Positive pole piece and preparation method thereof, lithium ion battery and electric device

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a positive pole piece and a preparation method thereof, a lithium ion battery and a power utilization device.The positive pole piece comprises a positive current collector and a positive coating located on at least one surface of the positive current collector, the positive electrode coating comprises a first positive electrode coating close to the positive electrode current collector and a second positive electrode coating far away from the positive electrode current collector, and materials of the first positive electrode coating comprise a first positive electrode active material, a first positive electrode conductive agent and a first positive electrode binder; the material of the second positive electrode coating comprises a second positive electrode active material, a second positive electrode conductive agent and a second positive electrode binder; the first positive electrode active material is carbon-coated lithium iron phosphate, and the second positive electrode active material is nitrogen-doped lithium iron phosphate. According to the application, collaborative optimization of the rate capability, low-temperature performance and cycling stability of the lithium battery can be realized, so that the battery design efficiency is improved.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and particularly to a positive electrode sheet, a preparation method thereof, a lithium-ion battery, and an electrical device using the same. Background Art

[0002] Currently, lithium iron phosphate (LiFePO4), as a typical representative of the positive electrode material for lithium-ion batteries, has become the mainstream choice for commercial lithium-ion batteries and is widely used in fields such as smart phones, laptop computers, electric vehicles, and energy storage systems due to its excellent comprehensive performance. This material has a relatively high theoretical specific capacity (170 mAh / g), a stable discharge voltage platform (3.4 V vs. Li / Li), excellent cycle life, outstanding thermal stability, high safety, and is also environmentally friendly. -1 ) and a stable discharge voltage platform (3.4 V vs. Li / Li + ), excellent cycle life, outstanding thermal stability, high safety, and is also environmentally friendly.

[0003] In the process of implementing the embodiments of the present disclosure, it is found that at least the following problems exist in the related art:

[0004] Lithium iron phosphate still has problems such as low electronic conductivity (~10 -8 -10 -10 S cm -1 ) and a small lithium ion diffusion coefficient (~10 -18 cm 2 s -1 ), resulting in limitations in its kinetic performance under high-rate charge and discharge and low-temperature environments, and affecting its wide application under high-power and extreme working conditions.

[0005] Although the related art has improved the conductivity and rate performance of lithium iron phosphate to a certain extent, there are still limitations, and there are still technical problems such as an overly thick carbon coating affecting ion transport performance, excessive doping leading to lattice distortion and thus affecting cycle stability, and too small particle size resulting in enhanced interfacial side reactions.

[0006] It should be noted that the information disclosed in the above background art section is only used to enhance the understanding of the background of the present application, and thus may include information that does not constitute the prior art known to those of ordinary skill in the art. Summary of the Invention

[0007] To provide a basic understanding of some aspects of the disclosed embodiments, a simple summary is given below. This summary is not a comprehensive review, nor is it intended to identify key / important elements or delineate the scope of protection of these embodiments, but rather serves as a preface to the subsequent detailed description.

[0008] Embodiments of the present disclosure provide a positive electrode tab and a method for preparing the same, a lithium-ion battery, and an electrical device, so as to achieve the coordinated optimization of the rate performance, low-temperature performance, and cycle stability of a lithium battery, thereby improving the efficiency of battery design.

[0009] The positive electrode tab includes: a positive electrode current collector and a positive electrode coating located on at least one surface of the positive electrode current collector. The positive electrode coating includes: a first positive electrode coating close to the positive electrode current collector and a second positive electrode coating far from the positive electrode current collector. The materials of the first positive electrode coating include: a first positive electrode active material, a first positive electrode conductive agent, and a first positive electrode binder. The materials of the second positive electrode coating include: a second positive electrode active material, a second positive electrode conductive agent, and a second positive electrode binder; the first positive electrode active material is carbon-coated lithium iron phosphate, and the second positive electrode active material is nitrogen-doped lithium iron phosphate; and, the first positive electrode coating and the second positive electrode coating are also coated on the other surface of the positive electrode current collector, so as to form a four-layer positive electrode active material coating structure on both sides of the positive electrode current collector.

[0010] Optionally, the characteristic peak intensity in the X-ray photoelectron spectroscopy of the first positive electrode active material satisfies: I C / I Fe ≥0.5. There is no characteristic peak in the Fourier transform infrared spectroscopy of the first positive electrode active material in the range of 1350-1370 cm -1 , and a characteristic peak appears in the range of 945-1065 cm -1 ;

[0011] The characteristic peak intensity in the X-ray photoelectron spectroscopy of the second positive electrode active material satisfies: 0.05≤I N / I Fe ≤3.0. Characteristic peaks appear in both the range of 1350-1370 cm -1 and the range of 945-1065 cm -1 in the Fourier transform infrared spectroscopy of the second positive electrode active material;

[0012] wherein, I Fe represents the intensity of the Fe2p characteristic peak in the X-ray photoelectron spectroscopy, and the binding energy range of the Fe2p characteristic peak is 710.0 eV to 730.0 eV; I N represents the intensity of the N1s characteristic peak in the X-ray photoelectron spectroscopy, and the binding energy range of the N1s characteristic peak is 395.0 eV to 405.0 eV; I C represents the intensity of the C1s characteristic peak in the X-ray photoelectron spectroscopy, and the binding energy range of the C1s characteristic peak is 282.0 eV to 285.5 eV; in the Fourier transform infrared spectroscopy, 1350-1370 cm -1 and 945-1065 cm -1Corresponding to the stretching vibration of the C-N bond and PO4 3- asymmetric stretching vibration.

[0013] Optionally, characteristic peaks appear in the Raman spectra of the first positive electrode active material and the second positive electrode active material in the range of 940-970 cm -1 .

[0014] Optionally, the D 50 of both the first positive electrode active material and the second positive electrode active material is 600 nm to 10.0 μm.

[0015] Optionally, the tap density of the positive electrode sheet is 2.0-2.8 g / cm 3 .

[0016] Optionally, both the first positive electrode conductive agent and the second positive electrode conductive agent are conductive carbon black and carbon nanotubes, and both the first positive electrode binder and the second positive electrode binder are polyvinylidene fluoride.

[0017] Optionally, the mass ratio of the first positive electrode active material, the first positive electrode conductive agent, and the first positive electrode binder is 91%-96%:1%-4%:1%-5%, and the mass ratio of the second positive electrode active material, the second positive electrode conductive agent, and the second positive electrode binder is 92%-96%:1%-4%:1%-4%.

[0018] In some embodiments, the method for preparing the positive electrode sheet includes:

[0019] Mixing the first positive electrode active material, the first positive electrode conductive agent, and the first positive electrode binder in a solvent according to the mass ratio to obtain the material for the first positive electrode coating, and mixing the second positive electrode active material, the second positive electrode conductive agent, and the second positive electrode binder in a solvent according to the mass ratio to obtain the material for the second positive electrode coating;

[0020] Coating the material for the first positive electrode coating on one surface of the positive electrode current collector and then drying to obtain the first positive electrode coating;

[0021] Coating the material for the second positive electrode coating on the first positive electrode coating and then drying to obtain the second positive electrode coating;

[0022] Cold pressing the positive electrode current collector, the first positive electrode coating, and the second positive electrode coating to obtain the positive electrode sheet.

[0023] In some embodiments, the lithium-ion battery includes the positive electrode sheet as described in the present application.

[0024] In some embodiments, the electrical device includes the lithium-ion battery as described in the present application.

[0025] The positive electrode sheet, its preparation method, lithium-ion battery, and electrical device provided by the embodiments of the present disclosure can achieve the following technical effects:

[0026] By adopting a double-layer electrode sheet structure design, that is, carbon-coated lithium iron phosphate is used on the side close to the positive electrode current collector to construct a stable electron conduction network, reduce the contact resistance, and improve the cycle stability; nitrogen-doped lithium iron phosphate is used on the side close to the separator to enhance the electron conductivity and reduce the diffusion barrier of lithium ions in the lattice, improve the kinetic performance, thereby enhancing the rate performance and low-temperature adaptability.

[0027] The above general description and the following description are only exemplary and explanatory, and are not used to limit this application. Description of the Drawings

[0028] One or more embodiments are exemplarily illustrated by corresponding drawings. These exemplary illustrations and the drawings do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings are shown as similar elements. The drawings do not constitute a proportional limitation, and among them:

[0029] Figure 1 is a schematic structural diagram of a positive electrode sheet provided by an embodiment of the present application;

[0030] Figure 2 is a schematic structural diagram of another positive electrode sheet provided by an embodiment of the present application;

[0031] Figure 3 is an X-ray photoelectron spectroscopy diagram of the positive electrode coating provided by an embodiment of the present application;

[0032] Figure 4 is a Fourier transform infrared spectroscopy diagram of the positive electrode coating provided by an embodiment of the present application;

[0033] Figure 5 is a flowchart of the preparation method of the positive electrode sheet provided by an embodiment of the present application. Detailed Embodiments

[0034] In order to be able to understand the characteristics and technical content of the embodiments of the present disclosure in more detail, the implementation of the embodiments of the present disclosure will be described in detail below in conjunction with the drawings. The attached drawings are only for reference and explanation, and are not used to limit the embodiments of the present disclosure. In the following technical description, for the sake of explanation, sufficient understanding of the disclosed embodiments is provided through multiple details. However, one or more embodiments can still be implemented without these details. In other cases, well-known structures and devices can be shown in a simplified manner.

[0035] In the description, claims, and above-mentioned accompanying drawings of the embodiments of the present disclosure, terms such as "first" and "second" are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so as to implement the embodiments of the present disclosure described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion.

[0036] In the embodiments of the present disclosure, the orientation or positional relationship indicated by terms such as "upper", "lower", "inner", "middle", "outer", "front", and "back" is based on the orientation or positional relationship shown in the accompanying drawings. These terms are mainly used to better describe the embodiments of the present disclosure and their implementations, and are not used to limit that the indicated devices, elements, or components must have a specific orientation or be constructed and operated in a specific orientation. And, in addition to being able to represent an orientation or positional relationship, some of the above terms may also be used to represent other meanings. For example, the term "upper" may also be used to represent a certain attachment relationship or connection relationship in some cases. For those of ordinary skill in the art, the specific meanings of these terms in the embodiments of the present disclosure can be understood according to specific circumstances.

[0037] In addition, the terms "arranged", "connected", and "fixed" should be understood in a broad sense. For example, "connected" can be a fixed connection, a detachable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or there is internal communication between two devices, elements, or components. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present disclosure can be understood according to specific circumstances.

[0038] Unless otherwise specified, the term "plurality" means two or more.

[0039] In the embodiments of the present disclosure, the character " / " indicates that the objects before and after are in an "or" relationship. For example, A / B means: A or B.

[0040] The term "and / or" is an associative relationship describing an object, indicating that three relationships can exist. For example, A and / or B means: A or B, or, A and B these three relationships.

[0041] It should be noted that, without conflict, the embodiments in the embodiments of the present disclosure and the features in the embodiments can be combined with each other.

[0042] In related technologies, traditional modification methods mainly focus on the following aspects: (1) Particle size regulation - by reducing the particle size of LiFePO4, shortening the lithium-ion diffusion path, and improving the rate performance; (2) Carbon coating modification - coating conductive carbon (such as a carbon layer formed by pyrolysis of carbon sources such as glucose and citric acid) on the surface of LiFePO4 to improve electronic conductivity and reduce the contact resistance between particles, thereby improving the rate performance and cycle life; (3) By introducing hetero-valent or homo-valent metal / non-metal elements (such as Mg 2+ , Ti 4+ or N, S doping) into the LiFePO4 lattice, the electronic structure of the material can be adjusted, the energy level distribution can be optimized, and the electronic and ionic conductivities can be improved.

[0043] However, although the above methods have improved the conductivity and rate performance of lithium iron phosphate to a certain extent, there are still limitations. In this regard, as shown in Figure 1 and Figure 2 , an embodiment of the present application discloses a positive electrode plate, which includes: a positive electrode current collector and a positive electrode coating located on one surface of the positive electrode current collector. Specifically, the positive electrode coating includes: a positive electrode current collector 1 and a positive electrode coating located on at least one surface of the positive electrode current collector 1. Specifically, the positive electrode coating includes: a first positive electrode coating 2 close to the positive electrode current collector 1 and a second positive electrode coating 3 far from the positive electrode current collector 1.

[0044] Specifically, the surface of the positive electrode current collector 1 facing the separator has a positive electrode coating.

[0045] Figure 1 The shown positive electrode plate has a positive electrode coating only on one surface of the positive electrode current collector 1. When using this positive electrode plate to prepare an electrical device, the surface with the positive electrode coating faces the separator.

[0046] Figure 2 The shown positive electrode plate has positive electrode coatings on both surfaces of the positive electrode current collector 1, so as to form a four-layer positive electrode active material coating structure on both sides of the positive electrode current collector 1; when using this positive electrode plate to prepare an electrical device, any surface of the positive electrode plate with a positive electrode coating faces the separator.

[0047] The material of the first positive electrode coating includes: a first positive electrode active material, a first positive electrode conductive agent, and a first positive electrode binder. The material of the second positive electrode coating includes: a second positive electrode active material, a second positive electrode conductive agent, and a second positive electrode binder. The first positive electrode active material is carbon-coated lithium iron phosphate. The second positive electrode active material is nitrogen-doped lithium iron phosphate. And, the first positive electrode coating and the second positive electrode coating are also coated on the other surface of the positive electrode current collector, so as to form a four-layer positive electrode active material coating structure on both sides of the positive electrode current collector.

[0048] The positive electrode sheet of the embodiment of the present application adopts a double-layer electrode sheet structure design, that is, carbon-coated lithium iron phosphate is used on the side close to the positive electrode current collector to construct a stable electron conduction network, reduce the contact resistance, and improve the cycle stability; nitrogen-doped lithium iron phosphate is used on the side close to the separator to enhance the electron conductivity and reduce the diffusion barrier of lithium ions in the lattice, improve the kinetic performance, and thus enhance the rate performance and low-temperature adaptability.

[0049] Specifically, as Figure 3 and Figure 4 shown, the characteristic peak intensity in the X-ray photoelectron spectroscopy of the first positive electrode active material satisfies: I C / I Fe ≥0.5. In the Fourier transform infrared spectrum of the first positive electrode active material, no characteristic peak appears at 1350-1370 cm -1 , and characteristic peaks appear at 945-1065 cm -1 .

[0050] Specifically, as Figure 3 and Figure 4 shown, the characteristic peak intensity in the X-ray photoelectron spectroscopy of the second positive electrode active material satisfies: 0.05≤I N / I Fe ≤3.0. In the Fourier transform infrared spectrum of the second positive electrode active material, characteristic peaks appear at both 1350-1370 cm -1 and 945-1065 cm -1 .

[0051] As Figure 3 and 4 shown, where I Fe represents the intensity of the Fe2p characteristic peak in the X-ray photoelectron spectroscopy, and the binding energy range of the Fe2p characteristic peak is 710.0 eV-730.0 eV; I N represents the intensity of the N1s characteristic peak in the X-ray photoelectron spectroscopy, and the binding energy range of the N1s characteristic peak is 395.0 eV-405.0 eV; I C represents the intensity of the C1s characteristic peak in the X-ray photoelectron spectroscopy, and the binding energy range of the C1s characteristic peak is 282.0 eV-285.5 eV; in the Fourier transform infrared spectrum, 1350-1370 cm -1 and 945-1065 cm -1 correspond to the stretching vibration of the C-N bond and the asymmetric stretching vibration of PO4 3- , respectively.

[0052] Optionally, characteristic peaks appear at 940-970 cm -1 in the Raman spectra of the first positive electrode active material and the second positive electrode active material.

[0053] By selecting appropriate proportions of C and N, the kinetic performance and cycling performance of the lithium-ion battery are balanced, and ion transport is improved.

[0054] Optionally, the D of the first positive electrode active material and the second positive electrode active material 50 are both 600 nm to 10.0 μm.

[0055] By selecting appropriate D of the active material 50 , the volume expansion of the material can be reduced, and the occurrence of side reactions can be inhibited, thereby improving the cycling stability of the battery.

[0056] Optionally, the compaction density of the positive electrode sheet is 2.0 to 2.8 g / cm 3 .

[0057] By selecting appropriate compaction density of the positive electrode sheet, a balance is achieved between enhanced electron transport and hindered ion diffusion, thereby improving the kinetic performance.

[0058] In summary, through the precise regulation of the particle size of the active material and the compaction density of the positive electrode sheet, this design can improve the rate performance and low-temperature performance of the battery while ensuring excellent cycle life, achieving the synergistic optimization of the comprehensive electrochemical performance of the lithium iron phosphate battery, thereby improving the efficiency of battery design.

[0059] Optionally, the first positive electrode conductive agent and the second positive electrode conductive agent are both conductive carbon black and carbon nanotubes, and the first positive electrode binder and the second positive electrode binder are both polyvinylidene fluoride.

[0060] Optionally, the mass ratio of the first positive electrode active material, the first positive electrode conductive agent, and the first positive electrode binder is 91%-96%:1%-4%:1%-5%, and the mass ratio of the second positive electrode active material, the second positive electrode conductive agent, and the second positive electrode binder is 92%-96%:1%-4%:1%-4%. More preferably, the mass ratio of the first positive electrode active material, the first positive electrode conductive agent, and the first positive electrode binder is 96:2:2, and the mass ratio of the second positive electrode active material, the second positive electrode conductive agent, and the second positive electrode binder is 96:2:2.

[0061] Combined Figure 5 As shown, the embodiments of the present application also disclose a method for preparing a positive electrode sheet, including:

[0062] Step 501: Mix the first positive electrode active material, the first positive electrode conductive agent, and the first positive electrode binder in a solvent in a mass ratio to obtain the material for the first positive electrode coating, and mix the second positive electrode active material, the second positive electrode conductive agent, and the second positive electrode binder in a solvent in a mass ratio to obtain the material for the second positive electrode coating.

[0063] In a specific embodiment, the mass ratio is 96:2:2. The solvent can be N-methylpyrrolidone.

[0064] Step 502: Coat the material of the first positive electrode coating on one surface of the positive electrode current collector and then dry it to obtain the first positive electrode coating.

[0065] The method for preparing carbon-coated lithium iron phosphate can adopt existing methods, which will not be elaborated here.

[0066] Step 503: Coat the material of the second positive electrode coating on the first positive electrode coating and then dry it to obtain the second positive electrode coating.

[0067] The method for preparing nitrogen-doped lithium iron phosphate can adopt existing methods, which will not be elaborated here.

[0068] Step 504: Cold press the positive electrode current collector, the first positive electrode coating, and the second positive electrode coating to obtain the positive electrode plate.

[0069] The embodiment of the present application also discloses a lithium-ion battery, including the positive electrode plate of the foregoing embodiment. In addition, the lithium-ion battery further includes: a negative electrode plate, an electrolyte, and a separator.

[0070] Specifically, the capacity N / P ratio of the negative electrode plate to the positive electrode plate is 1.02 to 1.2. The negative electrode plate includes: a negative electrode current collector and negative electrode coatings located on two opposite surfaces of the negative electrode current collector. Among them, the material of the negative electrode coating includes: a negative electrode active material, a negative electrode conductive agent, a thickener, and a negative electrode binder.

[0071] Optionally, the negative electrode active material includes at least one of artificial graphite, natural graphite, soft carbon, and hard carbon.

[0072] Optionally, the negative electrode conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon black.

[0073] Optionally, the thickener is sodium carboxymethyl cellulose.

[0074] Optionally, the negative electrode binder includes at least one of sodium carboxymethyl cellulose, polyacrylic acid, polyacrylonitrile, polystyrene-acrylic acid, and styrene-butadiene rubber.

[0075] Specifically, the preparation method of the negative electrode plate is as follows:

[0076] Add the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder into deionized water according to the mass ratio and stir to form the material of the negative electrode coating. Then coat the material of the negative electrode coating on two opposite surfaces of the negative electrode current collector, and after drying and cold pressing, form the negative electrode plate.

[0077] In a specific embodiment, the mass ratio of the negative electrode active material, the negative electrode conductive agent, the thickening agent, and the negative electrode binder is: 96:1.5:1:1.5.

[0078] Specifically, the electrolyte includes: a lithium salt, a solvent, and an additive.

[0079] Optionally, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluoro(oxalato)borate, and lithium bis(trifluoromethylsulfonyl)imide.

[0080] Optionally, the solvent includes at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, and ethyl methyl carbonate.

[0081] Optionally, the additive includes at least one of fluoroethylene carbonate, difluoroethylene carbonate, vinylene sulfate, vinylene sulfite, vinylene carbonate, and vinyl carbonate.

[0082] Specifically, the preparation method of the electrolyte is as follows:

[0083] Mix the lithium salt, the solvent, and the additive evenly according to the mass ratio to obtain the electrolyte.

[0084] In a specific embodiment, the mass ratio of the lithium salt, the solvent, and the additive is: 10:77:15.

[0085] Optionally, a separator with a high porosity can be selected.

[0086] Specifically, the lithium-ion battery in the embodiment of the present application is a cylindrical battery, and it can be assembled by the following method:

[0087] Roll the positive electrode sheet and the negative electrode sheet respectively after rolling and slitting, and wind them together with the separator to obtain a cylindrical battery core. Then, weld the battery core with a connecting piece and install it into a battery case. Then, perform the processes of injecting liquid, sealing, and forming to obtain the lithium-ion battery. The case of the lithium-ion battery is a cylinder. Among them, winding is carried out in the order of the positive electrode sheet, the separator, the negative electrode sheet, and the separator from the inside to the outside. The surface of the positive electrode current collector coated with the positive electrode coating faces outward.

[0088] The battery in the embodiment of the present application has the following discharge characteristics: After the battery charged to 100% SOC charging rate with 0.1C is left standing at 25°C for 6 hours, it is discharged at a rate of 0.1C to 2.5V, and the corresponding discharge capacity is Q1; the discharge capacity when discharged at 10C to 2.5V is Q2; the retention rate of the discharge capacity Q2 / Q1 ≥ 50%.

[0089] The cylindrical battery according to the embodiment of the present application also has the following discharge characteristics: After the battery charged to 100% SOC charging rate with 0.1C is left standing at 25°C for 6 hours, at 25°C, it is discharged at a rate of 0.5C to 2.5V, and the corresponding discharge capacity is Q 25 ; After the battery charged to 100% SOC charging rate with 0.1C is left standing at -40°C for 6 hours, and at an ambient temperature of -40°C, it is discharged at a discharge rate of 5C to a voltage of 1.5V or the temperature of the electrochemical device rises to 65°C, the discharge capacity of the electrochemical device in the constant current discharge operation is Q -40 , the retention rate of the discharge capacity Q -40 / Q 25 ≥50%.

[0090] In addition, the cylindrical battery according to the embodiment of the present application has the following characteristics: After the battery with a 100% charging rate is cycled 1000 times at a rate of 1C at 25°C, the capacity attenuation rate is less than 20%.

[0091] In addition, the embodiment of the present application also discloses an electrical device, including the lithium-ion battery as described in the present application.

[0092] The technical solution of the present application will be further described below with specific embodiments.

[0093] Example 1

[0094] I. Preparation of the positive electrode sheet

[0095] The first positive electrode active material (lithium iron phosphate coated with carbon LiFePO4@C-3%, D 50 is 3.5μm), conductive carbon black, carbon nanotubes and polyvinylidene fluoride (PVDF) are fully stirred and mixed evenly in an N-methylpyrrolidone solvent system to obtain the material for the first positive electrode coating. The second positive electrode active material (lithium iron phosphate doped with nitrogen LiFePO4@N-2.0%, D 50 is 3.5μm), conductive carbon black, carbon nanotubes and polyvinylidene fluoride (PVDF) are fully stirred and mixed evenly in an N-methylpyrrolidone solvent system to obtain the material for the second positive electrode coating; then, the material for the first positive electrode coating is first coated on a positive electrode current collector (aluminum foil) with a thickness of 12.0μm. After the positive electrode sheet is dried, the material for the second positive electrode coating is coated on the first positive electrode coating. Finally, after drying and cold pressing, a positive electrode sheet is formed, and the compaction density is 2.45 g / cm 3 .

[0096] Specifically, the carbon-coated lithium iron phosphate is prepared according to the following method:

[0097] 1. Preparation of the precursor

[0098] Weigh iron phosphate (FePO4) and lithium carbonate (Li2CO3) according to a molar ratio of 100:100 and put them into a ball milling tank, and perform high-energy ball milling for 4 hours to obtain a uniformly mixed precursor.

[0099] 2. Two-step sintering doping

[0100] The first stage: Low-temperature pre-sintering

[0101] Place the ball-milled precursor in a tube furnace, and under the protection of argon (Ar), pre-sinter at 350 °C for 2 hours to remove part of the crystal water and impurities in the precursor, and at the same time improve the reaction activity of the material.

[0102] The second stage: High-temperature solid-phase synthesis

[0103] Add glucose as a carbon source, and the molar ratio of glucose to the precursor is 2% (that is, the molar ratio of iron phosphate, lithium carbonate and glucose is 100:100:4). Under the protection of argon (Ar) or nitrogen (N2), continue to heat up to 700 °C and hold for 12 hours. Glucose pyrolyzes at high temperature to form a uniform carbon coating layer, further improving the conductivity and stability of the material. Finally, LiFePO4@C-2.0% with high crystallinity is formed, where 2.0% represents the molar percentage of glucose in the precursor.

[0104] 3. Final product

[0105] After calcination, the obtained product is crushed and screened to prepare a carbon-coated lithium iron phosphate material with a particle size of 1-8 μm, that is, LiFePO4@C-2.0%.

[0106] Specifically, nitrogen-doped lithium iron phosphate is prepared according to the following method:

[0107] 1. Preparation of precursor

[0108] Weigh iron phosphate (FePO4), lithium carbonate (Li2CO3) and urea (CO(NH2)2) according to a molar ratio of 100:100:4 and put them into a ball milling tank, and perform high-energy ball milling for 4 hours to obtain a uniformly mixed precursor.

[0109] 2. Two-step sintering doping

[0110] The first stage: Low-temperature pre-sintering

[0111] Place the ball-milled precursor in a tube furnace, and under the protection of argon (Ar), pre-sinter at 350 °C for 2 hours. During this process, urea (CO(NH2)2) thermally decomposes to generate NH3 gas, promoting the preliminary embedding of nitrogen into the precursor lattice.

[0112] The second stage: High-temperature solid-phase synthesis

[0113] Under the protection of argon (Ar) or nitrogen (N₂), the temperature is continuously raised to 700 °C and held for 12 hours, enabling nitrogen (N) element to be doped into the LiFePO₄ lattice while promoting the crystallization of the material to form LiFePO₄@N-2.0%, where 2.0% represents the molar percentage of urea in the precursor.

[0114] 3. Final product

[0115] After calcination, the obtained product is crushed and sieved to prepare a nitrogen-doped lithium iron phosphate composite material with a particle size of 1 - 8 μm, namely LiFePO₄@N-2.0%.

[0116] II. Preparation of the negative electrode sheet

[0117] Calculated by mass percentage, the materials of the negative electrode coating include: 96.0% graphite, 1.5% carbon nanotubes, 1.0% thickener sodium carboxymethyl cellulose (CMC), and 1.5% binder styrene-butadiene rubber (SBR). The above substances are added to deionized water and stirred evenly to form the material of the negative electrode coating with a solid content of 40%. Then, the material of the negative electrode coating is coated on two opposite surfaces of the negative electrode current collector (copper foil), and after drying and cold pressing, a negative electrode sheet is formed with a compaction density of 1.5 g / cm 3 。

[0118] III. Preparation of the electrolyte

[0119] The lithium salt lithium hexafluorophosphate (LiPF₆), organic solvents ethylene carbonate (EC), dimethyl carbonate (DMC), the first type of additive fluoroethylene carbonate (FEC), the second type of additive vinylene sulfate (DTD), and the third type of additive vinylene carbonate (VC) are mixed evenly in a ratio of 10.0:20.0:55.0:2.0:8.0:5.0 by mass percentage to obtain the electrolyte.

[0120] IV. Separator

[0121] A separator with a high porosity is selected. The thickness of the base film PE in the separator is 9 μm, the thickness of the ceramic coating on both sides of the base film is 1.0 μm, and the thickness of the PVDF coating is 1.0 μm.

[0122] V. Assembly of the lithium-ion battery

[0123] The positive electrode sheet and the negative electrode sheet are respectively roll-pressed and slit, and then wound together with the separator to obtain a 4680 cylindrical battery core. Subsequently, after the battery core is welded to the connecting piece and placed in the battery case, after the processes of liquid injection, sealing, and formation, the lithium-ion battery of Example 1 is obtained. The case of this lithium-ion battery is cylindrical, and its size parameters are a diameter of 46.0 mm and a length of 80.0 mm.

[0124] Example 2

[0125] The difference between this example and Example 1 is that the molar percentage of urea (CO(NH2)2) in the added precursor is 4.0%, that is, LiFePO4@N-4.0%, and the others are the same as those in Example 1.

[0126] Example 3

[0127] The difference between this example and Example 1 is that the molar percentage of urea (CO(NH2)2) in the added precursor is 6.0%, that is, LiFePO4@N-6.0%, and the others are the same as those in Example 1.

[0128] Example 4

[0129] The difference between this example and Example 1 is that the molar percentage of urea (CO(NH2)2) in the added precursor is 8.0%, that is, LiFePO4@N-8.0%, and the others are the same as those in Example 1.

[0130] Example 5

[0131] The difference between this example and Example 1 is that the molar percentage of urea (CO(NH2)2) in the added precursor is 10.0%, that is, LiFePO4@N-10.0%, and the others are the same as those in Example 1.

[0132] Example 6

[0133] The difference between this example and Example 3 is that the molar percentage of added glucose is 4.0%, that is, LiFePO4@C-4.0%, and the others are the same as those in Example 3.

[0134] Example 7

[0135] The difference between this example and Example 3 is that the molar percentage of added glucose is 6.0%, that is, LiFePO4@C-6.0%, and the others are the same as those in Example 3.

[0136] Example 8

[0137] The difference between this example and Example 3 is that the molar percentage of added glucose is 8.0%, that is, LiFePO4@C-8.0%, and the others are the same as those in Example 3.

[0138] Example 9

[0139] The difference between this example and Example 3 is that the molar percentage of added glucose is 10.0%, that is, LiFePO4@C-10.0%, and the others are the same as those in Example 3.

[0140] Example 10

[0141] The difference between this example and Example 3 is that the molar percentage of added glucose is 12.0%, i.e., LiFePO4@C - 12.0%, and the others are the same as in Example 3.

[0142] Example 11

[0143] The difference between this example and Example 8 is that the compaction of the positive electrode sheet is 2.25 g / cm 3 , and the others are the same as in Example 8.

[0144] Example 12

[0145] The difference between this example and Example 8 is that the compaction of the positive electrode sheet is 2.35 g / cm 3 , and the others are the same as in Example 8.

[0146] Example 13

[0147] The difference between this example and Example 8 is that the compaction of the positive electrode sheet is 2.55 g / cm 3 , and the others are the same as in Example 8.

[0148] Example 14

[0149] The difference between this example and Example 8 is that the D of the active material of the positive electrode sheet 50 is 0.7 μm, and the others are the same as in Example 8.

[0150] Example 15

[0151] The difference between this example and Example 8 is that the D of the active material of the positive electrode sheet 50 is 1.8 μm, and the others are the same as in Example 8.

[0152] Example 16

[0153] The difference between this example and Example 8 is that the D of the active material of the positive electrode sheet 50 is 7.8 μm, and the others are the same as in Example 8.

[0154] Comparative Example 1

[0155] The difference between this comparative example and Example 8 is that the lithium iron phosphate of the positive active material is not nitrogen-doped and carbon-coated, and the others are the same as in Example 8.

[0156] Comparative Example 2

[0157] The difference between this comparative example and Example 8 is that only the active material in the material of the second positive electrode coating near the separator side is nitrogen-doped, and the molar percentage of urea (CO(NH2)2) in the added precursor is 6.0%, that is, LiFePO4@N-6.0%, while the active material in the material of the first positive electrode coating near the current collector side is not carbon-coated, and the others are the same as in Example 8.

[0158] Comparative Example 3

[0159] The difference between this example and Example 8 is that only the active material in the material of the first positive electrode coating near the current collector side is carbon-coated, and the molar percentage of added glucose is 8.0%, that is, LiFePO4@C-8.0%, while the active material in the material of the second positive electrode coating near the separator side is not nitrogen-doped, and the others are the same as in Example 8.

[0160] The lithium-ion battery of the present application is tested in the following manner:

[0161] First, discharge the lithium-ion battery at a constant current to 2.5 V to ensure it is in a safe state, reducing the risk of short circuit or thermal runaway during disassembly. Inside a glove box (protected by argon or other inert atmosphere), carefully disassemble the battery and remove the positive electrode sheet of the cylindrical battery cell. Use tweezers or a suitable tool to peel the sheet, avoiding damage to the active material layer. Then, cut the removed positive electrode sheet into appropriate sizes and soak it in an anhydrous dimethyl carbonate (DMC) solution for 30 minutes to dissolve and remove the residual electrolyte and possible by-products. After taking out the sheet, gently wipe the surface with lint-free wiping paper, then replace the fresh DMC solution and repeat the soaking-wiping process three times to ensure no residual contaminants on the sheet surface. Subsequently, rinse the sheet with absolute ethanol and wipe again to further remove the solvent and impurities. After cleaning, place the sheet in the glove box and let it stand for 48 hours to ensure it is completely dry, preventing interference from solvent residues in subsequent tests. The dried sheet is gently scraped with a plastic spatula or blade to obtain the positive active material layer. It is necessary to distinguish between the upper and lower layers. The active material powder in the upper layer (at the position of 70 - 90% of the sheet thickness) and the active material powder in the lower layer (at the position of 20 - 40% of the sheet thickness) are respectively transferred to centrifuge tubes containing absolute ethanol and ultrasonically dispersed in an ultrasonic cleaner for 30 minutes to further remove possible residual electrolyte, binder, and impurities. After ultrasonic treatment, centrifuge the sample (set the rotation speed at 5000 rpm for 2 minutes), discard the supernatant, redisperse the powder with absolute ethanol, ultrasonically treat for another 10 minutes, and then centrifuge again. Repeat this process three times to ensure the purity of the powder sample. Finally, collect the precipitate and transfer it to a vacuum drying oven, dry it at 80 °C for 12 hours to ensure complete removal of the residual solvent. The dried powder is filled into a sealed bag or sealed sample box, immediately taken out of the glove box, and quickly perform XPS, Raman characterization, and active material particle D 50 testing.

[0162] 1. Specific measurement method for the intensity (height) of elemental characteristic peaks in the XPS spectrum

[0163] The XPS test uses a PHI-5000 VersaProbe device, with AlKα (1486.6 eV) as the X-ray source and the power set at 150 W (15 kV × 10 mA). The test includes: full-spectrum scanning (0 - 1100 eV, step size 1 eV), background subtraction using Shirley background correction, and using C1s (284.8 eV) as an internal standard for data normalization and elemental quantitative analysis.

[0164] 2. Specific measurement method for Raman spectroscopy

[0165] A 532 nm laser was used as the excitation light source, and the laser power was set to 1 - 5 mW to avoid sample ablation. The sample was evenly dispersed on a silicon substrate. Raman spectra were collected in the range of 500 - 1500 cm -1 , and the spectral resolution was set to 1 cm -1 . The spectra were accumulated and scanned 3 - 5 times to improve the signal-to-noise ratio. The instrument used an XYZ automatic displacement platform for precise focusing and was calibrated with a silicon wafer (520.7 cm -1 ) before testing to ensure data accuracy.

[0166] 3. Specific measurement method for the D50 of the positive electrode active material particles

[0167] Take a small amount of powder and evenly spread it on the conductive tape. Then, take clear particle images of at least 5 different regions under a scanning electron microscope (SEM). Import the images using ImageJ or Nanomeasure software. After calibrating the scale, manually or automatically measure the projected diameters of more than 200 particles. Arrange the data in ascending order and plot the cumulative distribution curve. Take the particle size value corresponding to a cumulative percentage of 50% as D50. At the same time, interference data from obvious aggregates need to be excluded to ensure accuracy.

[0168] 4. Test method for the compaction density of the positive electrode sheet

[0169] First, cut the negative electrode sheet that has been rinsed with dimethyl carbonate and vacuum dried into 6 square samples of standard size (2.0 cm × 2.0 cm). Then, wipe off the active material on the front and back of 3 of the square samples, rinse them with ethanol, dry them, weigh them, and calculate the average mass M1. At the same time, use a micrometer to measure the average thickness L1 of the samples. Then, weigh the mass of the other 3 square samples and calculate the average mass M2. At the same time, measure the average thickness L2 of the samples. Calculate the thickness of the electrode sheet: L2 - L1, in cm; calculate the compaction density of the electrode sheet: Unit: g / cm 3 .

[0170] 5. Test method for the discharge performance at different rates at 25°C

[0171] Place the battery in an incubator at 25°C for 4 h and perform the test according to the following steps:

[0172] (1) Charge at a constant current and constant voltage to 3.65 V under the condition of 0.1C, with a cut-off current of 0.01C, and let it stand for 30 min.

[0173] (2) Discharge at a constant current to a cut-off of 2.5 V under the condition of 0.1C, and the capacity is Q1. Let it stand for 30 min.

[0174] (3) Constant current and constant voltage charge to 3.65 V under the condition of 0.1 C, with the cut-off current of 0.01 C, and stand for 30 min.

[0175] (4) Constant current discharge to 2.5 V cut-off under the condition of 10 C, the capacity is Q2, and stand for 30 min.

[0176] (5) The calculation method of 10 C capacity retention rate is: Q2 / Q1×100.

[0177] 6. Low temperature rate performance test method

[0178] Place the lithium-ion battery with 100% SOC in a 25 °C constant temperature oven for 6 h, and then discharge it to 2.5 V at a rate of 0.5 C at 25 °C, the discharge capacity is Q 25 . Place the lithium-ion battery with 100% SOC in a -40 °C constant temperature oven for 6 h, and then discharge it to 1.5 V at a rate of 5 C at -40 °C or until the device temperature rises to 65 °C, the discharge capacity is Q -40 , and count the discharge capacity retention rate Q 25 / Q -40 ×100.

[0179] 7. Cycle performance test method

[0180] Place the battery in a 25 °C constant temperature oven for 4 h, and perform the test according to the following steps:

[0181] (1) First cycle constant current and constant voltage charge: Constant current charge to 3.65 V at 0.1 C, and then switch to constant voltage charge until the current drops to 0.01 C.

[0182] (2) After charging, stand for 10 minutes.

[0183] (3) Perform constant current discharge, discharge to 2.5 V at a rate of 0.1 C.

[0184] (4) Repeat the above charging and discharging process: Constant current charge to 3.65 V at a rate of 1 C. Stand for 30 minutes again. Constant current discharge to 2.5 V at a rate of 1 C.

[0185] (5) Repeat the above charging and discharging process for a total of 1000 cycles.

[0186] (6) Count the discharge capacity Q1 and Q of the battery after 1 cycle and 1000 cycles 1000 , and count the capacity attenuation rate of the battery (Q1 - Q 1000 ) / Q1×100.

[0187] The test results of the above examples and comparative examples are shown in Table 1.

[0188] Table 1 Test results

[0189]

[0190] As shown in Table 1, by comparing the test results of Examples 1 to 10, it can be seen that the lithium-ion battery of Example 8 has the most excellent room-temperature rate performance, low-temperature rate performance, and cycle stability. At this time, I N / I Fe and I C / I Fe are 0.35 and 2.3. Deviating from this ratio will lead to a decline in the kinetic performance of the lithium-ion battery. In addition, as the value of I N / I Fe gradually increases, the cycle performance of the lithium-ion battery shows a significant downward trend, while the value of I C / I Fe has a relatively small impact on the cycle performance of the lithium-ion battery. This is attributed to the fact that the performance of the double-layer coated lithium iron phosphate electrode is jointly determined by the synergistic effect of nitrogen doping in the upper layer and carbon coating in the lower layer: when the nitrogen doping amount in the upper layer is low (I N / I Fe <0.35), nitrogen atoms form Fe-N bonds by substituting lattice oxygen, significantly improving the electronic conductivity and optimizing the Li + diffusion path. However, excessive doping (I N / I Fe >0.35) will lead to a decline in rate performance due to the surge of Fe-Li anti-site defects and the increase of grain boundary impedance, and will also result in poor lattice stability, affecting the cycle stability; when the carbon coating amount in the lower layer is low (I C / I Fe <2.3), the carbon layer improves ion transport by constructing a three-dimensional conductive network and expanding the electrode-electrolyte contact interface, while an excessive coating amount (I C / I Fe >2.3) will hinder the kinetic performance due to the reduction of porosity and the increase of Li + tortuosity. In addition, the coated carbon layer can play a structural buffering role to ensure mechanical stability, and even has a relatively small impact on the cycle stability of the battery in the case of high coating.

[0191] Comparing Example 8 with Examples 11 to 13, it can be seen that as the compaction density of the electrode gradually increases, the kinetic performance of the lithium-ion battery shows a trend of first increasing and then decreasing. This indicates that the compaction density of the lithium iron phosphate positive electrode needs to achieve a balance between enhanced electron transport and hindered ion diffusion. Appropriately increasing the compaction density can optimize the contact between particles, improve the electronic conductivity, and shorten the lithium-ion diffusion path, thereby enhancing the kinetic performance. However, when the compaction density is too high, the electrode structure may be damaged and the electrolyte penetration is limited, resulting in a decline in the kinetic performance.

[0192] Comparing Comparative Example 8 with Examples 14 to 16, it can be seen that as the D50 of the active material of the positive electrode sheet gradually increases, the kinetic performance of the battery shows a downward trend, while the cycling performance is improved. This indicates that an increase in the D50 of the active material can reduce the electrode / electrolyte interface area and extend the lithium ion diffusion path, thereby reducing the kinetic performance. However, a larger particle size can effectively reduce the volume expansion of the material and inhibit the occurrence of side reactions, thereby improving the cycling stability of the battery.

[0193] Comparing Comparative Example 8 with Comparative Examples 1 to 3, it can be seen that whether nitrogen doping and carbon coating are not carried out, or only nitrogen doping or carbon coating is carried out, the kinetic performance of the lithium battery is lower than that of the case where nitrogen doping and carbon coating are carried out simultaneously.

[0194] In summary, through the design of a double-layer positive electrode sheet, using nitrogen-doped lithium iron phosphate in the upper layer and carbon-coated lithium iron phosphate in the lower layer, and optimizing the compaction density and particle size, the rate performance, low-temperature performance, and cycling performance of lithium iron phosphate can be improved.

[0195] The above description and the drawings fully illustrate the embodiments of the present disclosure so that those skilled in the art can practice them. Other embodiments may include structural and other changes. The embodiments represent possible variations. Unless explicitly required, the individual components and functions are optional, and the order of operations may vary. Some parts and features of some embodiments may be included in or replace parts and features of other embodiments. The embodiments of the present disclosure are not limited to the structures already described and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A positive electrode plate, comprising a positive current collector and a positive electrode coating located on at least one surface of the positive current collector, characterized in that, The positive electrode coating includes: a first positive electrode coating close to the positive electrode current collector and a second positive electrode coating far from the positive electrode current collector. The materials of the first positive electrode coating include: a first positive electrode active material, a first positive electrode conductive agent, and a first positive electrode binder. The materials of the second positive electrode coating include: a second positive electrode active material, a second positive electrode conductive agent, and a second positive electrode binder. The first positive electrode active material is lithium iron phosphate coated with carbon, and the second positive electrode active material is nitrogen-doped lithium iron phosphate. Moreover, the first positive electrode coating and the second positive electrode coating are also coated on the other surface of the positive electrode current collector, so that a four-layer positive electrode active material coating structure is formed on both sides of the positive electrode current collector.

2. The positive electrode sheet according to claim 1, wherein In the X-ray photoelectron spectroscopy of the first positive electrode active material, the characteristic peak intensity satisfies: I C / I Fe ≥0.

5. In the Fourier transform infrared spectrum of the first positive electrode active material, no characteristic peak appears at 1350-1370 cm -1 , and characteristic peaks appear at 945-1065 cm -1 . In the X-ray photoelectron spectroscopy spectrum of the second positive electrode active material, the characteristic peak intensity satisfies: 0.05 ≤ I N / I Fe ≤ 3.

0. In the Fourier transform infrared spectroscopy spectrum of the second positive electrode active material, characteristic peaks appear at both 1350 - 1370 cm -1 and 945 - 1065 cm -1 ; Among them, I Fe represents the intensity of the Fe2p characteristic peak in the X-ray photoelectron spectroscopy diagram, and the binding energy range of the Fe2p characteristic peak is 710.0 eV to 730.0 eV; I N represents the intensity of the N1s characteristic peak in the X-ray photoelectron spectroscopy diagram, and the binding energy range of the N1s characteristic peak is 395.0 eV to 405.0 eV; I C represents the intensity of the C1s characteristic peak in the X-ray photoelectron spectroscopy diagram, and the binding energy range of the C1s characteristic peak is 282.0 eV to 285.5 eV; in the Fourier transform infrared spectroscopy diagram, 1350-1370 cm -1 and 945-1065 cm -1 correspond to the stretching vibration of the C-N bond and the asymmetric stretching vibration of PO4 3- respectively.

3. The positive electrode sheet according to claim 1, characterized in that, Characteristic peaks appear in the Raman spectra of the first positive electrode active material and the second positive electrode active material in the range of 940-970 cm -1 .

4. The positive electrode sheet according to claim 1, wherein The D of the first positive electrode active material and the second positive electrode active material 50 are both 600 nm to 10.0 μm.

5. The positive electrode sheet according to claim 1, characterized in that, The tap density of the positive electrode sheet is 2.0 to 2.8 g / cm 3 .

6. The positive electrode sheet according to claim 1, characterized in that, Both the first positive electrode conductive agent and the second positive electrode conductive agent are conductive carbon black and carbon nanotubes, and both the first positive electrode binder and the second positive electrode binder are polyvinylidene fluoride.

7. The positive electrode sheet according to claim 1, wherein, The mass ratio of the first positive electrode active material, the first positive electrode conductive agent, and the first positive electrode binder is 91%-96%:1%-4%:1%-5%, and the mass ratio of the second positive electrode active material, the second positive electrode conductive agent, and the second positive electrode binder is 92%-96%:1%-4%:1%-4%.

8. A method for preparing a positive electrode sheet according to any one of claims 1 to 7, characterized in that, Comprising: Mixing the first positive electrode active material, the first positive electrode conductive agent, and the first positive electrode binder in a solvent according to the mass ratio to obtain the material of the first positive electrode coating, and mixing the second positive electrode active material, the second positive electrode conductive agent, and the second positive electrode binder in a solvent according to the mass ratio to obtain the material of the second positive electrode coating; Coating the material of the first positive electrode coating on one surface of the positive electrode current collector and then drying it to obtain the first positive electrode coating; Coating the material of the second positive electrode coating on the first positive electrode coating and then drying it to obtain the second positive electrode coating; Cold pressing the positive electrode current collector, the first positive electrode coating, and the second positive electrode coating to obtain a positive electrode plate.

9. A lithium-ion battery, characterized in that, Comprising the positive electrode plate according to any one of claims 1-8.

10. An electrical device, characterized in that, Comprising the lithium ion battery according to claim 9.

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