Positive pole piece, preparation method thereof and cylindrical battery

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a positive pole piece, a preparation method of the positive pole piece and a cylindrical battery, the positive pole piece comprises a positive current collector and a positive material active substance layer located on the surface of the positive current collector, the positive material active substance layer comprises active particles, at least part of the active particles are embedded into the positive electrode current collector, an embedded layer is formed in the positive electrode current collector, and the average embedded depth H of the embedded layer is 0.5-4 [mu] m; wherein the active particles comprise a first type of particles and a second type of particles, the average particle size Da50 of the first type of particles is larger than the average particle size Db50 of the second type of particles, the ratio of the average particle size Da50 of the first type of particles to the average particle size Db50 of the second type of particles is 1.33-5.6, and the volume ratio E of the first type of particles to the whole active particles is 0.5-0.9. The compaction density P of the positive pole piece is 3.1-3.8 g / cm < 3 >, and the embedding factor Z of the active particles is calculated according to the following formula: Z = P * E * Da50 / Db50, and Z is more than or equal to 4 and less than or equal to 15.

Description

Technical Field

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

[0002] At present, lithium-ion batteries are a common energy storage device and can be used in various application scenarios such as vehicles, mobile phones, power tools, energy storage power stations, etc. to provide energy. In the related art, the positive electrode sheet of a lithium-ion battery includes a positive electrode current collector (aluminum foil) and a positive electrode active material layer coated on the surface of the positive electrode current collector. During the production process of a lithium-ion battery, the bonding quality between the positive electrode active material particles in the positive electrode active material layer and the positive electrode current collector directly affects the performance and safety of the battery. As the core material of the positive electrode current collector, the bonding strength, contact area, and embedding depth between the aluminum foil and the positive electrode active material particles are the key factors determining the battery performance.

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

[0004] The bonding method between the positive electrode active material particles and the positive electrode current collector in the related art often has the problem of insufficient bonding strength, resulting in that during the charge and discharge process of the battery, especially under high-rate charge and discharge and long-cycle conditions, the positive electrode active material particles are prone to fall off from the aluminum foil, affecting the cycle stability and energy density of the battery. At the same time, the contact area between the positive electrode active material particles and the aluminum foil is limited, restricting the electron transfer efficiency, thereby affecting the rate performance and cycle stability of the battery.

[0005] 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

[0006] To have a basic understanding of some aspects of the disclosed embodiments, a simple summary is given below. This summary is not a general review, nor is it intended to identify key / important elements or delineate the protection scope of these embodiments, but rather serves as a preface to the subsequent detailed description.

[0007] The embodiments of the present disclosure provide a positive electrode sheet, a preparation method thereof, and a cylindrical battery to improve the bonding strength, contact area, and embedding depth between the positive electrode current collector and the positive electrode active material particles.

[0008] In some embodiments, the positive electrode plate includes a positive electrode current collector and a positive electrode material active substance layer located on the surface of the positive electrode current collector. The positive electrode material active substance layer includes active particles. At least part of the active particles are embedded in the positive electrode current collector to form an embedded layer with the positive electrode current collector. The average embedded depth H of the embedded layer is 0.5 - 4 μm.

[0009] Among them, the active particles include a first type of particles and a second type of particles. The average particle size D a50 of the first type of particles and the average particle size D b50 of the second type of particles have a ratio of 1.33 - 5.6. The volume ratio E of the first type of particles in the entire active particles is 0.5 - 0.9. The tap density P of the positive electrode plate is 3.1 - 3.8 g / cm 3 . The embedding factor Z of the active particles is calculated by the following formula:

[0010] Z = P * E * D a50 / D b50 , and 4 ≤ Z ≤ 15.

[0011] Optionally, the positive electrode plate is cut into circular pieces with a diameter of 0.7 cm. The circular pieces are observed by cross-section polishing - scanning electron microscope CP - SEM at a magnification of 1000 times. The number of active particles embedded in the positive electrode current collector per 100 μm length in the circular pieces is 1 - 10.

[0012] Optionally, X particles with the deepest embedding in the current collector are obtained in every 100 μm, where 1 ≤ X ≤ 5. If X > 5, it is calculated as 5 particles. The average embedding depth H is calculated by the following formula:

[0013] H = (D1 + … + Dx) / X;

[0014] Among them, Dx is the embedding depth of the Xth active particle embedded in the positive electrode current collector.

[0015] Optionally, the average particle size D a50 of the first type of particles is 8 - 14 μm, the average particle size D b50 of the second type of particles is 2.5 - 6.0 μm. The ratio of the average embedding depth H to the average particle size D a50 of the first type of particles in the active particles is 1 - 4:10.

[0016] Optionally, the single-sided thickness B of the positive electrode material active substance layer is 35 - 55 μm. Among them, the ratio of the average embedding depth H to the single-sided thickness B is 0.01 ≤ H / B ≤ 0.1.

[0017] Optionally, the active particles in the positive electrode material active substance layer include one or more of lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, and lithium nickel cobalt manganese aluminate.

[0018] Optionally, the first type of particles includes polycrystalline particles, and the second type of particles includes single-crystalline or polycrystalline particles.

[0019] Optionally, the active particles contain Ni, Co, Mn, and Al elements; and further include one or more of Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La, and Y elements.

[0020] Optionally, the positive electrode current collector includes aluminum foil with a thickness of 12-16 μm. The positive electrode material active substance layer further includes carbon black and polyvinylidene fluoride (PVDF). Among them, the mass ratio of carbon black, PVDF, and active particles is 1-10:1-10:80-98.

[0021] Optionally, the areal capacity of the positive electrode plate ranges from 2.55 to 3.55 mAh / cm 2 , and the contact resistance R between the positive electrode plate and the positive electrode current collector ranges from 0.0012 to 0.0225 Ω·cm 2 .

[0022] Optionally, wipe off the positive electrode material active substance layer on the surface of the positive electrode current collector with N-methylpyrrolidone (NMP), and use a scanning electron microscope (SEM) to observe the surface of the positive electrode current collector. For every 0.1 mm 2 of the positive electrode current collector, the number of pits counted is the number of active particles embedded in the positive electrode current collector, which is 80-500.

[0023] In some embodiments, the method for preparing the positive electrode plate includes:

[0024] Mix the active particles, the first type of particles, and the second type of particles according to a mass ratio of 8:2;

[0025] Add the mixed active particles, carbon black, and polyvinylidene fluoride (PVDF) into N-methylpyrrolidone (NMP) respectively, and mix and stir with a homogenizer. The solid content is 60-75%, and a positive electrode slurry is obtained;

[0026] Coat the positive electrode slurry on at least one surface of the positive electrode current collector, and after baking and rolling, the positive electrode plate is obtained.

[0027] In some embodiments, the cylindrical battery includes the positive electrode plate as described in this application.

[0028] The positive electrode plate, its preparation method, and the cylindrical battery provided by the embodiments of the present disclosure can achieve the following technical effects:

[0029] The current collector aluminum foil in the positive electrode tab usually has an oxide layer on its surface. The existence of the oxide layer will result in a relatively large contact impedance of the positive electrode tab, affecting the charge and discharge performance of the battery. In this regard, in this application, at least part of the active particles are embedded into the positive electrode current collector to achieve the purpose of piercing the oxide layer, so that the active particles are in direct contact with the positive electrode current collector, thereby reducing the contact impedance between the active material region and the positive electrode current collector. And during the charge and discharge process of the battery, especially under high-rate charge and discharge and long-cycle conditions, the positive electrode active material particles and the positive electrode current collector can still have good bonding strength, improving the cycle stability and energy density of the battery.

[0030] At the same time, by changing the compaction density of the positive electrode tab, the volume ratio of the two types of particles in the active particles, and the volume ratio of the first type of particles in the whole active particles, the range of the embedding factor is adjusted, so as to control the embedding depth of the embedding layer formed by the active particles embedded into the positive electrode current collector, increase the contact area between the positive electrode current collector and the active particles, reduce the contact impedance, increase the electron transfer efficiency, and significantly improve the charge and discharge performance and cycle stability of the battery.

[0031] In addition, by controlling the upper limit of the value of the embedding factor, it is possible to avoid serious deformation of the positive electrode current collector and reduce the risk of the positive electrode tab breaking during the rolling process.

[0032] 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

[0033] 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 scale limitation, and among them:

[0034] Figure 1 is a schematic structural diagram of a cylindrical battery provided by an embodiment of the present disclosure;

[0035] Figure 2 is a schematic structural diagram of another cylindrical battery provided by an embodiment of the present disclosure;

[0036] Figure 3 is a schematic structural diagram of active particles embedded in a positive electrode current collector provided by an embodiment of the present disclosure;

[0037] Figure 4 is a flowchart of a preparation method of a positive electrode tab provided by an embodiment of the present disclosure;

[0038] Figure 5 is a flowchart of another preparation method of a positive electrode tab provided by an embodiment of the present disclosure.

[0039] Reference numerals:

[0040] 1 - Cap; 2 - Steel shell; 21 - Battery core; 3 - Negative electrode sheet; 4 - Separator; 5 - Positive electrode sheet. Detailed implementation manners

[0041] In order to be able to understand the features 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 with reference to the accompanying drawings. The accompanying drawings are for reference and illustration purposes only and are not used to limit the embodiments of the present disclosure. In the following technical description, for the sake of explanation, multiple details are provided to provide a full understanding of the disclosed embodiments. 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.

[0042] The terms "first", "second", etc. in the specification, claims and above-mentioned drawings of the embodiments of the present disclosure 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.

[0043] In the embodiments of the present disclosure, the orientation or positional relationship indicated by the terms "upper", "lower", "inner", "middle", "outer", "front", "rear", etc. 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 the 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.

[0044] In addition, the terms "arranged", "connected", "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.

[0045] Unless otherwise specified, the term "plurality" means two or more.

[0046] In the embodiments of the present disclosure, the character " / " indicates an "or" relationship between the front and rear objects. For example, A / B means: A or B.

[0047] The term "and / or" is an associative relationship describing an object, indicating that there can be three relationships. For example, A and / or B means: A or B, or, the three relationships of A and B.

[0048] 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.

[0049] In the related art, according to the packaging form, lithium-ion batteries can be divided into three forms: square, cylindrical, and soft-pack. Among them, cylindrical lithium batteries have gradually become a research hotspot for lithium batteries due to their good consistency, high production efficiency, and strong heat dissipation ability at the system level. As shown in Figure 1 and Figure 2 , the cylindrical battery is encapsulated with a cylindrical steel shell 2, and the cap 1 is located at the top of the battery. The inside of the steel shell 2 is used to accommodate the battery core 21, and the battery core 21 is formed by winding a positive electrode sheet 5, a negative electrode sheet 3, and a separator 4. Specifically, the positive electrode sheet 5 includes a positive electrode current collector (aluminum foil) and a positive electrode material active substance layer coated on at least one surface thereof. The negative electrode sheet 3 includes a negative electrode current collector (copper foil) and a negative electrode material active substance coating coated on at least one surface thereof.

[0050] To solve the technical problems existing in the related art, the present application provides a positive electrode sheet, including a positive electrode current collector and a positive electrode material active substance layer located on the surface of the positive electrode current collector. The positive electrode material active substance layer includes active particles. Among them, at least part of the active particles are embedded in the positive electrode current collector (aluminum foil) and form an embedded layer with the positive electrode current collector. The average embedding depth H of the embedded layer (that is, the depth at which at least part of the active particles are embedded in the aluminum foil) is 0.5 - 4 μm; among them, the active particles include a first type of particles and a second type of particles. The average particle size D a50 of the first type of particles is greater than the average particle size D b50 of the second type of particles. The ratio of the average particle size D a50 of the first type of particles to the average particle size D b50 of the second type of particles is 1.33 - 5.6 (preferably 1.8 - 4.0). The volume ratio E of the first type of particles in the entire active particles is 0.5 - 0.9. The tap density P of the positive electrode sheet is 3.1 - 3.8 g / cm 3 . The embedding factor Z of the active particles is calculated by the following formula:

[0051] Z = P * E * D a50 / D b50 , and 4 ≤ Z ≤ 15;

[0052] As shown inFigure 3 As shown, when using the positive electrode tab provided by the embodiments of the present disclosure, since the current collector aluminum foil in the positive electrode tab usually has an oxide layer covering its surface, the presence of the oxide layer will cause a relatively large contact impedance of the positive electrode tab, affecting the charge and discharge performance of the battery. In response to this, the present application embeds at least part of the active particles into the positive electrode current collector to achieve the purpose of piercing the oxide layer, so that the active particles are in direct contact with the positive electrode current collector, thereby reducing the contact impedance between the active material region and the positive electrode current collector. And during the charge and discharge process of the battery, especially under high-rate charge and discharge and long cycle conditions, the positive electrode active material particles and the positive electrode current collector can still have good bonding strength, improving the cycle stability and energy density of the battery.

[0053] At the same time, by changing the compaction density of the positive electrode tab, the volume ratio of the two types of particles in the active particles, and the volume ratio of the first type of particles in the entire active particles, the range of the embedding factor is adjusted, so as to control the embedding depth of the embedding layer formed by the active particles embedded in the positive electrode current collector, increasing the contact area between the positive electrode current collector and the active particles, reducing the contact impedance, and significantly improving the electron transfer efficiency, the charge and discharge performance, and the cycle stability of the battery.

[0054] In addition, by controlling the upper limit of the value of the embedding factor, it is possible to avoid serious deformation of the positive electrode current collector, reducing the risk of the positive electrode tab breaking during the rolling process.

[0055] Optionally, the positive electrode tab of the present application is cut into circular pieces with a diameter of 0.7 cm, and the circular pieces are observed under a cross-section polishing-scanning electron microscope CP-SEM at a magnification of 1000 times. The number of active particles embedded in the positive electrode current collector per 100 μm length in the circular piece is 1-10.

[0056] In this way, by controlling the number of embedded particles within 1-10 particles / 100 μm, it can not only ensure sufficient contact area between the particles and the aluminum foil (reducing the contact impedance), but also avoid excessive embedding causing damage to the aluminum foil structure. If it exceeds 10, there will be too many depressions on the surface of the aluminum foil, which may cause stress concentration and lead to brittle fracture of the electrode tab; if it is less than 1, it will result in insufficient effect of piercing the oxide layer and limited electron transfer efficiency.

[0057] X particles with the deepest embedding in the current collector are obtained in every 100 μm, where 1≤X≤5. If X>5, it is calculated according to 5 particles, and the average embedding depth H is calculated by the following formula:

[0058] H=(D1+…+Dx) / X;

[0059] where Dx is the embedding depth of the Xth active particle embedded in the positive electrode current collector.

[0060] Optionally, the active particles in the active material layer of the positive electrode material of the present application include one or more of lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, and lithium nickel cobalt manganese aluminate.

[0061] The active particles of the positive electrode material of the present application are divided into two types of particles with different sizes. The average particle size D of the first type of particles a50 is 8 - 14 μm, which can ensure sufficient mechanical strength to pierce the aluminum foil oxide layer, while avoiding uneven roller pressing of the electrode sheet or perforation of the aluminum foil caused by over - sized particles. The average particle size D of the second type of particles b50 is 2.5 - 6.0 μm, which can fill the gaps between large particles, improve the compaction density of the electrode sheet (3.1 - 3.8 g / cm 3 ), and at the same time assist in embedding into the aluminum foil to enhance the overall bonding force.

[0062] Among them, the first type of particles of the present application can be polycrystalline particles, and the second type of particles can be single - crystal or polycrystalline particles. At the same time, the active particles contain Ni, Co, Mn, and Al elements. In addition, the active particles of the present application also include one or more of Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La, and Y elements. In this way, the conductivity of the positive electrode active particles can be further improved.

[0063] Optionally, the ratio of the average embedding depth H to the average particle size D of the first type of particles in the active particles of the present application a50 is 1 - 4:10, preferably 1.33 - 5.6. In this way, the synergistic effect of large and small particles can be optimized, and the balance between the embedding depth and the flexibility of the electrode sheet can be achieved.

[0064] Optionally, the areal capacity range of the positive electrode sheet of the present application is 2.55 - 3.55 mAh / cm 2 , and this range can match the specific capacity characteristics of high - nickel ternary materials (such as NCMA). On the premise of ensuring cycle stability, the optimal balance between energy density and electrode sheet thickness can be achieved.

[0065] Optionally, the contact impedance R between the positive electrode sheet and the positive electrode current collector of the present application ranges from 0.0012 - 0.0225 Ωcm 2 . In this way, by reducing the impedance to this range through the embedding structure, the present application can significantly improve the electron transfer efficiency, thereby improving the high - rate performance of the battery.

[0066] The single-sided thickness B of the positive electrode active material layer of the present application is 35 - 55 μm. Among them, the ratio of the average embedding depth H to the single-sided thickness B is 0.01 ≤ H / B ≤ 0.1. In this way, by controlling the ratio of the average embedding depth H to the single-sided thickness B of the electrode, the structural stability of the positive electrode active material is ensured. Among them, when H / B ≥ 0.01, the embedding depth is sufficient to pierce the oxide layer, which will significantly reduce the contact impedance. When H / B ≤ 0.1, it can avoid the deterioration of the flexibility of the electrode due to excessive embedding.

[0067] Optionally, the positive electrode current collector of the present application includes aluminum foil, and the thickness of the aluminum foil is 12 - 16 μm. In this way, when the thickness of the aluminum foil ≥ 12 μm, it can ensure the mechanical strength of the aluminum foil and prevent fracture or deformation during the rolling process. When the thickness of the aluminum foil ≤ 16 μm, it can control the volumetric energy density of the battery and avoid the reduction of the proportion of active materials caused by the over-thick current collector.

[0068] Optionally, the positive electrode active material layer of the present application further includes carbon black and polyvinylidene fluoride (PVDF). Among them, the mass ratio of carbon black, PVDF, and active particles is 1 - 10:1 - 10:80 - 98. In this way, carbon black, as a conductive agent, accounts for 1 - 10% of the components, which can ensure the continuity of the conductive network, improve the rate performance (such as the 5C capacity retention rate in Test 1), and at the same time avoid the decrease in adhesiveness caused by excessive amount. PVDF, as a binder, accounts for 1 - 10% of the components, which can balance the flexibility of the electrode and the particle fixing effect, and prevent the active materials from falling off during charge and discharge. The active particles account for 80 - 98% of the components, which can maximize the proportion of active materials, ensure the battery capacity, and at the same time leave necessary voids to relieve volume expansion.

[0069] In a specific application of the present application, the positive electrode active material layer on the surface of the positive electrode current collector is wiped off by N-methylpyrrolidone (NMP), and then the surface of the positive electrode current collector is observed using a scanning electron microscope (SEM). For every 0.1 mm 2 of the positive electrode current collector, the number of pits is counted, which is the number of active particles embedded in the positive electrode current collector, and is 80 - 500. In this way, the particle embedding density is reflected by the number of pits: when the number of pits ≥ 80, it can ensure that the oxide layer is fully damaged and reduce the contact impedance. When the number of pits ≤ 500, it can avoid the excessive damage to the surface of the aluminum foil resulting in a decrease in mechanical strength.

[0070] Combined Figure 4 As shown, the embodiments of the present disclosure provide a method for preparing a positive electrode sheet, including:

[0071] Step 401: Mix the active particles, the first type of particles, and the second type of particles in a mass ratio of 8:2.

[0072] Step 402: Add the mixed active particles, carbon black, and polyvinylidene fluoride (PVDF) into N-methylpyrrolidone (NMP) respectively, and mix and stir them with a homogenizer. The solid content is 60-75%, obtaining the positive electrode slurry.

[0073] Step 403: Coating the positive electrode slurry on at least one surface of the positive electrode current collector, and after baking and rolling, obtaining the positive electrode plate.

[0074] Combined Figure 5 As shown, the embodiments of the present disclosure provide another method for preparing a positive electrode plate, including:

[0075] Step 501: By volume percentage, take 80% of the NCMA ternary material with an average particle size D 50 of 10.2 μm and 20% of the NCMA ternary material with an average particle size D 50 of 3.2 μm for mixing.

[0076] Step 502: By weight percentage, take 97% of the mixed NCMA material, 1.5% of conductive carbon black, and 1.5% of polyvinylidene fluoride (PVDF) binder as the solid substances of the positive electrode slurry. Disperse the solid substances in N-methyl-2-pyrrolidone, with a solid content of 65%, and mix evenly in a homogenizer to obtain the positive electrode slurry.

[0077] Step 503: Coating the positive electrode slurry on at least one surface of the positive electrode current collector, and then through baking and rolling, obtaining a positive electrode plate with a compaction density of 3.5 g / cm 3 positive electrode plate.

[0078] In addition, the embodiments of the present disclosure provide a cylindrical battery, including the positive electrode plate, negative electrode plate, and separator located between the two as described in this application. The positive electrode plate, negative electrode plate, and separator are wound to form a battery core. The diameter of the battery core is 20-60 mm, the height is 60-200 mm, and the ratio of the height to the diameter is greater than 1.7. In addition, the cylindrical battery of this application also includes a cap and a steel shell. The inside of the steel shell is used to accommodate the battery core, and the top is encapsulated with a cap.

[0079] The cylindrical battery of this application is tested in the following ways:

[0080] Test 1: Rate performance test

[0081] Take a cylindrical battery of this application, whose positive electrode material is a ternary material, and the voltage window is 2.5-4.2V. For different positive electrode materials, the voltage window needs to be adjusted accordingly. Place the battery in a constant temperature oven at 25°C for more than 4 hours, and test it according to the following steps:

[0082] (1)Discharge the battery at a constant current of 0.1C until it cuts off at 2.5V, and let it stand for 10 min;

[0083] (2)Charge the battery at a constant current of 0.1C until it cuts off at 4.2V, then charge it at a constant voltage until it cuts off at 0.01C, and let it stand for 10 min;

[0084] (3)Discharge the battery at a constant current of 0.1C until it cuts off at 2.5V, and let it stand for 10 min, then read the capacity value C0 at this time;

[0085] (4)Charge the battery at a constant current of 0.1C until it reaches 4.2V, then charge it at a constant voltage until it cuts off at 0.01C, and let it stand for 10 min;

[0086] (5)Discharge the battery at a constant current of 0.5C until it cuts off at 2.5V, and let it stand for 10 min;

[0087] (6)Charge the battery at a constant current of 0.1C until it reaches 4.2V, then charge it at a constant voltage until it cuts off at 0.01C, and let it stand for 10 min;

[0088] (7)Discharge the battery at a constant current of 1C until it cuts off at 2.5V, and let it stand for 10 min;

[0089] (8)Charge the battery at a constant current of 0.1C until it reaches 4.2V, then charge it at a constant voltage until it cuts off at 0.01C, and let it stand for 10 min;

[0090] (9)Discharge the battery at a constant current of 2C until it cuts off at 2.5V, and let it stand for 10 min;

[0091] (10)Charge the battery at a constant current of 0.1C until it reaches 4.2V, then charge it at a constant voltage until it cuts off at 0.01C, and let it stand for 10 min;

[0092] (11)Discharge the battery at a constant current of 3C until it cuts off at 2.5V, and let it stand for 10 min;

[0093] (12)Charge the battery at a constant current of 0.1C until it reaches 4.2V, then charge it at a constant voltage until it cuts off at 0.01C, and let it stand for 10 min;

[0094] (13)Discharge the battery at a constant current of 4C until it cuts off at 2.5V, and let it stand for 10 min;

[0095] (14)Charge the battery at a constant current of 0.1C until it reaches 4.2V, then charge it at a constant voltage until it cuts off at 0.01C, and let it stand for 10 min;

[0096] (15)Discharge the battery at a constant current of 5C until it cuts off at 2.5V, and let it stand for 10 min;

[0097] (16) Charge the battery at a constant current of 0.1C until 4.2V, then charge at a constant voltage until cutoff at 0.01C, and let it stand for 10 min;

[0098] (17) Discharge the battery at a constant current of 6C until cutoff at 2.5V, and let it stand for 10 min;

[0099] (18) Charge the battery at a constant current of 0.1C until 4.2V, then charge at a constant voltage until cutoff at 0.01C, and let it stand for 10 min;

[0100] (19) Discharge the battery at a constant current of 8C until cutoff at 2.5V, and let it stand for 10 min;

[0101] (20) Obtain the rate performance of a single battery, i.e., the capacity retention rate, by the ratio of the discharge capacity at each rate to the discharge capacity of the first discharge.

[0102] Test Two: Cycling Stability Test

[0103] Take a cylindrical battery of the present application, whose positive electrode material is a ternary material, the voltage window is 2.5 - 4.2V, and for different positive electrode materials, the voltage window needs to be adjusted accordingly. Place the battery in a constant temperature oven at 25°C for more than 4h, and conduct the test according to the following steps:

[0104] (1) Discharge the battery at a constant current of 0.1C until cutoff at 2.5V, and let it stand for 5 min;

[0105] (2) Charge the battery at a constant current of 0.2C until cutoff at 4.2V, then charge at a constant voltage until cutoff at 0.05C, and let it stand for 5 min;

[0106] (3) Discharge the battery at a constant current of 0.2C until cutoff at 2.5V, and let it stand for 5 min, and read the capacity value C0 at this time;

[0107] (4) Charge the battery at a constant current of 1.0C until 4.2V, then charge at a constant voltage until cutoff at 0.05C, and let it stand for 5 min;

[0108] (5) Discharge the battery at a constant current of 2.0C until cutoff at 2.5V, and let it stand for 5 min;

[0109] (6) Repeat steps (4) and (5) 600 times;

[0110] (7) Obtain the cycling stability of a single battery, i.e., the capacity retention rate, by the ratio of the 600th discharge capacity to the first discharge capacity in steps (4) and (5).

[0111] Test Three: Contact Impedance Test

[0112] Using a 46-probe electrode resistance test system, input the thickness of the electrode sheet, the thickness of the current collector, and the volume resistivity of the current collector in advance. Then place the electrode sheet on the measurement stage. The position where the probe contacts is the intersection of the bold scale on the scale plate. After lowering the handle to the bottom, start the measurement. After the measurement is completed, lift the handle, move the position of the electrode sheet, and repeat the test 5 times.

[0113] Test Four: Particle Size Test

[0114] Cut the electrode sheet into circular pieces with a diameter of 0.7 cm. Under the observation of 1000 times magnification of CP-SEM, use software to measure the diameters of 100 large particles, and take the average value as the average particle diameter D of the large particles a50 , and the small particles are the same as above as D b50 .

[0115] Among them, the measurement method of the volume ratio of large and small particles includes:

[0116] Cut the electrode sheet into circular pieces with a diameter of 0.7 cm. Under the observation of 1000 times magnification of CP-SEM, in the area with a length of 50 μm and a width of 30 μm, count the number Y1 of large particles and the number Y2 of small particles. The volume V1 of the large particles is approximately calculated as V1 = π * D a50 3 / 6, and the volume V2 of the small particles is approximately calculated as V2 = π * D b50 3 / 6. Then the volume ratio E of large and small particles is E = (Y1 * V1) / (Y2 * V2).

[0117] Test Five: Measurement of Compaction Density

[0118] (1) Wash the sample electrode sheet with dimethyl carbonate 2 - 5 times and place it in an oven to dry to fully remove dimethyl carbonate;

[0119] (2) Cut a circular piece with an area of S from the double-sided coating area on the sample electrode sheet, weigh it to obtain the sum of the masses m1 of the foil and the coating, and use a micrometer to measure the sum of the thicknesses d1 of the foil and the coating;

[0120] (3) Scrape off all the powder on the surface of the circular piece, wipe the surface of the circular piece with NMP to fully remove the powder, and dry it thoroughly. Weigh the circular piece again to obtain the mass m2 of the foil, and use a micrometer to measure the thickness d2 of the foil again;

[0121] (4) Calculate the compaction density as (m1 - m2) / [S * (d1 - d2)] (unit: g / cm 3 ).

[0122] Test Six: Measurement of Areal Capacity per Side

[0123] Select fresh battery cells, disassemble the cells, take out the positive electrodes, wipe one side of them clean with NMP, and then cut them into electrode sheets with an area of K using a cutting machine to make button cells. Then, test and calculate through a BET battery tester. The test conditions are as follows: at 25°C, constant current charge at 0.1C to 4.2V, constant voltage at 4.2V to 0.01C; then discharge at 0.1C constant current to 2.5V to obtain the battery capacity. Then divide the battery capacity by the total area K of the positive electrode sheet coating to obtain the single-sided area capacity of the electrode sheet, with the unit of mAh / cm 2 。

[0124] Example 1

[0125] Example 1 of the present disclosure provides a method for preparing a positive electrode sheet, including:

[0126] (1) By volume percentage, take 80% of the NCMA ternary material with an average particle size of 10.2μm and 20% of the NCMA ternary material with an average particle size of 3.2μm for mixing.

[0127] (2) By weight percentage, take 97% of the mixed NCMA material, 1.5% of conductive carbon black, and 1.5% of polyvinylidene fluoride (PVDF) binder as the solid substances of the positive electrode slurry. Disperse the solid substances in N-methyl-2-pyrrolidone with a solid content of 65%, and mix evenly in a homogenizer to obtain the positive electrode slurry.

[0128] (3) Coat the positive electrode slurry on at least one surface of the positive electrode current collector, and then obtain a positive electrode sheet with a compaction density of 3.5 g / cm after baking and rolling. 3 Positive electrode sheet.

[0129] Example 2

[0130] Example 2 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the compaction density of the electrode sheet is 3.2 g / cm 3 。

[0131] Example 3

[0132] Example 3 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the compaction density of the electrode sheet is 3.7 g / cm 3 。

[0133] Example 4

[0134] Example 4 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the volume ratio of the first type of particles is 50%, and the ratio of the second type of particles is 50%.

[0135] Example 5

[0136] Example 5 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the volume ratio of the first type of particles is 70%, and the ratio of the second type of particles is 30%.

[0137] Example 6

[0138] Example 6 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the volume ratio of the first type of particles is 90%, and the ratio of the second type of particles is 10%.

[0139] Example 7

[0140] Example 7 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the average particle size of the first type of particles is 12.7 μm, and the average particle size of the second type of particles is 5.6 μm.

[0141] Example 8

[0142] Example 8 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the average particle size of the first type of particles is 12.7 μm, and the average particle size of the second type of particles is 3.2 μm.

[0143] Example 9

[0144] Example 9 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the average particle size of the first type of particles is 10.2 μm, and the average particle size of the second type of particles is 5.6 μm.

[0145] Comparative Example 1

[0146] Comparative Example 1 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the coated electrode sheet is not roll-pressed.

[0147] Comparative Example 2

[0148] Comparative Example 2 of the present disclosure provides a method for preparing a positive electrode sheet, which is different from Example 1 in that the volume ratio of the first type of particles is 0%, and the ratio of the second type of particles is 100%.

[0149] Table 1

[0150]

[0151] Table 2

[0152]

[0153] It can be seen from Comparative Examples 1-3 that by increasing the compaction density of the material and pressing more material into the foil, the contact area between the particles and the foil can be effectively increased, the contact impedance between the particles and the foil is reduced, and the rate performance and cycle stability of the battery cell are improved. It can be seen from Comparative Examples 4-6 that increasing the volume ratio of large particles can increase the ratio of the embedding depth H of the particles in the foil to the particle Da50, the contact area between the unit particles and the foil is larger, the contact impedance between the particles and the foil is reduced, and the rate performance and cycle stability of the battery cell are improved. It can be seen from Comparative Examples 7-9 that by increasing the ratio of the average particle size of large particles to that of small particles, the larger the average particle size of large particles, the deeper the embedding depth in the foil under a certain compaction density. The smaller the average particle size of small particles, the smaller the average particle size of small particles, which can be present in the gaps between large particles and be pressed into the foil together, increasing the contact area between the particles and the foil, reducing the contact impedance between the particles and the foil, and improving the rate performance and cycle stability of the battery cell. It can be seen from Comparative Example 1 that when the electrode sheet is not roll-pressed, the particles will not be embedded in the foil, so the contact impedance between the particles and the foil is large, and the rate and cycle stability are significantly reduced. It can be seen from Comparative Example 2 that when all the active particles are small particles, the embedding depth of the particles in the foil is limited under a certain compaction density, reducing the rate and cycle stability.

[0154] 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 only represent possible variations. Unless explicitly required, the individual components and functions are optional, and the order of operations can vary. Some parts and features of some embodiments may be included in or replaced with 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 only limited by the appended claims.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode material active material layer located on the surface of the positive electrode current collector, characterized in that: The positive electrode material active material layer includes active particles, wherein at least part of the active particles are embedded in the positive electrode current collector and form an embedded layer in the positive electrode current collector, and the average embedding depth H of the embedded layer is 0.5-4 μm; The active particles include first-type particles and second-type particles, and the average particle size D of the first-type particles is a50 Larger than the average particle size D of the second type of particles b50 , and the average particle size D of the first type of particles a50 The average particle size D of the second type of particles b50 The ratio of the first type of particles to the entire active particles is 1.33-5.6, the volume ratio E of the first type of particles to the entire active particles is 0.5-0.9, and the compaction density P of the positive electrode sheet is 3.1-3.8 g / cm 3 , the embedding factor Z of the active particles is calculated by the following formula: Z=P*E*D a50 / D b50 , and 4≤Z≤15.

2. The positive electrode sheet according to claim 1, characterized in that: The positive electrode sheet was cut into discs with a diameter of 0.7 cm, and the discs were observed by cross-sectional polishing-scanning electron microscopy CP-SEM at a magnification of 1000 times. The number of active particles embedded in the positive electrode current collector per 100 μm length in the disc was 1-10.

3. The positive electrode sheet according to claim 2, characterized in that: In every 100 μm, X particles with the deepest embedding in the current collector are obtained, where 1≤X≤5. If X>5, 5 particles are used for calculation, and the average embedding depth H is calculated by the following formula: H = (D1 + ... + Dx) / X; Wherein, Dx is the embedding depth of the Xth active particle embedded in the positive electrode current collector.

4. The positive electrode sheet according to claim 1, characterized in that: The average particle size D of the first type of particles a50 The average particle size D of the second type of particles is 8-14 μm. b50 The average embedding depth H is 2.5-6.0 μm, and the average particle size D of the first type of particles in the active particles is a50 The ratio is 1-4:

10.

5. The positive electrode sheet according to claim 1, characterized in that: The single-side thickness B of the positive electrode material active material layer is 35-55 μm, wherein the ratio of the average embedding depth H to the single-side thickness B is 0.01≤H / B≤0.

1.

6. The positive electrode sheet according to claim 1, characterized in that: The active particles in the positive electrode material active material layer include one or more of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and lithium nickel cobalt manganese aluminum oxide.

7. The positive electrode sheet according to claim 1, characterized in that: The first type of particles include polycrystalline particles, and the second type of particles include single crystal or polycrystalline particles.

8. The positive electrode sheet according to claim 1, characterized in that: The active particles contain Ni, Co, Mn and Al elements; and further contain one or more of Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La and Y elements.

9. The positive electrode sheet according to claim 1, characterized in that: The positive electrode current collector comprises aluminum foil, and the thickness of the aluminum foil is 12-16 μm; The positive electrode material active material layer also includes carbon black and polyvinylidene fluoride PVDF, wherein the mass ratio of carbon black, PVDF and active particles is 1-10:1-10:80-98.

10. The positive electrode sheet according to claim 1, characterized in that: The surface capacity of the positive electrode sheet is in the range of 2.55-3.55 mAh / cm 2 ; The contact impedance R between the positive electrode sheet and the positive electrode current collector is in the range of 0.0012-0.0225 Ωcm 2 .

11. The positive electrode sheet according to claim 1, characterized in that: The cathode material active material layer on the cathode current collector surface was wiped off with N-methylpyrrolidone NMP, and the surface of the cathode current collector was observed using a scanning electron microscope SEM. 2 The number of pits counted is the number of active particles embedded in the positive electrode current collector, which is 80-500.

12. A method for preparing a positive electrode sheet according to any one of claims 1 to 11, characterized in that: include: The first type of active particles and the second type of active particles are mixed in a mass ratio of 8:2; The mixed active particles, carbon black and polyvinylidene fluoride (PVDF) are added to N-methylpyrrolidone (NMP) respectively, and mixed and stirred by a homogenizer to obtain a positive electrode slurry with a solid content of 60-75%. The positive electrode slurry is coated on at least one surface of a positive electrode current collector, and after baking and rolling, the positive electrode sheet is obtained.

13. A cylindrical battery, characterized in that: Comprising the positive electrode sheet as described in any one of claims 1 to 11.

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