Secondary battery, electric device

Abstract

The application discloses a secondary battery and a power utilization device, and belongs to the technical field of batteries. The secondary battery provided by the application introduces a positive electrode additive containing a specific structure, a specific molar ratio of substances and a specific content of a substance range into a positive electrode active material layer, and the size of a gap between a core of a positive electrode active material and a carbon coating layer and the percentage range of the total area of cracks on the cross section of the core to the total area of the cross section of the core are limited; the obtained secondary battery has excellent rate performance, cycle life and storage life, has a low internal resistance, and can effectively reduce the gas production risk in the storage process.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0002] The rapid development of the power battery and energy storage battery industry has placed higher demands on the rate performance, cycle life, and storage life of lithium-ion batteries.

[0003] Currently, improving the rate performance, cycle life, and storage life of lithium-ion batteries from the perspective of materials such as cathode materials, anode materials, electrolytes, and separators has gradually reached a bottleneck. How to further improve the performance of lithium-ion batteries without changing the cathode materials, anode materials, electrolytes, and separators has become the current research focus. Summary of the Invention

[0004] The purpose of this application is to address the problems in the prior art where the cycle performance and rate performance of lithium-ion batteries need further improvement, and the serious gas generation caused by adopting other technologies to improve cycle performance. The application proposes a secondary battery and power device with good cycle life, storage life and rate performance, low internal resistance and low risk of gas generation during storage.

[0005] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a positive electrode and a negative electrode, wherein the positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector.

[0006] The positive electrode active material layer includes a lithium-containing positive electrode active material and a positive electrode additive;

[0007] The positive electrode additive includes a core and a carbon coating layer disposed on the surface of the core;

[0008] The core comprises TM, oxygen and lithium, wherein the molar ratio of TM to oxygen is 1:(1.5~3), and the TM comprises at least one of iron, cobalt, nickel, manganese and aluminum;

[0009] Based on the mass of lithium in the positive electrode active material, the mass percentage of lithium in the core is ≤21%;

[0010] There are gaps between the core and the carbon coating layer, with a gap size of 0.02~2μm;

[0011] The core has a cross-section with cracks, and the percentage of the total area of ​​the cracks on the cross-section is 2 to 50% of the total area of ​​the cross-section of the core.

[0012] As an embodiment of this application, the percentage of the total crack area on the cross-section of the core is 15-20% based on the total cross-sectional area of ​​the core.

[0013] As an embodiment of this application, the width B of the crack satisfies 20nm<B≤260 nm, and the length L satisfies 0.1μm≤L≤3.2μm.

[0014] As an embodiment of this application, the mass percentage of lithium in the core is 2-15% based on the mass of lithium in the lithium-containing positive electrode active material.

[0015] As an embodiment of this application, the thickness of the carbon coating layer is 0.01~0.2μm.

[0016] As an embodiment of this application, the average particle size of the positive electrode additive is 1~40μm.

[0017] As an embodiment of this application, in the 100μm×100μm area of ​​the positive electrode active material layer, the average number of positive electrode additive particles is 0.5~35.

[0018] As an implementation of this application, the kernel includes Li 1+x TM y O2, where 0 < x < 3 and 0.35 ≤ y < 2; TM includes at least one of iron, cobalt, nickel, manganese, and aluminum.

[0019] As an implementation of this application, the kernel includes Li 2.5 Fe 0.5 O2, Li2NiO2, Li3Co 0.5 O2, Li 1.33 Mn 0.67 O2Li 2.3 Fe 0.44 Al 0.005 O2, Li 2.4 Fe 0.5 Al 0.005 O2, Li 2.35 Fe 0.48 Al 0.005 O2, Li 2.25 Fe 0.41 Al 0.005 O2, Li 2.2 Fe 0.37 Al 0.005 O2, Li 1.1 Ni 0.9 Al 0.005 O2, Li 2.3 Co 0.44 Al 0.005 O2, Li 2.6 Fe 0.6 O2, Li 2.1 Fe 0.35 Al 0.03At least one of O2.

[0020] As an embodiment of this application, the lithium-containing positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, ternary materials, spinel materials, and lithium-rich manganese-based materials.

[0021] In a second aspect of this application, an electrical device is provided, including the aforementioned secondary battery.

[0022] The beneficial effects of this application are:

[0023] This application provides a secondary battery by introducing a positive electrode additive containing a specific structure and a specific molar ratio and content range of substances into the positive electrode active material layer. Furthermore, it defines the size of the gap between the core and the carbon coating layer of the positive electrode active material and the percentage range of the total area of ​​cracks on the core cross-section to the total area of ​​the core cross-section. The resulting secondary battery has excellent rate performance, cycle life, and storage life, as well as low internal resistance, and can effectively reduce the risk of gas generation during storage. Attached Figure Description

[0024] Figure 1 These are cross-sectional morphology images of the positive electrode sheet in the secondary battery prepared in Example 1, at different magnifications.

[0025] Figure 2 SEM morphology of the core of the positive electrode additive in the positive electrode sheet of the secondary battery prepared in Example 1: a-morphology image, b-calculation result of core crack area ratio;

[0026] Figure 3 SEM morphology of the core of the positive electrode additive in the positive electrode sheet of the secondary battery prepared in Example 16: a-morphology image, b-calculation result of core crack area ratio;

[0027] Figure 4 SEM morphology of the core of the positive electrode additive in the positive electrode sheet of the secondary battery prepared in Example 18: a-morphology image, b-calculation result of core crack area ratio;

[0028] Figure 5 The images show cross-sectional morphology of the positive electrode sheet in the secondary battery prepared in Comparative Example 6 at different magnifications.

[0029] Figure 6 This is a comparison chart of the cycling performance test results of the secondary batteries prepared in Example 1 and Comparative Example 6 at 25°C.

[0030] Figure 7 A comparison chart showing the test results of the storage capacity retention rate of the secondary batteries prepared in Example 1 and Comparative Example 6 at 60°C;

[0031] Figure 8 This is a comparison chart of the room temperature DCR test results of the secondary batteries prepared in Example 1 and Comparative Example 6;

[0032] Figure 9 This is a comparison chart of the rate discharge performance test results of the secondary batteries prepared in Example 1 and Comparative Example 6;

[0033] Figure 10 A schematic diagram illustrating the sampling process for testing the void size of the core and carbon coating. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0035] In this application, the technical features described in an open-ended manner include both closed technical solutions composed of the listed features and open technical solutions composed of the listed features.

[0036] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0037] In this application, the void size is defined as the vertical distance from any point on the surface of the carbon coating layer near the core to the core surface.

[0038] It should be noted that the size of the gap between the core and the carbon coating layer refers to the average gap size of the vertical distance from any point on the surface of the carbon coating layer closest to the core to the core surface.

[0039] In one embodiment of this application, a secondary battery is proposed, including a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector;

[0040] The positive electrode active material layer includes a lithium-containing positive electrode active material and a positive electrode additive;

[0041] The positive electrode additive includes a core and a carbon coating layer disposed on the surface of the core;

[0042] The core comprises TM, oxygen and lithium, wherein the molar ratio of TM to oxygen is 1:(1.5~3), and the TM comprises at least one of iron, cobalt, nickel, manganese and aluminum;

[0043] Based on the mass of lithium in the positive electrode active material, the mass percentage of lithium in the core is ≤21%;

[0044] There are gaps between the core and the carbon coating layer, with a gap size of 0.02~2μm;

[0045] The core has a cross-section with cracks, and the percentage of the total area of ​​the cracks on the cross-section is 2 to 50% of the total area of ​​the cross-section of the core.

[0046] This application research found that the secondary battery provided in this application introduces a positive electrode additive containing a specific structure and a specific molar ratio and content range of a specific substance into the positive electrode active material layer. Furthermore, it limits the size of the gap between the core and the carbon coating layer of the positive electrode active material and the percentage range of the total area of ​​cracks on the core cross-section to the total area of ​​the core cross-section. The resulting secondary battery has excellent rate performance, cycle life and storage life, while also having low internal resistance and low risk of gas generation during storage.

[0047] Specifically, on the one hand, the core contains TM and oxygen within a specific molar ratio range, and the mass percentage of lithium in the core, calculated based on the mass of lithium in the positive electrode active material, is within a specific range. This allows lithium ions to be released during the first charge of the secondary battery, providing active lithium and thus improving the cycle life and storage life of the secondary battery. On the other hand, when the size of the gap between the core and the carbon coating layer is within a specific range, and the percentage of the total crack area on the core cross-section to the total cross-sectional area of ​​the core is within a specific range, it can effectively store a portion of the electrolyte, thereby improving ion transport in the pores of the positive electrode during charge and discharge, reducing concentration polarization in the electrolyte on the surface of the positive electrode active material, improving the rate performance of the secondary battery, and reducing the internal resistance of the secondary battery. Furthermore, by controlling the percentage of the total crack area on the core cross-section to the total cross-sectional area of ​​the core, gas generation during storage can also be controlled, achieving a balance between the risk of gas generation and rate performance, resulting in a secondary battery with excellent overall performance.

[0048] It should be noted that the types of TM in the core and the molar ratio of TM to oxygen in the core were obtained through cross-sectional EDS testing. Specifically, a sample exposing the cross-section of the positive electrode sheet from the secondary battery was prepared by ion milling for SEM testing. The cross-section of the additive was observed by adjusting the magnification during the SEM test, and the EDS of the cross-section was measured. The types of TM were determined based on the EDS test results. The molar ratio of TM to oxygen in the core was obtained by averaging the EDS values ​​of TM and oxygen from multiple images.

[0049] For example, the molar ratio of TM to oxygen can be any point value or any two points within the range of 1:(1.5~3), such as one or any two of the following: 1:1.5, 1:1.6, 1:1.8, 1:2.0, 1:2.2, 1:2.4, 1:2.6, 1:2.8, 1:3.0.

[0050] In one embodiment, the molar ratio of TM to oxygen is 1:(1.8~2.4). For example, it can be one or any two of the following: 1:1.8, 1:1.9, 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4. This application has found that when the molar ratio of TM to oxygen is further selected within the above range, the average valence state of TM in the additive is lower, and there are fewer highly reactive oxygen free radicals generated during formation and charging. This reduces the oxidative decomposition of the electrolyte by high-valence TM and highly reactive oxygen free radicals during subsequent cycling and storage, thereby reducing gas generation in the secondary battery and resulting in a secondary battery with better overall performance.

[0051] It should be noted that, based on the mass of lithium in the positive electrode active material, the mass percentage of lithium in the core is calculated using the following formula after ICP and specific capacity tests of the positive and negative electrode sheets: Mass percentage of lithium in the core to lithium in the positive electrode active material Q1 = A1 * 3.861 / (C1 / E1) - 1, where A1 is the sum of the mass of lithium per unit area of ​​the positive electrode sheet and the mass of lithium per unit area of ​​the negative electrode sheet, in g / cm³. 2 A1*3.681 represents the capacity corresponding to the mass of lithium per unit area of ​​the wafer, in Ah / cm³. 2 C1 is the reversible capacity per unit area of ​​the positive electrode, expressed in Ah / cm². 2E1 represents the initial efficiency of the original cathode powder; for lithium iron phosphate, lithium manganese iron phosphate, and spinel materials, E1=1; for ternary materials, E1=0.891. C1 / E1 represents the capacity provided by lithium in the original cathode powder. A1 is obtained by testing the ICP of a specific area of ​​the cathode and anode plates after fully discharging the battery. C1 is obtained by testing the reversible capacity of the cathode plate per unit area at 45℃ and a small current of 0.04C after fully discharging the battery. C1 represents the capacity of the double-sided coated electrode. E1 is obtained by determining the type of cathode material based on the charge-discharge curve of the coin cell and the XRD of the cathode plate.

[0052] For example, based on the mass of lithium in the positive electrode active material, the mass percentage of lithium in the core can be any point value or any two points within the range of ≤21%, such as one or any two of the following: 0.1%, 1%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 21%.

[0053] It should be noted that the test method for the size of the gap between the core and the carbon coating layer is as follows: A sample exposing the cross-section of the positive electrode sheet from the secondary battery is prepared by ion milling for SEM testing, wherein the cross-section is perpendicular to the electrode surface; the positive electrode additive particles are located by adjusting the magnification during the SEM test, and the gap between the positive electrode additive core and the carbon coating layer is observed; the test sample particles are then determined, and the specific test steps are as follows:

[0054] S1. Determine the smallest rectangle that can enclose the cross-section of the positive electrode additive particles.

[0055] Determine the two points (a, b) with the farthest spacing between the carbon coating layers of the positive electrode additive particles, and use the line connecting the two points as the baseline (h). Translate this baseline along both sides in the direction perpendicular to the baseline until it is tangent to the outer side of the carbon coating layer. This will give you the two long sides (h1, h2) of the minimum rectangle. Connect the corresponding endpoints of the two long sides to get the required minimum rectangle.

[0056] S2. Determine the sampling points

[0057] The center point (O) of the rectangle is determined by connecting the two diagonals of the smallest rectangle. The diagonals intersect the inner surface of the carbon coating layer at 4 points. The long side and the wide side of the smallest rectangle are divided into 4 equal parts, that is, there are 3 division points on each of the two long sides and the two wide sides, for a total of 12 division points. The connecting line obtained by connecting the 12 division points to the center point of the rectangle intersects the inner surface of the carbon coating layer at 12 points. The 4 intersection points of the diagonals with the inner surface of the carbon coating layer and the 12 intersection points of the connecting line with the inner surface of the carbon coating layer are determined as sampling points.

[0058] S3. Calculate the gap size

[0059] Measure the shortest distance from each sampling point to the kernel surface (e.g., the distance between points a1 and a2), and calculate the average value to obtain the gap size.

[0060] For example, the gap size can be any point value or any two-point range within the range of 0.02 to 2 μm, such as one or any two of the following: 0.02 μm, 0.04 μm, 0.06 μm, 0.08 μm, 0.1 μm, 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, and 2 μm. In one embodiment, the gap size is 0.2 to 1.5 μm. For example, it can be one or any two of the following: 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, and 1.5 μm. This study found that when the pore size is further selected to be 0.2~1.5μm, the overall performance of the secondary battery is better.

[0061] It should be noted that the test method for the percentage of the total crack area on the cross-section of the core is as follows: a sample exposing the cross-section of the positive electrode sheet in the secondary battery is prepared by ion milling for SEM testing, and the cross-section is perpendicular to the surface of the electrode sheet; the crack is observed by adjusting the magnification during the SEM test, and the crack area and cross-sectional area can be obtained by identifying the grayscale difference between the crack and the cross-section of the core; then it is calculated according to the formula: percentage of the total crack area to the total cross-sectional area = total crack area / total area of ​​the cross-section of the core * 100%.

[0062] For example, the percentage of the total crack area on the cross-section of the core can be any point value or any two points within the range of 2% to 50%, such as 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, or any two of these ranges.

[0063] In one embodiment, the percentage of the total crack area on the cross-section of the core is 15-20%, based on the total cross-sectional area. For example, it can be a range of one or any two of 15%, 16%, 17%, 18%, 19%, and 20%.

[0064] This study found that the percentage of the total crack area on the cross-section affects the improvement of the rate performance and the reduction of internal resistance of the secondary battery, and also affects the stability of the cathode additive. Specifically, it affects gas generation and the dissolution rate of the TM electrode. When the percentage of the total crack area on the cross-section is selected to be 15-20% based on the total area of ​​the core cross-section, the secondary battery obtained has better rate performance, lower internal resistance, and less gas generation; that is, the secondary battery obtained has better overall performance.

[0065] In one embodiment, the width B of the crack satisfies 20nm≤B≤260 nm, and the length L satisfies 0.1μm≤L≤3.2μm.

[0066] It should be noted that the test method for crack width and length is as follows: After charging the secondary battery to 100% SOC, it is disassembled in a dry room to obtain the positive electrode sheet. The positive electrode sheet is then used to prepare a sample with exposed cross-section for SEM testing by ion milling, and the cross-section is perpendicular to the electrode surface. The crack is observed by adjusting the magnification during the SEM test, and the crack width and length are obtained by taking the average value of the measurements from multiple images.

[0067] For example, the width B of the crack can be any point value or any two points within the range of 20nm≤B≤260 nm, such as one or any two of the following: 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, 60nm, 80nm, 100nm, 120nm, 140nm, 160nm, 180nm, 200nm, 220nm, 240nm, and 260nm. The length L of the crack can be any point value or any two point values ​​within the range of 0.1μm≤L≤3.2μm, for example, it can be one or any two of the following: 0.1μm, 0.2μm, 0.4μm, 0.6μm, 0.8μm, 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2.0μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3.0μm, 3.2μm.

[0068] This study found that the values ​​of crack width B and length L can affect the area ratio of the crack in the cross section to a certain extent, thus affecting the overall performance of the secondary battery. Furthermore, the values ​​of crack width B and length L can also affect the morphology of the crack, thereby affecting the stability of the cathode additive. When the crack width B and length L are further selected within the range given in this application, the secondary battery has better cycle performance and lower risk of gas generation during storage.

[0069] In one embodiment, the crack width B satisfies 50 nm ≤ B ≤ 150 nm, and the length L satisfies 0.8 μm ≤ L ≤ 2.0 μm. For example, the crack width B can be any one or both of the following: 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm. The crack length L can be any one or both of the following: 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm. This application research found that when the crack width B and length L are further selected within the above ranges, the overall performance of the resulting secondary battery is superior.

[0070] In one embodiment, the mass percentage of lithium in the core is 2-15%, based on the mass of lithium in the lithium-containing positive electrode active material. For example, it can be one or any two of the following ranges: 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, and 15%. This application has found that when the mass percentage of lithium in the core is further selected within the above range, it not only ensures that the void size between the core and the carbon coating layer is within a suitable range, and that the area ratio of the corresponding cracks on the cross-section is within a suitable range, but also provides suitable active lithium replenishment for the secondary battery, thereby obtaining a secondary battery with excellent rate performance and low gas generation risk, as well as excellent cycle and storage life.

[0071] In one embodiment, the thickness of the carbon coating layer is 0.01~0.2μm.

[0072] It should be noted that the test method for the thickness of the carbon coating layer is as follows: after charging the secondary battery to 100% SOC, it is disassembled in a drying room to obtain the positive electrode sheet. The positive electrode sheet is then used to prepare a sample exposing the cross-section of the positive electrode sheet by ion milling for SEM testing. The thickness of the additive carbon coating layer is observed by adjusting the magnification during the SEM test, and the thickness is obtained by taking the average value of the carbon coating layer thickness in multiple images.

[0073] For example, the thickness of the carbon coating layer can be any point value or any two points within the range of 0.01~0.2μm, such as one or any two of 0.01μm, 0.02μm, 0.04μm, 0.06μm, 0.08μm, 0.1μm, 0.12μm, 0.14μm, 0.16μm, 0.18μm, and 0.2μm.

[0074] This study found that the carbon coating layer has a certain impact on the conductivity and stability of the cathode additive. When the thickness of the carbon coating layer is selected within the above range, it can effectively improve the conductivity and environmental stability of the cathode additive. At the same time, it can also serve as a rigid shell for the cathode additive, creating a gap of a specific size between the core and the core when the core contracts, thereby achieving the storage effect of electrolyte, thus improving the rate performance of the secondary battery and reducing the internal resistance of the secondary battery.

[0075] In one embodiment, the carbon coating layer has a mass percentage of 0.5-5% based on the mass of the cathode additive.

[0076] For example, the mass percentage of the carbon coating layer, based on the mass of the cathode additive, can be any point value or any two points within the range of 0.5% to 5%, such as one or any two of the following: 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%.

[0077] This study found that when the mass percentage of carbon coating in the cathode additive is 0.5-5%, the overall performance of the secondary battery can be significantly improved.

[0078] In one embodiment, the average particle size of the positive electrode additive is 1~40 μm.

[0079] It should be noted that the test method for the average particle size of the positive electrode additive is as follows: a sample exposing the cross-section of the positive electrode sheet in the secondary battery is prepared by ion milling for SEM testing; the size of the additive particles is observed by adjusting the magnification during the SEM test, and the average size of the additive particles in multiple images is obtained.

[0080] For example, the average particle size of the positive electrode additive can be any point value or any two points within the range of 1 to 40 μm, such as one or any two of 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm.

[0081] This study found that the average particle size of the added cathode additive not only affects its distribution in the cathode active material layer, but also affects the size of the voids between the core and the carbon coating layer after preparation, as well as the proportion of the total crack area on the cross-section. When the average particle size of the cathode additive is further selected to be 1~40μm, the resulting secondary battery has better rate performance and cycle performance, and lower internal resistance and gas generation risk.

[0082] In one embodiment, the average particle size of the positive electrode additive is 5-25 μm. For example, it can be any one or both of the following values: 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 24 μm, and 25 μm. This application has found that when the average particle size of the positive electrode additive is further selected to be 5-25 μm, the overall performance of the resulting secondary battery is superior. In one embodiment, the average number of positive electrode additive particles in a 100 μm × 100 μm area of ​​the positive electrode active material layer is 0.5-35.

[0083] It should be noted that the test method for the average number of positive electrode additive particles in a 100μm×100μm area of ​​the positive electrode active material layer is as follows: a sample exposing the cross-section of the positive electrode sheet in the positive electrode sheet of the secondary battery is prepared by ion milling for SEM testing; the number of additive particles is observed by adjusting the magnification during the SEM test, and the average value is obtained by taking multiple measurements and statistically analyzing them.

[0084] For example, in the 100μm×100μm area of ​​the positive electrode active material layer, the average number of positive electrode additive particles can be any point value or any two points within the range of 0.5 to 35. For example, it can be one or any two of the following: 0.5, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 35.

[0085] This study found that the average number of positive electrode additive particles within a 100μm × 100μm area of ​​the positive electrode active material layer reflects the amount of positive electrode additive added to a certain extent. The addition of positive electrode additives can provide active lithium, replenishing the active lithium consumed by the SEI film during the first charge of the secondary battery, thereby increasing the actual specific capacity of the positive electrode active material and thus improving the energy density of the secondary battery. Furthermore, excess active lithium in the positive electrode additives can be temporarily stored in the negative electrode, supplying the active lithium consumed by the subsequent growth of the SEI film, thereby improving the cycle performance and storage performance of the secondary battery. Cracks and voids in the positive electrode additives can store electrolyte, reducing concentration polarization on the positive electrode side during charge and discharge, thereby reducing the battery's internal resistance and improving rate performance.

[0086] In one embodiment, the average number of positive electrode additive particles in a 100μm × 100μm area of ​​the positive electrode active material layer can be 3 to 15. For example, it can be one or any two of the following values: 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15. This application has found that when the average number of positive electrode additive particles in a 100μm × 100μm area of ​​the positive electrode active material layer is further selected to be 3 to 15, the overall performance of the resulting secondary battery is superior.

[0087] In one embodiment, the kernel includes Li 1+x TM y O2, wherein 0 < x < 3, 0.35 ≤ y < 2; TM includes at least one of iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), and aluminum (Al).

[0088] In one embodiment, the kernel includes Li 2.5 Fe 0.5 O2, Li2NiO2, Li3Co 0.5 O2, Li 1.33 Mn 0.67 O2Li 2.3 Fe 0.44 Al 0.005 O2, Li 2.4 Fe 0.5 Al 0.005 O2, Li 2.35 Fe 0.48 Al 0.005 O2, Li 2.25 Fe 0.41 Al 0.005 O2, Li 2.2 Fe 0.37 Al 0.005 O2, Li 1.1 Ni 0.9 Al 0.005 O2, Li 2.3 Co 0.44 Al 0.005 O2, Li 2.6 Fe 0.6 O2, Li 2.1 Fe 0.35 Al 0.03 At least one of O2.

[0089] This application research found that selecting the core with the above-mentioned chemical formula enables the positive electrode additive in the subsequently prepared secondary battery to include a core containing TM, oxygen, and lithium, with a molar ratio of TM to oxygen of 1:(1.5~3), and the mass percentage of lithium in the core is ≤21% based on the mass of lithium in the positive electrode active material. Furthermore, by selecting a substance with the above-mentioned chemical formula as the core, and by adjusting the values ​​of x and y, different molar ratios of TM and oxygen and the mass percentage of lithium in the core can be obtained; simultaneously, the size of the voids between the obtained core and the carbon coating layer, as well as the proportion of the total crack area on the core cross-section, can also be controlled. In one embodiment, the lithium-containing positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, ternary materials, spinel materials, and lithium-rich manganese-based materials.

[0090] Exemplarily, the lithium iron phosphate includes doped or undoped lithium iron phosphate, and may have or not have a carbon layer coating; the lithium iron manganese phosphate includes LiFe 1-x Mn x PO4, where 0 < x < 1; the ternary material includes LiNi 1-x-y Co x Mn y O2 and / or LiNi 1-x-y Co x Al y O2, where 0 < x, y < 1; the spinel material includes LiNi 2-x Mn x O4, where 0 < x ≤ 2; the lithium-rich manganese-based material includes Li 1+x TM 1-x-y Mn y O2, where 0 < x, y < 1, and TM is one or more of Ni, Co, Ti, Ru, and Cr elements.

[0091] In one embodiment, the lithium-containing positive electrode active material includes LiFePO4 and / or LiFe 1-x Mn x PO4, where 0 < x < 1.

[0092] It is found in the research of this application that when further selecting LiFePO4 and / or LiFe 1-x Mn x PO4, where 0 < x < 1 as the lithium-containing positive electrode active material, the additive can supplement the lithium consumed in the SEI during the first formation, so that more lithium ions can be embedded into the lithium vacancies in the lattice of LiFePO4 and LiFe 1-x Mn x PO4 during discharge, enabling the LiFePO4 and LiFe 1-x Mn x PO4 battery to exhibit a higher specific capacity and effectively improve the energy density of the battery.And because the rate performance of LiFePO4 and LiFe 1-x Mn x PO4 itself is worse than that of the ternary material, the additive has a more significant improvement in the rate performance of LiFePO4 and LiFe 1-x Mn x PO4.

[0093] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder. This application places no restrictions on the conductive agent and the binder in the positive electrode active material layer, and any known conductive agent and adhesive can be used.

[0094] In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material. This application does not limit the negative electrode active material; any known negative electrode active material can be used.

[0095] In one embodiment, the secondary battery further includes a separator and an electrolyte.

[0096] In one embodiment, the electrolyte includes an organic solvent, a lithium salt, and additives. This application does not limit the organic solvent, lithium salt, and additives in the electrolyte; any known organic solvent, lithium salt, and additives can be used.

[0097] For example, the organic solvent includes at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate; the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; and the additive includes at least one of film-forming additives, conductive additives, and flame-retardant additives.

[0098] In one embodiment, the separator of the secondary battery is disposed between the positive and negative electrodes.

[0099] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0100] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0101] Example 1

[0102] This application provides a secondary battery, the preparation method of which includes the following steps:

[0103] (1) Preparation of positive electrode sheet

[0104] The positive electrode additive (with Li core) 2.5 Fe 0.5O2 (with a carbon coating thickness of 0.1 μm) and positive electrode active material (lithium iron phosphate) were mixed at a mass ratio of 1.5:100 to obtain a mixture. The mixture, conductive agent (a mixture of super P and carbon nanotubes at a mass ratio of 1:1), and binder (polyvinylidene fluoride) were then mixed at a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) solvent was added, and the mixture was stirred in a vacuum mixer until a uniform positive electrode slurry was obtained. The positive electrode slurry was uniformly coated onto a positive electrode current collector aluminum foil. After coating, it was dried at 100°C, and then rolled, slit, and cut into sheets to obtain the positive electrode sheet.

[0105] (2) Preparation of negative electrode sheet

[0106] The negative electrode active material (graphite), conductive agent (super P), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed at a mass ratio of 97:1:1:1. Deionized water was added and the mixture was stirred in a vacuum mixer to obtain a uniform negative electrode slurry. The negative electrode slurry was uniformly coated onto the negative electrode current collector copper foil. After coating, the foil was dried at 100°C, and then rolled, slit, and cut into sheets to obtain the negative electrode sheet.

[0107] (3) Preparation of electrolyte

[0108] The organic solvent is a mixture containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), wherein the volume ratio of EC, EMC, and DEC is 30:40:30. In an argon-atmosphere glove box with a water content of <10 ppm, thoroughly dried electrolyte salt LiPF6 is dissolved in the organic solvent to prepare a 1 mol / L solution, and then 2 wt.% of fluoroethylene carbonate (FEC) is added to obtain the electrolyte.

[0109] (4) Preparation of secondary batteries

[0110] The positive electrode, separator, and negative electrode are wound or stacked in sequence so that the separator is positioned between the positive and negative electrodes to isolate the two electrodes. Then the core is loaded into an aluminum-plastic film, baked to remove moisture, and then injected with electrolyte. After sealing, hot and cold pressing, formation, secondary sealing, and capacity testing, a secondary battery is obtained.

[0111] Among them, the cross-sectional morphology images of the positive electrode sheet after disassembly of the secondary battery prepared in Example 1 at different magnifications are shown in the figure below. Figure 1 As shown; the SEM morphology of the core of the positive electrode additive in the positive electrode sheet is as follows. Figure 2 As shown, where Figure 2 In the figure, a is the morphology diagram, and b is the calculated result of the proportion of the core crack area.

[0112] Examples 2-5

[0113] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the Li core of the positive electrode additive is adjusted. 1+x TM y The values ​​of x and y in O2 change the molar ratio of TM and oxygen.

[0114] Examples 6-7

[0115] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the Li core of the positive electrode additive is adjusted. 1+x TM y In O2, the selection of TM enables the change of the TM type.

[0116] Examples 8-11

[0117] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the average amount of positive electrode additives is adjusted to change Q1.

[0118] Examples 12-15

[0119] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the particle size of the positive electrode additive is adjusted to change the pore size.

[0120] Examples 16-18

[0121] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the Li core of the positive electrode additive is adjusted. 1+x TM y The amount of Al doping at the TM position in O2 changes Q2;

[0122] The SEM morphology images of the positive electrode additive cores in the positive electrode sheets of the secondary batteries prepared in Examples 16 and 18 after disassembly are shown below. Figures 3-4 As shown; the SEM morphology of the core of the positive electrode additive in the positive electrode sheet is as follows. Figure 2 As shown, where Figure 2 In the figure, a is the morphology diagram, and b is the calculated result of the proportion of the core crack area.

[0123] Examples 19-20

[0124] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the Li core of the positive electrode additive is adjusted. 1+x TM y The values ​​of x and y in O2 and the amount of Al doping at the TM position can change the crack width and length.

[0125] Examples 21-22

[0126] This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the preparation process of the positive electrode additive is adjusted to change the thickness of the carbon coating layer and Q3.

[0127] Examples 23-24

[0128] This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that the positive electrode active material is adjusted to change the type of positive electrode active material.

[0129] Comparative Examples 1-2

[0130] This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 lies in adjusting the Li core of the positive electrode additive. 1+x TM y The values ​​of x and y in O2 and the amount of Al doping at the TM position change the molar ratio of TM to oxygen.

[0131] Comparative Example 3

[0132] This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that the average amount of positive electrode additives is adjusted to change Q1.

[0133] Comparative Examples 4-5

[0134] This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that the particle size of the positive electrode additive is adjusted to change the porosity.

[0135] Comparative Example 6

[0136] This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that no positive electrode additive is added.

[0137] Among them, the cross-sectional morphology of the positive electrode sheet after disassembly of the secondary battery prepared in Comparative Example 6 is shown in the following figures at different magnifications. Figure 5 As shown.

[0138] Comparative Example 7

[0139] The difference between Comparative Example 7 and Comparative Example 6 is that the positive electrode active material is LiMn. 0.4 Fe 0.6 PO4.

[0140] Comparative Example 8

[0141] The difference between Comparative Example 8 and Comparative Example 6 is that the positive electrode active material is LiNi.0.6 Co 0.2 Mn 0.2 O2.

[0142] In the secondary batteries prepared in the examples and comparative examples, the type of core TM and its molar ratio with oxygen, the mass percentage of lithium in the core (Q1) based on the mass of lithium in the positive electrode active material, the size of the gap between the core and the carbon coating layer, the percentage of the total area of ​​cracks on the cross-section based on the total area of ​​the core's cross-section (Q2), the width B and length L of the cracks on the corresponding core cross-section, the thickness of the carbon coating layer, the mass percentage of the carbon coating layer based on the mass of the positive electrode additive (Q3), the particle size of the positive electrode additive, the average number of positive electrode additive particles in a 100μm×100μm area of ​​the positive electrode active material layer (Q4), the chemical formula of the core, and the type of lithium-containing positive electrode active material are shown in Tables 1-2.

[0143] Table 1. Parameters of Secondary Batteries

[0144]

[0145] Table 2 Parameter Table of Secondary Batteries

[0146]

[0147] The performance tests of the secondary batteries prepared in the examples and comparative examples are as follows:

[0148] 1. Cyclic Performance: 1C / 1C cycle charge / discharge tests were conducted in a 25℃ incubator. Constant current and constant voltage charging was performed with a current of 1C and a cutoff current of 0.05C. The constant current discharge current was 1C. A 5-minute interval was allowed between charge and discharge steps. The charge / discharge voltage range depended on the cathode material type: 2.5~3.65V for lithium iron phosphate, 2.5~4.25V for lithium manganese iron phosphate, 2.8~4.3 / 4.35 / 4.4 / 4.45V for medium-nickel ternary materials, and 2.8~4.1 / 4.15 / 4.2 / 4.25V for high-nickel ternary materials. The cycle capacity retention rate was calculated using the following formula: Cycle capacity retention rate for cycle X = (Discharge capacity for cycle X - Discharge capacity for cycle 1) / Discharge capacity for cycle 1 * 100%.

[0149] 2. Storage Performance: The battery's 1C capacity was tested at 25℃ and fully charged. After full charging, the battery was placed in a 60℃ oven for storage. After a certain number of days, the battery was removed and its 1C recovery capacity was tested at 25℃ and fully charged. This process was repeated. For the first 60 days, the oven capacity was tested every 15 days, and after 60 days, it was tested every 30 days. The storage capacity retention rate was calculated using the following formula: Storage X-day capacity retention rate = (Storage X-day discharge capacity - Storage 0-day discharge capacity) / Storage 0-day discharge capacity * 100%;

[0150] 3. DCR Test: At 25℃, adjust the battery to 90%, 50%, and 20% SOC sequentially according to its 1C capacity. After resting for 60 minutes, execute a 5C discharge pulse program for 10 seconds. Calculate the DCR based on the discharge pulse's start voltage, end voltage, and pulse current. The DCR calculation formula is as follows: DCR = (Pulse start voltage - Pulse end voltage) / Pulse current;

[0151] 4. Rate Performance: The battery is charged at a constant current and constant voltage of 1C (cutoff current is 0.05C), and then discharged at constant currents of 0.33C / 0.5C / 1C / 2C / 3C / 4C respectively. The discharge capacity at different rates is obtained, and the capacity retention rate at each rate is calculated. The formula for calculating the capacity retention rate at each rate is as follows: XC rate discharge capacity retention rate = (XC discharge capacity - 0.33C discharge capacity) / 0.33C discharge capacity * 100%; (XC is the corresponding rate)

[0152] 5. Gas generation test during 0% SOC storage: The battery was adjusted to 0% SOC at 25℃ using a 1C current. The battery was then stored in a 70℃ oven, and the volume of the secondary battery was tested every other day using the water displacement method. The volume expansion rate of the secondary battery after 4 days of storage was obtained; Volume expansion rate = (Volume after 4 days of storage - Initial volume) / Initial volume * 100%;

[0153] The results are shown in Table 3.

[0154] Table 3 Performance Test Table for Secondary Batteries

[0155]

[0156] As can be seen from Table 3, the secondary battery obtained using the technical solution provided in this application exhibits excellent overall performance. The overall performance of the secondary battery is improved for systems with lithium iron phosphate, lithium manganese iron phosphate, and ternary cathode active materials. Specifically, for systems with lithium iron phosphate cathode active materials, the internal resistance is low, with a 50% SOC DCR below 26.8 mΩ; the rate performance is excellent, with a 4C discharge retention rate above 76.8%; the cycle performance is excellent, with a 25℃ cycle capacity retention rate above 91.3% and a 60℃ cycle capacity retention rate above 82.5%; and the risk of gas generation during storage is low, with a volume expansion rate below 21.0% after 4 days of storage. As can be seen from Examples 1-22 and Comparative Examples 1-6, when the provided technical solution is outside the scope given in this application, the effects of this application cannot be achieved. This is evident from the performance test results of the secondary batteries prepared in Examples 1 and 6. Figures 6-9 This can also be seen intuitively in the text.

[0157] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application 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 this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a positive electrode and a negative electrode, wherein the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, characterized in that, The positive electrode active material layer includes a lithium-containing positive electrode active material and a positive electrode additive; The positive electrode additive includes a core and a carbon coating layer disposed on the surface of the core; The core comprises TM, oxygen and lithium, wherein the molar ratio of TM to oxygen is 1:(1.5~3), and the TM comprises at least one of iron, cobalt, nickel, manganese and aluminum; Based on the mass of lithium in the positive electrode active material, the mass percentage of lithium in the core is ≤21%; There are gaps between the core and the carbon coating layer, with a gap size of 0.02~2μm; The core has a cross-section with cracks, and the percentage of the total area of ​​the cracks on the cross-section is 2 to 50% of the total area of ​​the cross-section of the core.

2. The secondary battery according to claim 1, characterized in that, The percentage of the total area of ​​cracks on the cross-section of the core is 15-20% based on the total area of ​​the cross-section.

3. The secondary battery according to claim 1, characterized in that, The width B of the crack satisfies 20nm≤B≤260nm, and the length L satisfies 0.1μm≤L≤3.2μm.

4. The secondary battery according to claim 1, characterized in that, Based on the mass of lithium in the lithium-containing positive electrode active material, the mass percentage of lithium in the core is 2-15%.

5. The secondary battery according to claim 1, characterized in that, The thickness of the carbon coating layer is 0.01~0.2μm.

6. The secondary battery according to claim 1, characterized in that, The average particle size of the positive electrode additive is 1~40μm.

7. The secondary battery according to claim 1, characterized in that, In the 100μm×100μm area of ​​the positive electrode active material layer, the average number of positive electrode additive particles is 0.5~35.

8. The secondary battery according to claim 1, characterized in that, The kernel includes Li 1+x TM y O2, Where 0 < x < 3, 0.35 ≤ y < 2; TM includes at least one of iron, cobalt, nickel, manganese, and aluminum.

9. The secondary battery according to claim 8, characterized in that, Li 2.5 Fe 0.5 O2、Li2NiO2、Li3Co 0.5 O2、Li 1.33 Mn 0.67 O2Li 2.3 Fe 0.44 Al 0.005 O2、Li 2.4 Fe 0.5 Al 0.005 O2、Li 2.35 Fe 0.48 Al 0.005 O2、Li 2.25 Fe 0.41 Al 0.005 O2、Li 2.2 Fe 0.37 Al 0.005 O2、Li 1.1 Nor 0.9 Al 0.005 O2、Li 2.3 Co 0.44 Al 0.005 O2、Li 2.6 Fe 0.6 O2、Li 2.1 Fe 0.35 Al 0.03 O2 is a source of oxygen.

10. The secondary battery according to claim 1, characterized in that, The lithium-containing positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, ternary materials, spinel materials, and lithium-rich manganese-based materials. The spinel material includes LiNi. 2-x Mn x O4, where 0 <x≤2。 11. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 10.

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