Positive pole piece, lithium ion battery and electric device

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a positive pole piece, a lithium ion battery and an electric device. Wherein the positive pole piece comprises a positive current collector and a positive active material coating coated on at least one surface of the positive current collector, and is characterized in that the positive active material coating comprises large polycrystalline particles and small single crystal particles, and the positive pole piece meets the following relational expression: PD + K + n B-4. 20 is more than or equal to 0 and less than or equal to 0.30; wherein PD is the compaction density of the positive pole piece, K and n are contribution proportion coefficients of quantized single crystal small particles and polycrystalline large particles, A is the ratio of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles, and B is the mass percent of the polycrystalline large particles in the whole positive active material coating.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to a positive electrode, a lithium-ion battery, and an electrical device. Background Technology

[0002] Currently, with the rapid development of new energy vehicles and the energy storage industry, high-energy-density ternary lithium-ion batteries have become the market mainstream. However, while pursuing high specific capacity, high-nickel cathode materials also generally face challenges. High-nickel ternary cathode materials face the dual challenges of rapid lifespan degradation and insufficient thermal safety performance during long-term charge-discharge cycles. Fundamentally, structural instability of the cathode sheet during long-term repeated discharge is one of the core contributing factors. Currently, the industry generally improves battery energy density by increasing nickel content or increasing electrode compaction, but higher nickel content brings greater risks. High electrode compaction is a common choice in related technologies.

[0003] In the process of implementing the embodiments of this disclosure, at least the following problems were found in the related art:

[0004] Current technologies that improve electrode compaction density through gradation design only focus on optimizing porosity and fail to consider the stress distribution of monocrystalline and polycrystalline materials with different particle sizes. High-compaction electrodes achieved through this single improvement method are prone to particle breakage, leading to binder failure. Consequently, lithium-ion batteries, while maintaining high compaction, struggle to achieve good cycle life and rate performance, and exhibit relatively low thermal safety.

[0005] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0006] To provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. This summary is not intended as a general commentary, nor is it intended to identify key / important components or describe the scope of protection of these embodiments, but rather as a prelude to the detailed description that follows.

[0007] This disclosure provides a positive electrode sheet, a lithium-ion battery, and an electrical device. By establishing a system model that combines single-crystal particles and polycrystalline particles, it achieves high compaction of the lithium-ion battery while maintaining its structural stability. Furthermore, it effectively improves the cycle life and rate performance of the lithium-ion battery and enhances its thermal safety performance while ensuring high energy density.

[0008] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material coating coated on at least one surface of the positive current collector, wherein the positive active material coating includes polycrystalline large particles and single-crystal small particles, and the positive electrode sheet satisfies the following relationship:

[0009] 0≤|PD+K˙A+n˙B-4.20|≤0.30;

[0010] Wherein, PD is the compaction density of the positive electrode sheet, K and n are the contribution ratio coefficients of quantified single-crystal small particles and polycrystalline large particles, A is the ratio of the Dv50 particle size of single-crystal small particles to the Dv50 particle size of polycrystalline large particles, and B is the mass percentage of polycrystalline large particles in the entire positive electrode active material coating.

[0011] Optionally, the polycrystalline large particles and single-crystal small particles are lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or lithium nickel cobalt manganese aluminum oxide, with the general chemical formula LiNi. x Co y M 1-x-y O2, where M is Mn or Al, 0.80≤x≤0.95, 0≤y≤0.25.

[0012] Optionally, the D50 particle size of the polycrystalline large particles satisfies: 8μm≤D50≤13μm.

[0013] Optionally, the D50 particle size of the single crystal particles satisfies: 3μm≤D50≤5μm.

[0014] Optionally, the value range of the contribution ratio coefficient K satisfies: 0.16 < K ≤ 0.25; and the value range of the contribution ratio coefficient n satisfies: 0.57 < n ≤ 0.65.

[0015] Optionally, the ratio A of the Dv50 particle size of the single-crystal small particles to the Dv50 particle size of the polycrystalline large particles satisfies: 0.20≤A≤0.50; and the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating satisfies: 50wt%≤B≤90wt%.

[0016] Optionally, the polycrystalline large particles and single-crystal small particles may further include one or more doping elements selected from Al, Mg, Ti, Zr, Cr, Ce, Te, Sr, Y, La, F, B and P.

[0017] Optionally, the tap density TD of the mixed particles formed by the polycrystalline large particles and the single-crystal small particles satisfies: TD ≥ 2.0 g / cm³. 3 Furthermore, the specific surface area (BET) of the mixed particles satisfies: 0.5 m² / m³. 2 / g≤BET≤1.0m 2 / g.

[0018] Optionally, the positive current collector is an aluminum foil, the thickness H of which satisfies: 8μm≤H≤15μm, and the roughness Ra of which satisfies: 0.2μm≤Ra≤0.8μm.

[0019] Optionally, the thickness h of the positive electrode sheet satisfies: 100μm≤h≤300μm, and the porosity a of the positive electrode sheet satisfies: 15%≤a≤58%.

[0020] Optionally, the breakage rate d of the polycrystalline large particles of the positive electrode sheet after 500 cycles at room temperature satisfies: 5% ≤ d ≤ 15%.

[0021] In some embodiments, the lithium-ion battery comprises a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is the positive electrode as described in this application.

[0022] In some embodiments, the electrical device includes a lithium-ion battery as described in this application.

[0023] The positive electrode, lithium-ion battery, and power-consuming device provided in this disclosure can achieve the following technical effects:

[0024] By forming a system model that mixes polycrystalline large particles and monocrystalline small particles in the positive electrode active material coating, the polycrystalline structure enhances the ion diffusion kinetics of the system model, while the monocrystalline structure provides structural stability. Their synergistic effect effectively improves the cycle life and rate performance of lithium-ion batteries. Simultaneously, by adjusting the ratio A of the Dv50 particle size of the monocrystalline small particles to that of the polycrystalline large particles, and the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating, the compaction density PD of the positive electrode sheet is made to satisfy a specific numerical range. Since different types and sizes of monocrystalline small particles and polycrystalline large particles respond differently to external forces on the electrode sheet, the internal stress can be released by adjusting the appropriate gradation and particle size balance. The synergistically established system model can then predict high compaction of the electrode sheet, ensuring high energy density while reducing the risk of particle breakage, thereby improving the cycle life and thermal safety performance of lithium-ion batteries.

[0025] The above general description and the description below are exemplary and illustrative only and are not intended to limit this application. Attached Figure Description

[0026] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations and drawings do not constitute a limitation on the embodiments. Elements having the same reference numerals in the drawings are shown as similar elements. The drawings are not to be scaled. And wherein:

[0027] Figure 1 This is a schematic diagram of the structure of a lithium-ion battery provided in an embodiment of this disclosure;

[0028] Figure 2 This is a schematic diagram of the structure of a lithium-ion battery provided in an embodiment of this disclosure;

[0029] Figure 3 This is a schematic diagram of a lithium-ion battery provided in an embodiment of this disclosure;

[0030] Figure 4 This is a flat lay diagram of the positive electrode sheet of a battery disassembled according to an embodiment of this disclosure.

[0031] Figure label:

[0032] 1-Positive terminal; 10-Battery core; 11-Positive terminal post; 12-Negative terminal; 2-Shell; 3-Negative electrode sheet; 4-Separator; 5-Positive electrode sheet. Detailed Implementation

[0033] To provide a more detailed understanding of the features and technical content of the embodiments of this disclosure, the implementation of the embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. The accompanying drawings are for illustrative purposes only and are not intended to limit the embodiments of this disclosure. In the following technical description, for ease of explanation, several details are used to provide a full understanding of the disclosed embodiments. However, one or more embodiments may still be implemented without these details. In other cases, well-known structures and devices may be simplified in their depiction to simplify the drawings.

[0034] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this disclosure are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this disclosure described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0035] In this disclosure, the terms "upper," "lower," "inner," "middle," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for better description of the embodiments of this disclosure and their implementations, and are not intended to limit the indicated devices, elements, or components to having a specific orientation, or to require them to be constructed and operated in a specific orientation. Furthermore, some of the aforementioned terms may be used to indicate other meanings besides orientation or positional relationship; for example, the term "upper" may in some cases indicate a dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in the embodiments of this disclosure according to the specific circumstances.

[0036] Furthermore, the terms "set up," "connect," and "fix" should be interpreted broadly. For example, "connection" 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 a direct connection or an indirect connection through an intermediate medium, or it can be an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this disclosure according to the specific circumstances.

[0037] Unless otherwise stated, the term "multiple" means two or more.

[0038] In this embodiment of the disclosure, the character " / " indicates that the objects before and after it are in an "or" relationship. For example, A / B means: A or B.

[0039] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0040] It should be noted that, unless otherwise specified, the embodiments and features described in the present disclosure can be combined with each other.

[0041] Combination Figure 1 As shown, this embodiment of the present disclosure provides a lithium-ion battery, including a cylindrical casing 2, the interior of which is used to house a battery core, the top being a positive terminal 1, and the bottom being a negative terminal 12. A positive terminal post 11 is disposed on the positive terminal 1. Specifically, Figure 2 A schematic diagram of the battery core structure in this application is shown. Figure 3 A schematic diagram of the unfolded battery core in this application is shown. The lithium-ion battery includes a positive electrode 5, a negative electrode 3, a separator 4, and an electrolyte, wherein the positive electrode 5, the negative electrode 3, and the separator 4 are as follows: Figure 3 As shown, layers are stacked and then wound to form a shape like... Figure 2The cylindrical battery core 10 shown is initially wound to the electrode end at the cylindrical axis, and ends to the electrode end on the outer surface of the cylinder after winding. The positive electrode 5 includes a strip-shaped positive electrode foil, a positive electrode coating on the surface of the strip-shaped positive electrode foil, and a first empty foil area. The negative electrode 3 includes a strip-shaped negative electrode foil, a negative electrode coating on the surface of the strip-shaped negative electrode foil, and a second empty foil area. The first and second empty foil areas are perpendicular to the winding direction and are formed into the top or bottom end face of the lithium-ion battery by methods such as flattening or folding.

[0042] Furthermore, in order to achieve high-pressure compaction of lithium-ion batteries while maintaining structural stability, and to effectively improve cycle life and rate performance while ensuring high energy density, as well as enhance thermal safety performance, further research is needed.

[0043] In this regard, the present disclosure provides a positive electrode sheet, including a positive current collector and a positive active material coating coated on at least one surface of the positive current collector, wherein the positive active material coating includes polycrystalline large particles and single-crystal small particles, wherein the positive electrode sheet satisfies the following relationship:

[0044] 0≤|PD+K˙A+n˙B-4.20|≤0.30;

[0045] Wherein, PD is the compaction density of the positive electrode sheet, K and n are the contribution ratio coefficients of single-crystal small particles and polycrystalline large particles, the value range of K is: 0.16 < K ≤ 0.25, the value range of n is: 0.57 < n ≤ 0.65 (wherein, K is preferably 0.18, and n is preferably 0.59), A is the ratio of the Dv50 particle size of single-crystal small particles to the Dv50 particle size of polycrystalline large particles, that is, A = Dv50 single crystal / Dv50 polycrystalline, and B is the mass percentage of polycrystalline large particles in the entire positive electrode active material coating.

[0046] Using the positive electrode sheet provided in this disclosure, a system model is formed in the positive electrode active material coating by mixing polycrystalline large particles and single-crystal small particles. Polycrystalline particles enhance the ion diffusion kinetics performance of the system model, while single-crystal particles provide structural stability. Their synergistic effect effectively improves the cycle life and rate performance of the lithium-ion battery. Simultaneously, by adjusting the ratio A of the Dv50 particle size of the single-crystal small particles to the Dv50 particle size of the polycrystalline large particles, and the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating, the compaction density PD of the positive electrode sheet is made to satisfy a specific numerical range. Since different types and sizes of single-crystal small particles and polycrystalline large particles respond differently to external forces on the electrode sheet, internal stress can be released by adjusting the appropriate gradation and particle size balance. The synergistically established system model can then predict high compaction of the electrode sheet, ensuring high energy density as much as possible while reducing the risk of particle breakage, thereby improving the cycle life and thermal safety performance of the lithium-ion battery.

[0047] Optionally, the polycrystalline large particles and single-crystal small particles of this application are lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or lithium nickel cobalt manganese aluminum oxide, with the general chemical formula LiNi. x Co y M 1-x-y O2, where M is Mn or Al, 0.80≤x≤0.95, 0≤y≤0.25.

[0048] Optionally, the D50 particle size of the polycrystalline large particles in this application satisfies: 8μm≤D50≤13μm, wherein, more preferably, the D50 particle size of the polycrystalline large particles satisfies: 9μm≤D50≤12μm.

[0049] Optionally, the D50 particle size of the single crystal particles in this application satisfies: 3μm≤D50≤5μm.

[0050] Optionally, the ratio A of the Dv50 particle size of the single-crystal small particles to the Dv50 particle size of the polycrystalline large particles in this application satisfies: 0.20 ≤ A ≤ 0.50, wherein, more preferably, the ratio A satisfies: 0.25 ≤ A ≤ 0.45. The mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating satisfies: 50 wt% ≤ B ≤ 90 wt%, wherein, more preferably, the mass percentage B satisfies: 60 wt% ≤ B ≤ 80 wt%.

[0051] Optionally, the polycrystalline large particles and single-crystal small particles of this application may further include one or more doping elements selected from Al, Mg, Ti, Zr, Cr, Ce, Te, Sr, Y, La, F, B and P.

[0052] Optionally, the tap density TD of the mixed particles formed by the polycrystalline large particles and the single-crystal small particles satisfies: TD ≥ 2.0 g / cm³. 3 Furthermore, the specific surface area (BET) of the mixed particles satisfies: 0.5 m² / m³. 2 / g≤BET≤1.0m 2 / g.

[0053] Optionally, the positive current collector of this application is an aluminum foil, wherein the thickness H of the aluminum foil satisfies: 8μm≤H≤15μm, and the roughness Ra of the aluminum foil satisfies: 0.2μm≤Ra≤0.8μm.

[0054] Optionally, the thickness h (including the positive current collector) of the positive electrode sheet in this application satisfies: 100μm≤h≤300μm, and the porosity a of the positive electrode sheet satisfies: 15%≤a≤58%.

[0055] Optionally, the breakage rate d of the polycrystalline large particles of the positive electrode sheet of this application after 500 cycles at room temperature satisfies: 5% ≤ d ≤ 15%.

[0056] In some embodiments, the lithium-ion battery comprises a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode is the positive electrode as described in this application.

[0057] In some embodiments, this disclosure also provides an electrical device including a lithium-ion battery as described in this application.

[0058] The present invention will be further explained and illustrated below with reference to embodiments.

[0059] Example 1

[0060] This embodiment 1 provides a method for preparing a lithium-ion battery as follows:

[0061] 1. Preparation of the positive electrode sheet: Both polycrystalline large particles and single-crystal small particles of the positive electrode active material are used.

[0062] LiNi 0.80 Co 0.15 Al 0.05The ratio A of the Dv50 particle size of the single-crystal small particles to the Dv50 particle size of the polycrystalline large particles is 0.20; the polycrystalline large particles account for 50% of the total mass percentage of the positive electrode active material coating; the compaction density PD of the positive electrode sheet is 3.58, of which the polycrystalline large particles account for 70 wt% of the positive electrode active material, the active material layer accounts for 96% of the total positive electrode formulation, and 1.5% of polyvinylidene fluoride (PVDF) binder, 1.7% of Super-P (conductive carbon black) conductive agent and 0.8% of multi-walled carbon nanotube conductive agent are added; the above substances are added to the solvent NMP and stirred to obtain a positive electrode slurry with a solid content of 65%, which is then coated on both sides of the current collector to obtain the positive electrode sheet.

[0063] 2. Method for preparing negative electrode sheet: The negative electrode sheet includes a negative electrode current collector copper foil and a negative electrode active material layer coated on both sides of the current collector. The negative electrode coating comprises 96% artificial graphite and silicon carbide, 1.0% conductive agent, 1.0% thickener CMC and 2% binder polyacrylic acid, calculated by mass percentage. The above materials are added to deionized water and stirred to obtain a negative electrode slurry with a solid content of 40%. The slurry is then coated on both sides of the copper foil to obtain the negative electrode sheet.

[0064] 3. Electrolyte preparation method: Lithium salt LiPF6 is dissolved in an organic solvent, which is prepared by mixing ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate in a volume ratio of 1:1:1, and finally prepared into an electrolyte with a concentration of 1 mol / L.

[0065] 4. Preparation method of the diaphragm: The diaphragm includes a high porosity diaphragm, the polyethylene base membrane has a thickness of 9μm, the ceramic coating on both sides has a thickness of 1μm, the PVDF coating has a thickness of 1μm, and the air permeability of the diaphragm is ≤120s / 100mL.

[0066] 5. Assembly method: After the positive electrode sheet and negative electrode sheet are rolled, slit and die-cut respectively, they are wound together with the separator to form a battery core. The tabs are flattened, and then the positive and negative current collectors are welded on respectively. The negative current collector is welded to the steel shell and then installed in the battery casing. The insulating sheet is then installed on the positive current collector. Finally, the liquid injection, sealing and formation processes are completed to obtain the experimental battery.

[0067] Example 2

[0068] The difference between this embodiment and Embodiment 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.80 Co 0.1 Mn 0.1@LiF, the ratio A of the Dv50 particle size of its single crystal small particles to the Dv50 particle size of its polycrystalline large particles is 0.25; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 60%; the compaction density PD of the positive electrode sheet is 3.53, and all other aspects are the same as in Example 1.

[0069] Example 3

[0070] The difference between this embodiment and Embodiment 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.92 Co 0.02 Mn 0.04 Al 0.02 O2, the ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles is 0.34; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 70%; the compaction density PD of the positive electrode sheet is 3.61, and all other conditions are the same as in Example 1.

[0071] Example 4

[0072] The difference between this embodiment and Embodiment 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is Li(Ni) 0.90 Co 0.05 Mn 0.03 Al 0.02 ) 0.97 La 0.03 O2, the ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles is 0.35; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 80%; the compaction density PD of the positive electrode sheet is 3.58, and all other conditions are the same as in Example 1.

[0073] Example 5

[0074] The difference between this embodiment and Embodiment 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.92 Co 0.04 Mn 0.02 Mg 0.01 Ti 0.01 The ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles in O2@Al2O3 is 0.42; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 90%; the compaction density PD of the positive electrode sheet is 3.65, and all other conditions are the same as in Example 1.

[0075] Example 6

[0076] The difference between this embodiment and Embodiment 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.87 Co 0.06 Al 0.068 Nb 0.002 O2, the ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles is 0.45; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 50%; the compaction density PD of the positive electrode sheet is 3.70, and all other conditions are the same as in Example 1.

[0077] Example 7

[0078] The difference between this embodiment and Embodiment 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.93 Co 0.02 Mn 0.04 W 0.01 The ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles in O2@Li3BO3 is 0.50; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 90%; the compaction density PD of the positive electrode sheet is 3.55, and all other conditions are the same as in Example 1.

[0079] Comparative Example 1

[0080] The difference between this comparative example and Example 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.92 Co 0.04 Mn 0.02 Al 0.02 The ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles is 0.12; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 40%; the compaction density PD of the positive electrode sheet is 3.50, and all other conditions are the same as in Example 1.

[0081] Comparative Example 2

[0082] The difference between this comparative example and Example 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.90 Co 0.05 Mn 0.03 Al 0.02 The ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles is 0.60; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 93%; the compaction density PD of the positive electrode sheet is 3.85, and all other conditions are the same as in Example 1.

[0083] Comparative Example 3

[0084] The difference between this comparative example and Example 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.93 Co 0.02 Mn 0.03 Al 0.02 The ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles is 0.63; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 95%; the compaction density PD of the positive electrode sheet is 3.70, and all other conditions are the same as in Example 1.

[0085] Comparative Example 4

[0086] The difference between this comparative example and Example 1 is that the chemical formula of the positive electrode active material (polycrystalline large particles and single-crystal small particles) is LiNi. 0.88 Co 0.08 Mn 0.02 Al 0.0 The ratio A of the Dv50 particle size of the single crystal small particles to the Dv50 particle size of the polycrystalline large particles in O2@Li3PO4 is 0.58; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating is 40%; the compaction density PD of the positive electrode sheet is 3.55, and all other conditions are the same as in Example 1.

[0087] The composition and specific parameters of the positive electrode are shown in Table 1 below:

[0088] Table 1

[0089]

[0090] The data results comparing Examples 1-7 and Comparative Examples 1-4 in Table 1 show that this application has established a quantitative model for a monocrystalline / polycrystalline hybrid system. The model is based on the relationship 0 ≤ |PD + K˙A + n˙B - 4.20| ≤ 0.30, where PD is the compaction density of the cathode electrode, K and n are the contribution ratio coefficients of the monocrystalline small particles and the polycrystalline large particles (0.16 < K ≤ 0.25, 0.57 < n ≤ 0.65), A is the ratio of the Dv50 particle size of the monocrystalline small particles to that of the polycrystalline large particles, and B is the percentage of the mass of the polycrystalline large particles in the entire cathode active material coating. By considering the optimization of the cathode material from multiple dimensions, including gradation adjustment and monocrystalline / polycrystalline stress distribution, and by adjusting the relevant parameters, the structural stability of the cathode material, the cycle life of the lithium-ion battery, and its safety performance can be effectively improved. When the value of |PD+K˙A+n˙B-4.20| is 0.03, the cathode material exhibits excellent structural stability. After 500 cycles, the electrode particle breakage rate is only 6.21%, and the cycle capacity retention rate is the highest at 96.2%, with an overcharge pass rate of 100%. As shown in Table 1, comparing the data results of Examples 1-7 with Comparative Examples 1-4, by adjusting the polycrystalline ratio, the particle size ratio of single crystals to polycrystalline crystals, and the compaction density of the cathode material, and keeping the value of |PD+K˙A+n˙B-4.20| within the range of 0 to 0.30, the 1C / 1C, 500-cycle capacity retention rate of the lithium-ion battery is maintained above 85%, and the breakage rate of polycrystalline particles is also kept within an acceptable range. On the one hand, this avoids excessive particle breakage leading to uncontrolled oxygen release and potential thermal risks; on the other hand, it allows the material to have a certain gas generation capacity, enabling the battery to achieve safety protection by opening the CID under high-temperature testing. Compared to other examples, Example 5 shows a slightly lower cycle capacity retention rate because the positive electrode active material is predominantly polycrystalline and the electrode compaction density is slightly higher, resulting in some electrode particle breakage and more side reactions. Comparative Examples 1 and 4 show that the positive electrode active material has a low polycrystalline content (40%), mainly composed of single crystals. Although the electrode compaction density is low and the particle breakage rate is low, insufficient compaction leads to a loose internal structure, weak bonding, poor electronic conductivity, and tortuous ion migration paths. This makes the structure prone to collapse, resulting in more side reactions and a decrease in cycle life and safety performance. Comparative Examples 2 and 3 show that the ratio of single crystal to polycrystalline particle size is relatively large, with polycrystalline content exceeding 90% of the positive electrode active material and the electrode being produced under high voltage. This results in severe particle breakage in the positive electrode, significant oxygen release from the material, and severe side reactions with the electrolyte, significantly reducing the cycle life of the lithium-ion battery and posing a very high thermal safety risk.

[0091] The specific test methods and steps for each performance aspect are as follows:

[0092] Test 1: Capacity retention test, 25℃, 1C / 1C

[0093] Take a fresh battery, place it in a 25°C constant temperature chamber for more than 4 hours, and test it according to the following steps:

[0094] (1) Discharge the battery at a constant current of 0.1C until it is cut off at 2.5V, and let it stand for 5 minutes;

[0095] (2) Charge the battery at a constant current of 0.2C until it reaches 4.2V and then charge it at a constant voltage until it reaches 0.05C and then let it stand for 5 minutes.

[0096] (3) Discharge the battery at a constant current of 0.2C until it is cut off at 2.5V, and let it stand for 5 minutes. Then read the capacity value C0 at this time.

[0097] (4) Charge the battery at a constant current of 1.0C to 4.2V, and then charge it at a constant voltage of 0.05C until it stops, and let it stand for 5 minutes.

[0098] (5) Discharge the battery at a constant current of 1.0C until it is cut off at 2.5V, and let it stand for 5 minutes;

[0099] (6) Repeat steps (4) and (5) 500 times;

[0100] (7) The discharge capacity of the 500th cycle and the capacity retention rate of the 1st cycle.

[0101] Test 2: Particle breakage rate test

[0102] Take a battery that has undergone a capacity retention test, disassemble the battery, remove the positive electrode, and combine it with... Figure 4 As shown, a 1cm×1cm positive electrode sheet was taken from the middle of the length and width directions of the positive electrode sheet, and two more positive electrode sheets of the same size were taken from its left and right sides. After cleaning, these three positive electrode sheets were sent for testing and CP-SEM test with a resolution of 1.00K. A 200μm×100μm range centered on the current collector was selected for breakage rate statistics. Breakage rate = N_fragmented / N_total×100%, where N_fragmented represents the number of large polycrystalline particles that were broken. When the length of a single crack on a large polycrystalline particle exceeds 1 / 4 of the particle diameter or the number of cracks reaches two or more, the large polycrystalline particle is considered to be broken. N_total represents the total number of large polycrystalline particles within the statistical range.

[0103] Test 3: Overcharge Test

[0104] Take a cylindrical lithium battery prepared according to any of the above embodiments, place it in a constant temperature chamber at 25°C for more than 4 hours, and test it according to the following steps:

[0105] (1) Discharge the battery at a constant current of 0.1C until it is cut off at 2.5V, and let it stand for 5 minutes;

[0106] (2) Charge the battery at a constant current of 0.2C until it reaches 4.2V and then charge it at a constant voltage until it reaches 0.05C and then let it stand for 5 minutes.

[0107] (3) Discharge the battery at a constant current of 0.2C until it is cut off at 2.5V, and let it stand for 5 minutes;

[0108] (4) Temperature sensing wires are attached to the head, middle and bottom of the battery cell to collect temperature signals;

[0109] (5) Charge at 3C constant current to 6.0V, then switch to constant voltage charging for 1 hour, with a sampling interval of 1 second;

[0110] (6) Let stand for 30 minutes;

[0111] (7) A cell that does not catch fire or explode, and whose maximum temperature does not exceed 150°C, is considered to have passed. The pass rate is calculated after parallel testing of 5 cells.

[0112] The foregoing description and accompanying drawings fully illustrate embodiments of the present disclosure to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Individual components and functions are optional unless explicitly required, and the order of operation may vary. Parts and features of some embodiments may be included or substituted for parts and features of other embodiments. Embodiments of the present disclosure are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes may 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 sheet, comprising a positive current collector and a positive active material coating coated on at least one surface of the positive current collector, characterized in that, The positive electrode active material coating comprises polycrystalline large particles and single-crystal small particles, wherein the positive electrode sheet satisfies the following relationship: 0≤|PD+K˙A+n˙B-4.20|≤0.30; Wherein, PD is the compaction density of the positive electrode sheet, K and n are the contribution ratio coefficients of quantified single-crystal small particles and polycrystalline large particles, A is the ratio of the Dv50 particle size of single-crystal small particles to the Dv50 particle size of polycrystalline large particles, and B is the mass percentage of polycrystalline large particles in the entire positive electrode active material coating.

2. The positive electrode sheet according to claim 1, characterized in that, The polycrystalline large particles and single-crystal small particles are lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or lithium nickel cobalt manganese aluminum oxide.

3. The positive electrode sheet according to claim 1, characterized in that, The D50 particle size of the polycrystalline large particles satisfies: 8μm≤D50≤13μm.

4. The positive electrode sheet according to claim 1, characterized in that, The D50 particle size of the single crystal particles satisfies: 3μm≤D50≤5μm.

5. The positive electrode sheet according to claim 1, characterized in that, The value range of the contribution ratio coefficient K is: 0.16 < K ≤ 0.25; the value range of the contribution ratio coefficient n is: 0.57 < n ≤ 0.

65.

6. The positive electrode sheet according to claim 1, characterized in that, The ratio A of the Dv50 particle size of the single-crystal small particles to the Dv50 particle size of the polycrystalline large particles satisfies: 0.20≤A≤0.50; the mass percentage B of the polycrystalline large particles in the entire positive electrode active material coating satisfies: 50wt%≤B≤90wt%.

7. The positive electrode sheet according to claim 1, characterized in that, The polycrystalline large particles and single-crystal small particles also include one or more doping elements selected from Al, Mg, Ti, Zr, Cr, Ce, Te, Sr, Y, La, F, B and P.

8. The positive electrode sheet according to claim 1, characterized in that, The tap density TD of the mixed particles formed by the polycrystalline large particles and the single crystal small particles satisfies: TD ≥ 2.0 g / cm³ 3 Furthermore, the specific surface area (BET) of the mixed particles satisfies: 0.5 m² / m³. 2 / g≤BET≤1.0m 2 / g.

9. The positive electrode sheet according to claim 1, characterized in that, The positive current collector is an aluminum foil, the thickness H of which satisfies: 8μm≤H≤15μm, and the roughness Ra of which satisfies: 0.2μm≤Ra≤0.8μm.

10. The positive electrode sheet according to claim 1, characterized in that, The thickness h of the positive electrode sheet satisfies: 100μm≤h≤300μm, and the porosity a of the positive electrode sheet satisfies: 15%≤a≤58%.

11. The positive electrode sheet according to claim 1, characterized in that, The breakage rate d of the polycrystalline large particles of the positive electrode sheet after 500 cycles at room temperature satisfies: 5% ≤ d ≤ 15%.

12. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode is the positive electrode as described in any one of claims 1 to 11.

13. An electrical appliance, characterized in that, Including the lithium-ion battery as described in claim 12.

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