Lithium ion cell and battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion battery cell and a battery comprising the lithium ion battery cell. Comprising a positive plate, a diaphragm and a negative plate, the positive plate, the diaphragm and the negative plate are laminated and wound to form a roll core, and the roll core comprises a straight area and a bent area; the positive plate at least partially positioned in the straight region comprises a positive current collector and a positive active material layer positioned on at least one side surface of the positive current collector; the positive plate at least partially located in the bending area comprises a positive current collector, a positive active material layer located on at least one side surface of the positive current collector and a functional layer located on at least one side surface, deviating from the positive current collector, of the positive active material layer; the functional layer comprises a graphene substance. The battery provided by the invention has relatively good cycle performance and safety performance.

Description

Technical Field

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

[0002] With the development of lithium-ion battery technology, higher demands are being placed on battery power. However, high-power batteries present two main problems during high-rate discharge: First, lithium plating easily occurs in the bending areas of the battery, increasing both the risk of short circuits and thermal runaway. Second, the battery surface temperature is higher, leading to a greater temperature rise and a higher risk of thermal runaway. Therefore, it is crucial to address the issues of lithium plating in the bending areas of high-power batteries and to reduce the risk of thermal runaway caused by the large temperature rise on the battery surface. Summary of the Invention

[0003] The purpose of this invention is to improve the problems of easy lithium plating in the bending area and the high risk of thermal runaway due to large surface temperature rise in existing high-power batteries. This invention provides a lithium-ion battery cell and a battery including the lithium-ion battery cell. The positive electrode of the lithium-ion battery cell of this invention has a functional layer in a specific area. This functional layer contains graphene-like materials, which can reduce the overall resistance of the positive electrode and improve its heat dissipation capacity with minimal impact on the normal insertion and extraction of lithium ions, thereby effectively delaying the occurrence of thermal runaway. Furthermore, the functional layer in the bending area of ​​the core not only slows down the migration rate of lithium ions from the positive electrode to the negative electrode, thus effectively improving the problem of easy lithium plating in the bending area, but also improves the problem of heat accumulation caused by large stress in the bending area, thereby further reducing the risk of thermal runaway. Batteries including the lithium-ion battery cell of this invention have good cycle performance and safety performance.

[0004] High-power batteries in related technologies suffer from problems such as easy lithium deposition in bending areas and a high risk of thermal runaway. The inventors of this invention, through research, discovered that the causes of these problems are as follows: First, during high-rate discharge, the battery temperature rise mainly comes from the heat of electrochemical reactions involving lithium ions and the heat generated by power loss due to electrode impedance, especially in the positive electrode. The poor conductivity of the positive electrode active material leads to high impedance and slow heat transfer. Second, during high-rate charging, due to the rapid transport speed of lithium ions, and especially in the bending areas of the battery, where the positive electrode near the winding center corresponds to the negative electrode away from the winding center, the negative electrode cannot fully accommodate the lithium ions released from the positive electrode, leading to lithium ion deposition at that location. Furthermore, to ensure that the winding structure of the core does not easily loosen, the binding force in the bending area is greater than in the straight area. The contact between the positive electrode, separator, and negative electrode in the bending area is tighter, making it difficult for heat to dissipate in the bending area, resulting in heat accumulation and a higher risk of thermal runaway. Based on the above findings, the inventors of this invention conducted extensive targeted research and proposed the following solution:

[0005] The first aspect of the present invention provides a lithium-ion battery cell, the lithium-ion battery cell comprising a positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet, the separator, and the negative electrode sheet are stacked and wound to form a core, the core comprising a straight region and a bent region; the positive electrode sheet at least partially located in the straight region comprises a positive current collector and a positive active material layer located on at least one side surface of the positive current collector; the positive electrode sheet at least partially located in the bent region comprises the positive current collector, the positive active material layer located on at least one side surface of the positive current collector, and a functional layer located on at least one side surface of the positive active material layer opposite to the positive current collector; the functional layer comprises a graphene-like material.

[0006] Graphene-like materials possess excellent electrical conductivity and heat dissipation; however, their sheet-like structure hinders lithium-ion diffusion for two main reasons: First, graphene's sheet-like structure consists of numerous hexagons, the pore sizes of which are close to the size of lithium ions, making it difficult for them to pass through. Second, graphene's overlapping sheet-like structure significantly increases the transport path for lithium ions. Therefore, adding graphene-like materials to the cathode requires careful consideration. The inventors of this invention have discovered that if a coating containing graphene-like materials is specifically disposed on the positive electrode (e.g., disposed intermittently on the outer surface of the positive electrode), and the functional layer is located at least in the bending region of the battery, it is possible to improve lithium plating in the bending region of the battery while delaying the occurrence of thermal runaway, thereby improving the cycle performance and safety performance of the battery. This is because: the functional layer located in the bending region of the battery, due to the presence of graphene-like materials, can, on the one hand, utilize the hindering effect of graphene on lithium-ion diffusion to slow down the migration speed of lithium ions from the positive electrode to the negative electrode in the bending region, thus improving the lithium plating problem in the bending region; on the other hand, the graphene-like materials located on the surface of the positive electrode active material layer have good electrical conductivity and heat dissipation capabilities. Therefore, it can not only reduce the overall impedance of the positive electrode but also quickly conduct the heat generated during the continuous lithium insertion and extraction process of the positive electrode active material layer from the inside of the battery, thereby improving the battery's heat dissipation capacity and reducing the temperature rise of the battery during discharge. Furthermore, the bending area of ​​the wound cell is subjected to greater stress. Therefore, when the positive electrode active material layer in the bending area is under greater stress, granular powder shedding from the positive electrode active material layer in the bending area is likely to occur. This further exacerbates lithium plating in the bending area or causes active material dust to enter the battery, resulting in poor K-value during cycling. However, by setting layered graphene-like materials on the active layer in the positive electrode bending area, the bonding area between the layered material and the positive electrode active material layer is relatively large and the bonding is relatively strong. In addition, the good toughness of graphene-like materials can reduce the shedding of positive electrode active material layer in the bending area and the damage to the positive electrode sheet in the bending area, thereby further improving lithium plating and poor K-value of the battery.

[0007] A second aspect of the present invention provides a battery comprising the lithium-ion cell described in the first aspect of the present invention.

[0008] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0009] (1) By setting the position of the positive electrode functional layer, the present invention cleverly utilizes the ability of graphene-like materials to slow down the diffusion of lithium ions, significantly reduces the internal resistance of the positive electrode, improves the heat dissipation capacity of the positive electrode, and improves the problem of lithium plating in the bending area of ​​the battery, thereby improving safety performance and cycle performance.

[0010] (2) The battery of the present invention has a lower risk of thermal runaway and improves safety performance.

[0011] 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

[0012] Figure 1 The diagram shown is a schematic diagram of the positive electrode sheet in an example of the present invention.

[0013] Figure 2 The diagram shown is a schematic diagram of the positive electrode sheet in an example of the present invention.

[0014] Figure 3 The diagram shown is a schematic diagram of the positive electrode sheet in an example of the present invention.

[0015] Figure 4 The diagram shown is a schematic diagram of the core in an example of the present invention.

[0016] Figure 5 The image shown is an optical photograph of the cross-section of the battery after a thickness expansion rate test in Embodiment 1 of the present invention.

[0017] Figure 6 The image shown is an optical photograph of the cross-section of the battery in Comparative Example 3 of this invention after a thickness expansion rate test. Detailed Implementation

[0018] 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.

[0019] The first aspect of the present invention provides a lithium-ion battery cell, which may include a positive electrode, a separator, and a negative electrode. The positive electrode, the separator, and the negative electrode are stacked and wound to form a core, and the core includes a straight region and a bent region.

[0020] In this invention, the positive electrode sheet at least partially located in the flat region includes a positive current collector and a positive active material layer located on at least one side surface of the positive current collector. The positive electrode sheet at least partially located in the bent region includes the positive current collector, the positive active material layer located on at least one side surface of the positive current collector, and a functional layer located on at least one side surface of the positive active material layer opposite to the positive current collector. Figure 1 and Figure 2All of these are schematic diagrams of the positive electrode sheet in an embodiment of the present invention, wherein, Figure 1 This refers to the case where the positive electrode active material layer is set on only one side. Figure 2 This refers to the case where the positive electrode active material layer is set on both sides. From... Figure 1 and Figure 2 As can be seen, the positive electrode 1 located in the flat region 10 includes a positive current collector 1-1 and a surface located on at least one side of the positive current collector 1-1. Figure 1 The positive electrode active material layer 1-2 is disposed on one side surface of the positive electrode current collector 1-1; Figure 2 A positive electrode active material layer 1-2 is disposed on both sides of the positive electrode current collector 1-1. The positive electrode sheet 1 located in the bending region 20 includes the positive electrode current collector 1-1 and a positive electrode active material layer 1-2 disposed on at least one side of the positive electrode current collector. Figure 1 The positive electrode active material layer 1-2 is disposed on one side surface of the positive electrode current collector 1-1; Figure 2 The positive electrode active material layer 1-2 is disposed on both sides of the positive electrode current collector 1-1, and the functional layer 1-3 is located on at least one side of the positive electrode active material layer 1-2 that is away from the positive electrode current collector 1-1.

[0021] It is understandable that, due to the structural characteristics of the battery, there may be empty foil areas on the positive electrode sheet (i.e., there is no positive electrode active material layer on the positive electrode current collector). These empty foil areas may be located in the straight area of ​​the core or in the bent area of ​​the core.

[0022] Figure 2 and Figure 3 All of these are schematic diagrams of the positive electrode sheet in an embodiment of the present invention, wherein, Figure 2 This applies to cases where the functional layer is configured on both sides. Figure 3 This refers to the case where the function layer is set up on a single-layer side.

[0023] It is understandable that when the positive electrode active material layer 1-2 is disposed on one side of the positive electrode current collector 1-1, the functional layer 1-3 is located on the surface of the positive electrode active material layer 1-2 facing away from the positive electrode current collector 1-1 (e.g., Figure 1 (As shown). When the positive electrode active material layer 1-2 is disposed on both sides of the positive electrode current collector 1-1, the functional layer 1-3 can be disposed on the surface of the positive electrode active material layer 1-2 located on any side of the positive electrode current collector 1-1 that faces away from the positive electrode current collector 1-1 (e.g., ...). Figure 3 (as shown); it can also be disposed on the surface of the positive electrode active material layer 1-2 facing away from the positive electrode current collector 1-1 on both sides of the positive electrode current collector 1-1 (e.g. Figure 2 (As shown).

[0024] In this invention, the functional layer may include graphene-like materials.

[0025] In one example, the graphene-like material may include at least one of graphene, graphene oxide, and reduced graphene oxide.

[0026] Improving battery energy density is a hot research topic in related technologies. Currently, a relatively effective method is to use new materials, such as silicon-based materials for the negative electrode. However, silicon-based materials expand significantly during use, causing stretching and localized compression of the negative electrode sheet, which leads to battery deformation. The inventors of this invention have discovered that the deformation problem of batteries (especially silicon-doped batteries) can be further improved by setting recesses on the surface of the functional layer.

[0027] In this invention, the surface of the functional layer located on one side of the positive electrode sheet, facing away from the positive electrode current collector, has a plurality of recesses. The term "a plurality of" refers to a number of recesses greater than or equal to two.

[0028] First, the recesses on the surface of the functional layer provide a buffer space for the expansion of the silicon-based material and a space for releasing internal stress, thus effectively improving the battery deformation problem. Second, due to the high internal stress in the bending region, the electrolyte has poor wettability in this area, which easily leads to lithium plating. The recesses on the surface of the functional layer facilitate electrolyte wetting of the positive electrode and improve the electrolyte storage capacity of the functional layer, allowing for continuous electrolyte replenishment during battery cycling. This is especially true when the functional layer is located in the bending region, further reducing lithium plating caused by internal stress. Third, the recesses on the surface of the functional layer can create microscopic overhang regions. This is because the lithium ions in the recesses and non-recesses of the functional layer have different extraction rates, which can reduce the local lithium ion concentration in the positive electrode in the bending region to a certain extent, further improving the lithium plating problem in the bending region. Finally, the presence of graphene-like materials in the functional layer makes it highly compatible with the recesses. If recesses are directly placed on the surface of the positive electrode active material layer, there is a risk of powder shedding from the interior and edge areas of the recesses during the winding process, leading to increased self-discharge problems. However, placing recesses on the functional layer not only reduces the risk of positive electrode active material shedding but also increases the overall flexibility of the positive electrode sheet. This is because graphene-like materials have relatively high molecular bonding forces between their layered structures in the horizontal direction. Therefore, placing recesses on the surface of the functional layer allows graphene-like materials to reduce stress to a certain extent, thus making it less prone to powder shedding from the positive electrode active material layer.

[0029] In one example, the recess is located on the functional layer of the positive electrode sheet on the side near the winding center of the core. When the positive electrode sheet near the winding center corresponds to the negative electrode sheet away from the winding center, the positive active material layer is located on the outer edge of the arc relative to the negative active material layer. Therefore, there is more positive active material than negative active material, and the negative electrode sheet cannot completely accommodate the lithium ions released from the positive electrode sheet, making lithium deposition more likely. Furthermore, the compression of the active material in the bending area of ​​the positive electrode sheet results in a larger area of ​​increased positive active material per unit area than the area of ​​the negative electrode sheet in the bending area, further preventing the negative electrode sheet from completely accommodating the lithium ions released from the positive electrode sheet, thus causing lithium ions to deposit at that location. In addition, the portion of the positive electrode sheet near the winding center located in the bending area has greater compressive force, which increases the paste density per unit area; conversely, the portion of the negative electrode sheet away from the winding center located in the bending area has greater tensile force, resulting in a smaller amount of paste per unit area in that region, further increasing lithium ion deposition at that location. Therefore, by creating a recess in the functional layer on the side of the positive electrode near the winding center, the lithium ion extraction rate of the recess can be effectively balanced, the lithium ion concentration can be reduced, and thus the occurrence of lithium plating can be improved.

[0030] In this invention, the positive electrode active material layer may include a positive electrode active material. The particle size Dv10 of the positive electrode active material and the depth d of the recess satisfy: Dv10≤d≤25μm.

[0031] The inventors of this invention discovered that when the depth d of the recess is less than the particle size Dv10 of the positive electrode active material, the liquid storage effect of the recess and the effect of the constructed microscopic overhang region are not significant, and the improvement on the deformation and lithium plating problems in the bending area of ​​the battery is limited. However, when the depth d of the recess is greater than 25 μm, if the recess is obtained by mechanical pore formation, the capacity loss of the positive electrode is significant, which is detrimental to improving the battery's energy density. If the recess is formed by mechanical pressure rolling, excessive depth may lead to the risk of cracking of the positive electrode current collector.

[0032] In this invention, the particle size Dv10 of the positive electrode active material can be obtained by conventional methods in the art, such as laser particle size analyzer.

[0033] In this invention, the depth d of the recess can be 5μm-25μm, for example, 5μm, 10μm, 15μm, 20μm or 25μm.

[0034] In one example, the depth d of the recess is 10μm-20μm.

[0035] In this invention, the spacing between the recesses can be 20μm-500μm, for example, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 200μm, 300μm, 400μm or 500μm.

[0036] In one example, the spacing between the recesses is 300μm-400μm.

[0037] If the spacing between the recesses is too small (e.g., less than 20 μm), the over-compacted area of ​​the positive electrode will increase, raising the risk of breakage. Additionally, it may loosen the positive electrode active material layer, increasing the risk of detachment. Conversely, if the spacing between the recesses is too large (e.g., greater than 500 μm), the density of the recesses will be insufficient, thus diminishing their liquid storage effect and the effectiveness of the microscopic overhang regions they create, failing to effectively address the lithium plating problem.

[0038] In this invention, the area of ​​the orthographic projection of the recess onto the surface of the functional layer can be 900 μm. 2 -40000μm 2 For example, 900μm 2 1600μm 2 2500μm 2 3600μm 2 4900μm 2 6400μm 2 8100μm 2 10000μm 2 or 40000μm 2 .

[0039] In one example, the area of ​​the recess projected onto the surface of the functional layer is 8000 μm. 2 -10000μm 2 .

[0040] In this invention, the area of ​​the orthographic projection of the recess onto the surface of the functional layer refers to the area of ​​the orthographic projection of a recess onto the surface of the functional layer.

[0041] In this invention, the depth, spacing, and area of ​​the orthographic projection of the recesses onto the surface of the functional layer can be obtained using conventional methods in the art. For example, the depth of a recess is the maximum vertical distance from any point within the recess to the surface of the functional layer, which can be obtained using a 3D profilometer. The depths of at least 20 recesses on the outer surface of the functional layer are tested, and the average value is taken. The spacing between the recesses is the shortest distance between the edges of any two adjacent recesses, which can be obtained using a 3D profilometer. Specifically, at least 20 groups of adjacent recesses are selected on the surface of the positive electrode, the shortest distance between the edges of each group of recesses is measured, and the average value is taken. The area of ​​the orthographic projection of the recesses onto the surface of the functional layer can be measured and calculated using a 3D profilometer. Specifically, the parameters of at least 20 recesses are measured, their orthographic projection areas are calculated, and the average value is taken.

[0042] In this invention, the shape of the orthographic projection of the recess on the surface of the functional layer can be regular or irregular, both of which can achieve good results. For example, the shape of the orthographic projection of the recess on the surface of the functional layer is circular, or, for another example, the shape of the orthographic projection of the recess on the surface of the functional layer is rectangular.

[0043] In this invention, the Raman spectrum of the functional layer exhibits a D peak and a G peak. The intensity of the D peak is I. D With the intensity I of the G peak G The ratio I D / I G It can be 1-2, for example, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2.

[0044] In one instance, I D / I G It is 1.2-2.

[0045] In one instance, I D / I G It is 1.4-1.8.

[0046] The inventors of this invention discovered that when I in the Raman spectrum of the functional layer D / I G Within a specific range, this is beneficial for improving the ionic and electronic conductivity of the positive electrode. This is because the D peak (located at approximately 1300 cm⁻¹)... -1 ±100cm -1 The peak α is generally considered to be the disorder vibration peak of graphene, used to describe the degree of defects in graphene; while the G peak (located at approximately 1580 cm⁻¹) -1 ±100cm -1 ) is the main characteristic peak of graphene. If I D / I GIf the value is too small (e.g., less than 1), it indicates that the graphene is closer to the ideal graphene, but this results in higher costs and greater processing difficulties; while 1... D / I G An excessively large value (e.g., greater than 2) indicates excessive defects in the graphene, which is detrimental to its heat dissipation and conductivity. Raman spectroscopy tests were conducted using a Ramanor T-64000 Raman spectrometer with a beam diameter of 100 nm and an argon ion laser (wavelength 514.5 nm) as the light source.

[0047] In this invention, the region resistance of the positive electrode located in the flat region can be 100mΩ-10000mΩ, for example, 100mΩ, 200mΩ, 300mΩ, 400mΩ, 500mΩ, 600mΩ, 700mΩ, 800mΩ, 900mΩ, 1000mΩ, 2000mΩ, 3000mΩ, 4000mΩ, 5000mΩ, 6000mΩ, 7000mΩ, 8000mΩ, 9000mΩ or 10000mΩ. The area resistance of the positive electrode located in the bending region can be 50mΩ-2000mΩ, for example, 50mΩ, 60mΩ, 70mΩ, 80mΩ, 90mΩ, 100mΩ, 200mΩ, 300mΩ, 400mΩ, 500mΩ, 600mΩ, 700mΩ, 800mΩ, 900mΩ, 1000mΩ or 2000mΩ.

[0048] In one example, the area resistance of the positive electrode located in the flat region is 960mΩ-980mΩ. The area resistance of the positive electrode located in the bent region is 300mΩ-900mΩ.

[0049] The resistance of a conventional positive electrode (without a functional layer containing graphene or similar materials) is approximately 100 mΩ to 10000 mΩ. Adding a functional layer containing graphene or similar materials significantly reduces the resistance of the positive electrode. The resistance of the positive electrode affects the overall internal resistance of the battery; excessive resistance can lead to excessive polarization, thereby affecting the battery's cycle life, power performance, and safety.

[0050] In this invention, the area resistance of the positive electrode located in the straight region and the area resistance of the positive electrode located in the bent region can be tested by conventional methods in the art, such as an electrode resistance meter.

[0051] In this invention, the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer can be (4-200):1, for example, 4:1, 5:1, 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1, 100:1 or 200:1.

[0052] In one example, the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is (20-80):1.

[0053] Based on heat dissipation and ion conduction effects, the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer needs to be controlled. When the thickness ratio is too large (e.g., greater than 200:1), the thickness of the functional layer is relatively thin compared to the thickness of the positive electrode active material layer. In this case, the functional layer cannot achieve effective heat dissipation and reduce the internal resistance of the battery. Conversely, when the thickness ratio is too small (e.g., less than 4:1), it will severely affect the extraction of ions in the bending region, thereby affecting the capacity utilization of the positive electrode and the energy density of the battery.

[0054] In this invention, the thickness of the positive electrode active material layer can be 10μm-120μm, for example, 10μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 110μm or 120μm.

[0055] In this invention, the thickness of the functional layer can be 0.1μm-8μm, for example, 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm or 8μm.

[0056] In this invention, the thickness of the positive electrode active material layer and the thickness of the functional layer can be tested using conventional methods in the art, such as using a scanning electron microscope (SEM) to randomly select at least 10 sites without recesses on the cross-section of the positive electrode sheet, measure the thickness of the positive electrode active material layer at each site, and take the average value; similarly, at least 10 sites without recesses are randomly selected, the thickness of the functional layer at each site is measured, and the average value is taken.

[0057] In this invention, the functional layer may further include a dispersant. The dispersant may include at least one selected from polyvinyl alcohol (PVA), poly(ethyleneimine), polyethylene oxide, melamine, polyvinylpyrrolidone (PVP), and polypropylene oxide.

[0058] In this invention, based on the total mass of the functional layer, the content of the graphene-like material can be 95%-99.9% (e.g., 95%, 96%, 97%, 98%, 99%, 99.5%, or 99.9%), and the content of the dispersant can be 0.1%-5% (e.g., 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%).

[0059] In one example, based on the total mass of the functional layer, the content of the graphene-like material is 97%-99%, and the content of the dispersant is 1%-3%.

[0060] In this invention, the positive electrode active material may include at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, and lithium-rich manganese-based materials. The positive electrode active material layer may also include a positive electrode conductive agent, which may include at least one of conductive carbon black, carbon nanotubes, conductive carbon fibers, acetylene black, and Ketjen black. The positive electrode active material layer may also include a positive electrode binder, which may include at least one of polyvinylidene fluoride, styrene-butadiene rubber, polyacrylic acid, polytetrafluoroethylene, and polyethylene oxide.

[0061] In this invention, the core has bending areas located on both sides and straight areas connected to the bending areas on both sides. For example... Figure 4 The diagram shows a schematic of a core in an embodiment of the present invention. As can be seen from the diagram, the core has bending regions 20 on both sides and a straight region 10 connected to the bending regions 20 on both sides. Furthermore, the positive electrode sheet located in any of the bending regions 20 comprises n layers from the inside out. Along the direction from the inside out of the core, the dimension L of the functional layer on the nth layer of the positive electrode sheet in the length direction of the positive electrode sheet satisfies: L=k×π×[d1+d2+(n-1)×(d1+2×d2+d3)], where d1 is the thickness of the negative electrode sheet, d2 is the thickness of the separator, d3 is the thickness of the positive electrode sheet, n≥1, and k is 1-1.2.

[0062] The inventors of this invention discovered that when k is within a specific range, it indicates that the functional layer can cover the entire bending region, and a portion of the functional layer can also be located in the flat region adjacent to the bending region; in this case, the functional layer can effectively improve the lithium plating problem in the bending region. If k is less than 1, it indicates that the functional layer does not completely cover the bending region, leading to lithium plating in the areas of the bending region not covered by the functional layer. Furthermore, when k is within a specific range, the functional layer can also fully exert its function of increasing heat dissipation and conductivity without significantly adversely affecting the transport of lithium ions in the flat region, thereby effectively delaying the occurrence of thermal runaway.

[0063] It is understood that the positive electrode, negative electrode, and separator are stacked and then wound to form a core. Therefore, from the cross-section of the core, it includes several layers of positive electrode, negative electrode, and separator. In this invention, the thickness d1 of the negative electrode is the thickness of one layer of negative electrode, the thickness d2 of the separator is the thickness of one layer of separator, and the thickness d3 of the positive electrode is the thickness of one layer of positive electrode.

[0064] In this invention, the negative electrode sheet may include a silicon-based material. The silicon-based material includes at least one of silicon, silicon oxide, silicon carbon, and silicon alloys. The negative electrode sheet may also include a carbon-based material, including at least one of natural graphite, artificial graphite, mesophase carbon microspheres, soft carbon, and hard carbon. The negative electrode sheet may also include at least one of a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersant. The negative electrode conductive agent may include at least one of conductive carbon black, carbon nanotubes, conductive carbon fibers, acetylene black, and Ketjen black. The negative electrode binder may include at least one of polyvinylidene fluoride, styrene-butadiene rubber, polyacrylic acid, polytetrafluoroethylene, and polyethylene oxide. The negative electrode dispersant may include at least one of PVA, poly(ethyleneimine), polyethylene oxide, melamine, PVP, carboxymethyl cellulose, sodium carboxymethyl cellulose, and polypropylene oxide.

[0065] In this invention, the diaphragm can be a conventional choice in the art.

[0066] A second aspect of the present invention provides a battery comprising the lithium-ion cell described in the first aspect of the present invention.

[0067] In this invention, the battery may further include an electrolyte. The electrolyte can be a conventional choice in the art.

[0068] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0069] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0070] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0071] The following examples illustrate the battery of the present invention.

[0072] Example 1

[0073] The battery is prepared according to the following method:

[0074] (1) Preparation of positive electrode sheet

[0075] NCM811 ternary material (nickel-cobalt-manganese 811 ternary material, Dv10 of 3μm), polyvinylidene fluoride, and carbon nanotubes were mixed uniformly at a mass ratio of 97.3:1.3:1.4. N-methylpyrrolidone (NMP) was added to form a positive electrode active slurry. This positive electrode active slurry was coated onto both sides of an aluminum foil with a thickness of 10μm and dried at 100℃ for 5 min. Graphene and PVP were mixed uniformly at a mass ratio of 97:3, and NMP was added to form a functional layer slurry. This functional layer slurry was applied alternately to one side of the positive electrode active material layer using a gravure coating method. After drying at 100℃ for 5 min, it was rolled (compacted density 3.35 g / cm³). 3 The functional layer surface is rolled with mechanical force to create recesses, wherein the depth d of the recesses is 15 μm, the spacing between the recesses is 350 μm, and the shape of the orthographic projection of the recesses onto the functional layer surface is square, with an area of ​​9025 μm. 2 ;I D / I G The thickness is 1.6; the thickness of the positive electrode active material layer is 38 μm, the thickness of the functional layer is 0.5 μm, and the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is 76:1.

[0076] (2) Preparation of negative electrode sheet

[0077] Artificial graphite, silicon oxide (silicon content 64%), carboxymethyl cellulose, styrene-butadiene rubber, and carbon black were mixed evenly in a mass ratio of 76:19:1.5:1.5:2. Deionized water was added to form a negative electrode active layer slurry. This negative electrode active layer slurry was coated onto the surface of a 5 μm thick copper foil, dried at 100°C for 5 minutes, and then rolled (compacted density 1.45 g / cm³). 3 ), thus obtaining the negative electrode sheet.

[0078] (3) Battery fabrication

[0079] According to the conventional lithium-ion battery winding process, the positive electrode sheet prepared in step (1) and the negative electrode sheet prepared in step (2) are made into a core. The separator is a polyethylene film with a thickness of 7 μm, the electrolyte is (carbonate solvent + lithium hexafluorophosphate with a concentration of 1 mol / L), the functional layer is located in the bending area, and the functional layer is located on the outer surface of the positive active material layer on the side of the positive electrode sheet near the winding center.

[0080] Example 2

[0081] The procedure was carried out in accordance with Example 1, except that (1) the positive electrode was prepared and (2) the negative electrode was prepared, as follows:

[0082] (1) Preparation of positive electrode sheet

[0083] NCM811 ternary material (nickel-cobalt-manganese 811 ternary material, Dv10 of 2.9 μm), polyvinylidene fluoride, and carbon nanotubes were mixed uniformly at a mass ratio of 97.3:1.3:1.4. N-methylpyrrolidone (NMP) was added to form a positive electrode active slurry. This positive electrode active slurry was coated onto both sides of an aluminum foil with a thickness of 10 μm and dried at 100°C for 5 min. Graphene oxide and PVP were mixed uniformly at a mass ratio of 99:1, and NMP was added to form a functional layer slurry. This functional layer slurry was applied alternately to one side of the positive electrode active material layer using a gravure coating method. After drying at 100°C for 5 min, it was rolled (compacted density 3.35 g / cm³). 3 The functional layer surface is rolled with mechanical force to create recesses, wherein the depth d of the recesses is 10 μm, the spacing between the recesses is 300 μm, and the shape of the orthographic projection of the recesses onto the functional layer surface is square, with an area of ​​8100 μm. 2 ;I D / I G The thickness of the positive electrode active material layer is 1.4; the thickness of the positive electrode active material layer is 38 μm, the thickness of the functional layer is 1.5 μm, and the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is 25.3:1.

[0084] (2) Preparation of negative electrode sheet

[0085] Artificial graphite, silicon-carbon (70% silicon content), carboxymethyl cellulose, styrene-butadiene rubber, and carbon black were mixed evenly in a mass ratio of 76:19:1.5:1.5:2. Deionized water was added to form a negative electrode active layer slurry. This negative electrode active layer slurry was coated onto the surface of a 5 μm thick copper foil, dried at 100°C for 5 minutes, and then rolled (compacted density 1.45 g / cm³). 3 ), thus obtaining the negative electrode sheet.

[0086] Example 3

[0087] The procedure was carried out in accordance with Example 1, except that (1) the positive electrode was prepared and (2) the negative electrode was prepared, as follows:

[0088] (1) Preparation of positive electrode sheet

[0089] NCM811 ternary material (nickel-cobalt-manganese 811 ternary material, Dv10 of 2.5 μm), polyvinylidene fluoride, and carbon nanotubes were mixed uniformly at a mass ratio of 97.3:1.3:1.4. N-methylpyrrolidone (NMP) was added to form a positive electrode active slurry. This positive electrode active slurry was coated onto both sides of an aluminum foil with a thickness of 10 μm and dried at 100°C for 5 min. Reduced graphene oxide and PVP were mixed uniformly at a mass ratio of 95:5. NMP was added to form a functional layer slurry. This functional layer slurry was applied alternately to one side of the positive electrode active material layer using a gravure coating method. After drying at 100°C for 5 min, it was rolled (compacted density 3.35 g / cm³). 3 The functional layer surface is rolled and formed with mechanical force to create recesses, wherein the depth d of the recesses is 20 μm, the spacing between the recesses is 400 μm, and the shape of the orthographic projection of the recesses onto the functional layer surface is square, the area of ​​the orthographic projection being 10000 μm. 2 ;I D / I G The thickness is 1.8; the thickness of the positive electrode active material layer is 38 μm, the thickness of the functional layer is 1 μm, and the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is 38:1.

[0090] (2) Preparation of negative electrode sheet

[0091] Artificial graphite, silicon-carbon (70% silicon content), carboxymethyl cellulose, styrene-butadiene rubber, and carbon black were mixed evenly in a mass ratio of 76:19:1.5:1.5:2. Deionized water was added to form a negative electrode active layer slurry. This negative electrode active layer slurry was coated onto the surface of a 5 μm thick copper foil, dried at 100°C for 5 minutes, and then rolled (compacted density 1.45 g / cm³). 3 ), thus obtaining the negative electrode sheet.

[0092] Example 4

[0093] Used to verify the effect of "the presence or absence of a concave part".

[0094] The same procedure is followed as in Example 1, except that no recesses are created on the surface of the functional layer.

[0095] Example 5 group

[0096] This set of examples is used to verify the impact of changing the "position of the recess on the surface of the positive electrode".

[0097] This set of embodiments is based on Embodiment 1, except that the position of the recess on the surface of the positive electrode is changed, as follows:

[0098] Example 5a: A recess is created by rolling under mechanical force on the outer surface of the positive electrode active material layer and the surface of the functional layer;

[0099] Example 5b: A recess is created on the outer surface of the positive electrode active material layer by rolling under mechanical force (no recess is provided on the surface of the functional layer).

[0100] Example 6 group

[0101] This set of examples is used to verify the impact of changes in the "recess size".

[0102] This set of embodiments is based on Embodiment 1, except that the size of the recess is changed, as follows:

[0103] In Example 6a, the depth d of the recess is 5 μm (greater than Dv10), the spacing between the recesses is 20 μm, and the shape of the orthographic projection of the recesses onto the surface of the functional layer is square, with an area of ​​9025 μm². 2 ;

[0104] In Example 6b, the depth d of the recess is 25 μm, the spacing between the recesses is 500 μm, and the shape of the orthographic projection of the recess onto the surface of the functional layer is square, with an area of ​​900 μm. 2 ;

[0105] In Example 6c, the depth d of the recess is 2 μm (less than Dv10), the spacing between the recesses is 20 μm, and the shape of the orthographic projection of the recess onto the surface of the functional layer is square, with an area of ​​40000 μm. 2 .

[0106] Example 7 group

[0107] This set of examples is used to verify "I" D / I G The impact of the change.

[0108] This set of embodiments is based on Embodiment 1, except that the graphene is changed to alter I. D / I G The details are as follows:

[0109] In Example 7a, I D / I G It is 1.2;

[0110] In Example 7b, I D / I G The value is 2.

[0111] Example 8 group

[0112] This set of examples is used to verify the impact of changes in the "ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer".

[0113] This set of embodiments is based on Embodiment 1, except that the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is changed by altering the thickness of the functional layer, as detailed below:

[0114] Example 8a: The thickness of the functional layer is 0.3 μm, and the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is 126.7:1;

[0115] Example 8b: The thickness of the functional layer is 8 μm, and the ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is 4.75:1.

[0116] Example 9 group

[0117] This set of examples is used to verify the impact of the change in "L=k×π×[d1+d2+(n-1)×(d1+2×d2+d3)]".

[0118] This set of embodiments follows the same procedure as Embodiment 1, except that the coating size of the functional layer is changed to alter L = k × π × [d1 + d2 + (n-1) × (d1 + 2 × d2 + d3)], as detailed below:

[0119] Example 9a, k = 0.8;

[0120] Example 9b, k = 1.5.

[0121] Example 10

[0122] This was used to verify the impact of changes in the "position of the recess in the battery".

[0123] The same procedure is followed as in Example 1, except that the functional layer is located on the outer surface of the positive electrode active material layer on the side away from the winding center of the positive electrode sheet, that is, the recess is located on the side away from the winding center of the positive electrode sheet.

[0124] Example 11

[0125] The procedure is carried out in accordance with Example 1, except that the functional layer slurry is continuously coated on the surface of the positive electrode active material layer on one side (a recess is created on the surface of the functional layer by rolling under mechanical force).

[0126] Examples 1-8 and Example 10 all satisfy: L=k×π×[d1+d2+(n-1)×(d1+2×d2+d3)], where k is 1-1.2.

[0127] Comparative Example 1

[0128] The procedure was carried out in accordance with Example 4, except that the positional relationship between the positive electrode active material layer and the functional layer was changed, as follows:

[0129] The functional layer slurry was applied to one side of an aluminum foil with a thickness of 10 μm using a gravure coating method and dried at 100°C for 5 min. The positive electrode active slurry was then applied to both sides of the dried aluminum foil coated with the functional layer slurry and dried at 100°C for 5 min.

[0130] Comparative Example 2

[0131] The same procedure applies as in Example 1, except that the functional layer is located in the flat area (i.e., the functional layer is set in the flat area, and no functional layer is set in the bending area).

[0132] Comparative Example 3

[0133] Refer to Example 1, except that no functional layer is set, as follows:

[0134] The positive electrode active slurry was coated on both sides of an aluminum foil with a thickness of 10 μm, dried at 100°C for 5 min, and then rolled to obtain a positive electrode sheet.

[0135] Test Case I

[0136] Resistance test

[0137] The negative electrode sheets prepared in the examples and comparative examples were subjected to area resistance testing using an electrode resistance meter. The specific testing methods are as follows:

[0138] Take the negative electrode sheet and use the Yuaneng Technology PRCD resistivity meter to test the area resistance of the positive electrode sheet in the flat region and the bending region, as follows: place the test probe in the flat region to obtain the area resistance of the flat region; place the test probe in the bending region to obtain the area resistance of the bending region. Test each region 10 times, and take the average value of the area resistance of the bending region and record it in Table 1. In Examples 1-10, the area resistance of the flat region is in the range of 960mΩ-980mΩ; in Example 11, the area resistance of the flat region is in the range of 300mΩ-900mΩ.

[0139] Table 1

[0140] The area resistance (mΩ) of the region located in the bending area. Example 1 425 Example 2 382 Example 3 589 Example 4 427 Example 5a 423 Example 5b 427 Example 6a 425 Example 6b 426 Example 6c 428 Example 7a 301 Example 7b 856 Example 8a 546 Example 8b 226 Example 9a 423 Example 9b 422 Example 10 425 Example 11 420

[0141] Test Case II

[0142] (1) Temperature rise test of discharge rate

[0143] The temperature sensor is fixed to the largest surface of the battery, and a high-rate discharge surface temperature test is performed. The specific test steps are as follows:

[0144] The test temperature was 45℃, and the reference magnification was 0.5℃.

[0145] 1. Let stand for 2 hours;

[0146] 2. Discharge at 0.5C to the lower limit voltage (2.5V);

[0147] 3. Let stand for 60 minutes;

[0148] 4. Charge at 1.8C to the upper limit voltage (4.3V), then maintain constant voltage at 0.05C;

[0149] 5. Let stand for 30 minutes;

[0150] 6. Discharge the battery at a 9C rate to the lower limit voltage (2.5V), record the surface temperature of the battery, and record the maximum temperature value in the recorded data in Table 2.

[0151] (2) Thickness expansion rate test

[0152] The batteries prepared in the examples and comparative examples were subjected to thickness expansion rate testing. The specific testing steps are as follows:

[0153] The battery thickness is measured as t0. The battery is subjected to the following cycles: charged at 0.7C to the upper limit voltage (4.3V), cutoff current 0.025C, discharged at 0.5C to the lower limit voltage (2.5V), for 300 cycles. The battery thickness after the cycles is measured as t1 when fully charged. The thickness expansion rate is then calculated as (t1-t0) / t0×100%. The results are recorded in Table 2. Figure 5 The image shown is an optical photograph of the cross-section of the battery in Example 1 after a thickness expansion rate test. Figure 6 The image shown is an optical photograph of the cross-section of the battery in Comparative Example 3 after the thickness expansion rate test. By comparing the two images, it can be seen that setting a functional layer with graphene-like material in the bending area of ​​the battery and setting a recess on the functional layer can significantly improve the deformation problem of the battery.

[0154] (3) Lithium plating test

[0155] The batteries prepared in the examples and comparative examples were subjected to lithium plating tests. The specific test steps are as follows: charging at 0.7C to the upper limit voltage (4.3V), cutting off the current at 0.025C, discharging at 0.5C to the lower limit voltage (2.5V), cycling 300 times, disassembling the battery under full charge, and observing the lithium plating in the bending area of ​​the negative electrode. The results are recorded in Table 2. Among them, the lithium plating degree is ranked from light to heavy as follows: no lithium plating, slight lithium plating, lithium plating, and severe lithium plating. No lithium plating means: no lithium plating in the bending area; slight lithium plating means: the ratio of the area of ​​lithium plating in the bending area to the total area of ​​the bending area is greater than 0 and less than or equal to 5%; lithium plating means: the ratio of the area of ​​lithium plating in the bending area to the total area of ​​the bending area is greater than 0.5% and less than or equal to 30%; severe lithium plating means: the ratio of the area of ​​lithium plating in the bending area to the total area of ​​the bending area is greater than 30%.

[0156] (4) Short circuit test

[0157] The batteries prepared in the examples and comparative examples were subjected to Hipot short-circuit tests. The test voltage was 100V, the pressure was 140kg.f, and the test time was 1 second. The batteries with an output resistance of <20MΩ were considered defective, and the batteries with an output resistance of <20MΩ were considered good. The defect rate was calculated. 100 batteries were tested in each example and comparative example group, and the results were recorded in Table 2.

[0158] (5) Energy density

[0159] The energy density of the batteries prepared in the examples and comparative examples was tested. The specific testing steps are as follows:

[0160] Charge the battery at 0.7C to the upper limit voltage (4.3V), cut off the current at 0.025C, discharge at 0.5C to the lower limit voltage (2.5V), output discharge capacity, measure the length and width of the battery with a CCD instrument, measure the thickness of the battery with a PPG instrument, calculate the energy density using the formula: Energy density = Capacity × Operating voltage / (Length × Width × Thickness), and record the results in Table 2.

[0161] Table 2

[0162]

[0163]

[0164] As shown in Table 2, the lithium-ion battery prepared by the present invention, compared with the comparative example, can achieve a balance between lower temperature rise, thickness expansion rate, hit-pot defect rate, and higher energy density, and can significantly improve the lithium plating problem. A comparison between Example 11 and Example 1 shows that simultaneously setting a functional layer in the flat region of the core can further reduce the battery temperature rise. However, since the functional layer affects the insertion and extraction of lithium ions, the energy density of the battery in Example 11 is significantly lower than that of the battery in Example 1.

[0165] 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 lithium-ion battery cell, characterized in that, The lithium-ion battery cell includes a positive electrode, a separator, and a negative electrode. The positive electrode, the separator, and the negative electrode are stacked and wound to form a core. The core includes a straight area and a bent area. The positive electrode sheet, at least partially located in the flat region, includes a positive current collector and a positive active material layer located on at least one side surface of the positive current collector; The positive electrode sheet, which is at least partially located in the bending region, includes the positive current collector, the positive active material layer located on at least one side surface of the positive current collector, and a functional layer located on at least one side surface of the positive active material layer opposite to the positive current collector. The functional layer includes graphene-like materials.

2. The lithium-ion battery cell according to claim 1, wherein, The surface of the functional layer located on one side of the positive electrode sheet, away from the positive electrode current collector, has several recesses; Preferably, the recess is located on the functional layer of the positive electrode sheet on the side near the winding center of the core.

3. The lithium-ion battery cell according to claim 1 or 2, wherein, The graphene-like substances include at least one of graphene, graphene oxide, and reduced graphene oxide.

4. The lithium-ion battery cell according to claim 2, wherein, The positive electrode active material layer includes a positive electrode active material, and the particle size Dv10 of the positive electrode active material and the depth d of the recess satisfy: Dv10≤d≤25μm.

5. The lithium-ion battery cell according to claim 2, wherein, The depth d of the recess is 5μm-25μm; preferably 10μm-20μm. And / or, the spacing between the recesses is 20μm-500μm; preferably 300μm-400μm; And / or, the area of ​​the orthographic projection of the recess onto the surface of the functional layer is 900 μm. 2 -40000μm 2 Preferably 8000μm 2 -10000μm 2 .

6. The lithium-ion battery cell according to claim 1 or 2, wherein, The Raman spectrum of the functional layer has a D peak and a G peak, and the intensity of the D peak is I. D With the intensity I of the G peak G The ratio I D / I G The value is 1-2; preferably 1.2-2; more preferably 1.4-1.

8.

7. The lithium-ion battery cell according to claim 1 or 2, wherein, The area resistance of the positive electrode located in the flat region is 100mΩ-10000mΩ; preferably 960mΩ-980mΩ. And / or, the area resistance of the positive electrode located in the bending region is 50mΩ-5000mΩ; preferably 300mΩ-900mΩ.

8. The lithium-ion battery cell according to claim 1 or 2, wherein, The ratio of the thickness of the positive electrode active material layer to the thickness of the functional layer is (4-200):1; preferably (20-80):

1.

9. The lithium-ion battery cell according to claim 1 or 2, wherein, The core has bending areas on both sides and straight areas connected to the bending areas on both sides; the positive electrode sheet located in the bending area on either side includes n layers from the inside to the outside, and along the direction from the inside to the outside of the core, the dimension L of the functional layer on the nth layer of the positive electrode sheet in the length direction of the positive electrode sheet satisfies: L=k×π×[d1+d2+(n-1)×(d1+2×d2+d3)], where d1 is the thickness of the negative electrode sheet, d2 is the thickness of the separator, d3 is the thickness of the positive electrode sheet, n≥1, and k is 1-1.

2.

10. A battery, characterized in that, The battery comprises a lithium-ion cell according to any one of claims 1-9.

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