture analyzer (test temperature is 25°C to 170°C).

[0173] (2) Capacity Performance Test

[0174] The above-mentioned positive electrode sheet is combined with lithium sheet to form a coin cell. The coin cell is charged at 0.1C to 4.35V under 2.5-4.35V conditions, and then charged at a constant voltage of 4.35V to a current of 0.05C. After standing for 5 minutes, it is discharged at 0.1C to 2.5V. The ratio of the discharge capacity to the mass of the positive electrode material in the positive electrode sheet is the specific capacity (unit: mAh / g).

[0175] (3) Battery DC internal resistance (DCR) test

[0176] The battery was charged at 25°C at a rate of 0.33C to a voltage of 4.3V, and then constant voltage charging was performed. Charging was stopped when the charging current dropped to the charging termination current of 0.05C, and the battery was left to stand for 1 hour. Subsequently, the battery was discharged at a rate of 0.33C to a voltage of 2.5V, and the reversible capacity C0 was recorded.

[0177] The battery was charged at 25°C at a rate of 0.33C0 until the voltage reached 4.3V, followed by constant voltage charging. Charging was stopped when the charging current dropped to 0.05C0, and the battery was left to rest for 1 hour. Subsequently, the battery was discharged at 0.33C0 for 90 minutes, at which point the battery reached 50% SOC. The voltage at this point was recorded as U. 初始 Then continue discharging at a 4C0 rate for 30 seconds, denoted as I for the discharge current, and record the voltage at the 10th second of discharge as U. 结束 The DCR at the 10th second is calculated using the following formula: DCR = (U 初始 -U 结束 ) / I.

[0178] (4) Room temperature cycling performance test

[0179] At 25℃, the secondary battery is charged at a constant current of 1C to 4.3V, then charged at a constant voltage of 4.3V to a current of 0.05C. After resting for 5 minutes, it is discharged at 1C to 2.5V. The resulting capacity is recorded as the initial capacity C. a Repeat the above steps for the same battery, and record the battery's discharge capacity C after each cycle. n When C n / C a When 100% ≤ 80%, record the corresponding number of cycles.

[0180] Table 1

[0181] Serial Number <![CDATA[Dv 1 50(μm)]]> <![CDATA[Specific surface area (m 2 / g)]]> conductive layer <![CDATA[d1(nm)]]> w <![CDATA[Dv 2 50(nm)]]> <![CDATA[d2(nm)]]> n Example 1 4.9 1.232 carbon 20 0.8％ 600 410 11 Example 2 4.5 1.341 carbon 20 0.8％ 600 460 9 Example 3 2.4 2.515 carbon 20 0.8％ 600 430 5 Example 4 2.0 3.018 carbon 20 1.2％ 600 330 5 Example 5 1.6 3.772 carbon 20 0.8％ 600 460 3 Example 6 4.3 1.403 carbon 20 0.8％ 400 460 9 Example 7 4.7 1.284 carbon 20 0.8％ 800 460 9 Example 8 4.8 1.257 carbon 20 0.8％ 900 460 9 Example 9 4.5 1.341 silver 20 3.1％ 600 460 9 Example 10 4.5 1.345 gold 20 3.2％ 600 460 9 Example 11 4.4 1.372 carbon 10 0.5％ 600 460 9 Example 12 4.4 1.377 carbon 8 0.4％ 600 460 9 Example 13 4.2 1.437 carbon 100 3.3％ 600 460 7 Example 14 4.3 1.410 carbon 120 4％ 600 460 7 Example 15 4.2 1.435 carbon 300 6.8％ 600 460 5 Example 16 4.5 1.341 carbon 350 7.7％ 600 460 5 Example 17 4.8 1.254 carbon 20 0.7％ 600 500 9 Example 18 5.2 1.161 carbon 20 0.5％ 600 550 9 Example 19 4.9 1.227 carbon 20 1.2％ 400 300 15 Example 20 2.2 2.786 carbon 100 3.1％ 800 500 3 Comparative Example 1 0.6 10.058 / / / 600 / / Comparative Example 2 3.4 1.775 / / / 3400 / /

[0182] Table 2

[0183]

[0184] As shown in Tables 1 and 2, the positive electrode active material provided in this application has low hygroscopicity and high specific capacity. When applied to secondary batteries, it can significantly reduce the internal resistance of secondary batteries and improve their cycle life.

[0185] Based on the test results of Examples 1-5, the water content of the positive electrode sheet is related to the particle size of the positive electrode active material. A larger particle size results in a smaller specific surface area and a lower water content in the positive electrode sheet. The particle size and conductivity of the positive electrode active material can be adjusted by changing the thickness and total number of the conductive layer and lithium manganese iron phosphate layer. Based on the test results of Examples 2 and 6-8, when the coating layer remains constant, a smaller particle size of the core material is beneficial for further reducing the battery's internal resistance. Based on the test results of Examples 2 and 11-16, as the average thickness of the conductive layer and its proportion in the positive electrode active material increase, the conductivity of the positive electrode active material improves, thereby reducing the battery's internal resistance. An average thickness of the conductive layer within a suitable range allows the positive electrode active material to achieve both high specific capacity and low impedance. Based on the test results of Examples 2 and 17-18, when other conditions remain essentially unchanged, a smaller average thickness of the lithium iron phosphate layer is beneficial for further improving the conductivity of the positive electrode active material, thereby further reducing the battery's internal resistance. In addition, based on the test results of Examples 1-18, it can be seen that the water content of the positive electrode sheet and the internal resistance of the battery both have a certain impact on the cycle performance of the secondary battery. By adjusting the intrinsic structure of the positive electrode active material, the water content of the positive electrode sheet and the internal resistance of the battery can be kept within a suitable range, thereby effectively improving the cycle performance of the secondary battery.

[0186] In contrast, Comparative Example 1 used uncoated lithium manganese iron phosphate with smaller particle size, resulting in a significantly higher water content in the positive electrode compared to Examples 1-18, leading to a significant decrease in the cycle performance of the secondary battery. Comparative Example 2 used lithium manganese iron phosphate coated with a single conductive layer, which had a larger particle size, resulting in a lower water content in the positive electrode. However, due to the larger core particle size of the single-layer conductive layer, the conductive layer was insufficient to improve the overall conductivity of the material. Therefore, the internal resistance of the secondary battery in Comparative Example 2 was significantly higher than that in Examples 1-18, and its cycle performance was also unsatisfactory.

[0187] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode active material, comprising: a core material, the core material comprising lithium iron manganese phosphate particles; and a coating layer, the coating layer being coated on at least part of a surface of the core material, the coating layer comprising electrically conductive layers and lithium iron manganese phosphate layers being alternately stacked, and the outermost side of the coating layer being away from the core material being the electrically conductive layer. The lithium manganese iron phosphate particles have the general formula LiMn x Fe 1-x PO4, 0.1≤x≤0.9; The lithium manganese iron phosphate layer has the general formula LiMn y Fe 1-y PO4, 0.1≤y≤0.9, y 2. The positive electrode active material according to claim 1, wherein the sum of the total number of the electrically conductive layers and the total number of the lithium iron manganese phosphate layers is equal to or greater than 3.

3. The positive electrode active material according to claim 2, wherein the sum of the total number of the electrically conductive layers and the total number of the lithium iron manganese phosphate layers is 3 to 15.

4. The positive electrode active material according to claim 2, wherein the sum of the total number of the electrically conductive layers and the total number of the lithium iron manganese phosphate layers is 5 to 11.

5. The positive electrode active material according to any one of claims 1 to 4, wherein The volume distribution particle diameter Dv of the positive electrode active material 1 50 greater than or equal to 2 pm; and / or The specific surface area of the positive electrode active material is greater than or equal to 1 m 2 / g.

6. The positive electrode active material according to claim 5, wherein The volume distribution particle diameter Dv of the positive electrode active material 1 50 is 2 μm - 5 μm; and / or The specific surface area of the positive electrode active material is 1 m 2 / g-3m 2 / g.

7. The positive electrode active material according to any one of claims 1 to 6, wherein The volume distribution particle size Dv of the core material 2 50 less than or equal to 800 nm.

8. The positive electrode active material according to claim 7, wherein The volume distribution particle size Dv of the core material 2 50 is 400 nm - 800 nm.

9. The positive electrode active material according to any one of claims 1 to 8, wherein the thickness of a single layer of the electrically conductive layer is 10 nm to 300 nm.

10. The positive electrode active material according to claim 9, wherein the thickness of a single layer of the electrically conductive layer is 10 nm to 100 nm.

11. The positive electrode active material according to any one of claims 1 to 10, wherein the mass percentage content of the electrically conductive layer is 0.5% to 7% based on the total mass of the positive electrode active material.

12. The positive electrode active material according to claim 11, wherein the mass percentage content of the electrically conductive layer is 0.5% to 4% based on the total mass of the positive electrode active material.

13. The positive electrode active material according to any one of claims 1 to 12, wherein the thickness of a single layer of the lithium iron manganese phosphate layer is equal to or less than 500 nm.

14. The positive electrode active material according to claim 13, wherein the thickness of a single layer of the lithium iron manganese phosphate layer is 300 nm to 500 nm.

15. The positive electrode active material according to any one of claims 1 to 14, wherein the electrically conductive layer comprises one or more of electrically conductive carbon materials, metals.

16. The positive electrode active material according to claim 15, wherein the electrically conductive carbon materials comprise one or more of carbon nanotubes, graphene, carbon fibers, ketjen black, acetylene black, porous carbon materials, electrically conductive graphite.

17. The positive electrode active material according to claim 15, wherein the metals comprise one or more of silver, platinum, gold, palladium, rhodium, iridium, osmium, ruthenium.

18. A method for producing the positive electrode active material according to any one of claims 1 to 17, comprising: providing a core material, the core material comprising lithium iron manganese phosphate particles; The coating layer is formed on the surface of the inner core material by solid phase sintering, to obtain a positive electrode active material, the coating layer comprises conductive layers and lithium manganese iron phosphate layers which are alternately stacked, and the outermost side of the coating layer away from the inner core material is the conductive layer; The lithium manganese iron phosphate particles have the general formula LiMn x Fe 1-x PO4, 0.1≤x≤0.9; The lithium manganese iron phosphate layer has the general formula LiMn y Fe 1-y PO4, 0.1≤y≤0.9, y 19. The method of claim 18, wherein, The forming of the coating layer on the surface of the inner core material by solid phase sintering comprises: The conductive layers and the lithium manganese iron phosphate layers which are alternately stacked are formed on the surface of the inner core material by solid phase sintering; The conductive layer is obtained by sintering a conductive material precursor, the conductive material precursor comprises one or more of a carbon source and a metal source; The lithium manganese iron phosphate layer is obtained by sintering a lithium manganese iron phosphate precursor, the lithium manganese iron phosphate precursor comprises lithium manganese iron phosphate and a lithium source.

20. A positive electrode sheet comprising a positive electrode current collector and a positive electrode film layer on at least one side of the positive electrode current collector, the positive electrode film layer comprising the positive electrode active material according to any one of claims 1-17, or the positive electrode active material prepared according to the method of claim 18 or 19.

21. An electrode assembly comprising the positive electrode sheet according to claim 20.

22. A battery comprising a plurality of electrode assemblies according to claim 21.

23. An electric device comprising the battery according to claim 22, the battery being used to provide electric energy.

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