A secondary battery and an electric device

Abstract

The application relates to a secondary battery and a power consumption device, and belongs to the technical field of batteries. The secondary battery comprises a positive pole piece and a negative pole piece, the positive pole piece comprises a positive pole current collector and a positive pole active layer arranged on the surface of the positive pole current collector, the positive pole active layer comprises a positive pole active material; the negative pole piece comprises a negative pole current collector and a negative pole active layer arranged on the surface of the negative pole current collector, the negative pole active layer comprises a negative pole active material; and the secondary battery satisfies 0.14<=Z<=4.67; wherein Z=(n1 / n2)*(p1 / p2). The secondary battery can maintain high energy density, and has good fast charging performance and long cycle life.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a secondary battery and an electrical device. Background Technology

[0002] Cathode materials are a key component of lithium-ion batteries, largely determining their energy density. High-nickel layered oxide cathode materials, due to their high specific capacity, high voltage plateau, and high compaction density, are widely used in electric vehicles. However, even the energy density of high-nickel layered oxide cathode materials is insufficient to meet the ever-increasing demand for longer driving ranges in electric vehicles.

[0003] Currently, the energy density of cathode materials is mainly improved through high nickel content or high voltage. However, with high nickel content, as the nickel content in the cathode material increases, nickel located in the octahedron tends to enter lithium sites in the crystal lattice, leading to a decrease in the electrochemical activity of the cathode material. High voltage, while achieving high energy density, can cause cracking problems due to lattice collapse, gas release, or structural reconstruction. In other words, both high nickel content and high voltage can lead to poor cycle stability of the cathode material, resulting in a shorter battery cycle life. Therefore, there is an urgent need to develop an effective method to maintain high energy density while also possessing good fast-charging performance and long cycle life. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a secondary battery and electrical device.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, the present invention provides a secondary battery, the secondary battery comprising a positive electrode and a negative electrode, the positive electrode comprising a positive current collector and a positive active layer disposed on the surface of the positive current collector, the positive active layer comprising a positive active material; the negative electrode comprising a negative current collector and a negative active layer disposed on the surface of the negative current collector, the negative active layer comprising a negative active material;

[0007] The secondary battery satisfies: 0.14≤Z≤4.67;

[0008] Where Z = (n1 / n2)*(p1 / p2);

[0009] n1 units / 10000μm 2 The distribution density of the first active particles in the cross-section of the positive electrode active layer, wherein the first active particles are particles with a diameter greater than or equal to 8 μm in the positive electrode active material particles;

[0010] n2 per 10,000 μm 2 is the distribution density of the second active particles in the cross-section of the positive electrode active layer, and the second active particles are particles with a size less than 8 μm in the positive electrode active material particles;

[0011] p1 is the porosity of the positive electrode active layer;

[0012] p2 is the porosity of the negative electrode active layer.

[0013] As an embodiment of the present invention, the secondary battery satisfies: 1.24 ≤ Z ≤ 2.43.

[0014] As an embodiment of the present invention, the volume ratio of the first active particles to the second active particles in the unit volume of the positive electrode active layer is 1:(0.3 - 0.6).

[0015] As an embodiment of the present invention, n1 / n2 satisfies: 0.3 ≤ n1 / n2 ≤ 7.

[0016] As an embodiment of the present invention, the secondary battery satisfies at least one of the following:

[0017] a. n1 is 8 - 36;

[0018] b. n2 is 6 - 25;

[0019] c. p1 is 20% - 30%;

[0020] d. p2 is 35% - 45%.

[0021] As an embodiment of the present invention, the negative electrode sheet satisfies at least one of the following:

[0022] e. The particle size D of the negative electrode active material v50 is 3 - 12 μm;

[0023] f. The tap density of the negative electrode active layer is 1.4 - 1.8 g / cm 3 .

[0024] As an embodiment of the present invention, the positive electrode active material includes a core and a shell layer provided on the surface of the core; the core includes a compound with the chemical formula Li 1+a [Ni x Co y Mn z M b O2; wherein, 0.5 ≤ x < 1, 0 < y < 0.3, 0 ≤ z < 0.3, -0.1 ≤ a < 0.2, 0 < b < 0.3, and x + y + z + b = 1; M includes at least one of Ti, Zr, Sr, Sb, Nb, Ru, Pd, Y, Ce, and W.

[0025] As an embodiment of the present invention, the content of cobalt increases from the center of the core to the surface of the core.

[0026] As an embodiment of the present invention, the shell layer comprises a metal oxide, wherein the metal element X in the metal oxide comprises at least one of Al, W, Ca, Ce, Ti, Zr, and Y.

[0027] As an embodiment of the present invention, the shell layer satisfies: 2500≤w*h≤16500; where w ppm is the content of the metal element X in the positive electrode active material, and h nm is the thickness of the shell layer.

[0028] Secondly, the present invention provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] This invention regulates the distribution density of the first active particles (particles larger than or equal to 8 μm in the positive electrode active material) and the second active particles (particles smaller than 8 μm in the positive electrode active material) in the positive electrode active layer, and combines them with a positive electrode active layer and a negative electrode active layer with appropriate porosity, so that 0.14≤Z≤4.67, Z=(n1 / n2)*(p1 / p2). This not only effectively suppresses particle breakage of the positive electrode active material under high pressure to maintain the high energy density of the battery and improve the cycle performance of the battery, but also allows the electrode to be fully wetted by the electrolyte to reduce polarization and increase the diffusion rate of lithium ions. As a result, the secondary battery has high energy density while also having good fast charging performance and long cycle life. Detailed Implementation

[0031] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments and comparative examples. The purpose of this description is to provide a detailed understanding of the invention, not to limit its scope. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this invention. Unless otherwise specified, the experimental reagents and instruments involved in the implementation of this invention are commonly used reagents and instruments.

[0032] According to a first aspect of the present invention, a secondary battery is provided, the secondary battery comprising a positive electrode and a negative electrode, the positive electrode comprising a positive current collector and a positive active layer disposed on the surface of the positive current collector, the positive active layer comprising a positive active material; the negative electrode comprising a negative current collector and a negative active layer disposed on the surface of the negative current collector, the negative active layer comprising a negative active material;

[0033] The secondary battery satisfies: 0.14≤Z≤4.67;

[0034] Where Z = (n1 / n2)*(p1 / p2);

[0035] n1 units / 10000μm 2 The distribution density of the first active particles in the cross-section of the positive electrode active layer, wherein the first active particles are particles with a diameter greater than or equal to 8 μm in the positive electrode active material particles;

[0036] n2 units / 10000μm 2 The distribution density of the second active particles in the cross-section of the positive electrode active layer, wherein the second active particles are particles smaller than 8 μm in the positive electrode active material particles;

[0037] p1 is the porosity of the positive electrode active layer;

[0038] p2 is the porosity of the negative electrode active layer.

[0039] This invention regulates the distribution density of the first active particles (particles larger than or equal to 8 μm in the positive electrode active material particles) and the second active particles (particles smaller than 8 μm in the positive electrode active material particles) in the positive electrode active layer, and sets a positive electrode active layer and a negative electrode active layer with appropriate porosity, so that 0.14≤Z≤4.67, Z=(n1 / n2)*(p1 / p2). This not only effectively suppresses particle breakage of the positive electrode active material under high pressure to maintain the high energy density of the battery and improve the cycle performance of the battery, but also allows the positive and negative electrode sheets to be fully wetted by the electrolyte to reduce polarization and increase the diffusion rate of lithium ions. Thus, the secondary battery has high energy density while also having good fast charging performance and long cycle life.

[0040] Optionally, the value of Z can be any one or any two of the following: 0.14, 0.50, 1.02, 1.22, 2.45, 2.50, 3.30, 3.80, and 4.67; the secondary battery includes at least one of the following: wound soft-pack battery, wound aluminum-cased battery, wound cylindrical battery, stacked soft-pack battery, and stacked aluminum-cased battery; the negative electrode active material in the negative electrode sheet of the secondary battery includes at least one of the following: carbon-based material, silicon-based material, and tin-based material.

[0041] Optionally, to further optimize the battery's fast charging performance and long cycle life, the value of Z is set to 1.24 to 2.43.

[0042] As an embodiment of the present invention, the volume ratio of the first active particle and the second active particle per unit volume of the positive electrode active layer is 1:(0.3-0.6). Optionally, the volume ratio of the first active particle and the second active particle per unit volume of the positive electrode active layer can specifically be one or any two of 1:0.3, 1:0.4, 1:0.5, and 1:0.6. When the volume ratio of the first active particle to the second active particle in the positive electrode active layer is within the above range, it can improve the compaction density of the positive electrode sheet while ensuring that the porosity of the positive electrode sheet is within a reasonable range, and further optimize the compressive strength and energy density of the positive electrode sheet.

[0043] As an embodiment of the present invention, n1 / n2 satisfies: 0.3 ≤ n1 / n2 ≤ 7. Optionally, n1 / n2 can specifically be one or any two of the following: 0.3, 0.8, 1.5, 2.3, 3, 4, 5, 6, 7. When the value of n1 / n2 is within the above range, it can ensure that the material system has a better particle size distribution, improve the compaction density of the positive electrode sheet while ensuring kinetic performance. The reasonable particle size distribution also reduces the number of gaps between particles, which can reduce the contact resistance between particles, reduce the internal resistance and polarization of the battery, shorten the lithium-ion transport distance, and help improve the battery capacity and rate performance. The reasonable distribution can ensure that the positive electrode active material particles have strong compressive strength under high compaction density, reduce the cracking phenomenon caused by stress concentration in the positive electrode active material particles during rolling and cyclic charging and discharging, thereby reducing the exposed area of ​​active sites, reducing the generation of side reactions with electrolyte, and achieving the effect of improving battery cycle performance.

[0044] As an embodiment of the present invention, n1 is 8 to 36; optionally, n1 can specifically be one or any two of the following: 8, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, and 36. Studies have found that when n1 is 8 to 36, the positive electrode active material has a suitable specific surface area, which is beneficial to the battery performance.

[0045] As an embodiment of the present invention, n2 is 6 to 25; optionally, n2 can specifically be one or any two of 6, 8, 10, 12, 14, 16, and 25. Studies have found that when n2 is 6 to 25, the number of small particles in the battery system is moderate. On the one hand, this ensures a sufficient number of small particles to improve the kinetic performance of the positive electrode; on the other hand, controlling it within a certain upper limit prevents an excessive number of small particles from reducing the compaction density of the positive electrode, thereby ensuring that the battery system has a high energy density.

[0046] As an embodiment of the present invention, p1 is 20% to 30%; optionally, p1 can specifically be a value within the range of 20%, 22%, 24%, 26%, 28%, 30%, or any two of these ranges. The porosity of the positive electrode active material layer is related to the particle size, particle distribution, particle morphology, proportion of positive electrode active material, and the rolling process of the electrode sheet. A suitable porosity is beneficial for electrolyte wetting and reduces the diffusion energy during the lithium ion insertion / extraction process. When p1 is 20% to 30%, it is beneficial for the electrolyte to quickly wet the positive electrode sheet and ensures that lithium ions are more easily inserted / extracted.

[0047] As an embodiment of the present invention, p2 is 35% to 45%; specifically, p2 can be one or any two of 35%, 37%, 39%, 41%, 43%, and 45%. Studies have found that when p2 is 35% to 45%, it facilitates rapid wetting of the positive electrode by the electrolyte and ensures that lithium ions are more easily inserted and removed.

[0048] In some embodiments, the particle size D of the negative electrode active material v50 The size is 3–12 μm;

[0049] In some embodiments, the compaction density of the negative electrode active layer is 1.4–1.8 g / cm³. 3 .

[0050] Optionally, the particle size D of the above-mentioned negative electrode active material v50 Specifically, it can be a value within the range of one or any two of 3μm, 5μm, 7μm, 9μm, and 12μm. Research has found that when the particle size D of the negative electrode active material... v50 When the diameter is 3–12 μm, the channels for lithium ion insertion and extraction are shorter, which helps the lithium ion to quickly reach a fully intercalated state, thus resulting in better charge and discharge performance.

[0051] Optionally, the compaction density of the aforementioned negative electrode active layer can specifically be 1.4 g / cm³. 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 3 1.8g / cm 3 The value is within the range of one or both of these. Studies have found that when the compaction density of the negative electrode active layer is 1.4–1.8 g / cm³... 3 This facilitates the wetting of the electrolyte and also gives the battery a higher energy density.

[0052] As an embodiment of the present invention, the positive electrode active material includes a core and a shell disposed on the surface of the core, wherein the core comprises a material with the chemical formula Li. 1+a [Ni x Coy Mn z M b Compound of ]O2; wherein, 0.5 ≤ x < 1, 0 < y < 0.3, 0 ≤ z < 0.3, -0.1 ≤ a < 0.2, 0 < b < 0.3, and x + y + z + b = 1; M includes at least one of Ti, Zr, Sr, Sb, Nb, Ru, Pd, Y, Ce, and W. It has been found that when choosing a compound with the above chemical formula as the core of the positive electrode active material, the positive electrode active material has a high specific capacity per gram while having high long-term cycle stability. In some embodiments, 0.8 < x < 9.5, that is, the nickel content in the positive electrode active material accounts for more than 80% of the transition metal elements, and the nickel content is greater than 80%, which makes the positive electrode active material have a high specific capacity and is beneficial to improving the energy density of the battery.

[0053] As an embodiment of the present invention, the shell layer includes a metal oxide, and the metal element X in the metal oxide includes at least one of Al, W, Ca, Ce, Ti, Zr, and Y. The metal oxide coating layer can improve the cycle stability and safety of the battery and reduce the material cost.

[0054] In some embodiments, the cobalt content in the positive electrode active material increases from the center of the core to the surface of the core. When the cobalt content increases from the center of the core to the surface of the core, Co can improve the electronic conductivity of the material and reduce the degree of cation mixing and occupancy, thereby enhancing the structural stability and rate performance of the material. A higher surface Co content can improve the surface kinetic properties and surface structural stability, which helps to improve capacity utilization. At the same time, a higher surface Co content also means a lower surface Ni content, which can reduce the occurrence of side reactions between highly active Ni4+ and the electrolyte during charge and discharge, and improve the long-term cycle performance of the battery system. The distribution of cobalt on the particle surface and inside the positive electrode active material can be determined by preparing the cross-section of the positive electrode active material using a cross-section polisher (e.g., the IB-09010CP argon ion cross-section polisher from JEOL Corporation, Japan); then, referring to JY / T010-1996, scanning electron microscopy (e.g., the Sigma 300 scanning electron microscope from ZEISS Corporation, Germany) is used to scan the cross-section of the positive electrode active material; and finally, inductively coupled plasma atomic emission spectrometry (ICP) is used for detection. The ratio of the mass of cobalt in the surface region (the region extending 0.05L inward from the particle edge in the cross-sectional image of the obtained positive electrode active material, where L refers to the major axis length of the positive electrode active material particle) to the total mass of cobalt in the obtained cross-sectional image of the positive electrode active material is taken as the ratio of the mass of cobalt in the surface region of the positive electrode active material to the total mass of cobalt in the positive electrode active material. This allows us to determine the distribution of cobalt in the positive electrode active material.

[0055] In some embodiments, the shell layer also includes non-metallic oxides, such as boron oxide and silicon dioxide. Metallic oxides, as shell materials, prevent the electrolyte from having excessive contact with the core, reduce the occurrence of side reactions, thereby improving battery cycle stability, suppressing gas production, and improving safety performance.

[0056] As an embodiment of the present invention, the shell layer satisfies: 2500 ≤ w*h ≤ 16500; where w ppm is the content of the metal element X in the positive electrode active material, and h nm is the thickness of the shell layer. Optionally, w*h can specifically be a value within the range of 2500, 5000, 7500, 10000, 12500, 15000, and 16500, or any two of these ranges. When the content of the metal element X in the positive electrode active material and the thickness of the shell layer satisfy the above relationship, the coating layer on the surface of the positive electrode active material can maintain a suitable and uniform thickness. The coating layer covering the particles on the surface of the positive electrode active material can act as a physical barrier, protecting the positive electrode active material from electrolyte corrosion and inhibiting the occurrence of side reactions. A coating layer of suitable thickness can promote the rotation of interfacial ion charges, thereby improving interfacial dynamics and battery performance. When the w*h value is within the above range, it can protect the core of the positive electrode active material from electrolyte corrosion while also considering the migration rate of lithium ions during the insertion / extraction process.

[0057] Secondly, the present invention provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0058] To clearly understand the technical solution of the present invention, the present invention will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed by the present invention.

[0059] Example 1

[0060] This embodiment provides a secondary battery, including a positive electrode, a negative electrode, a separator (PE separator), and an electrolyte (1 mol / L LiPF6 solution, the solvent being composed of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1); the preparation method includes the following steps: stacking the positive electrode, negative electrode, and separator in sequence, with the separator positioned between the positive and negative electrodes, and then winding them to obtain a bare battery cell; then placing the bare battery cell in an outer aluminum-plastic film, baking to remove moisture, injecting the electrolyte, and vacuum packaging to obtain the secondary battery.

[0061] 1) The above-mentioned positive electrode sheet is prepared by the following method:

[0062] S1. Add nickel sulfate, manganese sulfate, cobalt sulfate and deionized water to the first reaction vessel and mix evenly (the molar ratio of elements Ni:Co:Mn = 90:5:5). Then add 0.4 mol / L ammonia water and 1 mol / L sodium hydroxide aqueous solution to the first reaction vessel to adjust the pH of the reaction system to 11.3 to obtain a mixed solution.

[0063] S2. Add Al2O3, RuO2, and ethanol solution to the second reactor and mix thoroughly (the molar ratio of elements Al:Ru:(Ni+Co+Mn)=0.02:0.02:1). Then add the mixture from S1 to the second reactor for co-precipitation reaction (the co-precipitation reaction is divided into a first stage and a second stage. In the first stage, the stirring speed is 400 rpm, the pH value of the reaction solution is 12.5, the ammonia value is 14 g / L, and the reaction time is 2.5 h; in the second stage, the stirring speed is 500 rpm, the pH value of the reaction solution is 12.0, the ammonia value is 8 g / L, and the reaction time is 3 h. The ammonia value is the concentration of ammonia water). A solid-liquid mixture is obtained. Then, the solid-liquid mixture is filtered, washed, dried, sieved, and demagnetized to obtain the precursor of the positive electrode material. The first and second stages of the co-precipitation reaction can obtain precursors with different particle size distributions.

[0064] S3. The positive electrode material precursor in S2 and Li2CO3 (with an elemental molar ratio of Li:(Ni+Co+Mn)=1.08:1) are mixed evenly and sintered in air at 750℃ for 13h. The mixture is then crushed to obtain the positive electrode active material. The positive electrode active material is then coated with alumina (Al2O3) and tempered at 550℃. After sieving, a positive electrode active material with a shell is obtained (where the mass fraction of the coating layer relative to the positive electrode active material is 0.02%).

[0065] S4. The positive electrode active material with shell, conductive agent (conductive carbon black) and binder (polyvinylidene fluoride) in S3 are mixed in a mass ratio of 97.3:1.8:0.9, and then mixed evenly with solvent (N-methylpyrrolidone) to form a positive electrode slurry. Then, the positive electrode slurry is evenly coated on both sides of the positive electrode current collector (aluminum foil) by a coating machine. After drying, rolling, cutting and slitting, the positive electrode sheet is obtained.

[0066] 2) The above negative electrode sheet is prepared by the following method:

[0067] The negative electrode active material (graphite), conductive agent (acetylene black), binder (styrene-butadiene rubber), and thickener (sodium carboxymethyl cellulose) are mixed in a mass ratio of 96:2:1:1, and then mixed evenly with deionized water to form a negative electrode slurry. The negative electrode slurry is then evenly coated on both sides of the negative electrode current collector (copper foil) using a coating machine. After drying, rolling, cutting, and slitting, the negative electrode sheet is obtained.

[0068] Examples 2-5

[0069] Examples 2-5 provide a secondary battery. The secondary battery differs from Example 1 in that, during the preparation of the positive electrode sheet, the reaction time of the first and second stages of the co-precipitation reaction in step S1 is adjusted to adjust the distribution density n1 of the first active particles in the positive active layer, the distribution density n2 of the second active layer, and the volume ratio v1 / v2 of the first active particles and the second active particles per unit volume. At the same time, by adjusting the rolling pressure during the preparation of the positive and negative electrode sheets, a certain compaction of the positive and negative electrode sheets is obtained, so that the porosity p1 of the positive active layer and the porosity p2 of the negative active layer are basically the same as those in Example 1.

[0070] Examples 6-7

[0071] Examples 6 and 7 provide a secondary battery, which differs from Example 3 in that, during the preparation of the positive electrode sheet, the reaction times of the first and second stages of the co-precipitation reaction in step S1 are adjusted so that the distribution density n1 of the first active particles in the positive active layer, the distribution density n2 of the second active layer, and the volume ratio v1 / v2 of the first and second active particles per unit volume are basically consistent with those in Example 3; at the same time, a certain compaction of the positive electrode sheet is obtained by adjusting the rolling pressure or roll gap parameters during the preparation of the positive electrode sheet, so as to adjust the porosity p1 of the positive active layer; and a certain compaction of the negative electrode sheet is obtained by adjusting the rolling pressure or roll gap parameters during the preparation of the negative electrode sheet, so that the porosity p2 of the negative active layer is basically consistent with that in Example 3.

[0072] Examples 8-11

[0073] Examples 8-11 provide a secondary battery, which differs from Example 3 in that the reaction times of the first and second stages of the co-precipitation reaction in step S1 of the positive electrode preparation method are adjusted so that the distribution density n1 of the first active particles in the positive electrode active layer, the distribution density n2 of the second active layer, and the volume ratio v1 / v2 of the first active particles and the second active particles per unit volume are basically the same as in Example 3; and the porosity p1 of the positive electrode active layer is basically the same as in Example 3 by adjusting the rolling pressure or roll gap parameters in the positive electrode preparation process; and the porosity p2 of the negative electrode active layer is adjusted by adjusting the rolling pressure or roll gap parameters in the negative electrode preparation process.

[0074] Examples 12-13

[0075] Examples 12 and 13 provide a secondary battery, which differs from Example 2 in that the negative electrode sheet is prepared by selecting a particle size D. v50 =3μm, particle size D v50 Graphite with a diameter of 12 μm is used as the negative electrode active material.

[0076] Examples 14-15

[0077] Examples 14 and 15 provide a secondary battery, which differs from Example 2 in that, during the preparation of the negative electrode sheet, the compaction density of the negative electrode active layer is adjusted by adjusting the equipment pressure or roller gap parameters during the rolling process.

[0078] Examples 16-17

[0079] Examples 16 and 17 provide a secondary battery, which differs from Example 2 in that the value of W*h is adjusted by adjusting the amount of Al2O3 added in step S3 during the preparation of the positive electrode.

[0080] Examples 18-19

[0081] Examples 18 and 19 provide a secondary battery, which differs from Example 2 in that, in the preparation process of the positive electrode sheet, the alumina in step S3 is replaced with zirconium dioxide and titanium dioxide respectively to coat the positive electrode active material.

[0082] Comparative Examples 1-2

[0083] Comparative Examples 1 and 2 provide a secondary battery, which differs from Example 3 in that the reaction times of the first and second stages of the co-precipitation reaction in step S1 of the positive electrode preparation method are adjusted so that the distribution density n1 of the first active particles in the positive active layer, the distribution density n2 of the second active layer, and the volume ratio v1 / v2 of the first and second active particles per unit volume are basically consistent with those in Example 3; and the porosity p1 of the positive active layer is controlled by adjusting the rolling pressure or roll gap parameters in the positive electrode preparation process; and the porosity p2 of the negative active layer is adjusted by adjusting the rolling pressure or roll gap parameters in the negative electrode preparation process.

[0084] Performance testing

[0085] 1. Distribution density test of the first and second active particles in the positive electrode sheet

[0086] The cross-sections of the positive electrode sheets in each embodiment and comparative example were tested by scanning electron microscopy. The SEM images (magnification of 1000x) of the test samples were analyzed using software or other tools with measurement functions. The number of first active particles (particle size ≥ 8 μm) and the number of second active particles (particle size < 8 μm) in the positive electrode active layer within the field of view were counted. Then, the distribution density of the first and second active particles was calculated.

[0087] 2. Porosity testing of positive and negative electrode sheets

[0088] Reference standards: GB / T 24533-2019 Graphite-based anode materials for lithium-ion batteries; Cetane absorption method in GB / T 33052-2016 Methods for determining porosity.

[0089] 3. Particle size D of the negative electrode active material v50

[0090] Reference standard: GB-T 19077-2016 Particle size analysis by laser diffraction.

[0091] 4. Compacted density of the negative electrode active layer

[0092] The thickness of the negative electrode sheet and the negative electrode current collector is measured using a precision micrometer or ten-thousand-meter. The electrode sheet is cut into small circular pieces of a certain area, and the surface density of the coating of the negative electrode sheet is obtained by weighing them using an analytical balance. The compaction density of the negative electrode active layer is calculated based on the thickness of the negative electrode sheet and the negative electrode current collector and the surface density of the coating of the negative electrode sheet.

[0093] 5. The thickness of the core surface shell in the positive electrode active material and the content of metal element X in the positive electrode active material.

[0094] Method for testing the thickness of the core surface shell in positive electrode active materials: After preparing the positive electrode sample, TEM testing is performed to obtain high-resolution transmission electron microscopy images of the positive electrode active materials. Based on the obtained high-resolution transmission electron microscopy images, the surface and bulk structure are analyzed to obtain the thickness data of the core surface shell in the positive electrode active materials.

[0095] 6. Test method for the content of metal element X in positive electrode active materials

[0096] Refer to inductively coupled plasma atomic emission spectrometry in EPA 6010D-2018.

[0097] Table 1. Parameters of the secondary batteries in each embodiment and comparative example.

[0098]

[0099] In Table 1, n1 and n2 are in units of units per 10000 μm. 2 v1 / v2 is the volume ratio of the first active particle and the second active particle per unit volume of the positive electrode active layer; w is the content of metal element X (Al, Ti or Zr) in the positive electrode active material, in ppm; h is the thickness of the shell layer, in nm.

[0100] The electrical performance of the secondary batteries prepared in the above embodiments and comparative examples was tested, as follows:

[0101] 1) 1 / 3C capacity test of secondary battery: The secondary battery is placed at a constant temperature of 25℃ for 2 hours. Within the voltage range of 2.8V to 4.25V, it is first charged at 1 / 3C to 4.25V, and then charged at a constant voltage of 4.25V until the current is ≤0.05mA. It is then placed at a constant voltage for 5 minutes. Then it is discharged at 1C to 2.8V. The capacity of the secondary battery is recorded, and the capacity test value is divided by the total mass of the secondary battery to obtain the capacity of the secondary battery at 1 / 3C.

[0102] 2) Cycle performance test of secondary battery: Under constant temperature conditions of 45℃ and voltage range of 2.8V to 4.2V, first charge the secondary battery to 4.2V at a 1C rate, then charge it at a constant voltage of 4.2V until the current is ≤0.05mA, and let it stand for 5 minutes; then discharge it to 2.8V at a 1C rate, and record the capacity as Dn (n=0, 1, 2, …). Repeat the above process until the capacity decays to 80% of the initial capacity, and record the number of cycles of the secondary battery.

[0103] 3) High-temperature gas generation test of lithium-ion secondary batteries: After the secondary batteries are fully charged to 4.25V at 1C, they are placed in a constant temperature chamber at 70℃ for 30 days; the initial volume and the volume after 30 days of standing are determined by the water displacement method, and the volume expansion rate of the secondary batteries is calculated according to the following formula.

[0104] Volume expansion rate (%) = (volume after standing for 30 days / initial volume - 1) × 100%.

[0105] 4) Fast charging performance test of lithium-ion secondary batteries: Under constant temperature conditions of 25℃ and a voltage range of 2.8V to 4.2V, the secondary battery is first charged to 4.2V at a 1C rate, and then charged at 4.2V at a constant voltage until the current is ≤0.05mA, and left to stand for 5 minutes; then discharged to 2.8V at a 1C rate, and the above process is repeated for 10 cycles. After that, the battery is disassembled and the lithium deposition on the surface of the negative electrode is observed. If no lithium deposition occurs, the rate is increased, and the battery is subjected to constant current charge and discharge tests at rate intervals of 1.5C, 1.8C, 2.1C, 2.4C, 2.7C, 3.0C... and then disassembled to observe the lithium deposition on the surface of the negative electrode until obvious lithium deposition appears on the negative electrode. The rate of the charge and discharge cycle after the appearance of lithium deposition is recorded.

[0106] Table 2 shows the performance of the secondary batteries in each embodiment and comparative example.

[0107]

[0108]

[0109] The test data above shows that when the secondary battery meets the conditions of 0.14≤Z≤4.67 and Z=(n1 / n2)*(p1 / p2), it exhibits superior cycle performance, safety performance, and fast-charging performance. Specifically, when the secondary battery meets these conditions, its cycle life is greater than 1100 cycles, its volume expansion rate is less than 50% (demonstrating good battery safety performance), and its direct charging rate without lithium plating is higher than 2C.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery, characterized in that, The device includes a positive electrode and a negative electrode. The positive electrode includes a positive current collector and a positive active layer disposed on the surface of the positive current collector, the positive active layer containing a positive active material. The negative electrode includes a negative current collector and a negative active layer disposed on the surface of the negative current collector, the negative active layer containing a negative active material. The secondary battery satisfies: 0.14≤Z≤4.67; Where Z = (n1 / n2) × (p1 / p2); n1 units / 10000μm 2 The distribution density of the first active particles in the cross-section of the positive electrode active layer, wherein the first active particles are particles with a diameter greater than or equal to 8 μm in the positive electrode active material particles, and 8 ≤ n1 ≤ 36; n2 units / 10000μm 2 The distribution density of the second active particles in the cross-section of the positive electrode active layer, wherein the second active particles are particles smaller than 8 μm in the positive electrode active material particles, and 6 ≤ n2 ≤ 25; p1 is the porosity of the positive electrode active layer, 20%≤p1≤30%; p2 is the porosity of the negative electrode active layer, 35%≤p2≤45%.

2. The secondary battery as described in claim 1, characterized in that, The secondary battery satisfies: 1.24≤Z≤2.43。 3. The secondary battery as described in claim 1, characterized in that, The volume ratio of the first active particle and the second active particle per unit volume of the positive electrode active layer is 1:(0.3-0.6).

4. The secondary battery as described in claim 1, characterized in that, 0.8≤n1 / n2≤6.

5. The secondary battery as described in claim 1, characterized in that, The negative electrode sheet satisfies at least one of the following: a. The particle size D of the negative electrode active material v50 The thickness is 3~12μm; b. The compaction density of the negative electrode active layer is 1.4~1.8 g / cm³. 3 .

6. The secondary battery as described in claim 1, characterized in that, The positive electrode active material includes a core and a shell layer provided on the surface of the core, and the core includes a compound with the chemical formula Li 1+a [Ni x Co y Mn z M b O2; wherein, 0.5 ≤ x < 1, 0 < y < 0.3, 0 ≤ z < 0.3, -0.1 ≤ a < 0.2, 0 < b < 0.3, and x + y + z + b = 1; M includes at least one of Ti, Zr, Sr, Sb, Nb, Ru, Pd, Y, Ce, and W.

7. The secondary battery as described in claim 6, characterized in that, The cobalt content increases from the center of the core to its surface.

8. The secondary battery as described in claim 6, characterized in that, The shell layer comprises a metal oxide, wherein the metal element X in the metal oxide includes at least one selected from Al, W, Ca, Ce, Ti, Zr, and Y.

9. The secondary battery as described in claim 8, characterized in that, The shell layer satisfies: 2500≤w×h≤16500; Wherein, w ppm is the content of the metal element X in the positive electrode active material, and h nm is the thickness of the shell layer.

10. An electrical appliance, characterized in that, The device includes the secondary battery as described in any one of claims 1 to 9, wherein the secondary battery serves as the power supply for the electrical device.

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