Negative electrode sheet and lithium ion secondary battery

Abstract

The present application relates to the technical field of battery, in particular to a negative electrode sheet and a lithium ion secondary battery. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector; the negative electrode active material layer comprises a first coating layer close to the negative electrode current collector and a second coating layer away from the negative electrode current collector; the thickness of the negative electrode sheet is 30-1000 μm; the first coating layer comprises a first negative electrode active material, and the second coating layer comprises a second negative electrode active material, wherein the second negative electrode active material comprises a second graphite material and hard carbon; the ratio of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material is L, and 1 L≤2.5; the outer surface of the negative electrode active material layer has a concave part. The negative electrode sheet has excellent lithium ion conductivity and capacity release, and the lithium ion secondary battery comprising the negative electrode sheet has high energy density and fast charging capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a negative electrode sheet and a lithium ion secondary battery comprising the same. BACKGROUND

[0002] Lithium ion battery (LIB) has great potential in electronic devices and electric vehicles. Current technology is committed to improving the energy density and fast charging capability of the battery.

[0003] For thick electrodes, the energy density of the battery can be improved to some extent, but it will make it difficult for electrolyte to soak, cause the lithium ion transport path to grow, the lithium ion concentration gradient to be too large, and the active material at the bottom layer of the electrode to be unable to fully play, etc., thereby leading to poor fast charging capability of the battery. The multi-layer composite electrode structure is a simple and efficient electrode structure design idea. The double-layer coating design in the prior art generally uses fast-charging graphite in the surface layer and high-capacity graphite (the particle size of high-capacity graphite is larger than that of fast-charging graphite) in the bottom layer. However, there is still a problem that the energy density and fast charging capability cannot be balanced in the actual use process.

[0004] Therefore, it is very important to simultaneously improve the energy density and fast charging capability of the battery. SUMMARY

[0005] The present application aims to overcome the above-mentioned problems in the prior art, and provides a negative electrode sheet and a lithium ion secondary battery comprising the same. The negative electrode sheet of the present application has a special structure design, which combines the manufacturing recess technology with the multi-layer coating technology, so that the negative electrode sheet has excellent lithium ion conduction capability and capacity development. The lithium ion secondary battery (hereinafter referred to as battery) comprising the negative electrode sheet of the present application has high energy density and fast charging capability.

[0006] The first aspect of the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector; the negative electrode active material layer comprises a first coating layer and a second coating layer stacked in the thickness direction of the negative electrode sheet, the first coating layer is close to the negative electrode current collector, and the second coating layer is away from the negative electrode current collector; the thickness of the negative electrode sheet is 30-1000 μm; the first coating layer comprises a first negative electrode active material, and the second coating layer comprises a second negative electrode active material; the second negative electrode active material comprises a second graphite material and hard carbon; the ratio of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material is L, and 1

[0007] In the related art, the negative electrode sheet uses fast-charging graphite for the surface layer and high-capacity graphite for the bottom layer, which cannot effectively balance high energy density and fast charging capability. The inventor of the present application has found that the cause of the problem is as follows: first, the negative electrode active material located in the surface layer is subjected to a large stress during rolling, resulting in a relatively low porosity of the surface layer, which makes the wettability of the electrolyte to the bottom layer of the negative electrode active material poor, thereby resulting in a low overall capacity of the battery and poor fast charging capability. Second, the particle size of the fast-charging graphite is smaller than that of the high-capacity graphite, and the limit compaction of the graphite particles with smaller particle size is also smaller; and the stress on the graphite particles located in the surface layer is greater than that on the graphite particles located in the bottom layer during the rolling process. Therefore, in order to minimize the damage to the structure of the graphite particles in the surface layer, the compaction density of the coating layer located in the bottom layer is lower, which affects the energy density of the battery.

[0008] Based on the above reasons, the inventor of the present application has conducted a large number of targeted experiments. It is found that: the negative electrode active material layer is divided into multiple layers, the average particle size of the active material located in the relative surface layer is greater than that of the active material located in the relative bottom layer, and the ratio of the average particle sizes is limited within a certain range. Through such setting, the problem of incomplete capacity exertion caused by uneven stress on the surface layer and the bottom layer during the rolling process can be effectively improved. In addition, hard carbon is a carbon material that is difficult to graphitize, and its internal crystal arrangement is disordered, with a large interlayer spacing (hard carbon interlayer spacing ≥ 0.34 nm). Therefore, it is more conducive to the insertion and extraction of lithium ions. It is also because of its special structure that the first coulomb efficiency and capacity of hard carbon are low. When the second negative electrode active material includes hard carbon, the advantages of hard carbon for rapid extraction of lithium ions can be fully utilized.

[0009] Through the above settings, the battery can to some extent balance the high energy density and fast charging performance. Further, a recess is provided on the outer surface of the negative electrode active material layer. The structure of the recess can increase the liquid storage space, improve the wettability of the electrolyte to the negative electrode sheet (shorten the lithium insertion channel, reduce the degree of detour, and make the lithium ions quickly pass through the surface layer to the inside of the negative electrode sheet), shorten the aging time, solve the problem of low porosity of the surface layer and insufficient capacity exertion of the active material in the bottom layer of the existing double-layer coating; at the same time, it can avoid the problem of lithium precipitation of the surface layer active material. The negative electrode sheet is coated in multiple layers, hard carbon material is used in the coating layer located in the relative surface layer, and a recess is provided on the outer surface of the negative electrode active material layer. The combination of the three can improve the lithium insertion capacity of the active material with large particle size in the surface layer and the active material with small particle size in the bottom layer, thereby improving the capacity exertion; and can compensate for the adverse effects of the disadvantages of hard carbon on the battery.

[0010] The second aspect of the present application provides a lithium ion secondary battery comprising the negative electrode sheet of the first aspect of the present application.

[0011] Compared with the prior art, the present application has at least the following advantages:

[0012] (1) The negative electrode sheet of the present application has excellent lithium ion conductivity and capacity release.

[0013] (2) The battery comprising the negative electrode sheet of the present application has high energy density, excellent rate performance and cycle stability.

[0014] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are not critical to the present application. The endpoints of the ranges and the values are provided as approximations of the values. The endpoints of the ranges and the values are not to be construed as being limited to the precise values recited as the exact dimensions are not critical to the present application. Any numerical range recited herein is intended to include all sub-ranges subsumed therein. For a numerical range having endpoints, the endpoints are included in the range. Any reference to a numerical value is intended to include values approximately close to the value. For a numerical range having endpoints, the endpoints are included in the range. Any reference to a numerical value is intended to include values approximately close to the value. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The distribution of the first coating and the second coating in an example of the present application is shown.

[0016] Figure 2 The scanning electron microscope (SEM) image of the surface of the negative electrode sheet in an example of the present application is shown.

[0017] Figure 3 The SEM image of the surface of the negative electrode sheet in an example of the present application is shown.

[0018] Figure 4 The width of the groove in an example of the present application is shown; wherein, Figure 4 (a)- Figure 4 The two long sides of the groove in (c) are straight lines, Figure 4 The two long sides of the groove in (d) are curved lines.

[0019] Figure 5 The distance between the grooves in an example of the present application is shown; wherein, Figure 5 (a) is the case where the two adjacent long sides are straight lines and parallel, Figure 5 (b) is the case where the two adjacent long sides are straight lines and not parallel, Figure 5 (c) is the case where the two adjacent long sides are curved lines.

[0020] Figure 6 The cross-sectional view of the recess in an example of the present application is shown; wherein, Figure 6 The recesses in (a)-6(f) have different depths. DETAILED DESCRIPTION

[0021] The specific embodiments of the present application are described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and explanatory in nature and are not intended to limit the application.

[0022] The first aspect of the present application provides a negative electrode sheet, which can include a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector. The negative electrode active material layer can include a first coating layer and a second coating layer stacked in the thickness direction of the negative electrode sheet. The first coating layer is close to the negative electrode current collector, and the second coating layer is away from the negative electrode current collector. Figure 1 As shown in the distribution diagram of the first coating layer and the second coating layer in an example of the present application, the negative electrode sheet includes a negative electrode current collector 1 and a negative electrode active material layer 2 located on one side surface of the negative electrode current collector 1. The negative electrode active material layer 2 includes a first coating layer 21 and a second coating layer 22 stacked in the thickness direction of the negative electrode sheet. The first coating layer 21 is close to the negative electrode current collector 1, and the second coating layer 22 is away from the negative electrode current collector 1. It should be noted that, Figure 1 Only the case of arranging the negative electrode active material layer on one side is given, and the negative electrode active material layer can also be located on both side surfaces of the negative electrode current collector.

[0023] In the present application, the thickness of the negative electrode sheet can be 30 μm-1000 μm, for example, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm or 1000 μm.

[0024] In an example, the thickness of the negative electrode sheet is 70 μm-210 μm.

[0025] In the present application, the first coating layer can include a first negative electrode active material, and the second coating layer can include a second negative electrode active material. The second negative electrode active material can include a second graphite material and hard carbon. The ratio of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material is L, 1

[0026] In an example, 1.05≤L≤1.4.

[0027] In the present application, the outer surface of the negative electrode active material layer can have a recess.

[0028] The inventors of the present application found that if the relationship between the ratio L of the average particle size of the second negative active material to the average particle size of the first negative active material and the depth of the recess is further regulated, the ability of the battery to balance high energy density and fast charging performance can be further improved, and the reason is that the average particle size of the second negative active material located in the relative surface layer is larger than the average particle size of the second negative active material located in the relative bottom layer, and the ratio of the two can reflect the difference in particle size. Within a certain range, the larger the ratio of the two, the greater the difference in particle size, the smaller the overall compaction density of the negative plate, the more conducive to the fast charging performance of the battery, but the thicker the battery, the lower the volume energy density; At this time, the depth of the recess does not need to be too deep. Because the greater the depth of the recess, the more active material mass is lost, which is not conducive to improving the energy density of the battery. The smaller the ratio of the two, the smaller the difference in particle size, the greater the overall compaction density of the negative plate, the more conducive to improving the energy density of the battery, but the fast charging performance will deteriorate accordingly. At this time, a deeper recess depth is needed to improve the infiltration of the electrolyte to the bottom negative plate. Therefore, by further regulating the relationship between L and the depth of the recess, it can fully exert the performance of the bottom and surface negative active materials within a certain range, improve the capacity and fast charging ability of the battery, and make the battery have high energy density and excellent rate performance.

[0029] In the present application, the depth of the recess is h, and the unit is μm. L and h can satisfy: 6≤L×h≤300, L×h is for example 6, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 200 or 300.

[0030] In an example, 14≤L×h≤100.

[0031] In an example, 16≤L×h≤86.

[0032] In the present application, the average particle size of the first negative active material can be 5-15 μm, for example 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm. The average particle size of the second negative active material can be 9-24 μm, for example 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm or 24 μm.

[0033] In the present application, the average particle size of the first negative electrode active material and the average particle size of the second negative electrode active material can be obtained by a method conventional in the art, for example, using an argon ion grinder, cutting along the thickness direction of the negative electrode sheet, and then using a scanning electron microscope (SEM) to obtain a cross-sectional micrograph of the negative electrode sheet along the thickness direction, randomly selecting at least 20 first negative electrode active material particles and 20 second negative electrode active material particles in the field of view of the micrograph, respectively measuring the particle size, and taking the average value, i.e. the average particle size of the first negative electrode active material and the average particle size of the second negative electrode active material.

[0034] In the present application, the porosity of the second coating layer is greater than the porosity of the first coating layer.

[0035] In the present application, the ratio of the porosity of the first coating layer to the porosity of the second coating layer can be 0.5-0.98, for example, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95 or 0.98.

[0036] In an example, the ratio of the porosity of the first coating layer to the porosity of the second coating layer is 0.7-0.9.

[0037] The porosity of the negative electrode active material layer can reflect the wettability of the electrolyte to the whole negative electrode sheet to a certain extent. The porosity of the second coating layer being greater than the porosity of the first coating layer can cause a certain change gradient of the porosity in the negative electrode sheet, thereby facilitating the wettability of the electrolyte. And when the ratio of the two is within a certain range, the change gradient of the porosity formed can form a driving force to promote the wettability of the electrolyte to the first coating layer located at the relatively bottom layer, thereby improving the capacity performance and fast charging performance of the whole battery.

[0038] In the present application, the porosity of the first coating layer can be 10%-40%, for example, 10%, 15%, 20%, 25%, 30%, 35% or 40%. The porosity of the second coating layer can be 20%-50%, for example, 20%, 25%, 30%, 35%, 40%, 45% or 50%.

[0039] In an example, the porosity of the first coating layer is 20%-25%. The porosity of the second coating layer is 25%-35%.

[0040] In the present application, the porosity of the first coating layer and the porosity of the second coating layer can be obtained by a method conventional in the art, for example, using laser scanning technology, scanning the cross section of the first coating layer and the second coating layer along the thickness direction by a laser beam, obtaining the cross-sectional topography information of the first coating layer and the second coating layer, reconstructing the pore structure inside the coating layer by reverse engineering method, and calculating the porosity.

[0041] In the present application, the first negative active material can include a first graphite material.

[0042] In the present application, the OI value of the second graphite material is greater than the OI value of the first graphite material.

[0043] The OI value of the graphite material is used to describe the degree of order of the interlayer arrangement, which can reflect the orientation of the crystal structure of the graphite material and has a greater impact on the electrochemical performance of the negative active material. When the OI value of the second graphite material is greater than the OI value of the first graphite material, it indicates that the capacity of the negative active material located in the relative surface layer is higher, which is beneficial to the capacity. The OI value of the first graphite material is smaller, which indicates that the negative active material located in the relative bottom layer is more conducive to the diffusion of lithium ions, thereby helping to maintain a high energy density while improving the rate performance of the battery.

[0044] In the present application, the OI value of the first graphite material can be 9-24, such as 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 or 24. The OI value of the second graphite material can be 12-26, such as 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 or 26.

[0045] In the present application, the OI value of the first graphite material and the OI value of the second graphite material can be obtained by conventional methods in the art, such as using an X-ray diffractometer (XRD).

[0046] In the present application, the specific capacity of the second graphite material is greater than the specific capacity of the first graphite material. When the specific capacity of the second graphite material is greater than the specific capacity of the first graphite material, the energy density of the battery can be improved without affecting the fast charging performance of the battery.

[0047] In the present application, the specific capacity of the first graphite material can be 320mAh / g-360mAh / g, such as 320mAh / g, 330mAh / g, 340mAh / g, 350mAh / g or 360mAh / g. The specific capacity of the second graphite material can be 340mAh / g-370mAh / g, such as 340mAh / g, 350mAh / g, 360mAh / g or 370mAh / g.

[0048] In the present application, the specific capacity of the first graphite material and the specific capacity of the second graphite material can be tested by a method conventional in the art, for example, assembling a half-cell by using the first graphite material or the second graphite material as a working electrode, respectively, using a lithium sheet as a reference electrode, and testing the above-mentioned half-cell curve by using an electrochemical workstation, i.e. the standard potential curve of the first graphite material or the second graphite material relative to the lithium sheet, specific capacity = electric capacity (unit mAh) / mass (unit g) of the first graphite material (or the second graphite material).

[0049] In the present application, the specific surface area of the first negative electrode active material is greater than the specific surface area of the second negative electrode active material. The specific surface area of the first negative electrode active material can be 1.2 m 2 / g-4 m 2 / g, for example, 1.2 m 2 / g, 1.5 m 2 / g, 2 m 2 / g, 2.5 m 2 / g, 3 m 2 / g, 3.5 m 2 / g, or 4 m 2 / g. The specific surface area of the second negative electrode active material can be 1 m 2 / g-3.5 m 2 / g, for example, 1 m 2 / g, 1.5 m 2 / g, 2 m 2 / g, 2.5 m 2 / g, 3 m 2 / g, or 3.5 m 2 / g.

[0050] In the present application, the specific surface area of the first negative electrode active material and the specific surface area of the second negative electrode active material can be tested by a method conventional in the art, for example, the BET (Brunauer-Emmett-Teller) method.

[0051] In the present application, the tap density of the second coating layer is greater than the tap density of the first coating layer. The tap density of the first coating layer can be 0.6 g / cm 3 -0.85 g / cm 3 , for example, 0.6 g / cm 3 , 0.65 g / cm 3 , 0.7 g / cm 3 , 0.75 g / cm 3 , 0.8 g / cm 3 , or 0.85 g / cm 3 . The tap density of the second coating layer can be 0.7 g / cm 3- 1 g / cm 3 , for example 0.7 g / cm 3 , 0.75 g / cm 3 , 0.8 g / cm 3 , 0.85 g / cm 3 , 0.9 g / cm 3 , 0.95 g / cm 3 or 1 g / cm 3 .

[0052] In the present application, the tap density of the first coating and the tap density of the second coating can be tested by methods conventional in the art, for example using a tap density meter.

[0053] In the present application, the tap density of the first coating and the tap density of the second coating can be tested by methods conventional in the art, for example using a tap density meter. 3 - 1.8 g / cm 3 , for example 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 or 1.8 g / cm 3 . The tap density of the second coating can be 1.5 g / cm 3 - 2 g / cm 3 , for example 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 , 1.8 g / cm 3 , 1.9 g / cm 3 or 2 g / cm 3 .

[0054] In the present application, the thickness of the first coating can be 10 pm - 80 pm, for example 10 pm, 15 pm, 20 pm, 30 pm, 40 pm, 50 pm, 60 pm, 70 pm or 80 pm. The thickness of the second coating can be 20 pm - 150 pm, for example 20 pm, 30 pm, 40 pm, 50 pm, 60 pm, 70 pm, 80 pm, 90 pm, 100 pm, 110 pm, 120 pm, 130 pm, 140 pm or 150 pm.

[0055] In the present application, the thickness of the first coating and the thickness of the second coating refer to the thickness of the first coating and the thickness of the second coating on one side of the anode current collector.

[0056] In the present application, the areal density of the second coating is greater than or equal to the areal density of the first coating. The areal density of the first coating is 2 mg / cm 2 - 15 mg / cm 2 , for example 2 mg / cm 2 , 3 mg / cm 2 , 4 mg / cm 2 , 5 mg / cm 2 , 6 mg / cm 2 , 7 mg / cm 2 , 8 mg / cm 2 , 9 mg / cm 2 , 10 mg / cm 2 , 11 mg / cm 2 , 12 mg / cm 2 , 13 mg / cm 2 , 14 mg / cm 2 , or 15 mg / cm 2 . The areal density of the second coating is 4 mg / cm 2 - 20 mg / cm 2 , for example 4 mg / cm 2 , 5 mg / cm 2 , 6 mg / cm 2 , 7 mg / cm 2 , 8 mg / cm 2 , 9 mg / cm 2 , 10 mg / cm 2 , 11 mg / cm 2 , 12 mg / cm 2 , 13 mg / cm 2 , 14 mg / cm 2 , 15 mg / cm 2 , 16 mg / cm 2 , 17 mg / cm 2 , 18 mg / cm 2 , 19 mg / cm 2 , or 20 mg / cm 2 .

[0057] In the present application, the recesses can include holes and / or grooves. When the recesses are holes, the number of the recesses (i.e., the holes) is greater than or equal to 2. When the recesses are grooves, the number of the recesses (i.e., the grooves) is greater than or equal to 1.

[0058] In an example, the recesses include holes. The shape of the holes in the orthographic projection of the surface of the negative electrode tab can be a regular figure or an irregular figure. As Figure 2SEM image of the surface of the negative electrode sheet in one example of the present application is shown. As can be seen from the image, the surface of the negative electrode sheet has a plurality of holes (i.e. the outer surface of the negative electrode active material layer has a plurality of holes).

[0059] In the present application, the hole diameter of the holes can be 40 μm to 200 μm, for example 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm or 200 μm.

[0060] In the present application, the hole diameter of the holes has the conventional meaning in the art. When the hole is a "regular circle" in the shape of the orthographic projection of the surface of the negative electrode sheet, the hole diameter is the diameter of the regular circle; when the hole is a non- "regular circle" (for example an ellipse or an irregular curved polygon) in the shape of the orthographic projection of the surface of the negative electrode sheet, the hole diameter is the diameter of the equivalent circle having an area equal to that of the non- "regular circle". The "hole diameter" can be tested by conventional means in the art, for example by SEM, selecting all or at least 10 holes in the field of view of the electron microscope, measuring the hole diameter and taking the average value.

[0061] In the present application, the hole spacing of the holes can be 50 μm to 1 mm, for example 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm or 1 mm.

[0062] In one example, the hole spacing of the holes is 100 μm to 300 μm.

[0063] In the present application, the hole spacing of the holes has the conventional meaning in the art, referring to the shortest distance between the edges of two adjacent holes on the surface of the negative electrode sheet. The "hole spacing" can be tested by conventional means in the art, for example by SEM, selecting all or at least 10 groups of adjacent holes in the field of view of the electron microscope, measuring the hole spacing and taking the average value.

[0064] In one example, the recess comprises a groove. The projection of the groove on the surface of the negative electrode sheet can be a regular figure or a non-regular figure. As Figure 3 SEM image of the surface of the negative electrode sheet in one example of the present application is shown. As can be seen from the image, the surface of the negative electrode sheet has a plurality of grooves (i.e. the outer surface of the negative electrode active material layer has a plurality of grooves).

[0065] In the present application, the width of the groove can be 20 μm-150 μm, for example, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm.

[0066] In an example, the width of the groove is 60 μm-120 μm.

[0067] In the present application, the width of the groove has the conventional meaning in the art. The projection of the groove on the surface of the negative electrode sheet comprises two long sides, and the width of the groove refers to the average distance from one long side to the other long side 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 width of the groove in an example of the present application; wherein, Figure 4 (a)- Figure 4 (c) the two long sides of the groove are straight lines, Figure 4 (d) the two long sides of the groove are curves. In Figure 4 (a) and Figure 4 (b), the two long sides are arranged in parallel, thus, in the width direction of the negative electrode sheet, the perpendicular distance from any point on one long side to the other long side is equal, at this time, the width of the groove is the perpendicular distance d from any point on one long side to the other long side in the length direction or the width direction of the negative electrode sheet. In Figure 4 (c), the two long sides 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 side to the other long side is not equal, at this time, the width of the groove can be taken as the average value, i.e. on one long side, 50 point positions are selected at equal distances (i.e. the distance between each point position is equal, so that the selected point positions can make the calculation result more accurate) based on the length of the long side, the width d corresponding to each point position is measured, and the average value is taken to obtain the width of the groove. In Figure 4 (d), the two long sides are curves, thus, in the width direction, the size from any point on one long side to the other long side is not equal, at this time, the width of the groove can also be taken as the average value, i.e. on one long side, 50 point positions are selected at random (since Figure 4 (d), the two long sides are curves, there is no Figure 4 (c), the relationship between the two long sides, thus, 50 point positions can be measured at random), the width d corresponding to each point position is measured, and the average value is taken to obtain the width of the groove. The "width of the groove" can be tested by conventional means in the art, for example, by SEM, all or at least 5 grooves are selected in the field of view of the electron microscope, the width is measured, and the average value is taken.

[0068] In the present application, the pitch of the groove can be 200 μm-3 mm, for example, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1 mm, 2 mm or 3 mm.

[0069] In an example, the pitch of the groove is 0.8 mm-2 mm.

[0070] 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.

[0071] In the present application, the pitch of the groove has the conventional meaning in the art, which refers 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 5 Fig. 1 shows a schematic diagram of the pitch of the groove in an example of the present application; wherein, Figure 5 (a) is the case where the two adjacent long edges are straight lines and parallel, Figure 5 (b) is the case where the two adjacent long edges are straight lines but not parallel, Figure 5 (c) is the case where the two adjacent long edges are curves. In Figure 5 In (a), the two adjacent long edges are straight lines and are arranged in parallel, thus, 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 D from any point on one long edge to the other long edge in the width direction. Figure 5 In (b), the two adjacent long edges are straight lines but are not arranged in parallel, thus, 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, the width D corresponding to each point is measured, and the average value is taken as the pitch. Figure 5 In (c), the two adjacent long edges are curves, thus, 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 5 In (c), the two long edges are curves, and there is no Figure 5 relationship between the two long edges in (b), thus, 50 points can be randomly selected for measurement), the width D corresponding to each point is measured, and the average value is taken as the pitch. The "pitch of the groove" can be tested by conventional means in the art, for example, by SEM, selecting all or at least 5 groups of adjacent grooves in the field of view of the electron microscope, measuring the pitch, and taking the average value.

[0072] By regulating the size of the recess, the wettability of the electrolyte to the negative plate can be further improved, which is beneficial to the capacity and the charge-discharge efficiency.

[0073] In the present application, the depth h of the recess can be 5-250 μm, for example, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, 200 μm, 210 μm, 220 μm, 230 μm, 240 μm or 250 μm.

[0074] In one example, the depth h of the recess is 10-70 μm.

[0075] In one example, the depth h of the recess is 10-25 μm.

[0076] In one example, the depth h of the recess is 20-45 μm.

[0077] In one example, the depth h of the recess is 40-75 μm.

[0078] It can be understood that for the wound battery, the negative plate has a single-sided area and a double-sided area (the single-sided area refers to an area in which the negative current collector has a negative active material layer only on one side surface; the double-sided area refers to an area in which the negative current collector has a negative active material layer on both side surfaces).

[0079] In the present application, the depth h of the recess refers to the maximum value of the vertical distance from any point in the recess (located in the double-sided area) to the outer surface of the negative plate. The depth h of the recess can be obtained by testing by means of the conventional method in the art, for example, by a 3D profilometer or SEM, selecting all the recesses or at least 10 recesses, measuring the depth of each recess, and taking the average value.

[0080] In the present application, as shown in Figure 6 FIG. 1 shows a cross-sectional schematic view of the recess in one example of the present application; wherein, Figure 6 The recesses in (a)-(f) have different depths. As can be seen from the figures, the outer surface of the negative active material layer 2 has a recess 3. In Figure 6 In (a), the recess 3 is located in the second coating layer 22 (i.e., the recess 3 does not penetrate through the second coating layer 22); in Figure 6 In (b), the recess 3 is located in the first coating layer 21 and the second coating layer 22 (i.e., the recess 3 penetrates through the second coating layer 22, but does not penetrate through the first coating layer 21); in Figure 6In (c), the recess 3 penetrates the first coating layer 21 and the second coating layer 22 on the side surface of the negative current collector 1 and passes through part of the negative current collector 1; in Figure 6 In (d), the recess 3 penetrates the negative current collector 1 from the side of the negative electrode tab and passes through part of the first coating layer 21 on the other side; in Figure 6 In (e), the recess 3 penetrates the negative current collector 1 from the side of the negative electrode tab and penetrates the first coating layer 21 on the other side and passes through part of the second coating layer 22; in Figure 6 In (f), the recess 3 penetrates the negative electrode tab.

[0081] In the present application, the shape of the cross section of the recess in the thickness direction of the negative electrode tab is not limited and can be rectangular or conical.

[0082] In the present application, the negative current collector can have an area (i.e., the single-sided area described above) where only one side surface has a negative active material layer. The ratio of the depth of the recess on the outer surface of the negative active material layer in the area to the thickness of the negative active material layer can be 0.33-0.5, for example, 0.33, 0.35, 0.4, 0.45, or 0.5.

[0083] The area where only one side surface of the negative current collector has a negative active material layer is the single-sided area. When the ratio of the depth of the recess on the outer surface of the negative active material layer in the single-sided area to the thickness of the negative active material layer is within a certain range, the structural stability of the negative electrode tab is higher and the adhesion between the negative active material layer and the negative current collector is stronger.

[0084] In an example, the negative electrode tab has the recess on both side surfaces. The positions of the recesses on the two sides of the negative electrode tab can be staggered or coincided with each other.

[0085] In the present application, the first graphite material and the second graphite material can each independently include artificial graphite and / or natural graphite.

[0086] In the present application, the first negative active material can further include at least one of hard carbon, soft carbon, mesocarbon microbeads, soft carbon-coated graphite material, and hard carbon-coated graphite material. The second negative active material can further include at least one of soft carbon, mesocarbon microbeads, soft carbon-coated graphite material, and hard carbon-coated graphite material.

[0087] In one example, the mass content of hard carbon in the second negative electrode active material is greater than the mass content of hard carbon in the first negative electrode active material. As previously described, hard carbon is beneficial to the intercalation and deintercalation of lithium ions, but has a low first coulombic efficiency and capacity. By regulating the mass content of hard carbon in the second negative electrode active material to be greater than the mass content of hard carbon in the first negative electrode active material, the advantages of hard carbon in facilitating the intercalation and deintercalation of lithium ions can be further fully realized, and the deficiencies of hard carbon in first coulombic efficiency and capacity can be made up for.

[0088] In the present application, the average particle size of the hard carbon can be 3 μm to 20 μm, for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm.

[0089] In the present application, the content of hard carbon, based on the total mass of the first negative electrode active material, is ≤ 6%, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, or 6%. The content of hard carbon, based on the total mass of the second negative electrode active material, can be 2% to 10%, for example, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0090] In one example, the content of hard carbon, based on the total mass of the first negative electrode active material, is ≤ 3%. The content of hard carbon, based on the total mass of the second negative electrode active material, is 3% to 6%.

[0091] In the present application, the first coating and the second coating can each independently further include a negative electrode conductive agent. The negative electrode conductive agent, for example, includes at least one of conductive carbon black, conductive graphite, carbon nanotubes (including multi-walled carbon nanotubes and / or single-walled nanotubes), and graphene.

[0092] In the present application, the first negative electrode active material and the second negative electrode active material can each independently further include a silicon-based material. The silicon-based material can include at least one of elemental silicon, a silicon-carbon material, a silicon-oxygen material, and a silicon alloy. The silicon-carbon material refers to a composite material including elemental silicon and elemental carbon. The silicon-oxygen material refers to a composite material including elemental silicon and elemental oxygen.

[0093] In the present application, the mass content of silicon element in the second negative electrode active material is greater than the mass content of silicon element in the first negative electrode active material. When the mass content of silicon element in the second negative electrode active material is greater than the mass content of silicon element in the first negative electrode active material, it is beneficial to the cycle stability of the battery. This is because the silicon-based material has a large volume expansion / contraction during the charging and discharging cycle of the battery, and when more silicon-based material is located on the surface of the negative electrode sheet, it can convert part of the volume expansion of the silicon-based material into expansion into the interior of the negative electrode sheet; and the outer surface of the negative electrode active material layer has a recess, and the second negative electrode active material includes hard carbon; the provision of the recess is beneficial to providing a certain buffer space for the expansion of the silicon-based material matrix, and the hard carbon is beneficial to the insertion and extraction of lithium ions, which can alleviate the insertion and extraction of lithium ions in the silicon-based material, making the silicon-based material more uniform and alleviating the deintercalation of lithium. At this time, the thickness expansion rate of the negative electrode sheet is smaller, the deformation rate of the battery is lower, and it is beneficial to the cycle stability.

[0094] In the present application, the mass content of silicon element in the first negative electrode active material can be 1%-30%, for example, 1%, 5%, 10%, 15%, 20%, 25% or 30%. The mass content of silicon element in the second negative electrode active material can be 2%-50%, for example, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%.

[0095] In an example, the mass content of silicon element in the first negative electrode active material is 2%-10%. The mass content of silicon element in the second negative electrode active material is 5%-15%.

[0096] In an example, the silicon-based material includes the silicon-carbon material. The particle size distribution of the silicon-carbon material can be Dv10 of 3-6 μm (for example, 3 μm, 4 μm, 5 μm or 6 μm), Dv50 of 6-12 μm (for example, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm or 12 μm), and Dv90 of 12-25 μm (for example, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm or 25 μm).

[0097] In the present application, the particle size distribution Dv10, Dv50 and Dv90 of the silicon-carbon material can be obtained by a method conventional in the art, for example, a laser particle size analyzer.

[0098] In the present application, the negative electrode current collector can include a metal layer and a carbon-coated layer located on at least one side surface of the metal layer. The carbon-coated layer can include at least one of conductive carbon black, conductive graphite, carbon nanotubes and graphene.

[0099] The second aspect of the present application provides a lithium ion secondary battery, which can comprise the negative electrode sheet according to the first aspect of the present application.

[0100] In the present application, the lithium ion secondary battery can have an elongation of the negative current collector in the first direction of 0.5-15% after 20 cycles, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%, wherein the cycles are performed under the following conditions: 5 min rest, 0.2 C discharge to 3.0 V, 5 min rest, 0.7 C charge to 4.53 V, cutoff 0.025 C, 5 min rest, 0.2 C discharge to 3.0 V, 5 min rest, 3.5 C for 20 cycles at 25°C±2°C. The lithium ion secondary battery can have a tensile strength of the negative current collector of 200-1000 MPa after 20 cycles, for example, 300 MPa, 400 MPa, 500 MPa, 600 MPa, 700 MPa, 800 MPa, 900 MPa, or 1000 MPa, wherein the cycles are performed under the following conditions: 5 min rest, 0.2 C discharge to 3.0 V, 5 min rest, 0.7 C charge to 4.53 V, cutoff 0.025 C, 5 min rest, 0.2 C discharge to 3.0 V, 5 min rest, 3.5 C for 20 cycles at 25°C±2°C.

[0101] In an example, the lithium ion secondary battery has a tensile strength of the negative current collector of 300-700 MPa after 20 cycles.

[0102] By controlling the elongation and tensile strength of the negative current collector, the energy density and cycle stability of the battery can be further improved, especially when the negative active material comprises silicon-based materials. The negative current collector with specific elongation and tensile strength means that it can better adapt to different shapes and sizes during the battery manufacturing process, so as to reduce the material damage caused by mechanical stress during the battery assembly process or the volume expansion of the active material (especially silicon-based materials) during the battery cycle process. It can also withstand greater stress without breaking when the battery is subjected to external force, which is crucial for maintaining the structural stability and cycle stability of the battery.

[0103] In the present application, the first direction refers to the direction perpendicular to the extension direction of the negative tab, and is also the length direction of the negative electrode sheet.

[0104] In the present application, the lithium ion secondary battery can further comprise a positive electrode sheet. The positive electrode sheet can comprise a positive current collector. The positive current collector can comprise an aluminum foil and / or a carbon-coated aluminum foil.

[0105] In the present application, the lithium ion secondary battery can have a positive electrode current collector with an elongation of 0.5%-5% in the first direction after 20 cycles, for example, 0.5%, 1%, 2%, 3%, 4%, or 5%; wherein the cycles are performed under the following conditions: 25℃±2℃, 5min rest, 0.2C discharge to 3.0V, 5min rest, 0.7C charge to 4.53V, cutoff 0.025C, 5min rest, 0.2C discharge to 3.0V, 5min rest, 3.5C cycle for 20 cycles. The lithium ion secondary battery can have a positive electrode current collector with a tensile strength of 100MPa-400MPa after 20 cycles, for example, 100MPa, 200MPa, 300MPa, or 400MPa; wherein the cycles are performed under the following conditions: 25℃±2℃, 5min rest, 0.2C discharge to 3.0V, 5min rest, 0.7C charge to 4.53V, cutoff 0.025C, 5min rest, 0.2C discharge to 3.0V, 5min rest, 3.5C cycle for 20 cycles.

[0106] In one example, the lithium ion secondary battery can have a positive electrode current collector with a tensile strength of 100MPa-250MPa after 20 cycles.

[0107] In the present application, the negative electrode sheet can have a tab-centered structure and / or a multi-tab structure. The positive electrode sheet can have a tab-centered structure and / or a multi-tab structure.

[0108] In the present application, the components of the battery other than the positive electrode sheet and the negative electrode sheet (for example, the separator and the electrolyte) can be conventional choices in the art.

[0109] 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.

[0110] 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.

[0111] In the following examples, the materials used are commercially available analytical pure unless otherwise specified.

[0112] The following examples are used to illustrate the lithium ion secondary battery of the present application.

[0113] Example 1

[0114] A battery was prepared according to the following method:

[0115] (1) Preparation of a negative electrode sheet

[0116] A first negative electrode active material (average particle size: 8.5 pm, specific surface area: 3.6 m 2 / g; including artificial graphite and silicon-carbon material, wherein the mass ratio of the artificial graphite and the silicon-carbon material was 98:2, the OI value of the artificial graphite was 10, and the specific capacity was 352 mAh / g; the mass content of silicon in the silicon-carbon material was about 60%), butadiene styrene rubber, and conductive carbon black were mixed at a mass ratio of 95:3:2, deionized water was added, and a first coating slurry was prepared; 2 A second negative electrode active material (average particle size: 9 pm, specific surface area: 3.4 m / g; including artificial graphite, hard carbon, and silicon-carbon material, wherein the mass ratio of the artificial graphite, the hard carbon, and the silicon-carbon material was 89:6:5, the OI value of the artificial graphite was 12, and the specific capacity was 354 mAh / g; the mass content of silicon in the silicon-carbon material was about 60%, and the average particle size of the hard carbon was 10 pm), butadiene styrene rubber, and conductive carbon black were mixed at a mass ratio of 95:3:2, deionized water was added, and a second coating slurry was prepared;

[0117] The ratio L of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material was 1.059; Lxh was 32.8;

[0118] The porosity of the first coating was 20.3%, the porosity of the second coating was 29.2%, and the ratio of the porosity of the first coating to the porosity of the second coating was 0.7;

[0119] The areal density of the first coating was 7.24 mg / cm 2 , and the areal density of the second coating was 7.24 mg / cm 2 ;

[0120] The thickness of the first coating was 24 pm, the thickness of the second coating was 30 pm, and the total thickness of the negative electrode sheet was 114 pm;

[0121] The ratio of the depth of the groove on the outer surface of the negative electrode active material layer in the single-sided area to the thickness of the negative electrode active material layer satisfied 0.33-0.5;

[0122] The mass content of silicon in the first negative electrode active material was 1.2%, and the mass content of silicon in the second negative electrode active material was 3%.

[0123] (2) Preparation of a positive electrode sheet

[0124] Lithium cobalt oxide, polyvinylidene fluoride and conductive carbon black were mixed in a mass ratio of 97.5:1.5:1, N-methyl pyrrolidone (NMP) was added to prepare a positive active material slurry, the positive active material slurry was uniformly coated on the surface of a carbon-coated aluminum foil, and the positive active material slurry was dried, roll-pressed and compacted to obtain a positive electrode sheet.

[0125] (3) Preparation of electrolyte

[0126] In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate, dimethyl carbonate and 1,2-propylene carbonate were mixed in a volume ratio of 1:1:1, 1M lithium hexafluorophosphate was added, and the mixture was stirred until it passed the water and free acid tests to obtain an electrolyte.

[0127] (4) Preparation of battery

[0128] The negative electrode sheet prepared in step (2), a separator (polyethylene substrate with boehmite ceramic layer and polyvinylidene fluoride adhesive layer on both sides) and the negative electrode sheet prepared in step (1) were wound to form a roll core, and the aluminum tab and the copper-nickel-plated tab were welded after hot pressing; the battery was formed, sorted, OCV tested after liquid injection, and the battery was obtained, wherein the positive electrode sheet and the negative electrode sheet were both tab-in-the-middle structures.

[0129] Example 2

[0130] Example 1 was followed, except that the negative electrode sheet was prepared in step (1) as follows:

[0131] The first negative active material (average particle size 10 μm, specific surface area 3 m 2 / g; including artificial graphite, hard carbon and silicon-carbon material, wherein the mass ratio of artificial graphite, hard carbon and silicon-carbon material is 94:1:5; the OI value of artificial graphite is 16, and the specific capacity is 354 mAh / g; the mass content of silicon in the silicon-carbon material is about 60%), butadiene rubber and conductive carbon black were mixed in a mass ratio of 95:3:2, deionized water was added, and a first coating slurry was prepared; the second negative active material (average particle size 12 μm, specific surface area 2.7 m 2 / g; including artificial graphite, hard carbon and silicon-carbon material, wherein the mass ratio of artificial graphite, hard carbon and silicon-carbon material is 87:3:10, the OI value of artificial graphite is 20, and the specific capacity is 355 mAh / g; the mass content of silicon in silicon-carbon material is about 60%, and the average particle size of hard carbon is 10 μm), styrene-butadiene rubber and conductive carbon black are mixed uniformly according to a mass ratio of 95:3:2, deionized water is added, and a second coating slurry is prepared; the first coating slurry and the second coating slurry are coated on the surface of the carbon-coated copper foil using a transfer coater, dried, and compacted by a rolling machine, and a groove (the depth h of the groove is 43 μm, the width is 96 μm, and the interval is 1.5 mm) is formed on the surface to obtain a negative electrode sheet;

[0132] The ratio L of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material is 1.2; Lxh is 51.6;

[0133] The porosity of the first coating layer is 22.4%, the porosity of the second coating layer is 25%, and the ratio of the porosity of the first coating layer to the porosity of the second coating layer is 0.9;

[0134] The area density of the first coating layer is 7.24 mg / cm 2 , and the area density of the second coating layer is 7.38 mg / cm 2 ;

[0135] The thickness of the first coating layer is 24 μm, the thickness of the second coating layer is 30 μm, and the total thickness of the negative electrode sheet is 114 μm;

[0136] The ratio of the depth of the groove on the outer surface of the negative electrode active material layer in the single-sided area to the thickness of the negative electrode active material layer satisfies 0.33-0.5;

[0137] The mass content of silicon in the first negative electrode active material is 3%, and the mass content of silicon in the second negative electrode active material is 6%.

[0138] Example 3

[0139] Refer to Example 1, except that the negative electrode sheet is prepared in step (1) as follows:

[0140] The first negative electrode active material (the average particle size is 15 μm, the specific surface area is 2.2 m 2 / g; including artificial graphite, hard carbon and silicon-carbon material, wherein the mass ratio of artificial graphite, hard carbon and silicon-carbon material is 87:3:10; the OI value of artificial graphite is 20, and the specific capacity is 356 mAh / g; the mass content of silicon in silicon-carbon material is about 60%), styrene-butadiene rubber and conductive carbon black are mixed uniformly according to a mass ratio of 95:3:2, deionized water is added, and a first coating slurry is prepared; the second negative electrode active material (the average particle size is 20 μm, the specific surface area is 1.8 m2 / g; including artificial graphite, hard carbon and silicon-carbon material, wherein the mass ratio of artificial graphite, hard carbon and silicon-carbon material is 81:4:15, the OI value of artificial graphite is 24, and the specific capacity is 358 mAh / g; the mass content of silicon in silicon-carbon material is about 60%, and the average particle size of hard carbon is 10 μm), butadiene styrene rubber and conductive carbon black are mixed uniformly according to a mass ratio of 95:3:2, deionized water is added, and a second coating slurry is prepared; the first coating slurry and the second coating slurry are coated on the surface of the carbon-coated copper foil using a transfer coater, dried, and compacted by a rolling machine, and a groove (the depth h of the groove is 25 μm, the width is 76 μm, and the pitch is 1 mm) is manufactured on the surface to obtain a negative electrode sheet;

[0141] wherein the ratio L of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material is 1.333; Lxh is 33.3;

[0142] The porosity of the first coating layer is 24.1%, the porosity of the second coating layer is 34.3%, and the ratio of the porosity of the first coating layer to the porosity of the second coating layer is 0.7;

[0143] The area density of the first coating layer is 7.24 mg / cm 2 , and the area density of the second coating layer is 7.96 mg / cm 2 ;

[0144] The thickness of the first coating layer is 24 μm, the thickness of the second coating layer is 30 μm, and the total thickness of the negative electrode sheet is 114 μm;

[0145] The ratio of the depth of the groove on the outer surface of the negative electrode active material layer in the single-sided area to the thickness of the negative electrode active material layer satisfies 0.33-0.5;

[0146] The mass content of silicon element in the first negative electrode active material is 6%, and the mass content of silicon element in the second negative electrode active material is 9%.

[0147] Example 4

[0148] Example 2 is referred to, except that the thickness of the first coating layer and the thickness of the second coating layer are regulated by changing the area density of the first coating layer and the area density of the second coating layer (the compaction density is controlled to be unchanged), and the depth, width and pitch of the groove are changed at the same time, specifically:

[0149] The area density of the first coating layer is 12.86 mg / cm 2 , and the area density of the second coating layer is 13.01 mg / cm 2 ;

[0150] The thickness of the first coating layer is 42 μm, the thickness of the second coating layer is 57 μm, and the total thickness of the negative electrode sheet is 204 μm.

[0151] L x h is 85.2;

[0152] The depth h of the groove is 71 μm, the width is 117 μm, and the interval is 1.8 mm.

[0153] Example 5

[0154] The first coating layer and the second coating layer are prepared according to Example 3, except that the thickness of the first coating layer and the thickness of the second coating layer are regulated by changing the areal density of the first coating layer and the areal density of the second coating layer (the compacted density is controlled to be constant), and the depth, width and interval of the groove are changed, specifically:

[0155] The areal density of the first coating layer is 12.86 mg / cm 2 , and the areal density of the second coating layer is 13.01 mg / cm 2 ;

[0156] The thickness of the first coating layer is 42 μm, the thickness of the second coating layer is 57 μm, and the total thickness of the negative electrode sheet is 204 μm;

[0157] L x h is 56;

[0158] The depth h of the groove is 42 μm, the width is 94 μm, and the interval is 1.5 mm.

[0159] Example 6

[0160] The first coating layer and the second coating layer are prepared according to Example 2, except that the thickness of the first coating layer and the thickness of the second coating layer are regulated by changing the areal density of the first coating layer and the areal density of the second coating layer (the compacted density is controlled to be constant), and the depth, width and interval of the groove are changed, specifically:

[0161] The areal density of the first coating layer is 4.37 mg / cm 2 , and the areal density of the second coating layer is 5.62 mg / cm 2 ;

[0162] The thickness of the first coating layer is 14 μm, the thickness of the second coating layer is 22 μm, and the total thickness of the negative electrode sheet is 78 μm;

[0163] L x h is 26.4;

[0164] The depth h of the groove is 22 μm, the width is 73 μm, and the interval is 1 mm.

[0165] Example 7

[0166] The first coating layer and the second coating layer are prepared according to Example 3, except that the thickness of the first coating layer and the thickness of the second coating layer are regulated by changing the areal density of the first coating layer and the areal density of the second coating layer (the compacted density is controlled to be constant), and the depth, width and interval of the groove are changed, specifically:

[0167] The areal density of the first coating layer is 4.37 mg / cm 2 , and the areal density of the second coating layer is 5.62 mg / cm 2 ;

[0168] The thickness of the first coating layer is 14 μm, the thickness of the second coating layer is 22 μm, and the total thickness of the negative electrode sheet is 78 μm;

[0169] L x h is 16;

[0170] The depth of the groove is 12 μm, the width is 61 μm, and the interval is 0.8 mm.

[0171] Example 8 group

[0172] This group of examples is used to verify the influence brought by the change of "L x h".

[0173] This group of examples is respectively made in reference to Example 5 and Example 7, except that L x h is regulated by changing the depth of the groove, specifically as follows:

[0174] Example 8a, made in reference to Example 5, except that the depth of the groove is 11 μm, the width is 60 μm, the interval is 0.8 mm, and L x h is 14.7;

[0175] Example 8b, made in reference to Example 7, except that the depth of the groove is 72 μm, the width is 120 μm, the interval is 1.8 mm, and L x h is 96 (this example only changes the depth of the groove located in the double-face area, and the ratio of the depth of the groove located in the single-face area to the thickness of the negative electrode active material layer still satisfies 0.33-0.5).

[0176] Example 9

[0177] Used to verify the influence brought by the change of "the ratio L of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material".

[0178] Made in reference to Example 1, except that L is regulated by changing the average particle size of the second negative electrode active material (at the same time, in order to make the change of L x h as small as possible, the depth, width and interval of the groove are adjusted), specifically as follows:

[0179] L is 2.353;

[0180] The average particle size of the second negative electrode active material is 20 μm, the specific surface area is 1.8 m 2 / g, and the OI value of the artificial graphite is 24, and the specific capacity is 358 mAh / g;

[0181] The porosity of the second coating is 34%, and the ratio of the porosity of the first coating to the porosity of the second coating is 0.6;

[0182] The depth h of the groove is 14 μm, the width is 63 μm, and the pitch is 0.8 mm.

[0183] Example 10

[0184] The influence brought by the change of "the mass content of hard carbon in the second negative electrode active material is greater than the mass content of hard carbon in the first negative electrode active material" is verified.

[0185] Reference is made to Example 1, except that the mass content of artificial graphite, hard carbon and silicon-carbon material in the first negative electrode active material and the second negative electrode active material is changed, specifically as follows:

[0186] In the first negative electrode active material, the mass ratio of artificial graphite, hard carbon and silicon-carbon material is 92:6:2; in the second negative electrode active material, the mass ratio of artificial graphite and silicon-carbon material is 95:5.

[0187] Example 11

[0188] The influence brought by the change of "the mass content of silicon element in the second negative electrode active material is greater than the mass content of silicon element in the first negative electrode active material" is verified.

[0189] Reference is made to Example 1, except that the mass content of artificial graphite, hard carbon and silicon-carbon material in the first negative electrode active material and the second negative electrode active material is changed, specifically as follows:

[0190] In the first negative electrode active material, the mass ratio of artificial graphite and silicon-carbon material is 95:5; in the second negative electrode active material, the mass ratio of artificial graphite, hard carbon and silicon-carbon material is 92:6:2.

[0191] In the first negative electrode active material, the mass content of silicon element is 3%, and in the second negative electrode active material, the mass content of silicon element is 1.2%.

[0192] Example 12 group

[0193] This group of examples is used to verify the influence brought by the change of "recess type".

[0194] This group of examples respectively refers to Examples 1-3, except that the recess is changed from groove to hole, specifically as follows:

[0195] Example 12a, reference is made to Example 1, the depth h of the hole is 32 μm, the hole diameter is 98 μm, and the pitch is 200 μm;

[0196] Example 12b was conducted in the same manner as Example 2, except that the depth h of the hole was 42 μm, the diameter of the hole was 113 μm, and the pitch was 300 μm.

[0197] Example 12c was conducted in the same manner as Example 3, except that the depth h of the hole was 26 μm, the diameter of the hole was 82 μm, and the pitch was 100 μm.

[0198] Example 13

[0199] This was conducted in order to verify the effects of changing the "width of the groove".

[0200] Example 1 was conducted, except that the width of the groove was changed as follows: the width of the groove was 146 μm.

[0201] Example 14

[0202] This was conducted in order to verify the effects of changing the "pitch of the groove".

[0203] Example 1 was conducted, except that the pitch of the groove was changed as follows: the pitch of the groove was 3 mm.

[0204] Example 15 group

[0205] This group of examples was conducted in order to verify the effects of changing the "ratio of the depth of the recess in the single-face region to the thickness of the negative electrode active material layer".

[0206] This group of examples was conducted in the same manner as Example 1, except that the "ratio of the depth of the recess in the single-face region to the thickness of the negative electrode active material layer" was controlled by changing the depth of the recess in the single-face region as follows:

[0207] Example 15a was conducted in the same manner as Example 15, except that the ratio of the depth of the recess in the single-face region to the thickness of the negative electrode active material layer was 0.24.

[0208] Example 15b was conducted in the same manner as Example 15, except that the ratio of the depth of the recess in the single-face region to the thickness of the negative electrode active material layer was 0.8.

[0209] Example 16 group

[0210] This group of examples was conducted in order to verify the effects of changing the "depth h of the recess". In this group of examples, only the depth of the recess in the double-face region was changed, and the depth of the recess in the single-face region was not changed.

[0211] This group of examples was conducted in the same manner as Examples 5 and 6, respectively, except that h was changed as follows:

[0212] Example 16a was conducted in the same manner as Example 6, except that the depth h of the groove was 5 μm, the width of the groove was 23 μm, the pitch was 1.5 mm, and L x h was 6.

[0213] Example 16b was performed with reference to Example 5, except that the recess was changed from a groove to a hole (this is because the depth of the recess in Example 16a was the thickness of the negative electrode tab, and if it were a groove, the negative electrode tab would be broken) and h was changed (in addition, in this example, a laser mode was used to punch the hole), the depth of the hole h was 204 pm, the hole diameter was 94 pm, the pitch was 1 mm, and Lxh was 272.

[0214] Example 17 group

[0215] This group of examples was performed with reference to Examples 16a and 16b, respectively, except that the average particle diameter of the first negative electrode active material and / or the second negative electrode active material was changed, as follows:

[0216] Example 17a was performed with reference to Example 16a, except that the average particle diameter of the second negative electrode active material was 10.5 pm, L was 1.05, and Lxh was 5.25;

[0217] Example 17b was performed with reference to Example 16b, except that the average particle diameter of the first negative electrode active material was 10 pm, the average particle diameter of the second negative electrode active material was 20 pm, L was 2, and Lxh was 408.

[0218] Examples 1-17 group all satisfied:

[0219] The tap density of the second coating layer was greater than the tap density of the first coating layer, and the tap density of the first coating layer was 0.6 g / cm 3 -0.85 g / cm 3 , and the tap density of the second coating layer was 0.7 g / cm 3 -1 g / cm 3 ;

[0220] The tap density of the second coating layer was greater than the tap density of the first coating layer, and the tap density of the first coating layer was 0.6 g / cm 3 -0.85 g / cm 3 , and the tap density of the second coating layer was 0.7 g / cm 3 -1 g / cm 3 ;

[0221] The particle size distribution of the silicon-carbon material was Dv10 of 3 pm-6 pm, Dv50 of 6 pm-12 pm, and Dv90 of 12 pm-25 pm.

[0222] Test Example I

[0223] (1) Current collector elongation test

[0224] The elongation of the positive and negative current collectors of the lithium ion secondary batteries prepared in the examples in the first direction was tested, and the specific test method was as follows:

[0225] The battery was placed at 25℃±2℃ for 5min, discharged to the lower limit voltage (3.0V) at 0.2C, placed for 5min, charged to the upper limit voltage (4.53V) at 0.7C, cut off at 0.025C, placed for 5min, discharged to the lower limit voltage (3.0V) at 0.2C, placed for 5min, and cycled at 3.5C for 20 cycles; then the battery was disassembled, the positive and negative electrode sheets were taken out, the positive and negative active material layers were peeled off, and the positive and negative current collectors were obtained; or the positive and negative current collectors of the empty foil area on the positive and negative electrode sheets were taken. According to the standard requirements, the positive and negative current collectors were respectively cut into 10mm long samples in the first direction (i.e. the direction perpendicular to the extension direction of the tab).

[0226] 1. The two ends of the sample were clamped, two marks were made at the center of the sample, the distance between the two marks was measured as the length before stretching;

[0227] 2. The sample was stretched, the strain was increased by 0.2%-0.5% each time, and the stress at each time was recorded; at the same time, the change in distance between the two marks was observed and measured;

[0228] 3. The distance between the two marks before the sample was broken was taken as the length after stretching;

[0229] The elongation was calculated according to the following formula: elongation=(length after stretching-length before stretching) / length before stretching×100%, and the results were recorded in Table 1.

[0230] (2) Tensile strength test of current collector

[0231] The tensile strength of the positive and negative current collectors of the lithium ion secondary batteries prepared in the examples was tested, and the specific test method was as follows:

[0232] The battery was placed at 25℃±2℃ for 5min, discharged to the lower limit voltage (3.0V) at 0.2C, placed for 5min, charged to the upper limit voltage (4.53V) at 0.7C, cut off at 0.025C, placed for 5min, discharged to the lower limit voltage (3.0V) at 0.2C, placed for 5min, and cycled at 3.5C for 20 cycles; then the battery was disassembled, the positive and negative electrode sheets were taken out, and the positive and negative current collectors of the empty foil area on the positive and negative electrode sheets were taken as samples. The tensile strength of the positive and negative electrode sheets was measured by the following method, specifically as follows:

[0233] 1. The sample was clamped between the upper and lower clamps of the testing machine, and the clamping was ensured to be stable;

[0234] 2. Start the testing machine and load slowly and uniformly until the sample is broken, record the load and strain data during the loading process, calculate the tensile strength of the sample according to the recorded data, and record the results in Table 1.

[0235] Table 1

[0236]

[0237] Note: " / " in Table 1 means no data. Since the ratio of the depth of the recess located in the single-face area to the thickness of the negative active material layer in Example 15b is large, it causes the breakage of the current collector, therefore, the elongation and tensile strength of the negative current collector are not tested.

[0238] Comparative Example 1

[0239] It is carried out according to Example 1, except that step (1) is to prepare a negative electrode sheet, specifically as follows:

[0240] The negative active material (average particle size of 9 μm, specific surface area of 3.4 m 2 / g; including artificial graphite, hard carbon and silicon-carbon material, wherein the mass ratio of artificial graphite, hard carbon and silicon-carbon material is 89:6:5, the OI value of artificial graphite is 12, and the specific capacity is 354 mAh / g; the mass content of silicon in silicon-carbon material is about 60%, and the average particle size of hard carbon is 10 μm), butadiene rubber and conductive carbon black are mixed uniformly at a mass ratio of 95:3:2, deionized water is added, and a negative active material slurry is prepared; the above negative active material slurry is coated on the surface of the carbon-coated copper foil, dried, and compacted by a roller, and grooves are made on the surface (the depth h of the groove is 31 μm, the width is 83 μm, and the pitch is 1.2 mm), to obtain a negative electrode sheet.

[0241] Comparative Example 2

[0242] It is carried out according to Example 1, except that the coating positions of the first coating slurry and the second coating slurry are exchanged, so that the first coating is away from the negative current collector, and the second coating is on the surface of the negative current collector (the recess is still on the surface of the negative electrode sheet).

[0243] Comparative Example 3

[0244] It is carried out according to Example 1, except that no hard carbon is added to the second negative active material, i.e., the second negative active material is a mixture of artificial graphite and silicon-carbon material at a mass ratio of 95:5.

[0245] In the above examples and comparative examples, in order to control the average particle size of the first negative active material and the average particle size of the second negative active material under the premise that the composition of each material in the first coating slurry and the second coating slurry remains unchanged, the average particle size of the artificial graphite and / or silicon-carbon material is adjusted, which is a routine operation in the art and will not be described in detail in the examples.

[0246] Test Example II

[0247] (1) Energy density test

[0248] The batteries prepared in the examples and comparative examples were subjected to energy density tests, and the specific test method was as follows:

[0249] The batteries were charged at 0.7C to the upper limit voltage (4.3V) with a cutoff current of 0.025C, and discharged at 0.5C to the lower limit voltage (2.5V), and the output discharge capacity and working voltage were obtained. The length, width and thickness of the battery were measured using a measuring instrument, and the energy density was calculated by the formula: energy density = discharge capacity x working voltage / (length x width x thickness). The results are shown in Table 2.

[0250] (2) Fast charging performance test

[0251] The batteries prepared in the examples and comparative examples were subjected to fast charging performance (3.5C and 4C) tests, and the specific test method for 3.5C was as follows:

[0252] The batteries were placed at 25°C±2°C for 5 min, discharged at 0.2C to the lower limit voltage (3.0V), placed for 5 min, charged at 0.7C to the upper limit voltage (4.58V) with a cutoff of 0.025C, placed for 5 min, discharged at 0.2C to the lower limit voltage (3.0V) (to test the initial capacity), placed for 5 min, charged at 3.5C to 4.37V, 2.8C to 4.37V, 2C to 4.53V, 1.5C to 4.58V with a cutoff of 0.05C; the voltage and initial thickness at full charge were measured and recorded; placed at 25°C±2°C for 5 min, discharged at 0.7C to 3V, placed for 5 min, charged at 3.5C to 4.37V, 2.8C to 4.37V, 2C to 4.53V, 1.5C to 4.58V with a cutoff of 0.05C, placed for 5 min, discharged at 0.7C to 3V, placed for 5 min, and cycled for 600 cycles. The full-charge voltage and thickness of the battery were then measured, the battery was disassembled, and the lithium precipitation on the surface of the negative electrode sheet was observed. The capacity retention rate, thickness expansion rate and lithium precipitation degree are shown in Table 2. The thickness expansion rate = (full-charge thickness of the battery after 600 cycles - initial thickness) / initial thickness x 100%), and the lithium precipitation degree from light to heavy is as follows: no lithium precipitation, slight lithium precipitation, lithium precipitation and severe lithium precipitation.

[0253] The specific test method for 4C was as follows:

[0254] The battery was rested at 25℃±2℃ for 5min, discharged to lower limit voltage (3.0V) at 0.2C, rested for 5min, charged to upper limit voltage (4.58V) at 0.7C, cut off at 0.025C, rested for 5min, discharged to lower limit voltage (3.0V) at 0.2C (initial capacity test), rested for 5min, charged to 4.37V at 4C, charged to 4.37V at 3C, charged to 4.53V at 2C, charged to 4.58V at 1.5C, cut off at 0.05C; the full state voltage and initial thickness were measured and recorded; rested at 25℃±2℃ for 5min, discharged to 3V at 0.7C, rested for 5min, charged to 4.37V at 4C, charged to 4.37V at 3C, charged to 4.53V at 2C, charged to 4.58V at 1.5C, cut off at 0.05C, rested for 5min, discharged to 3V at 0.7C, rested for 5min, cycled for 600 cycles, then measured the full voltage and thickness of the battery, disassembled the battery, and observed the lithium precipitation on the surface of the negative electrode sheet; the capacity retention rate, thickness expansion rate and lithium precipitation degree were recorded in Table 2.

[0255] Table 2

[0256]

[0257]

[0258] As can be seen from Table 2, the battery prepared from the negative electrode sheet of the present application has higher energy density, higher capacity retention rate, lower thickness expansion rate and better lithium precipitation performance under high-rate charging and discharging conditions compared with the comparative examples, indicating that the battery of the present application can balance high energy density and fast charging capability.

[0259] 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 disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector; the negative electrode active material layer includes a first coating layer and a second coating layer stacked along the thickness direction of the negative electrode sheet, the first coating layer being close to the negative electrode current collector and the second coating layer being away from the negative electrode current collector; the thickness of the negative electrode sheet is 30 μm to 1000 μm; The first coating layer includes a first negative electrode active material, the second coating layer includes a second negative electrode active material, the second negative electrode active material includes a second graphite material and hard carbon, and the ratio of the average particle size of the second negative electrode active material to the average particle size of the first negative electrode active material is L, and 1.05≤L≤1.4; The outer surface of the negative electrode active material layer has a concave portion; The depth of the concave portion is h, in μm; 16≤L×h≤86.

2. The negative electrode sheet according to claim 1, wherein: The average particle size of the first negative electrode active material is 5 μm-15 μm; and / or, the average particle size of the second negative electrode active material is 9 μm-24 μm; and / or, the average particle size of the hard carbon is 3 μm to 20 μm; And / or, based on the total mass of the second negative electrode active material, the content of the hard carbon is 2%-10%.

3. The negative electrode sheet according to claim 1, wherein: The porosity of the second coating layer is greater than the porosity of the first coating layer.

4. The negative electrode sheet according to claim 3, wherein: The ratio of the porosity of the first coating layer to the porosity of the second coating layer is 0.5-0.98; and / or, the porosity of the second coating layer is 20%-50%; And / or, the porosity of the first coating layer is 10%-40%.

5. The negative electrode sheet according to claim 1, wherein: The first negative electrode active material includes a first graphite material; and the OI value of the second graphite material is greater than the OI value of the first graphite material.

6. The negative electrode sheet according to claim 5, wherein: The OI value of the first graphite material is 9-24; the OI value of the second graphite material is 12-26.

7. The negative electrode sheet according to claim 1, wherein: The thickness of the negative electrode sheet is 70 μm-210 μm; and / or, the thickness of the first coating layer is 10 μm-80 μm; And / or, the thickness of the second coating layer is 20 μm-150 μm.

8. The negative electrode sheet according to claim 1, wherein: The recess comprises a hole and / or a groove.

9. The negative electrode sheet according to claim 8, wherein: The pore diameter is 40 μm-200 μm, and the spacing between the pores is 50 μm-1 mm; And / or, the width of the groove is 20 μm-150 μm, and the spacing between the grooves is 200 μm-3 mm.

10. The negative electrode sheet according to claim 1, wherein: The negative electrode current collector has a region having the negative electrode active material layer on only one surface thereof, and a ratio of the depth of the recessed portion on the outer surface of the negative electrode active material layer located in the region to the thickness of the negative electrode active material layer is 0.33-0.

5.

11. The negative electrode sheet according to claim 1, wherein: The first negative electrode active material includes at least one of hard carbon, soft carbon, mesophase carbon microbeads, soft carbon-coated graphite material, and hard carbon-coated graphite material; the second negative electrode active material also includes at least one of soft carbon, mesophase carbon microbeads, soft carbon-coated graphite material, and hard carbon-coated graphite material; The mass content of hard carbon in the second negative electrode active material is greater than the mass content of hard carbon in the first negative electrode active material.

12. The negative electrode sheet according to claim 1, wherein: The first negative electrode active material and the second negative electrode active material each independently include a silicon-based material, and the silicon-based material includes at least one of elemental silicon, silicon-carbon material, silicon-oxygen material and silicon alloy; The mass content of silicon in the second negative electrode active material is greater than the mass content of silicon in the first negative electrode active material; The mass content of silicon in the first negative electrode active material is 1%-30%, and the mass content of silicon in the second negative electrode active material is 2%-50%; The silicon-based material includes the silicon-carbon material, and the particle size distribution of the silicon-carbon material is Dv10 of 3 μm-6 μm, Dv50 of 6 μm-12 μm, and Dv90 of 12 μm-25 μm.

13. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises the negative electrode sheet according to any one of claims 1 to 12.

14. The lithium ion secondary battery according to claim 13, wherein The negative electrode current collector includes a metal layer and a carbon coating layer located on at least one side of the metal layer, and the carbon coating layer includes at least one of conductive carbon black, conductive graphite, carbon nanotubes and graphene.

15. The lithium ion secondary battery according to claim 14, wherein After 20 cycles of the lithium-ion secondary battery, the elongation of the negative electrode current collector in the first direction is 0.5%-15%; the cycle conditions are: at 25°C±2°C, standing for 5 minutes, discharging at 0.2C to 3.0V, standing for 5 minutes, charging at 0.7C to 4.53V, cutting off at 0.025C, standing for 5 minutes, discharging at 0.2C to 3.0V, standing for 5 minutes, and cycling at 3.5C for 20 cycles, wherein the first direction is a direction perpendicular to the extension direction of the negative electrode tab; And / or, after 20 cycles of the lithium-ion secondary battery, the tensile strength of the negative electrode current collector is 200 MPa-1000 MPa; the cycle conditions are: at 25°C±2°C, standing for 5 minutes, discharging at 0.2C to 3.0V, standing for 5 minutes, charging at 0.7C to 4.53V, cutting off at 0.025C, standing for 5 minutes, discharging at 0.2C to 3.0V, standing for 5 minutes, and cycling at 3.5C for 20 weeks.

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