Battery cell and lithium ion battery

Abstract

The invention provides a battery cell. The battery cell comprises a positive pole piece, a diaphragm and a negative pole piece which are sequentially stacked, the positive pole pieces and the negative pole pieces are alternately arranged, and the diaphragms are arranged between the adjacent positive pole pieces and negative pole pieces; the battery cell is provided with a first outermost side and a second outermost side along the thickness direction of the battery cell; the diaphragms sequentially comprise a first diaphragm, a second diaphragm, a third diaphragm,..., and an nth diaphragm from the first outermost side or the second outermost side to the middle part of the battery cell, and the total number of the diaphragms is 2n or 2n + 1; the porosity of the first separator is greater than the porosity of the nth separator. According to the battery cell disclosed by the invention, a lithium ion transmission channel of the outer-layer pole piece of the laminated battery cell structure can be effectively improved, and the problem of lithium precipitation at the edge of the outer-layer pole piece and the problem of local overheating of the battery cell are relieved, so that the cycle performance of the battery cell is effectively improved, the thickness change rate of the battery cell is improved, and the service life of the battery cell is prolonged.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a battery cell and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries using a winding process have lower space utilization due to the curvature at the winding corners, while stacked structures achieve higher volumetric energy density due to their efficient use of internal space. However, stacked cells also have certain drawbacks. The most prominent is that the stacking process lacks stretching restraint on the current collector and separator, resulting in weak contact restraint between each electrode layer. During repeated use, the stress release in different areas of the electrode can easily cause deformation, leading to uneven lithium-ion insertion and extraction. This is especially true for the outermost electrode layer in a stacked cell, where the outermost single-sided electrode and separator need to be separately laminated. When this laminated single-sided module is then stacked with the double-sided positive and negative electrode separator module, the adhesion strength between the separators is insufficient. This causes the separator's restraint on the outermost electrode layer to gradually weaken during cycling, making it prone to black spots and lithium plating on the electrode. It can also cause uneven local heat generation, thus affecting the cell's cycle life. Summary of the Invention

[0003] To address the aforementioned problems in the prior art, this invention provides a battery cell and a lithium-ion battery. The battery cell of this invention can effectively improve the lithium-ion transport channels of the outer electrode layer in a stacked battery cell structure, alleviate lithium plating problems at the edges of the outer electrode layer, and reduce localized overheating issues within the battery cell. This effectively improves the cycle performance of the battery cell, reduces the thickness variation rate of the battery cell, and ultimately extends the battery cell's lifespan.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] A first aspect of the present invention provides a battery cell comprising a positive electrode, a separator, and a negative electrode stacked sequentially; the positive electrode, the separator, and the negative electrode each have multiple layers, the positive electrode and the negative electrode are alternately arranged, and the separator is provided between adjacent positive electrode and negative electrode;

[0006] Along the thickness direction of the battery cell, the battery cell has a first outermost edge and a second outermost edge;

[0007] The separator, from the first outermost or the second outermost to the middle of the cell, includes a first separator, a second separator, a third separator, ..., an nth separator, wherein the total number of separators is 2n or 2n+1;

[0008] The porosity of the first diaphragm is greater than that of the nth diaphragm.

[0009] A second aspect of the present invention provides a lithium-ion battery, the lithium-ion battery comprising the cell described in the first aspect of the present invention.

[0010] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0011] This invention provides a battery cell and a lithium-ion battery. The battery cell of this invention, by adjusting the porosity of the separators at different locations in the cell structure, such that the porosity of the first separator is greater than that of the nth separator, can effectively improve the lithium-ion transport channels of the outermost electrode in the stacked battery cell structure, alleviate lithium plating problems at the edge of the outermost electrode, and reduce localized overheating issues within the cell. This effectively improves the cycle performance of the battery cell, reduces the thickness variation rate, and ultimately extends the battery cell's lifespan.

[0012] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0013] Figure 1 Schematic diagram of the battery cell provided by the present invention Figure 1 ;

[0014] Figure 2 Schematic diagram of the battery cell provided by the present invention Figure 2 .

[0015] Explanation of reference numerals in the attached figures:

[0016] 101-Positive electrode, 102-Separator, 103-Negative electrode, 104-First separator, 105-Nth separator, 106-First electrode, 107-Nth electrode. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0018] The outermost positive and negative electrode sheets and the separator are fabricated separately. The bonding between the separator and the positive and negative electrode sheets relies on the contact of the entire surface. Therefore, the overhang areas around the edges of the electrode sheets are prone to loose bonding. When all the electrode sheets are finally integrated, they are prone to loosening due to poor contact. This manifests as localized lithium plating on one side of the outermost electrode sheet during charging and discharging due to the large diffusion distance of lithium ions, especially at the edges of the electrode sheet. In the cell structure, the positive and negative electrode sheets located in the middle layer along the thickness direction are generally sandwiched between the separators on both sides. Whether in the middle area or at the edges, they will melt and bond with the separators on both sides under the high temperature and pressure of the fabrication process. Therefore, the bonding strength between the positive and negative electrode sheets in the middle layer and the separator is better than that between the positive and negative electrode sheets in the outer layer and the separator. This causes the binding force of the outer separator on the positive and negative electrode sheets to gradually weaken. In the later stages of cycling, the stress release and deformation in different areas of the electrode sheets will cause uneven lithium ion insertion and extraction, thus causing black spot lithium plating and affecting the cycle life of the cell.

[0019] Therefore, in order to improve the above problems, the present invention provides a battery cell and a lithium-ion battery including the battery cell. Using the battery cell of the present invention can effectively improve the lithium-ion transport channel of the outer electrode of the stacked battery cell structure, alleviate the lithium plating problem at the edge of the outer electrode and the local overheating problem of the battery cell, thereby effectively improving the cycle performance of the battery cell, improving the thickness change rate of the battery cell, and thus extending the service life of the battery cell.

[0020] The first aspect of the present invention provides a battery cell 100, such as Figure 1 As shown, it includes a positive electrode 101, a separator 102 and a negative electrode 103 stacked in sequence; the positive electrode 101 and the negative electrode 103 are alternately arranged, and the separator 102 is provided between adjacent positive electrode 101 and negative electrode 103.

[0021] Furthermore, such as Figure 2 As shown, along the thickness direction of the battery cell, the battery cell has a first outermost layer and a second outermost layer; the separator 102, from the first outermost layer or the second outermost layer to the middle of the battery cell 100, sequentially includes a first separator 104, a second separator, a third separator, ..., an nth separator 105. The total number of diaphragms is 2n or 2n+1 , ;

[0022] The first separator is located at both the first outermost and the second outermost sides of the cell along the thickness direction. That is, any separator located on either side of the outermost two sides of the cell along the thickness direction can be called the first separator. Similarly, the second separator, the third separator, ... the (n-1)th separator can be called the second separator, the third separator, ... the (n-1)th separator along the thickness direction of the cell.

[0023] The nth separator 105 is located at the center of the separators sequentially included along the thickness direction of the cell from the first outermost or second outermost layer, i.e., at the middle layer of the cell 100. Furthermore, it should be noted that when the total number of separators is 2n, the number of layers of the nth separator is 2, meaning different sides of the cell 100 include different nth separators, and two nth separator layers 105 are arranged adjacent to each other in the middle of the cell, such as... Figure 2 As shown; when the total number of separators is 2n+1, the number of layers of the nth separator is 1, that is, different sides of the cell 100 include the same nth separator;

[0024] Furthermore, the porosity of the first membrane is greater than that of the nth membrane; that is, the porosity of the membrane closest to the outermost part of the cell structure is greater than that of the membrane located at the center of the cell structure. The specific method for testing the membrane porosity is as follows: using the mercury porosimeter method, mercury is forced to overcome surface tension and enter the pore structure of the base membrane in the membrane by external pressure, so as to determine the pore size and pore distribution of the base membrane in the membrane. Then, the porosity of the membrane is finally calculated by the pore volume of the pore structure.

[0025] This invention adjusts the porosity of the separator at different locations within the battery cell structure, ensuring that the porosity of the separator near the outermost edge of the cell structure is greater than that of the separator at the center. This increases the number of ion channels for lithium-ion transport in the outer electrode, preventing uneven lithium-ion insertion / extraction due to long ion channel distances. It also improves lithium deposition at the edges of the outer electrode in stacked battery cells, effectively enhancing cycle performance, reducing thickness variation, and extending cycle life. Furthermore, by adjusting the porosity of the separator at different locations, the invention ensures that lithium-ion insertion / extraction units near the outermost electrode and those at the center exhibit similar dynamics, effectively mitigating uneven local heat generation within the cell, improving thermal stability and safety, and extending the cell's lifespan.

[0026] In one specific implementation, n = 4 to 20 (e.g., 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20).

[0027] In one specific embodiment, the porosity a1 of the first diaphragm is 40% to 60%; for example, the porosity a1 of the first diaphragm can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, or 60%. The first separator is located on the outermost layer along the thickness direction of the cell, and its porosity has the most significant impact on the cell performance. A suitable porosity of the outermost separator can simultaneously meet the requirements of electrolyte wetting and electronic insulation. Moreover, a moderate separator porosity also helps the separator withstand higher mechanical stress and volume changes during battery use. Therefore, by controlling the porosity a1 of the first separator within a suitable range, the electrochemical performance of the cell can be further improved. More importantly, by adjusting the porosity of the outermost separator within a specific high porosity range, the lithium-ion transport channels can be further improved, the edge lithium plating phenomenon caused by uneven insertion and extraction of ions can be mitigated, and the cycle performance of lithium-ion batteries can be enhanced. Furthermore, when the porosity a1 of the first separator is too large, lithium ions escape from the cathode side and enter the anode side too quickly, easily accumulating locally on the anode side, causing lithium dendrite precipitation, which can easily puncture the separator and create a safety hazard. When the porosity a1 of the first separator is too small, the difference between the porosity of the outer separator and the inner separator is small, which cannot improve the problem of uneven lithium ion insertion / extraction due to the long channel distance, thus affecting the improvement of lithium plating. In a preferred embodiment, the porosity a1 of the first separator is 40% to 48%.

[0028] In one specific embodiment, the porosity a2 of the nth diaphragm is 30% to 50%; for example, the porosity a2 of the nth diaphragm can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, or 50%. The porosity a2 of the nth separator is generally the smallest among all separators because it is located at the center of the cell along its thickness direction. Under the high temperature and pressure during cell fabrication, this separator will melt and adhere to the positive and negative electrode sheets on both sides, resulting in high adhesion and preventing lithium separation issues at the edges or in other parts. However, the porosity a2 of the nth separator cannot be too small. Excessive porosity restricts the flow of the electrolyte phase in the battery and reduces the migration channels for lithium ions, leading to increased polarization during charging and discharging, accelerated battery aging, and shortened cycle life. Therefore, by further controlling the porosity a2 of the nth separator within a suitable range, the electrochemical performance and cycle performance of the lithium-ion battery can be further improved, extending the cell's cycle life. In a preferred embodiment, the porosity a2 of the nth separator is 30% to 40%.

[0029] In one specific embodiment, the porosity a1 of the first diaphragm and the porosity a2 of the nth diaphragm satisfy the relationship 15% ≤ (2a1 + a2) / (n + 2) ≤ 55%; for example, the calculated result of (2a1 + a2) / (n + 2) can be 16%, 20%, 25%, 30%, 35%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, or 55%. In a preferred embodiment, 45% ≤ (2a1 + a2) / (n + 2) ≤ 55%. In this invention, by controlling the porosity a1 of the first separator to be greater than the porosity a2 of the nth separator, the lithium plating problem can be improved and the cycle performance of the lithium-ion battery can be enhanced. However, the porosity difference between the first separator a1 and the nth separator a2 should not be too large, and it is related to the number n of separators on any side along the cell thickness direction. Specifically, it needs to satisfy the relationship 15% ≤ (2a1 + a2) / (n + 2) ≤ 55%. When (2a1 + a2) / (n + 2) is greater than 55%, it indicates that the porosity a1 of the first separator and the porosity a2 of the nth separator are relatively large. Excessive porosity difference between the porosities a2 of the nth separator can lead to significant differences in rate performance between the positive and negative electrode units corresponding to different separator positions. This results in severe local polarization of the cell, accelerating electrode material degradation and shortening the cell's cycle life. Conversely, if (2a1+na2) / (n+2) is less than 15%, it indicates that the porosity difference between the first separator a1 and the nth separator a2 is too small. In this case, the problem of ion insertion / extraction distance in the outer electrode remains, and the improvement in lithium plating on the outer electrode is not significant enough. Therefore, by ensuring that the porosities a1 of the first separator and a2 of the nth separator satisfy the above relationship, the cell's cycle performance can be further improved.

[0030] In one specific embodiment, the porosity of the first separator, the second separator, the third separator, ..., the nth separator decreases sequentially, i.e., the porosity of the first separator > the porosity of the second separator > the porosity of the third separator > ... > the porosity of the nth separator. It should be noted that when the total number of separators is 2n, the porosity of the first outermost first separator and the second outermost first separator along the cell thickness direction can be the same or different. The same applies to the second separator, the third separator, ..., the nth separator on different sides, as long as the porosity of the first separator, the second separator, the third separator, ..., the nth separator on any side along the cell thickness direction decreases sequentially. In addition, when the total number of separators is 2n+1, the nth separator is also the innermost separator among the separators sequentially included on different sides along the cell thickness direction, so its porosity is the lowest among all separators.

[0031] The outermost electrode in the cell structure suffers from long ion channel distances and uneven insertion / extraction. While the next outermost electrode performs slightly better, it still exhibits the same issues. In fact, the closer the electrode is to the outermost electrode, the more severe the edge lithium plating problem becomes. Therefore, adjusting the porosity of the corresponding separator based on the location of different electrodes, allowing electrodes at different positions to be matched with separators of different porosities, can further improve the edge lithium plating phenomenon near the outermost electrode in a stacked cell structure. This effectively enhances the cell's cycle performance, reduces the cell's thickness variation rate, and extends its cycle life. Furthermore, by adjusting the porosity of the separator at different locations within the cell structure, the lithium-ion insertion / extraction units composed of different electrodes and separators at different locations can exhibit nearly identical kinetics. This further improves the problem of uneven local heat generation within the cell, enhancing the cell's thermal stability and safety performance, and extending its service life.

[0032] In this invention, the positive electrode includes a current collector and a coating on at least one side of the current collector surface; the positive electrode, from the first outermost or the second outermost to the middle of the cell, is sequentially a first electrode 106, a second electrode, a third electrode... the Nth electrode 107, where N = 3 to 21 (e.g., 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21), as shown. Figure 2 As shown, the porosity of the coating in the first electrode is greater than that in the coating in the Nth electrode. Furthermore, it should be noted that the porosity of the coating on the positive electrode can be tested by calculating the weight change of the electrode after immersion in the electrolyte.

[0033] This invention improves the cycle performance of the outermost electrode assembly unit of the battery cell by adjusting the pore structure of the active material in the outermost electrode coating to achieve higher porosity. This further improves the lithium-ion transport channels of the outermost electrode, providing more ion transport channels for lithium ions. Combined with a first separator with high porosity, this enhances the cycle performance of the outermost separator assembly unit of the battery cell.

[0034] In a preferred embodiment, the porosity of the coatings in the first electrode, second electrode, third electrode, ..., Nth electrode decreases sequentially, i.e., the porosity of the coating of the first electrode > the porosity of the coating of the second electrode > the porosity of the coating of the third electrode > ... > the porosity of the coating of the Nth electrode; moreover, it should be noted that the first electrode located on the outer side of the cell along the thickness direction and the first separator are stacked sequentially to form a single-sided separator assembly, while the Nth electrode located in the middle of the cell along the thickness direction and the nth separator and the corresponding negative electrode are stacked sequentially to form a double-sided separator assembly; the porosity of the coatings in the cell junction... In the later stages of cycling, the outermost first electrode and the first separator may become uneven in thickness due to insufficient adhesion. This invention addresses this issue by further adjusting the porosity of the coatings on the first, second, third, ... Nth electrode. This balances the problem of excessively long lithium-ion insertion / extraction distance caused by the uneven thickness of the separator components near the outermost layer, ultimately achieving uniform thickness of each separator component in the stacked cell structure during charging and discharging. This avoids lithium deposition at the edges of the outermost electrode of the cell structure, thereby improving the cycle performance and cycle life of the lithium-ion battery.

[0035] In one specific embodiment, the porosity of the coating in the first electrode is 20% to 35%; for example, the porosity of the coating in the first electrode can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, or 35%. A higher porosity in the first electrode is beneficial for improving the lithium plating problem of the outermost electrode and enhancing the cycle performance of the lithium-ion battery. However, the porosity of the coating in the first electrode should not be too high. When the porosity of the coating in the first electrode is greater than 35%, it may lead to an increased space for volume expansion and contraction of the electrode material during charging and discharging, thereby accelerating the shedding of active material, affecting cycle stability, and potentially shortening the cycle life of the battery due to the rapid degradation of the active material. Conversely, when the porosity of the coating in the first electrode is less than 20%, the lithium plating problem of the outer electrode of the cell cannot be better improved, and the cycle life of the lithium-ion battery will also be affected. In a preferred embodiment, the porosity of the coating in the first electrode is 25% to 30%.

[0036] In one specific embodiment, the porosity of the coating in the Nth electrode is 18% to 25%; for example, the porosity of the coating in the Nth electrode can be 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25%. The porosity of the coating in the Nth electrode should be smaller than that of the coating in the first electrode, but the porosity of the coating in the Nth electrode should not be too small. When the porosity of the coating in the Nth electrode is less than 18%, the limited lithium-ion transport will also lead to increased polarization during the charging and discharging process, accelerating battery aging and shortening cycle life. In a preferred embodiment, the porosity of the coating in the Nth electrode is 20% to 25%.

[0037] Furthermore, in this invention, the separator includes a base membrane and adhesive layers located on both sides of the base membrane; the areal density c1 of the adhesive layer of the first separator is 1.5 g / m³. 2 ~2.5g / m 2 For example, the areal density c1 of the adhesive layer of the first diaphragm can be 1.5 g / m³. 2 1.6g / m 2 1.7g / m 2 1.8g / m 2 1.9g / m 2 2g / m 2 2.1g / m 2 2.2g / m 2 2.3g / m 2 2.4g / m 2 2.5g / m 2 The areal density c2 of the adhesive layer of the nth diaphragm is 1.2 g / m³. 2 ~2g / m 2 For example, the areal density c2 of the adhesive layer of the nth diaphragm can be 1.2 g / m³. 2 1.3g / m 2 1.4g / m 2 1.5g / m 2 1.6g / m 2 1.7g / m 2 1.8g / m 2 1.9g / m 2 2g / m 2 In a preferred embodiment, the areal density c1 of the adhesive layer of the first diaphragm is 2 g / m³. 2 ~2.5g / m 2 The areal density c2 of the adhesive layer of the nth diaphragm is 1.5 g / m³. 2 ~2g / m 2The adhesive layers on both sides of the base membrane are coated with PVDF adhesive, and the areal density of the membrane adhesive layer is related to the concentration percentage of the PVDF adhesive. In addition, in one specific embodiment, a ceramic layer can be coated between the base membrane and the adhesive layer, and the ceramic layer is coated with ceramic adhesive.

[0038] The adhesive layer density of the separator determines the bonding strength between the separator and the positive and negative electrodes, and also determines the lithium-ion transport path during charging and discharging. When the adhesive layer density of the separator is too high, it means that there is too much adhesive on the separator surface, which will lead to a thicker swelling layer of the separator after high-temperature formation, making the lithium-ion transport path longer. However, the more adhesive on the separator surface, the better the contact between the separator and the positive and negative electrodes will be. Therefore, in order to improve the uneven lithium-ion insertion and extraction phenomenon of the positive and negative electrodes by combining the two characteristics, it can be achieved by further controlling the adhesive layer density of the separator, thereby improving the lithium plating caused by uneven local lithium insertion and extraction on the electrodes and improving the cycle life of lithium-ion batteries.

[0039] In this invention, the diaphragm includes a base membrane and adhesive layers located on both sides of the base membrane. The first diaphragm, second diaphragm, third diaphragm...nth diaphragm are also composed of their respective base membranes and adhesive layers on both sides of the base membrane. In one specific embodiment, the thickness d1 of the adhesive layer of the first diaphragm is 1μm to 3μm; for example, the thickness d1 of the adhesive layer of the first diaphragm can be 1μm, 1.5μm, 2μm, 2.5μm, or 3μm; the nth diaphragm... The thickness d2 of the adhesive layer of the membrane is 0.5μm to 2μm; for example, the thickness d2 of the adhesive layer of the nth diaphragm can be 0.5μm, 1μm, 1.5μm, or 2μm; the thickness D of the base membrane of the first diaphragm or the nth diaphragm is 4μm to 12μm; for example, the thickness D of the base membrane of the first diaphragm or the nth diaphragm can be 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, or 12μm.

[0040] The outermost first electrode and the first separator form a single-sided separator assembly, which is prone to curling and powder shedding after rolling. During the manufacturing process, dust-induced self-discharge can easily occur. Therefore, by adjusting the adhesive layer thickness d1 of the first separator to be slightly greater than the adhesive layer thickness d2 of the nth separator, the adhesive layer thickness of the outermost separator can be increased, which can significantly improve the self-discharge problem caused by powder shedding. Furthermore, by adjusting the thickness relationship of the adhesive layers in the separator, the adhesion between the outermost electrode and the separator can be improved, avoiding black spot lithium deposition caused by uneven stress release deformation in different areas of the electrode, thereby further improving the cycle performance of the cell and extending the cycle life of the cell.

[0041] Furthermore, the thickness d1 of the adhesive layer in the first separator and the thickness D of the base film satisfy the relationship d1 < (0.2~0.5)D. Although the thickness d1 of the adhesive layer in the first separator is slightly larger than the thickness d2 of the adhesive layer in the nth separator, the thickness d1 of the adhesive layer in the first separator should not be too large, because too large a thickness d1 of the adhesive layer in the first separator will also affect the lithium-ion transport distance. Therefore, by controlling the thickness d1 of the adhesive layer in the first separator and the thickness D of the base film to satisfy the relationship d1 < (0.2~0.5)D, the thickness of the outermost separator adhesive layer can be effectively avoided from being too thick, affecting the cycle performance of the cell. At the same time, the overall thickness of the cell structure can also be avoided from being too thick, affecting the overall performance of the lithium-ion battery.

[0042] Furthermore, the thickness d2 of the adhesive layer of the nth separator and the thickness D of the base film of the nth separator satisfy the relationship d2 > (0.1~0.2)D. Although the thickness d1 of the adhesive layer of the first separator is slightly larger than the thickness d2 of the adhesive layer of the nth separator, the thickness d2 of the adhesive layer in the nth separator should not be too small. If d2 is too small, it will affect the adhesion of the double-sided separator assembly inside the cell structure, resulting in poor contact between the separator and the electrode, affecting the transport of lithium ions in the separator assembly, and increasing the internal resistance of the cell. Furthermore, if the adhesive layer is too thin, it will result in insufficient adhesiveness, which may cause the electrode to deform, thereby accelerating the battery capacity decay, causing the battery to expand, affecting the battery cycle life, and even causing safety accidents. Therefore, by controlling the thickness d2 of the adhesive layer in the nth separator to satisfy the relationship d2 > (0.1~0.2)D, the present invention can effectively avoid the thickness d2 of the adhesive layer in the nth separator being too thin, thereby avoiding the shortening of the battery cycle life and the deterioration of the thickness change rate.

[0043] In this invention, the thickness L1 of the first diaphragm is 5μm to 16μm; for example, the thickness L1 of the first diaphragm can be 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or 16μm; the thickness M1 of the first electrode is 45μm to 85μm; for example, the thickness M1 of the first electrode can be 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, or 85μm.

[0044] In one specific implementation, L and M1 satisfy the relationship 55μm≤L1+M1≤90μm (for example, the calculated result of L1+M1 can be 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm).

[0045] It should be noted that L1+M1 is the total thickness of the composite single-sided separator assembly unit composed of the first separator and the first electrode, denoted as S1. The smaller the thickness of the composite unit of the outer separator and the electrode, the better the adhesion between the separator and the electrode, resulting in the shortest transport distance for lithium ions from the positive electrode to the negative electrode, minimizing the risk of lithium plating. This is more conducive to the diffusion of lithium ions within the outer positive and negative electrode unit structure, ensuring uniform lithium ion diffusion in each positive and negative electrode unit of the stacked cell and avoiding safety risks caused by uneven diffusion in certain areas. However, the thickness of the composite single-sided separator assembly unit should not be too large, as excessive thickness can lead to localized lithium ion accumulation, causing excessive cell polarization. This results in uneven reactions of the electrode materials during electrochemical reactions, accelerating material degradation and shortening the cell's cycle life. Therefore, this invention, by further controlling the total thickness S1 of the composite separator assembly unit composed of the first separator and the first electrode, can effectively improve the above problems, reducing the risk of cell polarization while improving the lithium plating problem on the outer layer of the cell.

[0046] In this invention, the thickness L2 of the nth diaphragm is 4μm to 14μm; for example, the thickness L2 of the nth diaphragm can be 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, or 14μm; the thickness M2 of the Nth electrode is 45μm to 85μm; for example, the thickness M2 of the Nth electrode can be 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, or 85μm.

[0047] In one specific implementation, L and M2 satisfy the relationship 60μm≤2L2+M2≤95μm (for example, the calculation result of 2L2+M2 can be 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm).

[0048] It should be noted that when the total number of separators is 2n, the total thickness of the composite double-sided separator assembly unit formed by the nth separator and the Nth electrode is denoted as S2. When the thickness of S2 is smaller, the polarization difference between different composite units in the entire cell will be larger, leading to uneven local heat generation within the cell and potential safety issues in the later stages of cycling. When the thickness of S2 is larger, local lithium-ion accumulation is more likely to occur within the composite unit formed by the inner separator and the electrode, also causing cell polarization. This results in uneven reactions of the electrode materials during electrochemical reactions, accelerating material degradation and shortening the cell's cycle life. Therefore, this invention, by further controlling the total thickness S2 of the composite double-sided separator assembly unit formed by the nth separator and the Nth electrode, can effectively improve the above problems, reduce the risk of cell polarization, improve the cell's safety performance in the later stages of cycling, and thus extend the cell's lifespan.

[0049] In this invention, L1, M1, L2, M2, and L satisfy the relationship L2 + M2 < L1 + M1, that is, the total thickness S1 of the composite single-sided separator assembly unit composed of the first separator and the first electrode, and the total thickness S2 of the composite double-sided separator assembly unit composed of the nth separator and the Nth electrode, satisfy the relationship S2 - L2 < S1. By adjusting the relationship between the thicknesses of the outer and inner separator assemblies, this invention can further reduce the risk of cell polarization and improve the lithium plating problem on the outer layer of the cell.

[0050] Furthermore, in this invention, the positive electrode sheet, the negative electrode sheet, and the separator can all be prepared using conventional materials in the art, and the active material in the coating of the positive electrode sheet can also be prepared using conventional positive electrode active materials in the art, which will not be elaborated here.

[0051] A second aspect of the present invention provides a lithium-ion battery, the lithium-ion battery comprising the cell described in the first aspect of the present invention.

[0052] According to the lithium-ion battery of the present invention, the lithium-ion battery further includes an electrolyte, which may be an electrolyte conventionally used in the art.

[0053] The lithium-ion battery of the present invention can be prepared using conventional methods in the art. Specifically, a positive electrode, a separator, and a negative electrode can be sequentially stacked to obtain a battery cell. This cell is then subjected to baking, electrolyte injection, formation, and encapsulation processes to obtain the aforementioned lithium-ion battery. All materials in this invention can be prepared using methods known in the art or obtained commercially.

[0054] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0055] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0056] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0057] Example 1

[0058] Used in the preparation of lithium-ion batteries

[0059] Step 1: Prepare the positive electrode sheet:

[0060] The main material of lithium cobalt oxide cathode, conductive carbon black and polyvinylidene fluoride were added to a mixing tank in a mass ratio of 97:1.5:1.5. Then, NMP solvent was added to prepare a cathode slurry with a solid content of 70% to 75%. The cathode slurry was then coated onto two sets of cathode current collector aluminum foils using a coating machine and dried at 120°C for 8 hours. By adjusting the rolling pressure, first and Nth electrodes with different thicknesses and porosities were obtained (the coating porosity of the first electrode was 25%, and the thickness M1 was 66 μm; the coating porosity of the Nth electrode was 20%, and the thickness M2 was 64 μm). Other cathode electrodes were rolled with a porosity between 20% and 25%, and the porosity of the other cathode electrodes was the same. Finally, all the obtained electrodes with different porosities were cut into strips, the cathode tab welding positions were washed out, the strips were cut, and the tab positions were die-cut to obtain the cathode electrodes.

[0061] The second step is to prepare the negative electrode sheet:

[0062] Graphite and silicon carbide (mass ratio 9:1), conductive carbon black (conductive agent), styrene-butadiene latex (binder), and sodium carboxymethyl cellulose (dispersant) are added to a mixing tank in a mass ratio of 94.5:2.0:2.0:1.5. Then, deionized water is added to prepare a negative electrode slurry. The slurry is stirred using a known batching process to obtain a solid content of 40% to 45%. The negative electrode slurry is then coated onto the negative electrode current collector aluminum foil using a skid coater, dried at 100°C, and then compacted using a roller press. The resulting electrode sheet is then cut into strips, laser-cleaned to create tab grooves, cut into pieces, and welded with nickel tabs to obtain the negative electrode sheet.

[0063] Step 4: Preparation of the diaphragm:

[0064] The intermediate diaphragm (nth diaphragm) is prepared by coating one side of the base membrane (base membrane thickness D is 8 μm) with a mixture of 8% PVDF adhesive (PVDF:NMP = 8:92) and 20% ceramic adhesive (Al2O3 powder: sodium butylnaphthalene sulfonate:deionized water = 20:0.2:79.8), in the order of ceramic adhesive first and then PVDF adhesive. The other side is coated only with PVDF adhesive. The base membrane is made of polyethylene. The final diaphragm has a porosity a2 of 30%, a thickness L2 of 6.5 μm, and a surface density c2 of 2 g / m³. 2 The thickness d2 of the adhesive layer is 2μm;

[0065] The outermost membrane (first membrane) was coated with a mixture containing 5% PVDF adhesive (PVDF:NMP = 5:95) and 20% ceramic adhesive (Al2O3 powder: sodium butylnaphthalene sulfonate:deionized water = 20:0.2:79.8) on one side of the base membrane (base membrane thickness D = 8 μm), followed by ceramic adhesive. The other side was coated only with PVDF adhesive. The base membrane was made of polyethylene. The resulting membrane had a porosity (a1) of 42%, a thickness (L1) of 8 μm, and a surface density (c1) of 1.5 g / m³. 2 The thickness d1 of the adhesive layer is 2μm;

[0066] The preparation steps for other membrane layers are the same as for the intermediate membrane layer, but different base membranes are used so that the porosity is between 30% and 42%. The porosity of each of the other membrane layers is the same.

[0067] Step 5, Lithium-ion battery fabrication: The positive electrode sheets (total number of positive electrode sheets is 5, N=3), separators (total number of separators is 8, n=4), and negative electrode sheets prepared above are stacked in a predetermined order, with the corresponding separators positioned between the corresponding positive and negative electrode sheets. This stacking process yields the desired result. Figure 2 The electrode assembly is shown. The electrode assembly is placed in an outer aluminum-plastic film, and after the moisture is removed at 80°C, the above-mentioned electrolyte is injected and it is sealed. After processes such as formation, degassing, and edge trimming, a lithium-ion battery is obtained.

[0068] Example 2 group

[0069] Example 2 was performed in accordance with Example 1, except that different base membranes were used, which changed the porosity of the first and nth diaphragms, as detailed in Table 1:

[0070] Table 1

[0071]

[0072] Example 3 Group

[0073] Example 3 was conducted in accordance with Example 1, except that the parameters of the rolling process during the preparation of the positive electrode were changed, thereby altering the porosity and thickness of the coatings corresponding to the first and nth electrodes. See Table 2 for details.

[0074] Table 2

[0075]

[0076] Example 4

[0077] Example 4 is carried out in accordance with Example 1, except that the other diaphragms are also set with porosity decreasing sequentially from the outside to the inside of the cell. Specifically, the porosity of the second diaphragm is 36%, the porosity of the third diaphragm is 33%, and the others are all carried out in accordance with Example 1.

[0078] Example 5

[0079] Example 5 is carried out in accordance with Example 1, except that the porosity of the other positive electrode plates is set to 23%, that is, the porosity of the second electrode plate is the same as that of the Nth electrode plate, and everything else is carried out in accordance with Example 1.

[0080] Example 6 group

[0081] Example 6 was performed in accordance with Example 1, except that the thickness of the adhesive layer of the first diaphragm and the nth diaphragm was changed, as detailed in Table 3:

[0082] Table 3

[0083]

[0084] Example 7 group

[0085] Example 7 was performed in accordance with Example 1, except that the specific composition of the PVDF adhesive was changed, which altered the areal density of the adhesive layers of the first and nth diaphragms, as detailed in Table 4.

[0086]

[0087]

[0088] Comparative Example 1

[0089] Comparative Example 1 was carried out in accordance with Example 1, except that the porosity of the positive electrode sheet at different locations was kept consistent. Specifically, the porosity of the coating on the positive electrode sheet was 25%.

[0090] Comparative Example 2

[0091] Comparative Example 2 was carried out in accordance with Example 1, except that the diaphragm at different locations maintained a consistent porosity, specifically, the porosity of the diaphragm was 35%.

[0092] Comparative Example 3

[0093] Comparative Example 3 was carried out in accordance with Example 1, except that the diaphragm and positive electrode at different locations maintained the same porosity. Specifically, the porosity of the coating on the positive electrode was 25%, and the porosity of the diaphragm was 35%.

[0094] Test case

[0095] (1) Lithium deposition test on the negative electrode surface of lithium-ion secondary battery under high current charging:

[0096] At 25°C, the lithium-ion secondary batteries prepared in the above examples and comparative examples were charged at a constant current of 7C to 4.5V, then charged at a constant voltage of 4.5V to a current of 0.05C, and then discharged at a constant current of 1C to 3.0V. This is one charge-discharge cycle.

[0097] The lithium-ion secondary battery was subjected to 30 high-current charge-discharge cycles as described above, then charged at a constant current of 7C to 4.5V, and then charged at a constant voltage of 4.5V to a current of 0.05C.

[0098] The battery was fully charged and disassembled to remove the negative electrode. The lithium deposition state on the negative electrode was observed, and the degree of lithium deposition was determined according to the following rules: A. No lithium deposition; B. Slight lithium deposition; C. Lithium formation in local areas; D. Lithium residue in most areas; E. Severe lithium deposition. The test results are recorded in Table 5.

[0099] (2) Battery cycle life test:

[0100] At 25°C, the lithium-ion secondary batteries prepared in the above examples and comparative examples were charged to 4.5V at a 7C rate and discharged to 3V at a 0.7C rate, and subjected to a full charge-discharge cycle test of 800T. The capacity retention rate (%) and cell thickness change rate (%) of the cells after 800T cycles were recorded, as well as the thickness of the cells at half charge and the thickness of the cells after full charge. The test results are recorded in Table 5.

[0101] Table 5

[0102]

[0103]

[0104] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A battery cell, characterized in that, The battery cell includes a positive electrode, a separator, and a negative electrode stacked in sequence; the positive electrode and the negative electrode are alternately arranged, and the separator is provided between adjacent positive electrode and negative electrode. Along the thickness direction of the battery cell, the battery cell has a first outermost edge and a second outermost edge; The separator, from the first outermost or the second outermost to the middle of the cell, includes a first separator, a second separator, a third separator, ..., an nth separator, wherein the total number of separators is 2n or 2n+1; The porosity of the first diaphragm is greater than that of the nth diaphragm.

2. The battery cell according to claim 1, wherein, The porosity a1 of the first diaphragm is 40% to 60%; the porosity a2 of the nth diaphragm is 30% to 50%. Preferably, the porosity a1 of the first diaphragm is 40% to 48%; the porosity a2 of the nth diaphragm is 30% to 40%. Preferably, the porosity a1 of the first diaphragm and the porosity a2 of the nth diaphragm satisfy the relationship 15%≤(2a1+na2) / (n+2)≤55%, and more preferably 45%≤(2a1+na2) / (n+2)≤55%. And / or, the porosity of the first diaphragm, the second diaphragm, the third diaphragm... the nth diaphragm decreases sequentially; And / or, n = 4 to 20.

3. The battery cell according to claim 1, wherein, The positive electrode includes a current collector and a coating on at least one side of the current collector; the positive electrode is sequentially arranged from the first outermost or the second outermost to the middle of the cell as a first electrode, a second electrode, a third electrode, ..., an Nth electrode, where N = 3 to 21; The porosity of the coating in the first electrode is greater than the porosity of the coating in the Nth electrode; Preferably, the porosity of the coating in the first electrode, the second electrode, the third electrode, ... the Nth electrode decreases sequentially.

4. The battery cell according to claim 3, wherein, The porosity of the coating in the first electrode is 20% to 35%, and the porosity of the coating in the Nth electrode is 18% to 25%. Preferably, the porosity of the coating in the first electrode is 25% to 30%, and the porosity of the coating in the Nth electrode is 20% to 25%.

5. The battery cell according to claim 1, wherein, The diaphragm includes a base membrane and adhesive layers located on both sides of the base membrane; the areal density c1 of the adhesive layer of the first diaphragm is 1.5 g / m³. 2 ~2.5g / m 2 The areal density c2 of the adhesive layer of the nth diaphragm is 1.2 g / m³. 2 ~2g / m 2 ; Preferably, the areal density c1 of the adhesive layer of the first diaphragm is 2 g / m³. 2 ~2.5g / m 2 The areal density c2 of the adhesive layer of the nth diaphragm is 1.5 g / m³. 2 ~2g / m 2 .

6. The battery cell according to claim 5, wherein, The thickness d1 of the adhesive layer of the first diaphragm is 1 μm to 3 μm; the thickness d2 of the adhesive layer of the nth diaphragm is 0.5 μm to 2 μm; the thickness D of the base film of the first diaphragm or the nth diaphragm is 4 μm to 12 μm. Preferably, the thickness d1 of the adhesive layer in the first diaphragm and the thickness D of the base film satisfy the relationship d1 < (0.2~0.5)D; Preferably, the thickness d2 of the adhesive layer in the nth diaphragm and the thickness D of the base film satisfy the relationship d2>(0.1~0.2)D.

7. The battery cell according to claim 3, wherein, The thickness L1 of the first diaphragm is 5 μm to 16 μm; the thickness M1 of the first electrode is 45 μm to 85 μm. Preferably, L1 and M1 satisfy the relationship 55μm≤L1+M1≤90μm.

8. The battery cell according to claim 7, wherein, The thickness L2 of the nth diaphragm is 4 μm to 14 μm; the thickness M2 of the Nth electrode is 45 μm to 85 μm. Preferably, L2 and M2 satisfy the relationship 60μm≤2L2+M2≤95μm.

9. The battery cell according to claim 8, wherein, The relation L2+M2<L1+M1 is satisfied.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the cell described in any one of claims 1 to 9.

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