Lithium-ion secondary battery and electric device

Abstract

A lithium-ion secondary battery and an electric device. The lithium-ion secondary battery comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer, the positive electrode film layer comprises a positive electrode active material, and the positive electrode active material comprises a lithium transition metal oxide and lithium transition metal phosphate; the volume-based particle size distribution curve of the positive electrode active material has at least two volume distribution peaks, the volume-based particle size corresponding to the maximum peak intensity of the volume distribution peak having the highest peak intensity is denoted as Dv1, and the volume-based particle size corresponding to the maximum peak intensity of the volume distribution peak having the second highest peak intensity is denoted as Dv2, where 3.0 μm≤Dv1≤10 μm, and 0.35 μm≤Dv2≤0.55 μm; and the porosity of the positive electrode film layer is 10-18%. The lithium-ion secondary battery has a relatively high energy density and a relatively long service life.

Description

Lithium-ion secondary batteries and electrical devices

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese patent application filed on November 13, 2024, entitled "Lithium-ion Secondary Battery and Power Consumption Device", application number: 202411621835.9, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of batteries, specifically to lithium-ion secondary batteries and electrical devices. Background Technology

[0004] In recent years, lithium-ion rechargeable batteries have been widely used in energy storage systems for hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. With the continuous expansion of applications for lithium-ion rechargeable batteries, the demand for their energy density and lifespan is also increasing. Therefore, related technologies for lithium-ion rechargeable batteries still need improvement. Summary of the Invention

[0005] In view of the above problems, this application provides a lithium-ion secondary battery and an electrical device, which has high energy density, long service life or suitable fast charging performance.

[0006] In a first aspect, this application provides a lithium-ion secondary battery. According to an embodiment of this application, the lithium-ion secondary battery includes a positive electrode and a negative electrode; the positive electrode includes a positive current collector and a positive electrode film disposed on the positive current collector, the positive electrode film including a positive electrode active material, the positive electrode active material including lithium transition metal oxide and lithium transition metal phosphate; the lithium transition metal oxide includes nickel, cobalt, manganese and / or aluminum, wherein, based on the total molar number of nickel, cobalt, manganese and / or aluminum, the molar percentage of nickel is 50% to 99%; the lithium transition metal phosphate includes manganese and iron, wherein, based on the total molar number of nickel, cobalt, manganese and / or aluminum, the molar percentage of nickel is 50% to 99%; The total molar percentage of manganese and iron is 50%–99%. The volumetric particle size distribution curve of the positive electrode active material has at least two volumetric distribution peaks. The volumetric distribution peak with the maximum peak intensity is denoted as the first peak, and the volumetric particle size corresponding to the maximum peak intensity of the first peak is denoted as Dv1. The volumetric distribution peak with the second maximum peak intensity is denoted as the second peak, and the volumetric particle size corresponding to the maximum peak intensity of the second peak is denoted as Dv2. Wherein, 3.0 μm ≤ Dv1 ≤ 10 μm; 0.35 μm ≤ Dv2 ≤ 0.55 μm; the porosity of the positive electrode film is 10%–18%.

[0007] By adjusting the volume particle size distribution of the positive electrode active material to meet the above conditions, the positive electrode active material can be tightly packed, increasing the actual packing density of the positive electrode active material, thereby increasing the compaction density of the positive electrode film and giving the positive electrode film the above porosity. The above settings can enable lithium-ion secondary batteries to have higher energy density and longer service life.

[0008] According to some specific embodiments of this application, 3.7μm≤Dv1≤9.2μm; 0.46μm≤Dv2≤0.49μm.

[0009] According to some specific embodiments of this application, the porosity of the positive electrode film is 12.3% to 17.9%.

[0010] Meeting the above-mentioned volumetric particle size distribution conditions and / or positive electrode film porosity can further improve the packing density of the positive electrode active material, thereby increasing the energy density and lifespan of lithium-ion secondary batteries.

[0011] According to an embodiment of this application, the compaction density of the positive electrode film is 2.5 g / cm³. 3 ~3.5g / cm 3 When the compaction density of the positive electrode film is within the above range, lithium-ion secondary batteries can have high energy density while also having a long service life.

[0012] According to an embodiment of this application, the areal density of the positive electrode film is 0.015 g / mm². 2 ~0.021g / mm 2 Having the areal density of the positive electrode film within the above-mentioned range is beneficial for improving the energy density of lithium-ion secondary batteries, while also ensuring a longer service life.

[0013] In some embodiments, the volumetric particle size distribution (Dv50) of lithium transition metal phosphate is 0.3 μm to 0.7 μm. In some embodiments, the volumetric particle size distribution (Dv50) of lithium transition metal oxide is 3 μm to 10 μm. This allows for a more compact packing of lithium transition metal oxide and lithium transition metal phosphate, further increasing the actual packing density of the positive electrode active material and the compaction density of the positive electrode sheet, thereby enabling the lithium-ion secondary battery to have higher energy density and / or longer lifespan.

[0014] According to an embodiment of this application, the manganese content in the positive electrode film layer is 13% to 21%. Limiting the manganese content within this range is beneficial to improving the stability and density of the positive electrode active material, thereby enhancing the cycle and storage performance of the lithium-ion secondary battery; and it can maintain a high voltage platform to improve the energy density of the lithium-ion secondary battery.

[0015] According to an embodiment of this application, the iron content in the positive electrode film layer is 0.1% to 20%. Limiting the iron content within this range, while maintaining energy density, is beneficial for improving the fast charging performance of lithium-ion secondary batteries at low SOC and the cycle performance of lithium-ion secondary batteries.

[0016] According to an embodiment of this application, the nickel content in the positive electrode film is 20% to 85%. Limiting the nickel content within the above range allows the positive electrode active material to have a higher capacity, thereby improving the energy density of the lithium-ion secondary battery, while also ensuring the structural stability of the positive electrode active material, resulting in a longer service life for the lithium-ion secondary battery.

[0017] According to embodiments of this application, the mass percentage of lithium transition metal phosphate in the positive electrode active material is less than or equal to 50%. According to some specific embodiments of this application, the mass percentage of lithium transition metal phosphate in the positive electrode active material is 5% to 40%. Within this content range, lithium transition metal phosphate can effectively improve the structural stability of the positive electrode active material, thereby enhancing the cycle performance and lifespan of the lithium-ion secondary battery. It can also significantly improve the fast-charging performance of the lithium-ion secondary battery at low SOC, and help reduce the cost of the lithium-ion secondary battery.

[0018] According to embodiments of this application, the lithium transition metal phosphate has an olivine structure. The olivine-structured lithium transition metal phosphate exhibits excellent structural stability, providing outstanding cycle stability and thermal stability for lithium-ion secondary batteries. During the charge and discharge process of the lithium-ion secondary battery, the volume change of the olivine-structured lithium transition metal phosphate is relatively small, and it undergoes almost no structural change at low temperatures (e.g., below 400°C). This allows the lithium-ion secondary battery to exhibit good performance over a wide temperature range. Consequently, the safety of the lithium-ion secondary battery can be significantly improved, greatly reducing the risk of thermal runaway. Furthermore, the olivine-structured lithium transition metal phosphate is easily doped and coated for modification, thereby further enhancing its conductivity and energy density.

[0019] According to embodiments of this application, the morphology of the lithium transition metal phosphate includes single crystals and / or polycrystalline structures. That is, the lithium transition metal phosphate can be single crystal particles, polycrystalline particles, or a mixture of single crystal particles and polycrystalline particles.

[0020] According to embodiments of this application, the lithium transition metal phosphate includes lithium manganese iron phosphate. Using lithium manganese iron phosphate is beneficial for improving the energy density, cycle life, thermal stability, and safety of lithium-ion secondary batteries, and the cost is relatively low.

[0021] According to embodiments of this application, the lithium transition metal phosphate includes the compound shown in Formula 2: Li x2 Mn y2 Fe y3 P z2 O a2 Q b2 Equation 2

[0022] Wherein, Q includes at least one of Zn, Al, Na, K, Ti, V, Mg, Nb, Mo, W, B, S, Si, N, F, Cl, and Br; 0.8≤x²≤1.2, 0.1≤y²≤0.9, 0.1≤y³≤0.9, 0.9≤z²≤1, 3.5≤a²≤4, and 0≤b²≤0.2.

[0023] By using the compound shown in Formula 2, the leaching of transition metal elements is reduced, while lithium-ion secondary batteries can achieve improved rate performance, cycle performance, and / or high-temperature stability, and the energy density is also increased.

[0024] According to embodiments of this application, in the lithium transition metal phosphate, the molar percentage of manganese, based on the total molar number of manganese and iron, is 0.65 to 0.75. Within this range, the lithium transition metal phosphate exhibits higher energy density, a higher voltage plateau, and better cycle life.

[0025] According to embodiments of this application, the lithium transition metal oxide has a layered structure. Layered lithium transition metal oxides have high compaction density, but are relatively expensive. During use in lithium-ion secondary batteries, they exhibit relatively more side reactions and relatively poor crystal structure stability. By rationally combining lithium transition metal phosphates and lithium transition metal oxides, the actual packing density of the positive electrode active material can be increased, thereby improving the energy density and lifespan of the lithium-ion secondary battery.

[0026] According to embodiments of this application, the lithium transition metal oxide comprises single-crystal particles. Using a single-crystal lithium transition metal oxide allows the lithium-ion secondary battery to have a higher voltage, which is beneficial for improving fast-charging performance.

[0027] According to embodiments of this application, the lithium transition metal oxide comprises a nickel-cobalt-manganese ternary cathode material and / or a nickel-cobalt-aluminum ternary cathode material. This results in high energy density, good cycle performance, and moderate cost.

[0028] According to some specific embodiments of this application, in the lithium transition metal oxide, based on the total molar number of nickel, cobalt, manganese, and / or aluminum, the molar percentage of nickel is 50%–60%, 60%–70%, or 80%–90%. In other words, the lithium transition metal oxide can be a 5-series ternary cathode material, a 6-series ternary cathode material, or an 8-series ternary cathode material. This satisfies the requirements for energy density and high voltage.

[0029] According to embodiments of this application, the lithium transition metal oxide comprises a compound of Formula 1: Li x1 (Ni a1 Co b1 Mn c1 ) d1 M 1-d1 O y1 A z1 Formula 1

[0030] Wherein, 0.8≤x1≤1.2, 0.3≤a1≤0.9, 0.01≤b1≤0.15, 0.15≤c1≤0.55, a1+b1+c1=1, 0.95≤d1≤1, 1.9≤y1≤2.1, 0≤z1≤0.1;

[0031] M includes one or more of Zr, Sr, B, Ti, Mg, Sn, Mo, W, Sb, Nb, La, and Al;

[0032] A includes one or more of S, N, F, Cl, Br, and I.

[0033] Lithium transition metal oxides, possessing the aforementioned chemical formula, can further enhance the energy density and lifespan of lithium-ion secondary batteries.

[0034] According to embodiments of this application, the lithium transition metal oxide and the lithium transition metal phosphate each independently include a core and a coating layer covering at least a portion of the surface of the core. The coating layer includes one or more of phosphates, pyrophosphates, carbon, doped carbon, oxides, and fast ion conductors.

[0035] Secondly, this application provides an electrical device. According to an embodiment of this application, the electrical device includes the battery described above. This battery possesses all the features and advantages of the lithium-ion secondary battery described above, which will not be repeated here.

[0036] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0037] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0038] Figure 1 is a schematic diagram of the structure of a lithium-ion secondary battery according to some embodiments of this application.

[0039] Figure 2 is a schematic diagram of the structure of an electrical device according to some embodiments of this application.

[0040] Figure 3 shows the particle size distribution curves of the positive electrode active material in some embodiments of this application.

[0041] Reference numerals: 1: Battery; 100: Electrode assembly; 200: Housing; 300: Cover plate. Detailed Implementation

[0042] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0044] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0045] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0046] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists, A and B exist simultaneously, and B exists. In addition, the character " / " in this document generally indicates that the related objects before and after it have an "or" relationship.

[0047] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0048] With the widespread application of lithium-ion rechargeable batteries, higher demands are being placed on their energy density, lifespan, fast-charging performance, and cost. Currently, the performance of lithium-ion rechargeable batteries is mainly limited by the positive electrode active material. Therefore, developing positive electrode active materials that meet the requirements of high energy density, long lifespan, good fast-charging performance, or low cost is an urgent problem to be solved.

[0049] In view of this, this application provides a lithium-ion secondary battery that, by employing a suitable positive electrode active material, achieves a balance between high energy density, low cost, and good service life.

[0050] The lithium-ion secondary batteries provided in this application include, but are not limited to, stacked lithium-ion secondary batteries and wound lithium-ion secondary batteries; their shape can be square, cylindrical, or other regular or irregular shapes; they can be hard-shell batteries, soft-pack batteries, or cylindrical batteries; and the batteries can exist in the form of individual battery cells or be assembled into battery modules, battery packs, etc. These lithium-ion secondary batteries can be used as power sources or energy storage systems in electrical devices such as vehicles, power tools, mobile terminals, and aircraft.

[0051] It is understood that the most basic unit of a lithium-ion secondary battery can include an electrode assembly and an electrolyte (specifically, an electrolyte solution in this embodiment). The electrode assembly typically includes a positive electrode, a negative electrode, and a separator. The positive and negative electrode sheets are stacked sequentially, with a separator between them for isolation, thus obtaining the electrode assembly. Alternatively, the electrode assembly can be obtained by winding the stacked electrodes. The electrode assembly is then placed in an outer packaging, filled with electrolyte, and encapsulated to obtain a lithium-ion secondary battery.

[0052] During the charging and discharging process of a lithium-ion secondary battery, lithium ions are repeatedly extracted and inserted between the positive active material in the positive electrode and the negative active material in the negative electrode. The electrolyte is immersed in the electrode assembly and mainly plays the role of conducting active ions between the positive and negative electrodes.

[0053] The lithium-ion secondary battery and power-consuming device provided in the embodiments of this application are described in detail below.

[0054] In a first aspect, this application provides a lithium-ion secondary battery. According to an embodiment of this application, the lithium-ion secondary battery includes a positive electrode and a negative electrode; the positive electrode includes a positive current collector and a positive electrode film disposed on the positive current collector, the positive electrode film including a positive electrode active material, the positive electrode active material including lithium transition metal oxide and lithium transition metal phosphate; the lithium transition metal oxide includes nickel, cobalt, manganese and / or aluminum, wherein, based on the total molar number of nickel, cobalt, manganese and / or aluminum, the molar percentage of nickel is 50% to 99%; the lithium transition metal phosphate includes manganese and iron, wherein, based on the total molar number of nickel, cobalt, manganese and / or aluminum, the molar percentage of nickel is 50% to 99%; The total molar percentage of manganese and iron is 50%–99%. The volumetric particle size distribution curve of the positive electrode active material has at least two volumetric distribution peaks. The volumetric distribution peak with the maximum peak intensity is denoted as the first peak, and the volumetric particle size corresponding to the maximum peak intensity of the first peak is denoted as Dv1. The volumetric distribution peak with the second maximum peak intensity is denoted as the second peak, and the volumetric particle size corresponding to the maximum peak intensity of the second peak is denoted as Dv2. Wherein, 3.0 μm ≤ Dv1 ≤ 10 μm; 0.35 μm ≤ Dv2 ≤ 0.55 μm; the porosity of the positive electrode film is 10%–18%.

[0055] Specifically, Dv1 can be 3.1μm, 3.2μm, 3.3μm, 3.4μm, 3.5μm, 3.6μm, 3.7μm, 3.8μm, 3.9μm, 4.0μm, 4.1μm, 4.2μm, 4.3μm, 4.4μm, 4.5μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc.; Dv2 can be 0.35μm, 0.38μm, 0.4μm, 0.4μm, etc. The thicknesses are 2μm, 0.45μm, 0.48μm, 0.5μm, 0.51μm, 0.52μm, 0.53μm, 0.54μm, 0.55μm, etc.; the porosity of the positive electrode film can be 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, etc.

[0056] In this paper, the particle size distribution curve of the positive electrode active material can be determined using a laser particle size analyzer in accordance with GB / T 19077-2016. The testing instrument can be the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK. For lithium transition metal oxides, deionized water can be used as the solvent, and the material can be sonicated for 5 minutes before testing; for lithium transition metal phosphates, alcohol can be used as the solvent, and the material can be sonicated for 30 minutes before testing.

[0057] The porosity of the positive electrode film refers to the proportion of void volume to the total volume in the positive electrode film. It can be detected using conventional methods in the field. As an example, a sample cup containing the sample can be placed in a true density analyzer, the testing system sealed, helium gas introduced according to a program, and the pressure of the gas in the sample chamber and expansion chamber measured. The true volume is then calculated using Bohr's Law (PV = nRT), thus obtaining the porosity of the sample under test.

[0058] By adjusting the volume particle size distribution of the positive electrode active material to meet the above conditions, the positive electrode active material can be tightly packed, increasing the actual packing density of the positive electrode active material, thereby increasing the compaction density of the positive electrode film and giving the positive electrode film the above porosity. The above settings can enable lithium-ion secondary batteries to have higher energy density and longer service life.

[0059] According to some specific embodiments of this application, 3.7μm≤Dv1≤9.2μm; 0.46μm≤Dv2≤0.49μm.

[0060] According to some specific embodiments of this application, the porosity of the positive electrode film is 12.3% to 17.9%.

[0061] Meeting the above-mentioned volumetric particle size distribution conditions and / or positive electrode film porosity can further improve the packing density of the positive electrode active material, thereby increasing the energy density and lifespan of lithium-ion secondary batteries.

[0062] According to an embodiment of this application, the compaction density of the positive electrode film is 2.5 g / cm³. 3 ~3.5g / cm 3 Specifically, the compaction density of the positive electrode film can be 2.5 g / cm³. 3 2.55g / cm 3 2.6g / cm 3 2.65g / cm 3 2.7g / cm 3 2.75g / cm 3 2.8g / cm 3 2.85g / cm 3 2.9g / cm 32.95g / cm 3 3.0g / cm 3 3.05g / cm 3 3.1g / cm 3 3.15g / cm 3 3.2g / cm 3 3.25g / cm 3 3.3g / cm 3 3.35g / cm 3 3.4g / cm 3 3.45g / cm 3 3.5g / cm 3 wait.

[0063] The compaction density of the positive electrode film can be detected using conventional methods in the art. As an example, when the positive electrode is coated on one side, the compaction density of the positive electrode film on one side of the positive electrode is m / (V1-V2). When the positive electrode is coated on both sides, the compaction density of the positive electrode film on one side of the positive electrode is m / [2×(V1-V2)], where m represents the weight of the positive electrode film, V1 represents the volume of the positive electrode, and V2 represents the volume of the positive current collector. Here, m can be obtained by subtracting the weight of the positive current collector from the weight of the positive electrode. The product of the surface area and thickness of the positive electrode is the volume V1 of the positive electrode, and the product of the surface area and thickness of the positive current collector is V2. The thickness of the positive current collector and the thickness of the positive electrode are obtained by measuring the thickness of the empty foil in the positive electrode tab area using a micrometer.

[0064] When the compaction density of the positive electrode film is within the above range, lithium-ion secondary batteries can have a high energy density while also having a long service life.

[0065] According to an embodiment of this application, the areal density of the positive electrode film is 0.015 g / mm². 2 ~0.021g / mm 2 Specifically, the areal density of the positive electrode film can be 0.0155 g / mm². 2 0.016g / mm 2 0.0165g / mm 2 0.017g / mm 2 0.0175g / mm 2 0.018g / mm 2 0.0185g / mm 2 0.019g / mm 2 0.0195g / mm 2 0.02g / mm 2 0.0205g / mm 2 0.021g / mm2 wait.

[0066] Areal density refers to the mass per unit area of ​​a material of a certain thickness. The areal density of the positive electrode film in this paper can be measured using conventional methods in the art. As an example, a positive electrode sheet of a predetermined area can be cut, and its mass can be weighed using an electronic balance. Then, the areal density of the positive electrode film = (mass of the positive electrode sheet of the predetermined area - mass of the positive current collector of the predetermined area) / predetermined area.

[0067] Having the areal density of the positive electrode film within the above range is beneficial for improving the energy density of lithium-ion secondary batteries, while also taking into account their service life.

[0068] It is understandable that by adjusting one or more parameters such as the volume distribution particle size, volume particle size distribution width, and mass content of lithium transition metal oxide and lithium transition metal phosphate, the volume particle size distribution curve of the positive electrode active material, the porosity of the positive electrode film, the compaction density of the positive electrode active material, the compaction density of the positive electrode film, and the areal density of the positive electrode film can be adjusted.

[0069] In some embodiments, the volume distribution particle size Dv50 of the lithium transition metal phosphate is 0.3 μm to 0.7 μm. Specifically, it includes 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, and 0.7 μm.

[0070] In some embodiments, the volumetric particle size Dv50 of the lithium transition metal oxide is 3 μm to 10 μm. Specifically, it includes sizes such as 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, and 10 μm.

[0071] This allows lithium transition metal oxides and lithium transition metal phosphates to be packed more tightly, further increasing the actual packing density of the positive electrode active material and the compaction density of the positive electrode sheet, thereby enabling lithium-ion secondary batteries to have higher energy density and / or longer service life.

[0072] The volumetric particle size distribution (Dv50) of materials (e.g., lithium transition metal oxides, lithium transition metal phosphates, etc.) has a well-known meaning in the art, representing the particle size corresponding to a cumulative volumetric distribution percentage of 50%, and can be determined using instruments and methods known in the art. For example, it can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK. During testing, deionized water can be used as the solvent for lithium transition metal oxides, and the material can be sonicated for 5 minutes before testing; alcohol can be used as the solvent for lithium transition metal phosphates, and the material can be sonicated for 30 minutes before testing.

[0073] According to embodiments of this application, the manganese content in the positive electrode film layer is 13% to 21%, specifically 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%, 20.5%, and 21%. Limiting the manganese content within the above range is beneficial for improving the stability and density of the positive electrode active material, thereby enhancing the cycle and storage performance of the lithium-ion secondary battery; and it also helps maintain a high voltage platform, thus improving the energy density of the lithium-ion secondary battery.

[0074] According to embodiments of this application, the iron content in the positive electrode film layer is 0.1% to 20%, specifically 0.1%, 0.5%, 1%, 0.5%, 10%, 15%, 20%, etc. Limiting the iron content within the above range, while maintaining energy density, is beneficial for improving the fast-charging performance of lithium-ion secondary batteries at low SOC states and the cycle performance of lithium-ion secondary batteries.

[0075] According to embodiments of this application, the nickel content in the positive electrode film layer is 20% to 85%, specifically 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, and 85%. Limiting the nickel content within this range allows the positive electrode active material to have a higher capacity, thereby improving the energy density of the lithium-ion secondary battery. Simultaneously, it maintains the structural stability of the positive electrode active material, resulting in a longer service life for the lithium-ion secondary battery.

[0076] In the lithium-ion secondary battery of this application embodiment, the positive electrode active material includes lithium transition metal phosphate and lithium transition metal oxide. Specifically, lithium transition metal oxide has higher capacity and compaction density, which is beneficial to improving the energy density of lithium-ion secondary batteries, while lithium transition metal phosphate has better structural stability, which is beneficial to improving the service life, cycle performance and fast charging performance at low SOC state of lithium-ion secondary batteries. At the same time, lithium transition metal phosphate has lower manufacturing cost, which is beneficial to reducing the cost of lithium-ion secondary batteries and improving economic efficiency.

[0077] According to embodiments of this application, the mass percentage of lithium transition metal phosphate in the positive electrode active material is less than or equal to 50%. According to some specific embodiments of this application, the mass percentage of lithium transition metal phosphate in the positive electrode active material is 5% to 40%. As an example, the mass percentage of lithium transition metal phosphate in the positive electrode active material can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc. Within this content range, lithium transition metal phosphate can effectively improve the structural stability of the positive electrode active material, thereby improving the cycle performance and lifespan of the lithium-ion secondary battery. It can also significantly improve the fast-charging performance of the lithium-ion secondary battery under low SOC conditions and help reduce the cost of the lithium-ion secondary battery. Figure 3 shows the particle size distribution curves of the positive electrode active material with different mass percentages of lithium transition metal phosphate. The mass percentages shown in Figure 3 represent the mass percentage of lithium transition metal phosphate in the positive electrode active material.

[0078] According to embodiments of this application, the lithium transition metal phosphate has an olivine structure. The olivine-structured lithium transition metal phosphate exhibits excellent structural stability, providing outstanding cycle stability and thermal stability for lithium-ion secondary batteries. During the charge and discharge process of the lithium-ion secondary battery, the volume change of the olivine-structured lithium transition metal phosphate is relatively small, and it undergoes almost no structural change at low temperatures (e.g., below 400°C). This allows the lithium-ion secondary battery to exhibit good performance over a wide temperature range. Consequently, the safety of the lithium-ion secondary battery can be significantly improved, greatly reducing the risk of thermal runaway. Furthermore, the olivine-structured lithium transition metal phosphate is easily doped and coated for modification, thereby further enhancing its conductivity and energy density.

[0079] According to embodiments of this application, the morphology of the lithium transition metal phosphate includes single crystals and / or polycrystalline structures. That is, the lithium transition metal phosphate can be single crystal particles, polycrystalline particles, or a mixture of single crystal particles and polycrystalline particles.

[0080] In this article, "single crystal" should be interpreted broadly, encompassing both single-crystal particles (i.e., primary particles) and quasi-single crystals (also known as near-single crystals). Quasi-single crystals (or near-single crystals) typically refer to particles formed by the aggregation of a small number of primary particles. Polycrystalline refers to secondary particles formed by the aggregation of multiple primary particles.

[0081] It is understandable that single-crystal lithium transition metal phosphate particles are composed of individual primary particles or agglomeration of a small number of primary particles, while polycrystalline lithium transition metal phosphate particles are composed of agglomeration of multiple primary particles. In other words, single-crystal and polycrystalline morphologies have a certain influence on lithium transition metal phosphate particles.

[0082] In some embodiments of this application, the lithium transition metal phosphate may include single-crystal particles. For example, the proportion of single-crystal particles in the lithium transition metal phosphate may be above 80% or 90%. This allows the lithium transition metal phosphate to have an appropriate specific surface area and hygroscopicity, making it more suitable for processing conditions and environmental requirements, and thus helping to reduce the production cost of lithium-ion secondary batteries.

[0083] In some embodiments of this application, the powder compaction density P1 of lithium transition metal phosphate at 30000N is 1.89 g / cm³. 3 The above is an example; the compaction density P1 of lithium transition metal phosphate powder at 30000N is 2.2 g / cm³. 3 Above and 2.8g / cm 3 Below or 2.2g / cm 3 Above and 2.65g / cm 3 The following; specifically, 1.89 g / cm³ 3 1.9g / cm 3 1.95g / cm 3 2g / cm 3 2.05g / cm 3 2.1g / cm 3 2.15g / cm 3 2.2g / cm 3 2.25g / cm 3 2.3g / cm 3 2.35g / cm 3 2.4g / cm 3 2.45g / cm 3 2.5g / cm 3 2.55g / cm 3 2.6g / cm 3 2.65g / cm 3 When lithium transition metal phosphates have the above-mentioned powder compaction density, it is beneficial for lithium-ion secondary batteries to have higher energy density.

[0084] The compaction density of powder materials (such as lithium transition metal phosphates, lithium transition metal oxides, etc.) can be measured according to GB / T24533-2009.

[0085] According to embodiments of this application, the lithium transition metal phosphate includes lithium manganese iron phosphate material. The term "lithium manganese iron phosphate material" should be interpreted broadly, including both unmodified intrinsic lithium manganese iron phosphate and modified lithium manganese iron phosphate material obtained by modifying intrinsic lithium manganese iron phosphate (e.g., doping, coating).

[0086] Using lithium manganese iron phosphate material is beneficial for improving the energy density, cycle life, thermal stability and safety of lithium-ion secondary batteries, and the cost is relatively low.

[0087] According to embodiments of this application, the lithium transition metal phosphate includes the compound shown in Formula 2: Li x2 Mn y2 Fe y3 P z2 O a2 Q b2 Equation 2

[0088] Wherein, Q includes at least one of Zn, Al, Na, K, Ti, V, Mg, Nb, Mo, W, B, S, Si, N, F, Cl, and Br; 0.8≤x²≤1.2, 0.1≤y²≤0.9, 0.1≤y³≤0.9, 0.9≤z²≤1, 3.5≤a²≤4, and 0≤b²≤0.2.

[0089] By using the compound shown in Formula 2, the leaching of transition metal elements is reduced, while lithium-ion secondary batteries can achieve improved rate performance, cycle performance, and / or high-temperature stability, and the energy density is also increased.

[0090] According to embodiments of this application, in the lithium transition metal phosphate, the molar percentage of manganese, based on the total molar number of manganese and iron, is 65% to 75%. As an example, the molar percentage of manganese in the lithium transition metal phosphate, based on the total molar number of manganese and iron, can specifically be 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, etc. Within the above ratio range, the lithium transition metal phosphate exhibits higher energy density, a higher voltage plateau, and better cycle life.

[0091] According to embodiments of this application, lithium transition metal phosphates can have a core-shell structure, meaning the lithium transition metal phosphate particles include a core and a shell covering the core, the core comprising the compound shown in Formula 2 above. Specifically, the shell may include one or more coating layers. Each coating layer has ionic conductivity and / or electronic conductivity. In practice, each coating layer can be fully or partially coated. This can further improve the high-temperature cycle performance, cycle stability, and high-temperature storage performance of lithium-ion secondary batteries.

[0092] According to embodiments of this application, each of the above-mentioned one or more coating layers independently comprises one or more of pyrophosphate, phosphate, carbon, doped carbon, oxide, boride, and polymer. The term "polymer" can be either an oligomer or a high polymer, and this application embodiment is not limited in this regard.

[0093] According to some specific embodiments of this application, the shell includes a coating layer, namely, the lithium transition metal phosphate particle includes a core and a first coating layer covering the core. According to other specific embodiments of this application, the shell includes two coating layers, namely, the lithium transition metal phosphate particle includes a core, a first coating layer covering the core, and a second coating layer covering the first coating layer. According to other specific embodiments of this application, the shell includes three coating layers, namely, the lithium transition metal phosphate particle includes a core, a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer.

[0094] In some specific embodiments, when the shell includes two coating layers, the first coating layer includes one or more selected from pyrophosphate, phosphate, oxide and boride, and the second coating layer may include one or more selected from carbon and doped carbon.

[0095] In some specific embodiments, when the shell includes three coating layers, the first coating layer includes pyrophosphate, the second coating layer includes one or more selected from phosphate, oxide and boride, and the third coating layer includes one or more selected from carbon and doped carbon.

[0096] According to embodiments of this application, the pyrophosphate includes M′ b (P2O7) c ; and / or,

[0097] Phosphates include X m (PO4) q ; and / or,

[0098] The doping elements in carbon include one or more of Group IIIA, Group VA, Group VIA, and Group VIIA; and / or,

[0099] Oxides include M″ d Oe ; and / or,

[0100] Borides include Z v B w ; and / or,

[0101] Polymers include one or more selected from polysaccharides and their derivatives, and polysiloxanes;

[0102] M′, X, and Z each independently include one or more elements selected from Groups IA, IIA, IIIA, IB, IIB, IVB, IVB, VB, VIIB, and VIII;

[0103] b is selected from the range 1 to 4, c is selected from the range 1 to 6; m is selected from the range 1 to 2, q is selected from the range 1 to 4;

[0104] M″ includes one or more elements selected from alkali metals, alkaline earth metals, transition metals, Group IIIA elements, Group IVA elements, lanthanides, and Sb.

[0105] d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5;

[0106] v is selected from the range 1 to 7, and w is selected from the range 1 to 2.

[0107] In some specific embodiments, M′, X, and Z each independently include one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, Mn, and Al; and / or,

[0108] The doping elements in carbon include one or more selected from nitrogen, phosphorus, sulfur, boron, and fluorine; and / or,

[0109] M″ includes one or more elements selected from Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce, and may optionally include one or more elements selected from Mg, Al, Si, Zn, Zr, and Sn; and / or,

[0110] The polysiloxane is selected from one or more of linear polysiloxanes and cyclic polysiloxanes; and / or,

[0111] The polysaccharide is selected from one or more plant polysaccharides and marine polysaccharides.

[0112] By using the above materials as a coating layer, the dissolution of transition metal elements can be further reduced, the specific capacity and compaction density of transition metal phosphates can be further improved, and the rate performance, high-temperature cycle performance and high-temperature storage performance of lithium-ion secondary batteries can be further improved.

[0113] In some embodiments, one or more of the above-mentioned pyrophosphate, phosphate, and oxide are in a crystalline state.

[0114] In some embodiments, the crystallinity of the pyrophosphate, phosphate, and oxide is each independently 10% to 100%, and more preferably 50% to 100%.

[0115] In this document, "crystalline" means a crystallinity of 50% or higher, i.e., 50%-100%. That is, the presence of crystalline pyrophosphate and crystalline phosphate in this application indicates a crystallinity of 50% to 100%.

[0116] Pyrophosphate and phosphate with a certain degree of crystallinity not only help to fully utilize the pyrophosphate coating to reduce the dissolution of transition metals and the excellent lithium-ion conduction ability of the phosphate coating, as well as the function of reducing interfacial side reactions, but also enable the pyrophosphate coating and phosphate coating to better match their crystal lattices, thereby achieving a tight bond between the coatings.

[0117] It should be noted that crystallinity can be adjusted, for example, by modifying the process conditions of the sintering process, such as sintering temperature and sintering time. Crystallinity can be measured using methods known in the art, such as X-ray diffraction, density method, infrared spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance absorption method. Specifically, a method for testing the crystallinity of lithium transition metal phosphates using X-ray diffraction may include the following steps:

[0118] A certain amount of lithium transition metal phosphate powder is taken, and the total scattering intensity is measured by X-rays. It is the sum of the scattering intensities of all matter in space. It is only related to the intensity of the primary rays, the chemical structure of the lithium transition metal phosphate powder, and the total number of electrons participating in diffraction, i.e., the mass, and is independent of the order state of the sample. Then, crystalline scattering and non-crystalline scattering are separated from the diffraction pattern. The crystallinity is the ratio of the scattering of the crystalline part to the total scattering intensity.

[0119] In some embodiments, the weight ratio of pyrophosphate to phosphate and the weight ratio of pyrophosphate to oxide in the casing are each independently from 1:3 to 3:1, optionally from 1:3 to 1:1. Thus, by maintaining a suitable weight ratio of pyrophosphate to phosphate or a suitable weight ratio of pyrophosphate to oxide, transition metal dissolution can be effectively reduced, as can the surface impurity lithium content and interfacial side reactions, thereby improving the high-temperature storage performance and high-temperature cycling performance of lithium-ion secondary batteries.

[0120] In some embodiments, one or more coating layers each independently comprise pyrophosphate, the interplanar spacing of which ranges from 0.293 nm to 0.470 nm, optionally from 0.297 nm to 0.462 nm or 0.293 nm to 0.326 nm, more preferably from 0.300 nm to 0.310 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.57°, optionally from 18.00° to 32.00° or 26.41° to 32.57°, more preferably from 19.211° to 30.846°, and further preferably from 29.00° to 30.00°.

[0121] In some embodiments, one or more coating layers each independently comprise a phosphate, the interplanar spacing of which ranges from 0.244 nm to 0.425 nm, optionally from 0.345 nm to 0.358 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°, optionally from 24.25° to 26.45°.

[0122] As an example, the first or second coating layer comprises a phosphate. Both the first and second coating layers in the lithium transition metal phosphate utilize crystalline materials, with their interplanar spacing and angles falling within the aforementioned ranges. This effectively reduces impurity phases in the coating layer, thereby improving the specific capacity, cycle performance, and rate performance of the material.

[0123] In some embodiments, one or more coating layers each independently comprise carbon, and the carbon is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of SP2 carbon to SP3 carbon in the carbon is any value in the range of 0.07-13, more preferably any value in the range of 0.1-10, and even more preferably any value in the range of 2.0-3.0.

[0124] In some embodiments, the molar ratio of SP2 carbon to SP3 carbon may be about 0.1, about 0.2, about 0.3, about 0.4, about 0.5, about 0.6, about 0.7, about 0.8, about 0.9, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9 or about 10, or any range of any of the above values.

[0125] In this application, "about" for a certain value represents a range, specifically a range of ±10% of that value.

[0126] By selecting the form of carbon in the carbon coating layer, the overall electrical performance of lithium-ion secondary batteries can be improved. Specifically, by using a mixture of SP2 and SP3 carbon forms and limiting the ratio of SP2 to SP3 carbon within a certain range, the following situations can be avoided: if the carbon in the coating layer is all amorphous SP3, the conductivity is poor; if it is all graphitized SP2, although the conductivity is good, there are few lithium-ion pathways, which is not conducive to lithium insertion / extraction. In addition, limiting the molar ratio of SP2 to SP3 carbon within the above-mentioned range can achieve both good conductivity and promote lithium-ion transport, thus benefiting the battery's functionality and cycle performance.

[0127] The mixing ratio of SP2 and SP3 carbon can be controlled by sintering conditions such as sintering temperature and sintering time. The molar ratio of SP2 to SP3 carbon can be determined by Raman spectroscopy. The specific test method is as follows: by dividing the Raman spectrum into peaks, the Id / Ig ratio is obtained (where Id is the peak intensity of SP3 carbon and Ig is the peak intensity of SP2 carbon), thus confirming the molar ratio.

[0128] In some embodiments, one or more coating layers independently include doped carbon, and the mass content of the dopant element in the doped carbon is less than 30%; alternatively, the mass content of the dopant element in the doped carbon is less than 20%. Dopant elements within the above-mentioned content range can sufficiently improve the conductivity of the pure carbon layer while effectively avoiding excessive surface activity due to excessive doping, thereby effectively controlling interfacial side reactions caused by excessive doping of the coating layer.

[0129] In some embodiments, one or more coating layers each independently include doped carbon, wherein the doping element is nitrogen and / or sulfur, and the mass content of the doping element in the doped carbon is 1% to 15%; or, the doping element is phosphorus, boron and / or fluorine, and the mass content of the doping element in the doped carbon is 0.5% to 5%; as an example, the doping element is nitrogen, phosphorus, sulfur, boron or fluorine.

[0130] Since nitrogen and sulfur atoms have a similar atomic radius to carbon atoms and are less likely to damage the carbon skeleton, when the doping amount of nitrogen and sulfur atoms is within the relatively wide range mentioned above, it can fully utilize the conductivity of the doped carbon layer and promote lithium-ion transport and lithium-ion desolvation capabilities.

[0131] Since phosphorus, boron and / or fluorine atoms have different atomic radii from carbon atoms, excessive doping can easily damage the carbon framework. Therefore, when the doping amount of phosphorus, boron and / or fluorine atoms is within the relatively small range mentioned above, it can fully utilize the conductivity of the doped carbon layer and promote lithium-ion transport and lithium-ion desolvation capabilities.

[0132] In some embodiments, the polysiloxane comprises the structural unit shown in formula (i) below.

[0133] R1 and R2 are independently selected from H, -COOH, -OH, -SH, -CN, -SCN, amino, phosphate ester, carboxylic acid ester, amide, aldehyde, sulfonyl, polyether segment, C1-C20 aliphatic hydrocarbon, C1-C20 halogenated aliphatic hydrocarbon, C1-C20 heteroaliphatic hydrocarbon, C1-C20 halogenated heteroaliphatic hydrocarbon, C6-C20 aromatic hydrocarbon, C6-C20 halogenated aromatic hydrocarbon, C2-C20 heteroaromatic hydrocarbon and C2-C20 halogenated heteroaromatic hydrocarbon;

[0134] In some embodiments, R1 and R2 are independently selected from H, amino, phosphate ester group, polyether segment, C1-C8 alkyl, C1-C8 haloalkyl, C1-C8 heteroalkyl, C1-C8 haloheteroalkyl, C2-C8 alkenyl and C2-C8 haloalkenyl.

[0135] In some embodiments, the polysiloxane further comprises a capping group, which includes one or more of the following functional groups: polyether, C1-C8 alkyl, C1-C8 haloalkyl, C1-C8 heteroalkyl, C1-C8 haloheteroalkyl, C2-C8 alkenyl, C2-C8 haloalkenyl, C6-C20 aromatic hydrocarbon, C1-C8 alkoxy, C2-C8 epoxy, hydroxyl, C1-C8 hydroxyalkyl, amino, C1-C8 aminoalkyl, carboxyl, and C1-C8 carboxylalkyl.

[0136] In some embodiments, the polysiloxane includes those selected from polydimethylsiloxane, polydiethylsiloxane, polymethylethylsiloxane, polymethylvinylsiloxane, polyphenylmethylsiloxane, polymethylhydrosiloxane, carboxyl-functionalized polysiloxane, epoxy-terminated polysiloxane, methoxy-terminated polydimethylsiloxane, hydroxypropyl-terminated polydimethylsiloxane, polymethylchloropropylsiloxane, hydroxyl-terminated polydimethylsiloxane, polymethyltrifluoropropylsiloxane, perfluorooctylmethylpolysiloxane, aminoethylaminopropylpolydimethylsiloxane, terminal polyether polydimethylsiloxane, and side-chain aminopropylpolysiloxane. The following are some of the following: aminopropyl-terminated polydimethylsiloxane, side-chain phosphate-grafted polydimethylsiloxane, side-chain polyether-grafted polydimethylsiloxane, 1,3,5,7-octamethylcyclotetrasiloxane, 1,3,5,7-tetrahydro-1,3,5,7-tetramethylcyclotetrasiloxane, cyclopentapolydimethylsiloxane, 2,4,6,8-tetramethylcyclotetrasiloxane, 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, cyclic polymethylvinylsiloxane, hexadecylcyclooctasiloxane, tetradecylcycloheptasiloxane, and cyclic polydimethylsiloxane.

[0137] In some embodiments, the number average molecular weight of the polysiloxane, polysaccharide, and polysaccharide derivative is independently below 300,000, optionally from 10,000 to 200,000, more preferably from 20,000 to 120,000, and even more preferably from 400 to 80,000.

[0138] The number-average molecular weights of polysiloxanes, polysaccharides, and polysaccharide derivatives can be determined by methods known in the art, such as gel permeation chromatography (GPC). A PL-GPC 220 high-temperature gel permeation chromatograph can be used as the instrument.

[0139] In some embodiments, the mass percentage of polar functional groups in the polysiloxane is α', where 0 ≤ α' < 50%, and optionally, 5% ≤ α' ≤ 30%.

[0140] "The mass percentage of polar functional groups in polysiloxanes" refers to the mass proportion of polar functional groups in R1, R2, and the end-capping groups in the polysiloxane. Polar functional groups include one or more of the following: -COOH, -OH, -SH, -CN, -SCN, amino (including -NH2, -NH-), phosphate ester group, carboxylic ester group (-COO-), amide group (-CONH-), aldehyde group (-CHO), sulfonyl group (-S(=O)2-), polyether segment, halogen, alkoxy, and epoxy. When the aforementioned polar functional groups are directly bonded to silicon atoms, α' represents the mass fraction of these polar functional groups in the polysiloxane. When the aforementioned polar functional groups are not directly bonded to silicon atoms, α' represents the sum of the mass fractions of the polar functional groups and the divalent to tetravalent methyl groups (e.g., -CH2, -CH-, -C-, etc.) directly bonded to them in the polysiloxane. Here, "divalent to tetravalent methyl groups" refers to the carbon atom directly bonded to the polar functional group and located between the polar functional group and the silicon atom, as well as other nonpolar functional groups bonded to the carbon atom. Taking polymethyltrifluoropropylsiloxane as an example, α' refers to the mass percentage of -CF3, excluding the ethylidene; taking polymethylchloropropylsiloxane as an example, α' refers to the mass percentage of -CH2Cl, excluding the ethylidene; taking hydroxypropyl-terminated polydimethylsiloxane as an example, α' refers to the mass percentage of -CH2OH. The mass percentage of polar functional groups in polysiloxanes can be determined by methods known in the art, such as titration (e.g., acid-base titration, redox titration, precipitation titration), infrared spectroscopy, and nuclear magnetic resonance spectroscopy.

[0141] In some embodiments, the substituents attached to the sugar units in the polysaccharide and the polysaccharide derivative each independently include one or more of the following functional groups: -OH, -COOH and their salts, -R-OH, -SO3H and their salts, -R-OH, -R-SO3H and their salts, sulfate ester group, alkoxy group, where R represents an alkylene group, optionally representing a C1 to C5 alkylene group.

[0142] Optionally, the substituents attached to the sugar units in the polysaccharide and the polysaccharide derivative each independently include one or more of the following functional groups: -OH, -COOH, -COOLi, -COONa, -COOK, -SO3H, -SO3Li, -SO3Na, -SO3K, -CH2-SO3H, -CH2-SO3Li, -CH2-SO3Na, -CH2-SO3K, methoxy, ethoxy.

[0143] The term "substituents attached to sugar units" includes all groups attached to the backbone of sugar units.

[0144] In some embodiments, the polysaccharide includes one or more selected from pectin, carboxymethyl starch, hydroxypropyl starch, dextrin, cellulose ether, carboxymethyl chitosan, hydroxyethyl cellulose, carboxymethyl cellulose, carboxypropyl methyl cellulose, guar gum, guar gum, gum arabic, lithium alginate, sodium alginate, potassium alginate, fucoidan, agar, carrageenan, carrageenan, xanthan gum, and fenugreek gum.

[0145] In some embodiments, the mass percentage of substituents attached to the sugar units in the polysaccharide and the polysaccharide derivative is independently 20% to 85%, optionally 30% to 78%. The mass percentage of substituents attached to the sugar units in the polysaccharide and the polysaccharide derivative can be determined by methods known in the art, such as titration (e.g., acid-base titration, redox titration, precipitation titration), infrared spectroscopy, and nuclear magnetic resonance spectroscopy.

[0146] According to embodiments of this application, the surface of a lithium transition metal phosphate is coated with one or more of carbon and carbon doping. Specifically, the surface of the lithium transition metal phosphate is coated with carbon. This improves the conductivity of the lithium transition metal phosphate.

[0147] In some embodiments, the doping element in the doped carbon includes one or more selected from nitrogen, phosphorus, sulfur, boron, and fluorine. This facilitates control over the properties of the doped carbon layer.

[0148] In some specific embodiments, the lithium transition metal phosphate includes a core and a shell covering the core, wherein the core includes the compound shown in Formula 2; the shell includes a first coating layer covering the core and a second coating layer covering the first coating layer, the first coating layer including pyrophosphate M′P2O7 and phosphate XPO4, wherein M and X each independently include one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb and Al; the second coating layer contains carbon.

[0149] Doping modification of lithium transition metal phosphates helps reduce the lattice change rate of lithium transition metal phosphates during lithium insertion / extraction, improves structural stability, significantly reduces transition metal dissolution and decreases oxygen activity on the particle surface, lowers the lithium-ion migration barrier, promotes lithium-ion migration, and improves the rate performance of lithium-ion secondary batteries. The first coating layer includes pyrophosphate and phosphate. Since the migration barrier of transition metals in pyrophosphate is relatively high (>1 eV), it can effectively reduce transition metal dissolution. Phosphate has excellent lithium-ion conductivity and can reduce surface impurity lithium content. Furthermore, since the second coating layer is a carbon-containing layer, it can effectively improve the conductivity and desolvation capability of lithium transition metal phosphates. In addition, the "barrier" effect of the second coating layer can further reduce the migration of transition metal ions into the electrolyte and reduce side reactions. Therefore, the above-mentioned lithium transition metal phosphates can effectively reduce transition metal dissolution during lithium insertion / extraction while promoting lithium-ion migration, thereby improving the rate performance, cycle performance, and high-temperature performance of lithium-ion secondary batteries.

[0150] In other specific embodiments, the lithium transition metal phosphate includes a core and a shell covering the core, wherein the core includes the compound shown in Formula 2; the shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer includes Li pyrophosphate. f Q′P2O7 and / or Q′ g (P2O7) h , 0≤f≤2, 1≤g≤4, 1≤h≤6, Li pyrophosphate f Q′P2O7 and / or Q′ g (P2O7) h Each of the Q′ elements independently includes one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al; the second coating layer includes crystalline phosphate XPO4, where X includes one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al; and the third coating layer contains carbon.

[0151] Therefore, by doping modification, the dissolution of transition metals can be effectively reduced, the lithium-ion migration barrier can be lowered, and the cycle performance and rate performance of lithium-ion secondary batteries can be improved. By using the above-mentioned coating layer, the surface lithium content can be reduced, the contact between the core and the electrolyte can be reduced, thereby reducing interfacial side reactions, reducing gas production, and improving the high-temperature storage performance, cycle performance and kinetic performance of lithium-ion secondary batteries.

[0152] According to embodiments of this application, maintaining the overall core system's electrical neutrality minimizes defects and impurity phases in lithium transition metal phosphates. If an excess of transition metal (e.g., manganese) exists in the lithium transition metal phosphate, the excess metal is likely to precipitate as elemental or form impurity phases within the crystal lattice due to the relatively stable structure of the material system. Maintaining electrical neutrality minimizes such impurity phases. Furthermore, maintaining system electrical neutrality can, in some cases, generate lithium vacancies in the material, thereby improving the kinetic properties of lithium transition metal phosphates.

[0153] In some embodiments, one or more covering layers in the shell that are furthest from the core each independently include one or more selected from polysiloxanes, polysaccharides, and polysaccharide derivatives.

[0154] This improves the uniformity of the coating, effectively blocks interfacial side reactions caused by high voltage, thereby enhancing the high-temperature cycling and storage performance of lithium transition metal phosphates. Furthermore, the coating layer has good ionic conductivity, which helps to improve the specific capacity of lithium transition metal phosphates while reducing heat generation in lithium-ion secondary batteries.

[0155] In some embodiments, based on the weight of the core, the coating amount of the shell (only one coating layer) can be from 0.1 wt% to 6 wt%. A coating amount within this range allows for sufficient coating of the core, while simultaneously improving the kinetic performance of the lithium-ion secondary battery without sacrificing the specific capacity of lithium transition metal phosphate.

[0156] In some embodiments, based on the kernel weight, the coating amount of the first coating layer is greater than 0 and less than or equal to 7 wt%, optionally greater than 0 and less than or equal to 6 wt%, more preferably greater than 0 and less than or equal to 5.5 wt%, or 4-5.6 wt%, and further optionally greater than 0 and less than or equal to 2 wt%.

[0157] In some embodiments, based on the kernel weight, the coating amount of the second coating layer is greater than 0 and less than or equal to 6 wt%, optionally greater than 0 and less than or equal to 5.5 wt%, more preferably 2-4 wt% or 3-5 wt%; and / or,

[0158] In some embodiments, based on the kernel weight, the coating amount of the third coating layer is greater than 0 and less than or equal to 6 wt%, optionally greater than 0 and less than or equal to 5.5 wt%, and more preferably greater than 0 and less than or equal to 2 wt%.

[0159] In some embodiments, the shell may further include a fourth covering layer covering the third covering layer and a fifth covering layer covering the fourth covering layer. In some embodiments, based on the weight of the core, the covering amount of the fourth covering layer and the fifth covering layer is independently 0.01wt% to 10wt%, optionally 0.05wt% to 10wt%, more preferably 0.1wt% to 5wt%, and further preferably 0.1wt% to 2wt%.

[0160] Within the aforementioned range, the coating amount of each coating layer can fully coat the core and further improve the kinetic performance of the lithium-ion secondary battery without sacrificing the specific capacity of lithium transition metal phosphate.

[0161] In some embodiments, the shell covers 40% to 90% of the surface of the core, optionally 60% to 80%. This allows for sufficient coverage of the core, thereby improving the kinetic performance of the lithium-ion secondary battery.

[0162] In some embodiments, the thickness of the shell (which is only a single covering layer) can be 1-15 nm.

[0163] In some embodiments, the thickness of the first coating layer is 1-10 nm, optionally 2-10 nm.

[0164] In some embodiments, the thickness of the second coating layer is 2-25 nm, optionally 2-15 nm, and more preferably 3-15 nm.

[0165] In some embodiments, the thickness of the third coating layer is 2-25 nm, optionally 5-25 nm.

[0166] In some embodiments, the thickness of the first coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm, or within any range of any of the above values.

[0167] In some embodiments, the thickness of the second coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, or any range of any of the above values.

[0168] In some embodiments, the thickness of the third coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 21 nm, about 22 nm, about 23 nm, about 24 nm, or about 25 nm, or any range of any of the above values.

[0169] The first coating layer, having the aforementioned thickness range, can further reduce the adverse effects on the kinetic properties of lithium transition metal phosphates.

[0170] The second coating layer has the aforementioned thickness range, which makes the surface structure of the second coating layer stable and reduces the side reactions with the electrolyte. Therefore, it can effectively reduce interfacial side reactions and thus improve the high-temperature performance of the battery.

[0171] The third coating layer, having the aforementioned thickness range, can improve the electrical conductivity of lithium transition metal phosphate and enhance the compaction density of the positive electrode sheet prepared using lithium transition metal phosphate.

[0172] The thickness of the coating layer is mainly tested by FIB. The specific method may include the following steps: randomly select a single particle from the lithium transition metal phosphate powder to be tested, cut a thin slice with a thickness of about 100 nm from the middle position or near the middle position of the selected particle, and then perform TEM test on the thin slice to measure the thickness of the coating layer. Measure 3-5 positions and take the average value.

[0173] In some embodiments, the lattice change rate of the lithium transition metal phosphate before and after complete lithium insertion / extraction is less than 50%, optionally less than 9.8%, more preferably less than 8.1%, further preferably less than 7.5%, even more preferably less than 6%, even more preferably less than 4%, even more preferably less than 3.8%, and even more preferably 2.0-3.8%.

[0174] Reducing the lattice change rate facilitates Li ion transport, meaning Li ions have greater mobility in lithium transition metal phosphates, which is beneficial for improving the rate performance of lithium-ion secondary batteries. The lattice change rate can be measured using methods known in the art, such as X-ray diffraction (XRD).

[0175] In some embodiments, the Li / Mn antisite defect concentration of the lithium transition metal phosphate is 5.3% or less, optionally 5.1% or less, more preferably 4% or less, further preferably 2.2% or less, even more preferably 2% or less, and even more preferably 1.5%-2.2% or less.

[0176] The so-called Li / Mn inversion defect refers to the defect in the Li transition metal phosphate lattice where Li... + With Mn 2+ The positions of the two sites are interchanged. The Li / Mn antisite defect concentration refers to the concentration of Li / Mn antisite defects in lithium transition metal phosphate. 2+ Interchangeable Li + L i+ Percentage of the total. Mn of the inversion defect. 2+ It will hinder Li + The transport of these defects, by reducing the concentration of Li / Mn antisite defects, is beneficial to improving the specific capacity and rate performance of lithium transition metal phosphates. The concentration of Li / Mn antisite defects can be measured by methods known in the art, such as XRD.

[0177] In some embodiments, the surface oxygen valence state of the lithium transition metal phosphate is below -1.55, optionally below -1.82, more preferably below -1.88, further preferably below -1.90 or from -1.98 to -1.88, even more preferably from -1.98 to -1.89, and even more preferably from -1.98 to -1.90.

[0178] By reducing the surface oxygen valence state, interfacial side reactions between lithium transition metal phosphates and the electrolyte can be reduced, thereby improving the battery's cycle performance and high-temperature stability. The surface oxygen valence state can be measured using methods known in the art, such as electron energy loss spectroscopy (EELS).

[0179] According to embodiments of this application, the lithium transition metal oxide has a layered structure. Layered lithium transition metal oxides have high compaction density, but are relatively expensive. During use in lithium-ion secondary batteries, they exhibit relatively more side reactions and relatively poor crystal structure stability. By rationally combining lithium transition metal phosphates and lithium transition metal oxides, the actual packing density of the positive electrode active material can be increased, thereby improving the energy density and lifespan of the lithium-ion secondary battery.

[0180] According to embodiments of this application, the lithium transition metal oxide comprises single-crystal particles. Using a single-crystal lithium transition metal oxide allows the lithium-ion secondary battery to have a higher voltage, which is beneficial for improving fast-charging performance.

[0181] According to embodiments of this application, the lithium transition metal oxide comprises a nickel-cobalt-manganese ternary cathode material. This results in high energy density, good cycle performance, and moderate cost.

[0182] According to some specific embodiments of this application, in the lithium transition metal oxide, based on the total molar number of nickel, cobalt, manganese, and / or aluminum, the molar percentage of nickel is 50%–60%, 60%–70%, or 80%–90%. In other words, the lithium transition metal oxide can be a 5-series ternary cathode material, a 6-series ternary cathode material, or an 8-series ternary cathode material. This satisfies the requirements for energy density and high voltage.

[0183] According to embodiments of this application, the lithium transition metal oxide comprises a compound of Formula 1:

[0184] Li x1 (Ni a1 Co b1 Mn c1 ) d1 M 1-d1 O y1 A z1 Formula 1

[0185] Wherein, 0.8≤x1≤1.2, 0.3≤a1≤0.9, 0.01≤b1≤0.15, 0.15≤c1≤0.55, a1+b1+c1=1, 0.95≤d1≤1, 1.9≤y1≤2.1, 0≤z1≤0.1;

[0186] M includes one or more of Zr, Sr, B, Ti, Mg, Sn, Mo, W, Sb, Nb, La, and Al;

[0187] A includes one or more of S, N, F, Cl, Br, and I.

[0188] Lithium transition metal oxides, possessing the aforementioned chemical formula, can further enhance the energy density and lifespan of lithium-ion secondary batteries.

[0189] According to embodiments of this application, a lithium transition metal oxide may include a core and a shell covering the core, the core including the compound shown in Formula 1; the shell may include one or more coating layers; each coating layer has ionic conductivity and / or electronic conductivity.

[0190] According to embodiments of this application, one or more coating layers each independently include one or more selected from phosphates, pyrophosphates, carbon, doped carbon, oxides, and fast ion conductors, specifically one or more selected from phosphates, pyrophosphates, and oxides.

[0191] According to embodiments of this application, one or more coating layers each independently include one or more elements selected from Al, Zr, Mg, Ba, Cd, Zn, Ti, Co, W, Y, Si, Sn, B, P, S, and C.

[0192] According to embodiments of this application, the fast ion conductor may include Li 3x3 La 2 / 3-x3 M 2 a3 TiN 2 z3 O3, Li 2+2x4 Zn 1- x4 GeO4, LiM 3 One or more of 2(PO4)3, M 2 Includes one or more elements selected from Ba and Sr, N 2 Includes one or more elements selected from Al and Zr, M 3 It is one or more elements among Zr, Ti, Ge, and Hf, where 0.04≤x3≤0.167, 0≤a3≤1, 0≤z3≤1, and -0.3≤x4≤0.8.

[0193] According to embodiments of this application, the shell of a lithium transition metal oxide may include a coating layer; specifically, the coating layer includes one or more selected from phosphates, pyrophosphates, and oxides.

[0194] According to embodiments of this application, the shell of a lithium transition metal oxide may include a first coating layer covering the core and a second coating layer covering the first coating layer; specifically, the first coating layer and the second coating layer each independently include one or more selected from phosphates, pyrophosphates, and oxides.

[0195] According to embodiments of this application, the first coating layer may include one or more of phosphates and oxides, and the second coating layer may include one or more of pyrophosphates and oxides. Specifically, the oxides in the first coating layer include oxides of one or more elements selected from Al, Zr, Mg, Ti, Co, Y, Ba, and Cd; the oxides in the second coating layer include oxides of one or more elements selected from B, Sn, S, and P.

[0196] According to embodiments of this application, based on the weight of the core, the coating weight of the lithium transition metal oxide shell can be 0.005wt% to 1wt%, specifically 0.01wt% to 0.5wt%.

[0197] According to embodiments of this application, the thickness of the lithium transition metal oxide shell can be 2nm to 200nm, specifically 5nm to 50nm.

[0198] It is understood that, in addition to the positive electrode active material mentioned above, the positive electrode film layer may also include at least one of the following: positive electrode binder, positive electrode conductive agent, and functional additive.

[0199] In some embodiments, the positive electrode binder may include a polyvinylidene fluoride homopolymer and / or copolymer. Specifically, the comonomer may include one or more of tetrafluoroethylene, hexafluoropropylene, and propylene. Specifically, the weight percentage of the comonomer is ≤1%. As an example, the positive electrode binder may include one or more of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-tetrafluoroethylene-propylene terpolymer, and polyvinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer.

[0200] In some embodiments, the positive electrode binder may include one or more of polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0201] In some embodiments, the weight-average molecular weight of the positive electrode binder may be from 300,000 to 2,000,000.

[0202] In some embodiments, the positive electrode conductive agent may include one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, and the embodiments of this application are not limited thereto.

[0203] In some embodiments, the functional additive may include one or more of the following: dispersant, plasticizer, pore-forming agent, dehydrating additive, deacidifying additive, and lithium supplement.

[0204] In some embodiments, the dispersant may include an abc-type block copolymer, wherein block A comprises polyvinylpyrrolidone, block B comprises polyacrylic acid, and block C comprises one or more of polytetrahydrofuran chains, polyethylene oxide chains, polyethylene glycol chains, polypropylene glycol chains, and polypropylene triol chains. Specifically, the ratio of the average degree of polymerization of block A to block B is greater than 10:1. Specifically, the ratio of the average degree of polymerization of block A to block C is (0.1-10):1. Specifically, the weight-average molecular weight of the dispersant is 2000 to 100,000.

[0205] In some embodiments, the plasticizer may include one or more of the following: strong solvent type (PP-SS) plasticizer, low temperature resistant type (PP-LT) plasticizer, low volatility type (PP-LV) plasticizer, low diffusion type (SP-LD) plasticizer, heat stable type (SP-Stab) plasticizer, and flame retardant type (SP-FR) plasticizer.

[0206] In some embodiments, strong solvent-based (PP-SS) plasticizers provide strong plasticizing properties and may include, for example, phthalates and non-phthalates (such as benzoates, tricresyl phosphates, etc.). Low-temperature resistant (PP-LT) plasticizers also provide good low-temperature resistance and may include, for example, aliphatic diesters. Low-volatility (PP-LV) plasticizers also have low volatility and may include, for example, trimellitic esters and polyesters. Low-diffusion (SP-LD) plasticizers also have low diffusion and may include, for example, polyesters. Heat-stabilized (SP-Stab) plasticizers also have heat-stabilizing properties and may include, for example, epoxy compounds. Flame-retardant (SP-FR) plasticizers also have flame-retardant properties and may include, for example, phosphate esters and halogenated hydrocarbons.

[0207] In some embodiments, the lithium replenishing agent may include one or more of lithium-rich oxides (such as Li2NiO2, Li5FeO4, etc.), nanocomposites and binary lithium compounds. Specifically, the number of Li atoms in the molecular formula of the lithium replenishing agent is ≥1.5.

[0208] 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.).

[0209] In some embodiments, the positive electrode sheet does not exclude additional functional layers besides the positive electrode film layer. For example, in some embodiments, the positive electrode sheet may also include a functional coating, which may be located between the positive current collector and the positive electrode film layer and / or on the surface of the positive electrode film layer facing away from the positive current collector. As an example, the functional coating may include one or more of conductive carbon, dehydrating additives, deacidifying additives, and lithium replenishing agents, and the embodiments of this application are not limited thereto. In some embodiments, the thickness of the functional coating may be 0.1 μm-10 μm.

[0210] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive active material composition, 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.

[0211] In some embodiments, 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, the negative electrode film layer comprising a negative electrode active material. As an example, the negative current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the opposite surfaces of the negative current collector.

[0212] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper 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 (such as copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver and silver alloys, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0213] In some embodiments, the thickness of the negative electrode current collector can be 3μm-20μm.

[0214] In some embodiments, the negative electrode active material may include one or more of carbon-based materials, silicon-based materials, tin-based materials, and lithium titanate. Carbon-based materials may include one or more of graphite (e.g., artificial graphite, natural graphite, etc.), soft carbon, and hard carbon. Silicon-based materials may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide compounds, and tin alloys.

[0215] In some embodiments, the negative electrode active material may include a carbon-based material or a combination of a carbon-based material and a silicon-based material.

[0216] In some embodiments, the negative electrode active material may include a carbon-based material, and the carbon-based material includes graphite, or a combination of graphite and hard carbon. Optionally, the graphite may have a porous structure. Optionally, the specific capacity of the graphite may be greater than or equal to 340 mAh / g.

[0217] In some embodiments, the negative electrode active material may include a combination of carbon-based materials and silicon-based materials, wherein the carbon-based materials include graphite or a combination of graphite and hard carbon, and the silicon content in the negative electrode active material is greater than 0 and less than or equal to 30 wt%, based on the total weight of the negative electrode active material.

[0218] In some embodiments, the negative electrode film layer may include a negative electrode binder. As an example, the negative electrode binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS), and the embodiments of this application are not limited thereto.

[0219] In some embodiments, the negative electrode film layer may further include a negative electrode conductive agent. As an example, the negative electrode conductive agent may include one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, and the embodiments of this application are not limited thereto.

[0220] In some embodiments, the negative electrode film layer may also include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC)).

[0221] In some embodiments, the porosity of the negative electrode film is 20% to 50%.

[0222] The negative electrode sheet does not exclude additional functional layers besides the negative electrode film layer. In some embodiments, the negative electrode sheet may also include a functional coating, which may be located between the negative electrode current collector and the negative electrode film layer and / or on the surface of the negative electrode film layer facing away from the negative electrode current collector. Optionally, the functional coating may include carbon.

[0223] In some embodiments, the negative electrode film layer may further include a lithium replenishing material. Specifically, the lithium replenishing material includes one or more of lithium foil, lithium strip, lithium powder, and pre-lithiation reagent. Specifically, the pre-lithiation reagent may include one or more of Li-aromatic hydrocarbons, complexes of Li-aromatic hydrocarbons and ether solvents, and may be selected from one or more of lithium naphthalene and lithium biphenyl dimethyl ether (DME).

[0224] In some embodiments, the negative electrode sheet may include a negative current collector and a first negative electrode film layer and a second negative electrode film layer respectively disposed on two surfaces of the negative current collector. The composition of the first negative electrode film layer and the second negative electrode film layer may be the same or different; the thickness of the first negative electrode film layer and the second negative electrode film layer may be the same or different.

[0225] In some embodiments, the thickness ratio of the first negative electrode film layer and the second negative electrode film layer is 5:95 to 95:5.

[0226] The negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, negative electrode conductive agent, negative electrode binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0227] In some embodiments, the negative electrode sheet may not include a negative electrode active material capable of lithium ion intercalation / deintercalation. For example, the negative electrode sheet may include a lithium sheet or a lithium alloy sheet; or, the negative electrode sheet may include a mesh or foam-like three-dimensional framework layer; or, the negative electrode sheet may include a negative current collector and a lithium-containing layer disposed on at least one surface of the negative current collector.

[0228] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte; it can be selected according to requirements. For example, the electrolyte may include one or more of liquid electrolytes (also known as electrolyte solutions), all-solid electrolytes, and gel electrolytes.

[0229] In some embodiments, the electrolyte is liquid and includes a lithium salt, a non-aqueous solvent for dissolving the lithium salt, and optional additives.

[0230] In some embodiments, the lithium salt may include one or more selected from LiPF6, LiBF4, LiN(SO2F)2, LiN(CF3SO2)2, LiClO4, LiAsF6, LiB(C2O4)2, LiBF2C2O4, and LiPO2F2.

[0231] In some embodiments, the concentration of lithium salt may be 0.5-1.5 mol / L.

[0232] In some embodiments, the non-aqueous solvent may include one or more selected from propylene carbonate, ethylene carbonate, butene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, ethyl acetate, methyl butyrate, ethyl butyrate, methyl propionate, ethyl propionate, propyl propionate, methyl acrylate, ethylene sulfite, propylene sulfite, dimethyl sulfite, diethyl sulfite, acid anhydride, N-methylpyrrolidone, acetonitrile, sulfolane, dimethyl sulfoxide, dimethyl sulfide, γ-butyrolactone, and tetrahydrofuran.

[0233] In some embodiments, the additive may include one or more of the following: negative electrode film-forming additive, positive electrode film-forming additive, additive to improve battery overcharge performance, additive to improve battery high-temperature performance, additive to improve battery low-temperature power performance, dehydration additive, deacidification additive, and additive capable of complexing transition metal ions.

[0234] As an example, additives may include one or more selected from cyclic carbonate compounds containing carbon-carbon double bonds, halogenated cyclic carbonate compounds, nitriles and polynitriles, phosphazene compounds, aromatic hydrocarbons and halogenated aromatic hydrocarbons, isocyanate compounds, acid anhydride compounds, sulfate compounds, sulfite compounds, sulfonate compounds, disulfonate compounds, borate compounds, phosphate compounds, amide compounds, carbodiimide compounds, crown ethers and azacrown ethers, and their respective derivatives. The species may optionally include one or more of vinylene carbonate (VC), 1,2,3-tris(2-cyanoethoxy)propane (TCP), 1-aza-12-crown 4-ether (A12C4), N,N'-dicyclohexylcarbodiimide, N,N'-diisopropylcarbodiimide, N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, tris(hexafluoroisopropyl)boronic acid ester, tris(2,2,3,3-tetrafluoropropyl)boronic acid ester, and tris(pentafluorophenyl)boronic acid ester.

[0235] In some embodiments, the content of the additive may be less than or equal to 10 wt%, based on the total weight of the electrolyte.

[0236] In some embodiments, this application does not have a particular limitation on the type of separator membrane, and any well-known porous structure separator membrane with good chemical and mechanical stability can be selected.

[0237] In some embodiments, the separator includes a porous substrate. The porous substrate may be made of at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyester, and polyimide. The porous substrate may be a single-layer film or a multi-layer composite film, without particular limitation. When the porous substrate is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0238] In some embodiments, the separator may further include a coating on at least one surface of the porous substrate. Specifically, the coating includes one or more of inorganic heat-resistant particles and organic heat-resistant particles.

[0239] In some embodiments, the porosity of the separator is 10%-40%.

[0240] In some embodiments, the thickness of the separator can be 3μm-20μm.

[0241] In some embodiments, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding process or a stacking process.

[0242] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode components and electrolyte described above. In some embodiments, the outer packaging of the lithium-ion secondary battery can be a rigid shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the lithium-ion secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0243] This application does not impose any particular limitation on the shape of the lithium-ion secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square-structured lithium-ion secondary battery 1 as an example. Referring to Figure 1, the outer packaging may include a casing 200 and a cover plate 300. The casing 200 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The casing 200 has an opening communicating with the receiving cavity, and the cover plate 300 can be placed over the opening to close the receiving cavity. The electrode assembly 100 described in this application is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 100. The battery 1 may contain one or more electrode assemblies 100, which can be selected by those skilled in the art according to specific practical needs.

[0244] Secondly, this application provides an electrical device. According to an embodiment of this application, the electrical device includes the battery described above. This battery possesses all the features and advantages of the lithium-ion secondary battery described above, which will not be repeated here.

[0245] According to embodiments of this application, the lithium-ion secondary 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.

[0246] Figure 2 shows an example of an electrical device. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc.

[0247] The embodiments of this application are described in detail below.

[0248] The lithium-ion secondary batteries of Examples 1-12 and Comparative Example 1 were all prepared according to the following method.

[0249] Preparation of positive electrode sheet

[0250] The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) shown in Table 1 were thoroughly mixed in an appropriate amount of solvent NMP at a mass ratio of 96.4:2.1:1.5 to form a positive electrode slurry. The positive electrode slurry was uniformly coated on both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0251] Preparation of negative electrode sheet

[0252] The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose, and the conductive agent acetylene black are thoroughly mixed in an appropriate amount of deionized water to form a uniform first negative electrode slurry (graphite:conductive agent:plasticizer:binder = 97:0.4:1.1:1.5 by mass) and a second negative electrode slurry (graphite:conductive agent:plasticizer:binder = 96.7:0.7:1.2:1.4 by mass). The first negative electrode slurry is uniformly coated on the surface of the negative electrode current collector copper foil and dried to obtain the first negative electrode film layer. The second negative electrode slurry is coated on the first negative electrode film layer, and after drying and cold pressing, the negative electrode sheet is obtained.

[0253] Preparation of electrolyte

[0254] Ethyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0255] Preparation of the separating membrane

[0256] Porous polyethylene film is used as the separator.

[0257] Battery manufacturing

[0258] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion secondary battery is obtained.

[0259] Performance testing:

[0260] Cycle life of lithium-ion secondary batteries:

[0261] Examples 1-7, 9-12, and Comparative Example 1: At 25°C, the battery was charged at a constant current of 0.5C to 4.4V, and then charged at a constant voltage until the current reached 0.05C. At this point, the battery was fully charged, and the charging capacity was recorded as the first charge capacity. After the battery was left to stand for 5 minutes, it was discharged at a constant current of 0.5C to 2.5V. This constituted one charge-discharge cycle, and the discharge capacity was recorded as the first discharge capacity. The battery was subjected to cyclic charge-discharge tests using the above method, and the discharge capacity after each cycle was recorded until the battery's discharge capacity decreased to 80% of the first discharge capacity. The number of cycles at this point was used to characterize the battery's cycle performance. The higher the number of cycles, the better the battery's lifespan.

[0262] Example 8: At 25°C, the battery was charged at a constant current of 0.5C to 4.25V, and then charged at a constant voltage until the current reached 0.05C. At this point, the battery was fully charged, and the charging capacity was recorded, which is the first charge capacity. After letting the battery rest for 5 minutes, it was discharged at a constant current of 0.5C to 2.5V. This completes one charge-discharge cycle, and the discharge capacity was recorded, which is the first discharge capacity. The battery was subjected to cyclic charge-discharge tests using the above method, and the discharge capacity after each cycle was recorded until the battery's discharge capacity decreased to 80% of the first discharge capacity. The number of cycles at this point characterizes the battery's cycle performance. The higher the number of cycles, the better the battery's lifespan.

[0263] The cycle life of lithium-ion secondary batteries is shown in Table 2.

[0264] Table 1

[0265] Table 2

[0266] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A lithium-ion secondary battery, wherein, Including positive electrode plates and negative electrode plates; The positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes lithium transition metal oxide and lithium transition metal phosphate. The lithium transition metal oxide includes nickel, cobalt, manganese and / or aluminum, wherein the molar percentage of nickel is 50% to 99% based on the total molar number of nickel, cobalt, manganese and / or aluminum. The lithium transition metal phosphate includes manganese and iron, wherein, based on the total molar number of manganese and iron, the molar percentage of manganese is 50% to 99%. The volumetric particle size distribution curve of the positive electrode active material has at least two volumetric distribution peaks. The volumetric distribution peak with the maximum peak intensity is denoted as the first peak, and the volumetric particle size corresponding to the maximum peak intensity of the first peak is denoted as Dv1. The volumetric distribution peak with the second maximum peak intensity is denoted as the second peak, and the volumetric particle size corresponding to the maximum peak intensity of the second peak is denoted as Dv2. 3.0μm≤Dv1≤10μm; 0.35μm≤Dv2≤0.55μm; The porosity of the positive electrode film is 10% to 18%.

2. The lithium-ion secondary battery according to claim 1, wherein, 3.7μm≤Dv1≤9.2μm; 0.46μm≤Dv2≤0.49μm.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The porosity of the positive electrode film is 12.3% to 17.9%.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein The compacted density of the positive electrode film layer is 2.5 g / cm 3 ~ 3.5 g / cm 3 .

5. The lithium-ion secondary battery according to any one of claims 1 to 4, wherein The face density of the positive electrode film layer is 0.015 g / mm 2 ~ 0.021 g / mm 2 .

6. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein The volume distribution particle size Dv50 of the lithium transition metal phosphate is 0.3 μm to 0.7 μm; and / or The volumetric particle size Dv50 of the lithium transition metal oxide is 3 μm to 10 μm.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, wherein In the positive electrode film layer, the mass percentage of manganese is 13% to 21%.

8. The lithium-ion secondary battery according to any one of claims 1 to 7, wherein In the positive electrode film layer, the mass percentage of iron is 0.1% to 20%.

9. The lithium-ion secondary battery according to any one of claims 1 to 8, wherein In the positive electrode film layer, the mass percentage of nickel is 20% to 85%.

10. The lithium-ion secondary battery according to any one of claims 1 to 9, wherein In the positive electrode active material, the mass percentage of the lithium transition metal phosphate is less than or equal to 50%.

11. The lithium-ion secondary battery according to any one of claims 1 to 10, wherein In the positive electrode active material, the mass percentage of the lithium transition metal phosphate is 5% to 40%.

12. The lithium-ion secondary battery according to any one of claims 1 to 11, wherein The lithium transition metal oxide has a layered structure, and the lithium transition metal phosphate has an olivine structure.

13. The lithium-ion secondary battery according to any one of claims 1 to 12, wherein The lithium transition metal oxide comprises single-crystal particles.

14. The lithium-ion secondary battery according to any one of claims 1 to 13, wherein The lithium transition metal oxide includes nickel-cobalt-manganese ternary cathode materials and / or nickel-cobalt-aluminum ternary cathode materials.

15. The lithium-ion secondary battery according to any one of claims 1 to 14, wherein The lithium transition metal oxide includes the compound shown in Formula 1: Li x1 (Ni a1 Co b1 Mn c1 ) d1 M 1-d1 O y1 A z1 Formula 1 Wherein, 0.8≤x1≤1.2, 0.3≤a1≤0.9, 0.01≤b1≤0.15, 0.15≤c1≤0.55, a1+b1+c1=1, 0.95≤d1≤1, 1.9≤y1≤2.1, 0≤z1≤0.1; M includes one or more of Zr, Sr, B, Ti, Mg, Sn, Mo, W, Sb, Nb, La, and Al; A includes one or more of S, N, F, Cl, Br, and I.

16. The lithium-ion secondary battery according to any one of claims 1 to 15, wherein In the lithium transition metal oxide, the molar percentage of nickel is 50%–60%, 60%–70%, or 80%–90% based on the total molar number of nickel, cobalt, manganese, and / or aluminum.

17. The lithium-ion secondary battery according to any one of claims 1 to 16, wherein The lithium transition metal phosphate includes the compound shown in Formula 2: Li x2 Mn y2 Fe y3 P z2 O a2 Q b2 , Formula 2 Wherein, Q includes at least one of Zn, Al, Na, K, Ti, V, Mg, Nb, Mo, W, B, S, Si, N, F, Cl, and Br; 0.8≤x²≤1.2, 0.1≤y²≤0.9, 0.1≤y³≤0.9, 0.9≤z²≤1, 3.5≤a²≤4, 0≤b²≤0.

2.

18. The lithium-ion secondary battery according to any one of claims 1 to 17, wherein In the lithium transition metal phosphate, based on the total molar number of manganese and iron, the molar percentage of manganese is 65% to 75%.

19. The lithium-ion secondary battery according to any one of claims 1 to 18, wherein The lithium transition metal oxide and the lithium transition metal phosphate each independently include a core and a coating layer covering at least a portion of the surface of the core.

20. The lithium-ion secondary battery according to claim 19, wherein, The coating layer includes one or more of the following: phosphate, pyrophosphate, carbon, doped carbon, oxide, and fast ion conductor.

21. An electrical device, comprising: The lithium-ion secondary battery includes any one of claims 1 to 20.

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