Battery cell and lithium ion secondary battery

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a battery cell and a secondary battery. According to the battery cell, the elongation A of the diaphragm, the elongation B of the negative electrode current collector and the thickness H of the negative electrode active layer are regulated and controlled to meet the condition that A * B / H is greater than or equal to 0.0003 and less than or equal to 0.0063, so that the deformation of the negative electrode plates in the arc region can be reduced, and the contact between the electrode plates is improved, thereby improving the circulation and rate performance and the safety of the battery cell.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries, and particularly to a battery cell and a lithium-ion secondary battery. Background Art

[0002] The wound lithium-ion secondary battery is widely used in various electronic devices because its size can be customized, and it has high energy density and low production cost. However, the wound structure of the lithium-ion secondary battery also determines that the electrode sheets in the arc area are prone to deformation under the stress of repeated expansion during the charge and discharge cycles. Especially at high voltages, significant volume changes in the electrode materials, such as the swelling of the negative electrode during lithium intercalation and the structural changes of the positive electrode, will further exacerbate the stress concentration, resulting in a greater squeezing effect on the electrode sheets in the arc area and making them more likely to deform, thereby causing the failure of the battery cell and ultimately affecting the performance and lifespan of the battery cell. Summary of the Invention

[0003] In view of this, this application provides a battery cell and a lithium-ion secondary battery, aiming to solve to a certain extent the problem that the electrode sheets in the arc area of the existing battery cell are prone to deformation.

[0004] According to an embodiment of this application, in a first aspect, a battery cell is provided. The charging cut-off voltage of the battery cell is ≥4.48V; the battery cell includes a wound stack of a positive electrode sheet, a separator, and a negative electrode sheet; the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector; the battery cell satisfies: 0.0003 ≤ (A * B) / H ≤ 0.0063;

[0005] A is the elongation rate of the separator;

[0006] B is the elongation rate of the negative electrode current collector;

[0007] H is the thickness of the negative electrode active layer, in μm.

[0008] In some alternative embodiments, it satisfies: 0.0005 ≤ (A * B) / H ≤ 0.0044.

[0009] In some alternative embodiments, 30% ≤ A ≤ 180%.

[0010] Further, in some alternative embodiments, 65% ≤ A ≤ 160%.

[0011] In some alternative embodiments, 1.8% ≤ B ≤ 7%.

[0012] Further, in some alternative embodiments, 3.6% ≤ B ≤ 8%.

[0013] In some alternative embodiments, 20 ≤ H ≤ 60.

[0014] Further, in some alternative embodiments, 30 ≤ H ≤ 45.

[0015] In some alternative embodiments, the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes graphite and a silicon-carbon material; the OI value of the negative electrode sheet is C; the particle size of the negative electrode active layer is D0, in μm; it satisfies: 0.4 ≤ (A * C) / D0 ≤ 5.1, and / or, 0.02 ≤ (B * C) / D0 ≤ 0.26.

[0016] Further, in some alternative embodiments, 0.55 ≤ (A * C) / D0 ≤ 2.

[0017] Further, in some alternative embodiments, 0.03 ≤ (B * C) / D0 ≤ 0.1.

[0018] In some alternative embodiments, 8 ≤ C ≤ 32.

[0019] Further, in some alternative embodiments, 10 ≤ C ≤ 20.

[0020] In some alternative embodiments, 10 ≤ D0 ≤ 19.

[0021] Further, in some alternative embodiments, 12 ≤ D0 ≤ 16.5.

[0022] In some alternative embodiments, the battery cell satisfies: 1700 ≤ H / (E * K) ≤ 4800; E is the pore size of the separator, in μm; K is the porosity of the separator.

[0023] Further, in some alternative embodiments, it satisfies: 2200 ≤ H / (E * K) ≤ 4200.

[0024] In some alternative embodiments, 0.02 ≤ E ≤ 0.1.

[0025] In some alternative embodiments, 10% ≤ K ≤ 48%.

[0026] Further, in some alternative embodiments, 25% ≤ K ≤ 38%.

[0027] In some alternative embodiments, the average particle size of the silicon-carbon material is D1, in μm; the separator includes a base film, and the tensile strength of the base film is Q1, in kgf / cm 2 ; the negative electrode current collector includes a (111) crystal plane, and the intensity of the XRD diffraction peak of the (111) crystal plane is Q2; it satisfies: 4000000 ≤ (Q1 * Q2) / D1 ≤ 58000000.

[0028] Further, in some alternative embodiments, 14000000 ≤ (Q1 * Q2) / D1 ≤ 37000000.

[0029] In some alternative embodiments, 1000 ≤ Q1 ≤ 3800.

[0030] Further, in some alternative embodiments, 1800 ≤ Q1 ≤ 3100.

[0031] In some alternative embodiments, 30000 ≤ Q2 ≤ 80000.

[0032] Further, in some alternative embodiments, 54000 ≤ Q2 ≤ 71000.

[0033] In some alternative embodiments, 3.1 ≤ D ≤ 15.8.

[0034] Further, in some alternative embodiments, 6 ≤ D ≤ 10.

[0035] In some alternative embodiments, the negative electrode current collector includes a copper foil.

[0036] In some alternative embodiments, the silicon-carbon material includes spherical silicon-carbon, the spherical silicon-carbon includes silicon-carbon single particles and silicon-carbon secondary particles, and the average particle size D1 of the silicon-carbon material, the average particle size D2 (unit: μm) of the silicon-carbon single particles, and the average particle size D3 (unit: μm) of the silicon-carbon secondary particles satisfy: 0.43 ≤ D1 / (D2 + D3) ≤ 0.7.

[0037] In some alternative embodiments, 2 ≤ D2 ≤ 11;

[0038] In some alternative embodiments, 5 ≤ D3 ≤ 17.

[0039] In some alternative embodiments, grooves are provided on the surface of the negative electrode active layer facing away from the negative electrode current collector, and the ratio of the volume of the grooves to the volume of the negative electrode active layer is M, 0.2% ≤ M ≤ 2%;

[0040] The air permeability of the separator is N, 100 sec / 100 cc ≤ N ≤ 400 sec / 100 cc;

[0041] Satisfy: 0.5 ≤ M * N ≤ 8.

[0042] In some alternative embodiments, the separator includes a base film and an adhesive layer, the adhesive layer is provided on at least one surface of the base film, and the maximum thickness of the adhesive layer is denoted as L, 0.5 μm ≤ L ≤ 4 μm;

[0043] The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector. A concave portion is provided on the surface of the positive electrode active layer facing away from the positive electrode current collector. The positive electrode active layer at the concave portion is recessed toward the positive electrode current collector. The recessed depth of the concave portion is G, and 10 μm ≤ G ≤ 60 μm;

[0044] Satisfy: 14 ≤ G + L ≤ 41.

[0045] According to an embodiment of the present application, in a second aspect, a lithium-ion secondary battery is provided, including the battery cell described in the first aspect.

[0046] The technical solution of the present application has the following advantages:

[0047] For the battery cell provided by the present application, by regulating that the diaphragm elongation rate A, the negative electrode current collector elongation rate B, and the negative electrode active layer thickness H satisfy 0.0003 ≤ A * B / H ≤ 0.0063, the deformation of the negative electrode sheet in the arc region can be reduced, and the contact between the electrode sheets can be improved, thereby improving the cycle and rate performance and safety of the battery cell.

[0048] The additional aspects and advantages of the embodiments of the present application will be partially described and shown in the subsequent description, or will be explained through the implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0050] Figure 1 is a cross-sectional view of the battery cell in the thickness direction in an embodiment of the present application.

[0051] Figure 2 is a cross-sectional view of the negative electrode sheet in the thickness direction in an embodiment of the present application.

[0052] Figure 3 is Figure 2 the top view of

[0053] Figure 4 is a cross-sectional view of the positive electrode sheet in the thickness direction in an embodiment of the present application.

[0054] Figure 5 is Figure 4 the top view of

[0055] Figure 6 isFigure 4 Partial enlarged view of the concave portion.

[0056] Among them, the description of the drawings is as follows:

[0057] 10. Diaphragm; 11. Negative current collector; 12. Negative active layer; 13. Groove; 21. Positive current collector; 22. Positive active layer; 23. Concave portion; G. Depression depth; S1. Straight region; S2. Arc region. Specific embodiments

[0058] The following embodiments are provided to better further understand the present application, which is not limited to the described best mode, and does not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior art features falls within the protection scope of the present application.

[0059] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0060] In an embodiment of the present application, a battery cell is provided, and the charging cut-off voltage of the battery cell ≥ 4.48V, for example, 4.48V, 4.5V or 4.53V. The term "charging cut-off voltage" in the present application has the conventional meaning in the art, and generally refers to the maximum voltage value that the battery cell can safely reach during the charging process.

[0061] The following Figures 1 to 6 describes the embodiments of the present application.

[0062] As Figure 1 shown, the battery cell includes a wound positive electrode sheet, a diaphragm and a negative electrode sheet, and forms a straight region S1 and arc regions S2 located at both ends of the straight region S1; the positive electrode sheet of the battery cell includes a positive current collector 21 and a positive active layer 22 provided on at least one surface of the positive current collector 21; the negative electrode sheet includes a negative current collector 11 and a negative active layer 12 provided on at least one surface of the negative current collector 11, and the negative active layer 12 includes a negative active material, a binder and a conductive agent, wherein the binder can be at least one of styrene-butadiene rubber (SBR) or polyacrylic acid (PAA), etc.

[0063] On the one hand, during the winding and forming process of the battery cell, the deformation of the negative active layer at the bending part (i.e., the arc area) of the negative electrode sheet is larger than that of the negative active layer in the flat area. Among them, the negative active layer on the outer side (away from the winding center direction) of the arc area is stretched to both sides, which easily causes cracks or even powder falling in the outer negative active layer. While the negative active layer on the inner side (towards the winding center direction) of the arc area is squeezed towards the middle, thus causing extrusion deformation of the negative electrode sheet and the separator in the arc area, resulting in a reduction in the thickness of the separator and the negative electrode sheet. On the other hand, at high voltage (charging cut-off voltage ≥ 4.48V), the insertion and extraction speed of lithium ions will accelerate, leading to an increase in the temperature rise of the negative active layer and causing excessive expansion of the negative active layer, and further causing an increase in the overall expansion of the battery cell. And because the battery cell is wound from the inside out, the bending deformation of the negative active layer in the arc area itself is superimposed with the excessive expansion of the negative active layer under high voltage. In summary, the overall deformation displacement of the negative electrode sheet will gradually accumulate to the outermost layer, making the multi-layer negative electrode sheets near the outermost layer more likely to break, and ultimately causing abnormal charge and discharge of the battery cell.

[0064] Furthermore, the present application studies and finds that the negative active layer is sandwiched between the negative current collector and the separator. Since the negative active layer contains adhesive substances, the negative active layer can be effectively adhered to the negative current collector, and a bonding force will be generated between the separator and the negative active layer after hot pressing, so that the separator and the negative current collector can simultaneously apply binding forces to both surfaces of the negative active layer when stretching themselves, that is, the separator and the negative current collector can jointly act to limit the expansion and displacement of the negative active layer.

[0065] However, at the same time, on the one hand, at high voltage (charging cut-off voltage ≥ 4.48V), the temperature rise of the battery cell intensifies, which will cause a significant reduction in the viscosity of the adhesive in the negative active layer and the glue layer in the separator, and further lead to a weakening of the binding forces applied by the separator and the negative current collector to both surfaces of the negative active layer, resulting in a weakening of the force suppressing the expansion and displacement of the negative active layer, thus causing an increase in the deformation of the negative active layer, and it may cause cracks and gradual fractures in the negative current collector. On the other hand, the greater the thickness of the negative active layer, the greater the total expansion displacement per unit area, making the tensile or extrusion force applied by the negative active layer on the separator and the negative current collector greater. However, usually the elongation rate of the separator is much higher than that of the negative current collector. When the expansion displacement of the negative active layer reaches the critical point, it may cause cracks and gradual fractures in the negative current collector.

[0066] To solve the above problems existing in the related art, in some embodiments, the present application provides an electric core, the charging cut-off voltage of the electric core ≥ 4.48V; the electric core includes a stacked and wound positive electrode sheet, a separator, and a negative electrode sheet; the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector; the electric core satisfies: 0.0003 ≤ A * B / H ≤ 0.0063;

[0067] A is the elongation of the separator;

[0068] B is the elongation of the negative electrode current collector;

[0069] H is the thickness of the negative electrode active layer, with the unit of μm.

[0070] In the present application, by regulating that the elongation A of the separator, the elongation B of the negative electrode current collector, and the thickness H of the negative electrode active layer satisfy 0.0003 ≤ (A * B) / H ≤ 0.0063. In this way, by regulating that the thickness of the negative electrode active layer is positively correlated with the elongations of the separator and the negative electrode current collector respectively, those skilled in the art can adjust the thickness of the negative electrode active layer within the above range to reduce the probability of the negative electrode sheet breaking under high voltage, thereby reducing the deformation of the negative electrode sheet in the arc region, making the contact surface between the electrode sheets smoother, and thus improving the cycle and rate performance and safety of the electric core.

[0071] Further, in some embodiments, the elongation A of the separator, the elongation B of the negative electrode current collector, and the thickness H of the negative electrode active layer satisfy 0.0005 ≤ (A * B) / H ≤ 0.0044, thereby ensuring a better fit among the thickness of the negative electrode active layer, the negative electrode current collector, and the separator, effectively reducing the relative displacement amount between the negative electrode active layer and the negative electrode current collector and the separator, further reducing the probability of the negative electrode sheet breaking under high voltage, and further effectively improving the cycle and rate performance and safety of the electric core.

[0072] Exemplarily, the value of (A * B) / H can be 0.0004, 0.0005, 0.001, 0.0017, 0.002, 0.0025, 0.003, 0.0035, 0.004, 0.0044, 0.005, 0.0055, 0.0063, etc. or within the range formed by any two of the above values.

[0073] Specifically, the elongation refers to the percentage of the total deformation ΔL of the gauge section after the specimen is stretched and fractured to the original gauge length L.

[0074] In this application, the test method for the diaphragm elongation rate A includes: cutting the diaphragm longitudinally into samples with a size of 15 mm ± 0.2 mm, using a WD-D3 type electronic universal testing machine with a gap of 50 mm between the upper and lower fixtures, clamping both ends of the sample in the fixtures respectively, starting the test at a speed of 100 mm / min until the equipment stops the test, recording the elongation rate, conducting the test 3 times with an error not exceeding 10%, and taking the average value.

[0075] In some embodiments, the diaphragm elongation rate A satisfies: 30% ≤ A ≤ 180%. The larger the diaphragm elongation rate, the greater the binding effect on the negative active layer, and the less likely the arc area of the negative electrode sheet is to break. However, if the diaphragm elongation rate is too large, it will cause too high pressure inside the battery cell, deformation of the diaphragm, loss of isolation function, and then lead to internal short circuit of the battery cell, affecting the cycle life and safety of the battery. When the diaphragm elongation rate is too small, the phenomenon of plate shaping is likely to occur inside the battery cell, which not only cannot alleviate the fracture problem in the arc area of the negative electrode sheet, but also leads to a decrease in battery capacity.

[0076] Furthermore, in some embodiments, by controlling the diaphragm elongation rate A within the range of 65% - 160%, a strong binding effect can be formed on the negative active layer, further reducing the deformation of the arc area of the negative electrode sheet, and then reducing the probability of the negative electrode sheet breaking.

[0077] Exemplarily, the elongation rate A of the diaphragm can be 30%, 50%, 65%, 80%, 100%, 115%, 130%, 150%, 160%, 180%, etc. or within the range composed of any two of the above values.

[0078] The test method for the elongation rate B of the negative current collector includes: cutting the negative current collector longitudinally into samples with a size of 15 mm ± 0.2 mm, using a WD-D3 type electronic universal testing machine with a gap of 50 mm between the upper and lower fixtures, clamping both ends of the sample in the fixtures respectively, starting the test at a speed of 50 mm / min until the equipment stops the test, and recording the elongation rate, conducting the test 3 times with an error not exceeding 10%, and taking the average value.

[0079] In some embodiments, the elongation rate B of the negative current collector satisfies: 1.8% ≤ B ≤ 10%. During the charge and discharge process of the battery, the negative current collector needs to withstand alternating stress. If the elongation rate of the negative current collector is too small, the negative current collector is likely to wrinkle and break under the action of stress, resulting in a decline in battery performance and even causing safety problems. For a negative current collector with too large an elongation rate, uneven lithium intercalation is likely to occur during the battery cycle, accelerating cycle failure, leading to a decline in battery performance and a shorter cycle life.

[0080] Furthermore, in some embodiments, by controlling the elongation rate B of the negative electrode current collector within the range of 3.6% - 8%, not only can the stress-bearing capacity and lithium intercalation capacity of the negative electrode current collector be ensured, but also a strong binding effect on the negative electrode active layer can be formed, further reducing the deformation of the arc region of the negative electrode sheet, and thus reducing the probability of the negative electrode sheet breaking.

[0081] Exemplarily, the elongation rate B of the negative electrode current collector can be 1.8%, 2.5%, 3%, 3.6%, 4.4%, 5%, 5.5%, 6%, 6.5%, 7%, 8%, 9%, 10%, etc. or within the range composed of any two of the above values.

[0082] The testing method for the thickness H of the negative electrode active layer includes: discharging the lithium-ion secondary battery to 0% SOC and then disassembling to take out the negative electrode sheet. Immerse it in dimethyl carbonate (DMC) solvent for 12 h and then rinse it with DMC to remove the lithium salt attached to the negative electrode sheet. Grind the cross-section of the cleaned negative electrode sheet with an argon ion milling device, use SEM to photograph the cross-sectional morphology, measure the single-sided paste thickness (i.e., the distance from the foil to the surface of the electrode), and the error of the 3 tests does not exceed 10%, and take the average value.

[0083] In some embodiments, the thickness H of the negative electrode active layer satisfies: 20 ≤ H ≤ 60. The thicker the negative electrode active layer, the greater the probability of the arc region of the negative electrode sheet breaking. At the same time, the distance for the electrolyte to infiltrate into the electrode is longer, the ion migration distance is longer, and the polarization internal resistance of the battery increases, affecting the charging speed and energy density of the battery. If the negative electrode active layer is too thin, it cannot fully cover the current collector, resulting in a smaller reaction area, a decrease in battery capacity, and the negative electrode side is also prone to corrosion and mechanical damage, affecting the battery life.

[0084] Furthermore, in some embodiments, by controlling the thickness H of the negative electrode active layer within the range of 30 μm - 45 μm, the energy density and rate performance of the battery can be balanced. At the same time, it can also avoid the excessive increase in swelling caused by too large a thickness of the negative electrode active layer, resulting in an increase in the deformation of the negative electrode current collector and causing cracks or fractures in the negative electrode current collector.

[0085] Exemplarily, the thickness H of the negative electrode active layer can be 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, etc. or within the range composed of any two of the above values.

[0086] The negative electrode active layer includes negative electrode active materials, and the negative electrode active materials include graphite and silicon-carbon materials. The OI value of the negative electrode sheet reflects the degree of orientation of the graphite material, that is, the degree of orderliness of the arrangement of graphite sheets. The test method for the OI value includes: dissecting the battery to obtain the negative electrode sheet, soaking it in dimethyl carbonate (DMC) for 12 h to remove the residual electrolyte, cutting the negative electrode sheet to obtain a sample sheet with an area of 2 cm * 2 cm, using XRD for step scanning, with a scanning range of 52° - 79° and a step width of 0.0131°. The diffraction peak that appears at 2θ = 54° - 55° in the obtained diffraction pattern is the (004) peak of graphite, and its intensity is denoted as I004. The diffraction peak that appears at 2θ = 77° - 78° is the (110) peak of graphite, and its intensity is denoted as I110. The OI value of the negative electrode sheet is I004 / I110.

[0087] The larger the OI value of the negative electrode sheet, the greater the expansion of the negative electrode active layer in its thickness direction, and correspondingly, the smaller the extension in the direction parallel to the surface of the negative electrode active layer. At this time, there is no need to use a separator or a negative electrode current collector with a high elongation rate. That is, the OI value is negatively correlated with the separator elongation rate A or the negative electrode current collector elongation rate B. The larger the OI value, the smaller A or B. Vice versa. In some embodiments, the OI value C of the negative electrode sheet satisfies: 8 ≤ C ≤ 32. As an example, the value of C can be 8, 10, 12, 14, 16, 18, 20, 24, 26, 30, 32, etc., or within the range composed of any two of the above values.

[0088] The particle size D0 of the negative electrode active layer refers to the particle size corresponding to when the cumulative volume distribution of all solid particles in the negative electrode active layer reaches 50%, with the unit of μm. D0 is tested using a laser particle size analyzer. The test method includes: discharging the lithium-ion secondary battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking it in dimethyl carbonate (DMC) solvent for 12 h and then rinsing it with DMC to remove the lithium salts attached to the electrode sheet, soaking it in water to detach the negative electrode paste from the current collector, then drying it in an oven at 80°C, and then grinding the large paste with a mortar for 1 min to make it finer to obtain a composite material, and using a Malvern particle size tester for measurement.

[0089] The larger the particle size D0 of the negative electrode active layer, the greater the degree of expansion of the negative electrode sheet, and the OI value of the negative electrode sheet correspondingly increases. That is, the OI value is positively correlated with D0. Vice versa. In some embodiments, the particle size D0 of the negative electrode active layer satisfies: 10 ≤ D0 ≤ 19. As an example, the value of D0 can be 10, 11, 12, 13, 14, 15, 16.5, 17, 19, etc., or within the range composed of any two of the above values.

[0090] In some embodiments, by adjusting the OI value C of the negative electrode sheet to satisfy 0.4 ≤ (A * C) / D0 ≤ 5.1 with the particle size D0 of the negative electrode active layer and the diaphragm elongation rate A, the expansion of the negative electrode sheet can be further reduced, the probability of the negative electrode sheet breaking can be decreased, and at the same time, the compaction density of the negative electrode sheet can be taken into account to ensure the energy density and rate performance of the battery.

[0091] If the value of (A * C) / D0 is too small, it indicates that the diaphragm elongation rate is small, the OI value of the negative electrode sheet is small, and the particle size of the negative electrode active layer is large. At this time, the expansion of the negative electrode active layer in the direction parallel to its surface is large, while the binding effect of the diaphragm on the negative electrode active layer is small, and the negative electrode active particles expand greatly, which easily causes the negative electrode sheet to break, and the lithium-ion diffusion path becomes longer, and the active sites decrease, resulting in a decrease in the rate performance of the battery; if the value of (A * C) / D0 is too large, it indicates that the diaphragm elongation rate is large, the OI value of the negative electrode sheet is large, and the particle size of the negative electrode active layer is small. At this time, the expansion of the negative electrode active layer in its thickness direction is large, and coupled with the small compaction density of the negative electrode, the energy density of the battery decreases and the expansion rate increases.

[0092] Further, when the OI value C of the negative electrode sheet satisfies 0.55 ≤ (A * C) / D0 ≤ 2 with the particle size D0 of the negative electrode active layer and the diaphragm elongation rate A, the energy density, rate performance, and safety of the battery can be ensured to reach the best combination.

[0093] Exemplarily, the value of (A * C) / D0 can be 0.4, 0.45, 0.5, 0.55, 0.75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5.1, etc. or within the range composed of any two of the above values.

[0094] In some embodiments, by adjusting the OI value C of the negative electrode sheet to satisfy 0.02 ≤ (B * C) / D0 ≤ 0.26 with the particle size D0 of the negative electrode active layer and the elongation rate B of the negative electrode current collector, the expansion of the negative electrode sheet can be further reduced, the probability of the negative electrode sheet breaking can be decreased, and at the same time, the compaction density of the negative electrode sheet can be taken into account to ensure the energy density and rate performance of the battery.

[0095] If the value of (B * C) / D0 is too small, it indicates that the elongation rate of the negative electrode current collector is small, the OI value of the negative electrode sheet is small, and the particle size of the negative electrode active layer is large. At this time, the expansion of the negative electrode active layer in the direction parallel to its surface is large, while the binding effect of the negative electrode current collector on the negative electrode active layer is small, and the negative electrode active particles expand greatly, which easily causes the negative electrode sheet to break, and the lithium-ion diffusion path becomes longer, and the active sites decrease, resulting in a decrease in the rate performance of the battery; if the value of (B * C) / D0 is too large, it indicates that the elongation rate of the negative electrode current collector is large, the OI value of the negative electrode sheet is large, and the particle size of the negative electrode active layer is small. At this time, the expansion of the negative electrode active layer in its thickness direction is large, and coupled with the small compaction density of the negative electrode, the energy density of the battery decreases and the expansion rate increases.

[0096] Furthermore, when the OI value C of the negative electrode sheet, the particle size D0 of the negative electrode active layer, and the elongation rate B of the negative electrode current collector satisfy: 0.03 ≤ (B * C) / D0 ≤ 0.1, the energy density, rate performance, and safety of the battery can be ensured to reach the best combination.

[0097] Exemplarily, the value of (B * C) / D0 can be 0.02, 0.03, 0.05, 0.07, 0.08, 0.09, 0.1, 0.13, 0.15, 0.17, 0.2, 0.23, 0.26, etc. or within the range composed of any two of the above values.

[0098] The battery separator is usually composed of a base film and a coating layer. The base film mainly uses materials with relatively high molecular weights such as polyethylene (PE) and polypropylene (PP). The coating layer is located on at least one surface of the base film, and its main materials include ceramics, polyvinylidene fluoride (PVDF), and polymethyl methacrylate (PMMA), etc. Alumina and boehmite are two commonly used ceramic materials. Coating a ceramic layer on the base film can enhance the battery manufacturing effect. PVDF and PMMA are commonly used coating materials for the adhesive layer. Coating an adhesive layer on the base film can improve the high-temperature resistance, chemical corrosion resistance, and mechanical properties of the battery. Of course, a ceramic layer and an adhesive layer can also be coated on the base film simultaneously to combine the advantages of both.

[0099] The separator can block electron conduction but can conduct ion transport. This is mainly due to the voids on the separator serving as the transport channels for lithium ions. Therefore, the size and number of voids in the separator can affect the kinetic performance of the electrode sheet. When the negative electrode active layer is relatively thick, it takes more time to achieve the infiltration of the electrolyte, making the ion diffusion distance longer. At this time, the voids in the separator can be increased to increase the migration rate of the carriers, thereby improving the rate performance of the battery and reducing polarization. Therefore, in some embodiments, by regulating the pore diameter E (unit: μm) and porosity K of the separator and the thickness H (unit: μm) of the negative electrode active layer to satisfy: 1700 ≤ H / (E * K) ≤ 4800, this can further reduce the polarization of the battery cell while taking into account the energy density of the battery and improve the rate performance of the battery.

[0100] Furthermore, in some embodiments, when the pore diameter E (unit: μm) and porosity K of the separator and the thickness H (unit: μm) of the negative electrode active layer satisfy: 2200 ≤ H / (E * K) ≤ 4200, the energy density, rate performance, and safety of the battery can achieve the best combination.

[0101] Exemplarily, the value of H / (E * K) can be 1700, 2000, 2200, 2500, 2700, 3000, 3200, 3500, 3700, 4000, 4200, 4500, 4800, etc. or within the range composed of any two of the above values.

[0102] It is understandable that the pore diameter E of the separator reflects the size of the voids in the separator, the porosity K of the separator reflects the amount of voids in the separator, and E*K represents the ability of the separator to transport lithium ions.

[0103] The pore diameter E is measured by a pore diameter tester. In some embodiments, 0.02 ≤ E ≤ 0.1. Exemplarily, the pore diameter of the separator can be 0.02 μm, 0.03 μm, 0.04 μm, 0.05 μm, 0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, 0.1 μm, etc. or within the range composed of any two of the above values.

[0104] The test method for the porosity K includes: cutting the separator to obtain a separator sample with an area greater than 0.1 m 2 or a separator sample with a weight greater than 3 g, measuring the thickness of the sample, and recording the thickness data; after folding, putting the sample into a sample cup, using a true density meter to test the true volume V0 of the sample, calculating the apparent volume V1 = measured sample thickness * sample length * sample width, and the sample porosity K = (V1 - V0) / V1 * 100%.

[0105] In some embodiments, 10% ≤ K ≤ 48%. Exemplarily, the porosity of the separator can be 10%, 15%, 20%, 25%, 28%, 30%, 35%, 38%, 40%, 43%, 48%, etc. or within the range composed of any two of the above values.

[0106] During the cycling process of the lithium-ion secondary battery, lithium is repeatedly deintercalated and intercalated in the negative electrode sheet, making the expansion of the negative electrode sheet periodic. The expansion stress accumulates on the separator. After being stretched, the separator is not easy to retract, so it will continue to stretch until the separator becomes thinner or there is looseness at the interface with the electrode sheet, affecting the battery performance.

[0107] The research of this application finds that the tensile strength of the base film can affect the deformation of the separator when stressed. The greater the tensile strength of the base film, the stronger the deformation ability of the separator and the less likely it is to break. In some embodiments, the tensile strength Q1 of the base film satisfies 1000 ≤ Q1 ≤ 3800, especially when 1800 ≤ Q1 ≤ 3100, it can ensure that the separator is not easy to break, and at the same time prevent the contact force between the separator and the electrode sheet from being insufficient due to excessive elongation rate of the separator, reducing the battery stability.

[0108] Exemplarily, the tensile strength Q1 of the base film can be 1000 kgf / cm 2 、1500 kgf / cm 2 、1800 kgf / cm 2 、2000 kgf / cm 2 、2200 kgf / cm2 , 2500 kgf / cm 2 , 2800 kgf / cm 2 , 3000 kgf / cm 2 , 3300 kgf / cm 2 , 3500 kgf / cm 2 , 3800 kgf / cm 2 etc. or within the range formed by any two of the above values.

[0109] Tensile strength is the ability of a material to resist fracture under static tension or the maximum tensile force (tensile stress) that the material can withstand without fracture, which reflects the fracture resistance of the material. In this application, the test method for the tensile strength Q1 of the base film includes: longitudinally cutting the diaphragm into samples with a size of 15 mm ± 0.2 mm, using a WD-D3 type electronic universal testing machine with a gap of 50 mm between the upper and lower clamps, clamping both ends of the sample in the clamps respectively, starting the test at a speed of 100 mm / min until the equipment stops testing, recording the tensile strength, testing 3 times with an error not exceeding 10%, and taking the average value.

[0110] The negative current collector can be a copper foil. The crystal structure of copper belongs to the face-centered cubic crystal system. In this crystal structure, copper atoms are arranged in a specific way to form different crystal planes. It can be understood that a crystal plane refers to a plane passing through the centers of atoms in crystallography. During the spontaneous growth process of a crystal, a polyhedral shape composed of planes with different orientations can be developed, and the planes in these polyhedral shapes are called crystal planes. Different crystal planes have different atomic arrangement densities and surface energies, so they also exhibit different physical and chemical properties.

[0111] It is worth mentioning that the (111) crystal plane of copper is one of its most closely packed crystal planes, which is composed of three layers of atoms arranged alternately. The specific crystal plane parameters can be accurately measured by an X-ray powder diffraction (XRD) instrument. According to the intensity of the diffraction peak of the copper foil (111) crystal plane, the lattice structure of the copper foil can be reflected, and then the strength and ductility of the copper foil can be characterized.

[0112] In this application, the (111) crystal plane parameters of the copper foil are tested using a Shimadzu XRD-6100 type X-ray diffractometer. The diffraction peak that appears at the position of 2θ = 43.2° - 43.4° in the obtained diffraction pattern is the diffraction peak of the copper foil (111) crystal plane, and the peak intensity is denoted as Q2.

[0113] In some embodiments, the intensity Q2 of the diffraction peak of the (111) crystal plane of the copper foil satisfies: 30000 ≤ Q2 ≤ 80000. In particular, when 54000 ≤ Q2 ≤ 71000, it can ensure that the copper foil has good crystallinity, high strength, and good ductility, while also preventing the processing difficulty from increasing due to the too high strength of the copper foil. Exemplarily, the intensity of the diffraction peak of the (111) crystal plane of the copper foil can be 30000, 35000, 40000, 50000, 54000, 60000, 66000, 71000, 76000, 80000, etc. or within the range composed of any two of the above values.

[0114] Further, the average particle size of the silicon-carbon material in the negative electrode active layer is D1, in μm. It should be noted that D1 is obtained by taking pictures of 2 different regions at a magnification of 5K in SEM, measuring the maximum diameters of all silicon-carbon material particles in these 2 pictures, and taking their average value. The test method includes:

[0115] After discharging the lithium-ion secondary battery to 0% SOC, disassemble and take out the negative electrode sheet, soak it in dimethyl carbonate (DMC) solvent for 12 h, then rinse it with DMC to remove the lithium salts attached to the negative electrode sheet. Use an argon ion milling machine CP to laser cut the negative electrode sheet. At a magnification of 5K in the backscattering mode of a scanning electron microscope (SEM), take pictures of 2 different regions, measure the maximum diameters (i.e., the farthest distances) of all silicon-carbon material particles in these 2 pictures, and take their average value, which is defined as the particle size D1 of the silicon-carbon material.

[0116] The particle size D1 of the silicon-carbon particles will affect the performance of the battery cell. The larger the particle size, the larger the gap between the particles in the negative electrode sheet, and the smaller the tap density of the negative electrode sheet, thus affecting the energy density of the negative electrode sheet. At the same time, it will also cause a larger expansion of the negative electrode sheet during the cycling process, which is easily transmitted to the negative electrode current collector and the separator, resulting in a larger extension of the negative electrode current collector and a thinner separator thickness. When the particle size D1 of the silicon-carbon particles is small, the specific surface area is large, there are more negative reactions with the electrolyte, and the contact impedance between the particles is also large, affecting the rate performance of the battery. In some embodiments, controlling the particle size D1 of the silicon-carbon material to satisfy 3.1 μm ≤ D1 ≤ 15.8 μm, especially 6 μm ≤ D1 ≤ 10 μm, can balance the tap density and expansion degree of the negative electrode sheet, so that the battery simultaneously has good rate performance, energy density, and safety.

[0117] Exemplarily, the particle size D1 of the silicon-carbon material can be 3.1 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 15.8 μm, etc. or within the range composed of any two of the above values.

[0118] Based on this, in some embodiments, the tensile strength Q1 of the base film, the diffraction peak intensity Q2 of the (111) crystal plane of the copper foil, and the particle size D1 of the silicon-carbon material satisfy: 4000000 ≤ Q1 * Q2 / D1 ≤ 58000000. That is, the particle size of the silicon-carbon material is positively correlated with the tensile strength of the base film and the diffraction peak intensity of the (111) crystal plane of the copper foil, enabling those skilled in the art to adjust the particle size of the silicon-carbon material within the above range. On the one hand, it can reduce the probability of the negative electrode sheet and the separator breaking under high voltage and reduce the deformation of the negative electrode sheet in the arc area. On the other hand, it can ensure that the battery has good cycle and rate performance at the same time. On the third hand, it can also ensure that the negative electrode sheet has high strength and improve the processability.

[0119] Further, in some embodiments, the tensile strength Q1 of the base film, the diffraction peak intensity Q2 of the (111) crystal plane of the copper foil, and the particle size D1 of the silicon-carbon material satisfy 14000000 ≤ Q1 * Q2 / D1 ≤ 37000000, which can achieve the best combination of the energy density, rate performance, and safety of the battery.

[0120] Exemplarily, the value of Q1 * Q2 / D1 can be 4000000, 6000000, 10000000, 14000000, 18000000, 22000000, 26000000, 30000000, 33000000, 37000000, 40000000, 45000000, 50000000, 55000000, 58000000, etc. or within the range composed of any two of the above values.

[0121] In some embodiments, the silicon-carbon material includes spherical silicon-carbon, and the spherical silicon-carbon includes silicon-carbon single particles and silicon-carbon secondary particles. It should be noted that the silicon-carbon single particles are primary particles, while the silicon-carbon secondary particles are formed by the aggregation of silicon-carbon primary particles.

[0122] The larger the average particle size D2 of the silicon-carbon single particles, the greater the compaction density of the negative electrode sheet and the higher the energy density of the negative electrode. However, the lithium-ion diffusion path is also longer, the specific surface area decreases, and the active sites decrease, resulting in a decline in the rate performance of the battery. At the same time, it also makes the silicon-carbon particles larger and the negative electrode expand more; vice versa.

[0123] In some embodiments, the average particle size D2 of the silicon-carbon single particles satisfies 2μm ≤ D2 ≤ 11μm. Exemplarily, D2 can be 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, etc. or within the range composed of any two of the above values.

[0124] The average particle size D2 of a single silicon carbide particle is obtained by taking pictures of two different regions at a magnification of 5K in SEM, measuring the maximum diameters of all single silicon carbide particles in these two pictures, and taking their average value. The test method includes:

[0125] After discharging the lithium-ion secondary battery to 0% SOC, disassemble it to take out the negative electrode sheet. Immerse the negative electrode sheet in dimethyl carbonate (DMC) solvent for 12 h and then rinse it with DMC to remove the lithium salts attached to the negative electrode sheet. Use an argon ion milling machine CP to laser cut the negative electrode sheet. Take pictures of two different regions at a magnification of 5K in the backscattering mode of a scanning electron microscope (SEM), measure the maximum diameters (i.e., the farthest distances) of all single silicon carbide particles in these two pictures, take their average value, and define it as the particle size D2 of the single silicon carbide particle.

[0126] The larger the average particle size D3 of the silicon carbide secondary particle, the more single particles are agglomerated, there are more electron transport channels, the electron conductivity is better. At the same time, after the particles are agglomerated, the compaction increases, the energy density increases, the contact area with graphite is larger, and the binding force of the negative electrode active material is better. However, the lithium insertion / extraction path of the silicon carbide particles in the inner layer becomes longer, and the rate performance decreases. Similarly, the swelling is also larger. Vice versa.

[0127] In some embodiments, the average particle size D3 of the silicon carbide secondary particle satisfies 5 μm ≤ D3 ≤ 17 μm. Exemplarily, D3 can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, etc. or within the range composed of any two of the above values.

[0128] The average particle size D3 of the silicon carbide secondary particle is obtained by taking pictures of two different regions at a magnification of 5K in SEM, measuring the maximum diameters of all silicon carbide secondary particles in these two pictures, and taking their average value. The test method includes:

[0129] After discharging the lithium-ion secondary battery to 0% SOC, disassemble it to take out the negative electrode sheet. Immerse the negative electrode sheet in dimethyl carbonate (DMC) solvent for 12 h and then rinse it with DMC to remove the lithium salts attached to the negative electrode sheet. Use an argon ion milling machine CP to laser cut the negative electrode sheet. Take pictures of two different regions at a magnification of 5K in the backscattering mode of a scanning electron microscope (SEM), measure the maximum diameters (i.e., the farthest distances) of all silicon carbide secondary particles in these two pictures, take their average value, and define it as the particle size D3 of the silicon carbide secondary particle.

[0130] The average particle size D1 of the silicon-carbon material reflects the quantity ratio of the single silicon-carbon particles and the secondary silicon-carbon particles. By controlling that the average particle size D1 of the silicon-carbon material, the average particle size D2 (unit: μm) of the single silicon-carbon particles, and the average particle size D3 (unit: μm) of the secondary silicon-carbon particles satisfy: 0.43 ≤ D1 / (D2 + D3) ≤ 0.7, the battery can have the advantages of high energy density, good rate performance, and high safety at the same time.

[0131] If the value of D1 / (D2 + D3) is too small, it indicates that there are more single silicon-carbon particles and fewer secondary particles, resulting in a lower energy density of the negative electrode sheet. At the same time, the specific surface area of the silicon-carbon particles is large, there is more contact with the electrolyte, the side reaction is large, and there are fewer secondary silicon-carbon particles. The contact area between a single particle and graphite decreases, and the adhesion to the negative electrode becomes poor, resulting in easier powder falling during the expansion process of the negative electrode.

[0132] If the value of D1 / (D2 + D3) is too large, it indicates that there are fewer single silicon-carbon particles and more secondary particles. The lithium insertion and extraction of the silicon-carbon particles are slower, the rate performance is reduced, and the risk of lithium deposition increases at high rates.

[0133] Please refer to Figure 2 and Figure 3 In some embodiments, a groove 13 is provided on the surface of the negative electrode active layer 12 facing away from the negative electrode current collector 11. By using a laser to process linear grooves, linear holes, blind holes, through holes, etc. with a certain depth and width on the surface of the negative electrode sheet, the contact area between the negative electrode sheet and the electrolyte can be increased, which is beneficial to the infiltration of the electrolyte, making the electrochemical reaction more sufficient, thereby improving the capacity and energy density of the battery. At the same time, it can also shorten the migration distance of lithium ions in the negative electrode sheet and improve the charge and discharge rate of the battery. In some embodiments, the volume ratio of the groove to the volume of the negative electrode active layer is M, 0.2% ≤ M ≤ 2%. Exemplarily, the value of M can be 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, etc. or within the range composed of any two of the above values.

[0134] It should be noted that the volume ratio M of the groove on the negative electrode sheet can be measured by the following method: Use an elemental analysis microscope to measure the groove depth, groove width, and groove spacing, calculate the number of grooves within a distance of 10 cm, calculate the total area of the grooves, and then multiply by the length of the grooves to obtain the total volume of the grooves. The volume of the negative electrode active layer is obtained by multiplying the coating thickness H by the coating width and the length of 10 cm. Finally, divide the total volume of the grooves by the volume of the negative electrode active layer to obtain M.

[0135] Selecting a separator with high air permeability helps the rapid transfer of the electrolyte between the positive and negative electrodes of the battery, thereby improving the kinetic performance of the battery cell. The air permeability N of the separator can be tested using a separator air permeability tester. In some embodiments, 100 sec / 100 cc ≤ N ≤ 400 sec / 100 cc. Exemplarily, the air permeability of the separator can be 100 sec / 100 cc, 150 sec / 100 cc, 200 sec / 100 cc, 250 sec / 100 cc, 300 sec / 100 cc, 350 sec / 100 cc, 400 sec / 100 cc, etc. or within the range formed by any two of the above values.

[0136] This application's research found that when the volume ratio M of the grooves on the negative electrode sheet and the air permeability N of the separator satisfy: 0.5 ≤ M * N ≤ 8, it is possible to further ensure the kinetic performance of the battery cell while reducing the probability of the pole piece breaking. Exemplarily, the value of M * N can be 0.5, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, etc. or within the range formed by any two of the above values.

[0137] Since the adhesive layer in the battery separator will show uneven softening and melting during hot pressing in the arc region, forming some spaces. Especially for the sprayed adhesive layer, which is not flat itself, this reserves some gaps between the pole pieces, which is beneficial to releasing the stress generated by expansion, reducing the expansion stress on the pole pieces, and improving the cycle performance and charge-discharge capacity of the battery cell. The greater the thickness of the separator adhesive layer, the smaller the expansion stress on the pole pieces, and the more beneficial it is to improve the cycle and rate performance of the battery.

[0138] The thickness L of the separator adhesive layer is tested by the following method: After discharging the lithium-ion secondary battery to 0% SOC, disassemble it, take the separator in the overhang area, grind the cross-section with an argon ion milling device, take a cross-sectional morphology photograph at a magnification of SEM 5K, and take the maximum value of the adhesive layer thickness.

[0139] In some embodiments, 0.5 μm ≤ L ≤ 4 μm. Exemplarily, the thickness L of the separator adhesive layer can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, etc. or within the range formed by any two of the above values.

[0140] Please refer to Figures 4 to 6, the positive electrode sheet includes a positive current collector 21 and a positive active layer 22 provided on at least one surface of the positive current collector 21. A recess 23 is provided on the surface of the positive active layer 22 facing away from the positive current collector 21, and the positive active layer 22 at the recess 23 is recessed toward the positive current collector 21. By providing a recess on the surface of the positive electrode sheet, on the one hand, the gap between the electrode sheets can be increased, providing a place for the electrolyte to infiltrate, making the electrochemical reaction more sufficient, thereby improving the capacity and energy density of the battery. At the same time, it can also shorten the migration distance of lithium ions in the positive electrode sheet and improve the charge and discharge rate of the battery. On the other hand, it also provides more space for the expansion of the active material of the electrode sheet and alleviates the change in the thickness of the battery.

[0141] It should be noted that the recess on the positive electrode sheet can be formed by mechanical rolling. The present application does not specifically limit the shape of the recess, and any recess shape in the prior art can be applied to the present application.

[0142] Use an elemental analysis microscope to measure the depression depth G of the recess on the surface of the positive electrode sheet. In some embodiments, 10μm ≤ G ≤ 60μm. Exemplarily, the depression depth G can be 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, etc. or within the range composed of any two of the above values.

[0143] The present application has found through research that when the depression depth G of the recess on the positive electrode sheet and the adhesive layer thickness L satisfy: 14 ≤ G + L ≤ 41, the expansion stress on the electrode sheet can be further reduced, the probability of electrode sheet breakage can be decreased, and the cycle and rate performance of the battery cell can be improved. Exemplarily, the value of G + L can be 14, 17, 21, 24, 28, 31, 34, 38, 41, etc. or within the range composed of any two of the above values.

[0144] According to the second aspect of the present application, a secondary battery is provided, including the battery cell described in the first aspect.

[0145] The secondary battery of the present application can reduce the deformation of the negative electrode sheet in the arc area, improve the contact between the electrode sheets, and thus has good cycle and rate performance and high safety.

[0146] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application. For those not specifying specific experimental steps or conditions in the examples and comparative examples, the operations or conditions of the conventional experimental steps described in the literature in the art can be followed. For the reagents or instruments not indicating the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase. In all examples and comparative examples of the present application, the unit % represents mass percentage.

[0147] Example 1

[0148] This example provides a method for preparing a lithium-ion secondary battery, which includes the following steps:

[0149] First step: Preparation of the positive electrode sheet

[0150] Lithium cobaltate, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode paste; the positive electrode paste is coated on both the front and back surfaces of the aluminum foil, and after baking and rolling, a positive electrode sheet with a thickness of 100 μm is obtained.

[0151] Second step: Preparation of the negative electrode sheet

[0152] The negative electrode active material (i.e., silicon carbide and graphite, with silicon carbide accounting for 5% of the weight of graphite), SP, lithium carboxymethyl cellulose (CMC-Li), and polyacrylic acid (PAA) are mixed in a mass ratio of 97:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode paste; among them, the average particle size D1 of the silicon carbide particles is 7.7 μm, the silicon carbide particles include silicon carbide single particles and silicon carbide secondary particles, the average particle size D2 of the silicon carbide single particles is 4.8 μm, and the average particle size D3 of the silicon carbide secondary particles is 9.3 μm.

[0153] The negative electrode paste is coated on both the front and back surfaces of the copper foil (the elongation rate B is 3.6%, and the intensity Q2 of the XRD diffraction peak of its (111) crystal plane is 70152), and after baking and rolling, a negative electrode sheet with a thickness of 110 μm is prepared, and the thickness H of the single-sided negative electrode active layer is 44 μm. The OI value C of the negative electrode sheet is 12.3, and the particle size D0 of the negative electrode active layer is 14.6 μm.

[0154] Subsequently, laser scribing is performed on the negative electrode sheet to form a groove, and the ratio M of the volume of the groove to the volume of the single-sided negative electrode active layer is 0.68%.

[0155] A slot with a fixed size is provided at a certain position of the negative electrode sheet, and the copper-plated nickel tab is laser-welded into this slot.

[0156] Third step: The positive and negative electrode sheets are slit, formed into sheets, and wound with a separator to obtain a core. A positive electrode embossing roller is added to the winding equipment so that concave portions are formed on the surface of the positive electrode active layer facing away from the positive electrode current collector, and the depression depth G of the concave portions is 30 μm; then, through encapsulation, baking, liquid injection, formation, second sealing, sorting, and OCV, a lithium-ion secondary battery is obtained.

[0157] Among them, the electrolyte is a commercially available conventional electrolyte, and the lithium salt is LiFP 6 ; the separator includes a first adhesive layer, a base film (the tensile strength Q1 is 2204 kgf / cm2 ) a ceramic layer and a second adhesive layer (with a thickness L of 2 μm), the ceramic layer facing the positive electrode sheet; the elongation rate A of the separator is 66%, the air permeability N is 236 sec / 100 cc, the pore size E is 0.042 μm, and the porosity K is 38%.

[0158] In this embodiment, (A * B) / H = 0.00054, (A * C) / D0 = 0.556, (B * C) / D0 = 0.03, (Q1 * Q2) / D1 = 2.01 * 10^7, H / (E * K) = 2757, D1 / (D2 + D3) = 0.55, M * N = 1.6, G + L = 32.

[0159] The preparation methods of Examples 2 - 45 and Comparative Examples 1 - 6 are basically the same as those of Example 1, and the differences are as follows:

[0160] Example 1 group

[0161] This group of examples is used to verify the influence brought by the change of "(A * B) / H", which is the relationship between the elongation rate A of the separator, the elongation rate B of the negative electrode current collector, and the thickness H of the negative electrode active layer.

[0162] This group of examples is carried out with reference to Example 1. The difference is that the value of (A * B) / H is changed, specifically as follows:

[0163] In Example 2, the elongation rate A of the separator is 160%, the elongation rate B of the negative electrode current collector is 6.8%, the thickness H of the negative electrode active layer is 30 μm, then (A * B) / H = 0.00363.

[0164] In Example 3, the elongation rate A of the separator is 120%, the elongation rate B of the negative electrode current collector is 4.4%, the thickness H of the negative electrode active layer is 30 μm, then (A * B) / H = 0.00176.

[0165] In Example 4, the elongation rate A of the separator is 30%, the elongation rate B of the negative electrode current collector is 8%, the thickness H of the negative electrode active layer is 30 μm, then (A * B) / H = 0.0008.

[0166] In Example 5, the elongation rate A of the separator is 160%, the elongation rate B of the negative electrode current collector is 6.8%, the thickness H of the negative electrode active layer is 25 μm, then (A * B) / H = 0.00435.

[0167] In Example 6, the elongation rate A of the separator is 160%, the elongation rate B of the negative electrode current collector is 6.8%, the thickness H of the negative electrode active layer is 60 μm, then (A * B) / H = 0.00181.

[0168] Example 7, the diaphragm elongation rate A is 160%, the negative electrode current collector elongation rate B is 3.6%, and the negative electrode active layer thickness H is 44 μm, then (A * B) / H = 0.00131.

[0169] Example 8, the diaphragm elongation rate A is 110%, the negative electrode current collector elongation rate B is 8%, and the negative electrode active layer thickness H is 44 μm, then (A * B) / H = 0.002.

[0170] Example 9, the diaphragm elongation rate A is 110%, the negative electrode current collector elongation rate B is 8%, and the negative electrode active layer thickness H is 30 μm, then (A * B) / H = 0.00293.

[0171] Example 10, the diaphragm elongation rate A is 96%, the negative electrode current collector elongation rate B is 1.8%, and the negative electrode active layer thickness H is 50 μm, then (A * B) / H = 0.00035.

[0172] Example 11, the diaphragm elongation rate A is 180%, the negative electrode current collector elongation rate B is 10%, and the negative electrode active layer thickness H is 30 μm, then (A * B) / H = 0.006.

[0173] Example 2 group

[0174] This group of examples is used to verify the influence brought by the change of "the relationship between the diaphragm elongation rate A and the OI value C of the negative electrode sheet and the particle size D0 of the negative electrode active layer (A * C) / D0" and "the relationship between the negative electrode current collector elongation rate B and the OI value C of the negative electrode sheet and the particle size D0 of the negative electrode active layer (B * C) / D0".

[0175] This group of examples is carried out with reference to Example 1. The difference is that the values of (A * C) / D0 and (B * C) / D0 are changed, specifically as follows:

[0176] Example 12, the OI value C of the negative electrode sheet is 10, and the particle size D0 of the negative electrode active layer is 16.5 μm, then (A * C) / D0 = 0.727, (B * C) / D0 = 0.035.

[0177] Example 13, the OI value C of the negative electrode sheet is 20, and the particle size D0 of the negative electrode active layer is 12 μm, then (A * C) / D0 = 2.0, (B * C) / D0 = 0.097.

[0178] Example 14, the OI value C of the negative electrode sheet is 15, and the particle size D0 of the negative electrode active layer is 14.6 μm, then (A * C) / D0 = 1.233, (B * C) / D0 = 0.06.

[0179] Example 15: The OI value C of the negative electrode sheet is 8, and the particle size D0 of the negative electrode active layer is 19 μm. Then, (A * C) / D0 = 0.505, and (B * C) / D0 = 0.024. Moreover, the average particle size D1 of the silicon-carbon particles is 11 μm. The silicon-carbon particles include silicon-carbon single particles and silicon-carbon secondary particles. The average particle size D2 of the silicon-carbon single particles is 9 μm, and the average particle size D3 of the silicon-carbon secondary particles is 16 μm. Then, D1 / (D2 + D3) = 0.44.

[0180] Example 16: The OI value C of the negative electrode sheet is 32, and the particle size D0 of the negative electrode active layer is 10 μm. Then, (A * C) / D0 = 4.8, and (B * C) / D0 = 0.256. Moreover, the average particle size D1 of the silicon-carbon particles is 6 μm. The silicon-carbon particles include silicon-carbon single particles and silicon-carbon secondary particles. The average particle size D2 of the silicon-carbon single particles is 2 μm, and the average particle size D3 of the silicon-carbon secondary particles is 7 μm. Then, D1 / (D2 + D3) = 0.67.

[0181] Example 17: The OI value C of the negative electrode sheet is 12.3, and the particle size D0 of the negative electrode active layer is 16.5 μm. Then, (A * C) / D0 = 0.373, and (B * C) / D0 = 0.06.

[0182] Example 18: The OI value C of the negative electrode sheet is 32, and the particle size D0 of the negative electrode active layer is 10 μm. Then, (A * C) / D0 = 5.760, and (B * C) / D0 = 0.256.

[0183] Example 19: The OI value C of the negative electrode sheet is 12.3, and the particle size D0 of the negative electrode active layer is 16.5 μm. Then, (A * C) / D0 = 0.895, and (B * C) / D0 = 0.019.

[0184] Example 20: The OI value C of the negative electrode sheet is 32, and the particle size D0 of the negative electrode active layer is 10 μm. Then, (A * C) / D0 = 3.84, and (B * C) / D0 = 0.32.

[0185] Example Group 3

[0186] This group of examples is used to verify the influence brought about by the change of "(Q1 * Q2) / D1", which is the relationship between the tensile strength Q1 of the base film, the XRD diffraction peak intensity Q2 of the (111) crystal plane of the copper foil, and the average particle size D1 of the silicon-carbon material.

[0187] This group of examples is carried out with reference to Example 1. The difference is that the value of Q2 * Q1 / D1 is changed, specifically as follows:

[0188] Example 21: The tensile strength Q1 of the base film is 2204 kgf / cm 2, the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 54350, the average particle size D1 of the silicon-carbon material is 7.7 μm, then Q2 * Q1 / D1 is 1.56 * 10^7.

[0189] Example 22, the tensile strength Q1 of the base film is 3095 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 61505, the average particle size D1 of the silicon-carbon material is 7.7 μm, then Q2 * Q1 / D1 is 2.47 * 10^7.

[0190] Example 23, the tensile strength Q1 of the base film is 1857 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 61505, the average particle size D1 of the silicon-carbon material is 7.7 μm, then Q2 * Q1 / D1 is 1.48 * 10^7.

[0191] Example 24, the tensile strength Q1 of the base film is 3095 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 70152, the average particle size D1 of the silicon-carbon material is 6 μm, then Q2 * Q1 / D1 is 3.62 * 10^7.

[0192] Example 25, the tensile strength Q1 of the base film is 3095 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 70152, the average particle size D1 of the silicon-carbon material is 10 μm, then Q2 * Q1 / D1 is 2.17 * 10^7.

[0193] Example 26, the tensile strength Q1 of the base film is 1392 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 34946, the average particle size D1 of the silicon-carbon material is 12 μm, then Q2 * Q1 / D1 is 4.05 * 10^6.

[0194] Example 27, the tensile strength Q1 of the base film is 3688 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 77856, the average particle size D1 of the silicon-carbon material is 5 μm, then Q2 * Q1 / D1 is 5.74 * 10^7.

[0195] Example 28, the tensile strength Q1 of the base film is 1392 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the copper foil (111) crystal plane is 34946, the average particle size D1 of the silicon-carbon material is 15 μm, then Q2 * Q1 / D1 is 3.24 * 10^6.

[0196] Example 29, the tensile strength Q1 of the base film is 3796 kgf / cm 2 , the XRD diffraction peak intensity Q2 of the (111) crystal plane of the copper foil is 77856, and the average particle size D1 of the silicon-carbon material is 5 μm, then Q2 * Q1 / D1 is 5.91 * 10^7.

[0197] 4 groups of examples

[0198] This group of examples is used to verify the influence brought by the change of "the relationship H / (E * K) between the thickness H of the negative electrode active layer, the diaphragm pore size E, and the diaphragm porosity K".

[0199] This group of examples is carried out with reference to Example 1. The difference is that the value of H / (E * K) is changed, specifically as follows:

[0200] Example 30, the diaphragm pore size E is 0.06 μm, the diaphragm porosity K is 32%, then H / (E * K) is 2292.

[0201] Example 31, the diaphragm pore size E is 0.042 μm, the diaphragm porosity K is 25%, then H / (E * K) is 4190.

[0202] Example 32, the diaphragm pore size E is 0.088 μm, the diaphragm porosity K is 11%, then H / (E * K) is 4545.

[0203] Example 33, the diaphragm pore size E is 0.088 μm, the diaphragm porosity K is 25%, then H / (E * K) is 2000.

[0204] Example 34, the diaphragm pore size E is 0.06 μm, the diaphragm porosity K is 15%, then H / (E * K) is 4889.

[0205] Example 35, the diaphragm pore size E is 0.06 μm, the diaphragm porosity K is 45%, then H / (E * K) is 1630.

[0206] 5 groups of examples

[0207] This group of examples is used to verify the influence brought by the change of "the relationship M * N between the ratio M of the groove volume on the negative electrode to the volume of the negative electrode active layer and the diaphragm air permeability N".

[0208] This group of examples is carried out with reference to Example 1. The difference is that the value of M * N is changed, specifically as follows:

[0209] Example 36, the ratio M of the groove volume on the negative electrode to the volume of the negative electrode active layer is 0.36%, the diaphragm air permeability N is 150 sec / 100 cc, then M * N is 0.5.

[0210] Example 37, the ratio M of the volume of the groove on the negative electrode to the volume of the negative electrode active layer is 1.8%, and the air permeability N of the separator is 326 sec / 100 cc, so M * N is 5.9.

[0211] Example 38, the ratio M of the volume of the groove on the negative electrode to the volume of the negative electrode active layer is 2%, and the air permeability N of the separator is 400 sec / 100 cc, so M * N is 8.0.

[0212] Example 39, the ratio M of the volume of the groove on the negative electrode to the volume of the negative electrode active layer is 0.2%, and the air permeability N of the separator is 150 sec / 100 cc, so M * N is 0.3.

[0213] Example 40, the ratio M of the volume of the groove on the negative electrode to the volume of the negative electrode active layer is 2.2%, and the air permeability N of the separator is 400 sec / 100 cc, so M * N is 8.8.

[0214] 6 groups of examples

[0215] This group of examples is used to verify the influence brought by the change of "the relationship G + L between the thickness L of the glue layer in the separator and the depression depth G of the concave part on the positive electrode plate".

[0216] This group of examples is carried out with reference to Example 1. The difference is that the value of G + L is changed as follows:

[0217] Example 41, the thickness L of the glue layer in the separator is 0.5 μm, and the depression depth G of the concave part on the positive electrode plate is 15 μm, so G + L is 15.5 μm.

[0218] Example 42, the thickness L of the glue layer in the separator is 3 μm, and the depression depth G of the concave part on the positive electrode plate is 35 μm, so G + L is 38 μm.

[0219] Example 43, the thickness L of the glue layer in the separator is 3 μm, and the depression depth G of the concave part on the positive electrode plate is 38 μm, so G + L is 41 μm.

[0220] Example 44, the thickness L of the glue layer in the separator is 4 μm, and the depression depth G of the concave part on the positive electrode plate is 38 μm, so G + L is 42 μm.

[0221] Example 45, the thickness L of the glue layer in the separator is 0.5 μm, and the depression depth G of the concave part on the positive electrode plate is 13 μm, so G + L is 13.5 μm.

[0222] 1 group of comparative examples

[0223] This group of comparative examples is used to verify the influence brought by the change of "the relationship (A * B) / H between the elongation rate A of the separator, the elongation rate B of the negative electrode current collector, and the thickness H of the negative electrode active layer".

[0224] This group of embodiments is carried out with reference to Embodiment 10, the difference being that the value of (A*B) / H is changed, specifically as follows:

[0225] For Comparative Example 1, the diaphragm elongation A is 60%, then (A*B) / H = 0.00022.

[0226] For Comparative Example 3, the negative current collector elongation B is 1.5%, then (A*B) / H = 0.00029.

[0227] Comparative Example 2 group

[0228] This group of comparative examples is used to verify the influence brought about by the change of "(A*B) / H", which is the relationship among the diaphragm elongation A, the negative current collector elongation B, and the negative active layer thickness H.

[0229] This group of embodiments is carried out with reference to Embodiment 11, the difference being that the value of (A*B) / H is changed, specifically as follows:

[0230] For Comparative Example 2, the diaphragm elongation A is 200%, then (A*B) / H = 0.00667.

[0231] For Comparative Example 4, the negative current collector elongation B is 12%, then (A*B) / H = 0.0072.

[0232] Comparative Example 3 group

[0233] This group of comparative examples is used to verify the influence brought about by the change of "(A*B) / H", which is the relationship among the diaphragm elongation A, the negative current collector elongation B, and the negative active layer thickness H.

[0234] This group of embodiments is carried out with reference to Embodiment 1, the difference being that the value of (A*B) / H is changed, specifically as follows:

[0235] For Comparative Example 5, the diaphragm elongation A is 30%, the negative current collector elongation B is 1.8%, and the negative active layer thickness H is 19 μm, then (A*B) / H = 0.00028.

[0236] For Comparative Example 6, the diaphragm elongation A is 220%, the negative current collector elongation B is 18%, and the negative active layer thickness H is 62 μm, then (A*B) / H = 0.00639.

[0237] Further, for convenient reference, the values of the parameters in the above embodiments and comparative examples are presented in Tables 1 - 3. Among them, " / " represents the absence of this item. A is the diaphragm elongation rate, B is the copper foil elongation rate, H is the thickness of the negative electrode active layer, Q1 is the tensile strength of the base film, Q2 is the XRD diffraction peak intensity of the (111) crystal plane of the copper foil, E is the pore size of the diaphragm, K is the porosity of the diaphragm, N is the air permeability of the diaphragm, M is the ratio of the volume of the groove on the negative electrode sheet to the volume of the negative electrode active layer, L is the thickness of the glue layer in the diaphragm, G is the depression depth of the concave part on the positive electrode sheet, C is the OI value of the negative electrode sheet, D0 is the particle size of the negative electrode active layer, D1 is the average particle size of the silicon-carbon material, D2 is the average particle size of a single silicon-carbon particle, D3 is the average particle size of the secondary silicon-carbon particle. The value of Q2*Q1 / D is expressed in scientific notation.

[0238] Table 1

[0239]

[0240]

[0241] Table 2

[0242]

[0243]

[0244] Table 3

[0245]

[0246]

[0247] Test Example

[0248] 1. 25°C cyclic expansion rate test:

[0249] Fully charge the battery and measure the thickness F1 of the battery before cycling. After 50 cycles, fully charge the battery and measure the thickness F2 of the battery cell. The expansion rate = (F2 - F1) / F1.

[0250] Cycling conditions: At a temperature of 0°C, constant current charging at 1C until the cut-off voltage of 4.5V, constant voltage charging until the cut-off current of 0.2C, constant current discharging at 0.7C, and the cut-off voltage of 3V.

[0251] 2. 25°C cyclic capacity retention rate test:

[0252] Charge at 1C to the cut-off voltage of 4.5V, constant voltage charge to the cut-off current of 0.05C, discharge at 1C to 3V, and repeat the above charge and discharge steps 300 times. The capacity retention rate is the discharge capacity at the 300th time divided by the maximum value of the discharge capacities in the first 3 discharges.

[0253] 3. Constant Current Charging Ratio Test at 25°C:

[0254] The battery is discharged at 0.2C to 3V at 25°C, and then charged at a constant current of 0.5C to 4.5V. Record the constant current charging capacity R1. Continue to charge at a constant voltage until the cut-off current of 0.05C. Record the constant voltage charging capacity R2. The constant current charging ratio = R1 / (R1 + R2).

[0255] 4. Volume Energy Density Test:

[0256] The formula for calculating the volume energy density (ED) is ED = E / V, where E is the discharge energy of the battery. The test method is as follows: Charge the lithium-ion secondary battery at a current of 0.2C to the upper limit voltage, then continue to charge at a constant voltage until the current drops to 0.02C. Then discharge at a current of 0.2C until the voltage reaches 3.0V. The discharged energy is E; V is the volume of the lithium-ion secondary battery, which is obtained by measuring the thickness, width, and length of the battery and calculating the product of the three.

[0257] The above test results are shown in Table 4.

[0258] Table 4

[0259]

[0260]

[0261] Combined with Tables 1 - 4, it can be seen that compared with Comparative Examples 1 - 6, the batteries in all Examples show higher energy density, and at the same time have better cycle and rate performance as well as good safety.

[0262] Compared with Example 10, the elongation rate of the separator in Comparative Example 1 becomes smaller, and the elongation rate of the copper foil in Comparative Example 3 is too small, which both reduce the restraint on the negative electrode active layer, so the swelling rate after cycling becomes larger. Compared with Example 11, the elongation rate of the separator in Comparative Example 2 becomes larger, and the elongation rate of the copper foil in Comparative Example 4 is too high, which both increase the restraint on the negative electrode active layer, and it is easy to cause battery cycle and swelling dives after accumulation. In Comparative Example 5, the thickness of the negative electrode active layer is too thin, affecting the energy density of the battery. At the same time, the elongation rates of the separator and the copper foil are both small, resulting in a small restraint force on the negative electrode active layer, serious battery swelling after cycling, and affected rate performance; while in Comparative Example 6, the negative electrode active layer is too thick, resulting in corresponding increased swelling and decreased battery rate performance.

[0263] This shows that by regulating the separator elongation rate A, the negative electrode current collector elongation rate B, and the negative electrode active layer thickness H to satisfy 0.0003 ≤ A * B / H ≤ 0.0063 in this application, the deformation of the negative electrode sheet in the arc area can be reduced, the contact between the electrode sheets can be improved, thereby improving the cycle and rate performance and safety of the battery.

[0264] Obviously, the above embodiments are merely examples given for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to exhaustively list all implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A battery cell, characterized in that: The charging cut-off voltage of the battery cell is ≥4.48V; The battery core comprises a stacked and wound positive electrode sheet, a separator and a negative electrode sheet; the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector; The battery cell satisfies: 0.0003≤(A*B) / H≤0.0063; A is the elongation of the diaphragm; B is the elongation of the negative electrode current collector; H is the thickness of the negative electrode active layer, in μm.

2. The battery cell according to claim 1, characterized in that: Satisfies: 0.0005≤(A*B) / H≤0.0044; and / or, 30%≤A≤180%, preferably 65%≤A≤160%; and / or, 1.8%≤B≤10%, preferably 3.6%≤B≤8%; And / or, 20≤H≤60, preferably 30≤H≤45.

3. The battery cell according to claim 1 or 2, characterized in that: The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes graphite and silicon-carbon material; the OI value of the negative electrode sheet is C; the particle size of the negative electrode active layer is D0, in μm; Satisfy: 0.4≤(A*C) / D0≤ 5.1, and / or, 0.02≤(B*C) / D0≤0.26; Preferably, 0.55≤(A*C) / D0≤2; Preferably, 0.03≤(B*C) / D0≤0.1; Preferably, 8≤C≤32, more preferably, 10≤C≤20; Preferably, 10≤D0≤19, more preferably, 12≤D0≤16.

5.

4. The battery cell according to claim 1, characterized in that: The battery cell satisfies: 1700≤H / (E*K)≤4800, preferably 2200≤H / (E*K)≤4200; E is the pore size of the diaphragm, in μm; K is the porosity of the diaphragm.

5. The battery cell according to claim 4, characterized in that: 20≤H≤60, preferably 30≤H≤45; and / or, 0.02≤E≤0.1; And / or, 10%≤K≤48%, preferably, 25%≤K≤38%.

6. The battery cell according to claim 3, characterized in that: The average particle size of the silicon-carbon material is D1, in μm; The diaphragm includes a base film, and the tensile strength of the base film is Q1, in kgf / cm 2 ; The negative electrode current collector includes a (111) crystal plane, and the intensity of the XRD diffraction peak of the (111) crystal plane is Q2; Satisfies: 4000000≤(Q1*Q2) / D1≤58000000; Preferably, 14000000≤(Q1*Q2) / D1≤37000000.

7. The battery cell according to claim 6, characterized in that: The negative electrode current collector comprises copper foil; and / or, 1000≤Q1≤3800, preferably, 1800≤Q1≤3100; and / or, 30000≤Q2≤80000, preferably, 54000≤Q2≤71000; and / or, 3.1≤D1≤15.8, preferably, 6≤D1≤10; And / or, the silicon-carbon material includes spherical silicon carbon, the spherical silicon carbon includes silicon carbon single particles and silicon carbon secondary particles, and the average particle size D1 of the silicon-carbon material, the average particle size D2 (in μm) of the silicon carbon single particles, and the average particle size D3 (in μm) of the silicon carbon secondary particles satisfy the following relationship: 0.43≤D1 / (D2+D3)≤0.7; Preferably, 2≤D2≤11; Preferably, 5≤D3≤17.

8. The battery cell according to claim 1, characterized in that: A groove is provided on the surface of the negative electrode active layer facing away from the negative electrode current collector, and the ratio of the volume of the groove to the volume of the negative electrode active layer is M, 0.2%≤M≤2%; The air permeability of the diaphragm is N, 100sec / 100cc≤N≤400sec / 100cc; Satisfies: 0.5≤M*N≤8.

9. The battery cell according to claim 1, characterized in that: The diaphragm includes a base film and a glue layer, wherein the glue layer is disposed on at least one side surface of the base film, and the maximum thickness of the glue layer is L, 0.5 μm≤L≤4 μm; The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer disposed on at least one side surface of the positive electrode current collector, a concave portion is disposed on the surface of the positive electrode active layer away from the positive electrode current collector, the positive electrode active layer at the concave portion is concave toward the positive electrode current collector, and the concave depth of the concave portion is G, 10 μm≤G≤60 μm; Satisfies: 14≤G+L≤41.

10. A lithium ion secondary battery comprising the battery cell according to any one of claims 1 to 9.

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