Negative plate and lithium battery

Abstract

The present invention provides a negative plate, comprising: a current collector having a first coating region as a main body and a second coating region located on at least one side of the first coating region, the first coating region being coated with an active slurry containing first graphite, the second coating region being coated with second graphite, the O / I of the second graphite being greater than the O / I of the first graphite, where O represents crystal orientation, and O represents crystal orientation; i represents the structure defect degree. By adopting the technical scheme, the second graphite instead of the first graphite is adopted in the second coating area, although the capacity of the battery is slightly reduced, the circulating swelling amount of the edge area of the negative plate can be obviously reduced, so that the pressing risk which is borne by the edge plate of the laminated core due to the limitation of the space of the shell and is continuously accumulated along with the increase of the number of circulating turns is reduced, and the service life of the battery is prolonged. Finally, the diving phenomenon of rapid attenuation of the capacity of the battery cell is relieved, and the overall cycle life of the battery is guaranteed. The invention also provides a negative plate and a lithium battery comprising the negative plate.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a negative electrode and a lithium battery. Background Technology

[0002] With the iteration of industry technologies and the upgrading of end-use applications (such as new energy vehicles and energy storage equipment), the market has placed higher demands on the driving range of lithium batteries. This demand has directly driven the technological research and development, process improvement, and large-scale application of long-life batteries. Among them, prismatic batteries, with their advantages of high structural strength, good energy density adaptability, and high pack integration efficiency, are widely used in medium and large capacity energy storage and power battery fields.

[0003] However, the structural characteristics of prismatic batteries present a key bottleneck in improving cycle life. For example... Figure 1 As shown, the metal casing of the square battery 100 is a rigid structure, which creates a fixed spatial constraint on the internal stacked cores. The large surface area A of the casing has a certain deformation redundancy, which can accommodate the space requirements caused by the expansion of the stacked cores. However, the top corner B and the surrounding edge C of the casing are rigid support structures, which cannot reserve sufficient expansion margin for the electrode sheets at the edge of the stacked cores.

[0004] During the charge-discharge cycle of the battery cell, the negative electrode plate undergoes lithium ion insertion and extraction, along with changes in thickness (refer to...). Figure 1 The expansion accumulates slowly and continuously in the Y direction (as described in the text). Due to the lack of necessary expansion space at the apex and surrounding areas of the casing, the electrode sheets at the edge of the stacked core will bear the pressure exerted by the casing, which accumulates with the number of cycles. When this pressure reaches a critical value, it will not only severely damage the microstructure of the negative electrode sheet, but also hinder the normal insertion and extraction of lithium ions in the negative electrode active material, ultimately leading to a rapid capacity drop in the cell and significantly shortening its overall cycle life. Summary of the Invention

[0005] The present invention provides the following technical solutions to solve the above-mentioned technical problems.

[0006] This invention provides a negative electrode sheet, comprising:

[0007] The current collector has a first coating area as the main body and a second coating area located on at least one side of the first coating area. The first coating area is coated with an active slurry containing a first graphite, and the second coating area is coated with a second graphite. The O / I ratio of the second graphite is greater than that of the first graphite, where O represents crystal orientation and I represents structural defect degree.

[0008] With the above technical solution, using the second graphite instead of the first graphite in the second coating area will cause a slight decrease in the battery capacity, but can significantly slow down the cyclic expansion amount in the edge area of the negative electrode sheet, thereby reducing the risk of compression on the edge electrode sheets of the stacked core due to the limited space of the casing and continuously accumulating with the increase of the number of cycles. Finally, it alleviates the "diving" phenomenon of the rapid attenuation of the cell capacity and ensures the overall cycle life of the battery.

[0009] According to another specific embodiment of the present invention, one end of the current collector is connected to the tab, and the second coating area is located on both sides of the tab and the first coating area along the first direction, wherein the first direction is perpendicular to the extension direction of the tab.

[0010] According to another specific embodiment of the present invention, the second coating area surrounds the first coating area on all sides.

[0011] According to another specific embodiment of the present invention, the shape of the second coating area is rectangular, the length of the rectangle is the same as the side length of the current collector where it is located, and the width of the rectangle accounts for 1%-3% of the side length of the current collector where it is located.

[0012] The present invention also provides a negative electrode sheet, comprising:

[0013] A current collector, including a first coating area and a second coating area as the main body. The first coating area is coated with an active slurry containing the first graphite, and the second coating area is coated with the second graphite, and the O / I of the second graphite is greater than the O / I of the first graphite, where O represents the crystal orientation and I represents the structural defect degree. The second coating area is coated at the four top corners of the current collector, and the shape of each is an isosceles triangle formed by cutting off with the two adjacent sides of the top corner as the waists, and the waist length of the isosceles triangle is 1%-3% of the side length of the current collector where it is located.

[0014] With the above technical solution, while maximizing the alleviation of this pressure problem, the amount of the second graphite is reduced to minimize the impact on the battery capacity as much as possible.

[0015] According to another specific embodiment of the present invention, the area of the second coating area accounts for 8%-12% of the area of the current collector.

[0016] According to another specific embodiment of the present invention, the O / I of the first graphite is 8-15, and the O / I of the second graphite is 22-30.

[0017] According to another specific embodiment of the present invention, the particle size of the second graphite simultaneously satisfies: 5μm < D10 < 6μm, 10μm < D50 < 15μm, and 20μm < D90 < 25μm.

[0018] According to another specific embodiment of the present invention, the specific capacity of the second graphite is 349-351 mAh / g.

[0019] The present invention also provides a lithium battery, including the negative electrode sheet in any of the above embodiments.

[0020] The lithium battery provided by this invention has a longer cycle life and is less prone to rapid capacity decay during long-term charge-discharge cycles, thus maintaining its electrochemical performance more stably. Attached Figure Description

[0021] Figure 1 A three-dimensional structural diagram of a square-shell battery cell is shown.

[0022] Figure 2 This diagram illustrates a negative electrode sheet in one embodiment of the present invention, where the second coating area is located on both sides of the first coating area;

[0023] Figure 3 This diagram illustrates a negative electrode sheet in another embodiment of the present invention, where the second coating area is located on both sides of the first coating area.

[0024] Figure 4 This diagram illustrates a negative electrode sheet in another embodiment of the present invention, where the second coating area is located around the first coating area;

[0025] Figure 5 This diagram illustrates a negative electrode sheet in another embodiment of the present invention, where the second coating area is located at the apex of an isosceles triangle.

[0026] Figure 6 A schematic diagram showing the degree of compression of the stacked cores after cycling in Comparative Examples 1-4 is shown;

[0027] Figure 7 The diagram shows the cyclic performance of the stacked cores in Comparative Examples 1-4;

[0028] Figure 8 The diagram shows the cyclic performance of the stacked cores in Examples 1-4. Detailed Implementation

[0029] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0030] The terms “first”, “second”, etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0031] It should be noted that in this specification, similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0032] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0033] During charging and discharging, lithium ions need to be inserted and extracted within the negative electrode. However, the apex and edges of the negative electrode are subjected to various stresses after being rolled into a stack and installed in a housing. Figure 1 The spatial constraints at the apex B and the surrounding edge C are illustrated. These constraints hinder lithium-ion insertion and extraction, and the electrode edges are continuously subjected to compressive forces, ultimately affecting the normal insertion and extraction of lithium ions and causing a rapid decline in battery capacity. Based on this, the present invention provides the following technical solution to address the aforementioned technical problems.

[0034] like Figures 2-5 As shown, the present invention provides a negative electrode sheet, comprising:

[0035] The current collector 1 has a first coating region 2 as the main body and a second coating region 3 located on at least one side of the first coating region 2. The first coating region 2 is coated with an active slurry containing first graphite, and the second coating region 3 is coated with second graphite. The O / I ratio of the second graphite is greater than that of the first graphite, where O represents crystal orientation and I represents structural defect degree.

[0036] To address the aforementioned technical problems, this invention employs a "regional coating" method. First, a first layer of graphite is coated onto the main functional area of ​​the negative electrode (i.e., the first coating area 2) to ensure that the core performance of the negative electrode, such as its charge-discharge capability, energy storage capacity, cycle and storage stability, DC internal resistance (DCR), and low-temperature discharge efficiency, remains unaffected. Then, a second layer of graphite with a higher O / I value is used in the second coating area 3 at the edge to mitigate the degree of cycle expansion in this region. It should be noted that the first layer of graphite is a commonly used type in conventional active slurries, and its selection is typically based on ensuring the aforementioned core performance characteristics of the battery. In the characterization of lithium-ion battery negative electrode materials (especially graphite-based materials), the O / I value is a key parameter for measuring the microstructural characteristics of graphite. A higher O / I value indicates a more regular crystal structure and fewer defects in the graphite, resulting in a smaller expansion rate during cycling; conversely, a lower O / I value indicates a more disordered crystal arrangement and denser defects, leading to a greater expansion rate during cycling.

[0037] Through the above design, the use of second graphite instead of first graphite in the second coating area 3 will cause a slight decrease in battery capacity, but it can significantly slow down the cyclic expansion of the negative electrode edge area, thereby reducing the risk of pressure that the stacked edge electrode is subjected to due to the limited space of the casing and which accumulates with the increase of the number of cycles. Ultimately, it can reduce the "plunge" phenomenon of rapid capacity decay of the cell, ensure the overall cycle life of the battery, and improve its cycle stability.

[0038] Furthermore, in the above embodiments, such as Figure 2 As shown, one end of the current collector 1 is connected to the tab 4. The second coating area 3 is along the first direction (e.g., Figure 2 The X direction (as shown in the image) is located on both sides of the tab 4 and the first coating area 2. The first direction X is perpendicular to the extension direction of the tab (e.g., the direction of extension of the tab). Figure 2 (The Z direction in the equation is perpendicular to the direction of the equation).

[0039] Normally, after the negative electrode, separator, and positive electrode are wound together to form a stack and installed in the housing, their height direction (reference) Figure 1 A certain amount of space is reserved at the upper end of the electrode (in the Z direction). This reserved space can serve as redundancy, acting as a buffer against expansion. In contrast, the negative electrode sheets distributed on both sides of the first coating area 2 along the first direction X are subjected to more severe compressive pressure. Based on this, in this embodiment, the second coating area 3 is set on both sides of the tab 4 along the first direction X, thereby enabling more precise suppression of expansion in high-stress areas.

[0040] Furthermore, such as Figure 4As shown, the second coating area 3 surrounds the first coating area 2. This design allows the second graphite with a high O / I value to form a surrounding protective structure, which can fully cover the edge area of ​​the first coating area 2, thereby providing a more comprehensive suppression of expansion at the edge of the negative electrode. This surrounding layout can evenly disperse the expansion stress generated during cycling, avoid electrode deformation or damage caused by local stress concentration, further reduce the risk of compression of the stacked cores under the constraint of the casing, more effectively alleviate the phenomenon of cell capacity "plummeting", and provide more comprehensive protection for battery cycle life.

[0041] Furthermore, in the above embodiments, the second coating area 3 is rectangular in shape. The length of the rectangle is the same as the side length of the current collector 1 to which it is located, and the width of the rectangle accounts for 1%-3% of the side length of the current collector 1 to which it is located. Figure 2 Taking this as an example, the length of the rectangle and the side length of the current collector 1 are both L1, and the width of the rectangle D2 accounts for 1%-3% of D1. This size setting can effectively reduce the risk of capacity decay while avoiding excessive battery capacity loss due to the excessive proportion of the second coating area 3, thus achieving a better balance between cycle stability and capacity retention.

[0042] Furthermore, considering that the pressure on the edge of the electrode is greater than that on the larger surface area (refer to...), Figure 1 Region A in the text), and at the four vertex corners (refer to...) Figure 1 The pressure condition at point B is the most prominent. To alleviate this pressure problem to the greatest extent possible while minimizing the impact on battery capacity by reducing the amount of second graphite, the present invention provides the following technical solution.

[0043] like Figure 5 As shown, the present invention also provides a negative electrode sheet, comprising:

[0044] The current collector 1 includes a first coating area 2 and a second coating area 3 as the main components. The first coating area 2 is coated with an active slurry containing first graphite, and the second coating area 3 is coated with second graphite. The O / I ratio of the second graphite is greater than that of the first graphite, where O represents crystal orientation and I represents structural defect degree. The second coating area 3 is coated at the four vertices of the current collector 1, and each area is shaped as an isosceles triangle formed by cutting off the two adjacent sides of the vertex. The length M1 of the isosceles triangle is 1%-3% of the side length of the current collector 1 at that location.

[0045] Studies have shown that setting the second coating area 3 as Figure 5 The four isosceles triangles shown can effectively solve the above problems. Specifically, the shape of the second coating area 3 is an isosceles right triangle.

[0046] Further, in each of the above embodiments, the area of the second coating region 3 accounts for 8%-12% of the area of the current collector 1. This design of the area ratio can, while fully exerting the function of the second graphite in suppressing edge expansion and ensuring cycle stability, minimize the loss of the overall battery capacity caused by the adoption of the second coating region 3, thereby achieving the balanced optimization of battery performance and capacity.

[0047] Further, in each of the above embodiments, the O / I of the first graphite is 8-15, and the O / I of the second graphite is 22-30. With this setting, by selecting the first graphite with an O / I value in the conventional range, the core performance of the first coating region 2 as the main body of the negative electrode sheet is ensured. Also, by means of the second graphite with a significantly higher O / I value, it is ensured that the second coating region 3 has better crystal regularity and a lower structural defect rate, thereby specifically enhancing the anti-expansion ability of the edge region and providing a specific and reliable guarantee for reducing the risk of electrode sheet compression during the cycle and improving the battery life.

[0048] Further, in each of the above embodiments, the particle size of the second graphite simultaneously satisfies: 5μm < D10 < 6μm, 10μm < D50 < 15μm, and 20μm < D90 < 25μm. With this setting, the second graphite particles can form a more compact packing structure during the coating process. On the one hand, it can enhance the structural stability of the second coating region 3 and reduce the degree of expansion during the cycle. On the other hand, the above particle size range helps to reduce the resistance of the coating layer and ensure the electron conduction performance of the edge region of the electrode sheet, thereby兼顾 the anti-expansion effect and the electrochemical performance of the second coating region, and avoiding affecting the overall performance of the battery due to improper design of the material microstructure.

[0049] Further, in each of the above embodiments, the specific capacity of the second graphite is 349-351 mAh / g. On the one hand, a specific capacity of 350 mAh / g can ensure that the second coating region 3 has sufficient electrochemical activity and meets the basic performance requirements of this region. On the other hand, compared with the first graphite whose primary goal is core performance (the specific capacity of the first graphite is generally 353-355 mAh / g), the second graphite can effectively reduce the degree of graphite expansion in the corresponding region through targeted optimization in the expansion characteristic direction (increasing the O / I value), while ensuring that its own basic performance is not affected, thereby avoiding the problem of too rapid capacity decay caused by overpressure at the edges and corners of the battery cell during the cycle. Preferably, the specific capacity of the second graphite is 350 mAh / g.

[0050] The present invention also provides a lithium battery, including the negative electrode sheet in any of the above embodiments.

[0051] The lithium battery provided by the present invention has a higher cycle life, and is not prone to the problem of rapid capacity decay during long-term charge-discharge cycles, and can more stably maintain its electrochemical performance.

[0052] The present invention will now be described in further detail through specific embodiments and in conjunction with the accompanying drawings.

[0053] Examples 1-4

[0054] The first graphite, conductive agent, and dispersant (the specific ratio of first graphite: conductive agent: CMC: SBR is 95.5:1.0:1.5:2.0) are mixed together, and the second graphite, conductive agent, and dispersant (the specific ratio of second graphite: conductive agent: CMC: SBR is 95.5:1.0:1.5:2.0) are mixed together. Then, deionized water is added separately and the mixtures are stirred and dispersed to form slurries. Subsequently, the above slurries are processed sequentially according to... Figures 2-5 The specific method shown involves coating the above-mentioned slurry onto the negative electrode current collector, followed by drying, rolling, slitting, and sheet forming to obtain the negative electrode sheets of Examples 1-4. That is, the negative electrode sheet of Example 1 is as follows... Figure 2 As shown, the negative electrode of Example 2 is as follows: Figure 3 As shown, the negative electrode sheet of Example 3 is as follows: Figure 4 As shown, the negative electrode sheet of Example 4 is as follows: Figure 5 As shown. The first graphite has an O / I ratio of 10 (specific capacity of 355 mAh / g), and the second graphite has an O / I ratio of 25. In the above embodiments, the width of the rectangle in the second coating area accounts for 2% of the side length of the current collector, the area of ​​the second coating area accounts for 10% of the current collector area, and the particle size of the second graphite is D10 = 5.5 μm, D50 = 12 μm, D90 = 22 μm, and the specific capacity is 350 mAh / g.

[0055] Comparative Examples 1-4

[0056] The first graphite, conductive agent, and dispersant (the specific ratio of first graphite: conductive agent: CMC: SBR is 95.5:1.0:1.5:2.0) were mixed, and then deionized water was added to each mixture and stirred to disperse them into a slurry. The slurry was then sequentially coated onto a negative electrode current collector, and subsequently dried, rolled, slit, and sheeted to obtain negative electrode sheets (Comparative Examples 1-4). The O / I ratio of the first graphite was the same as in Examples 1-4.

[0057] Lithium iron phosphate (LiFePO4), a conductive agent, and PVDF (PVDF) binder were mixed in a ratio of 97.5:0.5:2.0. Then, N-methylpyrrolidone was added and stirred to disperse the mixture into a positive electrode slurry. The positive electrode slurry was then coated onto a positive electrode current collector, dried, rolled, slit, and sheeted to obtain the positive electrode sheet.

[0058] The negative electrode sheets of Examples 1-4 and Comparative Examples 1-4 were assembled with the positive electrode and separator to form a stack. After assembly, processes such as liquid injection, wetting, formation, and sealing were performed. Finally, the electrochemical performance of the battery was tested. The test conditions were as follows: the battery was charged in a stepped manner at an ambient temperature of 25°C. Specifically, when charging from 0% to 80%, the charging current was 1C (1C current value is equal to 1 times the rated capacity of the battery, such as 2A for a 2Ah battery); when charging from 80% to 90%, the charging current was reduced to 0.5C; when charging from 90% to 100%, the charging current was further reduced to 0.2C. After charging was completed, the battery was discharged to 2.5V with a 1C current.

[0059] The electrochemical performance test results of Examples 1-4 are as follows: Figure 8 As shown in the figure, the curves from top to bottom correspond to Examples 4, 3, 2, and 1, respectively; the test results of Comparative Examples 1-4 are as follows. Figure 7 As shown, where, Figure 7 and Figure 8 The horizontal axis represents the number of cycles, and the vertical axis represents the capacity retention rate. Furthermore, Figure 6 The figure also shows the compression state of the stacked cores (i.e., cell1-cell4) in Comparative Examples 1-4 (from left to right: Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4). As can be seen from the figure, the edges are subjected to more severe compression.

[0060] Depend on Figure 8 It can be seen that, compared with the Comparative Examples 1-4 which were not coated with the second graphite, the battery capacity decay of Examples 1-4 was significantly reduced. Further comparison of the performance differences of Examples 1-4 shows that Example 4 has the best capacity retention performance, specifically Example 4 > Example 3 > Example 2 > Example 1.

[0061] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A negative electrode sheet, characterized in that, Comprising: A current collector having a first coated area as the main body and a second coated area located on at least one side of the first coated area. The first coated area is coated with an active paste containing first graphite, and the second coated area is coated with second graphite. The O / I of the second graphite is greater than that of the first graphite, where O represents crystal orientation and I represents structural defect degree.

2. The negative electrode sheet according to claim 1, characterized in that, One end of the current collector is connected to a tab. The second coated area is located on both sides of the tab and the first coated area along a first direction, where the first direction is perpendicular to the extension direction of the tab.

3. The negative electrode sheet according to claim 1, characterized in that, The second coated area surrounds the first coated area on all sides.

4. The negative electrode sheet according to claim 2 or 3, characterized in that, The shape of the second coated area is rectangular. The length of the rectangle is the same as the side length of the current collector where it is located, and the width of the rectangle accounts for 1%-3% of the side length of the current collector where it is located.

5. A negative electrode sheet, characterized in that, Comprising: A current collector including a first coated area and a second coated area as the main body. The first coated area is coated with an active paste containing first graphite, and the second coated area is coated with second graphite. The O / I of the second graphite is greater than that of the first graphite, where O represents crystal orientation and I represents structural defect degree. The second coated area is coated at the four top corners of the current collector, and the shape of each is an isosceles triangle formed by cutting off with the two adjacent sides of the top corner as the waists. The waist length of the isosceles triangle is 1%-3% of the side length of the current collector where it is located.

6. The negative electrode sheet according to any one of claims 1-5, characterized in that, The area of the second coated area accounts for 8%-12% of the area of the current collector.

7. The negative electrode sheet according to any one of claims 1-5, characterized in that, The O / I of the first graphite is 8-15, and the O / I of the second graphite is 22-30.

8. The negative electrode sheet according to any one of claims 1-5, characterized in that, The particle size of the second graphite simultaneously satisfies: 5μm < D10 < 6μm, 10μm < D50 < 15μm, and 20μm < D90 < 25μm.

9. The negative electrode sheet according to any one of claims 1-5, characterized in that, The specific capacity of the second graphite is 349-351 mAh / g.

10. A lithium battery, characterized in that, Comprising the negative electrode sheet according to any one of claims 1-9.

Images