Electrode assembly and preparation method thereof, battery and power utilization device

Abstract

The invention belongs to the technical field of batteries, and particularly relates to an electrode assembly, a preparation method thereof, a battery and a power utilization device. The electrode assembly comprises a pole piece, the pole piece comprises a current collector, an active layer and a tab, the active layer is arranged on at least one side of the current collector in the thickness direction, and the tab is arranged on one side of the current collector in the height direction; the liquid absorption capacity of the active layer is gradually reduced from one side of the current collector provided with the tab to the other opposite side of the current collector and from the current collector to the direction deviating from the current collector. According to the embodiment of the invention, the electrolyte wettability of the electrode assembly can be effectively improved by carrying out gradient / gradual change design on the liquid absorption capability of the electrode assembly in the specific direction.

Description

Technical Field

[0001] This application belongs to the technical field of batteries, and particularly relates to an electrode assembly, a preparation method thereof, a battery, and an electrical device. Background Art

[0002] As a carrier of electrical energy, batteries are widely used in various fields. According to different uses, various requirements are put forward for batteries. For example, for batteries applied in the power field to provide electrical energy for devices such as automobiles and electric two-wheelers, they need to have high energy density and excellent cycle performance.

[0003] The battery assembly is the core component of the battery. During the long-term storage and use of the battery, the electrolyte wettability of the electrode assembly usually gradually deteriorates, affecting the performance of the battery. Summary of the Invention

[0004] In view of the above problems, this application provides an electrode assembly, a preparation method thereof, a battery, and an electrical device, which can solve the problem of poor wettability of the electrode assembly, thereby reducing the battery resistance and improving the cycle performance of the battery.

[0005] In a first aspect, this application provides an electrode assembly, which includes a pole piece. The pole piece includes a current collector, an active layer, and a tab. The active layer is disposed on at least one side in the thickness direction of the current collector, and the tab is disposed on one side in the height direction of the current collector;

[0006] In the direction from the side of the current collector where the tab is disposed to the opposite side, the liquid absorption capacity of the area of the active layer close to the tab on one side is greater than or equal to the liquid absorption capacity of the area on the opposite side, and the liquid absorption capacity of at least one area of the active layer close to the tab on one side is greater than the liquid absorption capacity of the area on the opposite side;

[0007] In the direction from the current collector to the direction away from the current collector, the liquid absorption capacity of the area of the active layer close to the current collector on one side is greater than or equal to the liquid absorption capacity of the area on the opposite side, and the liquid absorption capacity of at least one area of the active layer close to the current collector on one side is greater than the liquid absorption capacity of the area on the opposite side.

[0008] In the present application, through a specific distribution design of the liquid absorption capacity of the active layer in the direction from the side of the current collector where the tab is provided to the opposite side, and in the direction from the current collector to the side away from the current collector, that is, the liquid absorption capacity of the area of the active layer close to the tab (or close to the current collector) is greater than or equal to that of the area on the opposite side, and the liquid absorption capacity of at least one area of the active layer close to the tab (or close to the current collector) is greater than that of the area on the opposite side, an electrode assembly with a three-dimensional matrix distribution structure can be formed. For the electrode assembly with this structure, the active layer has a higher liquid absorption capacity at the part close to the tab than at the opposite side. Since the tab is usually close to the top of the electrode assembly, after the electrode assembly is infiltrated with the electrolyte, the amount of electrolyte absorbed by the top of the active layer will be greater than that at the bottom of the active layer. During the long-term storage and use of the electrode assembly, even if there is a tendency for the electrolyte at the top to gather downward due to the action of gravity, a relatively large amount of electrolyte can still be retained, making the electrolyte wettability of the active layer more uniform in this direction, which can compensate for the problem that the electrolyte accumulates at the bottom of the active layer due to gravity, resulting in poor wettability.

[0009] Meanwhile, in the direction from the current collector to the side away from the current collector, the area of the active layer close to the current collector has a higher liquid absorption capacity. After infiltration with the electrolyte, the area of the active layer closer to the current collector will have a higher amount of electrolyte, which can compensate for the problem that the electrolyte wettability of the electrode plate decreases or it is difficult to be infiltrated due to the electrolyte being extruded out during the charge and discharge process of the electrode plate due to volume expansion or contraction.

[0010] Therefore, through the specific distribution design of the liquid absorption capacity of the active layer in the direction from the side of the current collector where the tab is provided to the opposite side, and in the direction from the current collector to the side away from the current collector, the embodiments of the present application can effectively improve the electrolyte wettability of the active layer, enabling the active layer to be uniformly infiltrated in different directions. Since the electrolyte wettability of the electrode assembly is mainly reflected by the active layer, the improvement of the electrolyte wettability of the active layer is beneficial to the improvement of the overall electrolyte wettability of the electrode assembly. The improvement of the electrolyte wettability will be beneficial to reducing the battery resistance and improving the cycle performance of the battery.

[0011] In some embodiments, in the direction from the side of the current collector where the tab is provided to the opposite side, the liquid absorption capacity of the area of the separator included in the electrode assembly close to the tab is greater than or equal to that of the area on the opposite side, and the liquid absorption capacity of at least one area of the separator close to the tab is greater than that of the area on the opposite side;

[0012] In the direction from the current collector to the side away from the current collector, the liquid absorption capacity of the active layer is greater than that of the separator included in the electrode assembly.

[0013] By varying the liquid absorption capacity of the separator in the direction from the side of the current collector where the tab is provided to the opposite side, the liquid absorption capacity of the separator can be made higher on the side closer to the tab, further improving the electrolyte wettability of the electrode assembly in this direction.

[0014] Meanwhile, in the direction from the current collector to the direction away from the current collector, the liquid absorption capacity of the active layer is greater than that of the separator contained in the electrode assembly, which can reduce the situation where the separator absorbs a large amount of electrolyte and causes insufficient electrolyte in the active layer, and improve the electrolyte wettability of the electrode sheet in this direction.

[0015] In some embodiments, in the direction from the side of the current collector where the tab is provided to the opposite side, at least one of the active layer and the separator contained in the electrode assembly includes at least two adjacent gradient units arranged in series, and the liquid absorption capacity of the gradient unit closer to the tab side is higher than that of the gradient unit on the opposite side. Optionally, the number of the gradient units is 2 to 10.

[0016] By arranging a plurality of gradient units in the direction from the side of the current collector where the tab is provided to the opposite side, the liquid absorption capacity of the active layer and / or the separator in this direction can be decreased layer by layer. The gradient unit at a lower position can provide a buffering effect for the downward movement of the electrolyte in the adjacent gradient unit at a higher position, thereby facilitating the solution of the problem that the electrolyte is concentrated at the bottom of the electrode assembly due to gravity during long-term storage and use of the electrode assembly, and improving the electrolyte wettability of the electrode assembly in this direction. At the same time, the experiments of the embodiments of the present application show that within a certain range, the more the number of gradient units arranged in this direction, the better the improvement effect on the electrolyte wettability of the electrode assembly.

[0017] In some embodiments, the difference in liquid absorption capacity between two adjacent gradient units is 1.1 to 5 times, optionally 1.2 to 2 times.

[0018] The difference in liquid absorption capacity between two adjacent gradient units by a certain multiple can enable the gradient unit at a lower position to well provide a buffering effect for the downward movement of the electrolyte in the adjacent gradient unit at a higher position, and improve the electrolyte wettability in the direction from the side closer to the tab of the active layer to the opposite side.

[0019] In some embodiments, in the direction from the side of the current collector where the tab is provided to the opposite side, the height of any one gradient unit accounts for 10% to 60% of the total height of the active layer or the separator, optionally 10% to 50%.

[0020] Setting the height of the gradient unit in the direction from the side of the current collector where the tab is provided to the opposite side within a suitable range is beneficial to increasing the number of gradient units, and further improving the electrolyte wettability of the electrode assembly in this direction.

[0021] In some embodiments, in the direction from the current collector to the side away from the current collector, the active layer includes at least two adjacent liquid absorption layers arranged in sequence, wherein the liquid absorption capacity of the liquid absorption layer closer to the current collector is higher than that of the liquid absorption layer on the opposite side. Optionally, the number of liquid absorption layers is 2 to 10.

[0022] By arranging multiple liquid absorption layers in the direction from the current collector to the side away from the current collector, the liquid absorption capacity of the active layer can gradually decrease in this direction. When the volume of the electrode tab changes and the electrolyte is squeezed out, the liquid absorption layer on the side away from the current collector can provide a buffering effect for the outward overflow tendency of the electrolyte in the adjacent liquid absorption layer closer to the current collector, thereby facilitating the solution of the problem that the electrolyte wettability of the electrode tab is reduced or difficult to wet due to volume expansion or contraction during charge and discharge, and improving the electrolyte wettability of the electrode assembly in this direction. At the same time, experiments show that within a certain range, the more the number of liquid absorption layers arranged in the direction from the current collector to the side away from the current collector, the better the improvement effect on the electrolyte wettability of the electrode assembly.

[0023] In some embodiments, in the direction from the current collector to the side away from the current collector, the thickness of any liquid absorption layer accounts for 10% to 60% of the total thickness of the active layer on the side of the current collector, optionally 10% to 50%.

[0024] Setting the thickness of the liquid absorption layer in the direction from the current collector to the side away from the current collector within a suitable range is beneficial to increasing the number of liquid absorption layers, and further improving the electrolyte wettability of the electrode assembly in this direction.

[0025] In some embodiments, the active layer contains a first electrolyte absorbent;

[0026] In the direction from the side of the current collector where the tab is provided to the opposite side, the mass content of the first electrolyte absorbent in the region of the active layer closer to the tab side is greater than or equal to the mass content of the first electrolyte absorbent in the region of the opposite side, and the mass content of the first electrolyte absorbent in at least one region of the active layer closer to the tab side is greater than the mass content of the first electrolyte absorbent in the region of the opposite side;

[0027] In the direction from the current collector to the side away from the current collector, the mass content of the first electrolyte absorbent in the region of the active layer closer to the current collector side is greater than or equal to the mass content of the first electrolyte absorbent in the region of the opposite side, and the mass content of the first electrolyte absorbent in at least one region of the active layer closer to the current collector side is greater than the mass content of the first electrolyte absorbent in the region of the opposite side.

[0028] By adding an electrolyte absorbent to the active layer and adjusting the mass content of the electrolyte absorbent to vary in a certain direction, the electrode assembly can have different absorption and retention capabilities for the electrolyte in different regions, and exhibit a certain distribution pattern of liquid absorption capacity along a specific direction, which is beneficial to improving the electrolyte wettability of the electrode assembly.

[0029] In some embodiments, the separator included in the electrode assembly comprises a base film and a separator coating provided on at least one side of the base film; the separator coating comprises a second electrolyte absorbent;

[0030] In the direction from the side where the tab of the current collector is provided to the opposite side, the mass content of the second electrolyte absorbent in the region of the separator coating closer to the tab is greater than or equal to the mass content of the second electrolyte absorbent in the opposite region, and the mass content of the second electrolyte absorbent in at least one region of the separator coating closer to the tab is greater than the mass content of the second electrolyte absorbent in the opposite region.

[0031] In some embodiments, in the direction from the current collector to the direction away from the current collector, the mass content of the first electrolyte absorbent in the active layer is greater than the mass content of the second electrolyte absorbent in the separator coating.

[0032] By adding an electrolyte absorbent to the separator coating and adjusting the mass content of the electrolyte absorbent to vary in a certain direction, the electrode assembly can have different absorption and retention capabilities for the electrolyte in different regions, and exhibit a certain distribution pattern of liquid absorption capacity along a specific direction, further improving the electrolyte wettability of the electrode assembly.

[0033] In some embodiments, the first electrolyte absorbent and the second electrolyte absorbent each independently include one or more of polymethyl methacrylate, polyacrylate methyl ester, polyacrylate ethyl ester, polyvinyl acetate, polyacrylamide, polyacrylic acid, and polyacrylonitrile.

[0034] Such polymer materials have high affinity for the electrolyte and can swell and absorb the electrolyte to form a gel after contacting the electrolyte. By using these polymer materials as the electrolyte absorbent and adjusting the distribution of these polymer materials in the active layer and the separator coating, the electrode assembly can effectively exhibit a gradient / gradual change in liquid absorption capacity along a specific direction, thereby improving the electrolyte wettability of the electrode assembly.

[0035] In some embodiments, the mass content of the first electrolyte absorbent in the active layer and the mass content of the second electrolyte absorbent in the separator coating are each independently greater than 0 and less than or equal to 8%; optionally greater than or equal to 1% and less than or equal to 6%.

[0036] Setting the mass content of the electrolyte absorbent in the active layer and the separator coating within a suitable range is not only beneficial to adjusting the liquid absorption capacity of different regions of the electrode assembly, but also will not cause a significant reduction in the active material in the active layer, thereby maintaining the stability of the electrode sheet capacity.

[0037] In a second aspect, the present application provides a method for preparing an electrode assembly, including:

[0038] Preparing an active layer on at least one side in the thickness direction of the current collector, and providing a tab on one side in the height direction of the current collector;

[0039] In the direction from the side of the current collector where the tab is provided to the opposite side, controlling the liquid absorption capacity of the region of the active layer close to the tab side to be greater than or equal to the liquid absorption capacity of the opposite side region, and the liquid absorption capacity of at least one region of the active layer close to the tab side to be greater than the liquid absorption capacity of the opposite side region;

[0040] In the direction from the current collector to the direction away from the current collector, controlling the liquid absorption capacity of the region of the active layer close to the current collector side to be greater than or equal to the liquid absorption capacity of the opposite side region, and the liquid absorption capacity of at least one region of the active layer close to the current collector side to be greater than the liquid absorption capacity of the opposite side region.

[0041] By making the active layer have different liquid absorption capacities for the electrolyte in a specific direction, it is possible to make up for the problem that during long-term storage and use, the electrolyte usually tends to concentrate at the bottom of the electrode assembly due to gravity, resulting in poor electrolyte wettability of the electrode assembly. At the same time, it can also make up for the problem that during charge and discharge, the electrolyte wettability of the electrode sheet is reduced due to the electrolyte being extruded by the volume expansion or contraction of the electrode sheet, and improve the electrolyte wettability of the electrode assembly.

[0042] In some embodiments, the method for preparing the electrode assembly further includes:

[0043] In the direction from the side of the current collector where the tab is provided to the opposite side, controlling the liquid absorption capacity of the region of the separator included in the electrode assembly close to the tab side to be greater than or equal to the liquid absorption capacity of the opposite side region, and the liquid absorption capacity of at least one region of the separator close to the tab side to be greater than the liquid absorption capacity of the opposite side region;

[0044] In the direction from the current collector to the direction away from the current collector, controlling the liquid absorption capacity of the active layer to be greater than the liquid absorption capacity of the separator.

[0045] By varying the liquid absorption capacity of the separator in the direction from the side where the current collector is provided with the tab to the opposite side, the liquid absorption capacity of the separator can be made higher on the side closer to the tab, further improving the electrolyte wettability of the electrode assembly in this direction. At the same time, in the direction from the current collector to the direction away from the current collector, the liquid absorption capacity of the active layer is greater than that of the separator contained in the electrode assembly, which can reduce the situation where the separator absorbs a large amount of electrolyte and causes insufficient electrolyte in the active layer, and improve the electrolyte wettability of the electrode sheet in this direction.

[0046] In a third aspect, the present application provides a battery, which includes the electrode assembly of the first aspect.

[0047] The above-mentioned electrode assembly has good electrolyte wettability. During long-term storage and use, its top and the inner side of the electrode sheet active layer close to the current collector can be well wetted by the electrolyte. After applying it to the battery, the battery will exhibit low resistance and excellent cycle performance.

[0048] In a fourth aspect, the present application provides an electrical device, which includes the battery of the third aspect.

[0049] The battery disclosed in the embodiments of the present application can be used in electrical devices powered by the battery or various energy storage systems with the battery as an energy storage element to provide electrical energy. The above battery exhibits the advantages of low resistance and good cycle performance. Therefore, after applying this battery to various electrical devices, it is beneficial to improve the use experience of various electrical devices. Description of the Drawings

[0050] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0051] Figure 1 Schematic diagram of the change in liquid absorption capacity of the electrode assembly according to an embodiment of the present application;

[0052] Figure 2 Schematic diagram of the gradient structure of the negative electrode sheet and the separator in Embodiment 1 of the present application;

[0053] Figure 3 Schematic diagram of the gradient structure of the negative electrode sheet in Embodiment 2 of the present application;

[0054] Figure 4 Schematic diagram of the gradient structure of the positive electrode sheet in Embodiment 3 of the present application;

[0055] Figure 5 Schematic diagram of the gradient structure of the positive electrode sheet in Embodiment 4 of the present application;

[0056] Figure 6 Schematic diagram of the gradient structure of the negative electrode sheet in Embodiment 5 of the present application;

[0057] Figure 7 Schematic diagram of the gradient structure of the positive electrode sheet in Embodiment 5 of the present application;

[0058] Figure 8 Schematic diagram of the gradient structure of the negative electrode sheet in Comparative Example 2 of the present application;

[0059] Figure 9 Schematic diagram of the gradient structure of the positive electrode sheet in Comparative Example 2 of the present application;

[0060] Figure 10 Schematic diagram of the gradient structure of the electrode assembly in Comparative Example 3 of the present application;

[0061] Figure 11 Schematic diagram of a battery cell according to an embodiment of the present application;

[0062] Figure 12 is Figure 11 exploded view of a battery cell according to an embodiment of the present application shown in;

[0063] Figure 13 Schematic diagram of a battery module according to an embodiment of the present application;

[0064] Figure 14 Schematic diagram of a battery pack according to an embodiment of the present application;

[0065] Figure 15 is Figure 14 exploded view of a battery pack according to an embodiment of the present application shown in;

[0066] Figure 16 Schematic diagram of an electrical device using a battery according to an embodiment of the present application as a power source;

[0067] Reference numerals:

[0068] D1 - from the side of the current collector where the tab is provided to the opposite side, D2 - from the current collector to the direction away from the current collector;

[0069] 10 - negative current collector, 11 - first negative active layer, 111 - first gradient unit, 112 - second gradient unit, 113 - third gradient unit, 114 - fourth gradient unit, 115 - fifth gradient unit, 116 - sixth gradient unit;

[0070] 20 - Positive current collector, 21 - First positive active layer, 211 - First a gradient unit, 212 - First b gradient unit, 213 - First c gradient unit, 214 - First d gradient unit, 215 - First e gradient unit, 216 - First f gradient unit;

[0071] 30 - Separator, 31 - Separator coating;

[0072] 01 - Housing, 02 - Cover plate, 03 - Electrode assembly, 04 - Battery cell, 05 - Battery module, 06 - Upper box body, 07 - Lower box body. Detailed implementation manners

[0073] The embodiments of the technical solutions of the present application will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to illustrate the technical solutions of the present application more clearly, and therefore are only examples and cannot be used to limit the protection scope of the present application.

[0074] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above drawings are intended to cover non-exclusive inclusion.

[0075] In the description of the embodiments of this application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "a plurality" is more than two, unless otherwise specifically defined.

[0076] Referring to "embodiments" herein means that the specific features, structures or characteristics described in connection with the embodiments may be included in at least one embodiment of this application. The phrase appears in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will explicitly and implicitly understand that the embodiments described herein may be combined with other embodiments.

[0077] In the description of the embodiments of this application, the term "and / or" is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this article generally represents an "or" relationship between the associated objects before and after.

[0078] In the description of the embodiments of the present application, the term "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or similar expressions thereof refer to any combination of these items, including any combination of single item(s) or plural item(s). For example, "at least one (item) of a, b, or c", or "at least one (item) of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0079] It should be understood that in various embodiments of the present application, the magnitude of the sequence numbers of the above - mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0080] The weight of the relevant components mentioned in the specification of the embodiments of the present application can not only refer to the specific content of each component, but also represent the proportional relationship of the weights between the components. Therefore, as long as the content of the relevant components in the specification of the embodiments of the present application is scaled up or down proportionally, it is within the scope disclosed in the specification of the embodiments of the present application. Specifically, the mass described in the specification of the embodiments of the present application can be mass units well - known in the chemical industry such as μg, mg, g, kg, etc.

[0081] As a carrier of electrical energy, batteries are widely used in various fields. According to different uses, the market has put forward various requirements for batteries. For example, for batteries applied in the power field to provide electrical energy for devices such as automobiles and electric two - wheelers, they need to have high energy density and excellent cycle performance.

[0082] Among various batteries, batteries using liquid as the electrolyte are more widely used. Such batteries usually include electrode sheets (positive electrode sheets, negative electrode sheets). A separator is arranged between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet, the negative electrode sheet, and the separator are wound or laminated into an electrode assembly (electrical core JR), and the electrode assembly is filled with electrolyte. The common electrode assembly is placed along the vertical direction, that is, the direction of gravity. After injecting the electrolyte into the housing equipped with the electrode assembly, the electrolyte infiltrates the positive electrode sheet, the negative electrode sheet, and the separator. However, during the long - term storage and use of the battery, due to the action of gravity, the electrolyte usually tends to concentrate at the bottom of the electrode assembly, and the wettability of the top of the electrode assembly is poor.

[0083] At the same time, to improve the energy density, related technologies usually increase the compaction density of the electrode sheet as much as possible. The increase in the compaction density will reduce the pores of the electrode sheet, making it difficult for the electrolyte to infiltrate the electrode sheet, especially in the inner side of the electrode sheet near the current collector where the electrolyte is difficult to infiltrate.

[0084] In addition, during the cyclic charge and discharge process of the battery, due to the repeated extraction and insertion of active metal ions, such as lithium ions, sodium ions, etc., into and out of the active material of the electrode sheet, the active material undergoes volume expansion and contraction. During the expansion and contraction process of the active material, the surface pressure of the electrode sheet increases, causing the electrolyte stored inside the electrode sheet to be extruded, resulting in a decrease in the electrolyte wettability of the electrode sheet. In particular, the electrolyte wettability in the inner layer of the current collector in the electrode sheet is lower than that on the surface layer of the electrode sheet.

[0085] Poor electrolyte wettability in the electrode assembly will increase the migration resistance of active metal ions, cause an increase in battery impedance, result in phenomena such as increased electrode polarization and lithium plating, and further lead to battery capacity attenuation and reduced battery cycle performance.

[0086] To address the problem of poor electrolyte wettability in the electrode assembly, the embodiments of the present application design an electrode assembly with a three-dimensional matrix gradient structure. This electrode assembly has a liquid absorption capacity with a specific distribution pattern along two specific directions, that is, the liquid absorption capacity of the area on the side of the active layer close to the tab (or close to the current collector) is greater than or equal to that of the opposite side area, and the liquid absorption capacity of at least one area on the side of the active layer close to the tab (or close to the current collector) is greater than that of the opposite side area. In this way, the directional infiltration of the electrolyte by the electrode assembly can be realized, compensating for the problem of uneven electrolyte distribution at the top and bottom of the electrode assembly caused by gravity, and improving the problem that it is difficult for the electrolyte to infiltrate the inner layer of the current collector in the electrode sheet, which is beneficial to reducing the battery resistance and improving the battery cycle performance.

[0087] The electrode assembly designed in the embodiments of the present application can be applied to fabricate batteries, and thus can be applied to various electrical devices, including but not limited to mobile phones, tablets, laptop computers, electric toys, power tools, battery cars, electric vehicles, ships, spacecraft, etc.

[0088] The following further elaborates the present application in conjunction with embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application.

[0089] [Electrode Assembly]

[0090] In the first aspect of the embodiments of the present application, an electrode assembly is provided. The electrode assembly includes an electrode sheet, and the electrode sheet includes a current collector, an active layer, and a tab. The active layer is disposed on at least one side in the thickness direction of the current collector, and the tab is disposed on one side in the height direction of the current collector;

[0091] From the side of the current collector where the tab is disposed to the opposite side direction, the liquid absorption capacity of the area on the side of the active layer close to the tab is greater than or equal to that of the opposite side area, and the liquid absorption capacity of at least one area on the side of the active layer close to the tab is greater than that of the opposite side area;

[0092] In the direction from the current collector to the direction away from the current collector, the liquid absorption capacity of the region on the side of the active layer close to the current collector is greater than or equal to that of the opposite region, and the liquid absorption capacity of at least one region on the side of the active layer close to the current collector is greater than that of the opposite region.

[0093] Regarding the direction and liquid absorption capacity therein, the detailed features are as follows.

[0094] 1) Direction

[0095] The current collector usually adopts a metal thin sheet or composite material thin sheet with a small thickness, and the active layer is provided on one or both sides in the thickness direction of the current collector. The height direction of the current collector refers to the direction perpendicular to the thickness direction of the current collector, which can be the width direction or the length direction of the plane perpendicular to the thickness direction of the current collector. The tab is provided on one side in the height direction of the current collector. Therefore, the direction from the side of the current collector where the tab is provided to the opposite side is consistent with the height direction of the current collector. At the same time, the tab is usually close to the top of the electrode assembly, and the electrode assembly is usually in a standing state. Therefore, the height direction of the current collector is usually consistent with the height direction of the electrode assembly and also consistent with the gravity direction. Therefore, the direction from the side of the current collector where the tab is provided to the opposite side is usually also the gravity direction.

[0096] The direction from the current collector to the direction away from the current collector is consistent with the thickness direction of the current collector, and more specifically, it can be the direction from the side of the active layer close to the current collector to the side of the active layer away from the current collector.

[0097] 2) Liquid absorption capacity

[0098] The liquid absorption capacity of a certain sample reflects the absorption and infiltration ability of the sample to the liquid. In the embodiments of the present application, the liquid absorption capacity of the sample refers to the absorption and infiltration ability of the sample to the electrolyte. Samples with different liquid absorption capacities show different liquid absorption speeds macroscopically. Therefore, the liquid absorption capacity can be tested by the following means:

[0099] Take a certain area (for example, 2*2 cm 2 ) of the sample to be tested, weigh it to obtain M1, soak it in the electrolyte for a certain time t (for example, 1 h) at a certain temperature (for example, room temperature 25°C), take out the sample, dry the surface of the sample with a dust-free paper to remove the excess electrolyte, and then weigh it to obtain M2. Then, the liquid absorption speed of the sample, that is, the liquid absorption amount per unit time, is m = (M2 - M1) / t. Then the liquid absorption capacity F = σ*m / THK. Where σ is the porosity of the sample and THK is the thickness of the sample. The larger F is, the faster the liquid absorption speed and the stronger the liquid absorption capacity.

[0100] For the pole piece with a double-sided active layer, in actual operation, it needs to be wiped into a single-sided one for testing to prevent the situation that the test results are not easy to compare due to different designs on both sides.

[0101] For the difference in liquid absorption capacity of different regions of the active layer in the direction from the side where the tab of the current collector is provided to the opposite side of the electrode sheet, the liquid absorption capacity of different regions can be tested by taking the same electrode sheet, sampling different regions in this aspect, and then using the above method to test the liquid absorption capacity of different regions.

[0102] For the difference in liquid absorption capacity of different regions of the active layer in the direction from the current collector to the direction away from the current collector, that is, the thickness direction of the electrode sheet, it is necessary to compare the liquid absorption capacity F of the complete electrode sheet total and the liquid absorption capacity F of the inner layer after scraping off the surface layer of the electrode sheet inner (The thickness after scraping is confirmed by a step thickness gauge) to make a distinction. If F inner > F total it indicates that in this direction, the liquid absorption capacity of the region of the active layer close to the current collector side is greater than that of the opposite side region.

[0103] The F of the electrode sheet is usually between 0.1 and 3.

[0104] It can be understood that the electrode sheets in the electrode assembly usually include a positive electrode sheet and a negative electrode sheet. Then, in a certain direction, the distribution of the liquid absorption capacity of the active layer "the liquid absorption capacity of the region of the active layer close to the tab (or close to the current collector) side is greater than or equal to that of the opposite side region, and the liquid absorption capacity of at least one region of the active layer close to the tab (or close to the current collector) side is greater than that of the opposite side region" includes that at least one of the positive electrode active layer and the negative electrode active layer has the above distribution of the liquid absorption capacity in this direction.

[0105] In a certain direction, "the liquid absorption capacity of the region of the active layer closer to the tab (or closer to the current collector) is greater than or equal to that of the opposite region, and the liquid absorption capacity of at least one region of the active layer closer to the tab (or closer to the current collector) is greater than that of the opposite region", including various situations, including but not limited to: 1) the liquid absorption capacity of the active layer decreases in this direction; 2) the liquid absorption capacity of the active layer has a changing trend of gradually decreasing within a certain region in this direction and then remaining unchanged within a certain region; 3) the liquid absorption capacity of the active layer has a changing trend of remaining unchanged within a certain region, then gradually decreasing, and then remaining unchanged in this direction. Among them, the decreasing liquid absorption capacity of the active layer includes one or both distribution forms of gradient decrease or gradual decrease. Among them, gradient decrease means that in this direction, the active layer can be divided into two or more gradient units, each gradient unit has a uniform or approximately uniform liquid absorption capacity, and the liquid absorption capacity of adjacent two gradient units gradually decreases in this direction. Gradual decrease means that in this direction, the liquid absorption capacity of each region of the active layer shows a continuous decreasing trend. Among them, in the direction from the side of the current collector where the tab is provided to the opposite side, the case of gradient decrease in the liquid absorption capacity of the active layer can refer to Figure 1 Figure a in Figure 1 Figure b in, where the darker the color in the figure, the higher the liquid absorption capacity.

[0106] In the embodiment of the present application, by specifically designing the distribution of the liquid absorption capacity of the active layer in the direction from the side of the current collector where the tab is provided to the opposite side and in the direction from the current collector to the side away from the current collector, that is, the liquid absorption capacity of the region of the active layer closer to the tab (or closer to the current collector) is greater than or equal to that of the opposite region, and the liquid absorption capacity of at least one region of the active layer closer to the tab (or closer to the current collector) is greater than that of the opposite region, an electrode assembly with a three-dimensional matrix distribution structure can be formed. For the electrode assembly with this structure, the active layer has a higher liquid absorption capacity at the part closer to the tab than the opposite side, and the tab is usually close to the top of the electrode assembly. After the electrode assembly is infiltrated with the electrolyte, the amount of electrolyte retained at the top of the active layer will be greater than that at the bottom of the active layer. During the long-term storage and use of the electrode assembly, even if there is a tendency for the electrolyte at the top to gather downward due to the action of gravity, a relatively large amount of electrolyte can still be retained, making the electrolyte wettability of the active layer more uniform in this direction, which can make up for the problem of poor wettability caused by the electrolyte concentrating at the bottom of the active layer due to gravity.

[0107] Meanwhile, in the direction from the current collector to the side away from the current collector, the region of the active layer closer to the current collector has a higher liquid absorption capacity. After being infiltrated with the electrolyte, the region of the active layer closer to the current collector will have a higher electrolyte retention capacity, which can compensate for the problem that the electrolyte infiltration of the electrode sheet is reduced or difficult to infiltrate due to the extrusion of the electrolyte caused by the volume expansion or contraction of the electrode sheet during charge and discharge.

[0108] Therefore, in the embodiments of the present application, by specifically designing the liquid absorption capacity distribution of the active layer in the direction from the side where the tab is provided on the current collector to the opposite side, and in the direction from the current collector to the side away from the current collector, the electrolyte wettability of the active layer can be effectively improved, enabling the active layer to be uniformly infiltrated in different directions. And the electrolyte wettability of the electrode assembly is mainly reflected by the active layer. Therefore, the improvement of the electrolyte wettability of the active layer is beneficial to the improvement of the overall electrolyte wettability of the electrode assembly. The improvement of the electrolyte wettability will be beneficial to reducing the battery resistance and improving the cycle performance of the battery.

[0109] In some embodiments, in the direction from the side where the tab is provided on the current collector to the opposite side, the liquid absorption capacity of the region of the separator included in the electrode assembly closer to the tab side is greater than or equal to that of the region on the opposite side, and the liquid absorption capacity of at least one region of the separator closer to the tab side is greater than that of the region on the opposite side;

[0110] In the direction from the current collector to the side away from the current collector, the liquid absorption capacity of the active layer is greater than that of the separator.

[0111] Generally, the electrode assembly includes a separator and electrode sheets. The electrode sheets include a positive electrode sheet and a negative electrode sheet. The separator and the electrode sheets are alternately arranged, and more specifically, they can be alternately arranged in the order of positive electrode sheet - separator - negative electrode sheet. For a stacked electrode assembly, it is stacked in this order; for a wound electrode assembly, after arranging the electrode sheets and the separator in this order, it is then wound.

[0112] For the separator, the test method for the liquid absorption capacity of its different regions can refer to the liquid absorption capacity test method and the determination method for the difference in the liquid absorption capacity of different regions of the active layer recorded above. Generally, the liquid absorption capacity of the separator is between 1 and 15.

[0113] By variably designing the liquid absorption capacity of the separator in the direction from the side where the tab is provided on the current collector to the opposite side, the liquid absorption capacity of the separator closer to the tab side can be made higher, further improving the electrolyte wettability of the electrode assembly in this direction.

[0114] Meanwhile, in the direction from the current collector to the side away from the current collector, the liquid absorption capacity of the active layer is greater than that of the separator included in the electrode assembly, which can reduce the situation that the separator absorbs a large amount of electrolyte and causes insufficient electrolyte in the active layer, and improve the electrolyte wettability of the electrode sheet in this direction.

[0115] It can be understood that in the direction from the current collector to the side away from the current collector, the liquid absorption capacity of the region of the active layer close to the current collector side is greater than or equal to that of the opposite side region, and the liquid absorption capacity of at least one region of the active layer close to the current collector side is greater than that of the opposite side region. At the same time, the liquid absorption capacity of the active layer is greater than that of the separator. Then, in this direction, the relationship of the liquid absorption capacity of the electrode assembly is: the region of the active layer close to the current collector side > the region of the active layer close to the separator side > the separator, as Figure 1 shown in Figure c of, the darker the color in the figure, the higher the liquid absorption capacity.

[0116] Meanwhile, combined with the distribution of the liquid absorption capacities of the active layer and the separator in the direction from the side where the tab is provided on the current collector to the opposite side, it can be understood that the region of the active layer close to the current collector side and close to the tab side has the highest liquid absorption capacity and the highest value, while the region of the separator away from the tab side has the lowest liquid absorption capacity.

[0117] In some embodiments, in the direction from the side where the tab is provided on the current collector to the opposite side, at least one of the active layer and the separator included in the electrode assembly includes at least two adjacent gradient units arranged in series, and the liquid absorption capacity of the gradient unit close to the tab side is higher than that of the gradient unit on the opposite side. Optionally, the number of the gradient units is 2 to 10, such as any one of 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range value between any two of them.

[0118] By arranging a plurality of gradient units in the direction from the side where the tab is provided on the current collector to the opposite side, the liquid absorption capacity of the active layer and / or the separator can be gradually decreased in this direction. The gradient unit at a lower position can provide a buffering effect for the downward movement of the electrolyte in the adjacent gradient unit at a higher position, thereby facilitating the solution of the problem that the electrolyte is concentrated at the bottom of the electrode assembly due to gravity during long-term storage and use of the electrode assembly, and improving the electrolyte wettability of the electrode assembly in this direction. At the same time, the experiments of the embodiments of the present application show that within a certain range, the more the number of the gradient units arranged in this direction, the better the improvement effect on the electrolyte wettability of the electrode assembly.

[0119] In some embodiments, the difference in liquid absorption capacity between two adjacent gradient units is 1.1 to 5 times, optionally 1.2 to 2 times. For example, it can be any one of the point values of 1.1 times, 1.2 times, 1.4 times, 1.6 times, 1.8 times, 2 times, 2.5 times, 3 times, 3.5 times, 4 times, 4.5 times, 5 times or the range value between any two of them.

[0120] The difference in liquid absorption capacity between two adjacent gradient units by a certain multiple can enable the gradient unit at a lower position to well buffer the downward movement of the electrolyte in the adjacent gradient unit at a higher position, and improve the electrolyte wettability in the direction from the side of the active layer close to the tab to the opposite side.

[0121] In some embodiments, in the direction from the side of the current collector where the tab is provided to the opposite side, the height of any one gradient unit accounts for 10% to 60% of the total height of the active layer or the separator, optionally 10% to 50%. For example, it can be any one of the point values of 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60% or the range value between any two of them.

[0122] Setting the height of the gradient unit in the direction from the side of the current collector where the tab is provided to the opposite side within a suitable range is beneficial to increasing the number of gradient units, and further improving the electrolyte wettability of the electrode assembly in this direction.

[0123] In some embodiments, in the direction from the current collector to the direction away from the current collector, the active layer includes at least two adjacent liquid absorption layers arranged, and the liquid absorption capacity of the liquid absorption layer closer to the current collector side is higher than that of the liquid absorption layer on the opposite side. Optionally, the number of liquid absorption layers is 2 to 10, for example, any one of the point values of 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range value between any two of them.

[0124] Considering that in the direction from the side of the current collector where the tab is provided to the opposite side, the active layer may include at least two adjacent gradient units arranged, and the liquid absorption capacity of the gradient unit closer to the tab side is higher than that of the gradient unit on the opposite side. Therefore, under certain conditions, a certain liquid absorption layer in the direction from the current collector to the direction away from the current collector coincides with a certain gradient unit in the direction from the side of the current collector where the tab is provided to the opposite side.

[0125] By arranging a plurality of liquid absorption layers in the direction from the current collector to the side away from the current collector, the liquid absorption capacity of the active layer can gradually decrease in this direction. When the volume of the electrode tab changes and the electrolyte is squeezed out, the liquid absorption layer on the side away from the current collector can provide a buffering effect for the outward overflow tendency of the electrolyte in the adjacent liquid absorption layer closer to the current collector side, thus facilitating the solution of the problem that during the charge and discharge process of the electrode tab, the electrolyte wettability of the electrode tab decreases or it is difficult to be wetted due to volume expansion or contraction squeezing out the electrolyte, and improving the electrolyte wettability of the electrode assembly in this direction. At the same time, experiments show that within a certain range, the more the number of liquid absorption layers arranged in the direction from the current collector to the side away from the current collector, the better the improvement effect on the electrolyte wettability of the electrode assembly.

[0126] In some embodiments, in the direction from the current collector to the side away from the current collector, the thickness of any liquid absorption layer accounts for 10% - 60% of the total thickness of the active layer on the current collector side, optionally 10% - 50%, for example, it can be any point value among 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60% or the range value between any two of them.

[0127] Setting the thickness of the liquid absorption layer in the direction from the current collector to the side away from the current collector within a suitable range is beneficial to increasing the number of liquid absorption layers, and further improving the electrolyte wettability of the electrode assembly in this direction.

[0128] In some embodiments, the active layer includes a first electrolyte absorbent;

[0129] In the direction from the side where the tab of the current collector is arranged to the opposite side, the mass content of the first electrolyte absorbent in the area of the active layer closer to the tab side is greater than or equal to the mass content of the first electrolyte absorbent in the area of the opposite side, and the mass content of the first electrolyte absorbent in at least one area of the active layer closer to the tab side is greater than the mass content of the first electrolyte absorbent in the area of the opposite side;

[0130] In the direction from the current collector to the side away from the current collector, the mass content of the first electrolyte absorbent in the area of the active layer closer to the current collector side is greater than or equal to the mass content of the first electrolyte absorbent in the area of the opposite side, and the mass content of the first electrolyte absorbent in at least one area of the active layer closer to the current collector side is greater than the mass content of the first electrolyte absorbent in the area of the opposite side.

[0131] The electrolyte absorbent refers to a material capable of absorbing the electrolyte. The active layer contains the electrolyte absorbent, that is, the electrolyte absorbent, active material, conductive agent, binder and other components together form the active layer. The mass content of the electrolyte absorbent in the active layer can be obtained by combining one or more of an inductively coupled plasma spectrometer (ICP), an energy dispersive X-ray spectrometer (EDS), a proton nuclear magnetic resonance spectrometer, and a carbon nuclear magnetic resonance spectrometer for testing.

[0132] By adding an electrolyte absorbent to the active layer and adjusting the mass content of the electrolyte absorbent to vary in a certain direction, the electrode assembly can have different absorption and retention capabilities for the electrolyte in different regions, and exhibit a certain distribution pattern of liquid absorption capacity along a specific direction, which is beneficial to improving the electrolyte wettability of the electrode assembly.

[0133] In some embodiments, the separator included in the electrode assembly comprises a base film and a separator coating provided on at least one side of the base film; the separator coating contains a second electrolyte absorbent;

[0134] In the direction from the side where the tab of the current collector is provided to the opposite side, the mass content of the second electrolyte absorbent in the region of the separator coating close to the tab is greater than or equal to the mass content of the second electrolyte absorbent in the opposite region, and the mass content of the second electrolyte absorbent in at least one region of the separator coating close to the tab is greater than the mass content of the second electrolyte absorbent in the opposite region.

[0135] In some embodiments, in the direction from the current collector to the direction away from the current collector, the mass content of the first electrolyte absorbent in the active layer is greater than the mass content of the second electrolyte absorbent in the separator coating.

[0136] The separator coating contains an electrolyte absorbent, that is, the electrolyte absorbent and other materials form a coating on at least one side of the base film. By adding an electrolyte absorbent to the separator coating and adjusting the mass content of the electrolyte absorbent to vary in a certain direction, the electrode assembly can have different absorption and retention capabilities for the electrolyte in different regions, and exhibit a certain distribution pattern of liquid absorption capacity along a specific direction, further improving the electrolyte wettability of the electrode assembly.

[0137] In some embodiments, the first electrolyte absorbent and the second electrolyte absorbent each independently include one or more of polymethyl methacrylate, poly(methyl acrylate), poly(ethyl acrylate), poly(vinyl acetate), polyacrylamide, polyacrylic acid, and polyacrylonitrile.

[0138] Such polymer materials have a high affinity for the electrolyte and can swell and absorb the electrolyte to form a gel after contacting the electrolyte. By using these polymer materials as electrolyte absorbents and adjusting the distribution of these polymer materials in the active layer and the separator coating, the liquid absorption capacity of the electrode assembly can be effectively made to exhibit a gradient / gradual change along a specific direction, thereby improving the electrolyte wettability of the electrode assembly.

[0139] In some embodiments, the mass content of the first electrolyte absorbent in the active layer and the mass content of the second electrolyte absorbent in the separator coating are each independently greater than 0 and less than or equal to 8%; optionally greater than or equal to 1% and less than or equal to 6%. For example, the mass content can be any one of the point values of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8% or the range value between any two of them.

[0140] Setting the mass content of the electrolyte absorbent in the active layer and the separator coating within a suitable range is not only beneficial to adjusting the liquid absorption capacity of different regions of the electrode assembly, but also does not cause a large reduction in the active material in the active layer, thereby maintaining the stability of the electrode sheet capacity.

[0141] In addition, the electrode sheet of the electrode assembly includes a positive electrode sheet and a negative electrode sheet, and the separator is disposed between the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and the negative electrode sheet. For other detailed technical features of the positive electrode sheet, negative electrode sheet, and separator in the electrode assembly, as well as the preparation method of the electrode assembly, reference can be made to the following content.

[0142] 1. Positive Electrode Sheet

[0143] The positive electrode sheet may include a positive electrode current collector and a positive electrode active layer provided on at least one side of the positive electrode current collector and containing a positive electrode active material, a binder, and a conductive agent.

[0144] Among them, the positive electrode active material is the key substance participating in the battery chemical reaction in the positive electrode sheet. The conductive agent is used to collect microcurrents between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector to improve the electronic conductivity. At the same time, the conductive agent can also promote the infiltration of the electrolyte into the positive electrode sheet. The binder can improve the binding strength between the substances in the positive electrode active layer and between the positive electrode active layer and the positive electrode current collector. The thickening agent is beneficial to increasing the viscosity of the positive electrode paste and improving the processing performance of the positive electrode paste. The positive electrode current collector is used to transmit electrons.

[0145] 1.1. Positive Electrode Active Material

[0146] Depending on the type of battery to which the electrode assembly is applied, different positive electrode active materials can be used for the positive electrode sheet. The electrode assembly of the embodiments of the present application can be applied to both lithium-ion batteries and sodium-ion batteries.

[0147] When the electrode assembly is applied to a lithium-ion battery, the positive electrode active material can include one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium nickel cobalt manganese oxide, lithium manganate, lithium nickelate, lithium cobaltate, and lithium ferrate.

[0148] When the electrode assembly is applied to a sodium-ion battery, the positive electrode active material can include one or more of layered oxides, polyanion compounds, and Prussian blue compounds. For example, the layered oxide can include Na x MO 2 , where M = Fe, Mn, Ni, Co, Cr, Sc, Ti, V, Cr, Cu, Zn and their combinations, and 0.4 ≤ x ≤ 1. The polyanion compound can include one or more of phosphates, pyrophosphates, sulfate-based, and anion-doped types.

[0149] The mass content of the positive electrode active material in the positive electrode active layer is 80% to 98%. For example, it can include but is not limited to any point value among 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98% or the range value between any two of them. It can be understood that setting the mass content of the positive electrode active material in the active layer contained in the positive electrode sheet at a relatively high level can improve the energy density of the battery.

[0150] 1.2. Conductive agent

[0151] The conductive agent can include one or more of acetylene black (SP), carbon nanotubes, conductive carbon black (super-P), Ketjen black, carbon fiber, and graphene.

[0152] The mass content of the conductive agent in the positive electrode active layer can be 0.5% to 5%, such as any point value among 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or the range value between any two of them, and can also be set to other contents according to needs.

[0153] 1.3. Binder

[0154] The binder includes but is not limited to one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, polyimide, polytetrafluoroethylene, polybutyl acrylate, polyacrylonitrile, carboxymethyl cellulose, carboxymethyl cellulose salt, polyacrylic acid, polyacrylate, polyvinyl alcohol, sodium alginate, cyclodextrin, styrene-butadiene rubber, vinyl acetate resin, acrylic resin, and chlorinated rubber.

[0155] The mass content of the binder in the positive electrode active layer is 0.5% to 5%, for example, any one of the point values of 0.5%, 1%, 2%, 3%, 4%, 5% or the range value between any two of them.

[0156] 1.4. Positive electrode current collector

[0157] The positive electrode active layer is disposed on at least one side of the positive electrode current collector, and optionally on both sides of the positive electrode current collector. The positive electrode current collector may include, but is not limited to, a metal current collector, a carbon current collector, a conductive resin current collector, a composite current collector of metal and resin, etc. More specifically, for example, aluminum, copper, nickel, titanium, iron and their respective alloys, stainless steel, carbon fiber, carbon nanotubes (CNT), graphite, etc. Optionally, the positive electrode current collector includes aluminum.

[0158] 2. Negative electrode sheet

[0159] The negative electrode sheet may include a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector and containing a negative electrode active material, a binder, and a conductive agent.

[0160] Among them, the negative electrode active material may include one or more of graphite, hard carbon, soft carbon, mesophase carbon microspheres, graphene, silicon, and silicon dioxide. The mass content of the negative electrode active material in the negative electrode active layer may include, but is not limited to, 90% to 98%, for example, any one of the point values of 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or the range value between any two of them.

[0161] The types of the conductive agent and the binder in the negative electrode sheet and their mass contents in the negative electrode active layer may refer to those of the positive electrode sheet, which will not be elaborated here.

[0162] The negative electrode active layer may also optionally include a thickening agent, such as carboxymethyl cellulose (CMC). The mass content of the thickening agent in the active layer includes, but is not limited to, 0.5% to 5%, for example, any one of the point values of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or the range value between any two of them.

[0163] The negative electrode current collector may include, but is not limited to, a metal current collector, a carbon current collector, a conductive resin current collector, a composite current collector of metal and resin, etc. More specifically, for example, copper, nickel, titanium, iron and their respective alloys, stainless steel, carbon fiber, carbon nanotubes (CNT), graphite, etc.

[0164] 3. Separator

[0165] The separator in the electrode assembly includes a base film and a separator coating disposed on at least one side of the base film.

[0166] The base film can be any porous structure diaphragm with electrochemical stability and mechanical stability, such as a single-layer or multi-layer film of one or more of glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF).

[0167] The separator coating usually also includes one or more of inorganic materials and organic materials. Among them, the inorganic materials can include one or more of alumina and boehmite, and these inorganic materials can improve the heat resistance, hardness, chemical stability, flame retardancy, etc. of the separator. The organic materials can include one or more of aramid, polyvinylidene fluoride, and polymethyl methacrylate (PMMA), and these organic materials can be used as binders to enhance the adhesiveness, or improve the chemical stability, heat resistance, etc. of the separator.

[0168] [Preparation method of electrode assembly]

[0169] The electrode assembly of the embodiment of the present application can be prepared by the following method.

[0170] The second aspect of this embodiment provides a preparation method of an electrode assembly, including:

[0171] Prepare an active layer on at least one side in the thickness direction of the current collector, and set a tab on one side in the height direction of the current collector;

[0172] From the side of the current collector where the tab is set to the opposite side, control the liquid absorption capacity of the area of the active layer close to the tab to be greater than or equal to the area of the opposite side, and the liquid absorption capacity of at least one area of the active layer close to the tab is greater than the area of the opposite side;

[0173] From the current collector to the direction away from the current collector, control the liquid absorption capacity of the area of the active layer close to the current collector to be greater than or equal to the area of the opposite side, and the liquid absorption capacity of at least one area of the active layer close to the current collector is greater than the area of the opposite side.

[0174] By making the active layer have different liquid absorption capacities for the electrolyte in a specific direction, it can compensate for the problem that the electrolyte usually tends to concentrate at the bottom of the electrode assembly caused by gravity during long-term storage and use, resulting in poor electrolyte wettability of the electrode assembly. At the same time, it can also compensate for the problem that the electrolyte wettability of the electrode sheet decreases due to the extrusion of the electrolyte caused by the volume expansion or contraction of the electrode sheet during charge and discharge, and improve the electrolyte wettability of the electrode assembly.

[0175] The change trend of the liquid absorption capacity of the active layer in different directions can be controlled by the following method:

[0176] Prepare an active layer containing a first electrolyte absorbent;

[0177] In the direction from the side of the current collector where the tab is provided to the opposite side, control the mass content of the first electrolyte absorbent in the region of the active layer close to the tab side to be greater than or equal to the mass content of the first electrolyte absorbent in the region of the opposite side, and the mass content of the first electrolyte absorbent in at least one region of the active layer close to the tab side is greater than the mass content of the first electrolyte absorbent in the region of the opposite side;

[0178] In the direction from the current collector to the direction away from the current collector, control the mass content of the first electrolyte absorbent in the active layer to decrease, control the mass content of the first electrolyte absorbent in the region of the active layer close to the current collector side to be greater than or equal to the mass content of the first electrolyte absorbent in the region of the opposite side, and the mass content of the first electrolyte absorbent in at least one region of the active layer close to the current collector side is greater than the mass content of the first electrolyte absorbent in the region of the opposite side.

[0179] By adding an electrolyte absorbent to the active layer and adjusting the mass content of the electrolyte absorbent to vary in a certain direction, the active layer can have different absorption and retention capabilities for the electrolyte in different regions, and exhibit varying liquid absorption capabilities along a specific direction, improving the electrolyte wettability of the electrode assembly.

[0180] In some embodiments, the method for preparing the electrode assembly further includes:

[0181] In the direction from the side of the current collector where the tab is provided to the opposite side, control the liquid absorption capacity of the separator included in the electrode assembly in the region close to the tab side to be greater than or equal to the liquid absorption capacity of the region of the opposite side, and the liquid absorption capacity of at least one region of the separator close to the tab side is greater than the liquid absorption capacity of the region of the opposite side;

[0182] In the direction from the current collector to the direction away from the current collector, control the liquid absorption capacity of the active layer to be greater than the liquid absorption capacity of the separator.

[0183] By variably designing the liquid absorption capacity of the separator in the direction from the side of the current collector where the tab is provided to the opposite side, the liquid absorption capacity of the separator can be higher on the side close to the tab, further improving the electrolyte wettability of the electrode assembly in this direction. At the same time, in the direction from the current collector to the direction away from the current collector, the liquid absorption capacity of the active layer is greater than the liquid absorption capacity of the separator included in the electrode assembly, which can reduce the situation where the separator absorbs a large amount of electrolyte and causes insufficient electrolyte in the active layer, improving the electrolyte wettability of the electrode in this direction.

[0184] The liquid absorption capacity of the separator can be set by the following method: prepare a separator coating on at least one side of the base film to obtain the separator; the separator coating contains a second electrolyte absorbent;

[0185] In the direction from the side of the current collector where the tab is provided to the opposite side, control the mass content of the second electrolyte absorbent in the area of the separator coating near the tab to be greater than or equal to the mass content of the second electrolyte absorbent in the area of the opposite side, and the mass content of the second electrolyte absorbent in at least one area of the separator coating near the tab is greater than the mass content of the second electrolyte absorbent in the area of the opposite side;

[0186] In the direction from the current collector to the direction away from the current collector, control the mass content of the first electrolyte absorbent in the active layer to be greater than the mass content of the second electrolyte absorbent in the separator coating.

[0187] By adding an electrolyte absorbent to the separator coating and adjusting the mass content of the electrolyte absorbent to vary in a certain direction, the electrode assembly can have different absorption and retention capabilities for the electrolyte in different regions, and exhibit a varying liquid absorption capacity along a specific direction, further improving the electrolyte wettability of the electrode assembly.

[0188] In the above preparation method, the method for preparing the active layer may include: mixing an active material, a conductive agent, a binder, a first electrolyte absorbent (other components may also be added as needed) with a solvent to obtain a pole piece slurry; coating the pole piece slurry on at least one side of the current collector, drying and compressing.

[0189] Since the mass content of the first electrolyte absorbent in the active layer varies in a certain direction, therefore, pole piece slurries containing different contents of the first electrolyte absorbent can be prepared in advance, and coated layer by layer and region by region in sequence, so as to form an active layer with a liquid absorption capacity varying according to a certain rule.

[0190] Similarly, for the preparation of the separator coating, the second electrolyte absorbent can be made into a slurry with other components and a solvent in the separator coating, and then coated on at least one side of the base film. Since the mass content of the second electrolyte absorbent in the separator coating varies in a certain direction, slurries containing different contents of the electrolyte absorbent can be prepared in advance, and coated layer by layer and region by region in sequence, so as to form a separator coating with a liquid absorption capacity varying according to a certain rule.

[0191] In addition, the preparation method of the electrode assembly further includes: arranging the pole pieces and the separator in sequence, and performing lamination or winding to obtain the electrode assembly. It can be understood that the pole pieces of the electrode assembly include a positive pole piece and a negative pole piece, and the separator is arranged between the positive pole piece and the negative pole piece and separates the positive pole piece and the negative pole piece. Therefore, arranging the pole pieces and the separator in sequence and performing the lamination or winding step more specifically includes arranging the positive pole piece and the negative pole piece alternately, arranging the separator between the positive pole piece and the negative pole piece and separating the two, and performing lamination or winding to form the electrode assembly.

[0192] [Battery]

[0193] The third aspect of the embodiments of the present application provides a battery, which includes the electrode assembly of the first aspect above.

[0194] The above electrode assembly has good electrolyte wettability. During long-term storage and use, its top and the inner side of the current collector near the active layer of the electrode can be well wetted by the electrolyte. After being applied to the battery, the battery will exhibit low resistance and excellent cycling performance.

[0195] The electrode assembly of the embodiments of the present application can be a stacked electrode assembly or a wound electrode assembly. Therefore, the battery can also be one or both of a stacked battery and a wound battery.

[0196] Generally, in addition to the electrode assembly, the battery also includes an electrolyte, an outer package, etc. At the same time, the battery of the embodiments of the present application can include one or more of a battery cell, a battery module, and a battery pack.

[0197] 1. Electrolyte

[0198] The electrolyte can be a carrier for ion transport in the battery. In the battery of the embodiments of the present application, the electrolyte used can be a solid electrolyte, such as a polymer electrolyte, an inorganic solid electrolyte, etc., but is not limited thereto; the electrolyte can also be an electrolytic solution.

[0199] As the above electrolytic solution, it includes a solvent and a metal salt dissolved in the solvent.

[0200] Among them, the solvent can be a non-aqueous organic solvent. For example, it can include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB), and preferably two or more.

[0201] The battery of the embodiments of the present application can include one or both of a lithium-ion battery and a sodium-ion battery. For a lithium-ion battery, the metal salt in its electrolytic solution includes a lithium salt; for a sodium-ion battery, the metal salt in its electrolytic solution includes a sodium salt.

[0202] The lithium salt can include LiPF 6 (lithium hexafluorophosphate), LiBF 4 (lithium tetrafluoroborate), LiClO 4 (lithium perchlorate), LiAsF 6(Lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO 2 F 2 (lithium difluorophosphate), LiDFOP (lithium difluoro(oxalato)phosphate), and LiTFOP (lithium tetrafluoro(oxalato)phosphate), or one or more of them.

[0203] The sodium salt may include sodium hexafluorophosphate (NaPF 6 ), sodium bis(fluorosulfonyl)imide (NaFSI), sodium trifluoromethanesulfonate (NaOTf), sodium sulfide (Na 2 S), sodium chloride (NaCl), sodium fluoride (NaF), sodium sulfate (Na 2 SO 4 ), sodium carbonate (Na 2 CO 3 ), sodium phosphate (Na 3 PO 4 ), sodium nitrate (NaNO 3 ), sodium difluoro(oxalato)borate (NaDFOB), sodium pyrophosphate (Na 4 P 2 O 7 ), sodium dodecylbenzenesulfonate (SDBS), sodium dodecyl sulfate (SDS), trisodium citrate, sodium metaborate (NaBO 2 ), sodium borate (Na 2 B 4 O 7 ), sodium molybdate (Na 2 MoO 4 ), sodium tungstate (Na 2 WO 4 ), sodium bromide (NaBr), sodium nitrite (NaNO 2 ), sodium iodate (NaIO 3 ), sodium iodide (NaI), sodium silicate (Na 2 SiO 3 ), sodium lignosulfonate, sodium oxalate (Na 2 C 2 O 4 ), sodium aluminate (NaAlO 2 ), sodium methyl sulfonate, sodium acetate (CH 3 COONa), sodium dichromate (Na 2 Cr 2 O 7 ), sodium hexafluoroarsenate (NaAsF 6 ), sodium tetrafluoroborate (NaBF 4 ), sodium perchlorate (NaClO4 one or more of those in

[0204] The electrolyte may also optionally contain other additives, such as one or more of vinylene carbonate (VC), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), succinonitrile (SN), adiponitrile (ADN), glutaronitrile (GLN), hexane trinitrile (HTN), 1,3 - propane sultone (1,3 - PS), ethylene sulfate (DTD), methylene methyl disulfonate (MMDS), 1 - propene - 1,3 - sultone (PST), 4 - methyl ethylene sulfate (PCS), 4 - ethyl ethylene sulfate (PES), 4 - propyl ethylene sulfate (PEGLST), propylene sulfate (TS), 1,4 - butane sultone (1,4 - BS), ethylene sulfite (DTO), dimethyl sulfite (DMS), diethyl sulfite (DES), cyclic quaternary ammonium sulfonate, tris(trimethylsilyl) phosphate (TMSP), and tris(trimethylsilyl) borate (TMSB), but not limited thereto.

[0205] The electrolyte may also include additives. For example, the additives may include negative electrode film - forming additives, positive electrode film - forming additives, and may also include additives that can improve certain battery performance, such as additives for improving the overcharge performance of the battery, additives for improving the high - temperature or low - temperature performance of the battery, etc.

[0206] The filling volume of the electrolyte in the battery can be determined according to the designed filling coefficient, where the filling coefficient is the mass of the electrolyte injected corresponding to the unit capacity, and the electrolyte filling volume = the designed capacity of the battery cell * the filling coefficient.

[0207] In the embodiments of the present application, the liquid injection coefficient of the battery can be set to 1 g / Ah to 5 g / Ah, optionally 1.2 g / Ah to 2 g / Ah, such as any one of 1 g / Ah, 1.2 g / Ah, 1.4 g / Ah, 1.6 g / Ah, 1.8 g / Ah, 2 g / Ah, 2.2 g / Ah, 2.4 g / Ah, 2.6 g / Ah, 2.8 g / Ah, 3 g / Ah, 3.2 g / Ah, 3.4 g / Ah, 3.6 g / Ah, 3.8 g / Ah, 4 g / Ah, 4.2 g / Ah, 4.4 g / Ah, 4.6 g / Ah, 4.8 g / Ah, 5 g / Ah or the range value between any two of them. The electrode assembly of the embodiments of the present application can guide and infiltrate the electrolyte, and has good electrolyte wettability even at a low liquid injection coefficient. Therefore, after applying the electrode assembly of the embodiments of the present application to the battery, it is beneficial to reduce the liquid injection coefficient of the battery and reduce the amount of electrolyte injection. Moreover, experiments show that the electrode assembly of the embodiments of the present application has an excellent improvement effect on the electrochemical performance of the battery with a low liquid injection coefficient.

[0208] 2. Outer packaging

[0209] The battery may include an outer packaging. The outer packaging can be used to encapsulate the electrode assembly including the positive electrode sheet, the negative electrode sheet, and the separator, as well as the electrolyte.

[0210] The outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.; it can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate, etc.

[0211] The shape of the outer packaging can be cylindrical, square or any other shape. For example, Figure 11 is a battery cell with a square structure as an example of the outer packaging shape.

[0212] Refer to Figure 12 As shown in the figure, the outer packaging may include a housing 01 and a cover plate 02. Among them, the housing 01 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing 01 has an opening communicating with the receiving cavity, and the cover plate 02 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can form an electrode assembly 03 through a winding process or a stacking process. One or more electrode assemblies 03 are encapsulated in the receiving cavity. The electrolyte infiltrates in the electrode assembly 03.

[0213] 3. Battery cell, battery module, battery pack

[0214] The battery in the embodiments of the present application can be at least one of a battery cell, a battery module, and a battery pack. According to different packaging forms, batteries are divided into battery cells, battery modules, and battery packs. Among them, a battery cell is the most basic unit of a secondary battery, including an electrode assembly and an electrolyte. The electrode assembly is usually composed of a positive electrode sheet, a negative electrode sheet, and a separator. The positive electrode sheet and the negative electrode sheet are alternately stacked, and a separator is arranged between the positive electrode sheet and the negative electrode sheet to play a role of isolation, obtaining a bare battery cell, or a bare battery cell can also be obtained after winding. The battery cell is placed in a housing, electrolyte is injected, and it is sealed to obtain a battery cell. The battery cell mainly works by the movement of metal ions in the electrolyte between the positive electrode sheet and the negative electrode sheet.

[0215] In some battery packaging technologies, one or more battery cells can be integrated into a battery module first, and then one or more battery modules are assembled into a battery pack. In some other battery packaging technologies, one or more battery cells can also be directly installed in a box to form a battery pack, removing the intermediate state of the battery module, thereby reducing the mass of the battery pack and increasing the energy density of the battery.

[0216] Reference Figure 13 , which is an exemplary battery module. In the battery module, a plurality of battery cells 04 can be arranged in sequence along the length direction of the battery module. Of course, they can also be arranged in any other way. Further, the plurality of battery cells 04 can be fixed by fasteners.

[0217] Optionally, the battery module can further include a housing with a receiving space, and a plurality of battery cells 04 are received in the receiving space.

[0218] Reference Figure 14 and Figure 15 , which is an exemplary battery pack. The battery pack can include a battery box and a plurality of battery modules 05 arranged in the battery box. The battery box includes an upper box body 06 and a lower box body 07. The upper box body 06 can cover the lower box body 07 and form a closed space for receiving the battery modules 05. The plurality of battery modules 05 can be arranged in the battery box in any way.

[0219] [Power-consuming device]

[0220] The embodiments of the present application further provide a power-consuming device, and the power-consuming device includes the battery in the third aspect above.

[0221] The battery disclosed in the embodiments of the present application can be used in power-consuming devices powered by batteries, or various energy storage systems with batteries as energy storage elements, for providing electric energy. The above battery exhibits the advantages of low resistance and good cycle performance. Therefore, after applying the battery to various power-consuming devices, it is beneficial to improve the use experience of various power-consuming devices.

[0222] The electrical device may include, but is not limited to, mobile phones, tablets, laptop computers, electric toys, electric tools, battery cars, electric vehicles, ships, spacecraft, etc. Among them, the electric toys may include fixed or mobile electric toys, such as game consoles, electric vehicle toys, electric ship toys, electric airplane toys, etc., and the spacecraft may include airplanes, rockets, space shuttles, spaceships, etc. As the electrical device, the battery cells, battery modules or battery packs in the battery can be selected according to its usage requirements.

[0223] Figure 16 is an example of an electrical device. The electrical device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electrical device for the battery, a battery pack or a battery module can be adopted.

[0224] The embodiments of the present application will be described in detail below. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those not specified in the embodiments regarding specific technologies or conditions, the technologies or conditions described in the literature in the art or according to the product specifications shall be followed. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0225] In the following embodiments and comparative examples, for simplicity, D1 is used to refer to the direction from the side where the tab is provided on the current collector to the opposite side; for the positive electrode sheet, D1 refers to the direction from the side where the positive tab is provided on the positive current collector to the opposite side; for the negative electrode sheet, D1 refers to the direction from the side where the negative tab is provided on the negative current collector to the opposite side. D2 is used to refer to the direction from the current collector to the direction away from the current collector; for the positive electrode sheet, D2 refers to the direction from the positive current collector to the direction away from the positive current collector; for the negative electrode sheet, D2 refers to the direction from the negative current collector to the direction away from the negative current collector.

[0226] At the same time, considering that in the D1 direction, the active layer may include at least two adjacent gradient units arranged in a decreasing liquid absorption capacity; and in the D2 direction, the active layer includes at least two adjacent liquid absorption layers arranged in a decreasing liquid absorption capacity. In this way, a certain liquid absorption layer in the D2 direction will coincide with a certain gradient unit in the D1 direction. Therefore, in the following embodiments and comparative examples, the "gradient unit" is uniformly used to represent each region with different liquid absorption capacities.

[0227] Example 1

[0228] This embodiment provides a lithium-ion battery, which includes an electrode assembly composed of a positive electrode sheet, a negative electrode sheet, and a separator, as well as an electrolyte and an outer package. The negative electrode sheet and the separator include polyethyl acrylate (electrolyte absorbent) with a gradient content. The electrode assembly is placed in the outer package, and the electrolyte infiltrates the electrode assembly.

[0229] The composition of the electrode assembly includes:

[0230] 1) Negative electrode sheet

[0231] The negative electrode sheet includes a negative electrode current collector copper foil with a thickness of 8 μm and a negative electrode active layer with a total thickness of 60 μm provided on two surfaces of the negative electrode current collector. At the same time, a negative electrode tab is provided on one side in the height direction of the negative electrode current collector.

[0232] In this embodiment, on one side of the negative electrode sheet, the negative electrode active layer is provided with a plurality of gradient units with a decreasing mass content gradient of polyethyl acrylate in the D1 and D2 directions, that is, a single-sided gradient design.

[0233] As Figure 2 shown, in the negative electrode sheet, the negative electrode active layer (the first negative electrode active layer 11 with a thickness of 30 μm) on one side of the negative electrode current collector 10 includes the first gradient unit 111 and the second gradient unit 112 stacked in the thickness direction of the negative electrode sheet, and the third gradient unit 113 and the fourth gradient unit 114 stacked. Among them, the first gradient unit 111 and the third gradient unit 113 are close to the negative electrode current collector 10, and the first gradient unit 111 and the third gradient unit 113 are arranged side by side in the height direction of the negative electrode current collector 10, and the first gradient unit 111 is close to the top of the negative electrode current collector 10; the second gradient unit 112 and the fourth gradient unit 114 are arranged side by side in the height direction of the negative electrode current collector 10, and the second gradient unit 112 is close to the top of the negative electrode current collector 10.

[0234] The first gradient unit 111, the second gradient unit 112, the third gradient unit 113, and the fourth gradient unit 114 have the same size (the same length, width, and thickness), and all include polyethyl acrylate, graphite, 2.5 wt% styrene-butadiene rubber SBR, 0.5 wt% acetylene black SP, and 1.2 wt% carboxymethyl cellulose CMC. Among them, the total mass content of polyethyl acrylate and graphite is 95.8 wt%, and the mass content of polyethyl acrylate in the first gradient unit 111 and the second gradient unit 112, and the third gradient unit 113 and the fourth gradient unit 114 are 5%, 4%, 4%, and 3% in sequence.

[0235] The negative electrode active layer (the second negative electrode active layer, with a thickness of 30 μm, Figure 2 not shown in the figure) on the other side of the negative electrode current collector 10 includes 95.8 wt% graphite, 2.5 wt% SBR, 0.5 wt% SP, and 1.2 wt% CMC.

[0236] The negative electrode sheet can be prepared as follows:

[0237] Four kinds of negative electrode slurries corresponding to the first gradient unit 111 and the second gradient unit 112, the third gradient unit 113 and the fourth gradient unit 114 are respectively prepared. The first gradient unit 111 and the third gradient unit 113 are coated on one side of the negative electrode current collector 10 through a LOM (Laminated Object Manufacturing) extrusion coating device, and then dried. Then, the second gradient unit 112 and the fourth gradient unit 114 are coated side by side beside the first gradient unit 111 and the third gradient unit 113 through the LOM extrusion coating device, and dried again.

[0238] The corresponding negative electrode slurries on the other side of the negative electrode current collector 10 are coated and dried.

[0239] After coating and drying are completed, cold pressing is carried out to obtain the negative electrode sheet.

[0240] 2) Positive electrode sheet

[0241] The positive electrode sheet includes a positive electrode current collector aluminum foil with a thickness of 15 μm and a positive electrode active layer with a total thickness of 80 μm provided on two surfaces of the positive electrode current collector. The positive electrode active layer contains 96 wt% lithium nickel cobalt manganate NCM111, 3 wt% polyvinylidene fluoride PVDF, and 1 wt% acetylene black SP.

[0242] NCM111, PVDF, and SP are mixed, and N-methylpyrrolidone is added and stirred to disperse to obtain the positive electrode slurry. The positive electrode slurry is coated on the aluminum foil, dried, and cold pressed to obtain the positive electrode sheet.

[0243] 3) Separator

[0244] The separator includes a base film (a PE film with a thickness of 12 μm), and a separator coating with a thickness of 3 μm provided on one side of the base film. Refer to Figure 2 , the separator coating 31 includes two gradient units with mass contents of ethyl acrylate of 2% and 1% respectively. At the same time, the separator coating 31 also includes 3% by mass of PVDF and the balance of alumina, and the mass content of ethyl acrylate increases from bottom to top along the height direction of the negative electrode current collector 10. That is, the gradient unit with a mass content of ethyl acrylate of 2% is close to the top of the negative electrode current collector 10.

[0245] 4) Lithium-ion battery

[0246] The positive electrode sheet, the separator, and the negative electrode sheet are arranged in sequence, so that the separator is in the middle of the positive electrode sheet and the negative electrode sheet to play a role of isolation, and the first negative electrode active layer 11 of the negative electrode sheet is close to the separator coating 13. After arrangement, it is wound to obtain an electrode assembly. The electrode assembly is placed in an outer package, and the prepared electrolyte (1M LiPF 6, EMC: The EC volume ratio is 7:3, and the liquid injection coefficient is 1.2 g / Ah), and then through processes such as encapsulation, liquid injection, formation, and exhaust, a wound hard-shell lithium-ion battery is obtained.

[0247] Example 2

[0248] The difference between this example and Example 1 is that on both sides of the negative electrode sheet, the negative electrode active layers are provided with a plurality of gradient units with a decreasing mass content gradient of polyethyl acrylate in the D1 and D2 directions, having a double-sided gradient design.

[0249] That is, in this example, the second negative electrode active layer of the negative electrode sheet has the same structure as the first negative electrode active layer, including four gradient units with mass contents of polyethyl acrylate of 5%, 4%, 4%, and 3% in sequence. The distribution positions of these four gradient units in the second negative electrode active layer are symmetrical to the first gradient unit 111 and the second gradient unit 112, the third gradient unit 113 and the fourth gradient unit 114, as Figure 3 shown.

[0250] Example 3

[0251] The difference between this example and Example 2 is that on one side of the positive electrode sheet, the positive electrode active layer is provided with a plurality of gradient units with a decreasing mass content gradient of polyethyl acrylate in the D1 and D2 directions, having a single-sided gradient design.

[0252] Specifically, in this example, the positive electrode sheet includes a positive electrode current collector aluminum foil with a thickness of 15 μm and a positive electrode active layer with a total thickness of 80 μm provided on both surfaces of the positive electrode current collector. At the same time, a positive electrode tab is provided on one side in the height direction of the positive electrode current collector.

[0253] Referring to Figure 4 , the positive electrode active layer (the first positive electrode active layer 21 with a thickness of 40 μm) on one side of the positive electrode current collector 20 includes, in the thickness direction of the positive electrode sheet, the stacked a-gradient unit 211 and b-gradient unit 212, and the stacked c-gradient unit 213 and d-gradient unit 214. Among them, the a-gradient unit 211 and the c-gradient unit 213 are close to the positive electrode current collector 20, and the a-gradient unit 211 and the c-gradient unit 213 are arranged side by side in the height direction of the positive electrode current collector 20, and the a-gradient unit 211 is close to the top of the positive electrode current collector 20; the b-gradient unit 212 and the d-gradient unit 214 are arranged side by side in the height direction of the positive electrode current collector 20, and the b-gradient unit 212 is close to the top of the positive electrode current collector 20.

[0254] The a-gradient unit 211, the b-gradient unit 212, the c-gradient unit 213, and the d-gradient unit 214 have the same size (the same length, width, and thickness), and all contain ethyl polyacrylate, NCM111, 3 wt% PVDF, and 1 wt% SP. The total mass content of ethyl polyacrylate and NCM111 is 96 wt%, and the mass content of ethyl polyacrylate in the a-gradient unit 211, the b-gradient unit 212, the c-gradient unit 213, and the d-gradient unit 214 is 5%, 4%, 4%, and 3% in sequence.

[0255] The positive electrode active layer (the second positive electrode active layer, with a thickness of 40 μm, Figure 4 not shown in the figure) on the other side of the positive electrode current collector contains 96 wt% NCM111, 3 wt% PVDF, and 1 wt% SP.

[0256] Example 4

[0257] The difference between this example and Example 3 is that the positive electrode active layers on both sides of the positive electrode sheet are provided with a plurality of gradient units with a decreasing mass content gradient of ethyl polyacrylate in both the D1 and D2 directions, having a double-sided gradient design.

[0258] That is, in this example, the second positive electrode active layer of the positive electrode sheet has the same structure as the first positive electrode active layer, and includes four gradient units with the mass content of ethyl polyacrylate being 5%, 4%, 4%, and 3% in sequence. The distribution positions of these four gradient units in the positive electrode sheet are symmetrical to those of the a-gradient unit 211, the b-gradient unit 212, the c-gradient unit 213, and the d-gradient unit 214, as Figure 5 shown.

[0259] Example 5

[0260] The difference between this example and Example 4 is that in the negative electrode sheet, two additional gradient units with the mass content of ethyl polyacrylate being 6% and 5% in sequence are added to both the first negative electrode active layer and the second negative electrode active layer; meanwhile, in the positive electrode sheet, two additional gradient units with the mass content of ethyl polyacrylate being 6% and 5% in sequence are added to both the first positive electrode active layer and the second positive electrode active layer.

[0261] Specifically, the structures of the negative electrode sheet and the positive electrode sheet in this example are as follows:

[0262] 1) Negative electrode sheet

[0263] Refer to Figure 6, in this embodiment, the first negative electrode active layer 11 (with a thickness of 30 μm) on one side of the negative electrode current collector 10 includes, in the thickness direction of the negative electrode sheet, the stacked first gradient unit 111 and second gradient unit 112, the stacked third gradient unit 113 and fourth gradient unit 114, and the stacked fifth gradient unit 115 and sixth gradient unit 116. Among them, the first gradient unit 111, the third gradient unit 113, and the fifth gradient unit 115 are close to the negative electrode current collector 10, and the first gradient unit 111, the third gradient unit 113, and the fifth gradient unit 115 are arranged in sequence along the height direction of the negative electrode current collector 10, and the first gradient unit 111 is close to the top of the negative electrode current collector 10; the second gradient unit 112, the fourth gradient unit 114, and the sixth gradient unit 116 are arranged side by side along the height direction of the negative electrode current collector 10, and the second gradient unit 112 is close to the top of the negative electrode current collector 10.

[0264] The first gradient unit 111, the second gradient unit 112, the third gradient unit 113, the fourth gradient unit 114, the fifth gradient unit 115, and the sixth gradient unit 116 have the same size (the same length, width, and thickness), and all contain ethyl acrylate, graphite, 2.5 wt% SBR, 0.5 wt% SP, and 1.2 wt% CMC. Among them, the total mass content of ethyl acrylate and graphite is 95.8 wt%, and the mass content of ethyl acrylate in the first gradient unit 111 and the second gradient unit 112, the third gradient unit 113, the fourth gradient unit 114, the fifth gradient unit 115, and the sixth gradient unit 116 is 6%, 5%, 5%, 4%, 4%, and 3% in sequence.

[0265] The second negative electrode active layer on the other side of the negative electrode current collector 10 has a structure symmetrical to that of its first negative electrode active layer.

[0266] 2) Positive electrode sheet

[0267] Refer to Figure 7 , in the positive electrode sheet of this embodiment, the positive electrode active layer (the first positive electrode active layer 21, with a thickness of 40 μm) on one side of the positive electrode current collector 20 includes, in the thickness direction of the positive electrode sheet, the stacked a-gradient unit 211 and b-gradient unit 212, the stacked c-gradient unit 213 and d-gradient unit 214, and the stacked e-gradient unit 215 and f-gradient unit 216. Among them, the a-gradient unit 211, the c-gradient unit 213, and the e-gradient unit 215 are close to the positive electrode current collector 20, and the a-gradient unit 211, the c-gradient unit 213, and the e-gradient unit 215 are arranged in sequence along the height direction of the positive electrode current collector 20, and the a-gradient unit 211 is close to the top of the positive electrode current collector 20; the b-gradient unit 212, the d-gradient unit 214, and the d-gradient unit 214 are arranged side by side along the height direction of the positive electrode current collector 20, and the b-gradient unit 212 is close to the top of the positive electrode current collector 20.

[0268] The first gradient unit 211, the second gradient unit 212, the third gradient unit 213, the fourth gradient unit 214, the fifth gradient unit 215 and the sixth gradient unit 216 have the same size, and each contains ethyl polyacrylate, NCM111, 3 wt% PVDF and 1 wt% SP. The total mass content of ethyl polyacrylate and NCM111 is 96 wt%, and the mass content of ethyl polyacrylate in the first gradient unit 211, the second gradient unit 212, the third gradient unit 213, the fourth gradient unit 214, the fifth gradient unit 215 and the sixth gradient unit 216 is 6%, 5%, 5%, 4%, 4% and 3% in sequence.

[0269] The second positive electrode active layer on the other side of the positive electrode current collector 20 has a structure symmetric to that of its first positive electrode active layer.

[0270] Example 6

[0271] The difference between this example and Example 4 is that the injection coefficient of the electrolyte is 2 g / Ah.

[0272] Comparative Example 1

[0273] The difference between this comparative example and Examples 1 to 5 is that the negative electrode sheet, the positive electrode sheet and the separator do not contain ethyl polyacrylate.

[0274] That is, in this comparative example, the negative electrode sheet includes a copper foil with a thickness of 8 μm and negative electrode active layers provided on both surfaces of the copper foil. The total thickness of the two negative electrode active layers is 60 μm, and each contains 95.8 wt% graphite, 2.5 wt% styrene-butadiene rubber SBR, 0.5 wt% SP and 1.2 wt% CMC.

[0275] The positive electrode sheet includes an aluminum foil with a thickness of 15 μm and positive electrode active layers provided on both surfaces of the aluminum foil. The total thickness of the two positive electrode active layers is 80 μm, and each contains 96 wt% NCM111, 3 wt% PVDF and 1 wt% SP.

[0276] Comparative Example 2

[0277] The difference between this comparative example and Example 5 is that the negative electrode active layer and the positive electrode active layer each have a plurality of gradient units with a decreasing mass content of ethyl polyacrylate only in the D1 direction, and there are no such plurality of gradient units with a decreasing mass content of ethyl polyacrylate in the D2 direction; at the same time, the separator coating of the separator does not contain ethyl polyacrylate.

[0278] Among them, the structures of the negative electrode sheet and the positive electrode sheet in this comparative example are as follows:

[0279] 1) Negative electrode sheet

[0280] Reference Figure 8, in this comparative example, the first negative electrode active layer 11 (with a thickness of 30 μm) on one side of the negative electrode current collector 10 of the negative electrode sheet includes a first gradient unit 111, a third gradient unit 113, and a fifth gradient unit 115, and the first gradient unit 111, the third gradient unit 113, and the fifth gradient unit 115 are arranged side by side in the height direction of the negative electrode current collector 10, and the first gradient unit 111 is close to the top of the negative electrode current collector 10.

[0281] The first gradient unit 111, the third gradient unit 113, and the fifth gradient unit 115 have the same size (the same length, width, and thickness), and all contain ethyl polyacrylate, graphite, 2.5 wt% SBR, 0.5 wt% SP, and 1.2 wt% CMC. Among them, the total mass content of ethyl polyacrylate and graphite is 95.8 wt%, and the mass content of ethyl polyacrylate in the first gradient unit 111, the third gradient unit 113, and the fifth gradient unit 115 is 6%, 5%, and 4% in sequence.

[0282] The second negative electrode active layer on the other side of the negative electrode current collector 10 has a structure symmetric to that of its first negative electrode active layer.

[0283] 2) Positive electrode sheet

[0284] Refer to Figure 9 , in the positive electrode sheet of this comparative example, the positive electrode active layer (the first positive electrode active layer 21, with a thickness of 40 μm) on one side of the positive electrode current collector 20 includes a first a gradient unit 211, a third c gradient unit 213, a fourth d gradient unit 214, and a fifth e gradient unit 215, and the first a gradient unit 211, the third c gradient unit 213, and the fifth e gradient unit 215 are arranged in sequence in the height direction of the positive electrode current collector 20, and the first a gradient unit 211 is close to the top of the positive electrode current collector 20.

[0285] The first a gradient unit 211, the third c gradient unit 213, and the fifth e gradient unit 215 have the same size, and all contain ethyl polyacrylate, NCM111, 3 wt% PVDF, and 1 wt% SP. Among them, the total mass content of ethyl polyacrylate and NCM111 is 96 wt%, and the mass content of ethyl polyacrylate in the first a gradient unit 211, the third c gradient unit 213, and the fifth e gradient unit 215 is 6%, 5%, and 4% in sequence.

[0286] The second positive electrode active layer on the other side of the positive electrode current collector 20 has a structure symmetric to that of its first positive electrode active layer.

[0287] Comparative Example 3

[0288] The difference between this comparative example and Example 5 is that both the negative electrode active layer and the positive electrode active layer are provided with a plurality of gradient units with a decreasing mass content of ethyl polyacrylate only in the D2 direction, and there are no such plurality of gradient units with a decreasing mass content of ethyl polyacrylate in the D1 direction; meanwhile, the separator coating of the separator contains uniformly distributed ethyl polyacrylate.

[0289] Specifically, the structures of the negative electrode sheet, positive electrode sheet, and separator in this comparative example are as follows:

[0290] 1) Negative electrode sheet

[0291] Reference Figure 10 , in this comparative example, the first negative electrode active layer 11 (with a thickness of 30 μm) on one side of the negative electrode current collector 10 includes a first gradient unit 111 and a second gradient unit 112 stacked in the thickness direction of the negative electrode sheet, where the first gradient unit 111 is close to the negative electrode current collector 10.

[0292] The first gradient unit 111 and the second gradient unit 112 have the same size and both contain ethyl polyacrylate, graphite, 2.5 wt% SBR, 0.5 wt% SP, and 1.2 wt% CMC. The total mass content of ethyl polyacrylate and graphite is 95.8 wt%, and the mass contents of ethyl polyacrylate in the first gradient unit 111 and the second gradient unit 112 are 4% and 3% respectively.

[0293] The second negative electrode active layer on the other side of the negative electrode current collector 10 has a structure symmetric to that of its first negative electrode active layer.

[0294] 2) Positive electrode sheet

[0295] Referring to Figure 10 , in the positive electrode sheet of this comparative example, the first positive electrode active layer 21 (with a thickness of 40 μm) on one side of the positive electrode current collector 20 includes a stacked a-gradient unit 211 and b-gradient unit 212 in the thickness direction of the positive electrode sheet, where the a-gradient unit 211 is close to the positive electrode current collector 20.

[0296] The a-gradient unit 211 and the b-gradient unit 212 have the same size and both contain ethyl polyacrylate, NCM111, 3 wt% PVDF, and 1 wt% SP. The total mass content of ethyl polyacrylate and NCM111 is 96 wt%, and the mass contents of ethyl polyacrylate in the a-gradient unit 211 and the b-gradient unit 212 are 4% and 3% respectively.

[0297] The second positive electrode active layer on the other side of the positive electrode current collector 20 has a structure symmetric to that of its first positive electrode active layer.

[0298] 3) Separator

[0299] Reference Figure 10, the separator of this comparative example includes a base film (a PE film with a thickness of 12 μm), and a separator coating with a thickness of 3 μm provided on one side of the base film. The separator coating 31 contains 2% by mass of ethyl polyacrylate.

[0300] Comparative Example 4

[0301] The difference between this comparative example and Comparative Example 1 is that the liquid injection coefficient of the electrolyte is 2 g / Ah.

[0302] The gradient unit designs of the positive electrode sheets, positive electrode sheets and separators in each example and comparative example are shown in the following table.

[0303] [Table 1]

[0304]

[0305] In Table 1, the "gradient design direction" refers to the gradient distribution direction of the mass content of ethyl polyacrylate in the positive electrode sheet or the negative electrode sheet, including D1 and D2.

[0306] "Single-sided" means that the active layer on one side of the positive electrode sheet or the negative electrode sheet is designed with a gradient unit in which the mass content of ethyl polyacrylate shows a gradient distribution, and "double-sided" means that the active layers on both sides of the positive electrode sheet or the negative electrode sheet are designed with gradient units in which the mass content of ethyl polyacrylate shows a gradient distribution.

[0307] "Gradient mass content of ethyl polyacrylate" means the mass content of ethyl polyacrylate contained in each gradient unit in the positive electrode sheet, negative electrode sheet or separator. In Table 1, the vertical data in the "gradient mass content of ethyl polyacrylate" cell represents the mass content distribution of ethyl polyacrylate along the D1 direction, and the horizontal data represents the mass content distribution of ethyl polyacrylate along the D2 direction.

[0308] [Liquid absorption capacity]

[0309] The liquid absorption capacities of the gradient units with different mass contents of ethyl polyacrylate in the positive electrode sheet, negative electrode sheet and separator are tested. To make the test operation more convenient and the results more intuitive, the following is to first prepare complete positive electrode sheets, negative electrode sheets or separators corresponding to each gradient unit, and then conduct the liquid absorption capacity test. The test method is as follows.

[0310] Sample preparation:

[0311] 1) Mix ethyl acrylate, graphite, SBR, SP, CMC and water to form a negative electrode slurry. Coat the two surfaces of a copper foil (8 μm thick) with this negative electrode slurry, dry it and then perform cold pressing to form a negative electrode active layer with a thickness of 30 μm on both surfaces of the copper foil. The content of each component in the negative electrode slurry is as follows: 2.5 wt% SBR, 0.5 wt% SP and 1.2 wt% CMC. The total mass content of ethyl acrylate and graphite is 95.8 wt%. The mass content of ethyl acrylate is 6%, 5%, 4%, 3%, 0% in sequence, obtaining samples 1, 2, 3, 4 and 5 corresponding to each gradient unit with different mass contents of ethyl acrylate in the negative electrode sheet, and the mass of each sample is the same.

[0312] 2) Mix NCM111, PVDF, SP and NMP (N-methylpyrrolidone) to form a positive electrode slurry. Coat the two surfaces of an aluminum foil (15 μm thick) with this positive electrode slurry, dry it and then perform cold pressing to form a positive electrode active layer with a thickness of 40 μm on both surfaces of the aluminum foil. The content of each component in the positive electrode slurry is as follows: 3 wt% PVDF and 1 wt% SP. The total mass content of ethyl acrylate and NCM111 is 96 wt%. The mass content of ethyl acrylate is 6%, 5%, 4%, 3%, 0% in sequence, obtaining samples a, b, c, d and e corresponding to each gradient unit with different mass contents of ethyl acrylate in the positive electrode sheet, and the mass of each sample is the same.

[0313] 3) Coat a separator coating with a thickness of 3 μm on a PE film with a thickness of 12 μm, and control the mass content of ethyl acrylate in the separator coating to be 2%, 1%, 0%, obtaining samples A, B and C corresponding to each gradient unit with different mass contents of ethyl acrylate in the separator, and the mass of each sample is the same.

[0314] Liquid absorption capacity test:

[0315] Cut the sample to be tested into a size of 2*2 cm 2 Weigh to obtain the mass M1. At room temperature of 25 °C, soak it in the electrolyte (the electrolyte composition is the same as that in Example 1) for 1 h, take out the sample, and dry the excess electrolyte on the surface with a dust-free paper. Then weigh to obtain M2. Then the liquid absorption amount per unit time (1 h) of the sample is m (g)=M2 - M1. Then the liquid absorption capacity F = σ*m / THK. Where σ is the porosity of the sample, THK is the thickness of the sample (mm), and the dimension of F is g / mm.

[0316] [Table 2]

[0317]

[0318] The test results show that at different mass contents of polyethyl acrylate, the samples have different liquid absorption capacities for the electrolyte. Therefore, in Examples 1 to 5, the positive electrode sheet, the negative electrode sheet, and the separator contain polyethyl acrylate with a gradient distribution of mass content, so the positive electrode sheet, the negative electrode sheet, and the separator have a gradient distribution of liquid absorption capacity.

[0319] Specifically, in Examples 1 to 5, the positive electrode sheet, the negative electrode sheet, and the separator contain polyethyl acrylate with a gradually decreasing mass content along the D1 direction, and the mass content of polyethyl acrylate at the top of the positive electrode sheet, the negative electrode sheet, and the separator is higher than that at their respective bottoms. Correspondingly, the liquid absorption capacity at the top of the positive electrode sheet, the negative electrode sheet, and the separator is higher than that at their respective bottoms, and along the D1 direction, the liquid absorption capacity of each of the positive electrode sheet, the negative electrode sheet, and the separator gradually decreases.

[0320] At the same time, in Examples 1 to 5, the positive electrode sheet, the negative electrode sheet, and the separator contain polyethyl acrylate with a gradient distribution of mass content in the D2 direction, and the mass content of polyethyl acrylate in the positive electrode active layer and / or the negative electrode active layer closer to the current collector side is higher than that of the separator. Correspondingly, along the D2 direction of the positive electrode sheet, the negative electrode sheet, and the separator, their liquid absorption capacities are distributed in a gradient.

[0321] [Electrochemical performance]

[0322] 1. Electrochemical performance tests were carried out on the lithium-ion batteries of Examples 1 to 5 and Comparative Examples 1 to 3 (the liquid injection coefficient is 1.2 g / Ah for all), and the test results are shown in Table 3 and Table 4 below.

[0323] [Table 3]

[0324]

[0325] [Table 4]

[0326] Battery internal resistance change rate DC impedance change rate Cycle number change rate Example 1 -10.2％ -9.7％ +3.4％ Example 2 -16.4％ -22.6％ +5.0％ Example 3 -43.9％ -25.8％ +14.4％ Example 4 -64.8％ -29.0％ +17.2％ Example 5 -67.7％ -32.3％ +24.8％ Comparative Example 1 0 0 0 Comparative Example 2 -3.0％ -3.2％ +1.0％ Comparative Example 3 -3.5％ -3.2％ +1.3％

[0327] Among them, the change rates of each performance in Table 4 refer to the change rates calculated based on Table 3 compared with Comparative Example 1.

[0328] The test results show that compared with Comparative Example 1 where no gradient design of liquid absorption capacity is carried out in both the D1 and D2 directions, in Examples 1 to 5, by simultaneously setting gradient units with a gradually decreasing mass content of polyethyl acrylate in the positive electrode sheet, the negative electrode sheet, and the separator along the D1 and D2 directions, the electrode assembly has a gradually decreasing liquid absorption capacity along these two directions, which can reduce the internal resistance and DC impedance of the battery, increase the number of battery cycles, and improve the cycling performance of the battery. Moreover, as the number of gradient units increases, the internal resistance and DC impedance of the battery are lower, and the cycling performance of the battery is better.

[0329] The improvement of battery performance in Examples 1 to 5 is mainly due to the fact that the liquid absorption capacity of the positive electrode sheet, negative electrode sheet, and separator at the top is greater than that at the bottom, which can make up for the problem that during long-term storage and use, the electrolyte tends to concentrate at the bottom of the electrode assembly due to gravity, resulting in poor wetting of the top of the electrode assembly. At the same time, the liquid absorption capacity of the positive electrode sheet and / or negative electrode sheet on the side close to the current collector is greater than that on the separator side, making up for the problem that during charge and discharge of the positive electrode sheet and negative electrode sheet, the volume expansion or contraction squeezes out the electrolyte, resulting in a decrease in the wetting of the electrolyte on the side of the electrode sheet close to the current collector or difficulty in wetting, and improving the wetting of the electrolyte inside the electrode sheet. The improvement of electrolyte wetting is beneficial to promoting the electrochemical reaction in the battery, accelerating the transmission of lithium ions, thereby reducing the internal resistance and DC impedance of the battery; and after long-term cycling, good electrolyte wetting is maintained at the top of the electrode assembly and inside the electrode sheet, thereby improving the cycling performance of the battery.

[0330] 2. Electrochemical performance tests were carried out on the lithium-ion batteries of Example 6 and Comparative Example 4 (the injection coefficient was 2 g / Ah for both), and the test results are shown in Tables 5 and 6 below.

[0331] [Table 5]

[0332]

[0333] [Table 6]

[0334] Battery internal resistance change rate DC impedance change rate Cycle number change rate Example 6 -5.9％ -5.3％ +2.7％ Comparative Example 4 0 0 0

[0335] Among them, the change rates of each performance in Table 6 refer to the change rates calculated from Table 5 compared with Comparative Example 4.

[0336] The test results show that at a high injection coefficient, the internal resistance, DC impedance, and cycling performance of the battery in Example 6 are also improved compared with Comparative Example 4.

[0337] Moreover, by combining and analyzing Table 6 and Table 4, it can be seen that at a low injection coefficient of 1.2 g / Ah, the internal resistance of the battery in Example 4 is reduced by 64.8% compared with Comparative Example 1, the DC impedance is reduced by 29.0%, and the number of cycles is increased by 17.2%; and in the case of the same electrode assembly structure, Example 6 uses a higher injection coefficient of 2 g / Ah, and its internal resistance is reduced by 5.9% compared with Comparative Example 4, the DC impedance is reduced by 5.3%, and the number of cycles is increased by 2.7%. The improvement effect of the battery performance in Example 4 at a low injection coefficient of 1.2 g / Ah is more obvious than that at a high injection coefficient of 2 g / Ah, indicating that by designing the positive electrode sheet, negative electrode sheet, and separator to have a gradient distribution of liquid absorption capacity in a specific direction, the improvement effect on the electrochemical performance of the battery with a low injection coefficient is better.

[0338] [Electrochemical Performance Test Method]

[0339] 1. Battery internal resistance IMP

[0340] At 25°C, charge the battery at a constant current of 0.33C to 4.2V, then switch to constant voltage charging at 4.2V until the current reaches 0.05C, and then discharge at 0.33C for 30 minutes. Then, use the positive and negative test leads of the TH2523A AC impedance tester to contact the positive and negative electrodes of the battery respectively, and test under an alternating current of 1000Hz, and directly read the battery internal resistance IMP from the test instrument.

[0341] 2. DC impedance

[0342] At 25°C, charge the battery at a constant current of 0.33C to 4.2V, then switch to constant voltage charging at 4.2V until the current reaches 0.05C, and then discharge at 0.33C to 2.5V, and record the discharge capacity C 0 . Continue to discharge at 0.33C until the battery discharge capacity reaches 30% of C 0 , and record the voltage V 1 ; then discharge at 2C (I 1 ) for 30s, and record the voltage V 2 . Divide the voltage drop during this discharge period by the current to calculate the DC impedance, that is, DCR(30%SOC DCR@2C 30s) = (V 1 - V 2 ) / I 1 .

[0343] 3. Cycling performance

[0344] At 25°C, charge the battery at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V until the current reaches 0.05C, and then discharge at 1C to 2.5V, and record the initial discharge capacity C 1 , and this is one charge-discharge process. Repeat the charge-discharge cycle like this until the capacity retention rate decays to 80% of the initial discharge capacity C 1 , and then stop the test and record the number of cycles.

[0345] 4. Porosity

[0346] The test method for the porosity σ of the sample involved in the liquid absorption capacity test is as follows: Cut 20 circular samples with a diameter of 14mm from the sample, record the weight and thickness, and calculate the volume density. Place the circular samples in the sample cup of the true density porosity tester, use helium replacement, and combine Archimedes' law and Boyle's law to obtain the true volume of the electrode sheet, and then calculate the true density. The porosity is equal to (1 - volume density / true density) * 100%.

[0347] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be covered within the scope of the claims and the specification of the present application. In particular, as long as there is no structural conflict, the technical features mentioned in each embodiment can be combined in any way. The present application is not limited to the specific embodiments disclosed in the text, but includes all technical solutions that fall within the scope of the claims.

Claims

1. An electrode assembly, characterized in that: The electrode assembly comprises a pole piece, the pole piece comprises a current collector, an active layer and a pole ear, the active layer is arranged on at least one side in the thickness direction of the current collector, and the pole ear is arranged on one side in the height direction of the current collector; In the direction from the side of the current collector where the tab is provided to the other side opposite thereto, the liquid absorption capacity of the region of the active layer close to the tab side is greater than or equal to the liquid absorption capacity of the region on the other side opposite thereto, and the liquid absorption capacity of at least one region of the active layer close to the tab side is greater than the liquid absorption capacity of the region on the other side opposite thereto; In the direction from the current collector to away from the current collector, the liquid absorption capacity of the area of ​​the active layer close to the current collector is greater than or equal to the liquid absorption capacity of the other side area, and the liquid absorption capacity of at least one area of ​​the active layer close to the current collector is greater than the liquid absorption capacity of the other side area.

2. The electrode assembly according to claim 1, characterized in that: In the direction from the side of the current collector where the tab is provided to the other side opposite thereto, the liquid absorption capacity of the region near the tab side of the separator contained in the electrode assembly is greater than or equal to the liquid absorption capacity of the region on the other side opposite thereto, and the liquid absorption capacity of at least one region of the separator near the tab side is greater than the liquid absorption capacity of the region on the other side opposite thereto; In a direction from the current collector to away from the current collector, the liquid absorption capacity of the active layer is greater than the liquid absorption capacity of the separator.

3. The electrode assembly according to claim 1 or 2, characterized in that: In the direction from one side of the electrode ear where the current collector is set to the other opposite side, at least one of the active layer and the diaphragm contained in the electrode assembly includes at least two adjacently arranged gradient units, wherein the liquid absorption capacity of the gradient unit close to the electrode ear is higher than the liquid absorption capacity of the gradient unit on the other opposite side; optionally, the number of the gradient units is 2 to 10.

4. The electrode assembly according to claim 3, characterized in that: The difference in liquid absorption capacity between two adjacent gradient units is 1.1 to 5 times, and optionally 1.2 to 2 times.

5. The electrode assembly according to claim 3 or 4, characterized in that: In the direction from one side of the current collector where the pole ear is arranged to the other opposite side, the height of any one of the gradient units accounts for 10% to 60% of the total height of the active layer or the separator, and optionally 10% to 50%.

6. The electrode assembly according to any one of claims 1 to 5, characterized in that: In the direction from the current collector to away from the current collector, the active layer includes at least two adjacently arranged liquid absorption layers, wherein the liquid absorption capacity of the liquid absorption layer close to the current collector is higher than the liquid absorption capacity of the liquid absorption layer on the other opposite side; optionally, the number of the liquid absorption layers is 2 to 10.

7. The electrode assembly according to claim 6, characterized in that: In the direction from the current collector to away from the current collector, the thickness of any one of the liquid absorbing layers accounts for 10% to 60% of the total thickness of the active layer arranged on one side of the current collector, and optionally 10% to 50%.

8. The electrode assembly according to any one of claims 1 to 7, characterized in that: The active layer comprises a first electrolyte absorber; in a direction from one side of the current collector where the pole ear is provided to the other side opposite thereto, the mass content of the first electrolyte absorber in a region of the active layer close to one side of the pole ear is greater than or equal to the mass content of the first electrolyte absorber in a region on the other side opposite thereto, and the mass content of the first electrolyte absorber in at least one region of the active layer close to one side of the pole ear is greater than the mass content of the first electrolyte absorber in a region on the other side opposite thereto; In the direction from the current collector to away from the current collector, the mass content of the first electrolyte absorber in the region of the active layer close to one side of the current collector is greater than or equal to the mass content of the first electrolyte absorber in the region on the other side, and the mass content of the first electrolyte absorber in at least one region of the active layer close to one side of the current collector is greater than the mass content of the first electrolyte absorber in the region on the other side.

9. The electrode assembly according to claim 8, characterized in that: The diaphragm contained in the electrode assembly comprises a base film and a diaphragm coating disposed on at least one side of the base film; the diaphragm coating comprises a second electrolyte absorbent; In the direction from one side of the pole ear to the other opposite side of the current collector, the mass content of the second electrolyte absorber in the area close to one side of the pole ear of the diaphragm coating is greater than or equal to the mass content of the second electrolyte absorber in the area on the other opposite side, and the mass content of the second electrolyte absorber in at least one area close to one side of the pole ear of the diaphragm coating is greater than the mass content of the second electrolyte absorber in the area on the other opposite side.

10. The electrode assembly according to claim 9, characterized in that: In a direction from the current collector to away from the current collector, the mass content of the first electrolyte absorber in the active layer is greater than the mass content of the second electrolyte absorber in the diaphragm coating.

11. The electrode assembly according to claim 9 or 10, characterized in that: The first electrolyte absorbent and the second electrolyte absorbent independently include one or more of polymethyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyvinyl acetate, polyacrylamide, polyacrylic acid, and polyacrylonitrile.

12. The electrode assembly according to any one of claims 9 to 11, characterized in that: The mass content of the first electrolyte absorber in the active layer and the mass content of the second electrolyte absorber in the diaphragm coating are independently greater than 0 and less than or equal to 8%; optionally greater than or equal to 1% and less than or equal to 6%.

13. A method for preparing an electrode assembly, characterized in that: include: An active layer is prepared on at least one side of the current collector in the thickness direction, and a tab is provided on one side of the current collector in the height direction; In the direction from the side of the current collector where the tab is provided to the other side opposite thereto, the liquid absorption capacity of the region of the active layer close to the tab side is controlled to be greater than or equal to the liquid absorption capacity of the region on the other side opposite thereto, and the liquid absorption capacity of at least one region of the active layer close to the tab side is greater than the liquid absorption capacity of the region on the other side opposite thereto; In the direction from the current collector to away from the current collector, the liquid absorption capacity of the area of ​​the active layer close to the current collector is controlled to be greater than or equal to the liquid absorption capacity of the other side area, and the liquid absorption capacity of at least one area of ​​the active layer close to the current collector is greater than the liquid absorption capacity of the other side area.

14. The method for preparing an electrode assembly according to claim 13, characterized in that: The method for preparing the electrode assembly further includes: In the direction from the side of the current collector where the electrode tab is provided to the other side opposite thereto, the liquid absorption capacity of the region near the electrode tab of the diaphragm contained in the electrode assembly is controlled to be greater than or equal to the liquid absorption capacity of the region on the other side opposite thereto, and the liquid absorption capacity of at least one region of the diaphragm near the electrode tab is greater than the liquid absorption capacity of the region on the other side opposite thereto; In a direction from the current collector to away from the current collector, the liquid absorption capacity of the active layer is controlled to be greater than the liquid absorption capacity of the separator.

15. A battery, characterized in that: The battery comprises the electrode assembly according to any one of claims 1 to 12.

16. An electrical device, characterized in that: The electrical device comprises the battery according to claim 15.

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