A lithium ion secondary battery

Abstract

The application relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises a negative electrode sheet and an electrolyte; the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating layer located on at least one side surface of the negative electrode current collector, and the negative electrode active coating layer comprises a negative electrode active material; the negative electrode active material comprises graphite material and silicon-carbon material; the graphite material comprises secondary particles, and the silicon-carbon material comprises primary spherical particles and secondary spherical particles formed by a plurality of primary spherical particles; the electrolyte comprises fluoroethylene carbonate, the mass content c2 of the fluoroethylene carbonate in the electrolyte is less than or equal to 30%, and the mass content of the element Si in the negative electrode active coating layer is 1.5%-15%; the average particle size d2 of the secondary particles is 6-20 mu m; and the average particle size d1 of the primary spherical particles is 1-6 mu m. The lithium ion secondary battery can have high energy density and excellent cycle stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a lithium ion secondary battery. BACKGROUND

[0002] Lithium ion batteries have great potential in electronic devices, electric vehicles and the like. The current technology is committed to improving the energy density of the battery. Silicon-based materials are considered as key materials for improving the energy density of the battery because they have a higher theoretical specific capacity (about 3600 mAh / g), which is much higher than the theoretical specific capacity of graphite materials (about 370 mAh / g). The current method for improving the energy density of the battery is mainly to increase the content of silicon-based materials in the negative electrode sheet, but silicon-based materials have a volume expansion problem. Usually, in order to improve the volume expansion of silicon-based materials, fluorinated ethylene carbonate is added to the electrolyte. Fluorinated ethylene carbonate can form a film on the negative electrode, thereby inhibiting the volume expansion of silicon-based materials, but too much fluorinated ethylene carbonate will cause high-temperature gas production, which is not conducive to the cycle performance of the battery. And with the increase of the content of silicon-based materials, the side reaction of the negative electrode sheet and the electrolyte is intensified, and the active lithium is also seriously consumed, which is also not conducive to the cycle performance of the battery.

[0003] Therefore, it is very important to simultaneously improve the energy density and cycle performance of the battery. SUMMARY

[0004] The present application aims to overcome the problem that the energy density and cycle performance cannot be considered in the prior art, and provides a lithium ion secondary battery (hereinafter referred to as a battery). The negative electrode sheet has a high silicon content and a suitable fluorinated ethylene carbonate content, and a specific graphite material and a silicon-carbon material are combined for use, so that the lithium ion secondary battery (hereinafter referred to as a battery) comprising the above negative electrode sheet has high energy density and excellent cycle stability.

[0005] The present inventors have found through a large number of studies that by improving the graphite material and the silicon-carbon material in the negative electrode, the silicon content of the negative electrode sheet can be significantly improved while ensuring good cycle stability of the battery, and the compaction density of the negative electrode sheet can be improved, thereby making the battery have high energy density. And by controlling the content of fluorinated ethylene carbonate in the electrolyte, the fluorinated ethylene carbonate will not adversely affect the high-temperature performance of the battery, further improving the cycle performance of the battery.

[0006] By regulating the average particle size of the primary spherical particles in the silicon-carbon material, the battery can have both high energy density and good cycle stability. This is because: the commonly used silicon-carbon material is in a block shape, and the average particle size is about 6-12 μm, and the silicon-carbon material has a large hardness. Therefore, the commonly used silicon-carbon particles have poor electrical conductivity, and it is difficult to achieve a large compaction density, which leads to the fact that in a high-silicon-content system, especially a silicon content of more than 5% in the negative electrode active coating, it is difficult for the commonly used silicon-carbon material to guarantee the excellent cycle performance of the battery. In the present application, the silicon-carbon material including primary spherical particles and secondary spherical particles formed by the primary spherical particles is used. The primary spherical particles with small particle size are beneficial to improve the energy density of the battery; however, due to the small particle size, the specific surface area is large, which leads to an increased risk of side reactions with the electrolyte, and the stability is poor. The secondary spherical particles with large particle size have a small surface area, so the risk of side reactions with the electrolyte is low, and the stability is good; however, due to the large particle size, it is difficult to achieve a large compaction density, so the energy density of the battery is affected. The combination of the primary spherical particles and the secondary spherical particles not only improves the electrical conductivity of the silicon-carbon material itself, but also improves the compaction of the negative electrode sheet, so that the silicon content in the negative electrode active coating can be more than 5%, thereby improving the energy density of the battery. In addition, it can also reduce the risk of side reactions between the silicon-carbon material and the electrolyte, which is beneficial to improve the cycle stability of the battery.

[0007] On the basis of the above silicon-carbon material, the inventors of the present application use a specific graphite material in combination, and by regulating the structure and particle size of the graphite material, the graphite material includes secondary particles formed by a plurality of primary particles, and the average particle size of the secondary particles is 6-20 μm, which can further improve the energy density of the battery. The secondary particles of the graphite material with a specific particle size have better pressure resistance than the primary particles of the graphite material, which can improve the energy density of the battery. Therefore, the graphite material has a high matching degree with the specific silicon-carbon material, which is beneficial to improve the energy density of the battery.

[0008] Based on this, the inventors of the present application propose the following scheme:

[0009] The application discloses a lithium ion secondary battery, which comprises a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating layer located on at least one side surface of the negative electrode current collector, and the negative electrode active coating layer comprises a negative electrode active material; the negative electrode active material comprises a graphite material and a silicon-carbon material; the graphite material comprises secondary particles, and the silicon-carbon material comprises primary spherical particles and secondary spherical particles formed by a plurality of the primary spherical particles; the mass content of element Si in the negative electrode active coating layer is 1.5%-15%; the average particle size d2 of the secondary particles is 6-20 mu m; the average particle size d1 of the primary spherical particles is 1-6 mu m; and the electrolyte comprises fluoroethylene carbonate, and the mass content c2 of fluoroethylene carbonate in the electrolyte is less than or equal to 30%.

[0010] Compared with the prior art, the battery has the advantages of high energy density, long cycle life and low cycle expansion rate.

[0011] The endpoints of the ranges and any values claimed herein are not to be construed as limiting. Ranges can be expressed as from about one particular value to about another; however, when such a range is recited, it will be understood that the end points of the range are specifically contemplated. Each individual value within the range is also contemplated. Ranges are also inclusive of the endpoints. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 The scanning electron microscope (SEM) image of the cross section of the negative electrode sheet in the example of the application is shown.

[0013] Figure 2 The schematic diagram (top view) of the groove on the surface of the negative electrode sheet in the example of the application is shown, wherein, Figure 2 The groove in (a) is continuously arranged. Figure 2 The groove in (b) is arranged in segments.

[0014] Figure 3 The schematic diagram of the width of the groove in the example of the application is shown, wherein Figure 3 The two long edges of the groove in (a) are straight lines, Figure 3 The two long edges of the groove in (c) are curved lines. Figure 3 The two long edges of the groove in (d) are curved lines.

[0015] Figure 4 The schematic diagram of the pitch of the groove in the example of the application is shown, wherein Figure 4 (a) is the case that the two adjacent long edges are straight lines and parallel, Figure 4 (b) is the case that the two adjacent long edges are straight lines and not parallel, Figure 4(c) for the case where the two adjacent long sides are curved.

[0016] Figure 5 Fig. 1 shows a schematic view of the structure of a core in an example of the present application.

[0017] Figure 6 Fig. 2 shows a schematic view of the structure of a positive electrode sheet in an example of the present application, wherein Figure 6 (a) is a plan view, Figure 6 (b) is a cross-sectional view along the thickness direction.

[0018] Figure 7 Fig. 3 shows a plan view of the first surface of a positive electrode sheet in an example of the present application. DETAILED DESCRIPTION

[0019] The following detailed description of the application is provided for the purpose of explanation and not limitation, as the application is further defined in the claims.

[0020] The present application provides a lithium ion secondary battery, which includes a negative electrode sheet and an electrolyte. The negative electrode sheet can include a negative electrode current collector and a negative electrode active coating layer on at least one side surface of the negative electrode current collector. The negative electrode active coating layer can include a negative electrode active material, which can include a graphite material and a silicon-carbon material. The graphite material, for example, includes artificial graphite and / or natural graphite. The silicon-carbon material refers to a composite material including elemental carbon and elemental silicon, for example, a material including pores of a porous carbon filled (including partially filled or completely filled) with silicon and / or oxidized silicon.

[0021] In the present application, the graphite material can include secondary particles. The secondary particles are formed of a plurality of primary particles, and the "plurality" refers to a number of the primary particles forming the secondary particles being equal to or greater than 2. The silicon-carbon material can include primary spherical particles and secondary spherical particles formed of a plurality of the primary spherical particles. The "plurality" refers to a number of the primary spherical particles forming the secondary spherical particles being equal to or greater than 2. As Figure 1 Fig. 5 shows a scanning electron microscope (SEM) image of a cross section of a negative electrode sheet along the thickness direction in an example of the present application, from which it can be seen that the silicon-carbon material includes primary spherical particles and secondary spherical particles formed of a plurality of the primary spherical particles. In the figure, the primary spherical particles are framed by a black circle, and the secondary spherical particles are framed by a black square.

[0022] In the present application, the mass content of elemental Si in the negative electrode active coating layer can be 1.5% to 15%, for example, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%.

[0023] In an example, the mass content of element Si in the negative active coating is 3.5% to 12%.

[0024] In the present application, the mass content of element Si in the negative active coating can be tested by conventional methods in the art, for example, after discharging the battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking in dimethyl carbonate (DMC) solvent for 12h, then rinsing with DMC solvent to remove the lithium salt attached to the negative electrode sheet, drying, then high temperature treatment of the negative electrode sheet at 400℃ in an inert atmosphere for 2h (for example, in a tube furnace, under nitrogen or argon atmosphere), the negative active coating can be peeled off from the negative current collector, and the negative active coating is collected as a test sample. Using a thermal gravimetric analyzer (for example, TGA 550 thermal gravimetric analyzer), the test sample amount is 5mg to 15mg, under air or oxygen atmosphere, the temperature is raised from room temperature (25℃) to 900℃ at a rate of 10℃ / min, and kept at 900℃ for 40min, so that the non-silicon components in the negative active coating volatilize while the silicon is fully oxidized to silicon dioxide. The residual material is the ash of the negative active coating, and the mass content of element Si in the negative active coating can be calculated according to the mass of the ash, and the calculation formula is as follows: mass content of element Si in the negative active coating = 7 x mass of ash / (15 x mass of test sample).

[0025] In the present application, the average particle size d2 of the secondary particles can be 6μm to 20μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0026] In an example, d2 is 10μm to 15μm.

[0027] In the present application, the average particle size d2 of the secondary particles can be obtained by a method conventional in the art, for example, after discharging the battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking in DMC solvent for 12 h, then rinsing with DMC solvent to remove the lithium salt attached to the negative electrode sheet, then using deionized water to wash the negative electrode active coating from the negative electrode current collector, removing the filtrate after ultrasonic treatment and centrifugation, drying, dispersing the obtained sample in deionized water containing nonylphenol polyoxyethylene ether (wherein the mass content of nonylphenol polyoxyethylene ether is 0.02%-0.03%), forming a mixture, ultrasonic treatment for 2 minutes, then using a Malvern particle size tester for testing, and the obtained median particle size Dv50 data is the average particle size d2 of the secondary particles. Due to the specific composition and particle size of the graphite material and the silicon-carbon material in the present application, the silicon-carbon material has little effect on the average particle size of the secondary particles of the graphite material, and therefore the data obtained by the above testing method is the average particle size of the secondary particles.

[0028] In the present application, the average particle size d1 of the primary spherical particles can be 1-6 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or 6 μm.

[0029] In an example, d1 is 3-5 μm.

[0030] In the present application, the average particle size d1 of the primary spherical particles can be obtained by a method conventional in the art, for example, after discharging the battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking in DMC solvent for 12 h, then rinsing with DMC solvent to remove the lithium salt attached to the negative electrode sheet, using an argon ion grinder CP to laser cut the negative electrode sheet, then using SEM observation (using high voltage mode (Back-scattered Electrons BSE)), in this mode, the contrast of the silicon-carbon material is brighter (which can be used to distinguish the graphite material and the conductive agent in the negative electrode active coating), measuring at 5K magnification, randomly selecting at least 20 primary spherical particles, measuring the particle size of each primary spherical particle, and taking the average value. If the number of primary spherical particles is less than 20 at 5K magnification, then another micrograph is taken until 20 primary spherical particles are measured.

[0031] In the present application, the electrolyte can include fluoroethylene carbonate (FEC). The mass content c2 of FEC in the electrolyte is ≤30%, for example, 30%, 25%, 20%, 15%, 10%, 9% or 8%.

[0032] In an example, c2 is 8%-30%.

[0033] In the present application, the mass content c2 of FEC in the electrolyte can be tested by a method conventional in the art, such as gas chromatography-mass spectrometry (GC-MS) method.

[0034] In the present application, the mass content of element Si in the silicon-carbon material can be 30%-80%, such as 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%.

[0035] In the present application, in the negative active coating, the number of the primary spherical particles accounts for 0.1-0.9 of the total number of the primary spherical particles and the secondary spherical particles, such as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9.

[0036] In an example, in the negative active coating, the number of the primary spherical particles accounts for 0.3-0.8 of the total number of the primary spherical particles and the secondary spherical particles.

[0037] In the present application, the number of the primary spherical particles and the number of the secondary spherical particles can be tested by a method conventional in the art, such as disassembling the negative electrode sheet after discharging the battery to 0% SOC, soaking in DMC solvent for 12 h, then rinsing with DMC solvent to remove the lithium salt attached to the negative electrode sheet, using argon ion grinder CP to cut the negative electrode sheet along the thickness direction, then using SEM to observe (using high voltage mode), obtaining the mirror image of the cross section of the negative electrode sheet along the thickness direction, observing under 1K magnification, selecting at least 20 mirror images of different cross sections, counting the number of the primary spherical particles and the number of the secondary spherical particles in each mirror image respectively, and taking the average value.

[0038] By regulating the number ratio of the primary spherical particles and the secondary spherical particles in the negative active coating, the silicon-carbon material can be arranged more closely, which is not only conducive to further improving the conductivity of the silicon-carbon material itself, but also can further improve the compaction of the negative electrode sheet, thereby improving the energy density of the battery. And when the primary spherical particles and the secondary spherical particles are mixed in a specific number ratio, the risk of side reaction between the silicon-carbon material and the electrolyte can be further reduced, which is conducive to improving the cycle stability of the battery.

[0039] In the present application, 1.33≤d2 / d1≤20, such as 1.33, 1.5, 1.67, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20.

[0040] In an example, 1.5≤d2 / d1≤15.

[0041] In an example, 2≤d2 / d1≤5.

[0042] For the high-silicon content battery system, the advantages of high-silicon content can be maximized by ensuring high compaction and low expansion of the negative electrode sheet. Therefore, the silicon-carbon material is mixed with the graphite material with secondary particles, and when the ratio of the average particle size of the secondary particles to the average particle size of the primary spherical particles is within a certain range, the graphite material and the silicon-carbon material are matched to make the consistency of the orientation of the negative active material better, which is conducive to reducing the cycle expansion rate of the negative electrode sheet. When d2 / d1 is too large (for example, greater than 20), the particle size of the graphite secondary particles is too large relative to the particle size of the silicon-carbon primary spherical particles, the conductive performance of the larger particle size graphite material is poor, which leads to poor dynamic performance of the negative electrode sheet, and is not conducive to the cycle life of the battery; or the smaller particle size silicon-carbon material is prone to agglomeration and has poor dispersibility. When d2 / d1 is too small (for example, less than 1.33), the particle size of the graphite secondary particles is too small relative to the particle size of the silicon-carbon primary spherical particles, the smaller particle size graphite material is not pressure-resistant, which affects the improvement of the compaction density of the negative electrode sheet, thereby affecting the energy density of the battery; or the larger particle size silicon-carbon material has poor conductive performance, which affects the dynamic performance of the negative electrode sheet, and the particles are not pressure-resistant and are easy to break.

[0043] In the present application, the OI value V OI of the negative active coating satisfies: 10≤V OI / s≤35 (for example, 10, 15, 20, 25, 30, or 35), and 2≤V OI / d1≤15 (for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15).

[0044] In an example, 12≤V OI / s≤22, and 2.5≤V OI / d1≤5.5.

[0045] In an example, 15≤V OI / s≤20, and 3.5≤V OI / d1≤4.

[0046] The OI value of the negative electrode active coating, the sphericity of the primary spherical particles, and the average particle size of the primary spherical particles can all affect the expansion rate of the negative electrode sheet in the thickness direction to some extent. Specifically, the OI value of the negative electrode active coating represents the overall orientation of the negative electrode active material, and the smaller the OI value, the smaller the expansion rate of the negative electrode active material in the thickness direction of the battery. The higher the sphericity of the primary spherical particles, the more comparable the expansion in each direction, i.e., the greater the expansion rate in the thickness direction of the battery. However, the sphericity cannot be too low, which would increase the active sites on the surface of the silicon-carbon material and intensify the side reactions with the electrolyte, which is not conducive to improving the cycle stability. The smaller the average particle size of the primary spherical particles, the greater the specific surface area, and the greater the side reactions with the electrolyte during the charging and discharging process of the battery, and the greater the expansion. However, the particle size of the primary spherical particles cannot be too large, and a larger particle size of the primary spherical particles is not conducive to improving the tap density of the negative electrode, which would affect the overall energy density of the battery. Therefore, within a certain range, when the OI value is smaller, the sphericity of the silicon-carbon material can be correspondingly smaller, and the particle size can also be correspondingly reduced; when the OI value is larger, the sphericity of the silicon-carbon material needs to be correspondingly increased, and the particle size also needs to be correspondingly increased. By regulating the relationship among the three, when a specific relationship is met, the battery can further improve the energy density and cycle stability.

[0047] In the present application, the OI value V of the negative electrode active coating OI may be 10-30, for example, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30.

[0048] In an example, V OI is 12-17.

[0049] In the present application, the OI value V of the negative electrode active coating OI may be tested by conventional methods in the art, for example, after discharging the battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking in DMC solvent for 12 h, then rinsing with DMC solvent to remove the lithium salt attached to the negative electrode sheet, using an X-ray powder diffraction instrument (e.g., Shimadzu XRD-6100), testing the negative electrode active coating to obtain a diffraction pattern, the intensity of the (004) peak at 2θ of 54°-55° is denoted as I 004 , the intensity of the (110) peak at 2θ of 77°-78° is denoted as I 110 , and V OI is I 004 / I 110 .

[0050] In the present application, the sphericity s of the primary spherical particles can be 0.7-1, for example, 0.7, 0.8, 0.9, or 1.

[0051] In the present application, the sphericity of the primary spherical particles can be tested by methods conventional in the art, for example, after discharging the battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking in DMC solvent for 12 h, then rinsing with DMC solvent to remove the lithium salt attached to the negative electrode sheet, then using deionized water to wash the negative electrode active coating from the negative electrode current collector, centrifuging after ultrasonic treatment to remove the filtrate, drying, and observing the obtained powder with SEM (using high voltage mode). Through image processing software (such as Image Pro Plus), image analysis is performed on the silicon-carbon material particles in the SEM mirror image at a certain magnification (such as 2500 times), at least 10 primary spherical particles of silicon-carbon material in the mirror image are selected, the perimeter and area of each particle are obtained, the perimeter equivalent radius and area equivalent radius are calculated respectively, then the sphericity = area equivalent radius / perimeter equivalent radius, and the average value is taken.

[0052] In the present application, the surface of the negative electrode active coating on at least one side surface of the negative electrode current collector can have a first recess. The first recess provided on the surface of the negative electrode active coating can provide a certain release space for the volume expansion of the negative electrode active material, can reduce the extension of the negative electrode current collector caused by the volume expansion of the negative electrode active material, and is beneficial to further improving the cycle stability of the battery.

[0053] In the present application, the first recess can include a recess hole or a groove. The first recess can be obtained by laser drilling or wire drawing technology. When the first recess is a groove, the groove can be continuously provided or segmented. As shown in Figure 2 which is a schematic diagram (top view) of the groove on the surface of the negative electrode sheet in an example of the present application, wherein, Figure 2 (a) the groove is continuously provided; Figure 2 (b) the groove is segmented. As can be seen from the figure, the surface of the negative electrode sheet (i.e. the surface of the negative electrode active coating) has several grooves, in Figure 2 (a) the groove is continuously provided in the width direction of the negative electrode sheet, in Figure 2 (b) the groove is segmented in the width direction of the negative electrode sheet. Figure 2 Only the case where the groove is provided in the width direction of the negative electrode sheet is given in the present application, and the groove can also be provided in the length direction of the negative electrode sheet.

[0054] In the present application, the depth of the first recesses can be 5-40 pm, for example 5 pm, 10 pm, 15 pm, 20 pm, 25 pm, 30 pm, 35 pm or 40 pm. The depth of the first recesses has the conventional meaning in the art, referring to the vertical distance from the lowest point in the first recess to the surface of the negative electrode sheet. The depth of the first recesses can be tested by conventional methods in the art, for example using a 3D profilometer, testing the depth of all first recesses or at least 20 first recesses on the surface of the negative electrode active coating, and taking the average value.

[0055] In an example, the depth of the first recesses is 15-31 pm.

[0056] In the present application, the width of the first recesses can be 40-200 pm, for example 40 pm, 50 pm, 60 pm, 70 pm, 80 pm, 90 pm, 100 pm, 150 pm or 200 pm.

[0057] In an example, the width of the first recesses is 60-100 pm.

[0058] When the first recesses are holes, the width of the first recesses refers to the hole diameter of the holes. The hole diameter of the holes has the conventional meaning in the art. When the shape of the holes in the orthographic projection on the surface of the negative electrode sheet is a “regular circle”, the hole diameter of the holes is the diameter of the regular circle; when the shape of the holes in the orthographic projection on the surface of the negative electrode sheet is a non-“regular circle” (for example an ellipse or an irregular curved polygon), the hole diameter of the holes is the diameter of an equivalent circle with the same area as the non-“regular circle”. The hole diameter of the holes can be tested by conventional means in the art, for example by a 3D profilometer, testing the hole diameter of at least 20 holes on the surface of the negative electrode active coating, and taking the average value.

[0059] When the first recesses are grooves, the width of the first recesses refers to the width of the grooves. The width of the grooves has the conventional meaning in the art. The orthographic projection of the grooves on the surface of the negative electrode sheet comprises two long sides, and the width of the grooves refers to the average distance from one long side to the other long side in the length direction or width direction of the negative electrode sheet. As shown in Figure 3 Fig. 1 shows a schematic diagram of the width of a groove in an example of the present application, wherein Figure 3 (a)- Figure 3 (c) the two long sides of the groove are straight lines, Figure 3 (d) the two long sides of the groove are curves. In Figure 3 (a) and Figure 3In (b), the two long edges are arranged in parallel, thus, in the width direction of the negative plate, the perpendicular distance from any point on one long edge to the other long edge is equal, at this time, the width of the groove is the perpendicular distance L1 from any point on one long edge to the other long edge in the length direction or the width direction of the negative plate. Figure 3 In (c), the two long edges of the groove are straight lines, but are not arranged in parallel, thus, in the width direction, the size from any point on one long edge to the other long edge is not equal, at this time, the width of the groove can be taken as the average value, that is, on one long edge, 50 point positions are selected at equal distances based on the length of the edge (that is, the distance between each point position is equal, so that the selected point positions can make the calculation result more accurate), the width L1 corresponding to each point position is measured, and the average value is taken as the width of the groove. Figure 3 In (d), the two long edges are curves, thus, in the width direction, the size from any point on one long edge to the other long edge is not equal, at this time, the width of the groove can also be taken as the average value, that is, on one long edge, 50 point positions are selected at equal distances (since the two long edges are curves, the relationship between the two long edges in (c) does not exist, thus, 50 point positions can be randomly selected for measurement), the width L1 corresponding to each point position is measured, and the average value is taken as the width of the groove. Figure 3 In (d), the two long edges are curves, thus, in the width direction, the size from any point on one long edge to the other long edge is not equal, at this time, the width of the groove can also be taken as the average value, that is, on one long edge, 50 point positions are selected at equal distances (since the two long edges are curves, the relationship between the two long edges in (c) does not exist, thus, 50 point positions can be randomly selected for measurement), the width L1 corresponding to each point position is measured, and the average value is taken as the width of the groove. Figure 3 The width of the groove can be tested by conventional means in the art, for example, by a 3D profilometer, the widths of all the grooves or at least 5 grooves on the surface of the negative active coating are tested, and the average value is taken.

[0060] In the present application, the interval of the first recesses can be 0.5 mm-5 mm, for example, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 2 mm, 3 mm, 4 mm or 5 mm.

[0061] In an example, the interval of the first recesses is 0.8 mm-1.5 mm.

[0062] When the first recesses are holes, the interval of the first recesses refers to the interval of the holes. The interval of the holes has the conventional meaning in the art. It refers to the shortest distance between the edges of two adjacent holes on the surface of the negative plate, and the interval of the holes can be tested by conventional means in the art, for example, by a 3D profilometer, all or at least 10 groups of adjacent holes are selected, the interval is measured, and the average value is taken.

[0063] When the first recess is a groove, the pitch of the first recess refers to the pitch of the groove. It can be understood that when there is only one groove on the surface of the negative electrode sheet, there is no pitch of the groove. The pitch of the groove has the conventional meaning in the art, referring to the average distance between the two adjacent long edges of two adjacent grooves in the length direction or the width direction of the negative electrode sheet. As shown in Figure 4 Fig. 1 shows a schematic diagram of the pitch of the groove in an example of the present application, wherein Figure 4 (a) is a case where the two adjacent long edges are straight lines and parallel, Figure 4 (b) is a case where the two adjacent long edges are straight lines and not parallel, Figure 4 (c) is a case where the two adjacent long edges are curves. In Figure 4 (a), the two adjacent long edges are straight lines and are arranged in parallel, so that in the width direction, the distance from any point on one long edge to the other long edge is equal. At this time, the pitch of the groove is the distance L2 from any point on one long edge to the other long edge in the width direction. In Figure 4 (b), the two adjacent long edges are straight lines, but are not arranged in parallel, so that in the width direction, the distance from any point on one long edge to the other long edge is not equal. At this time, the pitch of the groove can be taken as the average value, i.e. on one long edge, 50 points are selected at equal distances (i.e. the distance between each point is equal, so that the selected points can make the calculation result more accurate) based on the length of the long edge, and the width L2 corresponding to each point is measured to obtain the average value of the pitch. In Figure 4 (c), the two adjacent long edges are curves, so that in the width direction, the distance from any point on one long edge to the other long edge is not equal. At this time, the pitch of the groove can also be taken as the average value, i.e. on one long edge, 50 points are selected at random (since Figure 4 the two long edges in (c) are curves, there is no Figure 4 relationship between the two long edges in (b), so that 50 points can be randomly selected for measurement), and the width L2 corresponding to each point is measured to obtain the average value of the pitch. The pitch of the groove can be tested by conventional means in the art, for example, by a 3D profilometer, to test the pitch of all the grooves or at least 5 grooves on the surface of the negative electrode active coating, and take the average value.

[0064] In the present application, the compaction density of the negative electrode sheet can be 1.2 g / cm 3 -1.9 g / cm 3 , for example, 1.2 g / cm 3 , 1.3 g / cm 3 , 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 31.8 g / cm 3 or 1.9 g / cm 3 Here, the compaction density refers to the actual compaction density measured after the battery is disassembled and the negative electrode sheet is removed after the battery preparation is completed, which is different from the compaction density obtained after the negative electrode sheet is rolled during the battery preparation process. This is because after the negative electrode sheet is assembled into a battery, there will be physical and chemical rebounds, resulting in compaction rebound, so that the actual compaction density of the negative electrode sheet measured after the battery preparation is completed is smaller than the compaction density obtained after rolling.

[0065] In the present application, the compaction density of the negative electrode sheet can be obtained by conventional methods in the art, for example, after the battery is discharged to 0% SOC, the negative electrode sheet is disassembled and removed, soaked in DMC solvent for 12h, then rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet, then washed with deionized water to remove the residue on the surface of the negative electrode sheet, dried, at least 20 sites on the negative electrode sheet are selected, the thickness of the negative electrode sheet at each site is measured using a micrometer, and the average value h (unit: μm) is taken; the negative electrode sheet is punched into a circular sheet with a diameter of 44.3mm using a punching film tool, 10 circular sheets are taken, the mass of each circular sheet is weighed, the average value m (unit: mg) is taken, and the area density M1 = (m-m1) x 100 / 1540.25 is calculated, wherein m1 is the mass of the negative electrode current collector in the circular sheet, unit: mg, and the unit of area density is mg / cm 2 The area density M1 and the average thickness h are used to calculate the compaction density, the formula is as follows: compaction density = M1 x 20 / (h-thickness of negative electrode current collector), wherein the unit of the thickness of the negative electrode current collector is μm.

[0066] In the present application, the lithium ion secondary battery can further include a positive electrode sheet.

[0067] <Positive electrode sheet>

[0068] In the present application, the positive electrode sheet can include a positive electrode current collector and a positive electrode active coating layer located on at least one side surface of the positive electrode current collector. The positive electrode active coating layer can include a lithium supplementing agent, which can include a substance with a chemical formula of Li 5+x Fe y M 1 z O4 and / or a substance with a chemical formula of Li 2±a Ni b M c O2; wherein -0.5≤x≤5 (such as -0.5, 0, 1, 2, 3, 4 or 5), 0.8≤y≤1.2 (such as 0.8, 0.9, 1, 1.1 or 1.2), 0≤z≤1 (such as 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1), M 1at least one of Mo, Nb, Ti, Zr, Ni, Y, Mn, Cu, Mg and Zn; 0≤a≤0.5 (e.g. 0, 0.1, 0.2, 0.3, 0.4 or 0.5), 0≤b≤1 (e.g. 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1), 0≤c≤1 (e.g. 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1), M 2 at least one of Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg and Zn.

[0069] In an example, the lithium supplement agent comprises Li2NiO2 and / or Li5FeO4.

[0070] In the present application, the mass content c1 of the lithium supplement agent in the positive active coating can be 0.3%-1%, for example, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%.

[0071] In an example, c1 is 0.5%-0.8%.

[0072] The primary spherical particles in the silicon-carbon material have a small average particle size, resulting in a large contact area with the electrolyte. During the charge and discharge cycles of the battery, the volume expansion will cause the SEI (Solid Electrolyte Interface) film on the surface of the silicon-carbon material to break, and the recombination of the broken SEI film will consume active lithium. Although the silicon-carbon material used in the negative electrode sheet of the present application is a combination of primary spherical particles and secondary spherical particles, which can effectively reduce the consumption of active lithium. However, due to the nature of silicon material, the consumption of active lithium is difficult to completely avoid. Therefore, in order to further improve the cycle life of the battery, a lithium supplement agent can be added to the positive electrode sheet to increase the content of active lithium in the battery and improve the cycle life of the battery.

[0073] In the present application, the mass content c1 of the lithium supplement agent in the positive active coating can be tested by conventional methods in the art, for example, after discharging the battery to 0% SOC, disassembling and taking out the positive electrode sheet, soaking in DMC solvent for 12h, then rinsing with DMC solvent to remove the lithium salt attached to the positive electrode sheet, calcining in a muffle furnace at 400℃ for 3h, gently scraping the positive active coating from the surface of the positive current collector, measuring the mass content of characteristic elements (such as element Ni and / or element Fe) in the lithium supplement agent by inductively coupled plasma-optical emission spectrometer (ICP-OES), and then calculating the mass content of the lithium supplement agent. The specific operation method is carried out according to GB / T 30902-2014.

[0074] In the present application, the positive active coating can further comprise a positive active material. The positive active material comprises lithium cobaltate.

[0075] The related art generally repairs the SEI film by adding FEC in the electrolyte, thereby inhibiting the occurrence of side reactions and reducing the breakage of silicon-based materials (i.e., the silicon-carbon material in the present application) during the charging and discharging process of the battery. Moreover, the amount of FEC added needs to be increased as the amount of silicon doping increases. However, FEC is unstable at high temperatures and can decompose to generate a large amount of gas. Too much FEC can cause the battery to produce gas and explode, increasing the safety risk. Therefore, the positive plate in the present application adds a lithium supplement, which reserves excess active lithium to reduce the additional consumption of lithium ions during the breakage and recombination of the SEI film, thereby being able to reduce the amount of FEC added in the electrolyte while ensuring a high silicon content. Therefore, within a certain range, when the content of FEC increases, the content of the lithium supplement in the positive plate can be appropriately reduced. If the content of the lithium supplement is too high at this time, it will not only affect the capacity development of the positive plate, but also can cause the side reaction between FEC and the lithium supplement to intensify, increasing the safety risk. Or, when the content of FEC decreases, the content of the lithium supplement in the positive plate needs to be increased to improve the electrochemical performance of the lithium ion battery. When the two satisfy a specific relationship, the battery can further improve the cycle life and reduce the cycle expansion rate while ensuring safe use.

[0076] In the present application, the mass content c2 of FEC in the electrolyte (unit: %) and the mass content c1 of the lithium supplement in the positive active coating (unit: %) satisfy: c1 x c2 ≤ 30, for example, 30, 25, 20, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1.

[0077] In an example, 7 ≤ c1 x c2 ≤ 24.

[0078] When c1 x c2 is low (e.g., less than 7), the lithium supplement provided by the lithium supplement is insufficient to effectively supplement the additional consumption of lithium ions during the breakage and recombination of the SEI film, which is not conducive to the improvement of the electrochemical performance of the battery. When c1 x c2 is high (e.g., greater than 24), there is too much FEC and / or too much lithium supplement, and the risk of side reactions between the two increases, which is not conducive to safety performance.

[0079] In an example, 12 ≤ c1 x c2 ≤ 15.

[0080] It should be noted that only the numerical part is used in the calculation of “c1 x c2”, and the unit is not involved in the operation. For example, in Example 1, c1 is 0.6%, and c2 is 23%, then c1 x c2 = 0.6 x 23 = 13.8.

[0081] In the present application, the length of the positive active coating on the first surface of the positive current collector is greater than the length of the positive active coating on the second surface of the positive current collector. It can be understood that there are single-sided coating areas and double-sided coating areas in the positive sheet during coating, the single-sided coating area refers to the positive current collector in this area, only one side surface has a positive active coating; while the double-sided coating area refers to the positive current collector in this area, both side surfaces have a positive active coating. This results in the case that the lengths of the positive active coatings on the two side surfaces of the positive sheet are not equal. In the present application, the side with the relatively longer length of the positive active coating on the surface of the positive current collector is defined as the first surface, and the side with the relatively shorter length of the positive active coating is defined as the second surface. The area where the projection of the positive active coating on the first surface overlaps with the projection of the positive active coating on the second surface in the thickness direction of the positive sheet is the double-sided coating area, and the area where the above projections do not overlap is the single-sided coating area. Due to the special structure of the winding type battery, usually the first surface faces the winding center of the winding core, and the second surface is away from the winding center of the winding core.

[0082] In the present application, the surface of the positive active coating on the first surface has a second recess, and the surface of the positive active coating on the second surface has a protrusion. The structure of one side having a recess and the other side having a protrusion can be obtained by embossing the surface of the positive sheet. The present inventors have analyzed the stress of the positive active coating facing the winding center and away from the winding center in the winding core, and found that when the first surface facing the winding center is provided with a second recess and the second surface away from the winding center is provided with a protrusion, not only is it beneficial to the structural stability of the positive sheet itself, but also provides a better buffer space for the volume expansion of the negative sheet, and can improve the cycle life of the battery. As shown in Figure 5 Fig. 1 is a schematic structural diagram of the winding core in an example of the present application, from which it can be seen that the surface of the positive active coating on the first surface has a second recess, and the surface of the positive active coating on the second surface has a protrusion, the first surface faces the winding center of the winding core, and the second surface is away from the winding center of the winding core.

[0083] In the present application, the shape of the orthographic projection of the second recess and the protrusion on the surface of the positive sheet is not limited, which can be circular, elliptical, linear (including straight line or wavy line), polygonal, etc.

[0084] In the present application, the positive sheet includes a positive tab welding area, a paste coating area, and a blank foil area. The blank foil area refers to the area on the positive current collector other than the positive tab welding area, which is not coated with a positive active coating. The paste coating area includes a double-sided coating area and a single-sided coating area. As shown in Figure 6 Fig. 2 is a schematic structural diagram of the positive sheet in an example of the present application, wherein Figure 6 (a) is a top view,Figure 6 (b) is a cross-sectional view along the thickness direction. As can be seen from the figure, the positive electrode sheet includes a positive electrode tab welding area 3, a pasting area 4, and a hollow foil area 5, wherein the pasting area 4 includes a double-coated area 10 and a single-coated area 20.

[0085] In an example, the second recess and the protrusion are located in the pasting area.

[0086] In an example, the second recess and the protrusion are located in the double-coated area.

[0087] In an example, the second recess and the protrusion are located in the pasting area and in the double-coated area.

[0088] In the present application, the distance from the second recess to the edge of the positive electrode tab welding area is w1, 0mm < w1 < 10mm, for example 0.1mm, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm.

[0089] In the present application, the distance from the second recess to the edge of the first side of the pasting area is w2, 2mm < w2 < 40mm, for example 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm, 13mm, 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, 20mm, 25mm, 30mm, 35mm, or 40mm. The first side is the side on which the positive electrode tab welding area is disposed.

[0090] In the present application, the distance from the second recess to the edge of the second side of the pasting area is w3, 2mm < w3 < 25mm, for example 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm, 13mm, 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, 20mm, or 25mm. The second side is the side opposite to the side on which the positive electrode tab welding area is disposed.

[0091] In the present application, the distance from the second recess to the edge of the third side of the pasting area is w4, 0mm < w4 < 20mm, for example 0.1mm, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm, 13mm, 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, or 20mm. The third side is the side of the pasting area close to the winding head.

[0092] In the present invention, the distance from the second recess to the boundary line between the double-sided coating area and the single-sided coating area is w5, where 0 mm < w5 ≤ 20 mm, for example, 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm or 20 mm.

[0093] By controlling w1, w2, w3, w4 and w5, it is possible to ensure that the positive electrode sheet does not shed powder, which is beneficial to improving the cycle life of the battery.

[0094] As Figure 7 Shown is a top view schematic diagram of the first surface of the positive electrode sheet in an example of the present invention. As can be seen from the figure, the first surface has a plurality of second recesses 6. The distance from the second recess 5 to the edge of the positive electrode tab welding area 3 is wl. The distance from the second recess 6 to the edge of the first side of the paste application area 4 is w2. The distance from the second recess 6 to the edge of the second side of the paste application area 4 is w3. The distance from the second recess 6 to the edge of the third side of the paste application area 4 is w4. The distance from the second recess 6 to the boundary line between the double-sided coating area 10 and the single-sided coating area 20 is w5.

[0095] In the present invention, the depth of the second recess can be 3 μm - 40 μm, for example, 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm or 40 μm. The height of the convex portion can be 3 μm - 40 μm, for example, 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm or 40 μm.

[0096] In one example, the depth of the second recess is 10 μm - 30 μm. The height of the convex portion is 10 μm - 30 μm.

[0097] [[ID= ]]In the present invention, the depth of the second recess and the height of the convex portion have their conventional meanings in the art. The depth of the second recess refers to the vertical distance from the lowest point in the second recess to the surface of the positive electrode sheet. The height of the convex portion refers to the vertical distance from the highest point on the convex portion to the surface of the positive electrode sheet. The depth of the second recess and the height of the convex portion can be obtained by testing through conventional methods in the art, for example, by using a 3D profiler. At least 20 second recesses or 20 convex portions are selected on the surface of the positive electrode sheet. The depth of each second recess is measured, and the average value is taken to obtain the depth of the second recess; the height of each convex portion is measured, and the average value is taken to obtain the height of the convex portion.

[0098] In the present application, the width of the second recess can be 0.2 mm-8 mm, for example 0.2 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm or 8 mm. The width of the convex part can be 0.2 mm-8 mm, for example 0.2 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm or 8 mm.

[0099] In an example, the width of the second recess is 1 mm-3 mm. The width of the convex part is 1 mm-3 mm.

[0100] In the present application, when the shape of the projection of the second recess in the thickness direction of the positive electrode sheet is a regular circle, the width of the second recess is the diameter of the regular circle; when the shape of the projection of the second recess in the thickness direction of the positive electrode sheet is a non-“regular circle”, the width of the second recess is the equivalent diameter of a circle equal to the area of the non-regular circle. Similarly, when the shape of the projection of the convex part in the thickness direction of the positive electrode sheet is a regular circle, the width of the convex part is the diameter of the regular circle; when the shape of the projection of the convex part in the thickness direction of the positive electrode sheet is a non-“regular circle”, the width of the convex part is the equivalent diameter of a circle equal to the area of the non-regular circle. The width of the second recess and the width of the convex part can be obtained by methods conventional in the art, for example by a 3D profilometer, selecting at least 10 second recesses and 10 convex parts on the surface of the positive electrode sheet, measuring the width of each second recess and the width of each convex part, and averaging the values respectively.

[0101] In the present application, the pitch of the second recess can be 0.5 mm-8 mm, for example 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm or 8 mm. The pitch of the convex part can be 0.5 mm-8 mm, for example 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm or 8 mm.

[0102] In an example, the pitch of the second recess is 1 mm-3 mm. The pitch of the convex part is 1 mm-3 mm.

[0103] In the present application, the pitch of the second recess refers to the shortest distance between the edges of the orthographic projection of two adjacent second recesses on the surface of the positive electrode sheet, and similarly the pitch of the convex part refers to the shortest distance between the edges of the orthographic projection of two adjacent convex parts on the surface of the positive electrode sheet. The pitch of the second recess and the pitch of the convex part can be obtained by methods conventional in the art, for example by a 3D profilometer, selecting at least 10 groups of adjacent second recesses or convex parts on the surface of the positive electrode sheet, measuring the pitch, and averaging the values.

[0104] In the present application, the composition of the positive active coating layer can be a conventional selection in the art.

[0105] <electrolyte>

[0106] In the present application, the electrolyte can further include at least one of a lithium salt, an additive, and an organic solvent. The lithium salt, for example, includes at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPO2F2), lithium difluoro oxalate borate (LiDFOB), lithium bis-trifluoromethylsulfonylimide, lithium difluorobisoxalate phosphate, lithium bisoxalate borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methide, and lithium tetrafluoroborate. The additive, for example, includes at least one of vinylene carbonate, vinyl ethylene carbonate, vinyl sulfate, fluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, acetonitrile, propionitrile, butyronitrile, butanedinitrile, propanedinitrile, pentanedinitrile, hexanedinitrile, heptanedinitrile, octanedinitrile, coumadin, 1,2-bis(2-cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,2,6-hexanetricarbonitrile, acrylonitrile, crotonitrile, trans-butenedinitrile, glycerol tricarbonitrile, and trans-hexenedinitrile. The organic solvent, for example, includes a carbonate-based solvent and / or a carboxylate-based solvent. The carbonate-based solvent, for example, includes at least one of ethylene carbonate (EC) and / or fluorine-substituted or unsubstituted propylene carbonate (PC), dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The carboxylate-based solvent, for example, includes at least one of fluorine-substituted or unsubstituted ethyl acetate, propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, methyl propionate (PP), ethyl propionate, n-propyl propionate, methyl butyrate, ethyl butyrate, and n-ethyl butyrate.

[0107] In the present application, the lithium ion secondary battery can further include a separator, which can be a conventional selection in the art.

[0108] It should be noted that the "first", "second", and the like numerical designations in the present application are only used to distinguish different substances or usage manners, and do not represent the difference in order.

[0109] The present application will be described in detail below through examples. The examples described in the present application are only a part of the examples of the present application, rather than all the examples. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0110] In the following examples, the materials used are commercially available analytical pure unless otherwise specified.

[0111] The following examples are used to illustrate the lithium ion secondary battery of the present application.

[0112] Example 1

[0113] The battery was prepared according to the following method:

[0114] (1) Preparation of positive electrode sheet

[0115] Lithium cobaltate, a lithium supplement (Li2NiO2), a positive electrode conductive agent (conductive carbon black), and a positive electrode binder (polyvinylidene fluoride) were mixed in a mass ratio of 96.4:0.6:1:2, N-methyl pyrrolidone (NMP) was added, and the mixture was stirred uniformly to prepare a positive electrode slurry; the positive electrode slurry was coated on the first surface and the second surface of an aluminum foil (the coating length of the positive electrode slurry on the first surface of the aluminum foil was greater than that on the second surface), and the aluminum foil was baked and rolled to obtain a positive electrode sheet with a thickness of 100 μm; a positive electrode tab welding area with a fixed size was arranged on the coating area of the positive electrode sheet (the size of the positive electrode tab welding area in the width direction of the positive electrode sheet was 20 mm), and a nickel tab was laser-welded in the positive electrode tab welding area; the positive electrode sheet was then passed through a roller, and embossing treatment was performed from the first surface to the second surface in the double-coating area (and avoiding the positive electrode tab welding area) to obtain a second recess (first surface) and a convex part (second surface); the shape of the orthographic projection of the second recess and the convex part on the surface of the positive electrode sheet was circular.

[0116] In the above, the width of the second recess was 2 mm, the depth was 20 μm, and the interval was 2 mm; w1 was 7 mm, w2 was 27 mm, w3 was 15 mm, w4 was 7 mm, and w5 was 7 mm.

[0117] (2) Preparation of negative electrode sheet

[0118] A graphite material (artificial graphite, the average particle size d2 of secondary particles was 12.3 μm), a silicon-carbon material (the number of primary spherical particles accounted for 0.54 of the total number of primary spherical particles and secondary spherical particles, the average particle size d1 of primary spherical particles was 4.2 μm, the sphericity s of primary spherical particles was 0.95, and the mass content of element Si in the silicon-carbon material was 70%), a negative electrode conductive agent (carbon nanotube), a negative electrode dispersing agent (lithium carboxymethyl cellulose), and a negative electrode binder (polyacrylic acid) were mixed in a mass ratio of 88.6:8.4:0.4:0.1:2.5, deionized water was added, and a negative electrode slurry was prepared; the negative electrode slurry was coated on both sides of a carbon-coated copper foil, and the copper foil was baked and rolled to obtain a negative electrode sheet with a thickness of 110 μm; a first recess (groove) was manufactured on the surface of the negative electrode sheet using a laser.

[0119] In the above, the mass content of element Si in the negative electrode active coating was 5.9%, d2 / d1 was 2.93, V OI was 15.4, and VOI s is 16.21, V OI d1 is 3.67; the width of the groove is 80.2 μm, the depth is 20.3 μm, and the pitch is 1.3 mm.

[0120] (3) Preparation of electrolyte

[0121] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), ethylene carbonate, propylene carbonate and diethyl carbonate were mixed in a weight ratio of 1:3:6 to obtain an organic solvent; FEC and lithium salt LiPF6 were dissolved in the above organic solvent to obtain an electrolyte; wherein the mass content c2 of FEC in the electrolyte was 23%, and the mass content of LiPF6 in the electrolyte was 12.5%;

[0122] Wherein, c1 x c2 is 13.8.

[0123] (4) Preparation of battery

[0124] The positive electrode sheet prepared in step (1), the separator (including a polyethylene base film with a thickness of 4 μm, a ceramic layer with a thickness of 2 μm on one side surface of the base film, a polymethyl methacrylate glue layer with a thickness of 0.5 μm on the other side surface of the base film, and a polyvinylidene fluoride + polymethyl methacrylate glue layer with a thickness of 2.5 μm on the outer surface of the ceramic layer), and the negative electrode sheet prepared in step (2) were wound to obtain a roll core; the battery was obtained through packaging, baking, liquid injection, formation, second sealing, sorting and OCV.

[0125] Example 2

[0126] The battery was prepared according to the following method:

[0127] (1) Preparation of positive electrode sheet

[0128] Lithium cobaltate, lithium supplement (Li2NiO2), positive electrode conductive agent (conductive carbon black) and positive electrode binder (polyvinylidene fluoride) were mixed in a mass ratio of 96.5:0.5:1:2, N-methyl pyrrolidone (NMP) was added, and stirring was uniform to prepare a positive electrode slurry; the positive electrode slurry was coated on the first surface and the second surface of the aluminum foil (the coating length of the positive electrode slurry on the first surface of the aluminum foil was greater than that on the second surface), baked and rolled to obtain a positive electrode sheet with a thickness of 100 μm; a positive electrode tab welding area with a fixed size was arranged on the coating area of the positive electrode sheet (the size of the positive electrode tab welding area in the width direction of the positive electrode sheet was 15 mm), and a nickel tab was laser welded in the positive electrode tab welding area; then, the roll was passed again, and embossing treatment was performed from the first surface to the second surface in the double-sided coating area (and avoiding the positive electrode tab welding area) to obtain a second recess and a convex part; the shape of the orthographic projection of the second recess and the convex part on the surface of the positive electrode sheet was circular.

[0129] The second recess has a width of 1 mm, a depth of 10 μm, and a pitch of 1 mm; w1 is 5 mm, w2 is 20 mm, w3 is 10 mm, w4 is 5 mm, and w5 is 5 mm.

[0130] (2) Preparation of the negative electrode sheet

[0131] The graphite material (artificial graphite, the average particle size d2 of the secondary particles is 10.2 μm), the silicon-carbon material (the number of the primary spherical particles accounts for 0.33 of the total number of the primary spherical particles and the secondary spherical particles, the average particle size d1 of the primary spherical particles is 4.8 μm, the sphericity s of the primary spherical particles is 0.85, and the mass content of the element Si in the silicon-carbon material is 70%), the negative electrode conductive agent (carbon nanotube), the negative electrode dispersing agent (lithium carboxymethyl cellulose), and the negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 88.6:8.4:0.4:0.1:2.5, deionized water is added, and a negative electrode slurry is prepared; the above negative electrode slurry is coated on both sides of the carbon-coated copper foil, and after baking and rolling, a negative electrode sheet with a thickness of 110 μm is obtained; a first recess (groove) is manufactured on the surface of the negative electrode sheet using a laser;

[0132] The mass content of the element Si in the negative electrode active coating is 5.9%, d2 / d1 is 2.13, V OI is 16.9, V OI / s is 19.88, V OI / d1 is 3.52; the width of the groove is 60.6 μm, the depth is 15.5 μm, and the pitch is 0.8 mm.

[0133] (3) Preparation of the electrolyte

[0134] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), vinyl carbonate, propylene carbonate, and diethyl carbonate are mixed in a weight ratio of 1:3:6 to obtain an organic solvent; FEC and lithium salt LiPF6 are dissolved in the above organic solvent to obtain an electrolyte; the mass content c2 of FEC in the electrolyte is 30%, and the mass content of LiPF6 in the electrolyte is 12.5%;

[0135] c1 x c2 is 15.

[0136] (4) Preparation of the battery

[0137] The positive electrode sheet prepared in step (1), a separator (including a polyethylene-based film, a ceramic layer with a thickness of 2 μm on one side surface of the base film, a polymethyl methacrylate adhesive layer on the other side surface of the base film, and a polyvinylidene fluoride + polymethyl methacrylate adhesive layer on the outer surface of the ceramic layer), and the negative electrode sheet prepared in step (2) are wound to obtain a core; the battery is obtained through packaging, baking, liquid injection, formation, two-sealing, sorting, and OCV.

[0138] Example 3

[0139] The battery is prepared according to the following method:

[0140] (1) Preparation of a positive electrode sheet

[0141] Lithium cobaltate, a lithium supplement (Li2NiO2), a positive electrode conductive agent (conductive carbon black), and a positive electrode binder (polyvinylidene fluoride) are mixed in a mass ratio of 96.2:0.8:1:2, N-methyl pyrrolidone (NMP) is added, and stirring is performed until uniformity is achieved, to prepare a positive electrode slurry; the positive electrode slurry is coated on the first surface and the second surface of an aluminum foil (the coating length of the positive electrode slurry on the first surface of the aluminum foil is greater than that on the second surface), baking and rolling are performed, and a positive electrode sheet with a thickness of 100 μm is obtained; a positive electrode tab welding area with a fixed size is arranged on the coating area of the positive electrode sheet (the size of the positive electrode tab welding area in the width direction of the positive electrode sheet is 25 mm), and a nickel tab is laser-welded in the positive electrode tab welding area; the positive electrode sheet is further rolled, and embossing is performed from the first surface to the second surface in the double-coating area (and avoiding the positive electrode tab welding area), to obtain a second recess and a convex part; the shape of the orthographic projection of the second recess and the convex part on the surface of the positive electrode sheet is circular;

[0142] In the formula, the width of the second recess is 3 mm, the depth is 30 μm, and the interval is 3 mm; w1 is 9 mm, w2 is 34 mm, w3 is 20 mm, w4 is 10 mm, and w5 is 10 mm.

[0143] (2) Preparation of a negative electrode sheet

[0144] The graphite material (artificial graphite, the average particle size d2 of the secondary particles is 14.9 μm), the silicon-carbon material (the number of the primary spherical particles accounts for 0.75 of the total number of the primary spherical particles and the secondary spherical particles, the average particle size d1 of the primary spherical particles is 3.1 μm, the sphericity s of the primary spherical particles is 0.8, and the mass content of the element Si in the silicon-carbon material is 70%), the negative electrode conductive agent (carbon nanotube), the negative electrode dispersing agent (lithium carboxymethyl cellulose), and the negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 88.6:8.4:0.4:0.1:2.5, deionized water is added, and a negative electrode slurry is prepared; the above negative electrode slurry is coated on both sides of the carbon-coated copper foil, and after baking and rolling, a negative electrode sheet with a thickness of 110 μm is obtained; a first recess (groove) is manufactured on the surface of the negative electrode sheet using a laser;

[0145] wherein the mass content of the element Si in the negative electrode active coating is 5.9%, d2 / d1 is 4.81, V OI is 12.3, V OI / s is 15.38, V OI / d1 is 3.97; the width of the groove is 99.7 μm, the depth is 30.5 μm, and the pitch is 1.5 mm.

[0146] (3) Preparation of electrolyte

[0147] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), vinyl carbonate, propylene carbonate, and diethyl carbonate are mixed in a weight ratio of 1:3:6 to obtain an organic solvent; FEC and lithium salt LiPF6 are dissolved in the above organic solvent to obtain an electrolyte; wherein the mass content c2 of FEC in the electrolyte is 15%, and the mass content of LiPF6 in the electrolyte is 12.5%;

[0148] wherein c1 x c2 is 12.

[0149] (4) Preparation of battery

[0150] The positive electrode sheet prepared in step (1), the separator (including a polyethylene-based film, a ceramic layer with a thickness of 2 μm on one side of the base film, a polymethyl methacrylate adhesive layer on the other side of the base film, and a polyvinylidene fluoride + polymethyl methacrylate adhesive layer on the outer surface of the ceramic layer), and the negative electrode sheet prepared in step (2) are wound to obtain a roll core; after packaging, baking, liquid injection, formation, two-sealing, sorting, and OCV, a battery is obtained.

[0151] Example 4 group

[0152] This group of examples is used to verify the influence of the change of the average particle size d2 of the secondary particles.

[0153] The examples in this group are implemented with reference to Example 1, except that the average particle size d2 of the secondary particles is changed, as follows:

[0154] Example 4a, d2 is 8.2 μm; wherein d2 / d1 is 1.95, V OI is 29.3, V OI is 30.84, V OI / d1 is 6.98;

[0155] Example 4b, d2 is 19.7 μm; wherein d2 / d1 is 4.69, V OI is 10.5, V OI is 11.05, V OI / d1 is 2.5.

[0156] Example 5 group

[0157] The examples in this group are used to verify the effect of changing the average particle size d1 of the primary spherical particles.

[0158] The examples in this group are implemented with reference to Example 1, except that the average particle size d1 of the primary spherical particles is changed, as follows:

[0159] Example 5a, d1 is 1.1 μm; wherein d2 / d1 is 11.18, V OI / d1 is 14;

[0160] Example 5b, d1 is 5.9 μm; wherein d2 / d1 is 2.08, V OI / d1 is 2.61.

[0161] Example 6 group

[0162] The examples in this group are used to verify the effect of changing the value of the number of primary spherical particles accounting for the total number of primary spherical particles and secondary spherical particles.

[0163] The examples in this group are implemented with reference to Example 1, except that the proportion of the number of primary spherical particles is changed, as follows:

[0164] Example 6a, the number of primary spherical particles accounts for 0.14 of the total number of primary spherical particles and secondary spherical particles;

[0165] Example 6b, the number of primary spherical particles accounts for 0.88 of the total number of primary spherical particles and secondary spherical particles;

[0166] Example 6c, the silicon-carbon material is entirely primary spherical particles, i.e., the number of primary spherical particles accounts for 1 of the total number of primary spherical particles and secondary spherical particles;

[0167] Example 6d, the number of primary spherical particles accounts for 0.05 of the total number of primary spherical particles and secondary spherical particles.

[0168] Example 7 group

[0169] The examples in this group are used to verify the influence brought by the change of the mass content of element Si in the negative active coating.

[0170] The examples in this group are implemented according to Example 1, except that the mass content of element Si in the negative active coating is regulated by changing the mass content of graphite material and silicon-carbon material in the negative slurry, specifically as follows:

[0171] Example 7a, the mass ratio of graphite material, silicon-carbon material, negative conductive agent, negative dispersant and negative binder is 92.1:4.9:0.4:0.1:2.5, wherein the mass content of element Si in the negative active coating is 3.4%;

[0172] Example 7b, the mass ratio of graphite material, silicon-carbon material, negative conductive agent, negative dispersant and negative binder is 80:17:0.4:0.1:2.5, wherein the mass content of element Si in the negative active coating is 11.9%;

[0173] Example 7c, the mass ratio of graphite material, silicon-carbon material, negative conductive agent, negative dispersant and negative binder is 94.2:2.8:0.4:0.1:2.5, wherein the mass content of element Si in the negative active coating is 2%;

[0174] Example 7d, the mass ratio of graphite material, silicon-carbon material, negative conductive agent, negative dispersant and negative binder is 76:21:0.4:0.1:2.5, wherein the mass content of element Si in the negative active coating is 14.7%.

[0175] Example 8

[0176] This example is used to verify the influence brought by the change of the sphericity s of primary spherical particles.

[0177] According to Example 1, except that s is 0.7; wherein, V OI / s is 22.

[0178] Example 9 group

[0179] The examples in this group are used to verify the influence brought by the change of V OI / s and V OI / d1.

[0180] The present group of examples is carried out according to example 2 and example 3 respectively, except that V is regulated by changing the sphericity s and the average particle size dl of the primary spherical particles, in particular as follows: OI s and V OI dl, in particular as follows:

[0181] Example 9a, carried out according to example 2, except that dl is 3.1 pm and s is 0.8; wherein d2 / dl is 3.29, V OI s is 21.13, V OI dl is 5.45;

[0182] Example 9b, carried out according to example 3, except that dl is 4.8 pm and s is 0.95; wherein d2 / dl is 3.10, V OI s is 12.95, V OI dl is 2.56.

[0183] Example 10

[0184] The present group of examples is used to verify the influence of the change of the “type of the first recess”.

[0185] Carried out according to example 1, except that laser is used to manufacture the recess holes on the surface of the negative electrode sheet, wherein the width of the recess hole is 80.5 pm, the depth is 25.2 pm, and the pitch is 500 pm.

[0186] Example 11 group

[0187] The present group of examples is used to verify the influence of the change of the “lithium supplement agent”.

[0188] The present group of examples is carried out according to example 1, except that no lithium supplement agent is added in the positive electrode sheet or different types of lithium supplement agent are replaced, in particular as follows:

[0189] Example 11a, the lithium supplement agent is removed from the positive electrode slurry; that is, the mass ratio of lithium cobaltate, positive electrode conductive agent and positive electrode binder is 97:1:2;

[0190] Example 11b, the lithium supplement agent is replaced by Li5Fe04 of the same mass.

[0191] Example 12 group

[0192] The present group of examples is used to verify the influence of the change of the “mass content cl of the lithium supplement agent in the positive electrode active coating”.

[0193] The present group of examples is carried out according to example 1, except that cl is changed, in particular as follows:

[0194] Example 12a, c1 is 0.3%; wherein the mass ratio of lithium cobalt oxide, lithium supplement agent, positive electrode conductive agent and positive electrode binder is 96.7:0.3:1:2, c1xc2 is 6.9;

[0195] Example 12b, c1 is 1%; wherein the mass ratio of lithium cobalt oxide, lithium supplement agent, positive electrode conductive agent and positive electrode binder is 96:1:1:2, c1xc2 is 23.

[0196] Example 13

[0197] This example is used to verify the influence brought by “whether the second concave part is located on the surface of the positive electrode active coating of the first surface and whether the convex part is located on the surface of the positive electrode active coating of the second surface”.

[0198] Refer to Example 1, except that embossing treatment is performed from the second surface to the first surface, i.e. the surface of the positive electrode active coating located on the second surface has the second concave part and the surface of the positive electrode active coating located on the first surface has the convex part.

[0199] Example 14

[0200] This example is used to verify the influence brought by “whether the single-side coating area has the second concave part”.

[0201] Refer to Example 1, except that embossing treatment is performed on the single-side coating area and the double-side coating area (and avoiding the positive electrode tab welding area).

[0202] Example 15 group

[0203] This group of examples is used to verify the influence brought by the change of “w1, w2, w3, w4 and w5”.

[0204] This group of examples refers to Example 1, except that w1, w2, w3, w4 and w5 are changed, specifically as follows:

[0205] Example 15a, w1 is 0.5mm, w2 is 2mm, w3 is 2mm, w4 is 0.5mm, w5 is 0.5mm;

[0206] Example 15b, w1 is 10mm, w2 is 40mm, w3 is 25mm, w4 is 20mm, w5 is 20mm.

[0207] Example 16 group

[0208] This group of examples is used to verify the influence brought by the change of “c1xc2”.

[0209] This set of embodiments is based on Embodiments 2 and 3, with the difference being that c1×c2 is adjusted by changing the mass content c2 of fluoroethylene carbonate in the electrolyte, as detailed below:

[0210] Example 16a was carried out with reference to Example 2, except that c2 was 15% and c1×c2 was 7.5;

[0211] Example 16b is carried out with reference to Example 3, except that c2 is 30% and c1×c2 is 24.

[0212] Example 17

[0213] This embodiment is used to verify the effect of "not setting a first recess on the surface of the negative electrode active coating".

[0214] The procedure is carried out in accordance with Example 1, except that no grooves are made on the surface of the negative electrode.

[0215] In the above embodiments, the protrusions all satisfy the following conditions: height is 3μm-40μm, width is 0.2mm-8mm, and spacing is 0.5mm-8mm.

[0216] Example 18

[0217] This embodiment is used to verify the effect of "not setting a second concave and convex part on the surface of the positive electrode active coating".

[0218] The procedure was carried out in accordance with Example 1, except that no embossing was performed.

[0219] Comparative Example 1

[0220] The procedure was carried out in accordance with Example 1, except that the artificial graphite was primary particles with an average particle size of 6.1 μm.

[0221] Comparative Example 2

[0222] The procedure was carried out in accordance with Example 1, except that the average particle size d2 of the artificial graphite secondary particles was 22.6 μm.

[0223] Comparative Example 3

[0224] The experiment was conducted in accordance with Example 1, except that the mass ratio of graphite material, silicon carbide material, negative electrode conductive agent, negative electrode dispersant and negative electrode binder was 65.5:31.5:0.4:0.1:2.5, and the mass content of element Si in the negative electrode active coating was 22%.

[0225] Comparative Example 4 Groups

[0226] This comparative group is used to verify the effect of changing the "average particle size d1 of primary spherical particles".

[0227] The present comparative examples were carried out according to Example 1, except that the average particle size dl of the primary spherical particles was changed, as follows:

[0228] Comparative Example 4a, dl was 0.8 μm;

[0229] Comparative Example 4b, dl was 6.5 μm.

[0230] Comparative Example 5 group

[0231] The present comparative examples were used to verify the effect of changing the mass content c2 of fluoroethylene carbonate in the electrolyte.

[0232] The present comparative examples were carried out according to Example 1, except that c2 was changed, as follows:

[0233] Comparative Example 5a, c2 was 0, i.e. no fluoroethylene carbonate was added to the electrolyte;

[0234] Comparative Example 5b, c2 was 40%.

[0235] Test Examples

[0236] (1) 45°C high-temperature cycle test

[0237] The batteries prepared in the examples and comparative examples were subjected to a 45°C high-temperature cycle test, according to the following test method:

[0238] The batteries were left to stand in a 45°C constant-temperature room for 2 h, then charged at 3C constant current to 4.2 V, and then charged at 2C constant current and constant voltage to 4.48 V, with a cut-off at 0.05C, and left to stand for 10 min; then discharged at 0.7C to 3.0 V, and this cycle was repeated 500 times. The thickness of the battery at the end of the 500th cycle was h1, and the discharge capacity was C1. The thickness of the battery at the end of the first cycle was h0, and the discharge capacity was C0. The capacity retention rate after 500 cycles was C1 / C0, and the thickness expansion rate after 500 cycles was (h1-h0) / h0. The results are shown in Table 1.

[0239] (2) Volume energy density test

[0240] The batteries prepared in the examples and comparative examples were subjected to a volume energy density test, according to the following test method:

[0241] The batteries were charged to the upper limit voltage (4.48 V) at a current of 0.2C, and then charged at constant voltage until the current dropped to 0.02C, and then discharged at a current of 0.2C to 3.0 V. The energy of the discharge was recorded as E. The thickness, width and length of the battery were measured, and the product of the three was calculated to obtain the volume of the battery, recorded as V. The formula for calculating the volume energy density was VED = E / V. The results are shown in Table 1.

[0242] Table 1

[0243]

[0244]

[0245]

[0246] As can be seen from Table 1, the battery of the present application can balance high energy density and excellent cycle stability compared with the comparative examples. The energy density of the battery of Comparative Example 1 is low and the thickness expansion rate is high compared with Example 1 because the graphite material uses primary particles. The high temperature performance of the battery is poor in Comparative Example 5b because the content of fluoroethylene carbonate is high.

[0247] The above describes the preferred embodiments of the present application in detail, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as the disclosed content of the present application and belong to the protection scope of the present application.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The lithium ion secondary battery comprises a negative electrode sheet and an electrolyte. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating layer located on at least one side surface of the negative electrode current collector, wherein the negative electrode active coating layer comprises a negative electrode active material; the negative electrode active material comprises a graphite material and a silicon-carbon material; the graphite material comprises secondary particles; and the silicon-carbon material comprises primary spherical particles and secondary spherical particles formed by a plurality of the primary spherical particles. The electrolyte comprises fluoroethylene carbonate, and the mass content of fluoroethylene carbonate in the electrolyte is 8-30%, denoted as c2. The mass content of element Si in the negative electrode active coating layer is 1.5%-15%. The average particle size of the secondary particles is d2, and d2 is 6-20 μm. The average particle size of the primary spherical particles is d1, and d1 is 1-6 μm.

2. The lithium-ion secondary battery according to claim 1, wherein In the negative electrode active coating layer, the number of the primary spherical particles accounts for 0.1-0.9% of the total number of the primary spherical particles and the secondary spherical particles. The mass content of element Si in the negative electrode active coating layer is 3.5%-12%. The silicon-carbon material comprises a material formed by filling silicon and / or oxidized silicon in the pores of porous carbon. The mass content of element Si in the silicon-carbon material is 30%-80%.

3. The lithium-ion secondary battery according to claim 2, wherein In the negative electrode active coating layer, the number of the primary spherical particles accounts for 0.3-0.8% of the total number of the primary spherical particles and the secondary spherical particles.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein 1.33≤d2 / d1≤20. d2 is 10-15 μm. d1 is 3-5 μm.

5. The lithium-ion secondary battery according to claim 4, wherein 2≤d2 / d1≤5.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein The OI value V of the negative electrode active coating OI The sphericity s of the primary spherical particles and the average particle diameter dl of the primary spherical particles, in units of μm, satisfy: 10≤V OI / s≤35, and 2≤V OI / d1≤15.

7. The lithium-ion secondary battery according to claim 6, wherein 12≤V OI / s≤22, and 2.5≤V OI / d1≤5.

5.

8. The lithium-ion secondary battery according to claim 7, wherein 15≤V OI / s≤20, and 3.5≤V OI / d1≤4.

9. The lithium-ion secondary battery according to claim 6, wherein the OI value V of the negative electrode active coating OI is 10-30; The sphericity of the primary spherical particles is 0.7-1.

10. The lithium-ion secondary battery according to claim 9, wherein The OI value V of the negative electrode active coating OI is 12-17.

11. The lithium-ion secondary battery according to claim 1 or 2, wherein The surface of the negative electrode active coating layer located on at least one side surface of the negative electrode current collector has a first recess.

12. The lithium-ion secondary battery according to claim 11, wherein The depth of the first recess is 5-40 μm. The width of the first recess is 40-200 μm. The interval of the first recess is 0.5-5 mm.

13. The lithium-ion secondary battery according to claim 1 or 2, wherein The lithium ion secondary battery further comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active coating layer located on at least one side surface of the positive electrode current collector. The positive active coating includes a lithium supplement agent, the lithium supplement agent includes a substance with a chemical formula of Li5+xFe y M 1 z O4 and / or a substance with a chemical formula of Li2±aNibM 2 cO2, wherein -0.5≤x≤5, 0.8≤y≤1.2, 0≤z≤1, M 1 includes at least one of Mo, Nb, Ti, Zr, Ni, Y, Mn, Cu, Mg and Zn; 0≤a≤0.5, 0≤b≤1, 0≤c≤1; M 2 includes at least one of Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg and Zn.

14. The lithium-ion secondary battery according to claim 13, wherein The mass content of the lithium supplement agent in the positive electrode active coating layer is c1, and c1 is 0.3-1%.

15. The lithium-ion secondary battery according to claim 13, wherein The lithium supplement agent comprises Li2NiO2 and / or Li5FeO4.

16. The lithium-ion secondary battery according to claim 14, wherein The mass content of fluoroethylene carbonate in the electrolyte is c2, and c2 is %, and the mass content of the lithium supplement agent in the positive electrode active coating layer is c1, and c1 is %, and c1×c2≤30.

17. The lithium-ion secondary battery according to claim 16, wherein 7≤c1×c2≤24.

18. The lithium-ion secondary battery according to claim 17, wherein 12≤c1×c2≤15.

19. The lithium-ion secondary battery according to claim 1 or 2, wherein The lithium ion secondary battery further comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active coating layer located on at least one side surface of the positive electrode current collector. The length of the positive electrode active coating layer located on the first surface of the positive electrode current collector is greater than the length of the positive electrode active coating layer located on the second surface of the positive electrode current collector. An area where projections of the positive active coating on the first surface and the positive active coating on the second surface in the thickness direction of the positive plate overlap is a double-sided coating area, and an area where the projections do not overlap is a single-sided coating area; A surface of the positive active coating on the first surface has a second recess, and a surface of the positive active coating on the second surface has a protrusion.

20. The lithium-ion secondary battery according to claim 19, wherein, The positive plate comprises a positive tab welding area, a coating area, and a hollow foil area, and the second recess and the protrusion are located in the coating area.

21. The lithium-ion secondary battery according to claim 20, wherein The positive plate comprises a coating area, the coating area comprising the double-sided coating area and the single-sided coating area, and the second recess and the protrusion are located in the double-sided coating area.

22. The lithium-ion secondary battery according to claim 19, wherein, The depth of the second recess is 3 μm-40 μm, the width of the second recess is 0.2 mm-8 mm, and the interval of the second recess is 0.5 mm-8 mm; And / or, the height of the protrusion is 3 μm-40 μm, the width of the protrusion is 0.2 mm-8 mm, and the interval of the protrusion is 0.5 mm-8 mm.

23. The lithium-ion secondary battery according to claim 22, wherein, The positive plate comprises a positive tab welding area, a coating area, and a hollow foil area, and the coating area comprises the double-sided coating area and the single-sided coating area; The distance from the second recess to the edge of the positive tab welding area is w1, 0 mm < w1 ≤ 10 mm; And / or, the distance from the second recess to the edge of the first side of the coating area is w2, 2 mm ≤ w2 ≤ 40 mm, the first side being the side where the positive tab welding area is arranged; And / or, the distance from the second recess to the edge of the second side of the coating area is w3, 2 mm ≤ w3 ≤ 25 mm, the second side being the side opposite to the side where the positive tab welding area is arranged; And / or, the distance from the second recess to the edge of the third side of the coating area is w4, 0 mm < w4 ≤ 20 mm, the third side being the side of the coating area close to the winding starting end; And / or, the distance from the second recess to the junction line of the double-sided coating area and the single-sided coating area is w5, 0 mm < w5 ≤ 20 mm.

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