Electrode assembly, battery cell and secondary battery

Abstract

The present application relates to the technical field of secondary batteries, and in particular to an electrode assembly, a battery cell and a secondary battery. The electrode assembly comprises a first electrode sheet, a second electrode sheet and a separator. The first electrode sheet comprises a first electrode material and a second electrode material, the expansion rate of the first electrode material being greater than that of the second electrode material. The first electrode sheet has differently stressed positions, the greater the stress on a position, the greater the mass proportion of the first electrode material at the position.

Description

Electrode assemblies, battery cells and secondary batteries

[0001] Related applications

[0002] This application claims priority to Chinese patent application number 2024101621262, filed on February 4, 2024, entitled “Electrode Assembly, Battery Cell and Secondary Battery,” the entire text of which is hereby incorporated by reference. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to an electrode assembly, a battery cell and a secondary battery. Background Art

[0004] With the development of the economy and society, the requirements for battery life in consumer electronics such as mobile phones and electric vehicles are becoming increasingly stringent. The energy density of existing secondary batteries has gradually failed to meet the demand. Improving the specific capacity of positive and negative electrodes is an effective way to increase battery energy density. Using two different active materials in the electrode electrode is conducive to improving the specific capacity of the electrode electrode. Taking the negative electrode as an example, adding silicon-based materials to the traditional graphite negative electrode can increase the specific capacity of the negative electrode electrode. However, due to the large volume expansion of silicon-based materials during the lithium insertion process, the battery cycle life is affected. Therefore, how to improve the battery cycle life is an urgent problem that needs to be solved. Summary of the Invention

[0005] Based on this, according to various embodiments of the present application, a first aspect provides an electrode assembly, and its technical solution is as follows:

[0006] An electrode assembly includes a first electrode plate, a second electrode plate and a diaphragm, wherein the diaphragm is located between the first electrode plate and the second electrode plate; the first electrode plate includes a first electrode material and a second electrode material, the expansion rate of the first electrode material is greater than the expansion rate of the second electrode material, and the first electrode plate has positions with different stresses, and the mass proportion of the first electrode material at the position with greater stress is greater; wherein the mass proportion of the first electrode material at a certain position is the percentage of the mass of the first electrode material at that position to the total mass of the first electrode material and the second electrode material at that position.

[0007] According to various embodiments of the present application, a second aspect provides a battery cell, and its technical solution is as follows:

[0008] A battery cell comprises the electrode assembly as described above and an electrolyte.

[0009] According to various embodiments of the present application, a third aspect provides a secondary battery, the technical solution of which is as follows:

[0010] A secondary battery comprises a battery shell and the battery cell described above located in the battery shell.

[0011] Compared with traditional solutions, this application has the following beneficial effects:

[0012] This application adjusts the mass distribution of the first electrode material with a larger expansion rate in the first electrode plate according to the stress of the first electrode plate. When the stress of the first electrode plate at a certain position is greater than that of the first electrode plate at other positions, the mass proportion of the first electrode material at this position is higher than that of the first electrode material at other positions. The greater stress is used to provide a higher binding force to resist the stronger expansion of the first electrode plate, which is beneficial to suppress the repeated volume expansion and contraction of the first electrode plate during the charge and discharge process, reduce the damage and regeneration of the SEI film on the surface of the first electrode, and reduce the attenuation of the battery capacity. In addition, by suppressing the repeated volume expansion and contraction of the first electrode plate during the charge and discharge process, the problem of failure due to electrical contact caused by pulverization of the electrode material can also be reduced. Through the above means, the cycle life of the battery can be improved while increasing the specific capacity of the first electrode plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the conventional technology, the following briefly introduces the drawings required for use in the embodiments or the conventional technology descriptions. Obviously, the drawings described below are merely embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the disclosed drawings without any creative work.

[0014] FIG1 is a schematic structural diagram of an electrode assembly according to one embodiment;

[0015] FIG2 is a schematic structural diagram of an electrode assembly according to another embodiment;

[0016] FIG3 is a schematic structural diagram of an electrode assembly according to another embodiment;

[0017] FIG4 is a schematic structural diagram of a first electrode plate according to an embodiment;

[0018] FIG5 is a schematic diagram of a battery cell structure according to an embodiment;

[0019] FIG6 is a schematic diagram of the structure of a secondary battery according to one embodiment. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application.

[0022] the term

[0023] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0024] In this application, "multiple pieces", "multiple", "multiple times", "plurality", etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more than or equal to two.

[0025] In this application, the terms "optionally," "optional," and "optional" refer to options that are optional and may or may not be present, i.e., to the selection of either option from the two parallel options of "with" or "without." If multiple "options" appear in a technical solution, unless otherwise specified and there are no contradictions or constraints, each "option" is independent.

[0026] In this application, the terms "first," "second," "third," "fourth," etc., in the "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be understood as indicating or implying relative importance or quantity, nor as implicitly indicating the importance or quantity of the indicated technical features. Furthermore, "first," "second," "third," "fourth," etc., are only used for non-exhaustive enumeration and description purposes, and should be understood not to constitute a closed-ended limitation on quantity.

[0027] In this application, when a first feature is “above” or “below” a second feature, it can mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediary. Furthermore, when a first feature is “above,” “above,” or “above” a second feature, it can mean that the first feature is directly above or diagonally above the second feature, or simply means that the first feature is at a higher level than the second feature. When a first feature is “below,” “below,” or “below” a second feature, it can mean that the first feature is directly below or diagonally below the second feature, or simply means that the first feature is at a lower level than the second feature.

[0028] In the present application, when a first feature and a second feature are “stacked”, it can mean that the first feature and the second feature are in direct contact, or the first feature and the second feature are in indirect contact via an intermediate medium. When a first feature and a second feature are “co-wound”, it can mean that the first feature and the second feature are in direct contact, or the first feature and the second feature are in indirect contact via an intermediate medium.

[0029] In this application, the mass proportion of the first electrode material at a certain location is the percentage of the mass of the first electrode material at that location to the total mass of the first electrode material and the second electrode material at that location. The mass proportion of the third electrode material at a certain location is the percentage of the mass of the third electrode material at that location to the total mass of the third electrode material and the fourth electrode material at that location.

[0030] In this application, the stress on the first electrode plate refers to the stress on the first electrode plate at a certain position in the electrode assembly. The stress on the second electrode plate refers to the stress on the second electrode plate at a certain position in the electrode assembly.

[0031] To increase the specific capacity of traditional graphite anodes, silicon-based materials are being added to the graphite anode. Silicon has an extremely high theoretical specific capacity and can form a Li15Si4 alloy at room temperature, with a theoretical capacity of 3579 mAh / g. Silicon-carbon anodes offer advantages such as high specific capacity (nearly 10 times that of existing graphite anodes), low lithium insertion and deintercalation potential (0.2-0.4V vs. Li), non-toxicity, and abundant reserves (second only to oxygen in the abundance of the element in the Earth's crust). They are replacing traditional graphite anodes as a new generation of high-specific energy and high-energy density secondary battery anodes. However, silicon-based materials suffer from significant volume expansion during lithium insertion, which can reach 300% when fully intercalated. This repeated volume expansion and contraction during charge and discharge can easily lead to repeated destruction and regeneration of the SEI film on the electrode surface, causing battery capacity degradation. Furthermore, they can easily cause the anode material to pulverize and lose electrical contact, leading to failure. Both of these issues reduce battery cycle life, limiting industrial application. To suppress the volume expansion and contraction of silicon-based materials during lithium insertion and extraction, one approach is to coat the surface of the silicon-based material with a carbon layer. However, this rigid core-shell structure is brittle, and the surface carbon layer easily breaks during expansion and contraction, resulting in a reduced initial efficiency and shorter cycle life of the silicon-carbon anode. Clearly, this improvement is not sufficient for the practical application of silicon-carbon anodes.

[0032] In addition to the silicon-carbon negative electrode, the use of two different active materials in other electrode plates is a research direction to improve the energy density of the electrode plates. During the research process, due to the different expansion rates of the two different active materials, the volume expansion and contraction of the active material with a higher expansion rate also has the same problem of shorter cycle life as the above-mentioned silicon-carbon negative electrode.

[0033] In order to solve the above problems, the present application shifts the focus from the improvement of the electrode plates to the structural design of the electrode assembly. Please refer to Figure 1. In one embodiment, the electrode assembly 100 includes a first electrode plate 11 and a second electrode plate 12, wherein the number of first electrode plates 11 is multiple, and the multiple first electrode plates 11 are stacked along the Z1 direction, the number of second electrode plates 12 is multiple, and the multiple second electrode plates 12 are alternately stacked with the multiple first electrode plates 11, and the electrode assembly 100 also includes a diaphragm 13, the number of diaphragms 13 is multiple, and the multiple diaphragms 13 are respectively located between each first electrode plate 11 and the second electrode plate 12.

[0034] The plurality of first electrode plates 11 include a first electrode material and a second electrode material, wherein the expansion rate of the first electrode material is greater than the expansion rate of the second electrode material, and each first electrode plate 11 independently includes at least one of the first electrode material and the second electrode material.

[0035] The mass proportion of the first electrode material at different positions of the same first electrode plate is the same, denoted as M n , along the stacking direction Z1 shown in FIG1 , the M of different first electrode sheets n As the closer to the stacking center Q1 is, the larger it is. Optionally, the M of each first electrode plate n Each of the above-mentioned components is independently a ratio between 0% and 100%, for example, 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100%.

[0036] In this embodiment, the stress on the first electrode plate 11 closer to the stacking center Q1 is greater, and the stress on the first electrode plate 11 farther away from the stacking center Q1 is smaller. Among the multiple first electrode plates 11, the M of the first electrode plate 11 closer to the stacking center Q1 is greater. n The larger the M is, the farther away from the stacking center Q1 the first electrode plate 11 is. n The smaller the M is, the smaller the expansion rate is. The mass distribution of the first electrode material with a larger expansion rate in the plurality of first electrode plates 11 is adjusted according to the stress of the plurality of first electrode plates 11. When the stress on a certain first electrode plate 11 is greater than that on other first electrode plates 11, the M of the plate is nHigher stress provides a higher binding force to resist the stronger expansion of the first electrode plate 11, which helps to suppress the repeated volume expansion and contraction of the first electrode plate 11 during the charge and discharge process, reduce the damage and regeneration of the SEI film on the surface of the first electrode, and reduce the decay of battery capacity. In addition, by suppressing the repeated volume expansion and contraction of the first electrode plate 11 during the charge and discharge process, it can also reduce the problem of failure caused by electrical contact due to pulverization of the electrode material. Through these measures, the cycle life of the battery can be improved while increasing the specific capacity of the first electrode plate 11.

[0037] Optionally, the number of first electrode plates 11 closest to the stacking center Q1 is one, and the mass fraction of the first electrode material at each position of the plate is recorded as M0. The difference ΔM between the mass fractions of the first electrode materials of two adjacent first electrode plates 11 satisfies: ΔM = M0 × K, where K is the ratio of the difference in stress between the two adjacent first electrode plates to the stress of the first electrode plate closest to the stacking center Q1. For example, in the embodiment shown in FIG1 , the number of first electrode plates 11 is five, and the first electrode plate closest to the stacking center Q1 is 11A. Along the stacking direction Z1, starting from the first electrode plate 11A, the first electrode plate 11B and the first electrode plate 11C are sequentially followed. Along the opposite direction of the stacking direction Z1, starting from the first electrode plate 11A, the first electrode plate 11D and the first electrode plate 11E are sequentially followed. M0 is the mass fraction of the first electrode material at each location of the first electrode plate 11A, and N0 is the stress value of the first electrode plate 11A. M1 is the mass fraction of the first electrode material at each location of the first electrode plate 11B, and N1 is the stress value of the first electrode plate 11B. M2 is the mass fraction of the first electrode material at each location of the first electrode plate 11C, and N2 is the stress value of the first electrode plate 11C. M3 is the mass fraction of the first electrode material at each location of the first electrode plate 11D, and N3 is the stress value of the first electrode plate 11D. M4 is the mass fraction of the first electrode material at each location of the first electrode plate 11E, and N4 is the stress value of the first electrode plate 11E. ΔM represents the difference between M1 and M0, the difference between M2 and M1, the difference between M3 and M0, or the difference between M4 and M3. Among them, M1-M0=M0×(N1-N0) / N0, M2-M1=M0×(N2-N1) / N0, M3-M0=M0×(N3-N0) / N0, M4-M3=M0×(N4-N3) / N0.

[0038] It is understandable that the stress on the first electrode plate 11 can be measured by a pressure sensor. In this embodiment, N0 is measured to be 20N, N1 is 14N, N2 is 8N, N3 is 14N, and N4 is 8N. A linear relationship of N = -6x + 20 is established along the stacking direction Z1 and in the opposite direction of the stacking direction Z1, where x is related to the distribution of the first electrode plate 11. x is 0, representing the first electrode plate 11A, x is 1, representing the first electrode plate 11B and the first electrode plate 11D, and x is 2, representing the first electrode plate 11C and the first electrode plate 11E.

[0039] In other embodiments, the stress differences between two adjacent first electrode plates 11 are different or have different differences. According to the actual difference, the difference ΔM in the mass ratio of the first electrode material of the two adjacent first electrode plates is obtained.

[0040] The difference ΔM between the mass percentage of the first electrode material of each adjacent pair of first electrode sheets 11 may be related to or unrelated to the stress difference between the two adjacent first electrode sheets. In some embodiments, the difference ΔM between the mass percentage of the first electrode material of each adjacent pair of first electrode sheets 11 is the same. Optionally, each ΔM is a value within the range of 1% to 99%. For example, each ΔM is 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%. In some other embodiments, the ΔM of two adjacent first electrode sheets 11 is random. Optionally, each ΔM takes any value within the range of 1% to 99%, for example, each ΔM is independently 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%.

[0041] Optionally, the number of first electrode plates closest to the center of the stack is two, and the mass proportion of the first electrode material at each position of the two plates is recorded as M0'. Except for the two first electrode plates 11 closest to the center of the stack, the difference ΔM' in the mass proportion of the first electrode material of the other two adjacent first electrode plates satisfies: ΔM'=M0'×K', where K' is the ratio of the difference in stress between the two adjacent first electrode plates to the stress of one of the two first electrode plates closest to the center of the stack.

[0042] It can be understood that the number of first electrode plates closest to the stacking center is two, which means that the two first electrode plates 11 are at the same distance from the stacking center, are subjected to the same stress, and have the same mass proportion of the first electrode material at each position.

[0043] Please refer to Figure 2. In another embodiment, the electrode assembly 200 includes a first electrode plate 22 and a second electrode plate 21, wherein the number of the first electrode plates 22 is multiple, and the multiple first electrode plates 22 are stacked along the Z2 direction, the number of the second electrode plates 21 is multiple, and the multiple second electrode plates 21 are alternately stacked with the multiple first electrode plates 22. The electrode assembly 200 also includes a diaphragm 23, the number of the diaphragm 23 is multiple, and the multiple diaphragms 23 are respectively located between each first electrode plate 22 and the second electrode plate 21.

[0044] The plurality of first electrode plates 22 include a first electrode material and a second electrode material, wherein the expansion rate of the first electrode material is greater than the expansion rate of the second electrode material, and each first electrode plate 22 independently includes at least one of the first electrode material and the second electrode material.

[0045] The mass proportion of the first electrode material at different positions of the same first electrode plate is the same, denoted as M n , along the stacking direction Z2 shown in FIG2 , the M of different first electrode sheets n As Q2 gets closer to the stacking center, it becomes larger. Optionally, the M of each first electrode plate is n Each of the above-mentioned components is independently a ratio between 0% and 100%, for example, 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100%.

[0046] In this embodiment, there are four first electrode plates 22. The two first electrode plates 11 closest to the stacking center Q2 are first electrode plates 22A and first electrode plates 22B. Along the stacking direction Z2, the first electrode plate 22C is the next plate following the first electrode plate 22B. Along the opposite direction of the stacking direction Z2, the first electrode plate 22D is the next plate following the first electrode plate 22B. The mass fraction of the first electrode material at each location of the first electrode plate 22A is the same as the mass fraction of the first electrode material at each location of the first electrode plate 22B, both being M0'. The stress values ​​experienced by the first electrode plate 22A and the first electrode plate 22B are the same, both being N0'. The mass fraction of the first electrode material at each location of the first electrode plate 22C is M1', and the stress value experienced is N1'. The mass fraction of the first electrode material at each location of the first electrode plate 22D is M2', and the stress value experienced is N2'. ΔM1′ represents the difference between M1′ and M0′ or the difference between M2′ and M0′, wherein M1′-M0′=M0′×(N1′-N0′) / N0′, and M2′-M0′=M0′×(N2′-N0′) / N0′.

[0047] In other embodiments, except for the two first electrode plates 22 closest to the stacking center Q2, the stress differences between the other two adjacent first electrode plates 22 are different or there are different differences. According to the actual difference, the difference ΔM' in the mass ratio of the first electrode material of the two adjacent first electrode plates is obtained.

[0048] Except for the two first electrode plates 22 closest to the stacking center Q2, the difference ΔM' in the mass ratio of the first electrode material of the other two adjacent first electrode plates 22 may be related to the stress difference of the two adjacent first electrode plates, or may be unrelated. In some embodiments, the difference ΔM' in the mass ratio of the first electrode material of the other two adjacent first electrode plates 22 is the same. Optionally, each ΔM' is a value in the range of 1% to 99%. For example, each ΔM' is 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%. In some other embodiments, the difference ΔM' in the mass ratio of the first electrode material of the other two adjacent first electrode plates 22 is random. Optionally, each ΔM' can take any value within the range of 1% to 99%, for example, each ΔM' can independently be 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%.

[0049] In the embodiments shown in Figures 1 and 2, multiple first electrode sheets, multiple second electrode sheets, and multiple separators are stacked. In another embodiment, referring to Figure 3, multiple first electrode sheets, multiple second electrode sheets, and multiple separators are stacked and wound together to form an electrode assembly 300, with the electrode assembly 300 having a winding center Q3. In this case, the mass fraction of the first active material at different locations on the same first electrode sheet varies, with the mass fraction of the first active material increasing at locations closer to the winding center Q3. In general, the first electrode sheet has locations that are subject to different stresses, with the mass fraction of the first active material increasing at locations subject to greater stress. That is, when the stress on a first electrode sheet at a certain location is greater than that on other locations, the mass fraction of the first active material at that location is greater than that on other locations. The greater stress provides a higher binding force to resist the greater expansion of the first electrode sheet, which helps to suppress the repeated volume expansion and contraction of the first electrode sheet during charge and discharge, thereby improving the cycle life of the battery.

[0050] In the above embodiment, the number of the first electrode plates is multiple, and the number of the second electrode plates and the diaphragm is multiple. In other embodiments, the number of the first electrode plates is multiple, the number of the second electrode plates is one, and the number of the diaphragm is multiple. In other embodiments, the number of the first electrode plates is one, the number of the second electrode plates is one, and the number of the diaphragm is one. Regardless of whether the number of the first electrode plates is one or multiple, the first electrode material of the first electrode plate has a greater mass proportion at the location where the stress is greater. When the number of the first electrode plates is one, the first electrode plate includes the first electrode material and the second electrode material.

[0051] It can be understood that the content of the first electrode material and the second electrode material in the first electrode plate is designed according to the capacity requirement of the first electrode plate.

[0052] Optionally, the first electrode plate is a negative electrode plate, the first electrode material includes one or more of SiO, Si and SiC, and the second electrode material includes graphite.

[0053] In one example, the designed negative electrode capacity is required to be 500mAh / g. Based on the capacity of graphite being 350mAh / g and the capacity of Si being 1500mAh / g, the total capacity of 500mAh / g is achieved by controlling the content of Si and graphite.

[0054] Optionally, the first electrode plate is a positive electrode plate, the first electrode material includes one or more of Li, Na, K, Mg and Al, and the second electrode material includes one or more of lithium cobalt oxide, lithium nickel cobalt aluminum oxide and lithium iron phosphate.

[0055] Alternatively, referring to FIG4 , in one embodiment, the first electrode plate 11 includes a current collector 111 and an electrode active layer 112 located on the current collector 111, and the first electrode material and the second electrode material are located in the electrode active layer 112. In the embodiment shown in FIG4 , the electrode active layer 112 is located on both sides of the current collector 111. In other embodiments, the electrode active layer 112 may also be located on one side of the current collector 111.

[0056] In some examples, the first electrode sheet is a negative electrode sheet, which includes a current collector and a negative electrode active layer located above the current collector, with the first electrode material and the second electrode material located within the negative electrode active layer. In this case, the current collector is selected from copper foil. When multiple negative electrode sheets are provided, multiple current collectors are provided, each of which independently has a negative electrode active layer disposed on one or both sides. Optionally, the negative electrode active layer also includes a binder and a conductive agent.

[0057] In some examples, the first electrode sheet is a positive electrode sheet, which includes a current collector and a positive electrode active layer located above the current collector, with the first electrode material and the second electrode material located within the positive electrode active layer. In this case, the current collector is selected from aluminum foil. When multiple positive electrode sheets are provided, the current collectors are multiple, each of which independently has a positive electrode active layer disposed on one or both sides. Optionally, the positive electrode active layer also includes a binder and a conductive agent.

[0058] In the above embodiment, the first electrode plate includes a first electrode material and a second electrode material with different expansion rates, while the second electrode plate does not include an electrode material with different expansion rates. In other embodiments, the second electrode plate includes a third electrode material and a fourth electrode material with different expansion rates to increase the energy density of the second electrode plate.

[0059] In one embodiment, there are multiple second electrode plates, and the multiple second electrode plates are alternately stacked with the multiple first electrode plates. The multiple second electrode plates include a third electrode material and a fourth electrode material, wherein the expansion rate of the third electrode material is greater than the expansion rate of the fourth electrode material, and each second electrode plate independently includes at least one of the third electrode material and the fourth electrode material.

[0060] The mass proportion of the third electrode material at different positions of the same second electrode plate is the same, denoted as m n , along the stacking direction, m of different second electrode sheets n As it gets closer to the stacking center, it becomes larger. Optionally, the m of each second electrode plate is n Each of the above-mentioned components is independently a ratio between 0% and 100%, for example, 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100%.

[0061] In this embodiment, the stress on the second electrode plate closer to the stacking center is greater, and the stress on the second electrode plate farther away from the stacking center is smaller. Among the plurality of second electrode plates, the m of the second electrode plate closer to the stacking center is greater. n The larger the m, the further away from the center of the stack the second electrode plate is. n The smaller the size, the smaller the third electrode material. The mass distribution of the third electrode material with a larger expansion rate in the second electrode sheets is adjusted according to the stress of the second electrode sheets. When the stress on a certain second electrode sheet is greater than that on other second electrode sheets, the m of the sheet is nHigher stress provides a higher binding force to resist the stronger expansion of the second electrode plate, which helps to suppress the repeated volume expansion and contraction of the second electrode plate during the charge and discharge process, reduce the damage and regeneration of the SEI film on the second electrode surface, and reduce the battery capacity decay. In addition, by suppressing the repeated volume expansion and contraction of the second electrode plate during the charge and discharge process, it can also reduce the problem of failure caused by the pulverization of the electrode material due to electrical contact. Through these measures, the cycle life of the battery can be improved while increasing the specific capacity of the second electrode plate.

[0062] Optionally, the number of second electrode plates closest to the center of the stack is one, and the mass proportion of the third electrode material at each position of the plate is recorded as m0. The difference Δm between the mass proportions of the third electrode materials of two adjacent second electrode plates satisfies: Δm = m0 × k, where k is the ratio of the difference in stress between the two adjacent second electrode plates to the stress of the second electrode plate closest to the center of the stack.

[0063] It can be understood that the stress on the second electrode plate can be obtained by testing with a pressure sensor.

[0064] In other embodiments, the stress differences between two adjacent second electrode plates are different or have different differences. According to the actual difference, the difference Δm in the mass ratio of the third electrode material of the two adjacent second electrode plates is obtained.

[0065] The difference Δm between the mass percentages of the third electrode material between two adjacent second electrode sheets may or may not be related to the stress difference between the two adjacent second electrode sheets. In some embodiments, the difference Δm between the mass percentages of the third electrode material between two adjacent second electrode sheets is the same. Optionally, each Δm has a value within the range of 1% to 99%. For example, each Δm has a value within the range of 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%. In some other embodiments, the difference Δm between the mass percentages of the third electrode material between two adjacent second electrode sheets is random. Optionally, each Δm has any value within the range of 1% to 99%, for example, each Δm independently has a value of 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%.

[0066] Optionally, the number of second electrode plates closest to the center of the stack is two, and the mass proportion of the third electrode material at each position of the two plates is recorded as m0'. Except for the two second electrode plates closest to the center of the stack, the difference Δm' in the mass proportion of the third electrode material of the other two adjacent second electrode plates satisfies: Δm'=m0'×k', where k' is the ratio of the difference in stress between the two adjacent second electrode plates to the stress of one of the two second electrode plates closest to the center of the stack.

[0067] It can be understood that the number of second electrode plates closest to the stacking center is two, which means that the two second electrode plates are at the same distance from the stacking center, are subjected to the same stress, and have the same mass proportion of the third electrode material at each position.

[0068] In other embodiments, except for the two second electrode plates closest to the center of the stack, the stress differences between the other two adjacent second electrode plates are different or there are different differences. Based on the actual difference, the difference Δm' in the mass proportion of the third electrode material of the corresponding two adjacent second electrode plates is obtained.

[0069] Except for the two second electrode plates closest to the center of the stack, the difference Δm' in the mass ratio of the third electrode material of the other two adjacent second electrode plates may be related to the stress difference of the two adjacent second electrode plates, or may be unrelated. In some embodiments, the difference Δm' in the mass ratio of the third electrode material of the other two adjacent second electrode plates is the same. Optionally, each Δm' is a value in the range of 1% to 99%. For example, each Δm' is 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%. In some other embodiments, the difference Δm' in the mass ratio of the third electrode material of the other two adjacent second electrode plates is random. Optionally, each Δm' can take any value within the range of 1% to 99%, for example, each Δm' can independently be 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 99%.

[0070] In the above embodiment, multiple first electrode plates, multiple second electrode plates and multiple diaphragms are stacked. In another embodiment, multiple first electrode plates, multiple second electrode plates and multiple diaphragms are stacked and wound together. At this time, the mass proportion of the third active material at different positions of the same second electrode plate is different, and the mass proportion of the third electrode material at the position closer to the winding center is greater. In general, the second electrode plate has different stress-bearing positions, and the mass proportion of the third electrode material at the position with greater stress is greater, that is, when the stress on the second electrode plate at a certain position is greater than that on the second electrode plate at other positions, the mass proportion of the third electrode material at this position is higher than that of the third electrode material at other positions. The greater stress provides a higher binding force to resist the stronger expansion of the second electrode plate, which is beneficial to suppress the repeated volume expansion and contraction of the second electrode plate during the charge and discharge process, and improve the cycle life of the battery.

[0071] In the above embodiment, the number of second electrode plates is multiple. In other embodiments, the number of second electrode plates is single. Regardless of whether the number of second electrode plates is single or multiple, the mass proportion of the third electrode material is greater at locations of the second electrode plates subject to greater stress. When the number of second electrode plates is single, the second electrode plate includes the third electrode material and the fourth electrode material.

[0072] It can be understood that the contents of the third electrode material and the fourth active material in the second electrode plate are designed according to the capacity requirements of the second electrode plate.

[0073] Optionally, the first electrode material includes one or more of SiO, Si and SiC, the second electrode material includes graphite, the third electrode material includes one or more of Li, Na, K, Mg and Al, and the fourth electrode material includes one or more of lithium cobalt oxide, lithium nickel cobalt aluminum oxide and lithium iron phosphate.

[0074] Optionally, the second electrode plate includes a current collector and an electrode active layer located on the current collector, and the third electrode material and the fourth electrode material are located in the electrode active layer.

[0075] Optionally, the electrode active layer is located on one side or both sides of the current collector.

[0076] In order to improve the specific capacity and energy density of the electrode plates while improving the cycle performance of the battery, the present application shifts the focus from the improvement of the electrode plates to the structural design of the electrode assembly. Different from the traditional setting of the material and structure of the electrode plates themselves, the present application starts from the structure of the electrode assembly and combines the stress conditions of the electrode plates in the electrode assembly to adjust the mass distribution of the electrode materials with larger expansion rates in the electrode plates. At positions with greater stress, the mass of the electrode materials with larger expansion rates accounts for a larger proportion. The constraint provided by the stress suppresses the expansion of the electrode plates during repeated charging and discharging, making the electrode plates more stable, reducing lithium plating, reducing the pulverization of the electrode materials, increasing the battery capacity, and increasing the battery cycle life.

[0077] A second aspect of the present application provides a battery cell. Please refer to FIG5 . In one embodiment, the battery cell 10 includes the electrode assembly 100 and the electrolyte 101 as described above.

[0078] A third aspect of the present application provides a secondary battery. Please refer to FIG6 . In one embodiment, the secondary battery 1 includes a battery shell 20 and the battery cell 10 as described above located in the battery shell 20 .

[0079] The following is further described in conjunction with specific examples and comparative examples. Unless otherwise specified, the raw materials involved in the following specific examples and comparative examples can be sourced from commercial sources. The instruments used can be sourced from commercial sources unless otherwise specified. The processes involved can be selected conventionally by those skilled in the art unless otherwise specified.

[0080] Example

[0081] The negative electrode active material, SBR, and ultrafine graphite were mixed in a mass ratio of 9:5:5, and an appropriate amount of solvent NMP was added to prepare 5 parts of negative electrode slurries, wherein the negative electrode active materials of the 5 parts of negative electrode slurries were all composed of Si and graphite. The mass of Si in each part of the negative electrode slurry accounted for the percentage of the total mass of Si and graphite in the slurry. See Table 1. The 5 parts of the negative electrode slurry were respectively coated on both sides of 5 parts of copper foil with a thickness of 5 μm, and a negative electrode active layer with a thickness of 45 μm was formed on both sides of the 5 parts of copper foil, respectively, to obtain 5 negative electrode sheets.

[0082] Lithium cobalt oxide, acetylene black and PVDF were mixed in a mass ratio of 8:1.5:0.5, and an appropriate amount of solvent NMP was added to prepare 6 portions of positive electrode slurry. 4 portions of the positive electrode slurry were respectively coated on both sides of 4 portions of aluminum foil with a thickness of 8 μm, and a positive electrode active layer with a thickness of 60 μm was formed on both sides of the 4 portions of aluminum foil. The remaining 2 portions of the positive electrode slurry were respectively coated on one side of 2 portions of aluminum foil with a thickness of 8 μm, and a positive electrode active layer with a thickness of 60 μm was formed on one side of the 2 portions of aluminum foil, to obtain a total of 6 positive electrode sheets.

[0083] Using a 5 μm thick PE separator, five negative electrode sheets 11 and six positive electrode sheets 12 are stacked according to the structure shown in FIG1 , wherein the positive electrode sheet having a positive active layer on one side is located on the outermost side, and the electrode assembly is prepared by pressing.

[0084] The stress values ​​of 5 negative electrode sheets in the electrode assembly were tested. The results are shown in Table 1.

[0085] Testing: A soft-pack battery was assembled with an electrode assembly, electrolyte, and aluminum-plastic film. Charge and discharge were performed at the rated rate at 25°C: 3C constant current and constant voltage to the cutoff voltage, followed by discharge at a 1C rate to 3.0V. The ratio of capacity to initial capacity was recorded after 200, 500, 1000, and 1600 cycles. Results are shown in Table 1.

[0086] Table 1

[0087] Comparative Example

[0088] The preparation method of the electrode assembly of this comparative example is basically the same as that of the electrode assembly of the embodiment, with the main difference being that the mass of Si in each portion of the negative electrode slurry accounts for the percentage of the total mass of Si and graphite in the slurry, as shown in Table 2. The cycle life of the comparative example was tested with reference to the method of the embodiment, and the results are shown in Table 2.

[0089] Table 2

[0090] It can be seen that the electrode assembly of the embodiment can suppress the volume expansion of the negative electrode plate and has a better cycle life.

[0091] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. An electrode assembly, characterized in that: It includes a first electrode plate, a second electrode plate and a diaphragm, wherein the diaphragm is located between the first electrode plate and the second electrode plate; the first electrode plate includes a first electrode material and a second electrode material, the expansion rate of the first electrode material is greater than the expansion rate of the second electrode material, and the first electrode plate has positions with different stresses, and the mass proportion of the first electrode material at the position with greater stress is greater; wherein the mass proportion of the first electrode material at a certain position is the percentage of the mass of the first electrode material at that position to the total mass of the first electrode material and the second electrode material at that position.

2. The electrode assembly according to claim 1, wherein There are multiple first electrode plates, and the multiple first electrode plates include the first electrode material and the second electrode material, and each first electrode plate independently includes at least one of the first electrode material and the second electrode material.

3. The electrode assembly according to claim 2, characterized in that Multiple pieces of the first electrode sheet, the second electrode sheet and the diaphragm are stacked, and the mass proportion of the first electrode material at different positions of the same first electrode sheet is the same, denoted as M n , along the stacking direction, the M of different first electrode sheets n The closer to the stacking center, the larger the value.

4. The electrode assembly according to claim 3, characterized in that The number of first electrode plates closest to the center of the stack is one, and the mass proportion of the first electrode material at each position of the first electrode plate is recorded as M0. The difference in the mass proportion of the first electrode material of two adjacent first electrode plates is recorded as ΔM, and ΔM is related or unrelated to the difference in stress exerted on the two adjacent first electrode plates.

5. The electrode assembly according to claim 4, characterized in that ΔM is not related to the difference in stress between the two adjacent first electrode plates, and ΔM satisfies the following condition: each ΔM can take any value within the range of 1% to 99%.

6. The electrode assembly according to claim 4, characterized in that ΔM is related to the difference in stress between the two adjacent first electrode plates, and ΔM satisfies the following conditions: the difference ΔM in the mass ratio of the first electrode material of each two adjacent first electrode plates is the same, and each ΔM is a value within the range of 1% to 99%.

7. The electrode assembly according to claim 4, characterized in that ΔM is related to the difference in stress between the two adjacent first electrode plates, and ΔM satisfies the following condition: ΔM=M0×K, where K is the ratio of the difference in stress between the two adjacent first electrode plates to the stress of the first electrode plate closest to the center of the stack.

8. The electrode assembly according to claim 3, wherein: The number of first electrode plates closest to the center of the stack is two, and the mass ratio of the first electrode material at each position of the two first electrode plates is recorded as M0'. Except for the two first electrode plates closest to the center of the stack, the difference in the mass ratio of the first electrode material of the other two adjacent first electrode plates is recorded as ΔM', and ΔM' is related or unrelated to the difference in stress exerted on the two adjacent first electrode plates.

9. The electrode assembly according to claim 8, characterized in that ΔM' is not related to the difference in stress between the two adjacent first electrode plates, and ΔM' satisfies the following condition: each ΔM' can take any value within the range of 1% to 99%.

10. The electrode assembly according to claim 8, characterized in that ΔM' is related to the difference in stress between the two adjacent first electrode plates, and ΔM' satisfies the following conditions: the difference ΔM' in the mass ratio of the first electrode material of the other two adjacent first electrode plates is the same, and each ΔM' is a value within the range of 1% to 99%.

11. The electrode assembly according to claim 8, characterized in that ΔM' is related to the difference in stress between the two adjacent first electrode plates, and ΔM' satisfies the following condition: ΔM'=M0'×K', where K' is the ratio of the difference in stress between the other two adjacent first electrode plates to the stress of one of the two first electrode plates closest to the center of the stack.

12. The electrode assembly according to claim 2, characterized in that Multiple pieces of the first electrode sheet, the second electrode sheet and the separator are wound together, and the mass proportion of the first active material at different positions of the same first electrode sheet is different, and the mass proportion of the first electrode material at the position closer to the winding center is greater.

13. The electrode assembly according to any one of claims 1 to 12, characterized in that: The first electrode plate includes a current collector and an electrode active layer located on the current collector, and the first electrode material and the second electrode material are located in the electrode active layer.

14. The electrode assembly according to any one of claims 1 to 13, characterized in that: The second electrode plate includes a third electrode material and a fourth electrode material, the expansion rate of the third electrode material is greater than the expansion rate of the fourth electrode material, and the second electrode plate has positions with different stresses, and the mass proportion of the third electrode material at the position with greater stress is greater; wherein the mass proportion of the third electrode material at a certain position is the percentage of the mass of the third electrode material at that position to the total mass of the third electrode material and the fourth electrode material at that position.

15. The electrode assembly according to any one of claims 1 to 14, characterized in that: The first electrode material includes one or more of SiO, Si and SiC, and the second electrode material includes graphite.

16. The electrode assembly according to any one of claims 1 to 14, characterized in that The first electrode material includes one or more of Li, Na, K, Mg and Al, and the second electrode material includes one or more of lithium cobalt oxide, lithium nickel cobalt aluminum oxide and lithium iron phosphate.

17. The electrode assembly according to claim 14, wherein: The third electrode material includes one or more of SiO, Si and SiC, and the fourth electrode material includes graphite.

18. The electrode assembly according to claim 14, wherein: The third electrode material includes one or more of Li, Na, K, Mg and Al, and the fourth electrode material includes one or more of lithium cobalt oxide, lithium nickel cobalt aluminum oxide and lithium iron phosphate.

19. A battery cell comprising the electrode assembly according to any one of claims 1 to 18 and an electrolyte.

20. A secondary battery, characterized in that: The invention comprises a battery shell and the battery cell according to claim 19 located in the battery shell.

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